Electrochemical fabrication and electrocatalytic characteristics studies of gold nanopillar array electrode (AuNPE) for development of a novel electrochemical sensor

Electrochimica Acta 53 (2007) 720–728

Electrochemical fabrication and electrocatalytic characteristics studies of gold nanopillar array electrode (AuNPE) for development of a novel electrochemical sensor Chunmee Shin a , Woonsup Shin b,1 , Hun-Gi Hong a,∗,1 a

b

Department of Chemistry Education, Seoul National University, Seoul 151-742, South Korea Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, South Korea Received 23 March 2007; received in revised form 19 July 2007; accepted 21 July 2007 Available online 26 July 2007

Abstract Gold nanopillar array electrodes were prepared by electrochemical deposition of gold into the nanopores of anodic aluminum oxide membrane placed onto the gold thin film electrode surface, which was in advance modified with cysteamine self-assembled monolayer as an anchoring layer. The Au nanopillar electrode is electrochemically stable and consists of highly dense, upstanding pillars assembled on the cysteamine monolayer. The structural morphology and chemical composition of the nanoarray electrode was characterized by field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Electrochemical measurements indicate that the Au nanopillar electrode possesses high electrocatalytic activities in the reduction of hydrogen peroxide and molecular oxygen, especially in glucose oxidation due to its higher electroactive surface area. The electro-oxidation studies of several electroactive neurotransmitters demonstrate that the nanopillar electrode can be utilized as a promising material for the construction of novel electrochemical sensor. © 2007 Published by Elsevier Ltd. Keywords: Anodic aluminum oxide membrane; Gold nanopillar array electrode; Electrocatalytic behavior; Electrochemical deposition; Neurotransmitter detection

1. Introduction Since nanoporous anodic aluminum oxide (AAO) membrane was used as a template [1], the fabrication of nanostructures of various materials has received much attention for fundamental scientific researches as well as practical applications such as nanoelectronic devices [2], tunable photonic materials [3], magnetic recording devices [4], biomaterial separation membrane [5], and electrochemical biosensors [6]. The AAO template possesses well-defined inner structures like uniform length, diameter, and pore density of porous nanometer-size channels which can be controlled [7,8]. Therefore, the template method has been widely used to prepare nanostructured materials such as nanopillar [9], nanotubule [6,10], nanowell [11], and nanowire [12] composed of metals. In order to prepare metallic nanostructure within the nanoporous membrane, elec-

∗ 1

Corresponding author. Tel.: +82 2 880 9115; fax: +82 2 889 0749. E-mail address: [email protected] (H.-G. Hong). ISE member.

0013-4686/$ – see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.07.040

trochemical deposition of metal at constant current or potential has been employed [11,13] onto metallic conductive layer, as a cathode, thermally evaporated on one side of the AAO membrane. After the electroplating, the AAO membrane was easily removed in a NaOH solution to expose two-dimensional periodic arrays of metallic nanostructure. From the view of electrochemistry, the nanostructure array electrode seems to be an attractive candidate as a novel electroanalytical sensing device for studies of electrochemical detection and electrocatalytic behavior [14,15]. For example, Martin and Wirtz [12] showed that Au nanodisk electrode ensemble would be useful in the ultra trace detection of electroactive species due to its enhanced signal-tobackground ratio at low concentration of supporting electrolytes. Recently, Chen and coworkers reported a novel method for direct electrochemical fabrication of platinum nanopillar array on gold substrate using AAO as template and cysteamine as a molecular anchor without sputtered metal as an electric conductor [9]. In this approach, the Pt nanopillar array was directly grown onto an Au surface modified with a self-assembled monolayer (SAM) of cysteamine. The morphology of the resulting array electrode shows individually well-separated Pt nanopillars

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after chemical removal of AAO template. Most of the research works reviewed [1,16] up to date have been mainly focused and published on the preparation and fabrication of nanostructured materials using highly ordered porous alumina membranes as templates. However, there have been relatively less efforts to study electrochemical properties of the nanostructured material as a nanosized electrode array. Only a few studies [6,17] on electrocatalytic behavior and electrochemical selective detection of biological molecules have been reported using noble metal nanotubular array electrodes. In this work, we have prepared a noble gold nanopillar array electrode (AuNPE) onto the gold thin film electrode coated with a SAM, modifying the method described by Chen and coworkers [9], and probed to extend its potential electrochemical application. The surface structure and composition of the Au nanopillar array electrode was characterized by field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive XPS (EDX), and X-ray diffraction (XRD). Electrochemical measurements were performed to evaluate the characteristics of the Au nanopillar array electrode. The results from the characterization show that the nanopillar electrode can be useful as an electrochemical sensor for detection of chemical and biological compounds due to its higher surface area.

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The dried and pretreated gold thin film electrode was soaked for 12 h in a 20 mM ethanol solution of cysteamine at room temperature in darkness. The electrode was washed with ethanol first, then rinsed with deionized water thoroughly to remove physically adsorbed cysteamine, and dried with nitrogen gas. 2.3. Preparation of AuNPE As shown in Fig. 1, the electrochemical deposition of gold into the pores of a microporous AAO membrane was performed in a home-made three-electrode electrochemical cell containing 5 mM HAuCl4 /0.5 M H3 BO3 electrolyte solution. As a working electrode, the cysteamine-monolayer modified Au thin film electrode was first wet with the electrolyte solution, then completely covered with an AAO membrane. The brittle membrane was softly pressed close between a silicone O-ring and the Au electrode with two holding clips, mounted on the electrochemical cell. Gold was electrochemically deposited into the pores of the AAO membrane at a constant applied potential of −0.45 V versus Ag/AgCl (sat’d KCl) for 1.0 h. Before direct electrochemical deposition, the assembly of the microporous membrane and the thin film electrode has been soaked in the electrolyte solution with stirring for at least 30 min to ensure maximal wetting through micropores in the membrane to the cysteamine-

2. Experimental 2.1. Materials and chemicals AAO disc membranes (60 ␮m thickness, 100 nm pore diameter on filtering face) were purchased from Whatman Corp. HAuCl4 , H3 BO3 , d-(+)-glucose, cysteamine, l-ascorbic acid, uric acid, acetamidophenol, and dopamine were purchased from Sigma–Aldrich and were used as received. All other chemicals used were reagent grade. Water purified with a Nano Pure System (Barnsted) to a resistivity of 18.2 M was used to prepare all aqueous solutions. 2.2. Preparation of Au thin film electrode and cysteamine-monolayer deposition Gold was sputtered onto borosilicate glass slides (1 in. × 3 in.) using an aluminum mask with seven uniformly spaced circular holes (3 mm diameter) and narrow slits for electric contact. The slides were precleaned by immersing them in a hot sulfuric acid solution for 30 min, rinsing with copious amounts of deionized water and drying prior to transfer to the sputtering chamber. The glass was etched first with Ar plasma for 30 s; an approximately 30 nm thick layer of titanium was deposited, followed by a ca. 100 nm layer of gold. Prior to deposition of cysteamine monolayer, these thin gold film electrodes were cleaned in hot 30% H2 O2 /98% H2 SO4 (volume ratio 1:5) for 10 s, and were cycled electrochemically between – 0.20 and 1.20 V versus Ag/AgCl (sat’d KCl) in 0.5 M H2 SO4 until the typical cyclic voltammogram of a clean gold was obtained. All pretreatments were followed by rinsing with deionized water and drying in a stream of nitrogen.

Fig. 1. Schematic view of a home-made electrochemical cell for gold electrodeposition through the micropores of AAO membrane onto Au thin film electrode modified with cysteamine monolayer: (a) AAO membrane; (b) Au thin film electrode; (c) clip; (d) silicon O-ring seal; (e) electrochemical cell; (f) Ag/AgCl (KCl sat’d) reference electrode; (g) Pt counter electrode; (h) magnetic stirring bar.

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monolayer on the Au electrode. According to the report [9], this wetting is known to result in more even gold deposition into the interspace between the AAO membrane and the electrode surface. After gold particles deposition, the AAO membrane was removed in a 2 M NaOH solution for 2 h to expose a freestanding gold nanopillar array electrode. Finally, the nanopillar electrode was electrochemically cleaned by continuous potential cycling between −0.2 and 1.2 V versus Ag/AgCl in 0.5 M H2 SO4 solution until the typical cyclic voltammogram of gold was obtained. In this work, the roughness factor of the AuNPE was calculated from the ratio (area of AuNPE/area of bare Au thin film electrode) of areas under the Au oxide reduction peak of the cyclic voltammograms observed at both electrodes (scan rate, 0.1 V s−1 ) in 0.5 M H2 SO4 solution. 2.4. Instrumentation All electrochemical measurements were performed with an electrochemical analyzer (CH Instruments, Bipotentiostat 760B, and BAS 100B/W). Cyclic voltammetry experiments were conducted in a single compartment Pyrex cell with a keyhole-shaped bare Au (5 mm diameter, gold sputtered glass substrates) or Au nanopillar array electrode as working electrode, a platinum wire counter electrode, and a Ag/AgCl (sat’d KCl) reference electrode. Gold and titanium sputtering was carried out by a metal sputter system (MHS-1500, Muhan Vac. Tech. Co., Korea). FESEM images were recorded with a JSM 6700F microscope (JEOL). Surface analysis of the electrodes was performed by X-ray photoelectron spectroscopy (XPS) on Sigma Probe (ThermoVG, U.K.). The X-ray diffraction patterns were recorded using a Bruker D8 Discover (Germany) diffractometer. 3. Results and discussion 3.1. Fabrication of AuNPE and its electrochemical stability The simple method of direct electrochemical fabrication of platinum nanopillar array on Au electrode surface has been reported by Chen and coworkers [9] using AAO as template and cysteamine monolayer as a molecular linker. This approach has shown a remarkable construction of Pt nanopillar array structure in which each Pt nanopillar is individually separated between them on Au electrode surface after removal of the template. We have modified this method using Au-sputtered thin film on glass instead of a gold disc electrode. As shown in Fig. 1, the use of the keyhole-shaped Au-sputtered thin film electrode makes it easy not only to access the microporous AAO membrane but also to be mounted on the home-made electrochemical cell with clamps and O-ring seal. Fig. 2 shows the fabrication procedure to prepare AuNPE on the self-assembled monolayer (SAM) of cysteamine formed on Au thin film electrode. In the step (b), the Au thin film electrode (shown in the step (a)) was modified with cysteamine SAM, which was based on the formation of Au S bond. SAM of alkanethiolates on gold [18] have been used as a powerful method to prepare a chemical interface which is a stable and structurally well-defined monolayer with a controllable thickness and desirable function. Especially, cysteamine of SAM

Fig. 2. Schematic procedure for electrochemical fabrication of Au nanopillar array electrode: (a) Au-sputtered thin film electrode; (b) cysteamine selfassembled monolayer on Au thin film electrode; (c) AAO membrane mounted on cysteamine SAM; (d) Electrochemical deposition of Au through nanopores of the membrane; (e) Au nanopillar electrode after removal of the membrane.

has been extensively used as a prime layer on Au for fabrication of a variety of electrochemical biosensors [19]. In the step (c), the monolayer wetted with the electrolyte solution of AuCl4 − was covered with the AAO membrane. Chen and coworkers [9] reported that wetting the Au electrode surface before covering with the membrane was effective to even gold deposition between the SAM surface and the membrane, resulting in a gold base plate which interconnects Au nanopillars with each other through subsequent deposition. Recently, Ohsaka and his coworkers reported that Au nanoparticles are strongly adsorbed on the amine-terminated organic surface of cysteamine SAM on polycrystalline gold electrode [20]. These facts demonstrate that cysteamine plays a role of good molecular anchor based on the strong affinity of thiol- and amine-functionalities for Au. In addition, it maintains good mechanical contact between the electrode surface and the template. After the gold deposition and chemical etching of the template, the obtained AuNPE was used to investigate its electrochemical stability using cyclic voltammetry. The electrode did not show any detectable change in cyclic voltammogram for 2 h during continuous potential cycling with scan rate of 100 mV s−1 between −0.2 and 1.2 V versus Ag/AgCl in 0.5 M H2 SO4 solution. We believe that the stability is attributable to the properties of cysteamine as a strong molecular anchor. 3.2. Surface analysis of AuNPE Usually, the two sides of a commercially available AAO membrane have different pore diameters. According to manufacture’s data, the filtration side has about 100 nm of pore diameter for filtering purpose. During the gold deposition, the reverse side of the membrane was in contact with the amineterminated surface of Au thin film electrode because it provides more regular spacing to give nanopillar structure as a template. Fig. 3a shows typical FE-SEM image of the reverse side of the AAO membrane pores in which the diameter of the pore is ca.

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Fig. 3. FE-SEM images of top-view of porous AAO membrane (a), top-view (b), side-view (c), EDAX spectrum (d), and XPS spectrum (e) of as-prepared Au nanopillar array electrode. Scale bar = 1 ␮m. Inset in (e) is a X-ray photoelectron spectrum of Au4f.

200–260 nm. The micropores present relatively uniform pore size. Fig. 3b shows a FE-SEM image of top view of the AuNPE, after removal of the template, deposited on the cysteamine-SAM on Au thin film electrode. From the top view, most of Au nanopillars present relatively even pillar diameter of ca. 137 ± 22 nm. However, some nanopillars lengthier than the average size, were observed in the micrograph. The lengthy Au nanopillars seemed to be detached from the nanopillar array electrode in the process of dissolving away the template in 2 M NaOH solution. The average length (ca. 430 nm) of the grown nanopillars can be estimated from the cross-sectional FE-SEM image of the Au nanopillar electrode in Fig. 3c. From the side view image, two layers were observed on the top of the Si substrate. The first layer from the bottom was the gold substrate sputtered on the Si plate; the second layer was the grown Au nanopillar array electrode. EDX spectrum (Fig. 3d) revealed that only gold was present in the second layer consisted of nanopillar array electrode. X-ray photoelectron spectroscopy (XPS) was hired to probe the valence of Au nanopillar structure. As calculated on the base of the binding energy of C1s at 284.5 eV, the XPS spectrum (Fig. 3e) shows two double peaks (shown

in the inset) located at 84.0 and 87.7 eV due to Au4f7/2-5/2 ; at 334.0 and 353.0 eV due to Au4d5/2-3/2 representing the elemental Au(0) [21]. 3.3. X-ray diffraction study of AuNPE Fig. 4 shows typical X-ray diffraction patterns of as-prepared Au nanopillar array and gold thin film electrodes. The bare Au thin film electrode (curve a) represented only one XRD peak at 2θ degree of 38.2 due to the sputtered gold film on glass substrate. This peak is the characteristic of the face-centered cubic (fcc) polycrystalline Au structure with a preferential crystallite orientation of Au(1 1 1). The XRD patterns of the Au nanopillar array electrode (curve b and c) due to time course of gold electrodeposition at −450 mV versus Ag/AgCl were assigned to the (2 0 0), (2 2 0), and (3 1 1) reflections of the Au nanopillar structure, corresponding to at the diffraction angle of 44.4, 64.6, and 77.5, respectively. Although the intensities of the diffraction peaks are quite smaller than that of (1 1 1) peak, the pattern shows the same reflection characteristics as Au nanowires [22] synthesized by chemical reduction of gold seed particles via seed-mediated

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Fig. 4. X-ray diffraction patterns of a bare Au thin film electrode (a), Au nanopillar array electrode obtained after the electrochemical deposition of 30 min (b) and 1 h (c) at the potential of −0.45 V vs. Ag/AgCl (sat’d KCl) in 5 mM HAuCl4 /0.5 M H3 BO3 solution.

growth process and as Au nanotubules [23] fabricated by direct evaporating gold onto porous AAO film. 3.4. Electrochemical characterization of AuNPE Electrochemical properties of the prepared AuNPE have been mostly investigated by cyclic voltammetry and compared with

those of a bare Au disk electrode having the same geometric area (0.196 cm2 ). Fig. 5a shows the electrochemical responses observed at the both electrodes are similar in 0.5 M H2 SO4 . However, the reduction peak area at the AuNPE was larger than that at the bare Au electrode, indicating the higher electroactive surface area of the AuNPE. In order to estimate the electrode surface area, we used the value of 450 ␮C cm−2 reported [23,24] for reduction of surface gold oxide peak at about 0.9 V versus Ag/AgCl. The conversion factor value was theoretically calculated for the reduction of a monolayer of chemisorbed divalent oxygen on a polycrystalline gold surface. Guo et al. [23] used this value to estimate the electrode surface area of nanoarchitectured Au thin film having high dense hollow nanotubule arrays upstanding on gold surface. This conversion factor provides good estimation of surface area for nanostructured polycrystalline Au film because the value was basically obtained for polycrystalline gold surface. The faradaic charge estimated by integration of the reduction peak area shows that electroactive surface area of the Au NPE is 4.6 times higher than that of the bare Au electrode. Using reversible redox couples of Fe(CN)6 3−/4− and Ru(NH3 )6 2+/3+ , the diffusion-controlled characteristics at the AuNPE, the bare Au, and the cysteamine modified Au electrode has been studied in 0.1 M phosphate buffer solution. As shown in Fig. 5b and c, the diffusional redox peak potentials of the two redox couples at the AuNPE were almost the same as those at the bare Au electrode within an experimental error of ca. ±5 mV. This fact demonstrates that both electrodes follow the same electron transfer kinetics, indicating that the AuNPE behaves like a macroelectrode with a large surface area. Here, it is worthwhile to note that the AuNPE do not exhibit a sigmoidal current–potential response, which is well-known characteristics of radial diffusion at an ultramicroelectrode. In contrast, the AuNPE shows typical voltammograms

Fig. 5. Cyclic voltammograms at Au nanopillar array electrode (solid line), bare Au thin film electrode (dashed line), and Au film electrode modified with cysteamine monolayer (dotted line). Electrolyte solution in 0.5 M H2 SO4 only (a); 0.1 M PBS (pH 7.0) containing 1 mM Fe(CN)6 3−/4− (b) and 1 mM Ru(NH3 )6 2+/3+ (c). All scan rates: 0.1 V s−1 .

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due to planar diffusion of solution redox probes. This phenomenon is due to the overlapping of the diffusion layer at each nanopillar being too closely spaced each other in the AuNPE as shown in Fig. 3c. The larger (4.6 times) electrode surface area at the AuNPE results in 1.4 and 1.1 times higher redox peak currents for Fe(CN)6 3−/4− and Ru(NH3 )6 2+/3+ at the bare Au electrode, respectively. This is consistent with the fact that the diffusion-controlled reaction rate is not enhanced much by the increased surface area of the AuNPE. As shown in Fig. 5b and c, the large background current observed at the AuNPE is attributed to its large exposed surface area. This electrochemical property has been also reported in the study of nanoarchitectured metal film electrode with high electroactive surface areas [23]. The cysteamine-modified Au electrode also exhibits reversible voltammetric responses for the solution redox couples. There are noticeable decreases in the redox peak current due to the self-assembled monolayer of cysteamine as a barrier. However, the electron transfer of solution redox probes at electrode surface does not get slow as expected because the monolayer has very short chain and many structural defects, resulting in almost no change in the peak-to-peak separation (Ep ) for the redox couples. The cyclic voltammetric responses for the reduction of hydrogen peroxide and oxygen were examined in the phosphate buffer solution (pH 7.0). As shown in Fig. 6a, the irreversible cathodic peak at AuNPE in H2 O2 reduction was observed at −0.26 V, which is more positive by 0.35 V than that at bare Au occurring at −0.61 V. In addition, the peak current at the AuNPE is also larger than that at the bare Au. These voltammetric behaviors have been also observed in the reduction of molecular oxygen dissolved in the buffer solution as shown in Fig. 6b. Each CV at AuNPE and bare Au electrode shows two well-separated reduction peaks: the first peak is due to the two-electron reduction of O2 to H2 O2 at −0.06 and −0.25 V, and the second one is attributed to the twoelectron reduction of the electrogenerated H2 O2 to H2 O at −0.29 and −0.56 V, respectively. The positive potential shift observed in the two consecutive reduction processes can be explained by electrocatalytic effect of the nanopillar electrode. Ohsaka and

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his coworkers [25–27] have reported the similar electrochemical responses in O2 reduction at glassy carbon electrode on which Au nanoparticles were electrochemically deposited. This electrocatalytic activity of the nanostructured or nanoparticlebased electrode over their bulk counterparts is believed to be originated from quantum-sized dimension [28]. 3.5. Electrochemical oxidation of glucose at bare Au electrode and AuNPE For development of novel amperometric glucose sensor, electrode material possessing high electrocatalytic activity has been sought by several research groups using nanostructured Pt or Au [29–32]. In order to investigate electrocatalytic effect of AuNPE, electrochemical oxidation of glucose has been studied in a deaerated 0.1 M phosphate buffer solution (pH 7.0) at a scan rate of 0.1 V s−1 and compared with that at bare Au electrode. For comparison, background voltammetric responses of the supporting electrolyte are also displayed in dashed line. The glucose oxidation at bare Au electrode starts at ca. 0.1 V and shows a single small oxidation peak around 0.3 V as shown in Fig. 7a. However, the glucose oxidation at AuNPE begins at approximately −0.25 V and exhibits two oxidation peaks, one shoulder peak at about 0.1 V and the other large broad peak at ca. 0.3 V as shown in Fig. 7b. There is a substantial difference in the potential of beginning glucose oxidation process. The oxidation of glucose takes place at lower potential by 350 mV at AuNPE than at bare Au electrode. This negative oxidation potential shift indicates that AuNPE catalyzes the oxidation of glucose. In addition, the oxidation peak current observed at 0.3 V is about four times larger at AuNPE than at bare Au electrode. The oxidation current at both electrodes rapidly decreases as the electrode potential moves toward more positive direction than their oxidation peak potentials. This observation might be ascribed to the formation of higher Au-oxides (Au2 O3 ), resulting in blocking the electrocatalytic oxidation [29]. However, the oxidation current is quickly restored in the backward potential scan due to the reduction of higher Au-oxides formed in the positive scan.

Fig. 6. Cyclic voltammograms at Au nanopillar array electrode (solid line) and bare Au thin film electrode (dashed line). Electrolyte solutions: (a) 0.1 M PBS (pH 7.0) containing 1 mM H2 O2 ; (b) 0.1 M PBS saturated with O2 (30 min purging). All scan rates: 0.1 V s−1 .

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Fig. 7. Cyclic voltammograms at bare Au thin film electrode (a) and Au nanopillar array electrode (b) in 0.1 M PBS (pH 7.0) containing 15 mM glucose. Dashed line: background CV responses. All scan rates: 0.1 V s−1 .

3.6. Voltammetric behavior of biological compounds at AuNPE Fig. 8 shows typical cyclic voltammograms of several biologically important compounds obtained at AuNPE and bare Au

electrode in 0.1 M phosphate buffer solution. Generally, these electroactive compounds coexist in biological fluids such as blood and urine. Therefore, it is sometimes difficult to monitor one compound selectively due to the interferences from other electroactive constituents because they oxidize at electrode potentials close to each other. For instance, the voltammetric detection of dopamine or uric acid in the biological samples needs more sensitive and selective methods because the concentration of interfering ascorbic acid is relatively higher than that of the neurotransmitters. As shown in Fig. 8a and b, direct oxidation of dopamine and p-acetamidophenol at bare Au electrode undergoes quasi-reversible, diffusion controlled reaction. Their Ep values of ca. 75 and 375 mV observed at bare Au electrode are significantly greater than those values of 61 and 112 mV observed at AuNPE, respectively. Their redox peak currents also increase more at AuNPE than at the bare Au electrode. This improvement in reversibility and peak current demonstrates that the AuNPE of a high real surface area (roughness factor, 7.1 ± 1.1) can make electron transfer kinetics faster and enhance electrochemical response for kinetically slow reaction. For example, the oxidation of ascorbic acid and uric acid is typically irreversible and kinetically controlled sluggish reaction as shown in Fig. 8c and d. As expected, their oxidation peak potentials also shifted toward negative direction by ca. 40–70 mV at the AuNPE than at the bare Au electrode. These voltammetric characteristics of biological compounds observed at the AuNPE can be useful for the development of a novel nonenzymatic glucose sensor. As shown in Fig. 8, the oxidation peak potential of the interfering substances such as ascorbic acid, acetamidophenol, and uric acid is 0.05, 0.43, and 0.50 V, respectively. These potentials are quite distant from the glucose oxidation peak potential (ca. 0.3 V) observed at the AuNPE. This peak potential discrimination from those of interfering species

Fig. 8. Cyclic voltammograms at Au nanopillar array electrode (solid line) and bare Au thin film electrode (dashed line) in 0.1 M PBS (pH 7.0) containing 1 mM (a) dopamine (DO); (b) p-acetamidophenol (AP); (c) ascorbic acid (AA); (d) uric acid (UA). All scan rates: 0.1 V s−1 .

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might be important in preparation of highly selective amperometric glucose sensor because it allows us to make better choice of working potential. However, it should be emphasized that any current measurement at the working potential of an amperometric glucose sensor would include some faradaic current contributions from the interfering substance when the substance oxidizes at less positive potential than that of glucose. As a remedy to this problem, Park et al. [15] reported that mesoporous platinum film structure with high roughness factor (ca. 72) responds more selectively and sensitively to glucose in the presence of common interfering species like ascorbic acid and acetaminophenol. It would be expected that the improvement in roughness factor for an AuNPE greatly decreases interfering oxidation of the species because kinetically slow oxidation of glucose produces high faradaic current due to high surface area of an AuNPE with high roughness factor. Fig. 9 shows cyclic voltammograms obtained at bare Au electrode and AuNPE in the phosphate buffer solution containing equimolar concentration (0.5 mM) of dopamine and ascorbic acid. As shown in Fig. 9a, the bare electrode presents only one broad oxidation peak at 0.23 V versus Ag/AgCl for both analytes, indicating that the voltammetric discrimination of the two components is not possible. The single voltammetric peak observed in the mixture of the two analytes at bare Au electrode has been reported by Raj et al. [32] that it resulted from the fouling of the electrode surface by the oxidation products and the catalytic oxidation of ascorbic acid by the oxidized dopamine. However, the one broad oxidation peak observed at the bare Au electrode was separated into two well-developed voltammetric peaks at AuNPE. The peak at 0.04 and 0.24 V versus Ag/AgCl is attributed to the oxidation of ascorbic acid and dopamine, respectively. This large peak potential difference of ca. 200 mV can make it feasible to do simultaneous determination of dopamine and ascorbic acid at the AuNPE. There is one thing to be noted that the oxidation peak (0.08 V) of ascorbic acid at the bare Au electrode (shown in Fig. 8c) is shifted to 0.04 V at the AuNPE (shown in Fig. 9b). This negative potential shift is electrochemically advantageous to discriminate voltammetric signal of dopamine oxidation in the presence of ascorbic acid

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because the oxidized dopamine does not catalyze the oxidation of ascorbic acid [20]. This fact demonstrates that the AuNPE can be used as an alternative for selective detection of dopamine in the presence of ascorbic acid. 4. Conclusions We have prepared Au nanopillar array electrode on the Au thin film electrode modified with cysteamine self-assembled monolayer by electrochemical deposition using nanoporous AAO membrane as a template. The surface morphology and chemical composition of the AuNPE have been revealed by FESEM, EDX, XRD, and XPS. The as-prepared Au nanopillar electrode has shown highly dense Au nanopillar arrays upstanding on the cysteamine-monolayer and exhibited that most of the nanopillars are relatively uniform in their height and diameter. Cyclic voltammetry measurements show that this electrode is electrochemically quite stable and reveals much larger electroactive surface area compared with that of bare Au thin film electrode. As expected, the AuNPE showed high electrocatalytic activity not only in the reduction of hydrogen peroxide and molecular oxygen but also in the oxidation of glucose due to its nano-sized pillar array structure. Furthermore, the discrimination between the voltammetric signals of dopamine and ascorbic acid can be observed at the AuNPE in the electro-oxidation studies of several neurotransmitters. The voltammetric characteristics of biological compounds at the AuNPE demonstrate that the glucose oxidation can be separable from the oxidation of interfering substances such as ascorbic acid, acetamidophenol, and uric acid. However, it is still required to suppress the interfering oxidation signals from those species in order to develop a novel nonenzymatic glucose sensor. Currently, we are investigating the effect of electrode surface roughness factor on the electrocatalytic activity for the development of a novel nonenzymatic sensor. Acknowledgements This work was supported by grant no. R01-2004-000-109880 from the Basic Research Program of the Korea Science & Engineering Foundation. References

Fig. 9. Cyclic voltammograms at Au nanopillar array electrode (solid line) and bare Au thin film electrode (dashed line) in 0.1 M PBS (pH 7.0) containing equimolar concentration of 0.5 mM dopamine and ascorbic acid. All scan rates: 0.1 V s−1 .

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