Electroanalysis Using Macro-, Micro-, and Nanochemical Architectures

Jul 29, 2006 - Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK, and. Materials Department ...
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Anal. Chem. 2006, 78, 6102-6108

Electroanalysis Using Macro-, Micro-, and Nanochemical Architectures on Electrode Surfaces. Bulk Surface Modification of Glassy Carbon Microspheres with Gold Nanoparticles and Their Electrical Wiring Using Carbon Nanotubes Xuan Dai,† Gregory G. Wildgoose,† Chris Salter,‡ Alison Crossley,‡ and Richard G. Compton*,†

Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK, and Materials Department, University of Oxford, Parks Road, Oxford, OX1 3PH, UK

Gold nanoparticles (∼30-60 nm in diameter) were deposited onto the surface of glassy carbon microspheres (10-20 µm) through electroless plating to produce bulk (i.e., gram) quantities of nanoparticle surface-modified microspheres. The gold nanoparticle-modified powder was then characterized by means of scanning electron microscopy and cyclic voltammetry. The voltammetric response of a macroelectrode consisting of a film of gold nanoparticle-modified glassy carbon microspheres, bound together and “wired-up” using multiwalled carbon nanotubes (MWCNTs), was investigated. We demonstrate that by intelligently exploiting both nano- and microchemical architectures and wiring up the electroactive centers using MWCNTs in this way, we can obtain macroelectrode voltammetric behavior while only using ∼1% by mass of the expensive gold material that would be required to construct the equivalent gold film macrodisk electrode. The potential utility of electrodes constructed using chemical architectures such as this was demonstrated by applying them to the analytical determination of arsenic(III) concentration. An optimized limit of detection of 2.5 ppb was obtained. Recently, nanometer-scale metals and semiconductor particles have attracted considerable attention due to their superior functional properties for a wide range of technological applications, including catalysis, optics, microelectronics, and chemical/biological sensors. One area that metal nanoparticles have been widely used in is the field of electrochemistry where metal nanoparticles have found applications in electroanalysis1,2 and bioelectroanalysiss especially DNA sensing,3 as well as in electrocatalysis.4,5 Invariably, * To whom correspondence should be addressed. Tel: 00441865 275413. Fax: 00441865 275410. E-mail: [email protected]. † Physical and Theoretical Chemistry Laboratory. ‡ Materials Department. (1) Herna´andez-Santos, D.; Gonza´lez-Garcı´a, M. B.; Costa Garcı´a, A. Electroanalysis 2002, 14, 1225. (2) Katz, E.; Willner, I.; Wang, J. Electroanalysis 2004, 16, 19. (3) Fang, Y.; Xu, Y.; He, P. J. Biomed. Nanotechnol. 2005, 1, 276. (4) Arvia, A. J.; Salvarezza, R. C.; Triaca, W. E. J. New Mater. Electrochem. Syst. 2004, 7, 133. (5) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182.

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for catalytic and electroanalytical applications, the metal nanoparticles must be supported, either on the electrode itself or on other conducting or semiconducting supports such as carbon nanotubes.5 A number of approaches to supporting gold nanoparticles on microscale supports can be found in the literature, the most predominant being the fabrication of gold nanoparticles supported on polymer microspheres.6-9 Other examples include electroless deposition onto carbon nanotubes,10-12 silica spheres,13,14 and stainless steel beads.15 Gold nanoparticles have also been deposited onto glassy carbon electrode substrates using a variety of electrochemical and chemical methods, e.g., for the simultaneous voltammetric determination of dopamine and ascorbic acid16 and for the detection of the arsenite anion.17-21 Although gold is one of the most inert of all metallic elements, gold nanoparticulate materials are currently attracting considerable interest for use as a heterogeneous catalyst, both in industry where these catalysts have been observed to be active at room temperatures and in academia where much is still unknown about the exact influence of gold nanoparticle size, morphology, and the support material on the mechanisms of gold catalysisswhich are (6) Cuendias, A.; Backov R.; Cloutet, E.; Cramail H. J. Mater. Chem. 2005, 15, 4196. (7) Li, S.; Yang, X.; Huang, W. Macromol. Chem. 2005, 206, 1967. (8) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 2002, 14, 2232. (9) Pastoriza-Santos, I.; Gomez, D.; Perez-Juste, J.; Liz-Marzan, L. M.; Mulvaney, P. Phys. Chem. Chem. Phys. 2004, 6, 5056. (10) Ma, X.; Lun, N.; Wen, S. Diamond Relat. Mater. 2005, 14, 68. (11) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 124, 9058. (12) Ang, L.; Hor, T. S. A.; Xu, G.; Tung, C.; Zhao, S.; Wang, J. L. S. Chem. Mater. 1999, 11, 2115. (13) Kobayashi, Y.; Tadaki, Y.; Nagao, D.; Konno, M. J. Colloid Interface Sci. 2005, 283, 601. (14) Miyake, H.; Ye, S.; Osawa, N. Electro. Chem. 2002, 4, 973. (15) Lan, P.; Kumar, K.; Wnek, G. E.; Przybycien, M. J. Electrochem. Soc. 1999, 146, 2517. (16) Zhang, L.; Jiang, X. J. Electroanal. Chem. 2005, 583, 292. (17) Majid, E.; Hrapovich, S.; Liu, Y.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2006, 78, 762. (18) Simm, A. O.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 5051. (19) Simm, A. O.; Banks, C. E.; Compton R. G. Electroanalysis 2005, 17, 335. (20) Dai, X.; Nekrassova, O.; Hyde, M. E.; Compton, R. G. Anal. Chem. 2004, 76, 5924. (21) Simm, A. O.; Banks, C. E.; Wilkins, S. J.; Karousos, N. G.; Davis, J.; Compton, R. G. Anal. Bioanal. Chem. 2005, 381, 979. 10.1021/ac060582o CCC: $33.50

© 2006 American Chemical Society Published on Web 07/29/2006

often unknown themselves.22-24 In particular, gold nanoparticles have been shown to be efficient catalysts in oxidation reactions, including the room-temperature oxidation of carbon monoxide to carbon dioxide,25 hydrocarbons such as propene,26 cyclic alkanes and alkenes,27 and also in the water gas shift reaction.23 Another considerable advantage of using appropriately supported gold nanoparticulate materials in catalytic applications is that the large active surface area that results from using gold nanoparticles reduces the actual amount of this expensive material required. Thus, Hughes et al. have recently reported in Nature that, by supporting gold nanoparticles on a graphite powder support, they could achieve the catalytic oxidation of cyclohexene with up to 100% conversion, while only using ∼1% coverage of the graphite support with gold nanoparticles under relatively mild conditions (60-80 °C, 4-24 h).27 Metal nanoparticle-modified electrodes, can be considered as behaving as random arrays of microelectrodes, whose voltammetric behavior is strongly dependent on the separation distance between individual microelectrodes (viz. nanoparticles).28-32 As such metal nanoparticle-modified electrodes, and for the remainder of this work, we will only be concerned with gold nanoparticles, provide four unique possible advantages over macroelectrodes for electroanalysis: enhancement of mass transport, catalysis, high effective surface area, and control over the electrode microenvironment.1,33,34 A complete theoretical framework for dealing with such spatially heterogeneous electrodes has been developed by Davies et al. 28,29,35 This theory is “complete” in that it considers voltammetric behavior of regular and random arrays of microelectrodes for all cases ranging from instances where the distance between individual microelectrodes is large relative to the size of the microelectrode itself, to where the separation between individual microelectrodes is very small. They have shown that, in the two limiting cases where the interelectrode separation is very large compared to the size of the microelectrodes or in the limit of strongly overlapping diffusion domains (i.e., where the individual microelectrodes within the array are in very close proximity to one another), the array exhibits behavior analogous to that of the corresponding macroelectrode. This offers a further unique advantage of using nanoparticlemodified electrodes in that, under the right conditions, only a very (22) Cortie, M. B.; Van der Lingen, E. Mater. Forum 2002, 26, 1. (23) Haruta, M. Hyomen Kagaku, 2005, 26, 578. (24) Haruta, M. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol. 1, p 655. (25) Grisel, R.; Weststrate, K.-J.; Gluhoi, A.; Nieuwenhuys, B. E. Gold Bull. 2002, 35, 39. (26) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Top. Catal. 2004, 29, 95. (27) Hughes, M. D.; Xu, Y.-J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (28) Davies, T. J.; Compton, R. G. J. Electroanal. Chem. 2005, 585, 63. (29) Davies, T. J.; Banks, C. E.; Compton, R. G. J. Solid State Electrochem. 2005, 9, 797. (30) 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. (31) Simm, A. O.; Ward-Jones, S.; Banks, C. E.; Compton, R. G. Anal. Sci. 2005, 21, 667. (32) Cheng, W.; Dong, S.; Wang, E. Langmuir 2002, 18, 9947. (33) Hung, D. Q.; Nekrassova, O.; Compton, R. G. Talanta 2004, 64, 269. (34) Welch, C. E.; Compton R. G. Anal. Bioanal. Chem. 2006, 384, 601. (35) Amatore, C.; Save´ant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39.

Chart 1. Cartoon Showing the Construction of the Au-np-GC Microspheres Deposited as a Film onto the Surface of a Glassy Carbon Macroelectrode, Where the Film Is Stabilized and Wired Together Using MWCNTs

small amount of gold in the form of nanoparticles is required in order to obtain a response equivalent to a macrodisk of the bulk material. However, there are two major drawbacks to using gold nanoparticle-modified electrodes. The first is that, whether the nanoparticles are deposited onto the electrode surface using electrochemical or electroless deposition methods, the electrodes generally have to be modified individually, thereby limiting their applicability to the mass production of electrochemical sensors. The second limitation is in ensuring that, where it is desirable, the deposited nanoparticles are in good electrical contact with the underlying macroelectrode and are deposited in such a way as to ensure that the resultant microelectrode array behaves as if it were a macroelectrode. This is particularly important in sensor design and manufacture where the amount of gold to be used should be minimized for reasons of cost.28,31 To overcome these limitations, we present a facile method to fabricate gold nanoparticles onto conducting glassy carbon microspheres via electroless plating resulting in the production of bulk (i.e., gram) quantities of surface-modified carbon material. This gold nanoparticle-modified glassy carbon microsphere (Aunp-GC) material was characterized by means of scanning electron microscopy (SEM) and cyclic voltammetry (CV). By depositing the gold nanoparticle-modified glassy carbon spheres into a film on the electrode surface, which is stabilized and electrically “wired” to the underlying macroelectrode using multiwalled carbon nanotubes (MWCNTs), we illustrate an elegant example of designed chemical architecture across the macro-, micro-, and nanoscales (Chart 1). Finally, we illustrate the potential utility of constructing electrodes in this way by examining their response to the analytical detection of As(III). The construction and wiring of the Au-npGC-modified electrode using MWCNTs in this way was found to produce a voltammetric response characteristic of a gold macroelectrode while using only 1% of the equivalent amount of gold metal required. EXPERIMENTAL SECTION Reagents and Equipment. Chemical reagents were obtained as follows: gold(I) sodium thiosulfate (AuNa(S2O3)2‚xH2O, 99.9%) and glassy carbon spherical powder (10-20 µm, type 1) were purchased from Alfa Aesar (Heysham, UK). L-Ascorbic acid (C6H8O6, 99.7%) was supplied by BDH (Poole, UK). Sodium (meta) arsenite (NaAsO2, 99%) was purchased from Fluka (Buchs, Switzerland). Mineral oil and graphite powder (2-20-µm diameter) Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

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were supplied by Sigma-Aldrich (Gillingham, UK). Multiwalled carbon nanotubes (purity >95%, diameter 30 ( 15 nm, length 5-20 µm) were purchased from Nanolab (Brighton MA). All the reagents were used without further purification. All solutions and subsequent dilutions were prepared using purified water from Vivendi UHQ grade water system with a resistivity of not less than 18.2 MΩ cm. As(III) stock solution (10 mM) was prepared from NaAsO2 by dissolving 0.013 g of NaAsO2 in 10 cm3 of purified water. All the solutions were degassed prior to the experiments. Electrochemical measurements were recorded using an Autolab PGstat 30 computer-controlled potentiostat with a standard three-electrode setup. A modified GC electrode served as a working electrode (see below), a platinum wire was used as a counter electrode, and a saturated calomel reference electrode (SCE; Radiometer, Copenhagen, Denmark) completed the cell assembly. Between each modification (see below), the GC electrode was polished with alumina powder (Micropolish II, Buehler) using decreasing particle sizes from 1 to 0.3 µm. The electrode was sonicated for 10 min in deionized water after each stage of polishing. All experiments were carried at a temperature 202 °C. Scanning electron microscopy (FEG-SEM) images were recorded using a JEOL 6500F instrument. Electroless Deposition of Gold Nanoparticles. The gold nanoparticles were electrolessly deposited onto the surface of glassy carbon microspheres using the following protocol:15 75 mg of AuNa(S2O3)2‚xH2O and 45 mg of L-ascorbic acid were dissolved in 50 cm3 of water and adjusted to pH 6.4 using 1 M NaOH. A 50-mg sample of the glassy carbon powder was added to this solution. The mixture was then stirred at room temperature for 24 h.15 After which time, the mixture was filtered, washed with pure water to remove any unreacted species, and the gold nanoparticle-modified powder (GC-nAu) was allowed to air-dry prior to use. Construction of the Film-Modified Glassy Carbon Electrodes with Gold Nanoparticle-Modified Glassy Carbon Spheres (GC-CNT/Au-np-GC). The construction of films consisting of Au-np-GC and MWCNTs on the surface of glassy carbon macroelectrodes were performed as follows: varying amounts (see below) of multiwalled carbon nanotubes (1, 3, or 5 mg) and gold nanoparticle-modified glassy carbon spheres (1, 5, 10, 25, or 50 mg) were suspended in 1 cm3 of dimethylformamide (DMF) to form a “casting” suspension. The casting suspension was then briefly sonicated for 30 s in order to disperse the MWCNTs and GC-nAu powder. A 20-µL aliquot of this suspension was then pipetted onto the surface of a freshly polished glassy carbon electrode. The DMF solvent was evaporated off by placing the electrode in an oven at ∼90 °C for 10 min (Chart 1). RESULTS AND DISCUSSION Characterization of Gold Nanoparticle-Modified Glassy Carbon Spheres. The gold nanoparticle-modified glassy carbon spheres were first examined using SEM. Images of the unmodified spheres (Figure 1a) reveal that the glassy carbon powder consists of microspheres of ∼10-20 µm in diameter in agreement with the manufacturer’s specifications. Some agglomerates of smaller (∼1-2 µm) particles could occasionally be observed arising from the manufacturing process. After modification, gold nanoparticles can clearly be observed decorating the surface of the glassy carbon microspheres (Figure 1b). From Figure 1c, it can be seen that 6104 Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

Figure 1. SEM images of (a) unmodified glassy carbon microspheres, (b) gold nanoparticle modified glassy carbon microspheres, and (c) a higher magnification image of the gold nanoparticles on the surface of a glassy carbon microsphere.

the average diameter of the polycrystalline gold nanoparticles or agglomerates of smaller nanoparticles is in the range 30-60 nm while the formation of gold films is not observed. From Figure 1c, we were able to estimate that the surface coverage of gold nanoparticles on the glassy carbon spheres is at least 10% by area. To confirm that the nanoparticles were indeed formed from gold, we used a voltammetric procedure developed previously36,37 (36) Kozlowska, H. A.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429. (37) Trasatti, S.; Petrii, O. A. Pure and Appl. Chem. 1991, 63, 711.

Figure 2. Overlaid cyclic voltammograms of a film modified electrode consisting of either blank GC microspheres or Au-np-GC spheres in 0.05 M H2SO4 (scan rate 100 mV s-1).

to “fingerprint” the characteristic signal of gold nanoparticles. The voltammetric responses were recorded for the film-modified electrodes using either blank GC microspheres or the Au-np-GCmodified spheres in 0.05 M H2SO4 over the potential range 0 to + 1.5 V. In the absence of Au-np-GC spheres, no peak was observed either in anodic or cathodic scan. However, the GCCNT/Au-np-GC electrode exhibited a sharp reduction peak ∼+0.8 V, characteristic of the reduction of surface oxides of gold (Figure 2).36,37 Thus, we can conclude that the nanoparticles observed in Figure 1b and c are indeed formed from metallic gold and that they are in electrical contact with the macroelectrode surface. By measuring the area under the reductive peak, it is possible to determine the electroactive surface area of the gold nanoparticle-modified electrodes. Using the literature value36,37 of 400 µC cm-2 for the charge passed per unit area on the surface of bulk gold, we can estimate the electroactive surface area of the Aunps on the modified electrode to be ∼2.5 cm2. The average diameter of the Au-nps was found to be 45 nm from the SEM images above, and the size distribution and average diameter (12 µm) of the glassy carbon microspheres is known from previous studies.38 Armed with this knowledge, and estimating the number of microspheres on the GC macroelectrode surface from the volume casting suspension used to prepare the modified electrode, we can carry out a crude calculation of the number of Au-nps per centimeter squared on the modified electrode surface using the area under the gold oxide reduction peak above. Reassuringly, the number of Au-nps calculated in this way was found to be within a factor of 2 of the number of Au-nps per centimeter squared calculated using the SEM images alone and were of the order of 109 Au-nps cm-2. One of the well-established advantages of using finely divided metal substrates, e.g., metal nanoparticle electrodes compared with using bulk metal electrodes, is that a significantly greater electroactive surface area can be achieved. One common method of producing metal electrodes while minimizing the amount of material required is to plate the metal onto a less expensive conducting substrate as a thin film. By comparing the amount of gold metal required to plate a macroelectrode with a typical film thickness of 1 µm, such that the electrode has the same electroactive area as our GC-CNT/Au-np-GC-modified electrode, 2.5 cm2, we find that ∼100 times more precious gold metal is (38) Thompson, M.; Wildgoose, G. G.; Compton, R. G. ChemPhysChem. In press.

required to form a gold film-plated electrode than was used to construct the GC-CNT/Au-np-GC-modified electrode. Furthermore, not only does the GC-CNT/Au-np-GC-modified electrode possess a high electroactive surface area while using substantially less gold material, but more interestingly, in the following section we show that due to the design and construction of this electrode interface using micro- to nanoscale chemical architectures, this electrode produces a response equivalent to using a gold macroelectrode due to diffusion of electroactive species occurring on different temporal and spatial scales. Response of As(III) on GC-CNT/GC-nAu Electrodes. To further characterize the voltammetric behavior of the Au-np-GC particles, we examined their response to As(III) in the form of the arsenite anion. To date, gold has been found to be a superior substrate for the voltammetric determination of As(III),18-21,39-43 while As(III) has not been observed voltammetrically on unmodified carbon substrates including MWCNTs. To confirm this, control experiments were performed using a bare glassy carbon electrode and a glassy carbon electrode modified with a film of MWCNTs (shown in Figure 4). In every case, no voltammetric signal was observed corresponding to As(III). Note that the lack of observed As(III) voltammetry on a bare glassy carbon electrode also precludes the possibility of observing any As(III) voltammetric response on unmodified glassy carbon microspheres. The choice of using As(III) to characterize the Au-np-GC-modified electrodes thereby prevents any misleading signal arising from the carbon microsphere supports, MWCNT wires, or the underlying carbon electrode. We have chosen to investigate the response of the Aunp-GC microspheres in the form of the GC-CNT/Au-np-GC film electrodes for two reasons. The first is that the use of a Au-npGC-modified carbon paste electrode (CPE), although arguably simpler to construct, may actually result in the formation of a “partially blocked” microelectrode array system, in that the mineral oil binder used in the CPE is likely to coat at least part of the Au-GC microsphere surface, thereby passivating the Au-np “nanotrodes”. The effects of using a partially blocked array have been treated theoretically35,44-48 and are likely to result in potentially misleading interpretations of the voltammetric response at Aunp-GC-modified carbon paste electrodes. The second reason for using the GC-CNT/Au-np-GC electrode design is that, in our experience, it is difficult to immobilize carbon microspheres onto planar electrode surfaces without resorting to the use of polymer binders such as Nafion or the formation of chemical bonds between the carbon microparticle and the electrode surface.38 Often these immobilization methods can result in a decrease in the electroanalytical performance of (39) Forsberg, G.; O’Laughlin, J. W.; Megargle, R. G. Anal. Chem. 1975, 47, 1586. (40) Hua, C.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987, 201, 263. (41) Kopanica, M.; Novotny, L. Anal. Chim. Acta. 1998, 368, 211. (42) Feeney, R.; Kounaves, S. P. Anal. Chem. 2000, 72, 2222. (43) Feeney, R.; Kounaves, S. P. Talanta 2002, 58, 23. (44) Brookes, B. A.; Davies, T. J.; Fisher, A. C.; Evans, R. G.; Wilkins, S. J.; Yunus, K.; Wadhawan, J. D.; Compton, R. G. J. Phys. Chem. B 2003, 107, 1616. (45) Davies, T. J.; Brookes, B. A.; Fisher, A. C.; Yunus, K.; Wilkins, S. J.; Greene, P. R.; Wadhawan, J. D.; Compton, R. G. J. Phys. Chem. B 2003, 107, 6431. (46) Davies, T. J.; Brookes, B. A.; Compton, R. G. J. Electroanal. Chem. 2004, 566, 193. (47) Chevallier, F. G.; Davies, T. J.; Klymenko, O. V.; Jiang, L.; Jones, T. G. J.; Compton, R. G. J. Electroanal. Chem. 2005, 577, 211. (48) Chevallier, F. G.; Davies, T. J.; Klymenko, O. V.; Jiang, L.; Jones, T. G. J.; Compton, R. G. J. Electroanal. Chem. 2005, 580, 265.

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Figure 4. Overlaid cyclic voltammograms of increasing As(III) additions (0-500 µM in100 µM additions) in 1 M H2SO4 on a GCCNT/GC-nAu electrode (scan rate 100 mV s-1, dashed line is the blank response). Dotted line is the overlaid response of a glassy carbon electrode modified with a film of MWCNTs only to 500 µM As(III). Inset: An exploded view of the As(III) reduction wave to illustrate that the wave shape is characteristic of macroelectrode behavior of the GC-CNT/GC-nAu electrode (see text).

Figure 3. SEM images of (a) the surface of a glassy carbon electrode modified with multiwalled carbon nanotubes and gold nanoparticle-modified glassy carbon spheres (5 mg/50 mg) and (b) a higher magnification image of the GC-CNT/Au-np-GC electrode surface illustrating the wiring up of both the Au-nps and the glassy carbon microspheres within the film by the MWCNTs.

the electrode and result in problems similar to those discussed in the first point made above. Therefore, to examine the electroanalytical performance of the GC-nAu microspheres immobilized on the surface of a glassy carbon macroelectrode, the modified glassy carbon microspheres were deposited onto the surface of a GC electrode along with MWCNTs, as described in the Experimental Sectinabove. This allowed us to take advantage of two properties of MWCNTs, in that not only do they help to stabilize and bind the microparticles onto the electrode surface but they also help to “wire up” both the gold nanoparticles themselves and the GC-nAu microspheres to the glassy carbon macroelectrode. Figure 3a shows the surface morphology of the film-modified GC-CNT/GC-nAu electrode, while Figure 3b shows how the MWCNTs help to wire each Au-np-GC microsphere into electrical contact (an improvement on the point contacts required by simply close-packing spheres onto a surface) and to bind and stabilize the microspheres within the film structure. The response of such an electrode was examined first by cyclic voltammetry. Figure 4 illustrates a typical cyclic voltammetric response in the range from - 0.5 to + 0.4 V versus SCE (scan rate 100 mV s-1) of a GC-CNT/Au-np-GC electrode (optimized film ratio 5 mg of MWCNT/50 mg of Au-np-GC; see below) in 1 6106

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M H2SO4. It can be seen that in the absence of As(III) no redox processes were observed in the potential range studied (dashed line in Figure 4). However, a new reduction wave emerges at - 0.25 versus SCE upon the addition of 100 µM As(III) and can be attributed to the three-electron reduction of As(III) to As(0).11 On reversing the scan direction, an oxidative stripping wave at + 0.15 V versus SCE was observed corresponding to the subsequent reoxidation of As(0) to the parent As(III) species. The shape of the As(III) reduction wave, shown in the inset in Figure 4 for clarity, is not sigmoidal as would be expected from either an individual microdisk or a widely spaced microdisk array at this modest scan rate, but is distinctly peak shaped in character indicating that the electrode is behaving as if it were a macroelectrode. Attempts to fit the peak shape, in particular to show a t-1/2 decay of the current after the peak potential is passed, using commercially available software were unsuccessful due to the peak being superimposed onto a sloping baseline in the cathodic region. However, if we consider the electrode to truly be a macrodisk of area 2.5 cm2 and carry out a simple calculation to determine the magnitude of the peak current expected in the limit of irreversible kinetics using eq 1, where ip is the peak current/Å, R is the

ip ) (2.99 × 105)n(Rna)1/2AD1/2ν1/2Cbulk

(1)

transfer coefficient, n and na are the total number of electrons transferred and the number of electrons transferred in the rate determining step respectively, A is the electrode area/cm2, D is the diffusion coefficient of As(III)/cm2 s-1, ν is the scan rate/V s-1, and Cbulk is the bulk concentration of As(III)/mol cm-3, we find that the magnitude of the peak current calculated for the case of a macrodisk electrode is in excellent agreement with the peak current observed in Figure 4 with a magnitude of the order of 10-4 Å.

Scheme 1. Schematic Illustration of How Intelligent Chemical Architectural Design Can Take Advantage of the Synergy between Type 1 and Type 4 Behavior at Nano- and Microelectrode Arrays To Produce Macroelectrode Behavior while Using a Fraction of the Electrode Material Required (See Text)

This macroelectrode behavior is a result of the unique chemical architecture of the modified electrode surface and can be explained by considering the result of the two limiting cases, described by Davies and Compton, working in tandem.28 If we consider the Au-nps on the GC microspheres as a random array of widely spaced nanoelectrodes (as they are shown to be in Figure 3b), it is then intuitively obvious that we are in type 1 behavior as classified by Davies and Compton.28 (the electrodeelectrode separation within the array is very large compared to the size of the electrode), the nanoelectrodes are therefore diffusionally independent from one another on the time scale of the experiment and the nanoelectrode array behaves as if it were a microdisk with an area equal to the sum of the individual nanotrode areas within the array (Scheme 1). If we then consider each Au-np-GC microsphere (which is effectively acting as an individual microdisk now) on the surface of the GC electrode, noting that almost all microspheres are likely to be in electrical contact with the GC macroelectrode due to the MWCNT binder, it is apparent (Figures 3a and b) that the spacing between each individual “microelectrode” is very small compared to the size of the microspheres. We are therefore in the regime of type 4 behavior (characterized by very small interelectrode separation within the array, heavily overlapping diffusion zones, and resultant planar diffusion to the array), and the microelectrode array of Aunp-GC spheres behaves as if it were a macroelectrode of area equivalent to the area covered by the array, not the sum of individual microelectrodes as was found in type 1 behavior.28 Thus, we can generate a voltammetric response on the GCCHT/Au-np-GC electrode equivalent to a gold macrodisk electrode, yet from the crude calculations given under Characterization

of Gold Nanoparticle-Modified Glassy Carbon Spheres, we are using the equivalent of only ∼1% of the required gold material. We note that both oxidative and reductive waves were found to increase linearly with further additions of As(III) up to total concentration of 500 µM. Therefore, to further illustrate the potential utility of constructing electrodes using intelligently designed chemical architectures, we next seek to optimize the GC-CNT/Au-np-GC structure and the experimental conditions used for the analytical determination of AS(III) using anodic stripping voltammetry. Optimization of the Mass Ratio of MWCNTs and Gold Nanoparticle-Modified Microspheres in the GC-CNT/Au-npGC Electrode. Need for Efficient Electrical Wiring. Having established that the film-modified electrode exhibited a voltammetric response to additions of As(III), we attempted to optimize the ratio of MWCNTs/GC-nAu microspheres in the film. The composition ratios of multiwalled carbon nanotubes (1, 3, or 5 mg) to GC-nAu (1, 5, 10, 25, or 50 mg) used to make up the casting suspension in 1-cm3 DMF were varied and the subsequent response toward As(III) of the film-modified electrodes was examined using linear sweep voltammetry with 60-s deposition at -0.3 V versus SCE in 1 M H2SO4. When the mass of MWCNTs in the casting solution was fixed at either 1 or 3 mg, the sensitivity of the electrode response toward As(III) remained unchanged despite the mass of GC-nAu micropsheres being increased from 1 to 50 mg. This may tentatively be attributed to reduced electrical connectivity between the Au-np microspheres within the film. However, when the mass of MWCNTs was fixed at 5 mg, the sensitivity of the response toward As(III) increased with an increasing mass GC-nAu used in the casting solution, reaching a maximum at a ratio of 5 mg of MWCNTs/50 mg of GC-nAu microspheres. Increasing the mass of modified microspheres beyond 50 mg resulted in a loss of film stability and no significant increase in the sensitivity of the response to As(III). Therefore, unless otherwise stated, this composition ratio (5 mg of MWCNTs/ 50 mg of GC-nAu) was used throughout. Optimization of the As(III) Detection Protocol Using the GC-CNT/GC-nAu Electrode. Having established the optimal film composition above, we next attempted to optimize the limit of detection and the sensitivity of the electrode’s response to the determination of As(III) concentration. The 1 M solutions of HCl, H2SO4, HNO3, and H3PO4 were compared in respect to the response toward As(III) on the gold nanoparticle-modified electrodes using linear sweep voltammetry. It was found that using 1 M HCl or 1 M H2SO4 as background electrolyte produced similar sensitivities in the electrode’s response, while the best linear calibration plots and reproducible arsenic stripping peaks were obtained using 1 M H2SO4. Next we investigated the effect of using different deposition times of 30, 60, 120, 180, and 300 s using LSV with standard additions of As(III) to 1 M H2SO4. It was found that increasing the deposition time increased the observed peak heights in a linear fashion. Therefore, for trace As(III) determinations, longer deposition times (300 s) were applied. At higher concentrations of As(III), short deposition times were used to avoid saturating the electrode surface. An optimized scan rate of 100 mV s-1 was used throughout. Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

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of such chemical architectures may have wide-ranging applications not necessarily for electroanalysis but also for electrocatalysis and general electrode design where the amount of expensive electrode material needs to be minimized without affecting the overall performance of the system too adversely.

Figure 5. Overlaid linear sweep stripping voltammograms with increasing As (III) additions (from 0.1 to 5 µM) in 1 M H2SO4 on GCCNT/GC-nAu (5 mg/50 mg). Electrodeposition was carried out at -0.3 V vs SCE for 300 s, scan rate 100 mV s-1.

Using these optimal conditions for the GC-CNT/Au-np-GC (LSV, 1 M H2SO4, 300-s deposition at - 0.3 V vs SCE) with standard additions of 0.1 µM As(III), as shown in Figure 5, a sensitivity of 6 µA/µM As(III) was obtained with a limit of detection of 0.03 µM (2.5 ppb). The limit of detection obtained using the GC-CNT/Au-np-GC electrode is typically 1 magnitude of order higher than existing methods.18-20,42,43 However, this is still within sufficient limits of detection and sensitivities to comply with the requirements of the WHO maximum permissible limit for arsenic concentrations in drinking water of 10 ppb49 and merely serves as an example of the potential utility of electrodes constructed in this way. The intelligent and judicious exploitation (49) http://www.who.int/int-fs/en/fact210.html.

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CONCLUSION Gold nanoparticles have been supported on the surface of glassy carbon microspheres (10-20 µm) through electroless deposition. The resulting material was characterized by means of SEM and CV. A method of constructing electrodes from these modified carbon microspheres using MWCNTs as both a binder to stabilize the film and also to wire the assembly together is presented. The response of such electrodes toward the analytical determination of As(III) was compared. In particular the unique construction of these electrodes exploiting macro- to micro- to nanoscale chemical architectures was found to exhibit behavior of a corresponding macroelectrode, while using the equivalent of only ∼1% of the gold material required. This effect can be explained not simply by considering the well-known advantages and surface area effects of using nanoparticles over bulk materials but also by considering the effect of diffusion to the nano- to microelectrode array generated using this elegant chemical architecture. ACKNOWLEDGMENT X.D. thanks the Clarendon Fund of Oxford University and Abington Partners for financial support. G.G.W. thanks the BBSRC and Schlumberger Cambridge Research for funding. Received for review March 30, 2006. Accepted June 25, 2006. AC060582O