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Recent advancements in nanoelectrodes and nanopipettes used in combined scanning electrochemical microscopy techniques Christine Kranz* In recent years, major developments in scanning electrochemical microscopy (SECM) have significantly broadened
the
application
range
of
this
electroanalytical
technique
from
high-resolution
electrochemical imaging via nanoscale probes to large scale mapping using arrays of microelectrodes. A major driving force in advancing the SECM methodology is based on developing more sophisticated probes beyond conventional micro-disc electrodes usually based on noble metals or carbon microwires. Received 1st September 2013 Accepted 27th October 2013
This critical review focuses on the design and development of advanced electrochemical probes particularly enabling combinations of SECM with other analytical measurement techniques to provide
DOI: 10.1039/c3an01651j
information beyond exclusively measuring electrochemical sample properties. Consequently, this critical
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review will focus on recent progress and new developments towards multifunctional imaging.
Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Albert-Einstein-Allee 11, Ulm, Germany. E-mail: [email protected]; Fax: +49 731 5022764; Tel: +49 731 50227494
Dr Christine Kranz received her M.S. and Ph.D. degrees in Chemistry from the LudwigMaximilians University in Munich (1992) and the Technical University of Munich (1996), Munich, Germany, respectively. Aer spending a year as a postdoctoral fellow at the Vienna University of Technology, Institute of Analytical Chemistry (Austria), she accepted a position at the School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, where she was appointed as a senior research scientist (until June 2008). Since 2005 she has also been a member of the Center for Cell and Molecular Signaling at Emory University, School of Physiology. Since July 2008, she has been heading the surface science group and coordinating the biosensing research activities at the Institute of Analytical and Bioanalytical Chemistry, University of Ulm. In addition, she is the scientic coordinator of the Focused Ion Beam Center UUlm, which was established at the IABC in 2008. Her main research focus is in the eld of scanning probe microscopy in particular scanning electrochemical microscopy (SECM), multifunctional scanning probes (e.g. combination AFM–SECM, IRSECM, IR-AFM), miniaturized amperometric biosensor technology, and (FIB)-based microfabrication.
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1. The hype cycle of SECM and hybrid techniques Since its introduction more than 25 years ago, scanning electrochemical microscopy (SECM) has evolved from an expert tool into a versatile electroanalytical measurement technique documented in a steadily increasing number of published original contributions, review papers, and books/book chapters focusing on various aspects in life sciences, catalysis and materials science. In SECM similar to other scanning probe techniques, a micro- or nanoelectrode is scanned across the sample surface, while the recorded signal (e.g., potentiometric or voltammetric in nature) at the micro- or nanoelectrode is inuenced by the nature and reactivity of the sample and the distance between the electrode and the sample surface. An important aspect in the popularity of SECM is that several companies nowadays offer commercial SECM instrumentation. In fact, one may even state that SECM is not only on an upward trajectory, but also does not follow the otherwise ubiquitous concept of a hype cycle. The hype cycle was introduced by Gartner in 1995 for evaluating the evolution of technology products and their acceptance. This hype cycle was later adapted by Ozin1 and associated with nanotechnology, and also by Hillman2 for developments concerning the electrochemical quartz crystal microbalance. Consequently, why not apply the hype cycle to developments in the eld of scanning electrochemical microscopy? While Fig. 1 indicates the steady development of SECM in an adapted hype cycle, the eld of combined techniques combining SECM with complementary methods may be subject to a different trend. It appears that the number of researchers active
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Adapted Gartner hype cycle for SECM and associated combined techniques.
Fig. 1
in the eld of hybrid scanning probe microscopy (SPM) techniques involving SECM is not nearly growing as rapidly. In fact, some combined techniques such as atomic force microscopy combined with SECM (AFM–SECM) may already be beyond the peak of inated expectations, i.e., well on the way towards trough of disillusionment and slope of enlightenment. In contrast, other combined techniques such as SECM merged with scanning ion conductance microscopy (SICM) may not yet have reached their climax. Despite such different stages of development, expectations, and progress in SECM and combined techniques, there is a common aspect in all these approaches: the development, fabrication, and/or integration of increasingly sophisticated miniaturized electrodes into imaging assemblies, which are more generically referred to as probes or tips adopting the nomenclature of the SPM community.
2. Introduction to micro- and nanoelectrodes The unique features of micro- and nano-sized electrodes are advantageously deployed in many areas of modern electroanalytical chemistry3–5 ranging from biomedical research6,7 to trace analysis and energy related topics.8,9 In particular, their application as probes for in situ scanning probe microscopy (SPM) has been a rapidly developing eld in recent years. This circumstance clearly relates to the introduction of nanometersized electrodes and to advanced probe designs e.g., electrode arrays suitable for imaging experiments; these developments particularly facilitate not only high-resolution imaging, but also mapping across large sample areas. Thereby, quantitative and qualitative information can be obtained while overcoming some of the drawbacks of classical SECM studies related to constant height imaging with conventional disc-shaped, glass-embedded microelectrodes. In comparison with macroscopic systems, electrodes with dimensions smaller than the diffusion layer are not only characterized by enhanced mass transport due to the associated
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hemispherical diffusion prole, but also show decreased double layer charging currents due to their reduced surface area. Furthermore, a reduced ohmic potential (iR) drop is associated with the small faradaic currents in the nA and pA regions. Generally, an electrode is termed “microelectrode” or “ultramicroelectrode”, ((UME) UME is frequently found in the context of SECM), if one dimension is smaller than approximately 25 mm.10 The voltammetric behavior of electrodes is related to the time scale of the experiment. For microelectrodes with disc or spherical geometry, the thickness of the diffusion layer d ¼ OpDt (D: diffusion coefficient and t: time) exceeds the radius of the electrode at relatively short times, resulting in an enhanced mass transport and steady state current. These advantages of microelectrodes have been discussed in detail in many review articles, only a few are listed here.10–13 Since the rst reports indicating the unique features of miniaturized electrodes in electrochemistry and electroanalytical chemistry have been published, a signicant body of literature has been dedicated to theoretical aspects, fabrication strategies, characterization, and applications of micro- and nanoelectrodes. Typically, an electrode is classied as a nanoelectrode, if at least one dimension is smaller than 100 nm,4 although other denitions may be found.
2.1. Conventional SECM and related limitations Shortly aer the initial reports on the benecial properties of microelectrodes, scanning electrochemical microscopy (SECM) was introduced for obtaining spatially resolved electroanalytical information while scanning a microelectrode probe across the sample surface at a constant height (here referred to as “conventional” SECM). While Bard and co-workers have performed the rst SECM experiment actually using an electrochemical scanning tunneling microscope (EC-STM) measuring faradaic currents at the EC-STM tip,14 Engstrom and co-workers reported about at the same time on the voltammetric behavior of a microelectrode within the diffusion eld of a macroscopic electrode.15 In the following years, Bard and co-workers were evolving the instrumental development as well as the associated theoretical models for describing the steady-state current measured at the microelectrode being approached to a sample surface.16–20 Thus, the fundamentals of feedback mode SECM using an articial redox mediator added to the electrolyte solution, and of the generation–collection mode were introduced. In feedback mode SECM21 the term ‘feedback’ refers to a recycling effect of a redox-active species, the oxidized or reduced form of which (only one form is present in solution) is converted at the microelectrode and diffuses within the small gap between the probe and the sample to the conductive sample surface, where its original redox state is regenerated. The locally increased concentration of the redox species results in an increase in faradaic current recorded at the tip (positive feedback effect). In the case of an electrochemically passive (i.e., insulating) surface, the diffusion of redox-active species towards the microelectrode is increasingly limited with the decreasing distance from the tip to the sample surface, and with the increasing radius of the insulation around a disc-shaped microelectrode.
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Generation–collection mode22 does not require an articial redox mediator as the redox-active species generated from the sample is detected at the tip, which is in close proximity to the surface (sample generation–tip collection mode) or vice versa species generated at the tip can be detected at the conductive sample surface (tip generation–sample collection mode). In the rst decade of SECM (i.e., approx. 1986–1996), almost every experiment was conducted with micrometer-sized disc electrodes made from gold and platinum, which were sealed into an insulating glass sheathing providing an electroactive diameter in the range of 1–25 mm, although potentiometric probes and micro-sized ion selective electrodes have also been used. It should be noted that the fabrication and voltammetric behavior of nanoelectrodes has already been described in the literature at that time as well.23,24 Yet, the major limitation using such nanoelectrodes resulted from scanning the miniaturized electrode at a constant height across the sample surface, as the optimum distance depends on the presence and nature of the sample and correlates with the electroactive radius of the miniaturized electrode. Consequently, the smaller the imaging electrode, the closer it has to be positioned to and scanned across the sample surface. Hence, constant height imaging with sub-micron electrodes is more or less limited to possibly at and horizontally aligned samples. Furthermore, positioning of potentiometric microelectrodes25 or modied microelectrodes26,27 (e.g., amperometric enzymatic biosensors) cannot be readily achieved by recording approach curves. Clearly, further evolvement of SECM was reliant on advanced developments (i) enabling reliable maintenance of a constant distance rather than a constant height during imaging, (ii) providing improved lateral resolution, and (iii) simultaneously obtaining additional analytical information on the sample while mapping its electrochemical properties. Almost all of these developments require modication of the electrode design, which will be the focus of this review. Besides the developments discussed in this review, it should be mentioned here that rst approaches for scanning a microelectrode at a constant distance across a sample surface already date back to 1992.28 Bard and Wipf have developed a mode for amperometric measurements, where the microelectrode was oscillated in a sinusoidal motion perpendicular to the sample surface with small amplitude. The resulting current likewise oscillates at that frequency. However, the phase is dependent on the nature of the sample surface. Hence, the frequency and phase of the oscillating current allow separation of topographical from electrochemical information.29 Recently, an improved theoretical model associated with this operation mode was published taking tip-induced convection into account, which is a signicant parameter when imaging insulating samples.30
2.2. Conical and disc-shaped nanoelectrodes Fabrication of conical or disc-shaped nanoelectrodes can be readily achieved by electrochemical etching of microwires and insulation of the etched wires,23,31–34 which was initially used for fabricating scanning tunneling microscope tips suitable for usage in a liquid phase environment.35 Electrodes with radii of several
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nanometers have been produced by etching.23,34 Aer etching, the wire is encapsulated within an insulating material such as apiezon wax,35,36 electrophoretic paint,32,37,38 poly(ethylene),39 polyimide,33 Teon®40 or glass.23,41 Most sheathing materials require a heat treatment step for cross-linking or soening, which nally leads to shrinkage of the sheathing material and exposure of a conical electroactive area at the end of the etched wire. However, obtaining reproducible electrode dimensions, and characterizing their precise geometric shape remain a challenge.4 Nowadays, nanometer-sized electrodes for SECM experiments are mostly fabricated using a laser-assisted pipette puller, which was initially introduced for pulling micropipettes for patch-clamp experiments and for fabricating in vivo microelectrodes and ion selective microelectrodes.42 Shao et al. have demonstrated that laser-assisted pipette pulling is suitable for pulling glass pipettes with inserted metal microwires into glass-encapsulated nanoelectrodes.43 By controlling local heating of the glass and the pulling force, platinum nanoelectrodes with radii in the range of 2–500 nm may be obtained. As the pulled wire is completely encapsulated, the glass at the end of the tip was removed by etching in 40% HF or by micropolishing, which however may limit the reproducibility of the shape of the obtained electrodes. As no experimental details were provided in this initial report by Shao et al.,43 Schuhmann and co-workers have published an optimized procedure for obtaining tight-sealed, disc-shaped
Fig. 2 Shear-force mode SECM. (A) (a) SEM image of a needle-shaped Pt nanoelectrode and (b), (c) electrochemically etched carbon nanoelectrodes. (Reprinted from ref. 44 and 48 with permission from Wiley VCH and the American Chemical Society, respectively). (B) Principle of 4D shear-force mode. The tip is approached using vibration damping to a set threshold value, and the z-point of this closest approach is registered with the tip current representing the sample topography and local electrochemical activity. Without using the shear-force control, the tip is retracted in fixed increments, the current is recorded, and the data are stored. After the desired distance is obtained, the tip is moved to the next x,y position, and the sequence of the shear-force-controlled tip approach with tip retraction in fixed increments while recording the current is repeated. (C) Diffusion profile above a microelectrode. Line scans and x,z,I image representing the diffusion cloud of the electrochemically produced [Ru(NH3)6]2+ species. Reprinted from ref. 65 with permission from the American Chemical Society.
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nanoelectrodes (Fig. 2A(a)) providing detailed information on material and experimental parameters.44 However, exposing and polishing such needle-type nanoelectrodes ensuring a disc-shaped electrode requires an accurate control of the polishing process.45 Due to the pulling process, a needle-shaped electrode is obtained, which is suitable for shear-force based SECM, as described below. Carbon ber electrodes, which are frequently used for detecting biomolecules, have been fabricated by ameetching46 or electrochemical etching47,48 (see Fig. 2A(b) and (c)) and consecutively sealing as described for metal wires.
2.3. Fiber-shaped electrodes for shear-force mode SECM As previously mentioned, an operational limitation of conventional SECM is the fact that the faradaic current determined at the scanning probe simultaneously serves as the analytical signal, and as the parameter for determining the probe position vs. the sample surface. A breakthrough in current-independent positioning was the introduction of shear-force-based positioning of ber-shaped microelectrodes.49,50 Shear-force-based positioning was derived from near-eld scanning optical microscopy (NSOM), and is based on a tapered micro- or nanoelectrode, which is mechanically modulated via horizontal vibration. Short-range hydrodynamic forces in the vicinity of the sample dampen the vibration amplitude; with a given cut-off, this change in amplitude may then be used as error signal within a feedback loop regulating the tip position to maintain a constant distance between the tip and the sample surface. While the rst experiments were realized using optical detection of the shear force,49,51 nowadays most shear-force SECM experiments use non-optical positioning approaches derived from NSOM52,53 such as tuning fork-type resonators50,54,55 or piezo-based actuator–detector systems with a ber holder sandwiched between two piezoelectrically active plates, which Schuhmann and co-workers have adapted and implemented in SECM.56 By using the shear-force distance with the piezo-based actuator–detector system, they could demonstrate constant distance imaging with nanoelectrodes.57,58 Besides using closed feedback loop-based tip positioning based on the shear-force signal, an alternative approach (i.e., standing approach (STA)) was shown with the electrode continuously repositioned at a constant distance to the sample surface by successively recording shear-force approach curves.54,59,60 As stated above, micro- or nanoelectrodes used in shearforce SECM have to be excited via lateral vibration, hence, a ber-shaped geometry of the electrode is required, which is usually obtained via the pipette puller method. The spatial topographical resolution is determined by the diameter of the tapered probe. Shear-force SECM is frequently compared with non-contact AFM measurements, however given the geometric dimensions of micro- or nanometer-sized electrodes embedded in an insulator material, the achievable topographical resolution is a priori not comparable to AFM measurements or images recorded with nanopipettes used in SICM. Despite the fact that no detailed theoretical model has been derived for this current-independent positioning strategy, shear-force mode SECM operation circumvents several of the
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initial limitations of conventional SECM studies based on current-dependent positioning of the electrochemical probe. For example, it facilitates positioning of and imaging with miniaturized biosensors and potentiometric probes (e.g., ion selective electrodes).61–63 The applications of shear-force SECM range from corrosion research64 to applications in neuroscience, which were recently reviewed.7 Schuhmann and coworkers have introduced 4D shear force imaging,65 which allows recording of the tip currents at several yet constant distances to the sample surface at each point of the x,y-grid (Fig. 2B). Using this mode, diffusion proles may be visualized as shown in Fig. 2C. A redox-active species converted at a microelectrode was detected at the shear-force probe in generator/collector mode at different distances, thereby resulting in a map of the diffusion prole (Fig. 2C). Recently, the same authors have visualized the respiratory activity of HEK293 cells by 4D shear-force imaging along with redox competition mode.66 Only recently, the rst approach offering quantitative treatment of shear-force SECM using nanoelectrodes has been published.67 Shear-force distance control has been successfully introduced into SECM, and commercial SECM instruments nowadays offer shear-force mode modules. Recently Etienne et al. presented an automated SECM approach based on the non-optical shear-force for the investigation of large samples with signicant variations in topography.68 However, as additional complexity is introduced into the experiments by laterally vibrating the probe, future studies need to focus on a comprehensive theoretical treatment, in particular if nanometer-sized electrodes are used for high-resolution imaging.
2.4. So microelectrodes A conceptually striking and facile approach to maintain a constant distance to the sample surface without requiring an electronic feedback circuit was introduced in a collaborative effort by the groups of Wittstock and Girault,69 who developed microelectrodes with so polymer insulation called “so stylus microelectrodes”. If the insulation layer is brought into direct contact with the sample surface, the so electrode bends to a certain extent, and hence, the electroactive area has a certain distance to the sample surface, which is kept constant once the microelectrode is dragged in contact across the sample surface (Fig. 3A). Such so probes are obtained by fabricating a microchannel into a polymer sheet via laser ablation or in the case of gold electrodes via aerosol jet printing using materials such as polyethylene terephthalate (PET), or polyimide Kapton HN®, respectively. These channels are then lled with electroactive materials such as conductive carbon ink69 or nanoparticulate gold ink,70 which both need to be cured aer lling and then laminated with a polymer lm to encapsulate the electroactive material. Lamination is obtained with Parylene C, a biocompatible polymer frequently used for insulation of in vivo electrodes71 and AFM–SECM probes.72 The electroactive area of the so probe, which reects a sickle-like shape for carbon electrodes and band-like structures for gold electrodes, is then exposed by laser ablation or by simply cutting with a sharp scalpel (see Fig. 3A for the optical image). As the edge of
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Fig. 3 Soft microelectrode approach. (A) Scheme of a soft stylus microelectrode and an optical image showing the sickle-shaped carbon electrode (reprint from ref. 73 with permission of the American Chemical Society). (B) Schematic representation of a soft stylus electrode array. (C) Sketch of the fountain pen. (D) Scheme of surface modification during a line-scan on OEG-SAM. For modification an “on” potential is applied with a defined pulse length. (Reprinted from ref. 75 with permission of Wiley-VCH). (E) (a) SECM image of the microfabricated EPFL logo recorded with a fountain pen in feedback mode SECM; (b) optical image of the logo. Reprinted from ref. 77 with permission from the Royal Chemical Society.
the so insulator sheet is in contact with the sample surface, the exible electrode bends, and the thickness of the insulation layer, the angle of exposure of the electrode, and the angle under which the electrode is approached to the surface determine the electroactive area of the sample surface.73 d ¼ hP þ tL sinðaÞ; ðhP $ 0; noncontact regimeÞ d¼ d ¼ tL sinðaÞ; ðhP \0; contact regimÞ where a is the angle between the cross-sectional plane of the probe and the sample surface, hp ¼ hA lT (hA: height of the attachment point and lT: vertical extension of the unbent probe), and tL is the thickness of the polymer lm covering the carbon tracks. As stated in the Introduction, substantial efforts in the SECM community are targeted towards high-resolution imaging, however, many real-world applications may also require the opposite, i.e., scanning large sample areas yet trading off in spatial resolution.74 Hence, besides the constant distance capability another interesting aspect of the so stylus electrodes is the possibility of fabricating microelectrode arrays (Fig. 3B).70,73,75 In 2004, Unwin and coworkers demonstrated the rst approach using a planar micro-disc array for parallel SECM imaging.76 In contrast to the so electrode array, the microfabricated array of planar Pt-dot electrodes was kept stationary while the sample was scanned at a constant height across the electrode array. Hence, large-scale samples may be investigated in reasonable periods of time, which is important considering solvent evaporation during extended imaging times, and changes of the sample in the case of in situ processes (e.g., when imaging live biological surfaces/interfaces). Multiplexed linear arrays of eight individually addressable carbon or gold
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electrodes have been demonstrated so far (Fig. 3B), which were employed e.g. for imaging interdigitated electrode arrays and human ngerprints.74 As the array allows multiple scans in a single pass across the surface, the imaging time for an entire ngerprint using an array with eight electrodes (500 mm interelectrode spacing) requires only approx. 5 h, whereas a single electrode would require approx. 1–2 days. Complex patterns were fabricated and analyzed as shown in Fig. 3D. By locally generating reactive species (Br2/HOBr) within short potential pulses at predened times correlating with a pattern dening le (i.e., a digital image was converted into a black-and-white image with 69 80 pixel resolution), an image was created in an oligo(ethyleneglycol)-terminated SAM layer (OEG-SAM) and consecutively mapped.75 Microfabrication of so probes may also be extended toward fabricating the so-called fountain pen probes,77 which enable SECM measurements at dry sample surfaces. The concept of locally establishing an electrochemical micro-cell, and hence, avoiding that the entire sample has to be immersed in electrolyte solution during SECM experiments has recently gained substantial interest78 and can be found in the literature using different synonyms, as described below. The fountain pen is based on the same concept as the so microelectrode, however, one micro-channel is not lled with ink (Fig. 3C) and can be used for delivering a solution, while the second micro-channel serves as an integrated counter/reference electrode, which is perpendicularly connected via the same PET face. The third micro-channel is lled with carbon ink to serve as the working electrode. By adding another microchannel, Girault and Wittstock have demonstrated a push/pull probe79 and a coupled SECM/matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS) system using microchannels for delivering and pulling the solution from the sample surface with a microsyringe pump connected to the channels.80 Again, only the sample location of interest is in contact with the electrolyte solution by delivering a small volume of electrolyte solution to the sample surface with the push channel. Human ngerprints, which had been in contact with picric acid as a model explosive, were investigated with this approach providing complementary analytical information during spatially resolved analysis.80 However, as with any alternative concept in SPM there are some advantages and disadvantages associated with such so stylus-type electrodes. As the probe assembly is in direct contact with the sample, imaging not well adhered so coatings or samples that do not show the required adhesion properties such as single cells may be problematic – similar to contactmode AFM – due to the imposed friction forces. Wittstock et al. provided an estimate of these forces, which are in the microNewton range.81 In addition – and as stated by the authors – reliable microelectrochemistry relies on reliably fabricated and well-dened microelectrodes. As the material used here is conductive ink, which is usually manually applied to the microchannels, a variation in the electrochemical response of individual electrodes was observed.81 To date, only micrometersized electrodes and arrays with comparatively large electrode spacings have been demonstrated though this was the goal for
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large-scale imaging. In addition, individual electrodes within an array may reveal different response behavior as such inks initially contain organic components, which may not be entirely removed during the curing process. Hence, the quality of such electrodes usually does not compare to conventional SECM electrodes based on solid-state microelectrodes manufactured from noble metal wires or carbon bers tightly sealed into insulating glass. As stated by the authors, the Parylene C insulation, which is in contact with the sample surface, may also deteriorate due to the friction forces. In particular, if harder samples are scanned for extended periods of time, this may lead to imaging artifacts due to contamination at the sample surface. However, it has to be noted that the surface is renewable by simply cutting the end of the electrode, thereby rendering these probes suitable for long-term usage. As stated above, so stylus probe arrays and the opportunity to enhance the functionality by adding micro-channels as needed to the assembly render this approach highly suitable for large-scale SECM investigations.
2.5. Nanopipettes in SECM In contrast to the so stylus probes facilitating imaging of large sample areas within reasonable periods of time, routinely achieving both high-resolution topographical and electrochemical imaging remains a challenge. High-resolution SECM was demonstrated with nanoelectrodes in combination with current-independent positioning and even reported with constant height SECM imaging.82 In recent years, a signicant number of studies have used micro- and nanopipettes for SECM experiments. The versatility of nanopipettes in general and in SPM in particular has been recently reviewed by Baker et al.83 Hence, here only recent trends in using nanopipettes in hybrid SECM techniques will be discussed. Micro- and nanopipettes are used in scanning ion conductance microscopy (SICM), which was rst introduced by Hansma et al. in 1989.84 In SICM, ion currents are recorded with spatial resolution by applying a small potential between a reference electrode, which is located within a micro- or nanopipette, and a reference electrode immersed in the sample solution. In contrast to other SPM techniques, a more widespread usage of SICM was rather limited aer its introduction until some signicant instrumental developments including improved positioning routines have been introduced.85,86 In particular in life sciences, SICM has nowadays gained signicant attraction due to highresolution non-contact imaging of biological samples such as live cells and the ease to combine with uorescence microscopy. In SECM, for a long time, micro- and nanopipettes have been mainly used in combination with ion-selective electrodes, where the pipette is lled with an ion-selective cocktail and for liquid/ liquid interface studies of ion transfer or electron transfer mediated by electro-inactive or electroactive species between two immiscible electrolyte solutions. However, such measurements do not necessarily require spatially resolved scanning of the probe.87,88 Bard and co-workers have presented an approach using a micropipette, which was additionally modied with a concentric ring-electrode.89 A borosilicate micropipette was modied with a gold layer and consecutively insulated with
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electrophoretic paint. To form an insulating layer, the electrodeposited paint has to be cured, which leads to shrinking of the sheathing, and therefore, exposure of the ring-electrode around the micro-capillary. The pipette was positioned at a height of a few micrometers in an aqueous electrolyte solution, and ferrocenemethanol was constantly dispensed as electroactive species by applying a constant pressure to the pipette. As a proof-ofprinciple, a platinum disc electrode with an electroactive diameter of 100 mm embedded in glass could be imaged, as the redoxactive species was oxidized at the platinum surface and reduced at the ring-electrode. Given the size of the orice and micrometersized ring-electrode, the achieved lateral resolution was comparable to conventional SECM experiments. For a number of applications in materials science, e.g., for corrosion studies, mapping catalytic activity, and in battery research, immersion of the entire sample surface in electrolyte solution is detrimental, as the surface properties may change during the imaging experiment due to the contact with the electrolyte solution. Consequently, approaches using a small yet dened volume in contact with the sample surface, e.g. for pit corrosion studies, date back to 1995, when B¨ ohni et al. used a microcapillary with a rubber gasket sealing a small contact area where the sample surface was in contact with solution (Fig. 4A).90 Scanning of droplet cells using capillaries with orices of diameters ranging from 10–600 mm was rst described by Lohrengel and co-workers.91–93 Capillaries containing a reference and a counter electrode or gold capillaries containing a reference electrode while the gold serving as a counter electrode were used. With this approach, the sample surface is wetted with droplets having diameters in the range of approximately 10–100 mm due to surface tension.
Fig. 4 (A) Scheme of the droplet cell using a silicone rubber gasket to define the contact area; (B) principle of the scanning micropipette contact method (SMCM). (C) (a) Optical image of a 95–5% Al–Cu alloy; (b) SMCM image of the alloy (area in the yellow square in (a) recorded with a 1 mm diameter micropipette filled with 2 mM Fe(CN)63 and 0.1 M KNO3). The alloy was biased at a potential of 0.10 V vs. AgQRE. (Reprinted from ref. 94 with permission from the American Chemical Society).
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The size of the droplet remains constant, when the capillary is moved across the sample surface. The basic concept of this scanning droplet cell was re-visited by Unwin and co-workers in an elegant way and with an improved positioning routine for imaging, with the introduction of the scanning micropipette contact method (SMCM) as schematically shown in Fig. 4B.94 A moving micropipette with pipette openings in the range of 300–1000 nm is positioned at the sample surface such that the liquid meniscus initiates contact between the sample surface and the capillary due to attractive capillary forces. The micropipette is lled with electrolyte solution containing an electroactive species, while the reference electrode is placed in the upper part of the micropipette (Fig. 4B). Hence, a two-electrode cell is formed once the droplet made contact with the conductive or semi-conductive sample surface, which is connected as the working electrode. Positioning is achieved by detecting the current ow, if the liquid meniscus of the micropipette makes contact to the sample surface. Basal plane imaging of HOPG and the investigation of Al alloy containing 5% Cu were demonstrated by point-to-point measurements, while the pipette was retracted and then re-approached for the next measurement similar to hopping mode operation in SICM.86 Fig. 4C shows the optical image of the alloy (a) and (b) an SMCM image obtained with a micropipette (diam. 1.0 mm) lled with 2 mM Fe(CN)63 and 0.1 M KNO3. The basic principle described above was extended to using double-barrel pipettes fabricated from theta capillaries. Dualbarrel capillaries have been used, for example for surface modication.95 Unwin and his group recognized the potential for highresolution electrochemical imaging using double-barrel micro-
Fig. 5 (A) Scheme of scanning electrochemical cell microscopy using a dual barrel pipette; (B) if the tip is moved to the sample, the electrolyte meniscus is brought into contact with the surface, and a small amplitude oscillation is applied to the z-position of the tip. During scanning, the ion migration current across the meniscus (Icond), the z-extension of the piezoelectric positioner (z), and the surface activity current (Iact) are simultaneously recorded resulting in multiple parameters (images adapted from ref. 97 with permission of the American Chemical Society). (C) SECCM image of HOPG: (a) AC component of the conductance current, (b) topography (z-piezo position) and (c) redox activity of the surface recorded by the reduction of 2 mM [Ru(NH3)6]3+ (reproduced from ref. 98 with permission from Wiley-VCH).
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and nanopipettes in SECM, introducing the concept of scanning electrochemical cell microscopy (SECCM).96 Here, both barrels are lled with electrolyte solution, and each barrel contains a quasi-reference electrode or a combined reference-and-counter electrode (i.e., quasi-reference counter electrode; QRCE) (Fig. 5A). In contrast to the single barrel approach, a voltage may now be applied between the two reference electrodes, which leads to migration of ions across the meniscus between the two barrels. Again, if the dual-barrel pipette is moved towards the sample surface the meniscus at the end of the barrels initiates a contact, which changes the measured ion current. By adding an oscillation of the pipette, an AC component of the ionic current is obtained, as the meniscus is periodically altered. Similar to positioning in SICM, the alternating ionic current component strongly depends on the meniscus height. Hence, it is highly suitable as a signal for a feedback loop, which allows unsurpassed imaging in constant distance operation. When imaging a conductive or semi-conductive surface, coincidentally with topography and ion conductance measurements, the electrochemical properties may be mapped, if an additional potential is applied between the sample (WE) and the electrodes within the scanning dual-barrel pipette. Hence, simultaneously multiple information on topography, electroactivity and conductivity can be obtained (Fig. 5B). The spatial resolution and the quality of the electrochemical measurements strongly depend on the geometrical features of the pipettes including the diameter of the orice, which is currently reported as small as 100 nm.78 High-resolution imaging could be demonstrated by mapping the catalytic properties of individual platinum nanoparticles deposited onto carbon nanotubes with respect to their morphology towards the oxygen reduction reaction.97 By applying an additional constant potential between the substrate and the reference electrode within the barrel, the redox activity could thus be mapped, and the reactivity of individual nanoparticles was demonstrated to vary between NPs with similar size, thereby invoking that variations in morphology have a signicant effect on the reactivity. A recent review article by Unwin et al. highlights the versatility and high-resolution imaging capability of SECCM for studying carbon materials including HOPG (Fig. 5C), CNTs and graphene.78 The most signicant advantage of this approach is the fact that multi-parametric information may simultaneously be obtained at the nanoscale. Furthermore, in comparison with the conventional fabrication of disc nanoelectrodes, which usually requires a nal polishing step, nanopipettes have the advantage that no further mechanical treatment is necessary. In addition, formation of a small electrochemical cell by wetting the sample at the position of the pipette rather than immersing the entire sample reduces effects such as passivation or fouling of the surface, and thus enables studies at the surface of particularly sensitive samples.
3. Combined scanning probe techniques Combining the superior lateral resolution with complementary chemical information at high spatial resolution is an ongoing challenge in scanning probe microscopy. In recent years,
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signicant advancements have been made, which mainly relate to combining individual SPM techniques for obtaining complementary information during imaging of the sample surface. Biomedical research most obviously has the fundamental need to investigate and image molecular events in or at living cells, cell ensembles, or entire organisms yielding quantitative information e.g., on signaling molecules with spatial and temporal resolution. However, also in materials science (e.g., corrosion studies), and energy research (e.g., fuel cells and batteries) essential interest is geared towards analyzing multiple parameters in correlated measurements on continuously changing samples/surfaces. Early approaches combining individual SPM techniques have involved the combination of atomic force and scanning tunneling microscopy with near eld optical microscopy (STM-NSOM),99,100 and the combination of AFM with NSOM.101 Also, reports focusing on the combination of SICM with NSOM and confocal microscopy have been published.85,102 In this review, only combinations of SECM with AFM and SICM will be discussed in detail. Again, probably the most critical aspect ensuring the analytical utility of these combinations is the design and development of reliable probes, as the analytical functionality is usually directly integrated into these tips. The most convenient SPM techniques for combination with SECM are certainly AFM and SICM, although the combination of STM with SECM,103,104 and more recently, SECM with Kelvin probe force microscopy (KPFM), was reported.105 3.1. SICM–SECM In only a few years, signicant progress has been made in developing dual probes for combined high-resolution SICM–SECM imaging. In particular for imaging so samples,
Fig. 6 (A) Fabrication scheme of nanopipette/ring-electrodes for combined SICM–SECM. The pulled capillary is modified outside with a conductive layer and consecutively with an insulation layer. Depending on the insulation method, the conductive ring and orifice are exposed by FIB milling (the viewpoint is along the x-axis and normal to the xaxis). (B) SEM images of SICM–SECM probes (a) gold ring-electrode insulated with ALD, (b) gold electrode insulated with electrophoretic paint, (c) carbon ring/nanopore electrode insulated with Parylene C (adapted from ref. 107–109 with permission of the American Chemical Society and Royal Chemical Society, respectively). (C) Schematic representation of a hybrid SICM–SECM probe based on a dual barrel pipette with one barrel filled with pyrolytic carbon deposition.
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SICM provides the advantage of a non-contact SPM technique achieving a resolution comparable to dynamic mode AFM imaging.106 In addition, the distance control in SICM facilitates constant-distance SECM imaging. Combined SICM–SECM experiments may be readily realized by sputtering a conductive layer onto the pipette followed by insulation and exposure of the ring-electrode at the end of the capillary (see Fig. 6A), as previously demonstrated for microcapillary/microring-electrodes.89 Comstock et al. have presented a combined SICM–SECM probe by modifying a nanopipette with a gold layer of approx. 200 nm thickness, which was then insulated with aluminum oxide using an atomic layer deposition (ALD) process.107 The ringelectrode and the orice of the pipette were exposed by focused ion beam (FIB) milling resulting in a nanopipette with a diameter of approx. 100 nm, and a ring-electrode with a diameter of approx. 300 nm (Fig. 6B(a)). Proof-of-principle measurements of a FIB-fabricated nanostructured model sample providing insulating 400 nm wide trenches milled into gold-coated glass were obtained in AC mode SICM and feedback mode and generator–collector mode SECM. Although the ALD aluminum oxide layer was conformal, it might be of limited utility e.g., in strong basic or acidic solutions. In addition as stated by the authors, the deposition completely occludes the nanopipette, and occurs also inside the nanopipette. A similar approach to the micropipette/microring-electrode published by Bard and co-workers89 was presented by the teams of Matsue and Korchev using also electrophoretic paint as an insulation material.108 However, here, an additional FIB-milling step was applied to ensure a well-exposed and uniform ringelectrode with diameters down to 330 nm as shown in Fig. 6B(b). Imaging of immobilized enzyme spots including glucose oxidase and horseradish peroxidase was demonstrated. The SICM image revealed extremely pronounced topographical features at the GOD spot, which were interpreted as ‘caves’. The electrochemical response revealed such features in the presence of ferrocenemethanol [Fc(CH2OH)], which is oxidized at the ring-electrode to [Fc(CH2OH)]+ that serves as an electron acceptor for the GODcatalyzed glucose oxidation. Only if the dual probe was positioned at a close distance (approx. 100 nm) to the sample surface, the “cave” features could be resolved in the electrochemical response. Upon increasing the distance to 600 nm, the features disappeared in the SECM image due to the distance dependence of SECM in feedback mode. Recently, Baker and co-workers have shown fabrication strategies to obtain sub-micrometer carbon ring-electrodes, carbon ring/platinum disc electrodes or carbon ring/nanopore electrodes, which are suitable for SECM, SICM or combined SICM–SECM measurements (Fig. 6B(c)).109 Such bifunctional nanoprobes are obtained by pyrolyzing Parylene C at a temperature of 900 C in a nitrogen atmosphere. Parylene C is deposited by a chemical vapor deposition process, which may also be used for batch-level production; hence, this approach is certainly interesting, as the same material serves for establishing conductive and insulating coatings. Nanoring-electrodes, SICM–SECM probes as well as platinum disc/carbon ring-electrodes have been fabricated with this approach. Instead of sputtering a layer onto a nanopipette, an elegant approach for combined SICM–SECM probes was demonstrated
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in a collaborative effort by the teams of Unwin, Matsue, and Korchev.110 Capillaries may be readily coated or lled with carbonaceous deposits by in situ pyrolytic decomposition of methane gas,111 which was reported earlier for acetylene112 or butane gas.110 For combined SICM–SECM, one channel of a dual barrel pipette is lled with carbon using this pyrolysis process resulting in double-barrel carbon nanoprobes (DBCNPs). Hence it is important that the second channel is blocked during lling with the gas so that the second channel could be lled at a later stage with an appropriate electrolyte solution for SICM measurements. Successful fabrication of dual-barrel pipettes with the total diameter as small as 100 nm, and carbon nanoelectrodes with diameters ranging from 20 nm to 2 mm was demonstrated providing sufficient reproducibility. The second electrolyte lled barrel was used for topographical analysis based on the SICM distance control or may also be applied for deliberately delivering molecules to the sample surface. Besides some model samples such as a PET membrane with 100 nm pores, also PC 12 cells were imaged with this innovative combined probe. Also both barrels can be lled with carbon material using the pyrolysis process.113 Recently, Unwin and co-workers showed that combined SICM–SECM measurements may also be realized using potentiometric probes.114 The previously described approach of lling one barrel with a carbon electrode was used to further electrodeposit iridium oxide on the nanoscopic area, thereby resulting in a pH-sensitive metal/metal oxide electrode. The approach of depositing iridium oxide onto microelectrodes has previously been proven suitable for pH-microelectrodes in SECM.115 The utility of pH-SICM was demonstrated during a dissolution experiment on calcite microcrystals in aqueous solution leading to local pH changes.
Critical Review
macroscopic sample surface, rst attempts using a metallized cantilever, which was manually insulated at the chip, were performed with a biased Pt-coated cantilever inducing the dissolution of ferrocyanide crystals.116 Topological changes during the dissolution process were simultaneously monitored with the conductive AFM tip. The main benet of merging AFM with SECM is the direct correlation of topological information with the chemical surface activity, both at an excellent lateral resolution.117 In principle, the combination of AFM–SECM requires a bifunctional probe comparable to the combined scanning probes already mentioned herein. To date, the following fabrication approaches have been reported: (i) manually fabricated probes with conical or spherical electrodes located at the apex of the tip; (ii) microfabricated probes with the conductive electrode also constituting the AFM tip; (iii) insulated carbon nanotubes at the end of the AFM tip with an exposed electroactive area; (iv)
3.2. AFM–SECM AFM is among the most widely used and versatile SPM techniques, which may be applied in any environment ranging from imaging live cells in buffered solutions to routine characterization under high vacuum conditions in industrial wafer fabrication. In contrast to SICM, besides superior spatial resolution AFM offers both morphological and (micro)-mechanical information on the sample at the nanometer scale. Fundamentally, AFM is based on (short- and long-ranged) force interactions between the sharp tip at the end of the cantilever arm, which is characterized by a distinct spring constant k, and the sample surface. The tip is scanned in contact across the sample surface, while a computer-controlled feedback loop maintains either constant cantilever deection (i.e., short-range forces dominate) or constant cantilever amplitude (i.e., dynamic mode), usually via an optical read-out of the cantilever movement. Well-established reproducible silicon microfabrication of such cantilevers with tailored spring constants providing integrated sharp tips made from e.g., silicon nitride or silicon enables AFM probes at comparatively low costs, and has contributed to the circumstance that AFM is nowadays considered a routine characterization tool. In contrast to electrochemical AFM, where the AFM tip is used for imaging electrochemical processes induced at a conductive
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(A) Scheme of cantilever-shaped sub-microelectrode; (a) SEM image of a etched and bent Pt wire insulated with anodic electrophoretic paint resulting in a micro-sized conical electrode (reprinted from ref. 118 with permission from the American Chemical Society), (b) SEM image of a sub-micrometer-sized spherical gold electrode formed via a controlled arc discharge technique (reprinted from ref. 133 with permission from the American Chemical Society). (B) Batch microfabricated AFM–SECM probes with triangular or conical electrodes; (a) SEM image of a triangular AFM–SECM probe obtained via electron beam lithography (EBL) (reprint from ref. 128 with permission from the American Chemical Society); (b) conical AFM–SECM probe with a platinum silicide (PtxSiy) electrode fabricated with the molding technique. The metal is embedded between two Si3N4 thin films (reprinted from ref. 134 with permission from the American Chemical Society); (c) SEM image of an insulated boron doped diamond (BDD)-coated AFM– SECM tip fabricated from BDD AFM cantilevers, which were modified with Si3N4 and a 50 nm Cr protection layer. Prior to RIE etching, the Si3N4 and Cr protection layers were partially removed by FIB milling (reprinted from ref. 127 with permission from the Institute of Physics). (C) Scheme of CNT-based AFM–SECM probes. The CNT attached to a conductive AFM tip serves as a template for a metal-wire electrode by coating the CNT with e.g., gold prior to insulation and FIB milling. (a) Polymer-insulated disc-shaped nanoelectrode, which was exposed by FIB milling; (inset: zoomed SEM of the nanowire electrode; reprinted from ref. 135 with permission from the American Chemical Society). (c) TEM image of a polymer insulated multi-walled carbon nanotube with the electroactive area exposed by laser ablation (reprinted from ref. 136 with permission from the Royal Chemical Society). Fig. 7
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Fig. 8 Sketch of an AFM–SECM probe with electrodes located below the tip apex; (a) SEM image of a sub-micrometer-sized tip-integrated frame electrode insulated with a mixed layer of Si3N4 and SiO2. The exposure of the frame electrode and re-shaping of the centered Si3N4 AFM tip are obtained using bitmap-assisted FIB milling developed by Kranz and co-workers. (b) AFM–SECM probe with the integrated disc nanoelectrode fabricated via the semi-batch fabrication process developed by Kranz and co-workers. (c) AFM–SECM probe with a square nanoelectrode. A square electrode and a distance holder at the apex of the pyramid were obtained by only removing part of the SiO2. Inset: AFM–SECM probe with exposing the electrode by milling the SiO2 at an angle of approx. 15 (reprint from ref. 148 with permission from Elsevier). (d) SEM image of a batch-fabricated AFM–SECM probe with a micrometer-sized Pt ring-electrode and a SiC AFM tip obtained by standard microfabrication steps developed by Kranz and coworkers. (e) AFM tip integrated boron-doped-diamond ring-electrode insulated with Parylene C and exposed via FIB milling (reprinted from ref. 150 with permission from Elsevier). (f) L-shaped microelectrode with a flattened arm insulated with epoxy. A disc-shaped electrode is exposed via FIB milling producing a small epoxy AFM tip beside the Pt electrode (reprinted from ref. 145 with permission from the Electrochemical Society).
microfabricated bifunctional probes, which comprise a sharp non-conductive AFM probe, and an electrode that is located at a certain distance above the apex of the tip. An overview of different probe designs is presented in Fig. 7 and 8. All these approaches require a sharp tip for maintaining the lateral resolution of AFM, a force constant comparable to commercially available AFM probes, a well-dened geometry ensuring appropriate imaging quality, and for the electrochemical signal obtained at the electroactive area, pinhole-free insulation of the entire probe avoiding any kinds of leakage currents is required, which may lead to erroneous electrochemical information. 3.2.1. Manually fabricated probes with conical and spherical electrodes. The rst approach, which may be readily adapted without access to any microfabrication facility, is the manual preparation of cantilever-shaped conical micro- or nanoelectrodes, which was introduced by Unwin and MacPherson in 2000.118 Although it should be mentioned that bent microelectrodes sealed into glass were commercially available from Nanonics Imaging Ltd (Israel) already in the 1990s. Metal microwires (Au or Pt) are etched, bent, and attened aer the etching process to ensure sufficient reectivity for the laser signal used as optical read-out of the cantilever deection. Insulation is achieved by deposition of anodic or cathodic electrophoretic paint,118,119 which was initially used for
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insulating electrochemical STM tips (ECSTM).37 As the electrodeposited electrophoretic paint has to be cured for crosslinking, a conically shaped sub-micro- or nanoelectrode is exposed. For AFM–SECM probes with gold electrodes, insulation using cathodic paint resulted in entirely coated probes so that the electroactive area was exposed by applying a high voltage to the probe via an electric arc resulting in spherical or conical gold electrodes with radii ranging from 150 to 550 nm.119 An advantage of this probe design is that high-resolution electrochemical imaging may be achieved due to the exposed nanoscale electroactive area. Demaille and co-workers have developed a new technique, which was termed TARM (tip attached redox mode).120,121 The gold electrode of an AFM–SECM probe is modied with a tethered redox mediator via poly(ethylene glycol) (PEG) linkers. Instead of a free-diffusing mediator, electrons are now shuttled between the gold tip and the substrate acting as molecular sensors for probing the local electrochemical reactivity of nanometer-sized active sites. 3.2.2. Microfabricated AFM–SECM probes with conical electrodes. Microfabricated probes providing conical electrodes have been demonstrated by several research groups using standard lithographic patterning122–127 or electron beam lithography.128,129 The advantage of this approach is that the fabrication steps may be achieved at a wafer level. Standard microfabrication processes based on plasma-enhanced chemical vapor deposition (PECVD), reactive ion etching (RIE), and molding processes, which are used to fabricate commercial silicon nitride probes, were used in combination with the deposition of an additional metal layer prior to the etching steps. As shown by Weaver and co-workers, aer cantilevers and tips were processed from silicon nitride deposited by PECVD onto a Si wafer, EBL was used to pattern the gold contact line, contact pad, and the gold-coated triangular tip prior to applying an additional layer of silicon nitride for insulating the contact from the tip electrode to the contact pad. In the last step, the contact pad and the gold-coated tip were exposed using a resist mask and RIE. Simpler denition of an electroactive area has been demonstrated using Parylene C-coated or photoresistcoated metallized cantilevers either by mechanical exposure or via UV illumination, respectively.130,131 Another approach providing high-aspect AFM–SECM probes was shown by Wain et al. based on commercially available metallic needle probes, which were insulated with Parylene C. The nanoscopic electrode was again exposed by FIB milling.132 Clearly, various advantages and disadvantages may be argued for and against the active electrode area being located at the apex of a combined probe. As electrochemical imaging typically requires a constant distance to the sample surface, AFM–SECM imaging with probes providing an electroactive area at the end of the tip requires scanning of each sample twice: rstly, recording the topography in contact mode or dynamic mode, and then secondly, liing the probe to execute electrochemical imaging. In the case of ‘active’ surfaces such as in catalysis or biological (live) samples, the surface properties may dynamically change in the time frame of such sequenced imaging experiments, which complicates the correlation of electrochemical and topographical information. In addition, as
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the electroactive area is initially in direct contact with the sample, scanning the sample for recording the topography may already alter the exposed electroactive area by abrasion unless sufficiently robust electrode materials such as e.g., boron-doped diamond127 are used, by fouling processes at the electrode surface, or by picking up contamination from the sample surface. Furthermore, direct contact of the electroactive area limits the implementation of electrochemical sensing schemes. 3.2.3. Carbon-nanotubes as AFM–SECM probes. It can be clearly stated that the advantage of such AFM–SECM probes is that nanoscopic electrodes are readily achievable, which allows high-resolution SECM imaging. Unsurpassed resolution in SPM can be achieved with carbon nanotubes (CNTs) due to their high aspect ratios, large Young’s modulus, chemical stability, mechanical robustness, and sub-nanometer radius of curvature.137–139 For SPM measurements, CNTs have to be attached to the probe, which is usually realized by adhesive pick-up with an AFM or STM probe, or by direct growth of CNTs at the tip.140 Combined AFM–SECM probes and conductive AFM probes based on carbon nanotubes have been reported.139,141 Burt et al. have fabricated combined probes by attaching a single-walled nanotube (SWNT) to an AFM tip, which was consecutively modied with a conducting metal layer, insulated with poly(oxyphenylene) and silicon nitride, respectively. The electroactive area was then exposed by FIB-milling.135 Patil et al. have reported the fabrication of AFM–SECM probes by attaching multi-walled nanotubes (MWNTs) to the apex of gold-coated AFM tips using a micromanipulator.136 Insulation was achieved with thin layers of Parylene C, aer the gold layer was passivated with long-chained alkanethiols. The electrode was nally exposed via laser ablation. Although the fabrication was demonstrated, no imaging experiments with these probes were shown. As mentioned above, the high-aspect ratio and the possibility to establish electrodes with diameters as small as 10 nm are convincing arguments for this technology. As the CNTs bend when in contact with the sample surface, direct contact of the electroactive area with the surface is avoided. However if the bending changes, e.g., by dris of the deection signal, this may also lead to changes in the distance between the electrode and the sample. 3.2.4. AFM–SECM probes with recessed electrodes. If the electrochemically active electrode area is separated from the tip apex by positioning the electrode at a certain distance above the AFM tip (Fig. 8), the electrode is advantageously not in direct contact with the sample surface. Hence, as reported for SICM–SECM topographical data and electrochemical data may be simultaneously recorded. However, generating threedimensional multi-layered structures facilitating sharp probes and an electrode not located directly at the tip apex requires multiple microfabrication steps, which are not easily achieved if sub-micrometer electrodes are required. Only the deposition steps including the metallization and the insulation layer are readily obtained using batch processes, whereas the exposure of an electrode recessed from the tip apex usually requires more sophisticated fabrication steps including e.g., focus ion beam milling.142–150 FIB-assisted processing has nowadays matured into a routine procedure for three-dimensional nanofabrication
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and prototyping based on the distinct advantages of mask-less direct processing.151 Kranz and co-workers have fabricated the rst frame-electrodes with edge lengths in the range of 500–1000 nm (as determined by the pyramidal shape of the conventional AFM tips) starting from commercially available silicon nitride probes (Fig 8(a)).142 They have also developed a batch process for fabricating entire bifunctional AFM–SECM cantilevers at the wafer scale establishing disc-shaped nanoelectrodes with diameters in the range of 100 nm (Fig. 8(b), unpublished results) involving only one FIB-milling step, which cannot be obtained at the wafer level. A similar approach was pursued by Wittstock and Oesterschulze,148 although only the backside of a fabricated hollow pyramid was in this case metallized, and hence, the FIB-milling step was crucial in dening the electrode size (Fig. 8(c)). A difficulty with this approach may be related to reproducible electrode dimensions, as the size of the electrode depends on the milling step and variation in the metal layer thickness; however probes with electrodes as small as 20 nm in diameter have been reported. As FIB-milling was achieved at a certain angle, the edge of the insulating AFM tip – in this case SiO2 – made contact with the sample surface for providing the constant distance. Shin et al. have demonstrated a FIB-less batch process for fabricating bifunctional AFM–SECM cantilevers with recessed ring-electrodes (Fig. 8(d));152,153 however, only micrometer-sized dimension could be achieved thereby limiting the achievable SECM resolution. Nevertheless, high-resolution topographical images may be simultaneously recorded along with the electrochemical response, which was demonstrated e.g., in alternating current (AC) impedance imaging without an added redox mediator.154 Recent developments by the teams of Kranz and Nebel succeeded in fabricating AFM–SECM probes with a recessed borondoped diamond electrode (Fig. 8(e)).149,150 BDD is a highly interesting electrode material due to the signicant overpotential for both hydrogen and oxygen evolution, the chemical inertness, the mechanical stability of the material, and the reduced electrode fouling compared to conventional electrode materials.155 As the surface termination plays a signicant role for the electrochemical properties of the BDD electrode, postFIB-milling electrochemical termination steps have been developed to ensure a proper response from such integrated BDD electrodes. Micrometer-sized AFM–SECM probes with the electroactive area recessed from the tip have also been demonstrated by Davoodi et al.145 These probes were fabricated by coating a Pt wire with Parylene C, and then using FIB-fabrication by applying triangular milling steps, which exposed the Pt disc electrode with a polymer tip besides the electrode (Fig. 8(f)). Another approach was based on a glass-encapsulated microwire, which was ground and polished at a dened angle of 15– 20 .156 Similar to the previously discussed so stylus probes, a constant distance to the sample surface was provided once the edge of the insulation was in direct contact with the sample surface. However, given the mechanical properties of the sheathing material, only hard materials could be imaged with this probe design without adversely affecting the sample surface.
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AFM–SECM imaging of glass-embedded Pt microelectrodes. (A) Schematic of the imaging process. (B) (a) SEM image of the used AFM tip-integrated BDD electrode. (b) Topography recorded in contact mode revealing polishing features. (c) Simultaneously recorded electrochemical image in 10 mM Ru(NH3)63+/0.1 M KCl, Etip ¼ 400 mV vs. Ag/AgCl (reprint from ref. 150 with permission from Elsevier); (C) (a) SEM image of the nanowire AFM–SECM probe. (b) Topography recorded in tapping mode at a scan rate of 0.5 Hz; (c) electrochemical image recorded in 10 mM Ru(NH3)63+/0.1 M KNO3 with the sample biased at 400 mV vs. Ag/AgCl and the tip biased at 0.0 mV. Reprinted from 135 with permission from the American Chemical Society. Fig. 9
Fig. 9 shows examples of simultaneously recorded AFM– SECM images using Pt disc electrodes (diam. 25 mm and 2 mm, respectively) as model samples (Fig. 9A). The combined images shown in Fig. 9B were obtained with an AFM tip-integrated micrometer-sized BDD electrode (Fig. 9B(a)) in contact mode AFM and feedback mode SECM in 10 mM Ru(NH3)63+/0.1 M KCl. The topographical image (b) clearly reveals features from the grinding and polishing procedure, which are not evident in the electrochemical image (c) as the corrugations do not inuence the conductivity of the Pt. A small diamond particle is visible in the topography and also in the SECM image as the particle results in a reduced current signal as the conductive surface is blocked by the diamond particle.150 Fig. 9C shows combined images of a Pt microelectrode with a diameter of 2 mm. The images were recorded in tapping mode AFM and feedback mode SECM in 10 mM Ru(NH3)63+/0.1 M KNO3 with a combined nanowire probe bearing an electroactive area of 100 nm and an insulation layer of 200 nm. In both examples the electrochemical response matches excellently with the topographical features. Both examples reveal high-resolution topographical features as small scratches with nanometer dimensions are resolved. The electrochemical image of the latter examples shows some artifacts, which the authors relate to brief electrical contact between the tip and the protrusions on the Pt substrate.135 Similar to AFM probes with electrodes extending to the tip apex, bifunctional probes providing recessed electrode designs are characterized by a series of advantages and disadvantages. In general, most fabrication schemes for such probes rely on access to appropriate micro- and nanofabrication facilities. In
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addition, fabrication of nanometer-sized electrodes appears more challenging, and hence, most of the demonstrated probes with recessed electrodes are in the sub-micrometer dimension range concerning the electroactive area. Furthermore, profound theoretical descriptions of the electrochemical behavior of discelectrodes are much more prevalent in the literature compared to frame-type electrodes.157 Yet, a signicant advantage is certainly the fact that topographical and electrochemical responses may be obtained in a truly simultaneous fashion. Furthermore, the electroactive area may be readily modied with additional sensing and molecular recognition layers based on a wide variety of immobilization architectures, which was demonstrated by implementing e.g., a biosensor interface into such AFM probes.158,159 In addition, the force constants of commercial cantilevers, which are modied with a conductive and an insulation layer as described herein are only slightly modied, and hence, so samples may be imaged e.g., in dynamic mode160 or simultaneous force measurements may be envisaged. Independent of the fabrication route, dual imaging may be performed in contact or dynamic mode AFM, and in generator– collector mode or feedback mode SECM using such bifunctional probes. Similar to the development of other emerging analytical techniques, many of the measurements demonstrated with AFM–SECM so far have been conducted on model samples such as microstructured patterns with alternating conductive and insulating features,122,129,131,134,142,146,153 glassembedded disc microelectrodes,128,135,150,152,161 well-dened surfaces such as HOPG,120,124,147 and transport of redox-active molecules through articial membranes,162–164 rather than realworld samples. Almost all studies were conducted with amperometric probes, however similar to conventional SECM, potentiometric probes or modied electrodes such as amperometric biosensors may also be integrated into AFM–SECM probes. As an example solid-state pH electrodes (Sb/Sb-oxide) as shown in Fig. 10C or Ir/Ir-oxide, or tip-integrated biosensors can be integrated. Examples with more relevance in terms of investigating induced changes at the surface have been demonstrated by mapping dissolution processes116,165,166 and corrosion-related AFM–SECM studies.145,156,167 The investigation of the biphasic alloy Ti–6Al–4V147 as a relevant industrial electrode system has also been reported. Earlier during the development of SECM its utility for studying enzyme activity has been demonstrated.26 Kueng et al. have applied AFM–SECM imaging to map the activity of glucose oxidase (GOD),160 which was entrapped within so polymer spots, as initially described by Schuhmann and co-workers.168 Enzyme spots were simultaneously imaged in dynamic mode AFM and generator–collector mode SECM. In the presence of glucose, an increased current was detected above the enzyme spots due to the formation of hydrogen peroxide as the side-product of the enzymatic conversion. The activity of immobilized horseradish peroxidase HRP spots was also imaged using ferrocenemethanol (FMA) as the redox mediator.159 Hirata et al.131 have investigated the enzyme activity of GOD, which was immobilized on a cross-linked polyion complex layer (poly-L-lysine and poly(4-styrenesulfonate
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Examples of combined SECM approaches using nonamperometic probes. (A) (a) Scheme of an AFM tip-integrated biosensor; (b) topographical image recorded with a combined AFM SECM probe in tapping mode AFM (scan area: 2450 2450 nm, scan rate: 1 line per s, drive frequency: 32 kHz) reveals the pores (diam. 200 nm) of a track-etched membrane. This membrane separates the donor compartment containing glucose solution (3 mM) and an upper compartment (PBS puffer). (b) The tip-integrated biosensor (immobilized glucose oxidase) was biased at a potential of 650 mV vs. Ag/AgCl to oxidize H2O2, the side product of the GOD-catalyzed oxidation of glucose. The electrochemical image reflects the diffusion of glucose through the pores resulting in an increased current due to oxidation of H2O2. (Adapted from ref. 158 with permission from Wiley-VCH). (B) (a) Schematic view of tip-attached redox mediator (TARM) AFM–SECM probe. (b) Topography of a HOPG sample recorded with an FcPEGylated AFM–SECM probe in tapping mode (drive frequency: 8.66 kHz, amplitude set point 4 nm, scan rate: 1 line per s). The tip current is presented in (b). The tip potential was set to Etip + 0.30 V per SCE and the substrate potential was set to Esub ¼ + 0.2 V per SCE. Images were processed with a flattening algorithm (adapted from ref. 120 with permission from the American Chemical Society). (C) (a) Sketch of a combined SICM–SECM probe with a pH-sensitive nanoelectrode (b) SICM topography of a calcite microcrystal (bulk pH 6.85). A 350 mV bias was typically applied between the Ag/AgCl electrode inside the SICM barrel and the QRCE in the bulk solution for SICM measurements; a sinusoidal wave creating a 60 nm peak–peak amplitude (280 Hz) was applied to the z-position of the probe. The resulting ac magnitude iAC was determined via the lock-in amplifier. (b) Simultaneously recorded pH map close to the calcite microcrystal adapted from ref. 114 with permission from the American Chemical Society. (D) Scheme and SEM image of a potentiometric AFM–SECM probe with a pH microsensor based on antimony oxide developed by Kranz and coworkers; a corresponding potential-pH calibration curve is shown. Fig. 10
composite)) at HOPG electrodes. AFM was operated in noncontact mode, thus enabling imaging of the sample surface with a dual probe bearing the electroactive area at the tip apex. Ideally, both the techniques are matching in terms of resolution. This is a challenging task as it requires nanoscopic electrodes, which can be reproducibly fabricated either at the apex of the tip or recessed from the tip. In addition, such electrodes should show superior electrochemical characteristics and importantly the kinetics of the electron transfer (in feedback) has to be fast so that the electrochemical process can be imaged. Demaille and co-workers have published a series of papers focusing on high-resolution imaging.120,121,169–172 Instead
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of freely diffusing mediators, redox-tagged chains (e.g., oligonucleotides or polyethylene glycol) were implemented, which were either attached to the sample surface169,170,172 or to the apex of the combined AFM–SECM gold tip.120,121 The authors succeeded in imaging individual gold nanoparticles with dimensions of approx. 20 nm, which were functionalized by redoxlabeled PEG chains.172 As the AFM–SECM probe has the electrode at the apex of the combined tip, the electroactive groups of the chains may be oxidized or reduced, if the AFM–SECM probe is in contact. Several acronyms have been introduced for the approach using tethered redox molecules including molecule touching SECM– AFM (Mt/SECM–AFM) and the tip-attached redox mediator (TARM). Again, as any other discussed methodology, AFM–SECM also has some distinct advantages and disadvantages. A clear advantage is the possibility of obtaining high-resolution topographical and electrochemical imaging, which can be ideally directly correlated as the information may be obtained in a single scan. In addition, the SECM functionality can be directly implemented into any standard AFM system in combination with an external potentiostat so that the electrochemical data can be fed in an additional AD channel, which is offered all commercial AFM systems. A concern raised by Wittstock67 that AFM–SECM probes reveal imperfect shapes of steady-state voltammograms is not necessarily true and strongly depends on the quality of the insulation, which is also true for other probe designs discussed in this review. Despite the general advantages of AFM–SECM, a particular challenge of combined probes remains: the mounting and establishment of a suitable electrical contact in comparison with the other combined approaches discussed here. Needle-type electrodes, so stylus electrodes, nanocapillaries or combined SICM–SECM probes do not require that the entire probe including the contact should be immersed in solution, which instead is usually the case for AFM–SECM probes. Hence, substantial precautions are needed to ensure that the electrical contact with the tip-integrated electrode remains well insulated aer mounting the probe avoiding leakage currents, which is usually ensured by manually insulating the contact with insulating varnish, nail polish etc. In addition, given the dimensions of the integrated electrodes, background noise, additional resistance due to connections, etc. have to be considered.
4. Quo vadis, SECM? As stated in the Introduction, SECM has not yet reached the zenith of the hype cycle in terms of applications, probe design, instrumental developments and improvements, and the introduction of additional complementary imaging modalities. Important aspects facilitating the advancement of SECM are certainly (i) that several companies are nowadays offering commercial SECM instruments, and that (ii) SECM is considered a truly non-invasive SPM technique, which renders it highly suitable for fundamental studies in bioelectrochemistry, biocorrosion, fuel cell research, electrochemical nanotechnology, sensor technology, and materials science.
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Consequently, advancing the technology towards multifunctional SPM and combined analytical platforms integrating complementary analytical techniques such as various spectroscopies or mass spectrometry is clearly of interest. Trifunctional scanning probes integrating the AFM–SECM– NSOM functionality have already been demonstrated and applied for imaging neurites of living PC12 cells.173 Recently, also combinations of SECM with spectroscopic techniques such as attenuated total reection (ATR) IR spectroscopy (IR–ATR– SECM) have been reported.174,175 This concept may be readily extended toward a combination with AFM–SECM or by implementing an IR-transparent electrode for combined spectroelectrochemical measurements.176 Likewise, rst efforts towards the combination of SECM with mass spectrometry have been reported in the literature. Another interesting aspect of combined analytical methods involving SECM is that any electroanalytical measurement technique may in fact be applied in combination with SECM, thereby providing spatial resolution. Consequently, limitations of classical electroanalytical techniques typically providing bulk information and usually requiring large sample volumes may be circumnavigated. Hence, SECM measurements have been obtained in combination with amalgam microelectrodes,177–180 with potentiometric probes (micro-ISEs and pH-microelectrodes),25,61,115,181 with modied electrodes such as microbiosensors,26,182,183 and with electrocatalytically modied electrodes.184–186 Such modications are not limited to micrometer-sized electrodes, but may be further miniaturized and implemented into combined SPM probes, as shown in Fig. 10. For example, a pH-sensitive electrode was obtained by electrodeposition of hydrous iridium oxide onto the carbon electrode of a combined SICM–SECM probe. The dissolution of calcite, which is strongly pH-dependent, was imaged. An increase of the local pH at the calcite– water interface could be resolved. Such strategies will signicantly broaden the eld of applications for multifunctional SPM probes, and hence, contribute to the deviation from the hype cycle shown in Fig. 1. For a more comprehensive overview, the reader is directed to ref. 187. However, considering the range of potential combinations discussed herein, the reliable performance of each individual analytical technique in these combinations is of paramount importance. In addition, their applicability beyond academic research, i.e., demonstrating full functionality on real-world samples, is a prerequisite for a more widespread usage of such rather sophisticated technologies.
Acknowledgements J.-S. Moon is acknowledged for nanodisc AFM–SECM and potentiometric probe development presented here. Furthermore, all funding agencies supporting research at IABC related to AFM–SECM are greatly acknowledged.
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134 M. R. Gullo, P. L. T. M. Frederix, T. Akiyama, A. Engel, N. F. DeRooij and U. Staufer, Anal. Chem., 2006, 78, 5436– 5442. 135 D. P. Burt, N. R. Wilson, J. M. R. Weaver, P. S. Dobson and J. V. Macpherson, Nano Lett., 2005, 5, 639–643. 136 A. V. Patil, A. F. Beker, F. G. M. Wiertz, H. A. Heering, G. Coslovich, R. Vlijm and T. H. Oosterkamp, Nanoscale, 2010, 2, 734–738. 137 S. S. Wong, E. Joselevich, A. T. Woolley, C. L. Cheung and C. M. Lieber, Nature, 1998, 394, 52–55. 138 C. V. Nguyen, Q. Ye and M. Meyyappan, Meas. Sci. Technol., 2005, 16, 2138–2146. 139 N. R. Wilson and J. V. Macpherson, Nat. Nanotechnol., 2009, 4, 483–491. 140 J. H. Hafner, C. L. Cheung and C. M. Lieber, J. Am. Chem. Soc., 1999, 121, 9750–9751. 141 A. Patil, J. Sippel, G. W. Martin and A. G. Rinzler, Nano Lett., 2004, 4, 303–308. 142 C. Kranz, G. Friedbacher, B. Mizaikoff, A. Lugstein, J. Smoliner and E. Bertagnolli, Anal. Chem., 2001, 73, 2491–2500. 143 A. Lugstein, E. Bertagnolli, C. Kranz, A. Kueng and B. Mizaikoff, Appl. Phys. Lett., 2002, 81, 349–351. 144 A. Lugstein, E. Bertagnolli, C. Kranz and B. Mizaikoff, Surf. Interface Anal., 2002, 33, 146–150. 145 A. Davoodi, J. Pan, C. Leygraf and S. Norgren, Electrochem. Solid-State Lett., 2005, 8, B21–B24. 146 J. Wiedemair, B. Balu, J.-S. Moon, D. W. Hess, B. Mizaikoff and C. Kranz, Anal. Chem., 2008, 80, 5260–5265. 147 S. E. Pust, M. Salomo, E. Oesterschulze and G. Wittstock, Nanotechnology, 2010, 21, 105709/1–105709/12. 148 M. Salomo, S. E. Pust, G. Wittstock and E. Oesterschulze, Microelectron. Eng., 2010, 87, 1537–1539. 149 W. Smirnov, A. Kriele, R. Hoffmann, E. Sillero, J. Hees, O. A. Williams, N. Yang, C. Kranz and C. E. Nebel, Anal. Chem., 2011, 83, 4936–4941. 150 A. Eifert, W. Smirnov, S. Frittmann, C. Nebel, B. Mizaikoff and C. Kranz, Electrochem. Commun., 2012, 25, 30–34. 151 R. M. Langford, P. M. Nellen, J. Gierak and Y. Fu, MRS Bull., 2007, 32, 417–423. 152 H. Shin, P. J. Hesketh, B. Mizaikoff and C. Kranz, Sens. Actuators, B, 2008, 134, 488–495. 153 H. Shin, P. J. Hesketh, B. Mizaikoff and C. Kranz, Anal. Chem., 2007, 79, 4769–4777. 154 K. Eckhard, H. Shin, B. Mizaikoff, W. Schuhmann and C. Kranz, Electrochem. Commun., 2007, 9, 1311–1315. 155 J. H. T. Luong, K. B. Male and J. D. Glennon, Analyst, 2009, 134, 1965–1979. 156 A. Davoodi, A. Farzadi, J. Pan, C. Leygraf and Y. Zhu, J. Electrochem. Soc., 2008, 155, C474–C485. 157 O. Sklyar, A. Kueng, C. Kranz, B. Mizaikoff, A. Lugstein, E. Bertagnolli and G. Wittstock, Anal. Chem., 2005, 77, 764–771. 158 A. Kueng, C. Kranz, A. Lugstein, E. Bertagnolli and B. Mizaikoff, Angew. Chem., Int. Ed., 2005, 44, 3419–3422. 159 C. Kranz, A. Kueng, A. Lugstein, E. Bertagnolli and B. Mizaikoff, Ultramicroscopy, 2004, 100, 127–134.
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160 A. Kueng, C. Kranz, A. Lugstein, E. Bertagnolli and B. Mizaikoff, Angew. Chem., Int. Ed., 2003, 42, 3238–3240. 161 J. V. Macpherson and P. R. Unwin, Anal. Chem., 2001, 73, 550–557. 162 J. V. Macpherson, C. E. Jones, A. L. Barker and P. R. Unwin, Anal. Chem., 2002, 74, 1841–1848. 163 C. E. Gardner, P. R. Unwin and J. V. Macpherson, Electrochem. Commun., 2005, 7, 612–618. 164 A. Kueng, C. Kranz, A. Lugstein, E. Bertagnolli and B. Mizaikoff, Angew. Chem., Int. Ed., 2005, 44, 3419– 3422. 165 C. E. Jones, J. V. Macpherson and P. R. Unwin, J. Phys. Chem., 2000, 104, 2351–2359. 166 C. E. Jones, P. R. Unwin and J. V. Macpherson, ChemPhysChem, 2003, 4, 139–146. 167 A. Davoodi, J. Pan, C. Leygraf, R. Parvizi and S. Norgren, Mater. Corros., 2013, 64, 195–198. 168 C. Kurzawa, A. Hengstenberg and W. Schuhmann, Anal. Chem., 2002, 74, 355–361. 169 J. Abbou, A. Anne and C. Demaille, J. Am. Chem. Soc., 2004, 126, 10095–10108. 170 A. Anne, E. Cambril, A. Chovin and C. Demaille, Anal. Chem., 2010, 82, 6353–6362. 171 R. D. Rodriguez, A. Anne, E. Cambril and C. Demaille, Ultramicroscopy, 2011, 111, 973–981. 172 K. Huang, A. Anne, M. A. Bahri and C. Demaille, ACS Nano, 2013, 7, 4151–4163. 173 A. Ueda, O. Niwa, K. Maruyama, Y. Shindo, K. Oka and K. Suzuki, Angew. Chem., Int. Ed., 2007, 46, 8238–8241. 174 L. Wang, J. Kowalik, B. Mizaikoff and C. Kranz, Anal. Chem., 2010, 82, 3139–3145. 175 L. Wang, C. Kranz and B. Mizaikoff, Anal. Chem., 2010, 82, 3132–3138. 176 D. Neubauer, J. Scharpf, A. Pasquarelli, B. Mizaikoff and C. Kranz, Analyst, 2013, 138, 6746–6752. 177 S. Daniele, C. Bragato, I. Ciani and M. A. Baldo, Electroanalysis, 2003, 15, 621–628. 178 J. Mauzeroll, E. Hueske and A. J. Bard, Anal. Chem., 2003, 75, 3880–3889. 179 S. Daniele, I. Ciani, M. A. Baldo and C. Bragato, Electroanalysis, 2007, 19, 2067–2076. 180 M. Alpuche-Aviles, J. E. Baur and D. O. Wipf, Anal. Chem., 2008, 80, 3612–3621. 181 C. Wei, A. J. Bard, G. Nagy and K. Toth, Anal. Chem., 1995, 67, 1346–1356. 182 A. Kueng, C. Kranz and B. Mizaikoff, Biosens. Bioelectron., 2005, 21, 346–353. 183 M. Ciobanu, D. E. Taylor Jr, J. P. Wilburn and D. E. Cliffel, Anal. Chem., 2008, 80, 2717–2727. 184 J. Li and J. Yu, Bioelectrochemistry, 2008, 72, 102–106. 185 N. Baltes and J. Heinze, ChemPhysChem, 2009, 10, 174– 179. 186 O. G. Voronin, A. Hartmann, C. Steinbach, A. A. Karyakin, A. R. Khokhlov and C. Kranz, Electrochem. Commun., 2012, 23, 102–105. 187 Scanning Electrochemical Microscopy, Second Edition., ed. A. J. Bard and M. V Mirkin, CRC Press, 2013.
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CRITICAL REVIEW
Cite this: Analyst, 2014, 139, 336
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Recent advancements in nanoelectrodes and nanopipettes used in combined scanning electrochemical microscopy techniques Christine Kranz* In recent years, major developments in scanning electrochemical microscopy (SECM) have significantly broadened
the
application
range
of
this
electroanalytical
technique
from
high-resolution
electrochemical imaging via nanoscale probes to large scale mapping using arrays of microelectrodes. A major driving force in advancing the SECM methodology is based on developing more sophisticated probes beyond conventional micro-disc electrodes usually based on noble metals or carbon microwires. Received 1st September 2013 Accepted 27th October 2013
This critical review focuses on the design and development of advanced electrochemical probes particularly enabling combinations of SECM with other analytical measurement techniques to provide
DOI: 10.1039/c3an01651j
information beyond exclusively measuring electrochemical sample properties. Consequently, this critical
www.rsc.org/analyst
review will focus on recent progress and new developments towards multifunctional imaging.
Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Albert-Einstein-Allee 11, Ulm, Germany. E-mail: [email protected]; Fax: +49 731 5022764; Tel: +49 731 50227494
Dr Christine Kranz received her M.S. and Ph.D. degrees in Chemistry from the LudwigMaximilians University in Munich (1992) and the Technical University of Munich (1996), Munich, Germany, respectively. Aer spending a year as a postdoctoral fellow at the Vienna University of Technology, Institute of Analytical Chemistry (Austria), she accepted a position at the School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, where she was appointed as a senior research scientist (until June 2008). Since 2005 she has also been a member of the Center for Cell and Molecular Signaling at Emory University, School of Physiology. Since July 2008, she has been heading the surface science group and coordinating the biosensing research activities at the Institute of Analytical and Bioanalytical Chemistry, University of Ulm. In addition, she is the scientic coordinator of the Focused Ion Beam Center UUlm, which was established at the IABC in 2008. Her main research focus is in the eld of scanning probe microscopy in particular scanning electrochemical microscopy (SECM), multifunctional scanning probes (e.g. combination AFM–SECM, IRSECM, IR-AFM), miniaturized amperometric biosensor technology, and (FIB)-based microfabrication.
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1. The hype cycle of SECM and hybrid techniques Since its introduction more than 25 years ago, scanning electrochemical microscopy (SECM) has evolved from an expert tool into a versatile electroanalytical measurement technique documented in a steadily increasing number of published original contributions, review papers, and books/book chapters focusing on various aspects in life sciences, catalysis and materials science. In SECM similar to other scanning probe techniques, a micro- or nanoelectrode is scanned across the sample surface, while the recorded signal (e.g., potentiometric or voltammetric in nature) at the micro- or nanoelectrode is inuenced by the nature and reactivity of the sample and the distance between the electrode and the sample surface. An important aspect in the popularity of SECM is that several companies nowadays offer commercial SECM instrumentation. In fact, one may even state that SECM is not only on an upward trajectory, but also does not follow the otherwise ubiquitous concept of a hype cycle. The hype cycle was introduced by Gartner in 1995 for evaluating the evolution of technology products and their acceptance. This hype cycle was later adapted by Ozin1 and associated with nanotechnology, and also by Hillman2 for developments concerning the electrochemical quartz crystal microbalance. Consequently, why not apply the hype cycle to developments in the eld of scanning electrochemical microscopy? While Fig. 1 indicates the steady development of SECM in an adapted hype cycle, the eld of combined techniques combining SECM with complementary methods may be subject to a different trend. It appears that the number of researchers active
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Adapted Gartner hype cycle for SECM and associated combined techniques.
Fig. 1
in the eld of hybrid scanning probe microscopy (SPM) techniques involving SECM is not nearly growing as rapidly. In fact, some combined techniques such as atomic force microscopy combined with SECM (AFM–SECM) may already be beyond the peak of inated expectations, i.e., well on the way towards trough of disillusionment and slope of enlightenment. In contrast, other combined techniques such as SECM merged with scanning ion conductance microscopy (SICM) may not yet have reached their climax. Despite such different stages of development, expectations, and progress in SECM and combined techniques, there is a common aspect in all these approaches: the development, fabrication, and/or integration of increasingly sophisticated miniaturized electrodes into imaging assemblies, which are more generically referred to as probes or tips adopting the nomenclature of the SPM community.
2. Introduction to micro- and nanoelectrodes The unique features of micro- and nano-sized electrodes are advantageously deployed in many areas of modern electroanalytical chemistry3–5 ranging from biomedical research6,7 to trace analysis and energy related topics.8,9 In particular, their application as probes for in situ scanning probe microscopy (SPM) has been a rapidly developing eld in recent years. This circumstance clearly relates to the introduction of nanometersized electrodes and to advanced probe designs e.g., electrode arrays suitable for imaging experiments; these developments particularly facilitate not only high-resolution imaging, but also mapping across large sample areas. Thereby, quantitative and qualitative information can be obtained while overcoming some of the drawbacks of classical SECM studies related to constant height imaging with conventional disc-shaped, glass-embedded microelectrodes. In comparison with macroscopic systems, electrodes with dimensions smaller than the diffusion layer are not only characterized by enhanced mass transport due to the associated
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hemispherical diffusion prole, but also show decreased double layer charging currents due to their reduced surface area. Furthermore, a reduced ohmic potential (iR) drop is associated with the small faradaic currents in the nA and pA regions. Generally, an electrode is termed “microelectrode” or “ultramicroelectrode”, ((UME) UME is frequently found in the context of SECM), if one dimension is smaller than approximately 25 mm.10 The voltammetric behavior of electrodes is related to the time scale of the experiment. For microelectrodes with disc or spherical geometry, the thickness of the diffusion layer d ¼ OpDt (D: diffusion coefficient and t: time) exceeds the radius of the electrode at relatively short times, resulting in an enhanced mass transport and steady state current. These advantages of microelectrodes have been discussed in detail in many review articles, only a few are listed here.10–13 Since the rst reports indicating the unique features of miniaturized electrodes in electrochemistry and electroanalytical chemistry have been published, a signicant body of literature has been dedicated to theoretical aspects, fabrication strategies, characterization, and applications of micro- and nanoelectrodes. Typically, an electrode is classied as a nanoelectrode, if at least one dimension is smaller than 100 nm,4 although other denitions may be found.
2.1. Conventional SECM and related limitations Shortly aer the initial reports on the benecial properties of microelectrodes, scanning electrochemical microscopy (SECM) was introduced for obtaining spatially resolved electroanalytical information while scanning a microelectrode probe across the sample surface at a constant height (here referred to as “conventional” SECM). While Bard and co-workers have performed the rst SECM experiment actually using an electrochemical scanning tunneling microscope (EC-STM) measuring faradaic currents at the EC-STM tip,14 Engstrom and co-workers reported about at the same time on the voltammetric behavior of a microelectrode within the diffusion eld of a macroscopic electrode.15 In the following years, Bard and co-workers were evolving the instrumental development as well as the associated theoretical models for describing the steady-state current measured at the microelectrode being approached to a sample surface.16–20 Thus, the fundamentals of feedback mode SECM using an articial redox mediator added to the electrolyte solution, and of the generation–collection mode were introduced. In feedback mode SECM21 the term ‘feedback’ refers to a recycling effect of a redox-active species, the oxidized or reduced form of which (only one form is present in solution) is converted at the microelectrode and diffuses within the small gap between the probe and the sample to the conductive sample surface, where its original redox state is regenerated. The locally increased concentration of the redox species results in an increase in faradaic current recorded at the tip (positive feedback effect). In the case of an electrochemically passive (i.e., insulating) surface, the diffusion of redox-active species towards the microelectrode is increasingly limited with the decreasing distance from the tip to the sample surface, and with the increasing radius of the insulation around a disc-shaped microelectrode.
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Generation–collection mode22 does not require an articial redox mediator as the redox-active species generated from the sample is detected at the tip, which is in close proximity to the surface (sample generation–tip collection mode) or vice versa species generated at the tip can be detected at the conductive sample surface (tip generation–sample collection mode). In the rst decade of SECM (i.e., approx. 1986–1996), almost every experiment was conducted with micrometer-sized disc electrodes made from gold and platinum, which were sealed into an insulating glass sheathing providing an electroactive diameter in the range of 1–25 mm, although potentiometric probes and micro-sized ion selective electrodes have also been used. It should be noted that the fabrication and voltammetric behavior of nanoelectrodes has already been described in the literature at that time as well.23,24 Yet, the major limitation using such nanoelectrodes resulted from scanning the miniaturized electrode at a constant height across the sample surface, as the optimum distance depends on the presence and nature of the sample and correlates with the electroactive radius of the miniaturized electrode. Consequently, the smaller the imaging electrode, the closer it has to be positioned to and scanned across the sample surface. Hence, constant height imaging with sub-micron electrodes is more or less limited to possibly at and horizontally aligned samples. Furthermore, positioning of potentiometric microelectrodes25 or modied microelectrodes26,27 (e.g., amperometric enzymatic biosensors) cannot be readily achieved by recording approach curves. Clearly, further evolvement of SECM was reliant on advanced developments (i) enabling reliable maintenance of a constant distance rather than a constant height during imaging, (ii) providing improved lateral resolution, and (iii) simultaneously obtaining additional analytical information on the sample while mapping its electrochemical properties. Almost all of these developments require modication of the electrode design, which will be the focus of this review. Besides the developments discussed in this review, it should be mentioned here that rst approaches for scanning a microelectrode at a constant distance across a sample surface already date back to 1992.28 Bard and Wipf have developed a mode for amperometric measurements, where the microelectrode was oscillated in a sinusoidal motion perpendicular to the sample surface with small amplitude. The resulting current likewise oscillates at that frequency. However, the phase is dependent on the nature of the sample surface. Hence, the frequency and phase of the oscillating current allow separation of topographical from electrochemical information.29 Recently, an improved theoretical model associated with this operation mode was published taking tip-induced convection into account, which is a signicant parameter when imaging insulating samples.30
2.2. Conical and disc-shaped nanoelectrodes Fabrication of conical or disc-shaped nanoelectrodes can be readily achieved by electrochemical etching of microwires and insulation of the etched wires,23,31–34 which was initially used for fabricating scanning tunneling microscope tips suitable for usage in a liquid phase environment.35 Electrodes with radii of several
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nanometers have been produced by etching.23,34 Aer etching, the wire is encapsulated within an insulating material such as apiezon wax,35,36 electrophoretic paint,32,37,38 poly(ethylene),39 polyimide,33 Teon®40 or glass.23,41 Most sheathing materials require a heat treatment step for cross-linking or soening, which nally leads to shrinkage of the sheathing material and exposure of a conical electroactive area at the end of the etched wire. However, obtaining reproducible electrode dimensions, and characterizing their precise geometric shape remain a challenge.4 Nowadays, nanometer-sized electrodes for SECM experiments are mostly fabricated using a laser-assisted pipette puller, which was initially introduced for pulling micropipettes for patch-clamp experiments and for fabricating in vivo microelectrodes and ion selective microelectrodes.42 Shao et al. have demonstrated that laser-assisted pipette pulling is suitable for pulling glass pipettes with inserted metal microwires into glass-encapsulated nanoelectrodes.43 By controlling local heating of the glass and the pulling force, platinum nanoelectrodes with radii in the range of 2–500 nm may be obtained. As the pulled wire is completely encapsulated, the glass at the end of the tip was removed by etching in 40% HF or by micropolishing, which however may limit the reproducibility of the shape of the obtained electrodes. As no experimental details were provided in this initial report by Shao et al.,43 Schuhmann and co-workers have published an optimized procedure for obtaining tight-sealed, disc-shaped
Fig. 2 Shear-force mode SECM. (A) (a) SEM image of a needle-shaped Pt nanoelectrode and (b), (c) electrochemically etched carbon nanoelectrodes. (Reprinted from ref. 44 and 48 with permission from Wiley VCH and the American Chemical Society, respectively). (B) Principle of 4D shear-force mode. The tip is approached using vibration damping to a set threshold value, and the z-point of this closest approach is registered with the tip current representing the sample topography and local electrochemical activity. Without using the shear-force control, the tip is retracted in fixed increments, the current is recorded, and the data are stored. After the desired distance is obtained, the tip is moved to the next x,y position, and the sequence of the shear-force-controlled tip approach with tip retraction in fixed increments while recording the current is repeated. (C) Diffusion profile above a microelectrode. Line scans and x,z,I image representing the diffusion cloud of the electrochemically produced [Ru(NH3)6]2+ species. Reprinted from ref. 65 with permission from the American Chemical Society.
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nanoelectrodes (Fig. 2A(a)) providing detailed information on material and experimental parameters.44 However, exposing and polishing such needle-type nanoelectrodes ensuring a disc-shaped electrode requires an accurate control of the polishing process.45 Due to the pulling process, a needle-shaped electrode is obtained, which is suitable for shear-force based SECM, as described below. Carbon ber electrodes, which are frequently used for detecting biomolecules, have been fabricated by ameetching46 or electrochemical etching47,48 (see Fig. 2A(b) and (c)) and consecutively sealing as described for metal wires.
2.3. Fiber-shaped electrodes for shear-force mode SECM As previously mentioned, an operational limitation of conventional SECM is the fact that the faradaic current determined at the scanning probe simultaneously serves as the analytical signal, and as the parameter for determining the probe position vs. the sample surface. A breakthrough in current-independent positioning was the introduction of shear-force-based positioning of ber-shaped microelectrodes.49,50 Shear-force-based positioning was derived from near-eld scanning optical microscopy (NSOM), and is based on a tapered micro- or nanoelectrode, which is mechanically modulated via horizontal vibration. Short-range hydrodynamic forces in the vicinity of the sample dampen the vibration amplitude; with a given cut-off, this change in amplitude may then be used as error signal within a feedback loop regulating the tip position to maintain a constant distance between the tip and the sample surface. While the rst experiments were realized using optical detection of the shear force,49,51 nowadays most shear-force SECM experiments use non-optical positioning approaches derived from NSOM52,53 such as tuning fork-type resonators50,54,55 or piezo-based actuator–detector systems with a ber holder sandwiched between two piezoelectrically active plates, which Schuhmann and co-workers have adapted and implemented in SECM.56 By using the shear-force distance with the piezo-based actuator–detector system, they could demonstrate constant distance imaging with nanoelectrodes.57,58 Besides using closed feedback loop-based tip positioning based on the shear-force signal, an alternative approach (i.e., standing approach (STA)) was shown with the electrode continuously repositioned at a constant distance to the sample surface by successively recording shear-force approach curves.54,59,60 As stated above, micro- or nanoelectrodes used in shearforce SECM have to be excited via lateral vibration, hence, a ber-shaped geometry of the electrode is required, which is usually obtained via the pipette puller method. The spatial topographical resolution is determined by the diameter of the tapered probe. Shear-force SECM is frequently compared with non-contact AFM measurements, however given the geometric dimensions of micro- or nanometer-sized electrodes embedded in an insulator material, the achievable topographical resolution is a priori not comparable to AFM measurements or images recorded with nanopipettes used in SICM. Despite the fact that no detailed theoretical model has been derived for this current-independent positioning strategy, shear-force mode SECM operation circumvents several of the
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initial limitations of conventional SECM studies based on current-dependent positioning of the electrochemical probe. For example, it facilitates positioning of and imaging with miniaturized biosensors and potentiometric probes (e.g., ion selective electrodes).61–63 The applications of shear-force SECM range from corrosion research64 to applications in neuroscience, which were recently reviewed.7 Schuhmann and coworkers have introduced 4D shear force imaging,65 which allows recording of the tip currents at several yet constant distances to the sample surface at each point of the x,y-grid (Fig. 2B). Using this mode, diffusion proles may be visualized as shown in Fig. 2C. A redox-active species converted at a microelectrode was detected at the shear-force probe in generator/collector mode at different distances, thereby resulting in a map of the diffusion prole (Fig. 2C). Recently, the same authors have visualized the respiratory activity of HEK293 cells by 4D shear-force imaging along with redox competition mode.66 Only recently, the rst approach offering quantitative treatment of shear-force SECM using nanoelectrodes has been published.67 Shear-force distance control has been successfully introduced into SECM, and commercial SECM instruments nowadays offer shear-force mode modules. Recently Etienne et al. presented an automated SECM approach based on the non-optical shear-force for the investigation of large samples with signicant variations in topography.68 However, as additional complexity is introduced into the experiments by laterally vibrating the probe, future studies need to focus on a comprehensive theoretical treatment, in particular if nanometer-sized electrodes are used for high-resolution imaging.
2.4. So microelectrodes A conceptually striking and facile approach to maintain a constant distance to the sample surface without requiring an electronic feedback circuit was introduced in a collaborative effort by the groups of Wittstock and Girault,69 who developed microelectrodes with so polymer insulation called “so stylus microelectrodes”. If the insulation layer is brought into direct contact with the sample surface, the so electrode bends to a certain extent, and hence, the electroactive area has a certain distance to the sample surface, which is kept constant once the microelectrode is dragged in contact across the sample surface (Fig. 3A). Such so probes are obtained by fabricating a microchannel into a polymer sheet via laser ablation or in the case of gold electrodes via aerosol jet printing using materials such as polyethylene terephthalate (PET), or polyimide Kapton HN®, respectively. These channels are then lled with electroactive materials such as conductive carbon ink69 or nanoparticulate gold ink,70 which both need to be cured aer lling and then laminated with a polymer lm to encapsulate the electroactive material. Lamination is obtained with Parylene C, a biocompatible polymer frequently used for insulation of in vivo electrodes71 and AFM–SECM probes.72 The electroactive area of the so probe, which reects a sickle-like shape for carbon electrodes and band-like structures for gold electrodes, is then exposed by laser ablation or by simply cutting with a sharp scalpel (see Fig. 3A for the optical image). As the edge of
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Fig. 3 Soft microelectrode approach. (A) Scheme of a soft stylus microelectrode and an optical image showing the sickle-shaped carbon electrode (reprint from ref. 73 with permission of the American Chemical Society). (B) Schematic representation of a soft stylus electrode array. (C) Sketch of the fountain pen. (D) Scheme of surface modification during a line-scan on OEG-SAM. For modification an “on” potential is applied with a defined pulse length. (Reprinted from ref. 75 with permission of Wiley-VCH). (E) (a) SECM image of the microfabricated EPFL logo recorded with a fountain pen in feedback mode SECM; (b) optical image of the logo. Reprinted from ref. 77 with permission from the Royal Chemical Society.
the so insulator sheet is in contact with the sample surface, the exible electrode bends, and the thickness of the insulation layer, the angle of exposure of the electrode, and the angle under which the electrode is approached to the surface determine the electroactive area of the sample surface.73 d ¼ hP þ tL sinðaÞ; ðhP $ 0; noncontact regimeÞ d¼ d ¼ tL sinðaÞ; ðhP \0; contact regimÞ where a is the angle between the cross-sectional plane of the probe and the sample surface, hp ¼ hA lT (hA: height of the attachment point and lT: vertical extension of the unbent probe), and tL is the thickness of the polymer lm covering the carbon tracks. As stated in the Introduction, substantial efforts in the SECM community are targeted towards high-resolution imaging, however, many real-world applications may also require the opposite, i.e., scanning large sample areas yet trading off in spatial resolution.74 Hence, besides the constant distance capability another interesting aspect of the so stylus electrodes is the possibility of fabricating microelectrode arrays (Fig. 3B).70,73,75 In 2004, Unwin and coworkers demonstrated the rst approach using a planar micro-disc array for parallel SECM imaging.76 In contrast to the so electrode array, the microfabricated array of planar Pt-dot electrodes was kept stationary while the sample was scanned at a constant height across the electrode array. Hence, large-scale samples may be investigated in reasonable periods of time, which is important considering solvent evaporation during extended imaging times, and changes of the sample in the case of in situ processes (e.g., when imaging live biological surfaces/interfaces). Multiplexed linear arrays of eight individually addressable carbon or gold
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electrodes have been demonstrated so far (Fig. 3B), which were employed e.g. for imaging interdigitated electrode arrays and human ngerprints.74 As the array allows multiple scans in a single pass across the surface, the imaging time for an entire ngerprint using an array with eight electrodes (500 mm interelectrode spacing) requires only approx. 5 h, whereas a single electrode would require approx. 1–2 days. Complex patterns were fabricated and analyzed as shown in Fig. 3D. By locally generating reactive species (Br2/HOBr) within short potential pulses at predened times correlating with a pattern dening le (i.e., a digital image was converted into a black-and-white image with 69 80 pixel resolution), an image was created in an oligo(ethyleneglycol)-terminated SAM layer (OEG-SAM) and consecutively mapped.75 Microfabrication of so probes may also be extended toward fabricating the so-called fountain pen probes,77 which enable SECM measurements at dry sample surfaces. The concept of locally establishing an electrochemical micro-cell, and hence, avoiding that the entire sample has to be immersed in electrolyte solution during SECM experiments has recently gained substantial interest78 and can be found in the literature using different synonyms, as described below. The fountain pen is based on the same concept as the so microelectrode, however, one micro-channel is not lled with ink (Fig. 3C) and can be used for delivering a solution, while the second micro-channel serves as an integrated counter/reference electrode, which is perpendicularly connected via the same PET face. The third micro-channel is lled with carbon ink to serve as the working electrode. By adding another microchannel, Girault and Wittstock have demonstrated a push/pull probe79 and a coupled SECM/matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS) system using microchannels for delivering and pulling the solution from the sample surface with a microsyringe pump connected to the channels.80 Again, only the sample location of interest is in contact with the electrolyte solution by delivering a small volume of electrolyte solution to the sample surface with the push channel. Human ngerprints, which had been in contact with picric acid as a model explosive, were investigated with this approach providing complementary analytical information during spatially resolved analysis.80 However, as with any alternative concept in SPM there are some advantages and disadvantages associated with such so stylus-type electrodes. As the probe assembly is in direct contact with the sample, imaging not well adhered so coatings or samples that do not show the required adhesion properties such as single cells may be problematic – similar to contactmode AFM – due to the imposed friction forces. Wittstock et al. provided an estimate of these forces, which are in the microNewton range.81 In addition – and as stated by the authors – reliable microelectrochemistry relies on reliably fabricated and well-dened microelectrodes. As the material used here is conductive ink, which is usually manually applied to the microchannels, a variation in the electrochemical response of individual electrodes was observed.81 To date, only micrometersized electrodes and arrays with comparatively large electrode spacings have been demonstrated though this was the goal for
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large-scale imaging. In addition, individual electrodes within an array may reveal different response behavior as such inks initially contain organic components, which may not be entirely removed during the curing process. Hence, the quality of such electrodes usually does not compare to conventional SECM electrodes based on solid-state microelectrodes manufactured from noble metal wires or carbon bers tightly sealed into insulating glass. As stated by the authors, the Parylene C insulation, which is in contact with the sample surface, may also deteriorate due to the friction forces. In particular, if harder samples are scanned for extended periods of time, this may lead to imaging artifacts due to contamination at the sample surface. However, it has to be noted that the surface is renewable by simply cutting the end of the electrode, thereby rendering these probes suitable for long-term usage. As stated above, so stylus probe arrays and the opportunity to enhance the functionality by adding micro-channels as needed to the assembly render this approach highly suitable for large-scale SECM investigations.
2.5. Nanopipettes in SECM In contrast to the so stylus probes facilitating imaging of large sample areas within reasonable periods of time, routinely achieving both high-resolution topographical and electrochemical imaging remains a challenge. High-resolution SECM was demonstrated with nanoelectrodes in combination with current-independent positioning and even reported with constant height SECM imaging.82 In recent years, a signicant number of studies have used micro- and nanopipettes for SECM experiments. The versatility of nanopipettes in general and in SPM in particular has been recently reviewed by Baker et al.83 Hence, here only recent trends in using nanopipettes in hybrid SECM techniques will be discussed. Micro- and nanopipettes are used in scanning ion conductance microscopy (SICM), which was rst introduced by Hansma et al. in 1989.84 In SICM, ion currents are recorded with spatial resolution by applying a small potential between a reference electrode, which is located within a micro- or nanopipette, and a reference electrode immersed in the sample solution. In contrast to other SPM techniques, a more widespread usage of SICM was rather limited aer its introduction until some signicant instrumental developments including improved positioning routines have been introduced.85,86 In particular in life sciences, SICM has nowadays gained signicant attraction due to highresolution non-contact imaging of biological samples such as live cells and the ease to combine with uorescence microscopy. In SECM, for a long time, micro- and nanopipettes have been mainly used in combination with ion-selective electrodes, where the pipette is lled with an ion-selective cocktail and for liquid/ liquid interface studies of ion transfer or electron transfer mediated by electro-inactive or electroactive species between two immiscible electrolyte solutions. However, such measurements do not necessarily require spatially resolved scanning of the probe.87,88 Bard and co-workers have presented an approach using a micropipette, which was additionally modied with a concentric ring-electrode.89 A borosilicate micropipette was modied with a gold layer and consecutively insulated with
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electrophoretic paint. To form an insulating layer, the electrodeposited paint has to be cured, which leads to shrinking of the sheathing, and therefore, exposure of the ring-electrode around the micro-capillary. The pipette was positioned at a height of a few micrometers in an aqueous electrolyte solution, and ferrocenemethanol was constantly dispensed as electroactive species by applying a constant pressure to the pipette. As a proof-ofprinciple, a platinum disc electrode with an electroactive diameter of 100 mm embedded in glass could be imaged, as the redoxactive species was oxidized at the platinum surface and reduced at the ring-electrode. Given the size of the orice and micrometersized ring-electrode, the achieved lateral resolution was comparable to conventional SECM experiments. For a number of applications in materials science, e.g., for corrosion studies, mapping catalytic activity, and in battery research, immersion of the entire sample surface in electrolyte solution is detrimental, as the surface properties may change during the imaging experiment due to the contact with the electrolyte solution. Consequently, approaches using a small yet dened volume in contact with the sample surface, e.g. for pit corrosion studies, date back to 1995, when B¨ ohni et al. used a microcapillary with a rubber gasket sealing a small contact area where the sample surface was in contact with solution (Fig. 4A).90 Scanning of droplet cells using capillaries with orices of diameters ranging from 10–600 mm was rst described by Lohrengel and co-workers.91–93 Capillaries containing a reference and a counter electrode or gold capillaries containing a reference electrode while the gold serving as a counter electrode were used. With this approach, the sample surface is wetted with droplets having diameters in the range of approximately 10–100 mm due to surface tension.
Fig. 4 (A) Scheme of the droplet cell using a silicone rubber gasket to define the contact area; (B) principle of the scanning micropipette contact method (SMCM). (C) (a) Optical image of a 95–5% Al–Cu alloy; (b) SMCM image of the alloy (area in the yellow square in (a) recorded with a 1 mm diameter micropipette filled with 2 mM Fe(CN)63 and 0.1 M KNO3). The alloy was biased at a potential of 0.10 V vs. AgQRE. (Reprinted from ref. 94 with permission from the American Chemical Society).
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The size of the droplet remains constant, when the capillary is moved across the sample surface. The basic concept of this scanning droplet cell was re-visited by Unwin and co-workers in an elegant way and with an improved positioning routine for imaging, with the introduction of the scanning micropipette contact method (SMCM) as schematically shown in Fig. 4B.94 A moving micropipette with pipette openings in the range of 300–1000 nm is positioned at the sample surface such that the liquid meniscus initiates contact between the sample surface and the capillary due to attractive capillary forces. The micropipette is lled with electrolyte solution containing an electroactive species, while the reference electrode is placed in the upper part of the micropipette (Fig. 4B). Hence, a two-electrode cell is formed once the droplet made contact with the conductive or semi-conductive sample surface, which is connected as the working electrode. Positioning is achieved by detecting the current ow, if the liquid meniscus of the micropipette makes contact to the sample surface. Basal plane imaging of HOPG and the investigation of Al alloy containing 5% Cu were demonstrated by point-to-point measurements, while the pipette was retracted and then re-approached for the next measurement similar to hopping mode operation in SICM.86 Fig. 4C shows the optical image of the alloy (a) and (b) an SMCM image obtained with a micropipette (diam. 1.0 mm) lled with 2 mM Fe(CN)63 and 0.1 M KNO3. The basic principle described above was extended to using double-barrel pipettes fabricated from theta capillaries. Dualbarrel capillaries have been used, for example for surface modication.95 Unwin and his group recognized the potential for highresolution electrochemical imaging using double-barrel micro-
Fig. 5 (A) Scheme of scanning electrochemical cell microscopy using a dual barrel pipette; (B) if the tip is moved to the sample, the electrolyte meniscus is brought into contact with the surface, and a small amplitude oscillation is applied to the z-position of the tip. During scanning, the ion migration current across the meniscus (Icond), the z-extension of the piezoelectric positioner (z), and the surface activity current (Iact) are simultaneously recorded resulting in multiple parameters (images adapted from ref. 97 with permission of the American Chemical Society). (C) SECCM image of HOPG: (a) AC component of the conductance current, (b) topography (z-piezo position) and (c) redox activity of the surface recorded by the reduction of 2 mM [Ru(NH3)6]3+ (reproduced from ref. 98 with permission from Wiley-VCH).
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and nanopipettes in SECM, introducing the concept of scanning electrochemical cell microscopy (SECCM).96 Here, both barrels are lled with electrolyte solution, and each barrel contains a quasi-reference electrode or a combined reference-and-counter electrode (i.e., quasi-reference counter electrode; QRCE) (Fig. 5A). In contrast to the single barrel approach, a voltage may now be applied between the two reference electrodes, which leads to migration of ions across the meniscus between the two barrels. Again, if the dual-barrel pipette is moved towards the sample surface the meniscus at the end of the barrels initiates a contact, which changes the measured ion current. By adding an oscillation of the pipette, an AC component of the ionic current is obtained, as the meniscus is periodically altered. Similar to positioning in SICM, the alternating ionic current component strongly depends on the meniscus height. Hence, it is highly suitable as a signal for a feedback loop, which allows unsurpassed imaging in constant distance operation. When imaging a conductive or semi-conductive surface, coincidentally with topography and ion conductance measurements, the electrochemical properties may be mapped, if an additional potential is applied between the sample (WE) and the electrodes within the scanning dual-barrel pipette. Hence, simultaneously multiple information on topography, electroactivity and conductivity can be obtained (Fig. 5B). The spatial resolution and the quality of the electrochemical measurements strongly depend on the geometrical features of the pipettes including the diameter of the orice, which is currently reported as small as 100 nm.78 High-resolution imaging could be demonstrated by mapping the catalytic properties of individual platinum nanoparticles deposited onto carbon nanotubes with respect to their morphology towards the oxygen reduction reaction.97 By applying an additional constant potential between the substrate and the reference electrode within the barrel, the redox activity could thus be mapped, and the reactivity of individual nanoparticles was demonstrated to vary between NPs with similar size, thereby invoking that variations in morphology have a signicant effect on the reactivity. A recent review article by Unwin et al. highlights the versatility and high-resolution imaging capability of SECCM for studying carbon materials including HOPG (Fig. 5C), CNTs and graphene.78 The most signicant advantage of this approach is the fact that multi-parametric information may simultaneously be obtained at the nanoscale. Furthermore, in comparison with the conventional fabrication of disc nanoelectrodes, which usually requires a nal polishing step, nanopipettes have the advantage that no further mechanical treatment is necessary. In addition, formation of a small electrochemical cell by wetting the sample at the position of the pipette rather than immersing the entire sample reduces effects such as passivation or fouling of the surface, and thus enables studies at the surface of particularly sensitive samples.
3. Combined scanning probe techniques Combining the superior lateral resolution with complementary chemical information at high spatial resolution is an ongoing challenge in scanning probe microscopy. In recent years,
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signicant advancements have been made, which mainly relate to combining individual SPM techniques for obtaining complementary information during imaging of the sample surface. Biomedical research most obviously has the fundamental need to investigate and image molecular events in or at living cells, cell ensembles, or entire organisms yielding quantitative information e.g., on signaling molecules with spatial and temporal resolution. However, also in materials science (e.g., corrosion studies), and energy research (e.g., fuel cells and batteries) essential interest is geared towards analyzing multiple parameters in correlated measurements on continuously changing samples/surfaces. Early approaches combining individual SPM techniques have involved the combination of atomic force and scanning tunneling microscopy with near eld optical microscopy (STM-NSOM),99,100 and the combination of AFM with NSOM.101 Also, reports focusing on the combination of SICM with NSOM and confocal microscopy have been published.85,102 In this review, only combinations of SECM with AFM and SICM will be discussed in detail. Again, probably the most critical aspect ensuring the analytical utility of these combinations is the design and development of reliable probes, as the analytical functionality is usually directly integrated into these tips. The most convenient SPM techniques for combination with SECM are certainly AFM and SICM, although the combination of STM with SECM,103,104 and more recently, SECM with Kelvin probe force microscopy (KPFM), was reported.105 3.1. SICM–SECM In only a few years, signicant progress has been made in developing dual probes for combined high-resolution SICM–SECM imaging. In particular for imaging so samples,
Fig. 6 (A) Fabrication scheme of nanopipette/ring-electrodes for combined SICM–SECM. The pulled capillary is modified outside with a conductive layer and consecutively with an insulation layer. Depending on the insulation method, the conductive ring and orifice are exposed by FIB milling (the viewpoint is along the x-axis and normal to the xaxis). (B) SEM images of SICM–SECM probes (a) gold ring-electrode insulated with ALD, (b) gold electrode insulated with electrophoretic paint, (c) carbon ring/nanopore electrode insulated with Parylene C (adapted from ref. 107–109 with permission of the American Chemical Society and Royal Chemical Society, respectively). (C) Schematic representation of a hybrid SICM–SECM probe based on a dual barrel pipette with one barrel filled with pyrolytic carbon deposition.
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SICM provides the advantage of a non-contact SPM technique achieving a resolution comparable to dynamic mode AFM imaging.106 In addition, the distance control in SICM facilitates constant-distance SECM imaging. Combined SICM–SECM experiments may be readily realized by sputtering a conductive layer onto the pipette followed by insulation and exposure of the ring-electrode at the end of the capillary (see Fig. 6A), as previously demonstrated for microcapillary/microring-electrodes.89 Comstock et al. have presented a combined SICM–SECM probe by modifying a nanopipette with a gold layer of approx. 200 nm thickness, which was then insulated with aluminum oxide using an atomic layer deposition (ALD) process.107 The ringelectrode and the orice of the pipette were exposed by focused ion beam (FIB) milling resulting in a nanopipette with a diameter of approx. 100 nm, and a ring-electrode with a diameter of approx. 300 nm (Fig. 6B(a)). Proof-of-principle measurements of a FIB-fabricated nanostructured model sample providing insulating 400 nm wide trenches milled into gold-coated glass were obtained in AC mode SICM and feedback mode and generator–collector mode SECM. Although the ALD aluminum oxide layer was conformal, it might be of limited utility e.g., in strong basic or acidic solutions. In addition as stated by the authors, the deposition completely occludes the nanopipette, and occurs also inside the nanopipette. A similar approach to the micropipette/microring-electrode published by Bard and co-workers89 was presented by the teams of Matsue and Korchev using also electrophoretic paint as an insulation material.108 However, here, an additional FIB-milling step was applied to ensure a well-exposed and uniform ringelectrode with diameters down to 330 nm as shown in Fig. 6B(b). Imaging of immobilized enzyme spots including glucose oxidase and horseradish peroxidase was demonstrated. The SICM image revealed extremely pronounced topographical features at the GOD spot, which were interpreted as ‘caves’. The electrochemical response revealed such features in the presence of ferrocenemethanol [Fc(CH2OH)], which is oxidized at the ring-electrode to [Fc(CH2OH)]+ that serves as an electron acceptor for the GODcatalyzed glucose oxidation. Only if the dual probe was positioned at a close distance (approx. 100 nm) to the sample surface, the “cave” features could be resolved in the electrochemical response. Upon increasing the distance to 600 nm, the features disappeared in the SECM image due to the distance dependence of SECM in feedback mode. Recently, Baker and co-workers have shown fabrication strategies to obtain sub-micrometer carbon ring-electrodes, carbon ring/platinum disc electrodes or carbon ring/nanopore electrodes, which are suitable for SECM, SICM or combined SICM–SECM measurements (Fig. 6B(c)).109 Such bifunctional nanoprobes are obtained by pyrolyzing Parylene C at a temperature of 900 C in a nitrogen atmosphere. Parylene C is deposited by a chemical vapor deposition process, which may also be used for batch-level production; hence, this approach is certainly interesting, as the same material serves for establishing conductive and insulating coatings. Nanoring-electrodes, SICM–SECM probes as well as platinum disc/carbon ring-electrodes have been fabricated with this approach. Instead of sputtering a layer onto a nanopipette, an elegant approach for combined SICM–SECM probes was demonstrated
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in a collaborative effort by the teams of Unwin, Matsue, and Korchev.110 Capillaries may be readily coated or lled with carbonaceous deposits by in situ pyrolytic decomposition of methane gas,111 which was reported earlier for acetylene112 or butane gas.110 For combined SICM–SECM, one channel of a dual barrel pipette is lled with carbon using this pyrolysis process resulting in double-barrel carbon nanoprobes (DBCNPs). Hence it is important that the second channel is blocked during lling with the gas so that the second channel could be lled at a later stage with an appropriate electrolyte solution for SICM measurements. Successful fabrication of dual-barrel pipettes with the total diameter as small as 100 nm, and carbon nanoelectrodes with diameters ranging from 20 nm to 2 mm was demonstrated providing sufficient reproducibility. The second electrolyte lled barrel was used for topographical analysis based on the SICM distance control or may also be applied for deliberately delivering molecules to the sample surface. Besides some model samples such as a PET membrane with 100 nm pores, also PC 12 cells were imaged with this innovative combined probe. Also both barrels can be lled with carbon material using the pyrolysis process.113 Recently, Unwin and co-workers showed that combined SICM–SECM measurements may also be realized using potentiometric probes.114 The previously described approach of lling one barrel with a carbon electrode was used to further electrodeposit iridium oxide on the nanoscopic area, thereby resulting in a pH-sensitive metal/metal oxide electrode. The approach of depositing iridium oxide onto microelectrodes has previously been proven suitable for pH-microelectrodes in SECM.115 The utility of pH-SICM was demonstrated during a dissolution experiment on calcite microcrystals in aqueous solution leading to local pH changes.
Critical Review
macroscopic sample surface, rst attempts using a metallized cantilever, which was manually insulated at the chip, were performed with a biased Pt-coated cantilever inducing the dissolution of ferrocyanide crystals.116 Topological changes during the dissolution process were simultaneously monitored with the conductive AFM tip. The main benet of merging AFM with SECM is the direct correlation of topological information with the chemical surface activity, both at an excellent lateral resolution.117 In principle, the combination of AFM–SECM requires a bifunctional probe comparable to the combined scanning probes already mentioned herein. To date, the following fabrication approaches have been reported: (i) manually fabricated probes with conical or spherical electrodes located at the apex of the tip; (ii) microfabricated probes with the conductive electrode also constituting the AFM tip; (iii) insulated carbon nanotubes at the end of the AFM tip with an exposed electroactive area; (iv)
3.2. AFM–SECM AFM is among the most widely used and versatile SPM techniques, which may be applied in any environment ranging from imaging live cells in buffered solutions to routine characterization under high vacuum conditions in industrial wafer fabrication. In contrast to SICM, besides superior spatial resolution AFM offers both morphological and (micro)-mechanical information on the sample at the nanometer scale. Fundamentally, AFM is based on (short- and long-ranged) force interactions between the sharp tip at the end of the cantilever arm, which is characterized by a distinct spring constant k, and the sample surface. The tip is scanned in contact across the sample surface, while a computer-controlled feedback loop maintains either constant cantilever deection (i.e., short-range forces dominate) or constant cantilever amplitude (i.e., dynamic mode), usually via an optical read-out of the cantilever movement. Well-established reproducible silicon microfabrication of such cantilevers with tailored spring constants providing integrated sharp tips made from e.g., silicon nitride or silicon enables AFM probes at comparatively low costs, and has contributed to the circumstance that AFM is nowadays considered a routine characterization tool. In contrast to electrochemical AFM, where the AFM tip is used for imaging electrochemical processes induced at a conductive
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(A) Scheme of cantilever-shaped sub-microelectrode; (a) SEM image of a etched and bent Pt wire insulated with anodic electrophoretic paint resulting in a micro-sized conical electrode (reprinted from ref. 118 with permission from the American Chemical Society), (b) SEM image of a sub-micrometer-sized spherical gold electrode formed via a controlled arc discharge technique (reprinted from ref. 133 with permission from the American Chemical Society). (B) Batch microfabricated AFM–SECM probes with triangular or conical electrodes; (a) SEM image of a triangular AFM–SECM probe obtained via electron beam lithography (EBL) (reprint from ref. 128 with permission from the American Chemical Society); (b) conical AFM–SECM probe with a platinum silicide (PtxSiy) electrode fabricated with the molding technique. The metal is embedded between two Si3N4 thin films (reprinted from ref. 134 with permission from the American Chemical Society); (c) SEM image of an insulated boron doped diamond (BDD)-coated AFM– SECM tip fabricated from BDD AFM cantilevers, which were modified with Si3N4 and a 50 nm Cr protection layer. Prior to RIE etching, the Si3N4 and Cr protection layers were partially removed by FIB milling (reprinted from ref. 127 with permission from the Institute of Physics). (C) Scheme of CNT-based AFM–SECM probes. The CNT attached to a conductive AFM tip serves as a template for a metal-wire electrode by coating the CNT with e.g., gold prior to insulation and FIB milling. (a) Polymer-insulated disc-shaped nanoelectrode, which was exposed by FIB milling; (inset: zoomed SEM of the nanowire electrode; reprinted from ref. 135 with permission from the American Chemical Society). (c) TEM image of a polymer insulated multi-walled carbon nanotube with the electroactive area exposed by laser ablation (reprinted from ref. 136 with permission from the Royal Chemical Society). Fig. 7
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Fig. 8 Sketch of an AFM–SECM probe with electrodes located below the tip apex; (a) SEM image of a sub-micrometer-sized tip-integrated frame electrode insulated with a mixed layer of Si3N4 and SiO2. The exposure of the frame electrode and re-shaping of the centered Si3N4 AFM tip are obtained using bitmap-assisted FIB milling developed by Kranz and co-workers. (b) AFM–SECM probe with the integrated disc nanoelectrode fabricated via the semi-batch fabrication process developed by Kranz and co-workers. (c) AFM–SECM probe with a square nanoelectrode. A square electrode and a distance holder at the apex of the pyramid were obtained by only removing part of the SiO2. Inset: AFM–SECM probe with exposing the electrode by milling the SiO2 at an angle of approx. 15 (reprint from ref. 148 with permission from Elsevier). (d) SEM image of a batch-fabricated AFM–SECM probe with a micrometer-sized Pt ring-electrode and a SiC AFM tip obtained by standard microfabrication steps developed by Kranz and coworkers. (e) AFM tip integrated boron-doped-diamond ring-electrode insulated with Parylene C and exposed via FIB milling (reprinted from ref. 150 with permission from Elsevier). (f) L-shaped microelectrode with a flattened arm insulated with epoxy. A disc-shaped electrode is exposed via FIB milling producing a small epoxy AFM tip beside the Pt electrode (reprinted from ref. 145 with permission from the Electrochemical Society).
microfabricated bifunctional probes, which comprise a sharp non-conductive AFM probe, and an electrode that is located at a certain distance above the apex of the tip. An overview of different probe designs is presented in Fig. 7 and 8. All these approaches require a sharp tip for maintaining the lateral resolution of AFM, a force constant comparable to commercially available AFM probes, a well-dened geometry ensuring appropriate imaging quality, and for the electrochemical signal obtained at the electroactive area, pinhole-free insulation of the entire probe avoiding any kinds of leakage currents is required, which may lead to erroneous electrochemical information. 3.2.1. Manually fabricated probes with conical and spherical electrodes. The rst approach, which may be readily adapted without access to any microfabrication facility, is the manual preparation of cantilever-shaped conical micro- or nanoelectrodes, which was introduced by Unwin and MacPherson in 2000.118 Although it should be mentioned that bent microelectrodes sealed into glass were commercially available from Nanonics Imaging Ltd (Israel) already in the 1990s. Metal microwires (Au or Pt) are etched, bent, and attened aer the etching process to ensure sufficient reectivity for the laser signal used as optical read-out of the cantilever deection. Insulation is achieved by deposition of anodic or cathodic electrophoretic paint,118,119 which was initially used for
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insulating electrochemical STM tips (ECSTM).37 As the electrodeposited electrophoretic paint has to be cured for crosslinking, a conically shaped sub-micro- or nanoelectrode is exposed. For AFM–SECM probes with gold electrodes, insulation using cathodic paint resulted in entirely coated probes so that the electroactive area was exposed by applying a high voltage to the probe via an electric arc resulting in spherical or conical gold electrodes with radii ranging from 150 to 550 nm.119 An advantage of this probe design is that high-resolution electrochemical imaging may be achieved due to the exposed nanoscale electroactive area. Demaille and co-workers have developed a new technique, which was termed TARM (tip attached redox mode).120,121 The gold electrode of an AFM–SECM probe is modied with a tethered redox mediator via poly(ethylene glycol) (PEG) linkers. Instead of a free-diffusing mediator, electrons are now shuttled between the gold tip and the substrate acting as molecular sensors for probing the local electrochemical reactivity of nanometer-sized active sites. 3.2.2. Microfabricated AFM–SECM probes with conical electrodes. Microfabricated probes providing conical electrodes have been demonstrated by several research groups using standard lithographic patterning122–127 or electron beam lithography.128,129 The advantage of this approach is that the fabrication steps may be achieved at a wafer level. Standard microfabrication processes based on plasma-enhanced chemical vapor deposition (PECVD), reactive ion etching (RIE), and molding processes, which are used to fabricate commercial silicon nitride probes, were used in combination with the deposition of an additional metal layer prior to the etching steps. As shown by Weaver and co-workers, aer cantilevers and tips were processed from silicon nitride deposited by PECVD onto a Si wafer, EBL was used to pattern the gold contact line, contact pad, and the gold-coated triangular tip prior to applying an additional layer of silicon nitride for insulating the contact from the tip electrode to the contact pad. In the last step, the contact pad and the gold-coated tip were exposed using a resist mask and RIE. Simpler denition of an electroactive area has been demonstrated using Parylene C-coated or photoresistcoated metallized cantilevers either by mechanical exposure or via UV illumination, respectively.130,131 Another approach providing high-aspect AFM–SECM probes was shown by Wain et al. based on commercially available metallic needle probes, which were insulated with Parylene C. The nanoscopic electrode was again exposed by FIB milling.132 Clearly, various advantages and disadvantages may be argued for and against the active electrode area being located at the apex of a combined probe. As electrochemical imaging typically requires a constant distance to the sample surface, AFM–SECM imaging with probes providing an electroactive area at the end of the tip requires scanning of each sample twice: rstly, recording the topography in contact mode or dynamic mode, and then secondly, liing the probe to execute electrochemical imaging. In the case of ‘active’ surfaces such as in catalysis or biological (live) samples, the surface properties may dynamically change in the time frame of such sequenced imaging experiments, which complicates the correlation of electrochemical and topographical information. In addition, as
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the electroactive area is initially in direct contact with the sample, scanning the sample for recording the topography may already alter the exposed electroactive area by abrasion unless sufficiently robust electrode materials such as e.g., boron-doped diamond127 are used, by fouling processes at the electrode surface, or by picking up contamination from the sample surface. Furthermore, direct contact of the electroactive area limits the implementation of electrochemical sensing schemes. 3.2.3. Carbon-nanotubes as AFM–SECM probes. It can be clearly stated that the advantage of such AFM–SECM probes is that nanoscopic electrodes are readily achievable, which allows high-resolution SECM imaging. Unsurpassed resolution in SPM can be achieved with carbon nanotubes (CNTs) due to their high aspect ratios, large Young’s modulus, chemical stability, mechanical robustness, and sub-nanometer radius of curvature.137–139 For SPM measurements, CNTs have to be attached to the probe, which is usually realized by adhesive pick-up with an AFM or STM probe, or by direct growth of CNTs at the tip.140 Combined AFM–SECM probes and conductive AFM probes based on carbon nanotubes have been reported.139,141 Burt et al. have fabricated combined probes by attaching a single-walled nanotube (SWNT) to an AFM tip, which was consecutively modied with a conducting metal layer, insulated with poly(oxyphenylene) and silicon nitride, respectively. The electroactive area was then exposed by FIB-milling.135 Patil et al. have reported the fabrication of AFM–SECM probes by attaching multi-walled nanotubes (MWNTs) to the apex of gold-coated AFM tips using a micromanipulator.136 Insulation was achieved with thin layers of Parylene C, aer the gold layer was passivated with long-chained alkanethiols. The electrode was nally exposed via laser ablation. Although the fabrication was demonstrated, no imaging experiments with these probes were shown. As mentioned above, the high-aspect ratio and the possibility to establish electrodes with diameters as small as 10 nm are convincing arguments for this technology. As the CNTs bend when in contact with the sample surface, direct contact of the electroactive area with the surface is avoided. However if the bending changes, e.g., by dris of the deection signal, this may also lead to changes in the distance between the electrode and the sample. 3.2.4. AFM–SECM probes with recessed electrodes. If the electrochemically active electrode area is separated from the tip apex by positioning the electrode at a certain distance above the AFM tip (Fig. 8), the electrode is advantageously not in direct contact with the sample surface. Hence, as reported for SICM–SECM topographical data and electrochemical data may be simultaneously recorded. However, generating threedimensional multi-layered structures facilitating sharp probes and an electrode not located directly at the tip apex requires multiple microfabrication steps, which are not easily achieved if sub-micrometer electrodes are required. Only the deposition steps including the metallization and the insulation layer are readily obtained using batch processes, whereas the exposure of an electrode recessed from the tip apex usually requires more sophisticated fabrication steps including e.g., focus ion beam milling.142–150 FIB-assisted processing has nowadays matured into a routine procedure for three-dimensional nanofabrication
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and prototyping based on the distinct advantages of mask-less direct processing.151 Kranz and co-workers have fabricated the rst frame-electrodes with edge lengths in the range of 500–1000 nm (as determined by the pyramidal shape of the conventional AFM tips) starting from commercially available silicon nitride probes (Fig 8(a)).142 They have also developed a batch process for fabricating entire bifunctional AFM–SECM cantilevers at the wafer scale establishing disc-shaped nanoelectrodes with diameters in the range of 100 nm (Fig. 8(b), unpublished results) involving only one FIB-milling step, which cannot be obtained at the wafer level. A similar approach was pursued by Wittstock and Oesterschulze,148 although only the backside of a fabricated hollow pyramid was in this case metallized, and hence, the FIB-milling step was crucial in dening the electrode size (Fig. 8(c)). A difficulty with this approach may be related to reproducible electrode dimensions, as the size of the electrode depends on the milling step and variation in the metal layer thickness; however probes with electrodes as small as 20 nm in diameter have been reported. As FIB-milling was achieved at a certain angle, the edge of the insulating AFM tip – in this case SiO2 – made contact with the sample surface for providing the constant distance. Shin et al. have demonstrated a FIB-less batch process for fabricating bifunctional AFM–SECM cantilevers with recessed ring-electrodes (Fig. 8(d));152,153 however, only micrometer-sized dimension could be achieved thereby limiting the achievable SECM resolution. Nevertheless, high-resolution topographical images may be simultaneously recorded along with the electrochemical response, which was demonstrated e.g., in alternating current (AC) impedance imaging without an added redox mediator.154 Recent developments by the teams of Kranz and Nebel succeeded in fabricating AFM–SECM probes with a recessed borondoped diamond electrode (Fig. 8(e)).149,150 BDD is a highly interesting electrode material due to the signicant overpotential for both hydrogen and oxygen evolution, the chemical inertness, the mechanical stability of the material, and the reduced electrode fouling compared to conventional electrode materials.155 As the surface termination plays a signicant role for the electrochemical properties of the BDD electrode, postFIB-milling electrochemical termination steps have been developed to ensure a proper response from such integrated BDD electrodes. Micrometer-sized AFM–SECM probes with the electroactive area recessed from the tip have also been demonstrated by Davoodi et al.145 These probes were fabricated by coating a Pt wire with Parylene C, and then using FIB-fabrication by applying triangular milling steps, which exposed the Pt disc electrode with a polymer tip besides the electrode (Fig. 8(f)). Another approach was based on a glass-encapsulated microwire, which was ground and polished at a dened angle of 15– 20 .156 Similar to the previously discussed so stylus probes, a constant distance to the sample surface was provided once the edge of the insulation was in direct contact with the sample surface. However, given the mechanical properties of the sheathing material, only hard materials could be imaged with this probe design without adversely affecting the sample surface.
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AFM–SECM imaging of glass-embedded Pt microelectrodes. (A) Schematic of the imaging process. (B) (a) SEM image of the used AFM tip-integrated BDD electrode. (b) Topography recorded in contact mode revealing polishing features. (c) Simultaneously recorded electrochemical image in 10 mM Ru(NH3)63+/0.1 M KCl, Etip ¼ 400 mV vs. Ag/AgCl (reprint from ref. 150 with permission from Elsevier); (C) (a) SEM image of the nanowire AFM–SECM probe. (b) Topography recorded in tapping mode at a scan rate of 0.5 Hz; (c) electrochemical image recorded in 10 mM Ru(NH3)63+/0.1 M KNO3 with the sample biased at 400 mV vs. Ag/AgCl and the tip biased at 0.0 mV. Reprinted from 135 with permission from the American Chemical Society. Fig. 9
Fig. 9 shows examples of simultaneously recorded AFM– SECM images using Pt disc electrodes (diam. 25 mm and 2 mm, respectively) as model samples (Fig. 9A). The combined images shown in Fig. 9B were obtained with an AFM tip-integrated micrometer-sized BDD electrode (Fig. 9B(a)) in contact mode AFM and feedback mode SECM in 10 mM Ru(NH3)63+/0.1 M KCl. The topographical image (b) clearly reveals features from the grinding and polishing procedure, which are not evident in the electrochemical image (c) as the corrugations do not inuence the conductivity of the Pt. A small diamond particle is visible in the topography and also in the SECM image as the particle results in a reduced current signal as the conductive surface is blocked by the diamond particle.150 Fig. 9C shows combined images of a Pt microelectrode with a diameter of 2 mm. The images were recorded in tapping mode AFM and feedback mode SECM in 10 mM Ru(NH3)63+/0.1 M KNO3 with a combined nanowire probe bearing an electroactive area of 100 nm and an insulation layer of 200 nm. In both examples the electrochemical response matches excellently with the topographical features. Both examples reveal high-resolution topographical features as small scratches with nanometer dimensions are resolved. The electrochemical image of the latter examples shows some artifacts, which the authors relate to brief electrical contact between the tip and the protrusions on the Pt substrate.135 Similar to AFM probes with electrodes extending to the tip apex, bifunctional probes providing recessed electrode designs are characterized by a series of advantages and disadvantages. In general, most fabrication schemes for such probes rely on access to appropriate micro- and nanofabrication facilities. In
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addition, fabrication of nanometer-sized electrodes appears more challenging, and hence, most of the demonstrated probes with recessed electrodes are in the sub-micrometer dimension range concerning the electroactive area. Furthermore, profound theoretical descriptions of the electrochemical behavior of discelectrodes are much more prevalent in the literature compared to frame-type electrodes.157 Yet, a signicant advantage is certainly the fact that topographical and electrochemical responses may be obtained in a truly simultaneous fashion. Furthermore, the electroactive area may be readily modied with additional sensing and molecular recognition layers based on a wide variety of immobilization architectures, which was demonstrated by implementing e.g., a biosensor interface into such AFM probes.158,159 In addition, the force constants of commercial cantilevers, which are modied with a conductive and an insulation layer as described herein are only slightly modied, and hence, so samples may be imaged e.g., in dynamic mode160 or simultaneous force measurements may be envisaged. Independent of the fabrication route, dual imaging may be performed in contact or dynamic mode AFM, and in generator– collector mode or feedback mode SECM using such bifunctional probes. Similar to the development of other emerging analytical techniques, many of the measurements demonstrated with AFM–SECM so far have been conducted on model samples such as microstructured patterns with alternating conductive and insulating features,122,129,131,134,142,146,153 glassembedded disc microelectrodes,128,135,150,152,161 well-dened surfaces such as HOPG,120,124,147 and transport of redox-active molecules through articial membranes,162–164 rather than realworld samples. Almost all studies were conducted with amperometric probes, however similar to conventional SECM, potentiometric probes or modied electrodes such as amperometric biosensors may also be integrated into AFM–SECM probes. As an example solid-state pH electrodes (Sb/Sb-oxide) as shown in Fig. 10C or Ir/Ir-oxide, or tip-integrated biosensors can be integrated. Examples with more relevance in terms of investigating induced changes at the surface have been demonstrated by mapping dissolution processes116,165,166 and corrosion-related AFM–SECM studies.145,156,167 The investigation of the biphasic alloy Ti–6Al–4V147 as a relevant industrial electrode system has also been reported. Earlier during the development of SECM its utility for studying enzyme activity has been demonstrated.26 Kueng et al. have applied AFM–SECM imaging to map the activity of glucose oxidase (GOD),160 which was entrapped within so polymer spots, as initially described by Schuhmann and co-workers.168 Enzyme spots were simultaneously imaged in dynamic mode AFM and generator–collector mode SECM. In the presence of glucose, an increased current was detected above the enzyme spots due to the formation of hydrogen peroxide as the side-product of the enzymatic conversion. The activity of immobilized horseradish peroxidase HRP spots was also imaged using ferrocenemethanol (FMA) as the redox mediator.159 Hirata et al.131 have investigated the enzyme activity of GOD, which was immobilized on a cross-linked polyion complex layer (poly-L-lysine and poly(4-styrenesulfonate
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Examples of combined SECM approaches using nonamperometic probes. (A) (a) Scheme of an AFM tip-integrated biosensor; (b) topographical image recorded with a combined AFM SECM probe in tapping mode AFM (scan area: 2450 2450 nm, scan rate: 1 line per s, drive frequency: 32 kHz) reveals the pores (diam. 200 nm) of a track-etched membrane. This membrane separates the donor compartment containing glucose solution (3 mM) and an upper compartment (PBS puffer). (b) The tip-integrated biosensor (immobilized glucose oxidase) was biased at a potential of 650 mV vs. Ag/AgCl to oxidize H2O2, the side product of the GOD-catalyzed oxidation of glucose. The electrochemical image reflects the diffusion of glucose through the pores resulting in an increased current due to oxidation of H2O2. (Adapted from ref. 158 with permission from Wiley-VCH). (B) (a) Schematic view of tip-attached redox mediator (TARM) AFM–SECM probe. (b) Topography of a HOPG sample recorded with an FcPEGylated AFM–SECM probe in tapping mode (drive frequency: 8.66 kHz, amplitude set point 4 nm, scan rate: 1 line per s). The tip current is presented in (b). The tip potential was set to Etip + 0.30 V per SCE and the substrate potential was set to Esub ¼ + 0.2 V per SCE. Images were processed with a flattening algorithm (adapted from ref. 120 with permission from the American Chemical Society). (C) (a) Sketch of a combined SICM–SECM probe with a pH-sensitive nanoelectrode (b) SICM topography of a calcite microcrystal (bulk pH 6.85). A 350 mV bias was typically applied between the Ag/AgCl electrode inside the SICM barrel and the QRCE in the bulk solution for SICM measurements; a sinusoidal wave creating a 60 nm peak–peak amplitude (280 Hz) was applied to the z-position of the probe. The resulting ac magnitude iAC was determined via the lock-in amplifier. (b) Simultaneously recorded pH map close to the calcite microcrystal adapted from ref. 114 with permission from the American Chemical Society. (D) Scheme and SEM image of a potentiometric AFM–SECM probe with a pH microsensor based on antimony oxide developed by Kranz and coworkers; a corresponding potential-pH calibration curve is shown. Fig. 10
composite)) at HOPG electrodes. AFM was operated in noncontact mode, thus enabling imaging of the sample surface with a dual probe bearing the electroactive area at the tip apex. Ideally, both the techniques are matching in terms of resolution. This is a challenging task as it requires nanoscopic electrodes, which can be reproducibly fabricated either at the apex of the tip or recessed from the tip. In addition, such electrodes should show superior electrochemical characteristics and importantly the kinetics of the electron transfer (in feedback) has to be fast so that the electrochemical process can be imaged. Demaille and co-workers have published a series of papers focusing on high-resolution imaging.120,121,169–172 Instead
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of freely diffusing mediators, redox-tagged chains (e.g., oligonucleotides or polyethylene glycol) were implemented, which were either attached to the sample surface169,170,172 or to the apex of the combined AFM–SECM gold tip.120,121 The authors succeeded in imaging individual gold nanoparticles with dimensions of approx. 20 nm, which were functionalized by redoxlabeled PEG chains.172 As the AFM–SECM probe has the electrode at the apex of the combined tip, the electroactive groups of the chains may be oxidized or reduced, if the AFM–SECM probe is in contact. Several acronyms have been introduced for the approach using tethered redox molecules including molecule touching SECM– AFM (Mt/SECM–AFM) and the tip-attached redox mediator (TARM). Again, as any other discussed methodology, AFM–SECM also has some distinct advantages and disadvantages. A clear advantage is the possibility of obtaining high-resolution topographical and electrochemical imaging, which can be ideally directly correlated as the information may be obtained in a single scan. In addition, the SECM functionality can be directly implemented into any standard AFM system in combination with an external potentiostat so that the electrochemical data can be fed in an additional AD channel, which is offered all commercial AFM systems. A concern raised by Wittstock67 that AFM–SECM probes reveal imperfect shapes of steady-state voltammograms is not necessarily true and strongly depends on the quality of the insulation, which is also true for other probe designs discussed in this review. Despite the general advantages of AFM–SECM, a particular challenge of combined probes remains: the mounting and establishment of a suitable electrical contact in comparison with the other combined approaches discussed here. Needle-type electrodes, so stylus electrodes, nanocapillaries or combined SICM–SECM probes do not require that the entire probe including the contact should be immersed in solution, which instead is usually the case for AFM–SECM probes. Hence, substantial precautions are needed to ensure that the electrical contact with the tip-integrated electrode remains well insulated aer mounting the probe avoiding leakage currents, which is usually ensured by manually insulating the contact with insulating varnish, nail polish etc. In addition, given the dimensions of the integrated electrodes, background noise, additional resistance due to connections, etc. have to be considered.
4. Quo vadis, SECM? As stated in the Introduction, SECM has not yet reached the zenith of the hype cycle in terms of applications, probe design, instrumental developments and improvements, and the introduction of additional complementary imaging modalities. Important aspects facilitating the advancement of SECM are certainly (i) that several companies are nowadays offering commercial SECM instruments, and that (ii) SECM is considered a truly non-invasive SPM technique, which renders it highly suitable for fundamental studies in bioelectrochemistry, biocorrosion, fuel cell research, electrochemical nanotechnology, sensor technology, and materials science.
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Consequently, advancing the technology towards multifunctional SPM and combined analytical platforms integrating complementary analytical techniques such as various spectroscopies or mass spectrometry is clearly of interest. Trifunctional scanning probes integrating the AFM–SECM– NSOM functionality have already been demonstrated and applied for imaging neurites of living PC12 cells.173 Recently, also combinations of SECM with spectroscopic techniques such as attenuated total reection (ATR) IR spectroscopy (IR–ATR– SECM) have been reported.174,175 This concept may be readily extended toward a combination with AFM–SECM or by implementing an IR-transparent electrode for combined spectroelectrochemical measurements.176 Likewise, rst efforts towards the combination of SECM with mass spectrometry have been reported in the literature. Another interesting aspect of combined analytical methods involving SECM is that any electroanalytical measurement technique may in fact be applied in combination with SECM, thereby providing spatial resolution. Consequently, limitations of classical electroanalytical techniques typically providing bulk information and usually requiring large sample volumes may be circumnavigated. Hence, SECM measurements have been obtained in combination with amalgam microelectrodes,177–180 with potentiometric probes (micro-ISEs and pH-microelectrodes),25,61,115,181 with modied electrodes such as microbiosensors,26,182,183 and with electrocatalytically modied electrodes.184–186 Such modications are not limited to micrometer-sized electrodes, but may be further miniaturized and implemented into combined SPM probes, as shown in Fig. 10. For example, a pH-sensitive electrode was obtained by electrodeposition of hydrous iridium oxide onto the carbon electrode of a combined SICM–SECM probe. The dissolution of calcite, which is strongly pH-dependent, was imaged. An increase of the local pH at the calcite– water interface could be resolved. Such strategies will signicantly broaden the eld of applications for multifunctional SPM probes, and hence, contribute to the deviation from the hype cycle shown in Fig. 1. For a more comprehensive overview, the reader is directed to ref. 187. However, considering the range of potential combinations discussed herein, the reliable performance of each individual analytical technique in these combinations is of paramount importance. In addition, their applicability beyond academic research, i.e., demonstrating full functionality on real-world samples, is a prerequisite for a more widespread usage of such rather sophisticated technologies.
Acknowledgements J.-S. Moon is acknowledged for nanodisc AFM–SECM and potentiometric probe development presented here. Furthermore, all funding agencies supporting research at IABC related to AFM–SECM are greatly acknowledged.
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