Faraday Discussions Cite this: Faraday Discuss., 2014, 172, 27
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
PAPER
View Journal | View Issue
Single layer graphene as an electrochemical platform† Nicole L. Ritzert,a Wan Li,b Cen Tan,a Gabriel G. Rodr´ıguez-Calero,a a Joaqu´ın Rodr´ıguez-Lopez, Kenneth Herna´ndez-Burgos,a Sean Conte,a ´ ab a Joshua J. Parks, Daniel C. Ralph*b and He´ctor D. Abrun ˜ a* Received 4th April 2014, Accepted 9th May 2014 DOI: 10.1039/c4fd00060a
Over the past decade, there has been a great deal of interest in graphene with regards to its electrochemical behavior. Previous studies have focused on understanding fundamental processes such as charge transfer and molecular transport at the graphene–electrolyte interface as well as on applications of graphene in electronic, optical, and mechanical systems. We present illustrative examples of large area, single layer graphene platforms for applications such as optical and sensing devices as well as microfluidic systems. Three examples of graphene modified with thin polymer films are discussed. We have explored the use of graphene as an electrochemical platform for surface-generated electrogenerated chemiluminescence (ECL) using poly-[Ru(v-bpy)3]2+, where v-bpy is 4-vinyl, 40 -methyl 2,20 -bipyridine, as a model system. We found that while graphene can sustain ECL conditions, there was film degradation during ECL, as demonstrated by a decrease in ECL intensity upon potential cycling even in the presence of a graphene coating (“graphene blanket”). Using poly 3,4-ethylenedioxythiophene (EDOT), we demonstrate a facile method of fabricating electrochromic electrodes from large area graphene. The oxidation of NADH at graphene was catalyzed using an electrodeposited layer of 3,4-dihydroxybenzaldehyde as an effective redox mediator. In addition, we describe the fabrication and characterization of a microfluidic device based on a solution-gated field effect transistor which was able to detect changes of 60 mV per pH unit change in an inverted cell design. On the other hand, a 29 mV shift in the Dirac point per unit pH change was measured with our microfluidic devices, and a ca. 10% FET conductance change was measured when we continuously changed the pH in
a Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, 14853, USA. E-mail: hda1@ cornell.edu; Fax: +1-607-255-9864; Tel: +1-607-255-4720 b Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, NY, 14853, USA. E-mail: [email protected]; Fax: +1-607-255-6428; Tel: +1-607-255-9644
† Electronic supplementary information (ESI) available: Cyclic voltammograms for deposition of 3,4-DHB on graphene; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO in acetonitrile and in phosphate buffer; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO before and aer ECL with tripropylamine; Raman spectra of ITO/poly-[Ru(v-bpy)3]2+/graphene before and aer ECL; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO before and aer ECL with C2O42; cyclic voltammograms of immobilized 3,4-DHB at different sweep rates. See DOI: 10.1039/c4fd00060a
This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 27
View Article Online
Faraday Discussions
Paper
solution from 6.91 to 7.64 in the microfluidic channel, demonstrating local microfluidic pH sensing (albeit non-Nerstian) in real time.
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Introduction During the past decade since it was isolated through simple exfoliation,1,2 graphene has demonstrated interesting optical, mechanical, and electronic properties.3–6 Graphene has also found use in electrochemical systems, from sensing devices to fundamental studies of its charge transfer properties.7–11 Large area, single layer graphene12–15 is interesting because unlike graphene akes, it can be used as the sole electrode material, removing the need for a conductive substrate and thus allowing for measurements on graphene without complications from the underlying substrate. Single layer graphene also offers the opportunity for simultaneously probing two interfaces in proximity. Recent debate has focused on any differences in charge transfer processes at the edges and basal planes of graphene16,17 and the effects of impurities introduced during growth and processing on the purported catalytic properties of graphene.18 Our group has fabricated and electrochemically characterized large area, single layer graphene electrodes.12,19–21 We have used these electrodes to probe phenomena such as controlling mechanical and chemical defect sites19 and measuring the 2D surface diffusion coefficient of electroactive adsorbates.20 To demonstrate the utility of large area, single layer graphene in different electrochemical platforms, we have expanded on our previous studies by demonstrating optical and sensing devices based on single layer graphene electrodes. In this paper, we discuss graphene as a protective layer for thin polymer lms during electrogenerated chemiluminescence (ECL), as an electrode material for electrochromic devices based on electropolymerized, conducting polymer lms, and as a substrate for electrochemical sensing of biomolecules. We also describe the fabrication and characterization of a working microuidic device based on electrochemically-gated single layer graphene (FET) transistors. Electropolymerizable materials, such as those based on conducting, organic polymers, are of great interest in electrochromic displays, electrochemical energy storage, organic light emitting diodes, exible electronics, and photo-voltaic devices because of their long-term stability, low oxidation potential, high contrast ratio, fast charge/discharge rates, tunable electronic structure, and fast switching times.22–34 Graphene has been proposed as an alternative to commonly used support materials such as indium tin oxide (ITO) due to graphene's extraordinary electrical, physical, optical, and mechanical properties.1,3,4,35–37 We have developed methodologies for electropolymerizing electrochromic compounds, viz., 3,4-ethylenedioxythiophene (EDOT), onto large area graphene electrodes through the use of electrochemical techniques such as cyclic voltammetry, chronoamperometry, and galvanostatic methods. Electropolymerization provides advantages in terms of sample preparation, cost, and processing, especially when compared to other chemical and physical deposition methods. The use of electrochemical techniques provides control over material properties such as thickness, electronic structure, chemical structure, and morphology by simply changing the deposition parameters and conditions. In addition, by using
28 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
electropolymerization to modify the graphene electrodes, we ensure the conformal coating of the electrode material. ECL based on thin organic lms is useful in devices such as optical displays and small-molecule sensors.38–40 Briey, ECL is produced when anions and cations of an electroactive, luminescent species that are electrogenerated in close proximity react, provided that sufficient energy is provided to form an excited electronic state, as shown below for annihilation ECL: Rc + Rc+ / R* + R
(1)
R* / R + hn
(2)
Because these reactions produce visible light, the required energy is typically ca. 2 to 3 eV.38 Coreactant ECL is induced by a single potential step or a linear potential sweep. A coreactant is used to generate a very reactive species that provides the energy needed to excite the emitting species. Proposed mechanisms are available for a variety of coreactants.38–40 One example is the tris(2,20 -bipyridyl) ruthenium(II)/oxalate system, in which a reducing reagent is formed upon sweeping the potential to oxidizing conditions:41 [Ru(bpy)3]2+ / [Ru(bpy)3]3+ + e
(3)
[Ru(bpy)3]3+ + C2O42 / [Ru(bpy)3]2+ + C2O4c
(4)
C2O4c / CO2c + CO2
(5)
[Ru(bpy)3]2+ + CO2c / [Ru(bpy)3]+ + CO2
(6)
[Ru(bpy)3]3+ + [Ru(bpy)3]+ / [Ru(bpy)3]2+* + [Ru(bpy)3]2+
(7)
Coreactant ECL is useful in systems where electrolyte media, such as aqueous solutions, are unstable at potentials necessary to generate the reactive species or if one of the electrogenerated species is unstable. Polymer lms can degrade during ECL due to effects such as dissolution caused by enhanced solubility of the charged lm and mechanical breakdown of the lm due to incorporation of solvent or counter ions.38 Particularly, highly reactive radicals, which may react with the lm, are formed during coreactant ECL. In the following studies, we considered using graphene as a protective layer at the lm–electrolyte interface during ECL of polymer lms to prevent/mitigate mechanical loss, e.g., dissolution, of the lm. Large area graphene has already been studied as a protective layer for metals and semiconductors under thermally and electrochemically oxidizing conditions42–44 although passivation of metals such as copper was limited to several hours. We investigated ECL at ITO electrodes modied using poly-[Ru(v-bpy)3]2+, where v-bpy is 4-vinyl, 40 -methyl 2,20 bipyridine, as a model system since the ECL of surface-immobilized [Ru(bpy)3]2+ is well-documented45–49 and lms of electropolymerized [Ru(v-bpy)3]2+ can be readily grown.50 We rst considered coreactant ECL in order to probe electron transfer processes across graphene, requiring interaction of a solution species, viz., the coreactant, through the graphene layer with immobilized [Ru(v-bpy)3]2+ lms. This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 29
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Faraday Discussions
Paper
Another useful area of polymer-modied electrodes is in biosensing. Polymer lms have been used as antifouling agents as well as providing a so platform for antibody detection.51–53 In our studies, we considered sensors based on nicotinamide adenine dinucleotide (NADH) because NADH is involved in over 200 known enzymatic reactions,54 making it a popular analyte, and glassy carbon modied with polymerized 3,4-dihydroxybenzaldehyde (DHB) catalyzes its oxidation.55 NADH oxidation has already been documented at graphenecomposite electrodes.7,8,56,57 Graphene offers the ability to easily pattern electrodes in different shapes, including coating 3D structures and exible substrates, that may be advantageous in designing new biosensor congurations. In addition, and in the specic case of DHB as a surface modier, it needs to be pointed out that such lms only grow on carbon surfaces. In addition to polymer-modied graphene platforms, we report our effort to construct a graphene sensing platform on which we integrate the solution-gated eld effect transistor into a microuidic system to achieve local pH sensing. Graphene is a two dimensional material that is fully comprised of surfaces. The electrical conductance of graphene is thus sensitive to the changes of charge environment induced by analyte reaction or adsorption on the graphene surface, and is oen used as a readout for various graphene-based sensing devices. The high charge carrier mobility of graphene imparts lower noise and thus better sensing performance in graphene-based sensing device.58 In solution-gated graphene eld effect transistors,59,60 the graphene surface is directly exposed to the solution and the gate voltage is applied electrochemically to graphene through the Debye layer at the graphene–electrolyte interface. Due to the relatively large capacitance of the Debye layer, changes of the gate voltage induced by a chemical change in solution are expected to lead to signicant changes of the graphene charge carrier density, and consequently the graphene conductance. Solutiongated eld effect transistors have been extensively studied for pH sensing.59–62 A moderate response of 6–20 mV shi in the Dirac point per unit pH change was typically observed. The corresponding change of graphene conductance was found to be 3–13% per unit pH change, when compared to the conductance at pH 7 at gate voltages close to the Dirac point.
Experimental methods Materials All chemicals were used as received unless otherwise noted. Tripropylamine (TPrA) ($98%), 3,4-ethylenedioxythiophene (EDOT), tetrabutylammonium hexauorophosphate (TBAPF6) (for electrochemical analysis, $99.0%), tetrabutylammonium perchlorate (TBAP) (for electrochemical analysis, $99.0%), anisole (anhydrous, 99.7%), Tris buffer, and acetonitrile (MeCN) (anhydrous, 99.8%), were from Sigma-Aldrich (St. Louis, MO). Strem Chemicals (Newbury Point, MA) was the supplier of tris(2,20 -bipyridyl)ruthenium(II) chloride hexahydrate, [Ru(bpy)3]2+ (min. 98%). Puried [Ru(v-bpy)3](PF6)2 was from an available batch. Reduced nicotinamide adenine dinucleotide (NADH) (grade II, 98%) disodium salt was from Roche Diagnostics (Indianapolis, IN). Sodium oxalate (Na2C2O4), sodium acetate, sodium phosphate monobasic, sodium phosphate dibasic, sodium nitrate, acetone, isopropanol, and dichloromethane were all analytical reagent grade and from Mallinkrodt-Baker (Phillipsburg, NJ). 3,4-DHB (97%) was 30 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
from Pfaltz and Bauer (Waterbury, CT). Silicon wafers with a 300 nm layer of thermally grown oxide (Si/SiO2) were from Silicon Quest International (San Jose, CA), and 1.1 mm glass slides coated with indium tin oxide (ITO) (10 U cm2) were from Nanocs Inc. (New York, NY). Water (18 MU cm) puried using a Millipore (Billerica, MA) Milli-Q system was used to clean all glassware and to prepare all aqueous solutions. In our studies, “large area” refers to the size of the graphene sheet so that the graphene electrode was between 2 and 4 cm2 at the end of processing. Graphene was grown on 0.025 mm thick copper foil (99.8%, Alfa-Aesar, Ward Hill, MA) using the chemical vapor deposition (CVD) method as described previously.12,20,63 Graphene was grown using Cu foil initially ca. 2 8 cm2. Then, the foil was cut into smaller, rectangular pieces with dimensions between 1.5 and 2 cm along each side. A layer of poly(methyl methacrylate) (PMMA) was added to the graphene before etching the copper in order to provide support during transfer. Aer etching the copper and transferring to water at least six times, graphene–PMMA was transferred to Si/SiO2, ITO, modied ITO (vide infra), or a glass microscope slide and dried with nitrogen. To remove the PMMA, the chips were immersed rst in anisole for two to three hours, then in 50/50 (v/v) dichloromethane– acetone for at least 12 hours, and nally in isopropanol for at least two hours. The quality of the single layer graphene was veried by the 2D and G peaks at 2700 and 1600 cm1, respectively, in the Raman spectra,64,65 obtained using a Renishaw (Gloucestershire, UK) InVia Confocal Raman microscope with excitation wavelength of 488 nm. Raman spectra also indicated that the graphene samples contained some disorder, shown by a small D peak at 1350 cm1.
Electrochemical and ECL measurements All electrochemical measurements were performed in a three-electrode cell conguration with graphene as the working electrode. Electrical contact to graphene was made using gold evaporated on the graphene or using copper tape and indium wire directly on the graphene. Auxiliary and reference electrodes were a gold or platinum wire and an Ag/AgCl/1 M KCl electrode inside a salt bridge (3% agar with 0.2 M potassium nitrate), respectively. All potentials are reported vs. Ag/ AgCl unless noted otherwise. A silver wire quasi reference electrode (Ag QRE) was used in nonaqueous solutions. No calibration of the QRE was made because the potential was usually stable over the time required for each measurement. Electrodes were placed in a homemade Teon cell with a Plexiglas base, and an o-ring was used to expose an area of 0.5 cm2 graphene to solution. For ECL measurements, the base was an aluminum piece with a hole to allow any emitted light to pass through the bottom of the cell. In this conguration, any ECL emission was detected through the ITO electrode on the bottom of the cell opposite to the solution (Fig. 1). ECL measurements were performed using an EG&G Princeton Applied Research (Princeton, NJ) Model 173 Potentiostat-Galvanostat with a Model 175 Universal Programmer and Model 176 Current Follower. Data were recorded using a custom LabVIEW (National Instruments, Austin, TX) program and a National Instruments USB-6210 DAQ device. Emission was detected using a Hamamatsu Model R928 photomultiplier tube (PMT) operated at 750 V using a Model 556 high-voltage power supply from EG&G Ortec (Oak Ridge, TN). The PMT This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 31
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Faraday Discussions
Paper
Diagram of equipment for detecting ECL emission. ECL generated in the electrochemical cell is detected by the photomultiplier tube placed directly below the cell, opposite to the electrolyte solution.
Fig. 1
was connected to the DAQ device using an Ithaco (Ithaca, NY) Model 1211 current preamplier. Electrochromic measurements were performed at room temperature using a Model HSV-100 or HABF1510m potentiostat (Hokuto Denko). All other electrochemical measurements were performed using a Model 900 potentiostat (CH Instruments, Austin, TX). Polymer lm growth for electrochemical measurements Films of poly-[Ru(v-bpy)3]2+ were made through electroreductive polymerization50 by immersing an ITO electrode into a 0.5 mM solution of [Ru(v-bpy)3](PF6)2, in 0.1 M TBAPF6 in acetonitrile and cycling the potential between 0.2 V and 1.8 V vs. the Ag QRE, just past the second reduction wave of poly-[Ru(v-bpy)3]2+, which is the second ligand reduction. The potential was typically cycled at a scan rate of 100 mV s1. Aer electropolymerization, the electrode was removed from the [Ru(v-bpy)3]2+ solution and rinsed with acetone. ITO–poly-[Ru(v-bpy)3]2+, i.e., the ITO modied with poly-[Ru(v-bpy)3]2+, was then immersed in fresh electrolyte solution or graphene was transferred over the polymer layer as described previously. 3,4-DHB lms were grown through electrooxidative polymerization55 by cycling the potential of graphene on Si/SiO2 between 0.2 and +0.8 V vs. Ag/AgCl for 5 cycles in a solution of 1 mM 3,4-DHB in 0.1 M Tris buffer, pH 8.21, with 0.1 M NaNO3 (Fig. S1†). Aer rinsing with water, the electrode was immersed in fresh buffer or buffer containing NADH. Microuidic device fabrication and characterization Fabrication of graphene eld effect transistor. For the fabrication of graphene eld effect transistors, metal electrodes made of 5 nm Ti and 200 nm Au were rst patterned on a fused silica substrate using photolithography and electron beam 32 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
evaporation (Fig. 2a; step 1). CVD graphene was then transferred onto the fused silica substrate to make electrical contact with the metal electrodes using the same processes as those for ECL sample preparation, except that the PMMA supporting layer was kept on top of the graphene (Fig. 2a; step 2). In the next step, (Fig. 2a; step 3) the graphene was patterned into a rectangular stripe using photolithography and oxygen plasma. Since the PMMA was not removed in the earlier step, the oxygen plasma patterned graphene by rst removing the PMMA and then the underneath graphene. This fabrication scheme avoided the direct contact of photoresist with graphene and was capable of obtaining cleaner graphene surfaces. In the last step, the photoresist and PMMA on graphene strips were removed by heated 1165 solution, which consists mostly of N-methyl-2pyrrolidone, for 1 hour at 70–80 degrees. The devices were then rinsed with isopropanol and blown dry with N2 (Fig. 2a; step 4). Fig. 2b shows the AFM image of a graphene sample we patterned with the above method. The CVD graphene surface remained smooth and clean aer the patterning step. Microuidic solution-gated graphene FET construction and measurement schemes. For the microuidic solution-gated graphene FET devices, PDMS microuidic channels 200 mm in width and 40 mm in height with multiple inlet ports and one outlet reservoir were fabricated using standard methods. A closed, sealed reservoir was placed along the channel for reference electrode insertion (Fig. 3a). To construct the microuidic device, the polydimethylsiloxane (PDMS) block was treated with oxygen plasma and then bonded to a freshly fabricated graphene FET. The two compartments were carefully aligned to ensure that the microuidic channel was right above the graphene stripe (Fig. 3c, inset). A platinum wire used as a reference electrode was punched into the sealed reservoir and further sealed with epoxy (Fig. 3b and c). During measurement, buffer solutions of known pH were injected into the microuidic channel via the inlet port at a controlled ow rate of
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
PAPER
View Journal | View Issue
Single layer graphene as an electrochemical platform† Nicole L. Ritzert,a Wan Li,b Cen Tan,a Gabriel G. Rodr´ıguez-Calero,a a Joaqu´ın Rodr´ıguez-Lopez, Kenneth Herna´ndez-Burgos,a Sean Conte,a ´ ab a Joshua J. Parks, Daniel C. Ralph*b and He´ctor D. Abrun ˜ a* Received 4th April 2014, Accepted 9th May 2014 DOI: 10.1039/c4fd00060a
Over the past decade, there has been a great deal of interest in graphene with regards to its electrochemical behavior. Previous studies have focused on understanding fundamental processes such as charge transfer and molecular transport at the graphene–electrolyte interface as well as on applications of graphene in electronic, optical, and mechanical systems. We present illustrative examples of large area, single layer graphene platforms for applications such as optical and sensing devices as well as microfluidic systems. Three examples of graphene modified with thin polymer films are discussed. We have explored the use of graphene as an electrochemical platform for surface-generated electrogenerated chemiluminescence (ECL) using poly-[Ru(v-bpy)3]2+, where v-bpy is 4-vinyl, 40 -methyl 2,20 -bipyridine, as a model system. We found that while graphene can sustain ECL conditions, there was film degradation during ECL, as demonstrated by a decrease in ECL intensity upon potential cycling even in the presence of a graphene coating (“graphene blanket”). Using poly 3,4-ethylenedioxythiophene (EDOT), we demonstrate a facile method of fabricating electrochromic electrodes from large area graphene. The oxidation of NADH at graphene was catalyzed using an electrodeposited layer of 3,4-dihydroxybenzaldehyde as an effective redox mediator. In addition, we describe the fabrication and characterization of a microfluidic device based on a solution-gated field effect transistor which was able to detect changes of 60 mV per pH unit change in an inverted cell design. On the other hand, a 29 mV shift in the Dirac point per unit pH change was measured with our microfluidic devices, and a ca. 10% FET conductance change was measured when we continuously changed the pH in
a Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, 14853, USA. E-mail: hda1@ cornell.edu; Fax: +1-607-255-9864; Tel: +1-607-255-4720 b Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, NY, 14853, USA. E-mail: [email protected]; Fax: +1-607-255-6428; Tel: +1-607-255-9644
† Electronic supplementary information (ESI) available: Cyclic voltammograms for deposition of 3,4-DHB on graphene; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO in acetonitrile and in phosphate buffer; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO before and aer ECL with tripropylamine; Raman spectra of ITO/poly-[Ru(v-bpy)3]2+/graphene before and aer ECL; cyclic voltammograms of poly-[Ru(v-bpy)3]2+ on ITO before and aer ECL with C2O42; cyclic voltammograms of immobilized 3,4-DHB at different sweep rates. See DOI: 10.1039/c4fd00060a
This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 27
View Article Online
Faraday Discussions
Paper
solution from 6.91 to 7.64 in the microfluidic channel, demonstrating local microfluidic pH sensing (albeit non-Nerstian) in real time.
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Introduction During the past decade since it was isolated through simple exfoliation,1,2 graphene has demonstrated interesting optical, mechanical, and electronic properties.3–6 Graphene has also found use in electrochemical systems, from sensing devices to fundamental studies of its charge transfer properties.7–11 Large area, single layer graphene12–15 is interesting because unlike graphene akes, it can be used as the sole electrode material, removing the need for a conductive substrate and thus allowing for measurements on graphene without complications from the underlying substrate. Single layer graphene also offers the opportunity for simultaneously probing two interfaces in proximity. Recent debate has focused on any differences in charge transfer processes at the edges and basal planes of graphene16,17 and the effects of impurities introduced during growth and processing on the purported catalytic properties of graphene.18 Our group has fabricated and electrochemically characterized large area, single layer graphene electrodes.12,19–21 We have used these electrodes to probe phenomena such as controlling mechanical and chemical defect sites19 and measuring the 2D surface diffusion coefficient of electroactive adsorbates.20 To demonstrate the utility of large area, single layer graphene in different electrochemical platforms, we have expanded on our previous studies by demonstrating optical and sensing devices based on single layer graphene electrodes. In this paper, we discuss graphene as a protective layer for thin polymer lms during electrogenerated chemiluminescence (ECL), as an electrode material for electrochromic devices based on electropolymerized, conducting polymer lms, and as a substrate for electrochemical sensing of biomolecules. We also describe the fabrication and characterization of a working microuidic device based on electrochemically-gated single layer graphene (FET) transistors. Electropolymerizable materials, such as those based on conducting, organic polymers, are of great interest in electrochromic displays, electrochemical energy storage, organic light emitting diodes, exible electronics, and photo-voltaic devices because of their long-term stability, low oxidation potential, high contrast ratio, fast charge/discharge rates, tunable electronic structure, and fast switching times.22–34 Graphene has been proposed as an alternative to commonly used support materials such as indium tin oxide (ITO) due to graphene's extraordinary electrical, physical, optical, and mechanical properties.1,3,4,35–37 We have developed methodologies for electropolymerizing electrochromic compounds, viz., 3,4-ethylenedioxythiophene (EDOT), onto large area graphene electrodes through the use of electrochemical techniques such as cyclic voltammetry, chronoamperometry, and galvanostatic methods. Electropolymerization provides advantages in terms of sample preparation, cost, and processing, especially when compared to other chemical and physical deposition methods. The use of electrochemical techniques provides control over material properties such as thickness, electronic structure, chemical structure, and morphology by simply changing the deposition parameters and conditions. In addition, by using
28 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
electropolymerization to modify the graphene electrodes, we ensure the conformal coating of the electrode material. ECL based on thin organic lms is useful in devices such as optical displays and small-molecule sensors.38–40 Briey, ECL is produced when anions and cations of an electroactive, luminescent species that are electrogenerated in close proximity react, provided that sufficient energy is provided to form an excited electronic state, as shown below for annihilation ECL: Rc + Rc+ / R* + R
(1)
R* / R + hn
(2)
Because these reactions produce visible light, the required energy is typically ca. 2 to 3 eV.38 Coreactant ECL is induced by a single potential step or a linear potential sweep. A coreactant is used to generate a very reactive species that provides the energy needed to excite the emitting species. Proposed mechanisms are available for a variety of coreactants.38–40 One example is the tris(2,20 -bipyridyl) ruthenium(II)/oxalate system, in which a reducing reagent is formed upon sweeping the potential to oxidizing conditions:41 [Ru(bpy)3]2+ / [Ru(bpy)3]3+ + e
(3)
[Ru(bpy)3]3+ + C2O42 / [Ru(bpy)3]2+ + C2O4c
(4)
C2O4c / CO2c + CO2
(5)
[Ru(bpy)3]2+ + CO2c / [Ru(bpy)3]+ + CO2
(6)
[Ru(bpy)3]3+ + [Ru(bpy)3]+ / [Ru(bpy)3]2+* + [Ru(bpy)3]2+
(7)
Coreactant ECL is useful in systems where electrolyte media, such as aqueous solutions, are unstable at potentials necessary to generate the reactive species or if one of the electrogenerated species is unstable. Polymer lms can degrade during ECL due to effects such as dissolution caused by enhanced solubility of the charged lm and mechanical breakdown of the lm due to incorporation of solvent or counter ions.38 Particularly, highly reactive radicals, which may react with the lm, are formed during coreactant ECL. In the following studies, we considered using graphene as a protective layer at the lm–electrolyte interface during ECL of polymer lms to prevent/mitigate mechanical loss, e.g., dissolution, of the lm. Large area graphene has already been studied as a protective layer for metals and semiconductors under thermally and electrochemically oxidizing conditions42–44 although passivation of metals such as copper was limited to several hours. We investigated ECL at ITO electrodes modied using poly-[Ru(v-bpy)3]2+, where v-bpy is 4-vinyl, 40 -methyl 2,20 bipyridine, as a model system since the ECL of surface-immobilized [Ru(bpy)3]2+ is well-documented45–49 and lms of electropolymerized [Ru(v-bpy)3]2+ can be readily grown.50 We rst considered coreactant ECL in order to probe electron transfer processes across graphene, requiring interaction of a solution species, viz., the coreactant, through the graphene layer with immobilized [Ru(v-bpy)3]2+ lms. This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 29
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Faraday Discussions
Paper
Another useful area of polymer-modied electrodes is in biosensing. Polymer lms have been used as antifouling agents as well as providing a so platform for antibody detection.51–53 In our studies, we considered sensors based on nicotinamide adenine dinucleotide (NADH) because NADH is involved in over 200 known enzymatic reactions,54 making it a popular analyte, and glassy carbon modied with polymerized 3,4-dihydroxybenzaldehyde (DHB) catalyzes its oxidation.55 NADH oxidation has already been documented at graphenecomposite electrodes.7,8,56,57 Graphene offers the ability to easily pattern electrodes in different shapes, including coating 3D structures and exible substrates, that may be advantageous in designing new biosensor congurations. In addition, and in the specic case of DHB as a surface modier, it needs to be pointed out that such lms only grow on carbon surfaces. In addition to polymer-modied graphene platforms, we report our effort to construct a graphene sensing platform on which we integrate the solution-gated eld effect transistor into a microuidic system to achieve local pH sensing. Graphene is a two dimensional material that is fully comprised of surfaces. The electrical conductance of graphene is thus sensitive to the changes of charge environment induced by analyte reaction or adsorption on the graphene surface, and is oen used as a readout for various graphene-based sensing devices. The high charge carrier mobility of graphene imparts lower noise and thus better sensing performance in graphene-based sensing device.58 In solution-gated graphene eld effect transistors,59,60 the graphene surface is directly exposed to the solution and the gate voltage is applied electrochemically to graphene through the Debye layer at the graphene–electrolyte interface. Due to the relatively large capacitance of the Debye layer, changes of the gate voltage induced by a chemical change in solution are expected to lead to signicant changes of the graphene charge carrier density, and consequently the graphene conductance. Solutiongated eld effect transistors have been extensively studied for pH sensing.59–62 A moderate response of 6–20 mV shi in the Dirac point per unit pH change was typically observed. The corresponding change of graphene conductance was found to be 3–13% per unit pH change, when compared to the conductance at pH 7 at gate voltages close to the Dirac point.
Experimental methods Materials All chemicals were used as received unless otherwise noted. Tripropylamine (TPrA) ($98%), 3,4-ethylenedioxythiophene (EDOT), tetrabutylammonium hexauorophosphate (TBAPF6) (for electrochemical analysis, $99.0%), tetrabutylammonium perchlorate (TBAP) (for electrochemical analysis, $99.0%), anisole (anhydrous, 99.7%), Tris buffer, and acetonitrile (MeCN) (anhydrous, 99.8%), were from Sigma-Aldrich (St. Louis, MO). Strem Chemicals (Newbury Point, MA) was the supplier of tris(2,20 -bipyridyl)ruthenium(II) chloride hexahydrate, [Ru(bpy)3]2+ (min. 98%). Puried [Ru(v-bpy)3](PF6)2 was from an available batch. Reduced nicotinamide adenine dinucleotide (NADH) (grade II, 98%) disodium salt was from Roche Diagnostics (Indianapolis, IN). Sodium oxalate (Na2C2O4), sodium acetate, sodium phosphate monobasic, sodium phosphate dibasic, sodium nitrate, acetone, isopropanol, and dichloromethane were all analytical reagent grade and from Mallinkrodt-Baker (Phillipsburg, NJ). 3,4-DHB (97%) was 30 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
from Pfaltz and Bauer (Waterbury, CT). Silicon wafers with a 300 nm layer of thermally grown oxide (Si/SiO2) were from Silicon Quest International (San Jose, CA), and 1.1 mm glass slides coated with indium tin oxide (ITO) (10 U cm2) were from Nanocs Inc. (New York, NY). Water (18 MU cm) puried using a Millipore (Billerica, MA) Milli-Q system was used to clean all glassware and to prepare all aqueous solutions. In our studies, “large area” refers to the size of the graphene sheet so that the graphene electrode was between 2 and 4 cm2 at the end of processing. Graphene was grown on 0.025 mm thick copper foil (99.8%, Alfa-Aesar, Ward Hill, MA) using the chemical vapor deposition (CVD) method as described previously.12,20,63 Graphene was grown using Cu foil initially ca. 2 8 cm2. Then, the foil was cut into smaller, rectangular pieces with dimensions between 1.5 and 2 cm along each side. A layer of poly(methyl methacrylate) (PMMA) was added to the graphene before etching the copper in order to provide support during transfer. Aer etching the copper and transferring to water at least six times, graphene–PMMA was transferred to Si/SiO2, ITO, modied ITO (vide infra), or a glass microscope slide and dried with nitrogen. To remove the PMMA, the chips were immersed rst in anisole for two to three hours, then in 50/50 (v/v) dichloromethane– acetone for at least 12 hours, and nally in isopropanol for at least two hours. The quality of the single layer graphene was veried by the 2D and G peaks at 2700 and 1600 cm1, respectively, in the Raman spectra,64,65 obtained using a Renishaw (Gloucestershire, UK) InVia Confocal Raman microscope with excitation wavelength of 488 nm. Raman spectra also indicated that the graphene samples contained some disorder, shown by a small D peak at 1350 cm1.
Electrochemical and ECL measurements All electrochemical measurements were performed in a three-electrode cell conguration with graphene as the working electrode. Electrical contact to graphene was made using gold evaporated on the graphene or using copper tape and indium wire directly on the graphene. Auxiliary and reference electrodes were a gold or platinum wire and an Ag/AgCl/1 M KCl electrode inside a salt bridge (3% agar with 0.2 M potassium nitrate), respectively. All potentials are reported vs. Ag/ AgCl unless noted otherwise. A silver wire quasi reference electrode (Ag QRE) was used in nonaqueous solutions. No calibration of the QRE was made because the potential was usually stable over the time required for each measurement. Electrodes were placed in a homemade Teon cell with a Plexiglas base, and an o-ring was used to expose an area of 0.5 cm2 graphene to solution. For ECL measurements, the base was an aluminum piece with a hole to allow any emitted light to pass through the bottom of the cell. In this conguration, any ECL emission was detected through the ITO electrode on the bottom of the cell opposite to the solution (Fig. 1). ECL measurements were performed using an EG&G Princeton Applied Research (Princeton, NJ) Model 173 Potentiostat-Galvanostat with a Model 175 Universal Programmer and Model 176 Current Follower. Data were recorded using a custom LabVIEW (National Instruments, Austin, TX) program and a National Instruments USB-6210 DAQ device. Emission was detected using a Hamamatsu Model R928 photomultiplier tube (PMT) operated at 750 V using a Model 556 high-voltage power supply from EG&G Ortec (Oak Ridge, TN). The PMT This journal is © The Royal Society of Chemistry 2014
Faraday Discuss., 2014, 172, 27–45 | 31
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Faraday Discussions
Paper
Diagram of equipment for detecting ECL emission. ECL generated in the electrochemical cell is detected by the photomultiplier tube placed directly below the cell, opposite to the electrolyte solution.
Fig. 1
was connected to the DAQ device using an Ithaco (Ithaca, NY) Model 1211 current preamplier. Electrochromic measurements were performed at room temperature using a Model HSV-100 or HABF1510m potentiostat (Hokuto Denko). All other electrochemical measurements were performed using a Model 900 potentiostat (CH Instruments, Austin, TX). Polymer lm growth for electrochemical measurements Films of poly-[Ru(v-bpy)3]2+ were made through electroreductive polymerization50 by immersing an ITO electrode into a 0.5 mM solution of [Ru(v-bpy)3](PF6)2, in 0.1 M TBAPF6 in acetonitrile and cycling the potential between 0.2 V and 1.8 V vs. the Ag QRE, just past the second reduction wave of poly-[Ru(v-bpy)3]2+, which is the second ligand reduction. The potential was typically cycled at a scan rate of 100 mV s1. Aer electropolymerization, the electrode was removed from the [Ru(v-bpy)3]2+ solution and rinsed with acetone. ITO–poly-[Ru(v-bpy)3]2+, i.e., the ITO modied with poly-[Ru(v-bpy)3]2+, was then immersed in fresh electrolyte solution or graphene was transferred over the polymer layer as described previously. 3,4-DHB lms were grown through electrooxidative polymerization55 by cycling the potential of graphene on Si/SiO2 between 0.2 and +0.8 V vs. Ag/AgCl for 5 cycles in a solution of 1 mM 3,4-DHB in 0.1 M Tris buffer, pH 8.21, with 0.1 M NaNO3 (Fig. S1†). Aer rinsing with water, the electrode was immersed in fresh buffer or buffer containing NADH. Microuidic device fabrication and characterization Fabrication of graphene eld effect transistor. For the fabrication of graphene eld effect transistors, metal electrodes made of 5 nm Ti and 200 nm Au were rst patterned on a fused silica substrate using photolithography and electron beam 32 | Faraday Discuss., 2014, 172, 27–45
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 12 August 2014. Downloaded by University of Washington on 02/12/2014 17:20:07.
Paper
Faraday Discussions
evaporation (Fig. 2a; step 1). CVD graphene was then transferred onto the fused silica substrate to make electrical contact with the metal electrodes using the same processes as those for ECL sample preparation, except that the PMMA supporting layer was kept on top of the graphene (Fig. 2a; step 2). In the next step, (Fig. 2a; step 3) the graphene was patterned into a rectangular stripe using photolithography and oxygen plasma. Since the PMMA was not removed in the earlier step, the oxygen plasma patterned graphene by rst removing the PMMA and then the underneath graphene. This fabrication scheme avoided the direct contact of photoresist with graphene and was capable of obtaining cleaner graphene surfaces. In the last step, the photoresist and PMMA on graphene strips were removed by heated 1165 solution, which consists mostly of N-methyl-2pyrrolidone, for 1 hour at 70–80 degrees. The devices were then rinsed with isopropanol and blown dry with N2 (Fig. 2a; step 4). Fig. 2b shows the AFM image of a graphene sample we patterned with the above method. The CVD graphene surface remained smooth and clean aer the patterning step. Microuidic solution-gated graphene FET construction and measurement schemes. For the microuidic solution-gated graphene FET devices, PDMS microuidic channels 200 mm in width and 40 mm in height with multiple inlet ports and one outlet reservoir were fabricated using standard methods. A closed, sealed reservoir was placed along the channel for reference electrode insertion (Fig. 3a). To construct the microuidic device, the polydimethylsiloxane (PDMS) block was treated with oxygen plasma and then bonded to a freshly fabricated graphene FET. The two compartments were carefully aligned to ensure that the microuidic channel was right above the graphene stripe (Fig. 3c, inset). A platinum wire used as a reference electrode was punched into the sealed reservoir and further sealed with epoxy (Fig. 3b and c). During measurement, buffer solutions of known pH were injected into the microuidic channel via the inlet port at a controlled ow rate of
