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Integrated All-Diamond Ultramicroelectrode Arrays: Optimization of Faradaic and Capacitive Currents Waldemar Smirnov, Nianjun Yang,* Rene Hoffmann, Jakob Hees, Harald Obloh, Wolfgang M€uller-Sebert, and Christoph E. Nebel Fraunhofer Institute for Applied Solid State Physics (IAF), Freiburg 79108, Germany
bS Supporting Information ABSTRACT: Integrated all-diamond ultramicroelectrode arrays (UMEAs) were fabricated using standard photolithography processes. The array consisted of typically 45 ultramicroelectrodes with a diameter of 10 μm and with a center-to-center spacing of 60 μm. The quasi-reference and counter electrodes were made from conductive diamond and were integrated on a 5 5 mm2 chip. On the UMEA, a high ratio of faradaic current to capacitive current was achieved on heavily boron-doped and hydrogen-terminated diamond surfaces at slow scan rates and in high concentration of supporting electrolyte. A sensitive and reproducible detection of dopamine was achieved on hydrogen-terminated diamond UMEA at slow scan rates. The detection limit of dopamine in the presence of ascorbic acid was 1.0 nM, which is 50 100 times lower than that obtained on the macrosized boron-doped diamond electrodes. This array is promising for sensitive and reproducible detection of analytes in solutions with low detection limits.
M
icroelectrode arrays (MEAs) and ultramicroelectrode arrays (UMEAs) have been paid extensive attention during past decades. They amplify the signal of individual (ultra)microelectrodes but do not lose the beneficial characteristics of individual (ultra)microelectrodes, such as a reduced ohmic resistance, an enhanced mass transport, decreased charging currents, and decreased deleterious effects of solution resistance.1 To improve the sensor performance with respect to the sensitivity, detection limit, lifetime, and reproducibility, the selection of appropriate and optimized material for the fabrication of MEAs and UMEAs is known to be critical. MEAs and UMEAs from heavily boron-doped diamond have been fabricated since they combine the unique characteristics of (ultra)microelectrodes and the electrochemical features of borondoped diamond. It is known that heavily boron-doped diamond applied as working electrode shows low background currents, wide potential windows, long chemical stability, and a rich surface chemistry.3 Moreover, diamond surfaces can be terminated electrochemically with hydrogen or oxygen.4 The first diamond MEAs were fabricated in 2002 by Fujishima and co-workers using structured silicon substrates as templates.5 Later, patterned silicon nitrite6 and “as-grown” diamond with randomly microstructured topology7 were used for the fabrication of MEAs and UMEAs. They had different shape, spacing, and number of electrodes. In 2005, Compton and co-workers8 realized for the first time all-diamond UMEAs. These arrays have been utilized for the detection of metal ions,6,8,9 neuronal activity measurements,5,7 and implant r 2011 American Chemical Society
application.10 However, none of these diamond-based MEAs or UMEAs are market-available. To achieve sensitive detection of analytes in solutions with low detection limits by using MEAs or UMEAs, one has to optimize the ratio of faradaic current (signal, S) and capacitive current (background, B). The largest S/B ratio can be obtained when maximum faradaic current is achieved with minimum capacitive current.1 The capacitive and faradaic currents are known to be affected by the scan rates applied, the spacing between microelectrodes, and the concentration of supporting electrolyte. For diamond MEAs and UMEAs, they are also affected by the surface terminations and boron-doping densities of the transducer. Our goal was to make diamond UMEAs market-available and also to explore their highly sensitive and reproducible sensing applications. In this paper, we demonstrate the batch production of diamond UMEAs using standard photolithography techniques. For their sensing applications, we first optimized faradaic and capacitive currents on diamond UMEAs with respect to the effect of scan rates, the concentration of supporting electrolyte, surface termination, and boron-doping density of microelectrodes. The sensing performance of diamond UMEAs was then tested using dopamine as the target compound. After the optimization of faradaic and capacitive currents, a 50 100 times lower detection Received: June 22, 2011 Accepted: August 9, 2011 Published: August 09, 2011 7438
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limit was achieved on diamond UMEAs than that obtained on macrosized diamond electrodes.
’ EXPERIMENTAL SECTION Chemicals and Apparatus. Dopamine and ascorbic acid were purchased from Fisher Scientific in Schwerte, Germany. Other chemicals were analytical grade and used as received without further purifications. Electrochemical experiments were conducted on a VMP-3 potentiostat (BioLogic Science Instruments, Claix, France). The diamond ultramicroelectrodes were used as the working electrode; the counter and quasi-reference electrodes were oxidized boron-doped diamond films which were integrated on the sensor chip. The difference of the potential from Ag/AgCl reference electrode to diamond quasi-reference electrode was 200 mV, which did not vary with the types of solutions we used. A Faraday box was used to detect the lowest currents without electronic noise. The faradaic currents on diamond UMEAs were recorded in 1.0 mM Fe(CN)63 /4 /KCl solutions, and the capacitive currents were recorded in KCl solution. Diamond Samples. Boron-doped and undoped polycrystalline diamond films were grown on silicon wafers using microwave plasma enhanced chemical vapor deposition (CVD) in an ellipsoidal shaped bell jar reactor.11 Trimethylboron (TMB) was used as the boron source. The density of net uncompensated acceptors was evaluated by Mott Schottky analysis of capacitance voltage measurements of H-terminated diamond samples in 0.1 M KCl at a frequency of 1.0 kHz.12 Different surface terminations of diamond electrodes were achieved using a twoelectrode system. To achieve an oxidized surface, the electrodes were treated anodically in 2.0 M sulfuric acid at +3.5 or +10 V for 30 s. The hydrogen termination was realized electrochemically by a cathodic treatment where a voltage of 35 V was applied to the working electrode in a 2.0 M sulfuric acid solution for 30 s.4
’ RESULTS AND DISCUSSION Fabrication of Integrated All-Diamond UMEAs. Figure 1 shows the schematic plots of the fabrication of integrated alldiamond UMEAs. A movie of the fabrication process can be seen in the Supporting Information. As shown in Figure 1a, an insulating diamond (iD) film with a thickness of a few micrometers (8 10 μm) was first grown on a Si wafer of 2 in. diameter to electrically disconnect the Si substrate from the electrochemical sensor. The iD film was then polished, followed by deposition of a 200 500 nm thick boron-doped polycrystalline diamond (BDD). The BBD film was wet-chemically cleaned in the mixture of concentrated sulfuric acid (98%) and concentrated nitric acid (65%; v/v = 3:1) at 200 °C for 1.5 h, resulting in an oxidized surface. To generate SiO2 patterns on the BDD film, an evaporated SiO2 layer with a thickness of 350 nm was structured by means of optical photolithography and dry etching using SF6 gas (Figure 1b). These SiO2 patterns were then used as a mask to structure the BDD film during a dry etching process, which was conducted using reactive ion etching (RIE) in a gas mixture of oxygen and hydrogen (Figure 1c).13 In Figure 1d, a subsequent deposition of SiO2 on the whole wafer was done. The second optical photolithography step and dry etching of SiO2 using SF6 gas were then applied (Figure 1e). The protected parts by SiO2 patterns were the electrodes and the contacts of the chip. The areas which were not protected by SiO2 patterns were then
Figure 1. Schematic plots of the fabrication of integrated all-diamond ultramicroelectrode arrays. The short names of iD represent insulating diamond and of BDD boron-doped polycrystalline diamond.
Figure 2. (a) Photography of an integrated all-diamond ultramicroelectrode array chip. The yellow parts are metal contacts. The dark part with a semicircle is the counter electrode, the dark rectangle is the quasireference electrode, and the center electrode is the working electrode. (b) Schematic plot of the structure of the electrode. (c) Arrangement of the array which is composed of 45 ultramicroelectrodes (10 μm in diameter) in a 500 μm (diameter) circle.
overgrown with the iD film, as shown in Figure 1f and the video in the Supporting Information. The overgrowth of the iD film on the BDD film formed boron-doped diamond channels which were used for the electric connection between electrodes and the contacts. After wet-chemical removal of the SiO2 (Figure 1g), the third optical photolithography step was applied and then Ti/Pt/ Au (20/60/200 nm) metal layers were deposited on the contacts by the lift-off technique (Figure 1h). Figure 2a shows the picture of an integrated all-diamond fabricated UMEA. The realized chip has a size of 5 5 mm2 and has a reference electrode, a counter electrode, and an array of ultramicroelectrodes (working electrode). All electrodes were made from boron-doped diamond. As discussed above, these electrodes were connected with metal contacts via boron-doped diamond channels which were hermetically covered by insulating diamond (Figure 2b). Please note that only the open electrodes were exposed to liquids for electrochemical measurements (see the video in the Supporting Information). The arrangement of the ultramicroelectrodes is shown in Figure 2c. The diameter of ultramicroelectrodes was 10 μm, and the vertical and horizontal center-to-center spacing between electrodes was 60 μm. The total number of ultramicroelectrodes on one chip was 45. For one 2 in. wafer, more than 40 integrated diamond UMEA chips were obtained. 7439
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Figure 4. Effect of surface termination: cyclic voltammograms of 1.0 mM Fe(CN)63 /4 on (a) initially oxidized, (b) electrochemically oxidized, and (c) electrochemically hydrogen-terminated diamond ultramicroelectrode arrays. The boron concentration was (4.2 ( 2) 1020 cm 3, and the scan rate was 0.02 V s 1. Figure 3. Scan rate dependency: cyclic voltammograms in 0.1 M KCl solution with (solid lines) and without (dashed lines) 1.0 mM Fe(CN)63 /4 at a scan rate of (a) 0.02, (b) 0.2, (c) 2, and (d) 20 V s 1.
Optimization of Faradaic and Capacitive Currents on Diamond UMEA. The faradaic and capacitive currents at these
diamond UMEAs were optimized with respect to scan rates applied, surface termination of diamond ultramicroelectrodes, the boron-doping density of diamond ultramicroelectrodes, and the concentration of supporting electrolyte. Figure 3 shows the faradaic currents (solid lines) of 1.0 mM Fe(CN)63 /4 in 0.1 M KCl on diamond UMEAs over a range of scan rates. The redox signals show a scan rate dependency. With increasing scan rate, the magnitude of the faradaic currents increased and the shape of the voltammograms varied. At low scan rates ranging from 0.02 (a) to 0.2 V s 1 (b) and at fast scan rates of 20 V s 1 (d), peak-shaped voltammograms were obtained, indicating linear diffusion-limited transport of analytes. A sigmoidal-shaped voltammogram (c) was detected at a scan rate of 2 V s 1, which is consistent with hemisphere diffusion to the ultramicroelectrodes on the array. To calculate the diffusion layer thickness δ, we applied the equation12 δ = (2DΔE/v)1/2 (where v is the scan rate, D = 7.6 10 6 cm s 1 is the diffusion-coefficient of analytes, and ΔE is the potential range over which electrolysis occurs). For example to estimate the size of the diffusion layer thickness at E = +0.4 V, we used the value ΔE = +0.8 V since significant electrolysis current started at E = 0.4 V. The calculated values of δ at E = +0.4 V for the scan rates of 0.02, 0.2, 2, and 20 V s 1 were 250, 78, 25, and 7.8 μm, respectively. The center-to-center separation and the diameter of microelectrodes were 60 μm and 10 μm, respectively. Thus, the separation between electrodes was 50 μm. At a scan rate of 0.02 V s 1, δ = 250 μm, which is much larger than the spacing between electrodes, indicating a complete overlap of redox molecule diffusion of individual electrodes and subsequently a linear diffusion profile. At a higher scan rate of 0.2 V s 1, δ = 78 μm, which is only slightly greater than the separation of electrodes, still an overlap of adjacent diffusion profiles is dominating. When δ becomes larger than the diameter of a microelectrode but is still smaller than the separation between electrodes the voltammetric response is the response of an individual
microelectrode (sigmoidal curve) multiplied by the total number of electrodes in the array. This can be detected for example at a scan rate of 2 V s 1. However, further increase of the scan rate (e.g., to 20 V s 1) leads to even smaller values of δ (e.g., to 7.8 μm) than the size of microelectrodes (10 μm). In this case the linear diffusion dominates the mass transport, resulting in peakshaped voltammograms. Figure 3 also shows capacitive currents (dashed lines) on diamond UMEA in 0.1 M KCl over a range of scan rates. The capacitive currents increased linearly with scan rate. The S/B ratios for scan rates of 0.02, 0.2, 2, and 20 V s 1 were estimated to be 1817 ( 40, 215 ( 18, 28 ( 6, 10 ( 1, respectively. The highest ratio was achieved at the slowest scan rate. In this case the diffusion profiles at neighboring microelectrodes overlapped, and thus peak-shaped voltammograms were detected. The magnitude of the faradaic current is thus proportional to the geometric area, which is comprised of all microelectrodes and the insulating parts. On the other side, the contribution of the capacitive current to the total current was small since the capacitive current is proportional to the scan rate and to the electrochemical active area, which is only the area for all microelectrodes. This gives rise to the enhanced ratios of S/B, leading to increased sensitivity for analytes. Since the geometric area of an UMEA chip is always 50 1000 times larger than the electrochemical active area, a 50 1000 times better sensitivity is expected for such electrode arrays.1,14 It is known that the surface termination of diamond affects the voltammetric properties, which can be characterized using Fe(CN)63 /4 .2 Figure 4 shows cyclic voltammograms of 1.0 mM Fe(CN)63 /4 as detected on differently terminated UMEA surfaces. In this case different surface terminations were achieved electrochemically. Please note that the diamond ultramicroelectrodes were initially oxidized due to a wet-chemical cleaning in a mixture of concentrated sulfuric acid (98%) and concentrated nitric acid (65%; v/v = 3:1) at 200 °C for 1.5 h. A peak difference of the anodic to the cathodic wave, ΔEp, was measured to be 230 mV on the initially oxidized electrode (curve a). Further oxidation of the UMEA at +10 V for 30 s in 2.0 M sulfuric acid increased ΔEp to 480 mV in conjunction with a decrease in the peak current (curve b). After electrochemical hydrogenation, a decrease of ΔEp to 90 mV (curve c) and enlarged faradaic 7440
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Figure 5. Doping concentration dependency: cyclic voltammograms of 1.0 mM Fe(CN)63 /4 on hydrogen-terminated diamond ultramicroelectrode arrays at a scan rate of 0.02 V s 1. The boron concentration was (a) (3.8 ( 2) 1019, (b) (5.1 ( 2) 1019, and (c) (4.2 ( 2) 1020 cm 3.
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Figure 7. (a) Cyclic voltammogram (solid line) of the mixture 25 nM dopamine (I) in the presence of 25 nM ascorbic acid (II) on hydrogenterminated diamond UMEAs at a scan rate of 0.02 V s 1. The dashed line was the voltammogram in 0.1 M sulfuric acid. (b) Working curves for the detection of dopamine (I) and ascorbic acid (II) on diamond UMEAs.
Table 1. Comparison of Dopamine Detection on Macrosized Diamond Electrodes, Diamond Microelectrode Arrays, and Ultramicroelectrode Arrays performance for the detection of dopamine electrode
linear
detection
presence of
used
range
limit
ascorbic acid √
macroelectrode
0.1 1 μM
macroelectrode
50 nM 200 nM
macroelectrode
Figure 6. Concentration dependency: cyclic voltammograms in x = 2, 17, 50, and 100 mM KCl in the presence (a) and absence (b) of 1.0 mM Fe(CN)63 /4 at a scan rate of 0.02 V s 1. The boron concentration was (4.2 ( 2.6) 1020 cm 3, and the electrode surface was electrochemically hydrogen-terminated.
currents were obtained. These results indicate the fastest electron transfer on hydrogen-terminated diamond surfaces and the slowest on an electrochemically oxidized surface. We have also measured capacitive currents of the array in 0.1 M KCl solution. The S/B ratio on hydrogen-terminated surface was 3986 ( 58, while on the initially oxidized and on the electrochemically oxidized diamond electrodes the ratios were about 2160 ( 32 and 940 ( 17, respectively. Hence, a more sensitive measurement is expected on the hydrogen-terminated diamond UMEAs when scanned at slow scan rates. The boron concentration of ultramicroelectrodes affected the faradaic and capacitive currents as well. Figure 5 shows cyclic voltammograms of 1.0 mM Fe(CN)63 /4 at a scan rate of 0.02 V s 1 on diamond UMEAs with different boron-doping levels. ΔEp varied from about 200 mV at a boron concentration of 1019 cm 3 (curves a, b) to around 90 mV at a boron concentration of (4.2 ( 2) 1020 cm 3 (curve c) . ΔEp decreased when the doping level increased. The similar tendency for the variation of ΔEp as a function of boron-doping level has been
macroelectrode
5 100 μM
1.1 μM
microelectrode
0.5 100 μM
50 nM
UMEA UMEA
20 1000 nM 100 800 μM
√ √ √ √
ref 15 16 17 18 19 20 21
nanograss
5 120 μM
1.5 μM
UMEA
2.5 100 nM
1.0 nM
√
22 this work
reported previously.15 The highest faradaic current was detected on diamond UMEA with a boron concentration of (4.2 ( 2) 1020 cm 3. This is because that high boron-doping level (>1020 cm 3) gives metallic conduction of the diamond sample6 and a faster heterogeneous electron-transfer rate constant is expected,15 resulting in a smaller ΔEp and higher faradaic currents than those on other boron-doped diamond samples. The lowest capacitive current was detected on diamond UMEA with a boron concentration of (3.8 ( 2) 1019 cm 3 due to the thickest depletion layer.2 The highest S/B ratio was 4237 ( 45 on diamond UMEA with the boron concentration of (4.2 ( 2) 1020 cm 3. In addition, we compared the voltammograms of diamond UMEAs in solutions with and without adding excess supporting electrolyte. Figure 6a shows the voltammetric response of 1.0 mM Fe(CN)63 /4 on diamond UMEA in the solutions containing different amounts of supporting electrolyte (x = 2, 17, 50, and 100 mM KCl) at a scan rate of 0.02 V s 1. The faradaic currents decreased as the concentration of KCl decreases. We attribute this to the slow electrochemical generation of electroactive species at the electrode surface in the absence of excess supporting electrolyte, thereby reducing the flux of reactants to the electrode surface (and thus the current).14 In contrast, the capacitive currents, as shown in Figure 6b, did not vary much with the variation of the concentration of supporting electrolyte, 7441
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Analytical Chemistry which is one of most important benefits of microelectrodes.1 The highest ratio of S/B was 3660 ( 62 in the solution with 100 mM KCl as supporting electrolyte, in comparison with the values of 2375 ( 47, 2060 ( 29, and 80 ( 14 for the solutions with 50, 17, and 2 mM KCl, respectively. Detection of Dopamine in the Presence of Ascorbic Acid on Diamond UMEA. Dopamine, one important neurotransmitter, was used as the standard target compound for the testing of sensing performance of diamond UMEAs. The optimized conditions we applied for the detection of dopamine in the presence of ascorbic acid were an electrochemically hydrogen terminated surface, a boron-doping level of (4.2 ( 2) 1020 cm 3, and a scan rate of 0.02 V s 1. In 0.1 M sulfuric acid, dopamine was electrochemically oxidized on diamond UMEAs at 0.55 V vs C and then reduced at 0.10 V vs C. For ascorbic acid, it was electrochemically oxidized at about 1.2 V vs C. Figure 7 shows the cyclic voltammogram of the mixture of 25 nM dopamine in the presence of 25 nM ascorbic acid, where clear oxidation and reduction waves of dopamine (I) and ascorbic acid (II) were visible. The difference of anodic peak potential for dopamine to the potential of ascorbic acid was more than 500 mV, indicating that dopamine and ascorbic acid can be detected individually and simultaneously on diamond UMEAs. Figure 7b shows the calibration curves we obtained in the concentration range of 2.5 100 nM for the detection of dopamine (I) in the presence of ascorbic acid (II). The detection limit for dopamine was 1.0 nM. Table 1 summarizes the detection of dopamine by using diamond-based electrodes in the presence or absence of ascorbic acid. The electrodes included macrosized electrodes,16 19 MEA,20 UMEA,21,22 and diamond nanograss.23 Please note that diamond electrodes after modification with polymers and other species have not been included. Different linear ranges with different orders of detection limits for dopamine were reported at diamond electrodes. By using our diamond UMEAs at optimized conditions, we achieved the lowest detection limit (1.0 nM) for dopamine detection, which is 50 100 times lower than that reported. Therefore our diamond UMEAs will be promising for dopamine detection with a low concentration (0.01 1 μM) in individutal biological samples or in those the presence of other similar compounds such as ascorbic acid.
’ CONCLUSION In summary, batch production of integrated all-diamond ultramicroelectrode arrays were realized using standard photolithography processes. A higher ratio of faradaic current to capacitive current was achieved at slower scan rates, on hydrogen-terminated diamond surfaces with high boron-doping level (>1020 cm 3), and in solutions with excess supporting electrolyte. Fast electrontransfer process occurred on hydrogen-terminated, heavily borondoped diamond electrodes. A comparison of detection limits of dopamine on macrosized diamond electrodes with that on a diamond ultramicroelectrode array shows that a 50 1000 times better sensitivity than on a macrosized electrode is achieved on a diamond UMEA together with high reproducibility. Therefore integrated all-diamond ultramicroelectrode arrays will be a new platform for electrochemical and biochemical sensing in the future. ’ ASSOCIATED CONTENT
bS
Supporting Information. Video illustrating the fabrication process of integrated all-diamond ultramicroelectrode arrays.
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This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Dietmar Brink for photolithography, Gudrun Kaufel and Ralf Schmidt for SiO2 deposition and etching, Oliver A. Williams for fruitful discussions about diamond growth, and Liana Marek for photographing diamond electrode arrays. The work was supported by contract researchMethoden f€ur die Lebenswissenschaften by the Baden-W€urttemberg Stiftung (P-LS-Meth/23). ’ REFERENCES (1) (a) Compton, R. G.; Wildgoose, G. G.; Rees, N. V.; Streeter, I.; Baron, R. Chem. Phys. Lett. 2008, 459, 1. (b) Ordeig, O.; del Campo, J.; Munoz, F. X.; Banks, C. E.; Compton, R. G. Electroanalysis 2007, 19, 1973.(c) Wang, J. Analytical Electrochemistry, 2nd ed.; Wiley-VCH: Malden, MA, USA, 2000; p 130. (2) (a) Angus, J. C.; Pleskov, Y. V.; Eaton, S. C. Thin Film Diamond II. In Semiconductors and Semimetals, Vol. 77; Nebel, C. E., Ristein, J., Eds.; Elsevier Academic Press: Waltham, MA, USA, 2004; Chapter 3. (b) Swain, G. M. Thin Film Diamond II. In Semiconductors and Semimetals, Vol. 77; Nebel, C. E., Ristein, J., Eds.; Elsevier Academic Press: Waltham, MA, USA, 2004; Chapter 4. (3) (a) Yang, W.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J. E.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N., Jr; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253. (b) Nebel, C. E.; Rezek, B.; Shin, D.; Uetsuka, H.; Yang, N. J. Phys. D: Appl. Phys. 2007, 40, 6443. (4) Hoffmann, R.; Kriele, A.; Obloh, H.; Hees, J.; Wolfer, M.; Smirnov, W.; Yang, N.; Nebel, C. E. Appl. Phys. Lett. 2010, 97, No. 052103. (5) Tsunozaki, K.; Einaga, Y.; Rao, T. N.; Fujishima, A. Chem. Lett. 2002, 502. (6) Provent, C.; Haenni, W.; Santoli, E.; Rychen, P. Electrochim. Acta 2004, 49, 3737. (7) (a) Soh, K. L.; Kang, W. P.; Davidson, J. L.; Wong, Y. M.; Wisisoraat, A.; Swain, G.; Cliffel, D. E. Sens. Actuators, B 2003, 91, 39. (b) Soh, K. L.; Kang, W. P.; Davidson, J. L.; Basu, S.; Wong, Y. M.; Cliffel, D. E.; Bonds, A. B.; Swain, G. Diamond Relat. Mater. 2004, 13, 2009. (c) Soh, K. L.; Kang, W. P.; Davidson, J. L.; Wong, Y. M.; Cliffel, D. E.; Swain, G. Diamond Relat. Mater. 2008, 17, 240. (8) (a) Pagels, M.; Hall, C. E.; Lawrence, N. S.; Meredith, A.; Jones, T. G. J.; Godfried, H. P.; Pickles, C. S. J.; Wilman, J.; Banks, C. E.; Compton, R. G.; Jiang, L. Anal. Chem. 2005, 77, 3705. (d) T., G. J.; Godfried, H. P.; Pickles, C. S. J.; Wilman, J.; Banks, C. E.; Compton, R. G.; Jiang, L. Anal. Chem. 2005, 77, 3705. (b) Simm, A. O.; Banks, C. E.; Ward-Jones, S.; Davies, T. J.; Lawrence, N. S.; Jones, T. G. J.; Jiang, L.; Compton, R. G. Analyst 2005, 130, 1303. (c) Lawrence, N. S.; Pagels, M.; Meredith, A.; Jones, T. G.; Hall, C. E.; Pickles, C. S.; Godfried, H. P.; Banks, C. E.; Compton, R. G.; Jiang, L. Talanta 2006, 69, 829. (9) (a) Khamis, D.; Mahe, E.; Dardoize, F.; Devilliers, D. J. Appl. Electrochem. 2010, 40, 1829.(b) Madore, C.; Duret, A.; Haenni, W.; Perret, A. Proceedings of the Symposium on Microfabricated Systems and MEMS V, Pennington, NJ, USA, 2000; Electrochemical Society Proceeding Series; Electrochemical Society: Pennington, NJ, USA, 2000; p 159. (10) (a) Bonnauron, M.; Saada, S.; Rousseau, L.; Lissorgues, G.; Mer, C.; Bergonzo, P. Diamond Relat. Mater. 2008, 17, 1399. (b) Bonnauron, M.; Saada, S.; Mer, C.; Gesset, C.; Williams, O. A.; Rousseau, L.; Scorsone, E.; Mailley, P. Phys. Status Solidi A 2008, 205, 2126. (c) Carabelli, V.; 7442
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