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Vera La Ferrara Gabriella Rametta Antonella De Maria ENEA – Portici Research Center – Laboratory of Materials and Devices, Portici, Italy
Received July 25, 2014 Revised April 3, 2015 Accepted April 8, 2015
Research Article
AC electric field for rapid assembly of nanostructured polyaniline onto microsized gap for sensor devices Interconnected network of nanostructured polyaniline (PANI) is giving strong potential for enhancing device performances than bulk PANI counterparts. For nanostructured device processing, the main challenge is to get prototypes on large area by requiring precision, low cost and high rate assembly. Among processes meeting these requests, the alternate current electric fields are often used for nanostructure assembling. For the first time, we show the assembly of nanostructured PANI onto large electrode gaps (30–60 m width) by applying alternate current electric fields, at low frequencies, to PANI particles dispersed in acetonitrile (ACN). An important advantage is the short assembly time, limited to 5–10 s, although electrode gaps are microsized. That encouraging result is due to a combination of forces, such as dielectrophoresis (DEP), induced-charge electrokinetic (ICEK) flow and alternate current electroosmotic (ACEO) flow, which speed up the assembly process when low frequencies and large electrode gaps are used. The main achievement of the present study is the development of ammonia sensors created by direct assembling of nanostructured PANI onto electrodes. Sensors exhibit high sensitivity to low gas concentrations as well as excellent reversibility at room temperature, even after storage in air. Keywords: AC electric field / Ammonia sensor / Nanostructured polyaniline / Rapid assembly DOI 10.1002/elps.201400550
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction Polyaniline (PANI) plays a key role as optical, chemical and bio sensors because of its simple and reversible doping– dedoping chemistry. Indeed PANI bulk is often used for ammonia sensor at room temperature [1–4]. Ammonia is used in different industrial processes and its recognition is important because of related toxicity. When PANI is in presence of ammonia, the amine groups, in emeraldine salt form (conducting state), are deprotonated converting it to the emeraldine base form (insulating state) with corresponding drop in conductivity [5, 6]. In recent years, nanostructured PANI films have emerged as good ammonia sensing material compared to bulk counterparts, and showed better performance
Correspondence: Vera La Ferrara, ENEA, Laboratory of Materials and Devices, Portici, Italy E-mail: [email protected]
Abbreviations: ACEO, alternate current electroosmosis; ACN, acetonitrile; DLS, dynamic light scattering; EDL, electric double layer; FIB, focused ion beam; ICEK, induced-charge electrokinetic; PANI, polyaniline C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
due to high specific area. In the literature, different manufacturing procedures are described for sensing devices based on nanostructured PANI, like solid state, seeding polymerization and electrospinning [7–13]. There are some difficulties in those techniques, mainly for large area processing, such as no controlled assembly, time consuming and expensive process. Those disadvantages can be overcome if nanostructured PANI is assembled by applying AC or DC fields on particles suspended in electrolyte and dropped onto electrodes. However, in the literature, nanostructured PANI assembly is realized by electric field application through nanosized gaps but working device is never shown [14, 15]. The only working device, to our knowledge, has been realized by covering 2 m gap interdigitated electrodes in two minutes by applying DC electric field to PANI particles [16]. In our work, indeed, we show, for the first time, how to assemble quickly nanostructured PANI onto large electrode gaps (30– 60 m width) by applying AC electric field, at low frequencies, to PANI particles dispersed in acetonitrile (ACN). The main goals are about reducing assembly time and covering
Colour Online: See the article online to view Fig. 1 in colour.
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simultaneously large area gaps. It should be expected that the wider the gap, the greater the assembly time, but, although electrode gaps are microsized, the assembly time is fast, limited to 5–10 seconds, when appropriate frequency is applied. Performance-enhancing devices have been realized by applying different sinusoidal voltages and frequencies on PANI particles dispersed in ACN solvent dropped onto large electrode gaps. In such conditions, different induced forces, which act on motion of particles and allow rapid particle assembly, are generated. Those forces are dielectrophoresis (DEP) force, which polarizes particles, induced-charge electrokinetic (ICEK) flow, around the particles, and alternate current electroosmosis (ACEO) flow, which is generated at the electrodes [17–23]. Finally, those devices based on nanostructured PANI network, narrow or loose mesh shapes, are tested as ammonia sensors at room temperature. More especially, narrow mesh shape based devices show high relative response (100%) and reversible recovery at 5 ppm of ammonia gas as well as good reversibility in the conductivity signal even after storage in air.
2 Materials and methods 2.1 Experimental Polyaniline (PANI, emeraldine salt 2–3 wt% dispersion in xylene) and ACN (anhydrous, 99.8%), are purchased from Sigma–Aldrich. PANI, as reported from Aldrich data sheet, has conductivity of 10 S/cm, dielectric constant of 20 ε 0 (where ε 0 is the vacuum dielectric constant) and pristine
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particle radius about 200 nm. ACN has electrical conductivity of 7 S/cm and dielectric constant of 37.5 ε 0 . PANI is dispersed in ACN with concentration in the range from 32 g/ml to 8 g/ml. The suspensions are stirred at room temperature for 30 min. Each suspension is characterized by dynamic light scattering (DLS), (Zetasizer Nano ZS Malvern Instruments) using 12 L quartz cuvette. That apparatus employs a 4 mW He–Ne laser, wavelength 632.8 nm and the measurement angle of 173° with a noninvasive back scatter technology to calculate the apparent size (called hydrodynamic radius, RH ) of polymer in solvent. Resulting data are weighted on three measurements executed at two minutes intervals. Dynamic viscosities of PANI suspensions, , and ACN solvent, 0 , are measured by a viscometer (SV-10 Vibroviscometer) enabling the calculation of intrinsic viscosity, []. These parameters, together polymer molecular weight, M, allow for estimating of pristine conformation of commercial PANI dispersed in ACN, before applying electric field. Additionally, PANI suspension is dropped onto typical substrate (as shown in inset of Fig. 1A from optical microscope image) to evaluate pristine PANI morphology when no AC electric field is applied. As a result, PANI aggregates are randomly dispersed as shown in Fig. 1A. Devices based on nanostructured PANI network are realized by starting from silicon substrates (p-type, orientation ⬍100⬎, resistivity 10 ⍀cm, Siltronix, France). After standard cleaning, 240 nm thick SiO2 layer and two Cr/Au electrodes (20 nm/180 nm thick) are deposited by e-beam assisted evaporation onto 1 cm2 silicon. Electrode geometry consists of two millimeter pads and two micrometer fingers as shown in Fig. 1B. Different electrodes are realized by fixing area pads while width of
Figure 1. (A) In inset, optical image shows top view of PANI aggregates randomly dispersed when no electric field is applied. PANI/ACN suspension is dropped onto Cr/Au electrodes (140 m gap and 100 m electrode finger width) patterned on typical substrate SiO2 /Si. As shown in the inset, a region has been marked and analyzed by FIB to magnify random aggregates. (B) Schematic diagram view (not to scale) of experimental setup used for PANI assembly through electrode gap when AC electric field is applied. Effective electrode sizes are displayed on scheme.
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Figure 2. FIB images of device A, with 30 m electrode gap and 350 m electrode finger, are reported. On the left, image shows nanostructured PANI network across gap after AC electric field application (10 Vpp voltage and 5 kHz frequency). Different magnifications, on the right, have been acquired to zoom in on PANI network by showing a morphology like narrowmesh interconnected network.
the electrode fingers or gap size (ranging from 30 to 500 m both electrode fingers and gaps) are changed to find favorable conditions for rapid nanostructured PANI assembly. Few microliters of PANI/ACN suspension are deposited by casting onto substrates between the electrode gaps. Electrode pads are then connected to microprobes of a function generator (AGILENT 33220A) and substrates are placed under an optical microscope connected to a camcorder to control assembling process. Sinusoidal voltage (Vpeak-peak(pp) = 1–10 V) and frequency in the range of 3 mHz–3 MHz are used to achieve the best working conditions. Schematic view of the patterned substrate and electrical circuit diagram are shown in Fig. 1B. Several samples are made by using different diluted suspensions but only 16 g/ml concentration leads to good results. Indeed high concentrations have an excessive number of PANI particles which does not permit controlled manipulation while low concentrations have few particles not sufficient for assembling when wide gap is used. In order to achieve working devices, we have optimized parameters for 16 g/ml concentration by changing sinusoidal voltage, frequency, electrode finger width and gap size. No particle movement is detected below 5 Vpp at all frequencies for all the electrode geometry, while in the range 5–10 Vpp PANI partial accumulation occurs but continuous PANI assembly is evident only at a defined frequency. We have obtained rapid and continuous assembly for sensor devices by starting with 350 m electrode finger width and 30 or 60 m gap size. Working parameters for those geometries are Vpp = 10 V and frequency, f, 5 or 3 kHz depending on gap sizes. In Figs. 2 and 3 long well-arranged PANI nanostructures onto 30 and 60 m gap are shown (named devices A and B, respectively) at different magnifications. Devices based on those networks of nanostructured PANI are characterized by a focused
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ion beam (FIB) (FEI, QUANTA 2003D), where a focused gallium ion beam (30 keV energy), striking to the surface, gives high contrast images for inspection of morphologies. Devices are therefore characterized as ammonia sensors in a test chamber. A volt-amperometric technique, at constant bias, is then employed for sensor electrical characterization in a controlled gas-flow environment, pre-mixed with dry carrier in the desired percentage by mass flow meters and continuously controlled by means of an on-line Fourier transform infrared spectrometer [24]. All the tested devices are biased at 1 V under 5 ppm of NH3 and nitrogen as carrier. Total gas flow is set at 500 sccm.
2.2 Theory 2.2.1 Hydrodynamic radius, viscosity and polymer conformation DLS tecnique measures the intensity (I) of light scattered from particles, when laser light passes through the suspension. It allows evaluating apparent particle size in suspension calculated from the measured Brownian motion (diffusion coefficient) of the polymeric particles. That size is approximated to hydrodynamic radius, RH , hypothesized to be the radius of a sphere having the same diffusion behavior of polymer surrounded by solvent molecules. The measured data are the correlation curve which is fitted by an exponential function (Eq. (1) calculating the diffusion coefficient (D) which is proportional to the lifetime of the exponential decay [25, 26]. The correlation function, G ( ) , is: G ( ) = I + exp(−Dq 2 ),
(1)
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Figure 3. FIB images of device B, with 60 m electrode gap and 350 m electrode finger, are reported. On the left, image shows nanostructured PANI network across gap after AC electric field application (10 Vpp voltage and 3 kHz frequency). Different magnifications, on the right, have been acquired to zoom in on PANI network by showing a morphology like loose-mesh network.
where the decay rate Dq2 includes the diffusion coefficient D of the molecules, while the fitting parameter  is related to the ratio of coherent signal to incoherent noise and is the correlator time delay. Hydrodynamic radius is calculated from the Stokes–Einstein equation: D=
KT , 60 RH
(2)
where K is the Boltzmann constant, T is the temperature, 0 is the solvent viscosity and RH hydrodynamic radius. By viscometer 0 is (0.30 ± 0.01) mPa·s, and it is possible to calculate the hydrodynamic radius RH of PANI particles dispersed in ACN, equal to (314.8 ± 3.2) nm. Suspension is monitored for two hours showing a net stability. The suspension is further investigated for calculation of intrinsic viscosity [] providing information about fundamental properties of the solute and interaction with solvent, indeed it can be precisely related to the conformation of particles in solution. The intrinsic viscosity, [] of polymer solution is determined by Solomon–Ciuta equation (Eq. (3) where r is the relative viscosity, r = 0 , is viscosity of PANI/ACN suspension, measured by viscometer, equal to (0.31 ± 0.01) mPa·s, sp is the specific viscosity, sp = r − 1, and C is the suspension concentration. By considering only the measurement for a single concentration, giving rise to the so-called single-point procedures that avoid multiple measurements and extrapolation, the intrinsic viscosity is [27]: [] =
[2(sp − ln r )]1/2 C
(3)
resulting [] = (2.59 ± 0.20) ml/mg. From the value of intrinsic viscosity, the polymer molecular weight, M, is calculated according to the Einstein viscosity relation (Eq. (4)), C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
modeling the hydrated polymer molecules in terms of equivalent hydrodynamic spheres of volume Ve and radius RH [28]: [] =
2.5NVe , M
(4)
where N is Avogadro’s number and M is (75807 ± 8300) kDa. Value of intrinsic viscosity is related to the molecular weight of the polymer in a solvent by Mark–Houwink–Sakurada relation [29]: [] = KM␣ ,
(5)
where K and ␣ are related on polymer/solvent interactions, resulting ␣ ࣈ 0.44. That parameter contains the information about the shape of the molecules indicating that commercial PANI in ACN is such as a flexible polymer chain in “ideal solvent” which is also called “theta solvent”, where the free energies of solvent-solvent interactions, solvent–polymer interactions, and polymer–polymer interactions are all the same (␣ generally varies between 0.5 for a “theta solvent” and 0.8 for a “good solvent”). Polymer chains twist each other and the average shape of coiled molecule can be approximated to a sphere. That approximation is already stated for PANI particle dispersed in ACN [16]. 2.2.2 AC electric field application and related mechanisms When AC electric field is applied to PANI/ACN suspension dropped onto electrodes, different mechanisms can be induced. The phenomenon is complex and further investigation might be necessary. Preliminary, we can suppose that when low frequencies and large electrode gaps are used, there is a combination of three forces [17–23]. The first is the DEP force www.electrophoresis-journal.com
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which acts directly on particles dispersed in medium, polarizing and driving them towards highest electric field in short range manipulation. Local particle aggregates of larger diameter are also formed. The second force is ICEK around particle and/or particle aggregates and manipulates them via hydrodynamic drag. The third force, called AC electroosmotic flow (ACEO), generated at the electrodes and related fluid motion, occurs when AC field is applied in media with low electrical conductivity (⬍100 S/cm) and frequency (⬍10 kHz) [17,18], as in our working conditions. ACEO transports the particle aggregates near the electrode surface by acting on large range motion. In order to better understand the assembly mechanism, movies have been recorded during the electric field application. In the Supporting Information a typical video is uploaded where the device has 350 m electrode finger width and 60 m gap. When 3 kHz frequency is applied, assembly time is fast, limited to about 5 s and fluid motion is clearly visible. That motion speeds up strongly particle towards gap where the strongest electric field gradient is effective. As a result, PANI particles are finally captured by forming nanostructured continuous PANI assembly across gap and process stops because opposite electrodes are short-circuited [30]. The aggregation of particles is irreversible and assembled structures remain stable after voltage is turned off. Fluid motion detected by camera is related to ACEO flow which is maximum at defined frequency [17]. Calculation of maximum ACEO frequency, considering the time averaged electroosmotic velocity, v, conducts to the experimental frequency values which in this work have allowed the assembly of PANI. When a potential is applied to the electrode, the field attracts charges on the electrode surface, forming an induced electric double layer (EDL) [22]. In the case of ACEO, the induced EDL interacts with the tangential component of the electric field to induce bulk fluid motion. It has been shown that the time averaged electroosmotic velocity v can be estimated to be [17]: 1 εm V02 ⍀2 v = , 8 z(1 + ⍀2 )2
(6)
where V0 is the amplitude of the AC electric field, is the viscosity of the electrolyte, ε m is ACN permittivity and z is the distance from the center of the electrode gap for a parallel electrode. The nondimensional frequency ⍀ is given by: εm k, (7) ⍀ = z m 2 where k is the reciprocal Debye length, m is the ACN conductivity and is the angular frequency ( = 2 f and f is the frequency). Equation (7) gives a velocity profile which tends to zero at low and high frequency limits with a maximum velocity at ⍀ = 1. In this case, the frequency of maximum ACEO is: m D 1 . (8) f = εm 2 z Considering ACN Debye length, D , about 360 nm [31], and gap size of 30 or 60 m, the frequency which permits
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fast assembly is about 5 and 3 kHz respectively. Such values match with our experimental frequencies used to assembly PANI particles confirming that this frequency-dependent effect is strongly correlated with the ACEO flow.
3 Results and discussion 3.1 Device morphology Devices A and B, with 30 and 60 m gap sizes, and same electrode finger width (350 m), are shown at different magnification in Figs. 2 and 3. Nanostructured PANI network is formed along the gap with some connections which could be due to contact forces (like van der Waals) and/or welding due to current between adjacent nanoparticles. Morphology of device A (Fig. 2) appears like a narrow-mesh interconnected network while device B (Fig. 3) shows loose-mesh network. The morphology difference of two samples is due to gap size: when gap increases DEP force decreases and consequently the ACEO velocity too. PANI nanostructures, being slower, arrive and fix them to the border of the electrodes creating point of high electric field gradient for the other nanoparticles in solution, having the time to reach the tip of nucleation point and finally contacting the opposite electrode, forming a loose mesh network, such as device B.
3.2 Device characterization as ammonia sensor Devices are subsequently inserted in the gas test chamber under exposure of 5 ppm ammonia gas in nitrogen carrier, where electrode pads are biased at 1 V and electrical current is recorded. The relative response (S) of the NH3 sensors can be defined as: S=
I − I0 ⌬I = × 100, I0 I0
(9)
where I and I0 are the current of the device in NH3 and in nitrogen, respectively. Graphs, reported in Figs. 4A and 4B, show the sensitivity of ammonia decreases in response when the mesh network changes from narrow to loose. The sensors are cycled two times, introducing and removing ammonia every 10 min. When exposed to ammonia, deprotonation occurs and the conductivity changes from the conducting emeraldine salt form to the insulating emeraldine base form, leading to a decreasing in its conductivity. The desorption of ammonia, however, provides protonation of PANI by giving an increasing of conductivity. When PANI is interacting with ammonia, the following reversible reaction occurs: PANIH+ + NH3 ↔ PANI + NH+ 4,
(10)
+
where the H of PANI on surface reduces as PANI interacts with NH3 . The conductivity depends on both the ability to transport charge carriers along the polymer backbone and for carrier hopping between polymer chains. Any interaction
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Figure 5. Relative response of device A under 5 ppm NH3 in nitrogen carrier after storage in air. Device is cycled four times. After first cycle, relative response is about 75%. It is evident a slight decreasing of relative response compared with that of figure 4A but the signal is always highly reversible.
stable and smaller response with respect to device A. The aging of device A is also monitored with four cycles of ammonia exposure after one week storage in air (Fig. 5). Intensity of the relative response decreases, but reversibility is again evident during ammonia cycles exposure.
4 Concluding remarks Figure 4. Relative response of devices A and B under two cycles of 5 ppm NH3 in nitrogen carrier are shown. (A) Device A shows a relative response of 100%, after the first cycle in ammonia. The conductivity is highly stable and also reversible. (B) For device B the response is 30%. The conductivity shows a drift. Graphs show as the sensitivity to ammonia decreases in relative response and is not more reversible when the network of PANI changes from narrow-mesh (device A) to loose-mesh (device B).
with PANI, that alters either of these processes, will affect the conductivity. The chain disorder and deformation will reduce the degree of delocalization of electronic energy levels of PANI chain electronic states and change the band structures resulting in a decrease of conductivity [5]. Device A keeps relative response of about 100% (Fig. 4A), while for device B, the relative response is about 30% (Fig. 4B). Moreover, device A shows signal reversibility after the first cycle of ammonia exposure, gaining the conductivity signal upon the evacuation of ammonia from the test chamber. The morphology of the devices can explain reversibility and related relative response. The narrow-mesh of device A is a structure with great surface area and porosity, with a numerous active sites, allowing for easy diffusion of ammonia into and out of the network. Morphology of device B appearing loose-like mesh has clearly a small surface area, less recombination sites, resulting in not
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Easy, scalable and rapid assembly based on AC electric field application is described for realizing nanostructured PANI through microsized gap directly onto templates for sensor device application. PANI is assembled as a continuous network across the gap when defined frequency is applied, while partial PANI accumulation onto electrode fingers may only be observed at higher and lower frequencies respect to defined frequency. Frequency-dependent effect is strongly correlated with the ACEO flow velocity which, together DEP force and ICEK flow, conducts to fast PANI particles assembly although gap is microsized. Such assumption is verified both by calculating the frequency of maximum ACEO which corresponds to experimental one and by recording movie during the assembly. Devices are characterized, at room temperature, as ammonia sensor at 5 ppm concentration in nitrogen carrier reporting multiple cycles of exposure. In particular, devices based on narrow mesh PANI network shape, even after storage in air, show highly sensitive detection as well as excellent reversibility. These results are promising in sensor devices and AC electric field application is an advantageous technique for fast manufacturing conducting polymer in a wide range of micro-scaled devices. The authors declare no conflict of interest.
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Electrophoresis 2015, 36, 1459–1465
Vera La Ferrara Gabriella Rametta Antonella De Maria ENEA – Portici Research Center – Laboratory of Materials and Devices, Portici, Italy
Received July 25, 2014 Revised April 3, 2015 Accepted April 8, 2015
Research Article
AC electric field for rapid assembly of nanostructured polyaniline onto microsized gap for sensor devices Interconnected network of nanostructured polyaniline (PANI) is giving strong potential for enhancing device performances than bulk PANI counterparts. For nanostructured device processing, the main challenge is to get prototypes on large area by requiring precision, low cost and high rate assembly. Among processes meeting these requests, the alternate current electric fields are often used for nanostructure assembling. For the first time, we show the assembly of nanostructured PANI onto large electrode gaps (30–60 m width) by applying alternate current electric fields, at low frequencies, to PANI particles dispersed in acetonitrile (ACN). An important advantage is the short assembly time, limited to 5–10 s, although electrode gaps are microsized. That encouraging result is due to a combination of forces, such as dielectrophoresis (DEP), induced-charge electrokinetic (ICEK) flow and alternate current electroosmotic (ACEO) flow, which speed up the assembly process when low frequencies and large electrode gaps are used. The main achievement of the present study is the development of ammonia sensors created by direct assembling of nanostructured PANI onto electrodes. Sensors exhibit high sensitivity to low gas concentrations as well as excellent reversibility at room temperature, even after storage in air. Keywords: AC electric field / Ammonia sensor / Nanostructured polyaniline / Rapid assembly DOI 10.1002/elps.201400550
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction Polyaniline (PANI) plays a key role as optical, chemical and bio sensors because of its simple and reversible doping– dedoping chemistry. Indeed PANI bulk is often used for ammonia sensor at room temperature [1–4]. Ammonia is used in different industrial processes and its recognition is important because of related toxicity. When PANI is in presence of ammonia, the amine groups, in emeraldine salt form (conducting state), are deprotonated converting it to the emeraldine base form (insulating state) with corresponding drop in conductivity [5, 6]. In recent years, nanostructured PANI films have emerged as good ammonia sensing material compared to bulk counterparts, and showed better performance
Correspondence: Vera La Ferrara, ENEA, Laboratory of Materials and Devices, Portici, Italy E-mail: [email protected]
Abbreviations: ACEO, alternate current electroosmosis; ACN, acetonitrile; DLS, dynamic light scattering; EDL, electric double layer; FIB, focused ion beam; ICEK, induced-charge electrokinetic; PANI, polyaniline C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
due to high specific area. In the literature, different manufacturing procedures are described for sensing devices based on nanostructured PANI, like solid state, seeding polymerization and electrospinning [7–13]. There are some difficulties in those techniques, mainly for large area processing, such as no controlled assembly, time consuming and expensive process. Those disadvantages can be overcome if nanostructured PANI is assembled by applying AC or DC fields on particles suspended in electrolyte and dropped onto electrodes. However, in the literature, nanostructured PANI assembly is realized by electric field application through nanosized gaps but working device is never shown [14, 15]. The only working device, to our knowledge, has been realized by covering 2 m gap interdigitated electrodes in two minutes by applying DC electric field to PANI particles [16]. In our work, indeed, we show, for the first time, how to assemble quickly nanostructured PANI onto large electrode gaps (30– 60 m width) by applying AC electric field, at low frequencies, to PANI particles dispersed in acetonitrile (ACN). The main goals are about reducing assembly time and covering
Colour Online: See the article online to view Fig. 1 in colour.
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simultaneously large area gaps. It should be expected that the wider the gap, the greater the assembly time, but, although electrode gaps are microsized, the assembly time is fast, limited to 5–10 seconds, when appropriate frequency is applied. Performance-enhancing devices have been realized by applying different sinusoidal voltages and frequencies on PANI particles dispersed in ACN solvent dropped onto large electrode gaps. In such conditions, different induced forces, which act on motion of particles and allow rapid particle assembly, are generated. Those forces are dielectrophoresis (DEP) force, which polarizes particles, induced-charge electrokinetic (ICEK) flow, around the particles, and alternate current electroosmosis (ACEO) flow, which is generated at the electrodes [17–23]. Finally, those devices based on nanostructured PANI network, narrow or loose mesh shapes, are tested as ammonia sensors at room temperature. More especially, narrow mesh shape based devices show high relative response (100%) and reversible recovery at 5 ppm of ammonia gas as well as good reversibility in the conductivity signal even after storage in air.
2 Materials and methods 2.1 Experimental Polyaniline (PANI, emeraldine salt 2–3 wt% dispersion in xylene) and ACN (anhydrous, 99.8%), are purchased from Sigma–Aldrich. PANI, as reported from Aldrich data sheet, has conductivity of 10 S/cm, dielectric constant of 20 ε 0 (where ε 0 is the vacuum dielectric constant) and pristine
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particle radius about 200 nm. ACN has electrical conductivity of 7 S/cm and dielectric constant of 37.5 ε 0 . PANI is dispersed in ACN with concentration in the range from 32 g/ml to 8 g/ml. The suspensions are stirred at room temperature for 30 min. Each suspension is characterized by dynamic light scattering (DLS), (Zetasizer Nano ZS Malvern Instruments) using 12 L quartz cuvette. That apparatus employs a 4 mW He–Ne laser, wavelength 632.8 nm and the measurement angle of 173° with a noninvasive back scatter technology to calculate the apparent size (called hydrodynamic radius, RH ) of polymer in solvent. Resulting data are weighted on three measurements executed at two minutes intervals. Dynamic viscosities of PANI suspensions, , and ACN solvent, 0 , are measured by a viscometer (SV-10 Vibroviscometer) enabling the calculation of intrinsic viscosity, []. These parameters, together polymer molecular weight, M, allow for estimating of pristine conformation of commercial PANI dispersed in ACN, before applying electric field. Additionally, PANI suspension is dropped onto typical substrate (as shown in inset of Fig. 1A from optical microscope image) to evaluate pristine PANI morphology when no AC electric field is applied. As a result, PANI aggregates are randomly dispersed as shown in Fig. 1A. Devices based on nanostructured PANI network are realized by starting from silicon substrates (p-type, orientation ⬍100⬎, resistivity 10 ⍀cm, Siltronix, France). After standard cleaning, 240 nm thick SiO2 layer and two Cr/Au electrodes (20 nm/180 nm thick) are deposited by e-beam assisted evaporation onto 1 cm2 silicon. Electrode geometry consists of two millimeter pads and two micrometer fingers as shown in Fig. 1B. Different electrodes are realized by fixing area pads while width of
Figure 1. (A) In inset, optical image shows top view of PANI aggregates randomly dispersed when no electric field is applied. PANI/ACN suspension is dropped onto Cr/Au electrodes (140 m gap and 100 m electrode finger width) patterned on typical substrate SiO2 /Si. As shown in the inset, a region has been marked and analyzed by FIB to magnify random aggregates. (B) Schematic diagram view (not to scale) of experimental setup used for PANI assembly through electrode gap when AC electric field is applied. Effective electrode sizes are displayed on scheme.
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Figure 2. FIB images of device A, with 30 m electrode gap and 350 m electrode finger, are reported. On the left, image shows nanostructured PANI network across gap after AC electric field application (10 Vpp voltage and 5 kHz frequency). Different magnifications, on the right, have been acquired to zoom in on PANI network by showing a morphology like narrowmesh interconnected network.
the electrode fingers or gap size (ranging from 30 to 500 m both electrode fingers and gaps) are changed to find favorable conditions for rapid nanostructured PANI assembly. Few microliters of PANI/ACN suspension are deposited by casting onto substrates between the electrode gaps. Electrode pads are then connected to microprobes of a function generator (AGILENT 33220A) and substrates are placed under an optical microscope connected to a camcorder to control assembling process. Sinusoidal voltage (Vpeak-peak(pp) = 1–10 V) and frequency in the range of 3 mHz–3 MHz are used to achieve the best working conditions. Schematic view of the patterned substrate and electrical circuit diagram are shown in Fig. 1B. Several samples are made by using different diluted suspensions but only 16 g/ml concentration leads to good results. Indeed high concentrations have an excessive number of PANI particles which does not permit controlled manipulation while low concentrations have few particles not sufficient for assembling when wide gap is used. In order to achieve working devices, we have optimized parameters for 16 g/ml concentration by changing sinusoidal voltage, frequency, electrode finger width and gap size. No particle movement is detected below 5 Vpp at all frequencies for all the electrode geometry, while in the range 5–10 Vpp PANI partial accumulation occurs but continuous PANI assembly is evident only at a defined frequency. We have obtained rapid and continuous assembly for sensor devices by starting with 350 m electrode finger width and 30 or 60 m gap size. Working parameters for those geometries are Vpp = 10 V and frequency, f, 5 or 3 kHz depending on gap sizes. In Figs. 2 and 3 long well-arranged PANI nanostructures onto 30 and 60 m gap are shown (named devices A and B, respectively) at different magnifications. Devices based on those networks of nanostructured PANI are characterized by a focused
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ion beam (FIB) (FEI, QUANTA 2003D), where a focused gallium ion beam (30 keV energy), striking to the surface, gives high contrast images for inspection of morphologies. Devices are therefore characterized as ammonia sensors in a test chamber. A volt-amperometric technique, at constant bias, is then employed for sensor electrical characterization in a controlled gas-flow environment, pre-mixed with dry carrier in the desired percentage by mass flow meters and continuously controlled by means of an on-line Fourier transform infrared spectrometer [24]. All the tested devices are biased at 1 V under 5 ppm of NH3 and nitrogen as carrier. Total gas flow is set at 500 sccm.
2.2 Theory 2.2.1 Hydrodynamic radius, viscosity and polymer conformation DLS tecnique measures the intensity (I) of light scattered from particles, when laser light passes through the suspension. It allows evaluating apparent particle size in suspension calculated from the measured Brownian motion (diffusion coefficient) of the polymeric particles. That size is approximated to hydrodynamic radius, RH , hypothesized to be the radius of a sphere having the same diffusion behavior of polymer surrounded by solvent molecules. The measured data are the correlation curve which is fitted by an exponential function (Eq. (1) calculating the diffusion coefficient (D) which is proportional to the lifetime of the exponential decay [25, 26]. The correlation function, G ( ) , is: G ( ) = I + exp(−Dq 2 ),
(1)
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Figure 3. FIB images of device B, with 60 m electrode gap and 350 m electrode finger, are reported. On the left, image shows nanostructured PANI network across gap after AC electric field application (10 Vpp voltage and 3 kHz frequency). Different magnifications, on the right, have been acquired to zoom in on PANI network by showing a morphology like loose-mesh network.
where the decay rate Dq2 includes the diffusion coefficient D of the molecules, while the fitting parameter  is related to the ratio of coherent signal to incoherent noise and is the correlator time delay. Hydrodynamic radius is calculated from the Stokes–Einstein equation: D=
KT , 60 RH
(2)
where K is the Boltzmann constant, T is the temperature, 0 is the solvent viscosity and RH hydrodynamic radius. By viscometer 0 is (0.30 ± 0.01) mPa·s, and it is possible to calculate the hydrodynamic radius RH of PANI particles dispersed in ACN, equal to (314.8 ± 3.2) nm. Suspension is monitored for two hours showing a net stability. The suspension is further investigated for calculation of intrinsic viscosity [] providing information about fundamental properties of the solute and interaction with solvent, indeed it can be precisely related to the conformation of particles in solution. The intrinsic viscosity, [] of polymer solution is determined by Solomon–Ciuta equation (Eq. (3) where r is the relative viscosity, r = 0 , is viscosity of PANI/ACN suspension, measured by viscometer, equal to (0.31 ± 0.01) mPa·s, sp is the specific viscosity, sp = r − 1, and C is the suspension concentration. By considering only the measurement for a single concentration, giving rise to the so-called single-point procedures that avoid multiple measurements and extrapolation, the intrinsic viscosity is [27]: [] =
[2(sp − ln r )]1/2 C
(3)
resulting [] = (2.59 ± 0.20) ml/mg. From the value of intrinsic viscosity, the polymer molecular weight, M, is calculated according to the Einstein viscosity relation (Eq. (4)), C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
modeling the hydrated polymer molecules in terms of equivalent hydrodynamic spheres of volume Ve and radius RH [28]: [] =
2.5NVe , M
(4)
where N is Avogadro’s number and M is (75807 ± 8300) kDa. Value of intrinsic viscosity is related to the molecular weight of the polymer in a solvent by Mark–Houwink–Sakurada relation [29]: [] = KM␣ ,
(5)
where K and ␣ are related on polymer/solvent interactions, resulting ␣ ࣈ 0.44. That parameter contains the information about the shape of the molecules indicating that commercial PANI in ACN is such as a flexible polymer chain in “ideal solvent” which is also called “theta solvent”, where the free energies of solvent-solvent interactions, solvent–polymer interactions, and polymer–polymer interactions are all the same (␣ generally varies between 0.5 for a “theta solvent” and 0.8 for a “good solvent”). Polymer chains twist each other and the average shape of coiled molecule can be approximated to a sphere. That approximation is already stated for PANI particle dispersed in ACN [16]. 2.2.2 AC electric field application and related mechanisms When AC electric field is applied to PANI/ACN suspension dropped onto electrodes, different mechanisms can be induced. The phenomenon is complex and further investigation might be necessary. Preliminary, we can suppose that when low frequencies and large electrode gaps are used, there is a combination of three forces [17–23]. The first is the DEP force www.electrophoresis-journal.com
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which acts directly on particles dispersed in medium, polarizing and driving them towards highest electric field in short range manipulation. Local particle aggregates of larger diameter are also formed. The second force is ICEK around particle and/or particle aggregates and manipulates them via hydrodynamic drag. The third force, called AC electroosmotic flow (ACEO), generated at the electrodes and related fluid motion, occurs when AC field is applied in media with low electrical conductivity (⬍100 S/cm) and frequency (⬍10 kHz) [17,18], as in our working conditions. ACEO transports the particle aggregates near the electrode surface by acting on large range motion. In order to better understand the assembly mechanism, movies have been recorded during the electric field application. In the Supporting Information a typical video is uploaded where the device has 350 m electrode finger width and 60 m gap. When 3 kHz frequency is applied, assembly time is fast, limited to about 5 s and fluid motion is clearly visible. That motion speeds up strongly particle towards gap where the strongest electric field gradient is effective. As a result, PANI particles are finally captured by forming nanostructured continuous PANI assembly across gap and process stops because opposite electrodes are short-circuited [30]. The aggregation of particles is irreversible and assembled structures remain stable after voltage is turned off. Fluid motion detected by camera is related to ACEO flow which is maximum at defined frequency [17]. Calculation of maximum ACEO frequency, considering the time averaged electroosmotic velocity, v, conducts to the experimental frequency values which in this work have allowed the assembly of PANI. When a potential is applied to the electrode, the field attracts charges on the electrode surface, forming an induced electric double layer (EDL) [22]. In the case of ACEO, the induced EDL interacts with the tangential component of the electric field to induce bulk fluid motion. It has been shown that the time averaged electroosmotic velocity v can be estimated to be [17]: 1 εm V02 ⍀2 v = , 8 z(1 + ⍀2 )2
(6)
where V0 is the amplitude of the AC electric field, is the viscosity of the electrolyte, ε m is ACN permittivity and z is the distance from the center of the electrode gap for a parallel electrode. The nondimensional frequency ⍀ is given by: εm k, (7) ⍀ = z m 2 where k is the reciprocal Debye length, m is the ACN conductivity and is the angular frequency ( = 2 f and f is the frequency). Equation (7) gives a velocity profile which tends to zero at low and high frequency limits with a maximum velocity at ⍀ = 1. In this case, the frequency of maximum ACEO is: m D 1 . (8) f = εm 2 z Considering ACN Debye length, D , about 360 nm [31], and gap size of 30 or 60 m, the frequency which permits
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fast assembly is about 5 and 3 kHz respectively. Such values match with our experimental frequencies used to assembly PANI particles confirming that this frequency-dependent effect is strongly correlated with the ACEO flow.
3 Results and discussion 3.1 Device morphology Devices A and B, with 30 and 60 m gap sizes, and same electrode finger width (350 m), are shown at different magnification in Figs. 2 and 3. Nanostructured PANI network is formed along the gap with some connections which could be due to contact forces (like van der Waals) and/or welding due to current between adjacent nanoparticles. Morphology of device A (Fig. 2) appears like a narrow-mesh interconnected network while device B (Fig. 3) shows loose-mesh network. The morphology difference of two samples is due to gap size: when gap increases DEP force decreases and consequently the ACEO velocity too. PANI nanostructures, being slower, arrive and fix them to the border of the electrodes creating point of high electric field gradient for the other nanoparticles in solution, having the time to reach the tip of nucleation point and finally contacting the opposite electrode, forming a loose mesh network, such as device B.
3.2 Device characterization as ammonia sensor Devices are subsequently inserted in the gas test chamber under exposure of 5 ppm ammonia gas in nitrogen carrier, where electrode pads are biased at 1 V and electrical current is recorded. The relative response (S) of the NH3 sensors can be defined as: S=
I − I0 ⌬I = × 100, I0 I0
(9)
where I and I0 are the current of the device in NH3 and in nitrogen, respectively. Graphs, reported in Figs. 4A and 4B, show the sensitivity of ammonia decreases in response when the mesh network changes from narrow to loose. The sensors are cycled two times, introducing and removing ammonia every 10 min. When exposed to ammonia, deprotonation occurs and the conductivity changes from the conducting emeraldine salt form to the insulating emeraldine base form, leading to a decreasing in its conductivity. The desorption of ammonia, however, provides protonation of PANI by giving an increasing of conductivity. When PANI is interacting with ammonia, the following reversible reaction occurs: PANIH+ + NH3 ↔ PANI + NH+ 4,
(10)
+
where the H of PANI on surface reduces as PANI interacts with NH3 . The conductivity depends on both the ability to transport charge carriers along the polymer backbone and for carrier hopping between polymer chains. Any interaction
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Figure 5. Relative response of device A under 5 ppm NH3 in nitrogen carrier after storage in air. Device is cycled four times. After first cycle, relative response is about 75%. It is evident a slight decreasing of relative response compared with that of figure 4A but the signal is always highly reversible.
stable and smaller response with respect to device A. The aging of device A is also monitored with four cycles of ammonia exposure after one week storage in air (Fig. 5). Intensity of the relative response decreases, but reversibility is again evident during ammonia cycles exposure.
4 Concluding remarks Figure 4. Relative response of devices A and B under two cycles of 5 ppm NH3 in nitrogen carrier are shown. (A) Device A shows a relative response of 100%, after the first cycle in ammonia. The conductivity is highly stable and also reversible. (B) For device B the response is 30%. The conductivity shows a drift. Graphs show as the sensitivity to ammonia decreases in relative response and is not more reversible when the network of PANI changes from narrow-mesh (device A) to loose-mesh (device B).
with PANI, that alters either of these processes, will affect the conductivity. The chain disorder and deformation will reduce the degree of delocalization of electronic energy levels of PANI chain electronic states and change the band structures resulting in a decrease of conductivity [5]. Device A keeps relative response of about 100% (Fig. 4A), while for device B, the relative response is about 30% (Fig. 4B). Moreover, device A shows signal reversibility after the first cycle of ammonia exposure, gaining the conductivity signal upon the evacuation of ammonia from the test chamber. The morphology of the devices can explain reversibility and related relative response. The narrow-mesh of device A is a structure with great surface area and porosity, with a numerous active sites, allowing for easy diffusion of ammonia into and out of the network. Morphology of device B appearing loose-like mesh has clearly a small surface area, less recombination sites, resulting in not
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Easy, scalable and rapid assembly based on AC electric field application is described for realizing nanostructured PANI through microsized gap directly onto templates for sensor device application. PANI is assembled as a continuous network across the gap when defined frequency is applied, while partial PANI accumulation onto electrode fingers may only be observed at higher and lower frequencies respect to defined frequency. Frequency-dependent effect is strongly correlated with the ACEO flow velocity which, together DEP force and ICEK flow, conducts to fast PANI particles assembly although gap is microsized. Such assumption is verified both by calculating the frequency of maximum ACEO which corresponds to experimental one and by recording movie during the assembly. Devices are characterized, at room temperature, as ammonia sensor at 5 ppm concentration in nitrogen carrier reporting multiple cycles of exposure. In particular, devices based on narrow mesh PANI network shape, even after storage in air, show highly sensitive detection as well as excellent reversibility. These results are promising in sensor devices and AC electric field application is an advantageous technique for fast manufacturing conducting polymer in a wide range of micro-scaled devices. The authors declare no conflict of interest.
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