Anal. Chem. 2006, 78, 7048-7053
Electrochemical Characterization of Recessed Nanodisk-Array Electrodes Prepared from Track-Etched Membranes Takashi Ito,* Ahmad A. Audi, and Gregory P. Dible
Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, Kansas 66506
Recessed nanodisk-array electrodes (RNEs) fabricated from track-etched polycarbonate membranes (TEPCMs) having cylindrical nanopores (50 nm in diameter) were characterized using cyclic voltammetry (CV). Such electrodes were prepared by depositing a gold thin film onto a TEPCM via sputtering or thermal evaporation. CV of the RNEs showed the transition from linear to radial diffusion modes of redox-active molecules with decreasing scan rate. The resulting change in maximum faradic current, which is the peak current in a peak-shaped CV and the plateau current in a sigmoidal CV, provides a simple means for calculating the pore length and effective pore density within a RNE. This method permits us to assess the completeness of the seal between a TEPCM and gold film as well as the extent to which air bubbles block the nanopores. A recessed nanodisk-array electrode (RNE) consists of an array of recessed disk electrodes having nanometer-scale diameters (Scheme 1). The recessed disks are formed from an insulatorbased membrane that contains cylindrical nanometer-scale pores aligned parallel to each other, and an electrode blocks one end of the pores. RNEs have been used to synthesize arrays of metal, semiconductor, or conducting polymer nanowires via electrodeposition.1-7 In addition, the recessed electrode structure was used to study lateral diffusion of electroactive amphiphiles in bilayer assemblies8-10 and to fabricate chemical sensors that are * To whom correspondence should be addressed. Phone: 785-532-1451. Fax: 785-532-6666. E-mail:
[email protected]. (1) Martin, C. R. Science 1994, 266, 1961-1966. (2) Scho ¨nenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kru ¨ ger, M.; Bachtold, A.; Huber, R.; Birk, H.; Staufer, U. J. Phys. Chem. B 1997, 101, 5497-5505. (3) Demoustier-Champagne, S.; Stavaux, P.-Y. Chem. Mater. 1999, 11, 829834. (4) Nicewarner-Pen ˜a, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pen ˜a, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137-141. (5) Schuchert, I. U.; Toimil Molares, M. E.; Dobrev, D.; Vetter, J.; Neumann, R.; Martin, M. J. Electrochem. Soc. 2003, 150, C189-C194. (6) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129. (7) Cho, S. I.; Choi, D. H.; Kim, S.-H.; Lee, S. B. Chem. Mater. 2005, 17, 45644566. (8) Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1986, 108, 3118-3120. (9) Miller, C. J.; Widrig, C. A.; Charych, D. H.; Majda, M. J. Phys. Chem. 1988, 92, 1928-1936. (10) Goss, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988, 92, 1937-1942.
7048 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006
Scheme 1
insensitive to convection.11-13 So far, RNEs have been prepared by depositing metal onto one face of a track-etched polycarbonate membrane (TEPCM)2,3,5,7 or onto an anodic alumina membrane,4,14,15 or by using diblock copolymer films on conducting substrates.6 For all applications of RNEs, the formation of a good seal between a nanoporous membrane and metal film and the complete removal of air bubbles from the nanopores are required to obtain reproducible results. A bad seal (Scheme 1a) leads to creeping of the electrolyte solution between the membrane and the metal film,15 which forms a gap whose size often changes during the measurements. The gap size also varies between RNEs. Since air bubbles remaining within nanopores are gradually released, incomplete removal of air bubbles (Scheme 1b) results in a gradual increase in effective pore density during experiments as well as variation in effective pore density between RNEs. Electrochemical methods provide a convenient means to characterize recessed electrode structures. For example, capacitive currents measured with cyclic voltammetry (CV) could be used to qualitatively discuss the seal of an insulating membrane contain(11) (12) (13) (14) (15)
Morita, K.; Shimizu, Y. Anal. Chem. 1989, 61, 159-162. Shimizu, Y.; Morita, K. Anal. Chem. 1990, 62, 1498-1501. Vlassiouk, I.; Takmakov, P.; Smirnov, S. Langmuir 2005, 21, 4776-4778. Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1985, 107, 1419-1420. Brumlik, C. J.; Martin, C. R.; Tokuda, K. Anal. Chem. 1992, 64, 12011203.
10.1021/ac061043m CCC: $33.50
© 2006 American Chemical Society Published on Web 08/23/2006
ing micrometer- to submicrometer-scale pores to a metal film.15-17 In addition, changes in faradic current at different scan rates as a result of transition of diffusion modes were reported on a single recessed microdisk electrode fabricated by photolithography,17 on arrays of recessed micro- and submicrodisk electrodes,15,18 and on a single truncated cone-shaped nanopore electrode.19,20 In this paper, RNEs having nanometer-scale pore diameters were characterized using electrochemical methods. TEPCMs ∼10 µm thick and containing an array of cylindrical pores of 50-nm diameter were used as nanoporous membranes. One face of each membrane was coated with a gold thin film to construct the RNE structure. Sputtering and thermal evaporation were used for coating the TEPCMs. It is known that sputtering5,7 provides a good seal between the TEPCM and the gold film, whereas a gold layer coated via thermal evaporation3 often peels away from the polymer membrane. This study demonstrates that a series of CV measurements at different scan rates can be used to determine the pore length and effective pore density in a TEPCM-based RNE. These methods are also shown to provide valuable data on the quality of the seal between a TEPCM and gold film, as well as the extent to which air bubbles block the RNE nanopores. EXPERIMENTAL SECTION Chemicals and Materials. All solutions were prepared with water having a resistivity of 18 MΩ cm or higher (Barnstead Nanopure Systems). Potassium nitrate (Fisher Chemical), potassium ferricyanide (K3Fe(CN)6) (Acros Organics), and 1,1′-ferrocenedimethanol (Fc(CH2OH)2; Aldrich Chemical) were of reagent grade quality or better and used without further purification. Track-etch polycarbonate membranes (50-nm pore diameter, 6 × 108 pores/cm2, ∼10 µm thick, 25 mm in membrane diameter) and gold wire (99.99%) were obtained commercially (Whatman and Surepure Chemicals, respectively). The pore density of the TEPCMs was measured using contact-mode atomic force microscopy in air (AFM; Digital Instruments Multimode AFM) to be 6.6 × 108 pores/cm2, which was within its nominal precision ((15%) of the manufacturer’s specifications. Electrode Fabrication. RNEs were prepared by coating the rougher face of a TEPCM with gold via thermal evaporation (model DV-502, Denton Vacuum) or sputtering (EFFA Coater, Ernest F. Fullam). The thickness of the gold layer was >100 nm, as was confirmed using a surface profiler. The gold-coated TEPCM was mounted on a glass slide (1 × 1 cm2) that had been previously spin-coated with PDMS (Sylgard 184, Dow Corning). Electrical contact to the gold surface was made using conducting copper tape (Electron Microscopy Sciences). The gold-coated TEPCMs were used as the working electrode. Electrochemical Measurements. Electrochemical data were obtained using a CH Instruments model 618B electrochemical analyzer. Electrochemical experiments were carried out in a threeelectrode cell (Scheme 2) containing a Ag/AgCl (3 M KCl) reference electrode and a Pt counter electrode. The RNE (serving as the working electrode) was immobilized at the bottom of the cell using an O-ring. The diameter of the membrane area in
Scheme 2
contact with the solution was 6.6 mm, as defined by the inner diameter of the O-ring. Prior to electrochemical measurements, 0.1 M KNO3 was added to the chamber, and air bubbles within the nanopores were removed under reduced pressure.21,22 CV measurements were performed at sweep rates ranging from 2 mV/s to 10 V/s, but the CVs obtained appeared irreversible at scan rates higher than 5 V/s. CVs obtained at the second potential cycle are discussed. The maximum faradic current, which is the peak current in a peak-shaped CV and the plateau current in a sigmoidal CV, was measured by subtracting the extrapolated charging current. Chronoamperometry was performed by applying a potential step from 0 to +0.5 V (vs Ag/AgCl) for Fc(CH2OH)2 and from +0.5 to 0 V (vs Ag/AgCl) for Fe(CN)63-. Data fitting was performed using the Goal Seek tool in MS-Excel. Characterization of Electrode Structure. The TEPCM of a RNE prepared via sputtering was dissolved in CH2Cl2. A horizontally oriented metal substrate was lifted through the CH2Cl2 solution, leading to collection of the remaining gold film on the metal substrate. The surface structure of the gold film was measured by tapping-mode AFM. AFM tips used (75 kHz resonant frequency) were obtained from Budget Sensors. RESULTS AND DISCUSSION Diffusion Models for a RNE at Different Scan Rates. The diffusional flux of molecules to a recessed microdisk-array electrode is known to change according to the scan rate, spacing between pores, and pore geometry.15,18 The same model can also be applied to RNEs containing 50-nm diameter pores. Scheme 3 summarizes the diffusional flux of molecules at a recessed diskarray electrode (a, pore radius (cm); L, pore length (cm); N, the number of pores involving the redox reaction) at different time scales, that is, scan rate, v (V/s), in CV. For fast scans, only redox-active molecules within nanopores diffuse linearly to the electrode surface and react (Scheme 3a), and thus, a peak-shaped voltammogram is obtained. The peak current (ip (A)) is the sum of faradic currents from individual recessed electrodes and, thus, is given by the following equation,18
xnFDv RT
ip ) 0.446nF(Nπa2)C
(1)
where n is the number of electrons (n ) 1 for the redox-active (16) (17) (18) (19) (20)
Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625-2630. Henry, C. S.; Fritsch, I. Anal. Chem. 1999, 71, 550-556. Tokuda, K.; Morita, K.; Shimizu, Y. Anal. Chem. 1989, 61, 1763-1768. Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229-6238. Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2006, 78, 477-483.
(21) Monahan, J.; Gewirth, A. A.; Nuzzo, R. G. Anal. Chem. 2001, 73, 31933197. (22) Dai, J.; Ito, T.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 1302613027.
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006
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Scheme 3
Figure 1. Tapping-mode AFM image (700 × 700 nm2) of the gold film surface exposed after the removal of the TEPCM of a TEPCMbased RNE prepared via sputtering. The line profile from the AFM image is shown below.
molecules used in this study), F is Faraday’s constant (96485 C/mol), R is the gas constant (8.31 J/Kmol), T is temperature (298 K for 25 °C), D is the diffusion coefficient of the redox-active species (7.6 × 10-6 cm2/s for Fe(CN)63- and 6.4 × 10-6 cm2/s for Fc(CH2OH)2),23,24 and C (mol/cm3) is its concentration. Importantly, the peak current is also dependent on N, the number of nanopores participating, but not on L, the length of the channels. Thus, assuming the C and D within the pores are the same as those in solution bulk and there is no creeping of the solution between the TEPCM and gold, N can be determined from the slope in the relationship between of ip and v1/2. From N, the density of open nanopores (d (pores/cm2)) can be determined from
N ) πr2d
(2)
where r is the radius of the O-ring in Scheme 2. On the other hand, if the seal between the TEPCM and gold film is not complete, a larger ip will be observed because of the larger electroactive surface area formed by the creeping of the solution between the TEPCM and gold film. At slower scan rates, the redox-active molecules will radially diffuse from outside the pores (Scheme 3b), and thus, a sigmoidal voltammogram is observed. The limiting current of such voltammograms (ilim) can be written as follows:25
ilim )
4πnFCDa2N 4L + πa
(3)
This limiting current is observed only when the spacing between pores is large;26 otherwise, the radial diffusion regimes from (23) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (24) Fan, F.-R. F. J. Phys. Chem. B 1998, 102, 9777-9782. (25) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988, 249, 1-14.
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multiple pores will overlap (Scheme 3c), again giving a peakshaped voltammogram.15 In the absence of the overlap of individual radial diffusion layers, the limiting current in the sigmoidal CV at low scan rate will be very similar, regardless of scan rate, and should be also similar to that obtained using chronoamperometry. In such a case, the limiting current ilim can be used to determine L (see eq 3) if the pore number N (or the effective pore density d) is independently determined from the CVs at faster scan rates (see eq 1). The bottom gold electrodes of RNEs used here do not have a perfect disk shape. Figure 1 shows an AFM image of the surface structure of a bottom gold film after dissolving a gold-coated TEPCM in CH2Cl2. Bumps formed by gold deposited within TEPCM nanopores were observed. The diameter of the bumps was ∼70 nm, which is a little larger than the nanopore diameter (∼50 nm), probably because of the convolution of the AFM tip. The top of the bump is not flat, and instead, there is a hole at the center of each bump, which was formed by preferential gold deposition near the pore surface.7 However, the resulting roughness of the electrode surface is expected to be smaller than the height of the bumps (24 h once the air bubbles are removed, whereas the faradic current in a CV gradually increases when air bubbles are present in the nanopores. Sigmoidal voltammograms were observed at lower scan rates, and then redox peaks were observed at g0.4 V/s. This indicates that, as discussed above, radial diffusion is dominant at slower scan rates, and the contributions of linear diffusion become more significant with increasing scan rate. On the TEPCM-based RNEs, transition from sigmoidal (Scheme 3b) to peak-shaped CVs (Scheme 3c) at very slow scan rates was not observed because of the relatively large spacing between pores: the spacing between pores is 0.4 µm on average, calculated from the pore density (6 × 108 pores/cm2; provided by the manufacturer), which is 8 times larger than the pore diameter (50 nm). This is a big contrast to recessed microdisk-array electrodes formed by anodic alumina membranes having much higher pore density (∼65% porous), which did not
(4) (5)
The break in maximum current due to the transition of the diffusion modes is, however, subtle compared with that observed using a RNE based on an anodic alumina membrane in stirred solution.15 The subtle break in maximum current can be explained by the nonuniform pore length in a TEPCM due to variation in track tilt angle.2 This difference in pore length gives a different transition scan rate at each pore according to eqs 4 and 5, resulting in the subtle break in the maximum current. The effective pore density (d) was estimated from the change in peak current at the faster scan rates by using eqs 1 and 2. The average and standard deviation of the effective pore density of three different electrodes were (6.5 ( 0.6) × 108 pores/cm2, which is close to the pore density provided by the manufacturer (6 × 108 pores/cm2; nominal precision, ( 15%) and one measured by AFM (6.6 × 108 pores/cm2). In addition, the pore length calculated from the plateau current using eq 3 was (14 ( 2) µm, which is similar to the thickness of TEPCM measured by surface profilometry (11 µm). Similar trends were also observed for different redox-active molecules. Figure 3a shows typical cyclic voltammograms for 3.0 mM Fc(CH2OH)2 using a TEPCM-based RNE prepared via gold sputtering. The transition from sigmoidal to peak-shaped CVs was similarly observed with increasing scan rate. Figure 3c (filled circles) shows the relationship between v1/2 and the maximum oxidation current. The maximum oxidation current was similar at scan rates slower than 0.2 V/s and gradually increased at faster scan rates. Again, the plateau current (7.3 µA) is close to the limiting current measured with chronoamperometry (7.5 µA). The effective pore density and the pore length , calculated using data from three different electrodes, were (7.4 ( 0.6) × 108 pores/ cm2 and (12 ( 1) µm, respectively. These values are similar to those obtained in 1 mM Fe(CN)63-. These results indicate that Analytical Chemistry, Vol. 78, No. 19, October 1, 2006
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Figure 3. (a) Cyclic voltammograms for a TEPCM-based RNE in 3.0 mM Fc(CH2OH)2 and 0.1 M KNO3 at five different scan rates (5, 20 mV/s; 0.1, 0.4, and 1 V/s). Gold was deposited via sputtering, and almost all nanopores are filled with the solution. (b) Cyclic voltammograms for a TEPCM-based RNE in 2.9 mM Fc(CH2OH)2 and 0.1 M KNO3 at the five different scan rates. Gold was deposited via thermal evaporation. (c) Relationship between the square root of scan rate and voltammetric maximum current of 3 mM Fc(CH2OH)2. Filled circles: taken on the RNE prepared via gold sputtering (data from Figure 3a, but not all CVs are shown in Figure 3a). Open circles: taken on the RNE prepared via gold thermal evaporation (data from Figure 3b, but not all CVs are shown in Figure 3b). The solid and dashed lines were obtained by least-squares fitting of the data shown as filled circles using the theoretical equations, which gave d ) 7.0 × 108 pores/cm2 and L ) 11.2 µm. In panels a and b, “+” indicates zero current.
the CV measurements at different scan rates provide a simple, reproducible means for estimating the effective pore density and length of a RNE. Effect of Blocking of RNE Nanopores. When removal of air bubbles from the RNEs was not complete (e.g., due to insufficient time under reduced pressure), the faradic currents observed were smaller than those of a RNE having open pores. Figure 2b depicts representative CVs of 1.0 mM K3Fe(CN)6 obtained under these conditions for a TEPCM-based RNE prepared via gold sputtering. The transition from sigmoidal to peak-shaped CVs was similarly observed, as was the expected increase in maximum reduction current at higher scan rates (Figure 2c (open circles)). However, the slope of the increase in peak current (at higher scan rates) and the limiting current (at slower scan rates) are smaller in the presence of air-bubbles. The smaller slope and limiting current are expected from eqs 1 and 3, respectively. For this specific RNE, the effective pore density and 7052 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006
the pore length were 3.0 × 108 pores/cm2 and 12 µm, respectively. When these values are compared among different RNEs having nanopores blocked by air bubbles, the observed effective pore density varied in relation to the length of time the sample was exposed to vacuum; however, the pore length observed was always close to the film thickness (11 µm). This result indicates that the electrochemical measurements are suitable to determine the density of open pores in a RNE in the case of a good seal between the TEPCM and gold thin film. Cyclic Voltammograms on RNEs Prepared via Thermal Gold Evaporation. Figure 3b shows cyclic voltammograms in 3.0 mM Fc(CH2OH)2 for a TEPCM-based RNE prepared via thermal gold evaporation. At all scan rates, the capacitive current is much larger than those observed at RNEs prepared via gold sputtering. This large capacitive current originates from the creeping of the solution between the TEPCM and gold thin film due to weak adhesion between them.15 Indeed, gold films deposited via thermal evaporation can be easily peeled from the TEPCMs. The faradic current is also very large (Figure 3c, open circles), especially at higher scan rates. Increased faradic currents result from redox reactions occurring in the resulting gap. At slower scan rates (e5 mV/s), the CV approaches sigmoidal shape, and the current is close to the plateau current observed with RNEs prepared via gold sputtering. This is probably because the current at slow scan rates is mainly determined by mass transport of redox-active molecules through the TEPCM nanopores, which involves radial diffusion of redox-active molecules from the solution bulk into the nanopores. This is supported by the observation that the limiting current measured with chronoamperometry (7.9 µA) on the RNE prepared via the thermal evaporation method is close to the value observed with the RNE prepared via gold sputtering (7.5 µA, vide supra). Similar results (i.e., larger peak currents in CVs and similar limiting currents in chronoamperograms, compared to the theoretical predictions) have also been reported for electrodes modified with a track-etched polyester membrane mechanically contacted to the electrode surface.27 The distance between the membrane and electrode cannot be controlled accurately in these systems. For RNEs prepared via thermal gold evaporation, the larger capacitive and faradic currents in CVs have been observed for all RNEs prepared to date. In addition, the absolute value of these currents was different for individual electrodes, because the distance between a TEPCM and gold film is not controllable. In contrast, the limiting current in chronoamperometry was very similar among the different electrodes. This result indicates that the density of open pores (or pore length) in RNEs can be determined from the limiting current measured by chronoamperometry using eq 3, even in the presence of the gap due to the incomplete seal between a TEPCM and gold film, as reported previously.27
SUMMARY AND CONCLUSIONS This paper has demonstrated that CV measurements at different scan rates (0.002-10 V/s) provide a rapid, reproducible means for determining the effective pore density and pore lengths within RNEs. This method is applicable to characterizing RNEs (27) Kralj, B.; Dryfe, R. A. W. Phys. Chem. Chem. Phys. 2001, 3, 3156-3164.
having relatively low pore density, because such RNEs show the transition between linear and radial diffusion modes. In contrast, NREs having high pore densities cannot be characterized using the proposed method, because the overlap of radial diffusion layers prevents observation of this transition.15 Although the analysis methods used in this paper are not based on detailed computer simulation19,20 and although the distribution of the pores is random, the results are fairly reasonable, probably because the current measured is the sum of currents from a large number of recessed nanodisk electrodes. The electrochemical characterization method demonstrated here will provide those developing RNEs with a simple, rapid, and reproducible means to assess the quality of the devices being prepared.
ACKNOWLEDGMENT The authors thank Dr. Daniel A. Higgins (Department of Chemistry, Kansas State University) for access to the thermal evaporation system, Drs. Hongxing Jiang and Zhaoyang Fan (Department of Physics, Kansas State University) for providing the sputter coater used in early experiments, and Dr. Christopher T. Culbertson (Department of Chemistry, Kansas State University) for access to the surface profiler. They also thank Dr. Daniel A. Buttry (Department of Chemistry, University of Wyoming) for his suggestions. The authors gratefully acknowledge financial support from Kansas State University. Received for review June 7, 2006. Accepted July 26, 2006. AC061043M
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