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Cyclic Biamperometry at Micro-Interdigitated Electrodes Mehdi Rahimi and Susan R. Mikkelsen* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada ABSTRACT: Cyclic biamperometry was studied as an analytical method for use with commercially available, comb-type, coplanar microinterdigitated electrodes (μIDEs), using the ferri-/ferrocyanide redox couple as a model analyte. The μIDEs studied in this work were made of gold that had been deposited onto a Ti/W adhesion layer on borosilicate glass chips and had 5 and 10 μm bands with equal gap sizes. Close proximity of the two working electrodes, and their interdigitation, resulted in signal amplification by redox cycling. Results were compared with those obtained by cyclic voltammetry, where one of the two IDE electrodes was used as the working electrode and external reference and auxiliary electrodes were used. Amplification factors of almost 20 were achieved due to redox cycling. Attempts to apply cyclic voltammetry to the μIDEs, with one of the combs as the working and the other as the auxiliary electrode, were unsuccessful due to corrosion of the auxiliary electrode comb. Results of this study, and the electrochemically unique feature of biamperometry to probe but not change the net contents of the medium under examination, suggest the applicability of scanning biamperometry at μIDEs to the very small volumes and electrochemical cell dimensions that are now of great interest.
I
nterdigitated electrodes (IDEs) were introduced into modern electrochemistry in 1985, when coplanar comb-like microinterdigitated electrodes (μIDEs) were examined as an alternative to opposing planar electrodes in thin-layer cells for generation of steady-state currents at stationary solid electrodes.1 This miniature design, having 20 μm separations between the gold bands, allowed independent control of the potential of each electrode through the use of a bipotentiostat with its associated external auxiliary and reference electrodes.1 Since then, several reports concerning various designs and integration of μIDEs have entered the literature and have included studies of electronic conduction in polymers,2 as well as applications to biosensors,3 8 electrochemical sensors,9,10 bioassays,11 13 flow cell detectors,14 16 batteries,17,18 photodetectors,19 and charge transport metasurements in discotic liquid crystals.20 In recent years, μIDEs have been successfully integrated into ultrasensitive sensing devices: for example, copper ion has been detected at levels as low as 1.0 10 13 M,21 and dengue antibody has been detected in human serum even after 50 000-fold dilution.22 μIDEs are often used for impedance measurements23,24 in chemical sensors or biosensors based on electroactive polymers, since very large impedance changes accompany oxidation and reduction of these polymer films. Mechanically stable μIDEs with various interdigitation patterns and spacings are now commercially available. The dimensions of comb-like μIDEs are typically described by geometric (rather than electrochemical) measures of the widths of the finger-like bands and the gaps between bands, as well as the length of the fingers and the overall areas of each of the two electrodes. As the dimensions are reduced, especially the gap width between the bands, the product of an electrochemically reversible redox reaction may diffuse from one band to a differently polarized nearby band and react in the reverse r 2011 American Chemical Society
manner, in a phenomenon known as redox cycling.25 Electrode surface or solution modifications that hinder transport of redox species to/from the electrodes have been shown to decrease the rate of redox cycling.26 It is now known that the limiting current resulting from redox cycling in μIDEs increases as the microband width and the gap between the microbands are decreased.27,28 In 1990, it was shown that the collection efficiency between the anodes and the cathodes is almost unity when the gap size is less than 10 μm.29 Amplification factors as high as 104 have been reported with redox cycling using interdigitated nanofluidic channels that have 25 55 nm height, making detection of ∼100 molecules possible.16 Simulation studies have suggested that, although the limiting current at μIDEs is significantly amplified when the gaps between fingers of the two working electrodes is decreased from 10 μm to 100 nm, no further amplification is expected with smaller gaps.30 However, an earlier study using scanning electrochemical microscopy demonstrated single-molecule detection with a tip substrate gap of 10 nm.31 These studies suggest that both cell geometry and electrochemical kinetics determine the amplification factors that can be achieved by redox cycling. Biamperometry, also called amperometry with two polarizable electrodes, involves the application of a relatively small voltage (e100 mV) across two identical electrodes and measurement of the current resulting from redox reactions at the two electrode electrolyte interfaces. The biamperometric current flows when (a) both halves of the redox couple are present in the electrolyte solution and can undergo both reductive and oxidative reactions within the applied voltage window32 or (b) when Received: May 18, 2011 Accepted: August 26, 2011 Published: August 27, 2011 7555
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Figure 1. Photograph of one of the 5 μm μIDEs used in this work.
different redox reactions, of closely matched potentials, are used for the anodic and cathodic half-reactions.33 35 Biamperometry was first introduced by Hostetter and Roberts more than 90 years ago as a method for the detection of end points in redox titrations.36 Biamperometry has since seen a commercial application in a glucose sensor (the Accu-Chek from Roche Diagnostics); in this device, two identical palladium electrodes were used with ferri/ferrocyanide as mediator.37 Over the past 5 years, several reports concerning the use of fixed-voltage biamperometric detectors for flow injection systems have been reported, most of which use separated metallic electrodes, rather than an interdigitated electrode pair.38 47 Sufficiently reversible redox couples for biamperometric applications have included ferri/ferrocyanide, Fe2+/Fe3+, DPPH•/DPPH, VO3 / VO2+, ferricinium/ferrocene, Co3+/Co2+, I3 /I , and Br3 /Br . In a previous paper, we reported a fundamental study of cyclic biamperometry at macroscopic (1.5 mm diameter) gold disk electrodes.48 In that study, a triangular voltage waveform was applied across two metal electrodes to quantitate one form of a redox couple (ferri-/ferrocyanide) in the presence of the other form; results allowed a new explanation for the distribution of the applied voltage between the two electrode solution interfaces and demonstrated the promise of this method for selective quantitation. We now report results of a study of cyclic biamperometry at gold, comb-like μIDEs, where signal amplification by redox cycling is possible, and compare signals with those obtained using cyclic voltammetry with the same electrode system.
’ EXPERIMENTAL SECTION Materials and Instrumentation. Potassium ferrocyanide, potassium ferricyanide, and potassium chloride were purchased from Sigma-Aldrich in the highest available quality and were used as received. Solutions were prepared in distilled, deionized water (Barnstead NanoPure) and contained 0.100 M KCl. Comb-type gold microinterdigitated electrodes (μIDEs) were of 2 different sizes and were obtained from ABTech Scientific (Richmond VA). They consist of borosilicate glass chips patterned first with a 10 nm Ti/W adhesion layer followed by a 100 nm layer of gold. IME1025MAUU (referred to herein as the 10 μm μIDE) has a digit length of 2.99 mm, 25 digit pairs, a digit
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width of 10 μm, and an interdigit spacing of 10 μm; the geometric surface area of each of the two electrodes was calculated as 7.48 10 3 cm2. IME0525MAUU (referred to as the 5 μm μIDE) has a digit length of 3.00 mm, 25 digit pairs, a digit width of 5 μm, and an interdigit spacing of 5 μm; the geometric surface area of each comb electrode was calculated as 3.74 10 3 cm2. ABTech also supplied the related adapter for connection to the potentiostat. The μIDEs were used as received and were rinsed with water and blotted dry with KimWipe tissue between experiments. Figure 1 shows a photograph of the 10 μm μIDE; the connecting leads are coated by the manufacturer with an insulating polymer, so that only the interdigitated parts of the μIDEs are exposed to solutions. Gold disk (1.5 mm diameter) and Ag/AgCl reference electrodes were acquired from Bioanalytical Systems (West Lafayette, IN). The gold disk electrodes were polished with an aqueous slurry of 1 μm Buehler Micropolish II (Tech-Met Canada), sonicated in water (Branson 1200), and dried prior to use. A Pt wire auxiliary electrode (Fisher Scientific) was used where indicated. Electrochemical measurements were made using a CHI 650A potentiostat (CH Instruments). Measurement of the auxiliaryto-reference potential difference in the final reported CV experiments was conducted using a model N1-USB-6251 data acquisition system with LabView software (National Instruments). Micrographs were obtained using a transmittance light microscope (Olympus BX 41). Temperature control for electrochemical experiments (25.0 ( 0.1 °C) was accomplished using a Haake D1 circulating bath and a water-jacketed electrochemical cell with an approximate total volume of 50 mL. Methods. For cyclic biamperometry (CB) experiments, potentiostat leads for both auxiliary and reference electrodes were connected to one comb electrode, and the working electrode lead to the other comb electrode of the μIDEs. With these connections, the applied voltage (not potential) across the cell is controlled by the potentiostat. Scans were conducted between 0.40 and +0.40 V, beginning at 0.40 V. Results (as limiting currents) for steady-state biamperograms are reported. Cyclic voltammetry (CV) experiments, with the exception of the last CV experiment (Figure 4 and related text), employed only one of the μIDE comb electrodes, and this was connected to the potentiostat’s working electrode lead. The other comb was left at open curcuit to prevent any passage of current. External reference and auxiliary electrodes were used. Scans were initiated at 0.20 V and were conducted between 0.20 and +0.60 V vs Ag/AgCl. Results (as cathodic peak currents) are reported for steady-state voltammograms. For the final reported CV experiments with μIDEs, one comb of the μIDE was used as the working electrode, while the other comb was connected to the auxiliary electrode lead. An external Ag/AgCl reference electrode was used. For comparison, two 1.5 mm diameter gold disk electrodes were used with the same Ag/AgCl reference electrode to allow measurement of the auxiliary-to-reference potential difference during CV scans. Calculations that required diffusion coefficients employed values of 7.63 10 6 and 6.62 10 6 cm2/s for ferricyanide and ferrocyanide in 0.100 M KCl, respectively.49
’ RESULTS AND DISCUSSION A comparison of CB and CV scans taken using a 5 μm μIDE is shown in Figure 2, where both comb electrodes are used for the 7556
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Figure 2. Comparison of cyclic biamperometry (main plot) with cyclic voltammetry (inset) performed at a 5 μm μIDE. For CB, the solution contained 0.20 mM each of ferrocyanide and ferricyanide, while for CV the solution contained 0.20 mM ferricyanide in 0.100 M KCl. The scan rate for both was 2 mV/s.
CB scan (main graph, a biamperogram) and only one comb electrode is used with external auxiliary and reference electrodes for the CV scan (inset, a voltammogram). There are several differences between the results provided by the two methods. The shapes of the curves are noticeably different. The CV results (Figure 2 inset) show typical CV peaks and decays, with the average of the two peak potentials corresponding to the formal potential of the ferri-/ferrocyanide redox couple under these conditions. On the other hand, the sigmoid shape of the CB scan results (Figure 2, main graph) is typical of redox cycling, while the midpoint of the curve at zero volts applied is typical of earlier results for cyclic biamperometry that used large (1.5 mm diameter), well-separated electrodes.48 The comparative magnitudes of the peak (CV, ≈ 0.065 μA) and plateau (CB, ≈ 1 μA) currents support the idea of redox cycling in the biamperometric experiments. Under the same conditions, an investigation of the effect of scan rate was undertaken, using scan rates of 2, 4, 8, 16, 25, and 36 mV/s. Typical diffusion-related results were obtained for the voltammograms, with a linear dependence of cathodic peak current on the square root of scan rate (slopes were 0.0221 μA(s/mV)1/2 with R2 = 0.998 for the 5 μM and 0.0381 μA (s/mV)1/2 with R2 = 0.999 for the 10 μm μIDEs; values predicted by the Randles-Sevcik equation for semi-infinite linear diffusion50 are 0.0176 and 0.0352, respectively). Due to redox cycling, plateau currents obtained from the biamperograms, however, did not vary with scan rate but were much higher than the peak currents obtained from the voltammograms. Average plateau current values were 0.90 ( 0.02 μA for the 5 μM and 1.181 ( 0.008 μA for the 10 μm μIDEs. The invariance of the plateau current with scan rate is again indicative of the redox cycling that occurs during the biamperometric experiments. These results are plotted in Figure 3 as the ratio of limiting biamperometry current to peak voltammetry current, as a function of the square root of scan rate. The main graph, corresponding to the 5 μm μIDE, shows that, at the slowest scan rate (2 mV/s), the biamperometric signal is 15-fold higher than the voltammetric signal; as scan rate increases, the ratio decreases to about a
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Figure 3. Signal ratio (as CB plateau current/CV peak current) vs square root of scan rate. The main plot shows results for a 5 μm μIDE, while the inset shows results obtained for a 10 μm μIDE. Solution conditions were the same as those used for Figure 2.
5-fold higher value at 36 mV/s. Similar results were obtained at the 10 μm μIDE (Figure 3 inset), where the 19-fold higher biamperometric value (2 mV/s) decreased to a 5-fold higher value at 36 mV/s. At higher scan rates, the diffusional limitations of redox cycling are apparent with both μIDEs. However, at lower scan rates (and likely even more with fixed applied voltage/ potential), the signal amplification benefits of redox cycling are very apparent. Signal dependence on ferricyanide concentration was compared using both the 5 μm and the 10 μm μIDEs and a constant scan rate of 25 mV/s. Solutions used for CV experiments contained only ferricyanide at concentrations of 100, 200, 300, 400, and 500 μM, while solutions examined by CB contained ferricyanide at these concentrations as well as excess (5 mM) ferrocyanide. In all four cases, good linearity (R2 > 0.98) was observed in plots of peak current (CV) or plateau current (CB) against concentration. Linear regression showed that all four y-intercepts were very close to zero. However, the slopes of the regression lines were very different for CV and CB, with CB signals being much higher than those of CV. With the 5 μm μIDE, slopes were 0.0010 A/M (CV) and 0.0116 A/M (CB), and with the 10 μm μIDE, the slopes were 0.0016 A/M (CV) and 0.0107 A/M (CB). The ratios of slopes (CB/CV), 11.6 and 6.7 for the 5 and 10 μm μIDEs, respectively, again point to the amplification that can be achieved by redox cycling. It should be possible to achieve similar amplification factors by voltammetry using a bipotentiostat, in a four-electrode cell, where the potentials of the two comb electrodes are controlled with respect to an external reference electrode, and using an external auxiliary electrode. For example, the potential of one comb could be fixed at a reducing potential while the potential of the other comb is scanned from a reducing to an oxidizing potential. This type of experiment has been examined using feedback mode scanning electrochemical microscopy,51 with a bipotentiostat controlling the substrate (fixed) and tip (scanned) potentials. As the substrate potential was systematically changed between scans, the tip voltammograms were observed to shift vertically on the current axis, while the magnitudes of the plateau 7557
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Figure 4. Micrographs of a 10 μm μIDE (a) before and (b) after attempts to use one of the interdigitated electrodes as an auxiliary electrode for CV in a 3-electrode cell with an external Ag/AgCl reference electrode. The light bands are the gold electrodes, and the gaps between the electrodes are seen in a dark color.
currents remained essentially constant. In these experiments, the fixed substrate potential controls the direction of redox cycling (whether substrate acts as anode or cathode), while the bipotentiostat allows independent measurement of currents at the substrate and tip. We have examined the possibility of conventional 3-electrode CV at the μIDEs, with one comb electrode used as the working and the other as the auxiliary electrode, to allow redox cycling between working and auxiliary electrodes. CV of a 0.20 mM ferricyanide solution was performed, and voltammograms showed abnormal appearances at all scan rates tested. Micrographs were taken before and after the CV experiments, and are shown in Figure 4. The picture of the new μIDE (Figure 4a) shows clearly defined edges and spacings that are in agreement with the product specifications. Following the CV runs (Figure 4b), the bands appear to have deteriorated, and corrosion appears to be significant. In parallel, CV experiments were done using two commercially available 1.5 mm diameter gold disk electrodes and the same Ag/AgCl reference electrode, while monitoring the potential difference between the auxiliary and reference electrodes. Results show that, when the applied potential is 0.2 V, the
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auxiliary-to-reference electrode potential difference reaches +0.95 V. This is a sufficiently positive potential to cause corrosion of the Ti/W/Au structure that comprises the auxiliary electrode comb of the interdigitated electrodes. Thus, the use of one of the μIDE combs as an auxiliary electrode is not possible under these conditions. In addition, the large potential difference between the working and auxiliary electrodes may be expected to erode the selectivity of the measurements.16,52 Scanning methods including cyclic voltammetry are powerful tools for the elucidation of electrochemical reaction mechanisms, preceding and following chemical reactions as well as the measurement of kinetic constants and the determination of n values for redox reactions.53 In the absence of preceding or following homogeneous reactions, quantitative (analytical) applications have been limited by the nature of the data, where peak or plateau currents depend on the total quantity (reduced plus oxidized) of the electroactive species. With conventional scanning methods, and equal diffusion coefficients, the entire voltammogram shifts up or down on the current axis, and although its position depends on the [R]/[O] value (leading to decisions for appropriate fixed potential experiments), the magnitude of the peak or plateau current does not depend on the concentration of the limiting redox form. Scanning biamperometry, on the other hand, yields a signal that depends on the concentration of the limiting reactant (R or O) and offers new promise for applications in disposable sensors and lab-on-chip (flow injection) devices. Unlike bipotentiostat experiments, including feedback-mode scanning electrochemical microscopy, for which collection efficiencies are generally not unity, biamperometric experiments that use a single, sufficiently reversible redox couple for both anodic and cathodic reactions do not change the contents of the examined solution (or thin film) as a result of the measurement, because anodic and cathodic currents are always equal and opposite. Furthermore, the relatively small voltage difference between the two working electrodes (maximum (0.4 V in our experiments) contrasts with the large voltage difference measured between working and auxiliary electrodes in our parallel CV experiments at macroscopic electrodes (1.15 V). Auxiliary electrode corrosion, and the generation of reactive species at the auxiliary electrode, must be considered for very small electrochemical cells with closely spaced electrodes.16,52 It is a key feature of both scanning and fixed-potential biamperometry that the net [R]/[O] composition of the solution (or thin film) does not change as a result of the measurement, as long as one redox couple is used for both anodic and cathodic reactions. Examples of possible applications of scanning biamperometry at μIDEs include coated sensors, where the coating over both IDE combs is an organic thin film containing a hydrophobic redox couple54 or a redox polymer.55 Exposure of the sensor for a fixed time to an analyte solution would change the concentration of the limiting species, e.g., in a flow system. This could be followed by multiple scan cycles, since biamperometric measurement does not change the net [R]/[O] composition of the coating, allowing integration or averaging of the analytical signal. Another promising application involves the very small-scale electrochemical cells that are of interest to nanotechnologists; the need for only two electrodes, as well as the unique feature of biamperometry to probe but not change the [R]/[O] ratio in the examined medium, could be of particular interest at these small scales. Finally, the simpler circuitry favors portability, an important parameter for point-of-care and field testing devices.56,57 7558
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: (519) 746-0435.
’ ACKNOWLEDGMENT Financial support from the Natural Sciences and Engineering Research Council of Canada and from the Department of Chemistry, University of Waterloo, is gratefully acknowledged. ’ REFERENCES (1) Sanderson, D. G.; Anderson, L. B. Anal. Chem. 1985, 57, 2388–2393. (2) Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601–607. (3) Dzyadevych, S. V.; Arkhypova, V. N.; El’skaya, A. V.; Martelet, C.; Jaffrezic-Renault, N.; Soldatkin, A. P. Curr. Top. Anal. Chem. 2001, 2, 179–186. (4) Berggren, C.; Bjarnason, B.; Johansson, G. Electroanalysis 2001, 13, 173–180. (5) Iliev, I.; Kaisheva, A. EXS Front. Biosensorics II 1997, 81, 87–98. (6) Niwa, O.; Xu, Y.; Halsall, H. B.; Heineman, W. R. Anal. Chem. 1993, 65, 1559–1563. (7) Niwa, O.; Kurita, R.; Liu, Z.; Horiuchi, T.; Torimitsu, K. Anal. Chem. 2000, 72, 949–955. (8) Elsholz, B.; Worl, R.; Blohm, L.; Albers, J.; Feucht, T.; Grunwald, T.; Jurgen, B.; Schweder, T.; Hintsche, R. Anal. Chem. 2006, 78, 4794–4802. (9) Morita, M.; Niwa, O.; Horiuchi, T. Electrochim. Acta 1997, 42, 3177–3183. (10) Matsue, T. Trends Anal. Chem. 1993, 12, 100–108. (11) Kim, S. K.; Hesketh, P. J.; Li, C.; Thomas, J. H.; Halsall, H. B.; Heineman, W. R. Biosens. Bioelectron. 2004, 20, 886–893. (12) Laczka, O.; Garcia-Aljaro, C.; del Campo, F. J.; Pascual, F. X. M.; Mas-Gordi, J.; Baldrich, E. Anal. Chim. Acta 2010, 677, 156–161. (13) Ino, K.; Saito, W.; Koide, M.; Umemura, T.; Shiku, H.; Matsue, T. Lab Chip 2011, 11, 385–388. (14) Niwa, O.; Tabei, H.; Solomon, B. P.; Xie, F.; Kissinger, P. T. J. Chromatogr., B 1995, 670, 21–28. (15) Daniel, D.; Gutz, I. G. R. Talanta 2005, 68, 429–436. (16) Goluch, E. D.; Wolfrum, B.; Singh, P. S.; Zevenbergen, M. A. G.; Lemay, S. G. Anal. Bioanal. Chem. 2009, 394, 447–456. (17) Nishizawa, M.; Uchida, I. Electrochemistry (Tokyo) 1999, 67, 420–426. (18) Nishizawa, M.; Uchida, I. Electrochim. Acta 1999, 44, 3629–3637. (19) Berger, P. R. Proc. SPIE 2001, 4285, 198–207. (20) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater. Chem. 1999, 9, 2081–2086. (21) Yang, B.; Kong, J. Chem. Lett. 2009, 38, 1072–1073. (22) Fang, X.; Tan, O. K.; Tse, M. S.; Ooi, E. E. Biosens. Bioelectron. 2010, 25 (5), 1137–1142. (23) Narakathu, B. B.; Atashbar, M. Z.; Bejcek, B. Sens. Lett. 2011, 9 (2), 872–875. (24) Zhang, Y.; Zheng, X.; Zhang, T. Sens. Actuators, B 2011, B156 (2), 887–892. (25) Anderson, L. B.; Reilley, C. N. J. Electroanal. Chem. 1965, 10, 295–305. (26) Niwa, O.; Morita, M.; Tabei, H. Electroanalysis 1994, 6, 237–243. (27) Bard, A. J.; Crayston, J. A.; Kittlesen, G. P.; Varco, S. T.; Wrighton, M. S. Anal. Chem. 1986, 58, 2321–2331. (28) Aoki, K.; Morita, M.; Niwa, O.; Tabei, H. J. Electroanal. Chem. 1988, 256, 269–282. (29) Niwa, O.; Morita, M.; Tabei, H. Anal. Chem. 1990, 62, 447–452. (30) Yang, X.; Zhang, G. Sens. Actuators, B 2007, B126, 624–631.
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