Ordered array of diamond ultramicroband electrodes for electrochemical analysis

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Diamond & Related Materials 17 (2008) 240 – 246 www.elsevier.com/locate/diamond

Ordered array of diamond ultramicroband electrodes for electrochemical analysis K.L. Soh a , W.P. Kang b,⁎, J.L. Davidson b , Y.M. Wong b , D.E. Cliffel c , G.M. Swain d a b

Interdisciplinary Program in Materials Science, Vanderbilt University, Nashville, TN 37235, USA Department of Electrical and Computer Engineering, Vanderbilt University, Nashville, TN, USA c Department of Chemistry, Vanderbilt University, Nashville, TN, USA d Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA

Received 14 December 2006; received in revised form 30 November 2007; accepted 6 December 2007 Available online 15 December 2007

Abstract Boron-doped diamond ultramicroband arrays with different array densities and interelement spacings were fabricated using silicon technology and selective diamond deposition (SAD) technique to yield microvoltammetric electrodes. The electroactive ultramicroband elements were designed with one microscopic critical dimension to impart microelectrode behavior while the other dimension was made larger to yield an increase in signal current. Cyclic voltammetry studies in this work showed that with sufficient interelement separation, the ultramicroband arrays display sigmoidal pseudo-steady-state cyclic voltammograms characteristic of microband electrodes. The ultramicroband arrays yielded higher faradaic current per unit area, than either square ultramicroelectrode array or conventional planar diamond electrode from earlier reported work. This is due to enhanced mass transport to the ultramicroband elements at slow scan rates. Larger current density and higher signal-to-noise (S/N) ratio leads to better limits of detection, making it possible to fabricate a more sensitive electrode for applications such as electroanalysis, electrocatalysis, trace element analysis, mechanistic and fast transfer kinetics studies, electrochemistry in highly resistive media, as well as sensors in flow and biological system. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond ultramicroelectrode array; Ultramicroband array; Electrochemical analysis; Higher current density; Non-linear diffusion

1. Introduction Diamond has been shown to possess numerous attractive features for use as an electrode material in electroanalytical chemistry. Planar boron-doped diamond electrodes have been reported to exhibit low background current, stable and reproducible response over a long time period [1]. Large faradaic current in conjunction with reduced background current results in better S/N ratio in comparison with other carbon allotropes such as glassy carbon and highly-oriented pyrolytic carbon (HOPG) [2–4]. The

⁎ Corresponding author. Vanderbilt University, VU Station B 351661, Nashville, TN 37235-1661, USA. Tel.: +1 615 322 0952; fax: +1 615 343 6614. E-mail address: [email protected] (W.P. Kang). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.023

absence of carbon-oxygen functionalities at the surface of goodquality, hydrogen-terminated diamond electrode discourages fouling and also confers large overpotential for electrolysis of electrolyte [5,6]. This latter feature had been found to give rise to a wider working potential window, enabling its application in the electrochemical studies of a wide number of analytes [4,6,7]. All these attractive electrochemical properties of borondoped diamond electrodes provide the motivation to utilize this material in microlithographic fabrication of microelectrodes. Microelectrodes are smaller than normal electrodes and exhibit sigmoidal voltammetric curves rather than the conventional peak behavior of macroelectrodes. The peak behavior of macroelectrodes is the result of linear diffusion-limited transport of analytes resulting in a diffusional depletion gradient, whereas, the sigmoidal voltammogram of microelectrodes is

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total geometrical area of the microelectrode array (both electroactive and insulative area), the noise is only proportional to the total area of the active elements. The noise becomes smaller with decreasing active area size. This type of enhanced microelectrode behavior takes place only when the diffusion layer of each element is bigger than the area of the element itself and has a hemispherical profile but before it is large enough to converge to form a continuous double layer over the area of the entire array. In other words, at very short time interval, when the diffusion layer is thin, linear diffusion will be the predominant form of analyte transfer to the individual active area (Fig. 1 (a)), thus, peak-shaped behavior can be observed. At slightly longer time, the diffusion layer will enlarge and becomes more hemispherical (Fig. 1 (b)). Due to its small size, mass transport to the element is enhanced, aided by edge effect, resulting in the

Fig. 1. Schematic of diffusion profile.

due to steady-state diffusion to the microscopic area. When the size of the electroactive area is decreased, the mass transport to the active element will be enhanced due to hemispherical diffusion, thus, causing the microelectrode to approach a steady-state limiting current as evidenced by a sigmoidal profile [8–12]. The limiting signal current from a single microelectrode element will be small since current is proportional to electrode area. The eventual commercial application of such an electrode may require additional signal processing or instrumentation to filter out noise and interference. In this work, we report the fabrication and characterization of boron-doped diamond ultramicroelectrode arrays (D-UMEAs) with ultramicroband elements. The increased number of electroactive elements in an array will yield larger total signal current as opposed to that of a single element microelectrode, giving rise to a more practical current level. With sufficient interelement spacing, an ordered array of microelectrodes can improve the merits of detection such as S/N ratio, sensitivity and limits of detection. This is possible because while the faradaic current is proportional to the

Fig. 2. Fabrication processes for rD-UMEAs.

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hemispherical, quasi-steady-state diffusion profile. It is during this time interval where sigmoidal curve and microelectrode behavior is observed. At longer time, if the interelement spacing is insufficiently large, the overlap of diffusion layer will occur (Fig. 1 (c)). The predominant type of analyte transport will again become linear diffusion-limited in nature,

to the entire array of microlectrodes, obliterating the effect of the microelectrode structure and cause the entire geometrical area of the microelectrode to behave effectively like a macroelectrode. This will result in the loss of mass transport enhancement, mitigating the advantages of using a microelectrode array [8,10–17].

Fig. 3. SEM micrograph of (a) r20D-UMEA, (b) r100D-UMEA, (c) r20D-UMEA3, and (d) r100D-UMEA3.

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This work presents ultramicroelectrode arrays with two different interelement spacings (20 µm and 100 µm) and two different densities (spatial dispersion). The rectangular geometry, known as microband was selected for this study. The width was maintained at a dimension equivalent to a square ultramicroelectrode element's dimension in earlier reported work [18], while the length is larger than the width, designed with the purpose of further increasing the signal current in addition to using an array. Throughout this paper, the higher density ultramicroband array electrodes will be referred to as r20D-UMEA and r100D-UMEA, with the prefix ‘r’ denoting the rectangle geometry, while the subscript 20 and 100 identifies the interelement spacing. The lower density ultramicroband array electrodes will be referred to as r20D-UMEA3 and r100D-UMEA3, the subscript ‘3’ indicating the three parallel ultramicroband array. The square D-UMEA from the earlier work will be referred to as s20D-UMEA and s100D-UMEA. 2. Experimental 2.1. Fabrication of rD-UMEAs The fabrication processes for the diamond ultramicroband array was outlined in Fig. 2. First, a 0.5 µm-thick oxide was thermally grown on n+ silicon substrate. Next, a 0.5 µm-thick molybdenum (Mo) was sputtered on top of the oxide layer. After standard photolithography and microfabrication were performed, the mold was sonicated in a diamond powder suspension. Diamond powder on the surface was then removed by photoresist (PR) lift-off before diamond deposition. The diamond growth was performed at temperature = 800 °C, pressure = 120 Torr, methane and hydrogen flow rate = 18/478 SCCM and microwave power = 5000 W. Boron doping was achieved using a gaseous dopant; trimethylboron (TMB) = 20 SCCM (standard cubic centimeter). Diamond deposition duration was 18 minutes. Lastly, the Mo layer was removed and the electrodes were washed with DI water. The fabrication of planar diamond films and square diamond ultramicroelectrode array, using these same diamond growth parameters has been previously reported [17,18]. 2.2. Diamond electrode characterization method Electrochemical measurements were performed in a singlecompartment, three electrode, cylindrical cell (50 ml) similar to that used by Granger et al. [7]. The Ag/AgCl (3 M) reference electrode and the platinum (Pt) wire counter electrode were from CH Instruments, Inc. K4Fe(CN)6•3H20 and KNO3 were from Fisher Scientific. All solutions were prepared with DI water (18 MΩ). The rD-UMEAs were rinsed with DI water prior to use without further surface pretreatment. Electrochemical measurements were made using the Electrochemical Analyzer (Model 620A) from CH Instruments, Inc. The supporting electrolyte was 0.1 M KNO3 and the analyte was 1 to 5 mM Fe(CN)64-. The working electrodes were clamped to the bottom of the cell. Electrical contact was made from the backside of the working electrode via copper tape.

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3. Results and discussions Fig. 3 shows the SEM of the four rD-UMEAs. The electroactive elements of these working electrodes sit in the middle of a 1 cm2 die. The number of individual ultramicroband elements and total active area for all four electrodes are in Table 1. The original photomask design for the individual element was 10 × 75 μm2. However, as the 0.5 μm thick SiO2 layer was wetetched using buffered oxide etch (BOE), the individual molds were enlarged due to BOE undercutting the area unmasked by Mo and PR. The cyclic voltammograms (CVs) for the redox reaction of 1 mM Fe(CN)64- in 0.1 M KNO3 at all four rD-UMEAs, using scan rates of 10 mV/s and 5 V/s are shown in Fig. 4. In general, all electrodes show quasi-steady-state sigmoidal CV profiles at a slow scan rate of 10 mV/s but exhibited peak behavior at the faster scan rate of 5 V/s, indicating that linear diffusion-limited transport was occurring at the electrodes during short times. The r20D-UMEA electrode displayed peak behavior at a slower scan rate and behaved neither completely like a macro- nor a microelectrode. This may be because even though the width of the band is small enough to impart some degree of microelectrode behavior, the interelement spacing is too small to prevent overlap or interaction of diffusion layer at a long time scale, resulting in linear diffusion transport to the ultramicroband array, thus, the peak behavior. However, the r20D-UMEA3 with the same interelement spacing of 20 µm displayed sigmoidal behavior at long time scale, presumably because the lower density of electroactive ultramicrobands allowed enhancement of analyte flux to the much smaller array. Both r100D-UMEA and r100D-UMEA3 displayed good sigmoidal behavior at all scan rates, by virtue of sufficient interelement separation for the time scale of the experiment. The slope of the sigmoidal CVs at 10 mV/s for these rD-UMEAs ranged from 53 to 94 mV in comparison with the 56/n mVexpected for a reversible redox system at steady state [19,20]. The forward and reverse limiting currents have almost the same magnitude. The background current in 0.1 M KNO3 for all four ultramicroelectrode arrays were low, within a potential window of about 3.5 V. In summary, Fig. 4, (a), (b), (c) and (d), rD-UMEAs with the larger 100 µm interelement spacing consistently displayed quasi-steady-state behavior regardless of the density of the array, whereas, with a smaller interelement spacing of 20 µm, quasi-steady-state current can be achieved with a much lower density electrode due to enhanced mass

Table 1 Working electrode identifications, element dimensions on photomask, number of elements making up the relevant arrays, and the total active area of each electrode as per SEM Working lectrodes

Photomask element dimensions (μm)

Number of elements

Total active area (cm2)

rD-UMEA20 rD-UMEA100 r3D-UMEA20 r3D-UMEA100

10 × 75 10 × 75 10 × 75 10 × 75

33 × 26 10 × 14 3 3

0.012445118 0.000964319 0.000026966 0.000021958

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Fig. 4. Cyclic voltammograms for 1 mM Fe(CN)46 in 0.1 M KNO3 at (a) r20D-UMEA, (b) r100D-UMEA, (c) r20D-UMEA3, and (d) r100D-UMEA3. Scan rate: (i) 10 mV/s and (ii) 5 V/s.

transport to the r20D-UMEA3 electrode. Less hysteresis of the sigmoidal curve was observed with lower density electrodes (rD-UMEA3s) due to lower capacitive background current. The current limit at the ultramicroband electrodes was found to be scan rate dependent, consistent with other reported works [21–24]. As mentioned, the limiting currents approached quasisteady-state, non-linear diffusion at slow scan rates, but is affected by linear diffusion-limited transport at fast scan rate. Fig. 5 shows that the limiting currents obtained at slow scan rates were in good agreement with predicted results, as shown in Fig. 5. The curves with the solid data points were experimental results while the hollow data points were theoretical results calculated from this Eq. (24): 
 i1 ¼ 2pmnFDC1 1=ln 4Dtp2 =w2 ; with t = RT/Fν, where m is the number of individual elements, n is the number of electrons per mol of reaction, F is the Faraday

constant, D is the diffusion coefficient of the analyte, C is the bulk concentration of the analyte, l and w is the length and width of the microband element, respectively, and ν is the scan rate. The diffusion coefficient for Fe(CN)64- in this system was calculated to be 7.6 × 10- 6 cm2/s from earlier unpublished results using the Randles-Sevcik equation and CV data from a glassy carbon working electrode which had a diameter of 3 mm. This number was in good agreement with a value of D = 7.35 × 10- 6 cm2/s as reported in the handbook of chemistry and physics [25]. The effective electrochemically active area from a non-flat, multi-faceted, polycrystalline CVD diamond surface is not readily quantifiable, and the mechanism of electrical conduction is still under debate. Under such consideration, the diffusion coefficient of the analyte in this system was investigated and calculated using the CV data from a freshly polished, 3 mm diameter glassy carbon working electrode tested in the same system. The diffusion coefficient, D, is a function of temperature, pressure and is affected by molecular

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size and shape of the analyte. Since all conditions are preserved, it is fair to use the D value calculated from another standard conventional working electrode with a known surface active area, using the same system. There was ~ 2.4 fold difference between the predicted and the experimental value of the limiting currents for r20D-UMEA. This may be because the predicted values were modeled according to the steady-state hemicylindrical behavior but in actual fact this particular electrode was experiencing effects of linear diffusion-limited transfer which lacks the enhancement factor, thus lower experimental data were obtained. The predicted limiting currents from the equation above yielded higher values than experimental result at fast scan rates, for all four electrodes. This may be because the theory assumes enhanced flux of analyte due to non-linear mass transport for all

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Fig. 6. Graph of current density vs concentration of 1 mM Fe(CN)46 in 0.1 M KNO3 at various working electrodes. Scan rate = 10 mV/s.

scan rates when in reality it is the linear-diffusion mass transport that predominates at fast scan rate. The rD-UMEAs were tested for sensitivity and linearity for the voltammertric oxidation of Fe(CN)64- between a concentration range of 50 µM and 4.9 mM. The oxidation limiting current density was plotted against the concentration of Fe(CN)64-, and linear responses were obtained for all four rD-UMEAs, Fig. 6. Results for the square diamond ultramicroelectrode arrays (sDUMEAs), planar diamond film and glassy carbon from earlier works [17,18] were included for comparison. Fig. 6 shows that, in general, the rD-UMEAs with 100 μm interelement spacing had higher oxidation current density than the rest of the electrodes, with the r100D-UMEA3 possessing a steeper slope of the two due to enhanced mass transport to a smaller array area of the three parallel microbands. The plot for r20D-UMEA3 is comparable to that for s100DUMEA, even though the former has smaller interelement spacing. This again could be due to the fact that the former has a small array area with three ultramicrobands experiencing enhanced mass transport compared to the latter whose entire array of square elements comprised an area of about 0.1 × 0.25 cm2. In this case, the length of the ultramicroband geometry imparted a sufficiently large magnitude of limiting current at the r20D-UMEA3 while its microelectrode behavior was conserved by just one critical dimension, yielding an electrode which displayed current density comparable to that of s100D-UMEA. That said, the r20D-UMEA showed lower current density than its square counterpart, the s20D-UMEA. This may be because the larger length of the ultramicroband in concert with the small interelement spacing resulted in a more severe case of diffusion layer overlap, especially at slow scan rates. Thus, less mass transport enhancement to the r20D-UMEA results in lower current density. 4. Conclusion

Fig. 5. Graph of current vs log ν for 1 mM Fe(CN)46 in 0.1 M KNO 3 at (a) r20D-UMEA, (b) r100D-UMEA, (c) r20D-UMEA3, and (d) r100D-UMEA3. Hollow data points were theoretical values. Solid data points were from experimental results.

rD-UMEAs were fabricated with standard silicon microfabrication technology and boron-doped diamond were deposited via SAD technique. Electrochemical characterization shows that the electrodes conform to theoretical results at slow scan rates while deviation at high scan rate can be attributed to linear

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diffusion-limited transport. The r100D-UMEA3, r100D-UMEA, and r20D-UMEA3 displayed quasi-steady-state sigmoidal CVs, characteristic of microband microelectrode array while r20D-UMEA showed mixed behavior of macro/microelectrode. The electrodes exhibited useful linear voltammetric responses for the oxidation of Fe(CN)64-with the r100D-UMEA3 possessing the highest current density, followed closely by r100D-UMEA. Higher current density of the r100D-UMEAs leads to higher S/N ratio, thus improving limits of detection and sensitivity. Current density behavior for r20D-UMEA3 is comparable to that of s100D-UMEA, while r20DUMEA's current density fell below that of s20D-UMEA. All ultramicroelectrode arrays showed higher current density than the planar diamond electrode, and all diamond derived electrodes showed higher current density than conventional glassy carbon working electrodes. The advantages of boron-doped diamond as an electrode material have been applied to the concept of microelectrode array to yield voltammetric microelectrodes with higher current density for oxidation of Fe(CN)64-. These results suggest analytical applications of the rD-UMEAs in voltammetric/amperometric detection at slow scan rates. Electrocatalysis is also possible because the current amplication factor of a microelectrode array could make such processes more time and cost effective. Likewise, redox surface modification using microband arrays could yield good efficiency. However, because the microband geometry showed different diffusional profiles depending on factors such as width, length, interelement spacing, density and scan rate, care should be taken when intepreting the results obtained from different regimes of mass transport diffusion. Understanding the interplay of these factors will facilitate the fabrication of suitable microelectrodes for specific applications in electroanalysis [4–7], electrocatalysis [26–29], trace element analysis [30,31], mechanistic and fast transfer kinetics studies [32,33], electrochemistry in highly resistive media [34,35], as well as sensors in flow and biological system [36–39]. References [1] G.M. Swain, R. Ramesham, Anal. Chem. 65 (1993) 345–351. [2] J.W. Strojek, M.C. Granger, G.M. Swain, T. Dallas, M.W. Holtz, Anal. Chem. 68 (1996) 2031–2037. [3] R. DeClements, G.M. Swain, T. Dallas, M.W. Holtz, R.D. Herrick II, J.L. Stickney, Langmuir 12 (1996) 6578–6586. [4] S. Alehashem, F. Chambers, J.W. Strojek, G.M. Swain, R. Ramesham, Anal. Chem. 67 (1995) 2812–2821. [5] H.B. Martin, A. Argoitia, U. Landau, A.B. Anderson, J.C. Angus, J. Electrochem. Soc. 143 (1996) L133–L136. [6] J. Xu, M.C. Granger, Q. Chen, J.W. Strojek, T.E. Lister, G.M. Swain, Anal. Chem. 69 (1997) 591A–597A. [7] M.C. Granger, M. Witek, J. Xu, J. Wang, M. Hupert, A. Hanks, M.D. Koppang, J.E. Butler, G. Lucazeau, M. Mermoux, J.W. Strojek, G.M. Swain, Anal. Chem. 72 (2000) 3793–3804.

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