Gold Nanoparticles as Fine Tuners of Electrochemical Properties of

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Gold Nanoparticles as Fine Tuners of Electrochemical Properties of the Electrode/Solution Interface Wenlong Cheng, Shaojun Dong, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China Received May 31, 2002. In Final Form: September 11, 2002 Recently, a novel approach for preparing SERS and SPR substrates was developed, which indicates a potential application in tailoring the interfacial structure of an electrode surface. In this study, (3-mercaptopropyl)trimethoxysilane (MPTMS) was selected as a polymeric adhesive layer, and a low concentration of colloid Au solution was used to achieve a more accurate control over interface morphology at nanoscale dimensions due to slow self-assembling kinetics of gold nanoparticles. Subsequent seeding growth of these MPTMS-supported submonolayers of gold nanoparticles in Au3+/NH2OH aqueous solution enlarges particle size and eventually results in the generation of conductive gold films (similar to previous (3-aminopropyl)trimethoxysilane-supported gold films). Such tunable interface structure was evaluated by atomic force microscopy (AFM). Also, ac impedance spectroscopy (ACIS) and cyclic voltammograms were performed to evaluate electrochemical properties of the as-prepared interfaces by using Fe(CN)63-/4couples as a probe. Furthermore, relevant theories of microarray electrodes were introduced into this study to explain the highly tunable electrochemical properties of the as-prepared interfaces. As a result, it is concluded that the electrochemical properties toward Fe(CN)63-/4- couples are highly dependent on the active nanoelectrode (nanoparticles) area fraction and nanoparticles are fine-tuners of interfacial properties because the number density (numbers/unit area) and size of nanoparticles are highly tunable by selfassembling and seeding growth time scale control. This is in agreement with the theoretical expectations for a microarray electrode if a single nanoparticle tethered to a blocking SAM is taken as a nanoelectrode and 2-D nanoparticle assemblies are taken as nanoelectrode arrays.

Introduction Electron transfer between the solid and liquid phases is the fundamental act of electrochemistry and is affected directly by interfacial properties such as conductivity, roughness, cleanliness, and so on. Therefore, construction of well-defined and highly controllable electrochemical interfaces is significant for both fundamental and applied studies in electrochemistry. In the past, vacuum deposition and sputtering of metal have been two usual fabrication techniques of electrochemical platforms, such as microdisk or microelectrode arrays.1-3 However, such preparative methods usually need expensive instruments to achieve well-defined film structure and also are time-consuming processes. Moreover, noble metals such as Au and Ag do not adhere well to glass4 unless glass is plasma-cleaned in the vacuum system just before laying down the metal. With significant exposure to water, the film will be damaged, limiting its applicability. To enhance the gold adhesion to the substrate, Cr (or other metals) was used as a undercoating layer, which brings some problems related to Cr (Ti, W) diffusion along grain boundaries with the noble-metal surface.5-8 This phenomenon leads in* Corresponding author. Fax: [email protected].

+86-431-5689711. E-mail:

(1) Montenegro, M. I., Queiros, M. A., Daschbach, J. L., Eds. Microelectrodes: Theory and Applications; NATO ASI Series E: Applied Sciences; Kluwer: Dordrecht, The Netherlands, 1990. (2) Stulik, K.; Amatore, C.; Holub, K.; Marecek, V.; Kutner, W. Pure Appl. Chem. 2000, 72 (8), 1483. (3) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmit, P. P. Ultramicroelectrodes; Datatech Systems, Inc.: Morgato, NC, 1987. (4) Janata, J. Principles of Chemical Sensors; Plenum Press: New York, 1989. (5) Josowicz, M.; Janata, J.; Levy, M. J. Electrochem. Soc. 1988, 135, 112-115. (6) Tisone, T. C.; Drovek, J. J. Vac. Sci. Technol. 1972, 9, 271-275. (7) Ashwell, G. W. B.; Heckingbottom, R. J. Electrochem. Soc. 1981, 128, 649-654.

evitably to the altering of surface properties of the gold and may affect the electron transfer and adsorption processes,9 making them unsuitable for electrochemical experiments. Recently, an alternative colloid chemical approach was developed for fabrication of SERS and SPR substrates.10-14 This solution-based strategy is versatile in controlling surface morphology and indicates also potential applications in tailoring of electrochemical interfaces. Several reports focused on electrochemical studies on such electrochemical interfaces prepared by the colloid chemical strategy. For example, Evans et al. reported the preparation of self-assembled monolayers of a diacetylene compound on surface-confined gold nanoparticles.15 Interestingly, subsequent photopolymerization yields highly conjugated poly(diacetylene). The colloid films were proven to act as base interfaces of redox-active monolayers, which exhibit facile surface-confined electron-transfer processes.16,17 Also, such colloid interfaces could be used as (8) Holloway, P. H. Gold Bull. 1978, 12, 99-106. (9) Cohen, R. M.; Janata, J. J. Electroanal. Chem. Interfacial Electrochem. 1983, 151, 33-39. (10) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (11) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1999, 15, 866. (12) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314. (13) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (14) Jin, Y. D.; Kang, X. F.; Song, Y. H.; Zhang, B. L.; Cheng, G. J.; Dong, S. J. Anal. Chem. 2001, 73, 2843. (15) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. Adv. Mater. 1999, 11, 131. (16) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (17) Harnisch, J. A.; Pris, A. D.; Porter, M. D. J. Am. Chem. Soc. 2001, 123, 5829.

10.1021/la026022b CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002

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substrates for electrochemical deposition.18,19 Natan et al. found that nanometer-scale morphology of the colloid interfaces played a key role in protein electrochemistry and only 36-nm-diameter gold nanoparticles20 exhibited quasireversible voltammetry. These previous studies indicate that well-defined control over nanoparticle size and interspacing is significant for electrochemical applications. In this report, MPTMS was selected as a Au colloid binder layer for gold nanoparticle immobilization due to its stronger blocking properties to Fe(CN)63-/4- couples than (3-aminopropyl)trimethoxysilane (APTMS). The adsorption of gold nanoparticles on mercaptosilylated planar substrate21a and spherical silica particles21b,c demonstrated very strong interactions between sulfhydryl groups and gold nanoparticles. Such interactions might contain thousands of covalent bonds. The submonolayers of gold nanoparticles on the MPTMSfunctionalized ITO surface were also demonstrated to be rather robust, resistant of electrochemical scanning at least in the potential of -0.2 to 0.8 V in this study. Although the previous studies21a have dealt with attachment of gold nanoparticles on organosilane-coated substrate by virtue of several nanoscale characterization techniques, we used Au colloid with a lower concentration (∼3 nM) to provide a more accurate control over selfassembling kinetics of gold nanoparticles on MPTMS. Such subtle control over the nanoscale interface structure might be significant for potential electrochemical sensor applications. Like these APTMS-supported nanoparticle systems,10-14 MPTMS-supported nanoparticles result in similar particle enlargement and coalescence when treated with Au3+/NH2OH aqueous solution. As a result, the electrode interface undergoes changes from a MPTMSpassivated surface to a nanoparticle submonolayer to a completely conductive film. The mechanism for such changes can be explained using relevant theories of microelectrode arrays. Experimental Section Reagents and Materials. The following reagents were purchased from commercial vendors and used without further purification: (3-mercaptopropyl)trimethoxysilane (MPTMS), HAuCl4‚3H2O, Na3citrate, HONH3Cl, and KCl from Aldrich; potassium ferricyanide/ferrocyanide from Sigma. Solutions were prepared from ultrapure water purified with the Milli-Q plus system (Millipore Co.). Its resistivity was over 18 MΩ cm. Preparation of Colloid Au. All glassware used in the following procedures was cleaned in a bath of freshly prepared 3:1 HCl/HNO3 and rinsed thoroughly in H2O prior to use. The gold colloids were prepared by the conventional citrate reduction of HAuCl4 in water at near-boiling temperature.22 Transmission electron microscopy was used to examine particle size distribution, and an average diameter of 12 nm was confirmed (not shown). Instrumentation. All electrochemical experiments were carried out on an Autolab PGSTAT30 potentiostat (Utrecht, The Netherlands) in a conventional one-compartment cell. The cell was housed in a homemade Faraday cage to reduce stray electrical noise. Measurements used standard three-electrode systems. A (18) Bright, R. M.; Walter, D. G.; Musick, M. D.; Jackson, M. A.; Allison, K. J.; Natan, M. J. Langmuir 1996, 12, 810. (19) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (20) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154. (21) (a) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (b) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (c) Fleming, M. S.; Walt, D. R. Langmuir 2001, 17, 4836. (22) Frens, G. Nature 1973, 241, 20.

Cheng et al. Ag/AgCl electrode was used as the reference electrode, a Pt foil was used as the counter electrode, and the O-rings with 6 mm inner diameter were used to seal the ITO slides for all electrochemical experiments (geometry area ca. 0.283 cm2). Atomic force microscopy (AFM) images were taken by using a Nanoscope IIIa instrument operating in the tapping mode with standard silicon nitride tips. Typically, the surface was scanned at 2 Hz with the resolution of 256 lines/image (all the image sizes equal 1 µm2 in this article). Construction of a Series of MPTMS-Supported Nanoelectrode Arrays with Variable Active Area Fractions and Their AFM and Electrochemical Characterization. The ITO glass slides (1 cm2) were sonicated for 20 min in each of the following solvents: soapy water; water; acetone; methanol. After thorough cleaning, these slides were immersed in 2% (v:v) MPTMS methanol solution for 1 day. After exhaustive rinsing by methanol, these slides were heated to 80 °C to remove loosely bound MPTMS molecules. These MPTMS-derivatized ITO slides were then immersed in sealed vessels containing 2 mL of ∼3 nM colloidal Au (12 nm) solution for various times, rinsed thoroughly, and stored in pure water. Prior to AFM imaging and electrochemical measurements, the series of the MPTMS-supported nanoelectrode arrays were dried in argon. To achieve a reproducible interface structure, all the adsorptive process was done free of shaking at room temperature. Surface-confined gold nanoparticle/nanoelectrode seeding enlargement was done in 0.3 mM Au3+/NH2OH aqueous solution, and the solution was kept shaking at ∼120 rpm at room temperature.

Results and Discussion The self-assembly of gold nanoparticle on a sulfhydrylexposed surface is the reverse process of self-assembly of alkanethiol on gold surface. Unlike the self-assembling process of alkanethiol23,24 on a gold surface, the adsorption of gold nanoparticles on MPTMS follows a slower kinetics, which can be evaluated by tapping-mode AFM and electrochemistry. Variable Area Fractions of Nanoelectrode Arrays by Self-Assembling Kinetics Control of Spherical Gold Nanoparticles. As a standard nanometer-scale imaging tool, AFM can give reliable topographical information for the as-prepared nanoelectrode arrays. Figure 1 shows two typical AFM images of the MPTMS-derivatized ITO sides with different immersion times in the asprepared colloidal Au solution. Typical rms (standard deviation of the height values) roughness of organosilane films on glass is in the range of 1-3 nm (the value was given by the solfware provided by Digital Instruments Co. Ltd.) as demonstrated by previous reports.25,26 The line scans in all the micrographs give nearly the same height variation in the range of 11-16 nm, and the width changes from 20 to 30 nm, which confirms that a single feature corresponds to a single nanoparticle (also called nanoelectrode in this discussion). Obviously, the different derivatizing times result in different nanoelectrode surface coverage. Manual number counting in AFM images can give the number density (nanoelectrode numbers/unit area); as a result, nanoelectrode area fraction on MPTMSfunctionalized surface can be calculated on the basis of an average diameter of as-prepared nanoparticles. Our previous studies27 show that a quantitative equation can be derived from the AFM studies. Simply, we can prepare (23) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (24) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-assembly; Academic Press: San Diego, CA, 1991. (25) Karrash, S.; Dolder, M.; Schabert, F.; Ramsden, J.; Engel, A. Biophys. J. 1993, 65, 2437. (26) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210. (27) Cheng, W.; Dong, S. J.; Wang, E. Anal. Chem. 2002, 74, 3599.

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Figure 1. Tapping-mode AFM images of the MPTMS-supported nanoelectrode arrays prepared with two different immersion times in 3 nM Au colloid. Treating times are labeled on the graphs. Obviously, different immersion times result in different number densities of nanoelectrodes on the MPTMS surface. All images are 1000 nm × 1000 nm.

the desired nanoelectrode arrays with desired number density by virtue of the equation. Also, the nice fit of the AFM experimental data27 suggests that the adsorption of gold nanoparticles on MPTMS is diffusion controlled. Compared with alkanethiol organization on gold surface, the adsorptive kinetics of the as-produced gold nanoparticles on MTPMS is more sluggish, which might be attributed to large molecular volume and high viscosity of Au colloid. Further Area Fraction Control by Surface-Confined Seeding Growth. Both previous12-14 and present studies demonstrate that the preparation of films with g25% nanoelectrode coverage is difficult due to the repulsive forces between surface-confined nanoparticles and free nanoparticles in solution.13 A recently developed hydroxylamine seeding method12,14 makes the construction of the nanoelectrode arrays with higher area fraction possible. Here, MPTMS was used instead of APTMS as the polymeric adhesive layer for gold nanoparticle immobilization. Briefly, gold nanoparticles with a small size are tethered to the MPTMS-functionalized surface first by virtue of the thiolphilic nature of the gold nanoparticles; afterward catalyzed reduction of Au3+ by NH2OH takes place selectively on these MPTMS-supported nanoelectrodes upon immersion into Au3+/NH2OH aqueous solution. Such seeding enlargement of the tethered nanoelectrodes can help us plant more close-packed nanoelectrodes on the MPTMS insulating plane. Figure 2 shows

Figure 2. Tapping-mode AFM images of the MPTMS-supported nanoelectrode arrays prepared with 12-min self-assembly in 3 nM Au colloid and images of the arrays after immersion into 0.3 mM Au3+/NH2OH aqueous solution with shaking. Treating times are labeled on the graphs. All images are 1000 nm × 1000 nm.

the images of nanoelectrode arrays on MPTMS with different seeding times. The line scan analysis gives nanoelectrode height changes from 12-18 to 50 nm, which suggests the enlargement of nanoelectrodes since the same AFM tip was used for all these measurements. The deviation from the spherical shapes indicates that long seeding time leads to the coalescence of nanoelectrodes. The longer derivatizing times (∼30 min) created MPTMSsupported conducting gold films, which were similar to APTMS-supported conducting gold films.14 Studies of Cyclic Voltammetry (CV) of a Series of the As-Prepared Interfaces. Here, the CV technique is used to evaluate the reversibility of electrochemical reactions at a series of the as-prepared electrode interfaces. It has to be noted that faradaic responses observed on a microarray electrode depend on the distance between electrode elements and the time scale (i.e., scan rate) of the experiment. Here, scan rate was fixed to 50 mV/s,

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Figure 4. Peak-to-peak separation (left) and peak current (right) as a function of nanoelectrode area fraction for the series of nanoelectrode arrays prepared by self-assembly of gold nanoparticles on the MPTMS surface. Figure 3. Solid lines show CVs of a series of the nanoelectrode arrays prepared by self-assembly of gold nanoparticles on MTPMS with different immersion times (min), which were labeled on the graph. The dashed line shows CV of the asprepared nanoelectrode arrays (by 618 min) after 10 min immersion in 0.3 mM Au3+/NH2OH. The solution is 1 mM Fe(CN)3-/4- + 0.1 M KCl. The scan rate is 50 mV/s.

Fe(CN)63-/4- couples were used as electrochemical probes, and CVs at a series of the as-prepared nanoelectrode arrays were recorded. Figure 3 shows CVs at a series of the nanoelectrode arrays prepared by self-assembly of the asprepared colloid Au and surface-confined seeding enlargement in 1 mM Fe(CN)63-/4-. It is clearly observed that the longer the time scale is, the larger the redox peak current and the smaller the peak-to-peak separation. The quantitative analysis was illustrated in Figure 4, which shows increasingly reversibility with increasing nanoelectrode loading. That is, the electrode/solution interface becomes increasingly accessible to electrochemical probes with increasing nanoelectrodes loading. Interestingly, it is observed that changes of peak currents versus times follow the similar trend as exhibited in ref 27, demonstrating a slow diffusion-controlled adsorptive process. The treatment of the as-produced gold nanoparticle submonolayers with Au3+/NH2OH (10 min) resulted in larger faradaic currents and smaller peak separations, which is nearly identical to electrochemical properties of a gold macroelectrode. This phenomenon can be explained by using relevant theories28-30 of microarray electrodes. As evidenced by the above AFM studies, the immersion in Au3+/NH2OH leads to nanoelectrode enlargement/ coalescence; as a result, the nanoelectrode spacing becomes smaller, and thus, diffusional profiles of individual nanoelectrodes are almost completely overlapped at this scan rate. Therefore, the CV responses similar to those for a gold macroelectrode were observed. (28) Sabatani, F.; Rubinstein, I. J. Phys. Chem. 1982, 91, 6663. (29) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (30) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920.

ACIS Characterization of Changes of Interfacial Electrochemical Properties. The changes of interfacial electron-transfer properties resulting from changes of interfacial structures can be monitored more accurately by alternate current impedance spectroscopy (denoted as ACIS). A common feature of the ACIS plots of a series of the as-prepared nanoelectrode arrays is the semicircle seen at higher frequencies as shown in Figure 5a. The sizes of these semicircles determine the values of the interfacial charge-transfer resistance of Fe(CN)63-/4couples at the as-prepared nanoelectrode arrays, and these values can be estimated by fitting these data using Autolab software. The various interfacial charge-transfer resistances for a series of the as-prepared nanoelectrode arrays are listed in Table 1. These values decreased from 1362.7 to 149.7 Ω cm2 with increasing nanoelectrode number density, indicating increasingly facile heterogeneous electron transfer kinetics. The heterogeneous chargetransfer resistance can be continuously decreased to ∼10 Ω cm2 by the treatment with Au3+/NH2OH solution as shown in Figure 6 (Table 2). At adsorptive step of the gold nanoparticle, the increasing loading of gold nanoparticles means the increase of nanoelectrode area fractions when nanoparticles were taken as nanoelectrodes; whereas, at seeding growth step, particle enlargement/coalescence also means the increase of nanoelectrode area fractions. Thus, it is concluded that increasingly accessible electrochemical properties to Fe(CN)63-/4- should be attributed to increasing active nanoelectrode area fractions. This shows the gold nanoparticles tethered to MPTMS surface are different from alkanethiol, and they act as nanoscale conducting building blocks. Both CV and ACIS studies indicate that the electrontransfer kinetics of the as-prepared interfaces is active area fraction dependent. Self-assembly of gold nanoparticles and subsequent seeding growth on MPTMS provide an excellent strategy for control over the active area fractions at nanometer scale. A low concentration of Au colloid enhances accuracy in such interfacial control. Also, the above series of voltammetric studies demonstrate that

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Figure 5. ACIS of the MPTMS-derivatized ITO in 3 nM Au colloid with various immersion times as labeled in the graph (only five pairs of plots shown for clarity): (a) plots of the imaginary part vs the real part; (b) plots of the in-phase impedance vs ω-1/2 after correcting for solution resistance and double-layer capacitance. The solution is 1 mM Fe(CN)3-/4- + 0.1 M KCl. The electrode potential is 0.22 V vs Ag/AgCl, the frequency range is 100 kHz-0.1 Hz, and voltage amplitude is 5 mV. Table 1. Changes of Characteristic Parameters of Nanoelectrode Arrays with the Increase of Nanoelectrode Number Densitya

ITO ITO/SAM 2 min 7 min 13 min 23 min 53 min 88 min 138 min 618 min

Rct (Ω cm2)

no./cm2

active area (cm2)

R0 (nm)

Ra (nm)

115 1385.2 1362.7 737.4 558.1 434.9 344.3 263.4 224.1 149.7

1.51 × 1010 2.82 × 1010 3.84 × 1010 5.10 × 1010 7.75 × 1010 9.99 × 1010 1.25 × 1011 2.65 × 1011

0.0171 0.0319 0.0434 0.0576 0.0876 0.1129 0.1413 0.2996

19519 9458 6564 5608 4190 2531 2144 1449

3091 2552 1689 1367 1345 1240 850 805 793

a All the parameters are derived from ac impedance spectroscopy analysis.

the heterogeneous electron-transfer kinetics is very sensitive to the characteristic parameters of nanoelectrode arrays, such as element size and interelement spacing. This is in agreement with theoretical expectations of microarray electrodes.28-30 Time-dependent electrochemical properties of such systems give the proof that heterogeneous electron-transfer kinetics are determined by selfassembling kinetics of gold nanoparticles or seeding enlargement of surface-confined gold nanoparticles; that is, gold nanoparticles are fine-tuners of these interfacial properties. Patches Made of Nanoelectrodes. It is known that ACIS provides the most reliable quantitative information

Figure 6. ACIS studies of seeding growth of surface-confined nanoparticles in 0.3 mM Au3+/NH2OH aqueous solution: (a) ACIS plots of the imaginary part vs the real part; (b) plots of the in-phase impedance vs ω-1/2 after correcting for solution resistance and double-layer capacitance. Open circles shows the plot of nanoelectrode arrays by 12 min self-assembly. Solid circles, solid squares, open squares, and solid up triangles are corresponding to 3, 10, 15, and 25 min immersion, respectively. The solution is 1 mM Fe(CN)63-/4- + 0.1 M KCl. The electrode potential is 0.22 V vs Ag/AgCl, the frequency range is 100 kHz0.1 Hz, and voltage amplitude is 5 mV. Table 2. Changes of Characteristic Parameters of Nanoelectrode Arrays with Nanoelectrode Enlargement and Coalescence in 0.3 mM Au3+/NH2OH Solutiona

12 min seed 3 min seed 10 min

Rct (Ω cm2)

no./cm2

active area (cm2)

R0 (nm)

Ra (nm)

623.1 203.3 ∼10

3.76 × 1010 3.76 × 1010 3.76 × 1010

0.0425 0.1701 0.7379

6466 3520 1839b

1333 1167 1580b

a All the parameters are derived from ac impedance spectroscopy analysis. b The value is inaccurate due to the disappearing kinetics semicircle.

about the active area fraction and distribution of pinholebased microelectrode arrays.28,31 Similarly, it is logical to use ACIS to map nanoelectrode distribution at the asprepared nanoelectrode arrays. The ACIS data given by Autolab solftware could be converted to the plots of inphase impedance vs ω-1/2 (ω is the radial frequency) as shown in Figure 5b. These plots exhibit two nearly linear domains at high and low frequencies. Matsuda32 noted that the intersection of the lines corresponding to the high(31) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (32) Tokuda, K.; Gueahi, T.; Matsuda, H. J. Electroanal. Chem. 1979, 102, 41.

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and low-frequency domains of the Zf′ plot occurred at the “knee” frequency, i.e., the transition between high- and low-frequency domains. Simply, we can estimate Ra (the radius of electroacive domains) and R0 (the radius of the inactive domain surrounding the active domains) values by virtue of the “knee” frequencies of these in-phase plots. All the calculating procedures are the same as Finklea’s treatment,33 and the only difference is that the 1 - θ value represents pinhole area fraction in ref 33 and represents nanoelectrode area fraction (which can be estimated by AFM measurements) in this article. The “knee” frequency32,33 of the Zf′ plots in Figure 5b is increasing continuously from 29 to 568 Hz with increasing loading of nanoparticles, indicating increasingly merged diffusional profiles. Correspondingly, these estimated Ra and R0 values were listed in the Table 1. It is noted that both Ra and R0 values are decreasing with increasing nanoelectrode number density. The decrease of R0 value can be attributed to decrease of inter-nanoelectrode spacing. However, the decrease of Ra value seems inconsistent with the radius of nanoelectrode. Actually, due to limitation of scan rate or frequency, the ideally diffusional profiles of individual nanoelectrodes at the as-prepared nanoelectrode arrays cannot be attained by virtue of conventional electrochemical methods. The theoretical analysis for this has been described previously,27 and the above CV studies also supported the fact (the sigmodial CVs have never been obtained at conventional scan rates). This shows these Ra values derived from ACIS are not the radius of a single nanoelectrode but rather the average radius of the patches consisting of thousands of closely spaced nanoelectrodes. Actually, the AFM images in Figure 1 support the ACIS results, and the randomly distributed nanoelectrodes at the MPTMS plane tend to group, therefore behaving as patch assemblies at the electrochemical responses. Interestingly, such characteristic behaviors extremely resemble these pinhole-based microarray electrodes.31,33 Similarly, 3 min seeding growth in Au3+/NH2OH also shows such trend (Table 2), exhibiting similar decreasing patch and interpatch sizes (1 - θ value estimated from Figure 2, middle). Differently, such changes resulted from not an increase of nanoelectrode number density but rather nanoelectrode enlargement. The 10 min seeding growth, differently, causes a large change in the macroscopic electrochemical properties; namely, the ACIS kinetics semicircle disappears, indicating a very rapid kinetics process that cannot be observed at the conventional frequency range. This resembles the case for a gold macroelectrode. The above AFM studies demonstrate that longer treating times in Au3+/NH2OH resulted in the coalescence of nanoparticles; as a result, the diffusional profiles of nanoelectrodes have overlapped almost entirely. Further treatment with Au3+/NH2OH resulted in the generation of a completely conductive MPTMS-supported gold film. Therefore, arbitrary interfacial properties (from blocking to microarray electrode to completely conductive interfaces) can be achieved by combination of selfassembling time scale control and seeding growth control in Au3+/NH2OH. Application Potentiality. Electrochemistry at electrodes with microscopic dimensions constitutes one of the (33) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.

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most important frontiers in modern electrochemical science.1-3 Numberous methods for preparing microscopic electrodes have been devised, and it is now fairly easy to make electrodes with dimensions on the order of micrometers.1-3 However, the studies on nanoscopic electrodes are still on the horizon due to the difficulties in preparing truly nanoscopic electrodes.34-36 This study shows that a colloid chemical strategy might be an alternative strategy for construction of well-defined nanoelectrode arrays. Recently, Xia et al.38 described a strategy that combines physical templating capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. It is expected that the combined template-assisted self-assembly and seeding growth control can prepare patterned nanoelectrode domain or well-ordered nanoelectrode arrays. As demonstrated by this study, the heterogeneous electron-transfer rate constant is highly sensitive to the as-prepared interfaces. Such sensitivity to electrontransfer kinetics could prove to be an advantage in sensor applications, where a mediator, with fast electron-transfer kinetics, is used to shuttle electrons to a redox enzyme.29 The desired species can be differentiated selectively on the basis of different electron-transfer kinetics. To attain certain sensor requirements, we can prepare desired nanoelectrode arrays to fulfill the above requirements by the time-controlled self-assembly and seeding growth. It is known that such control ensures a good precision in regulating the electron-transfer kinetics toward the electrochemical species. Such colloid electrodes predict a myriad of application opportunities including the possibility of doing electrochemistry in highly resistive media38 and the possibility of investigating the kinetics of redox processes that are too fast to be studied at electrodes of conventional dimensions.39 Conclusions According to the theories of microarray electrodes,31-33 the electrochemical properties of an electrode interface are active electrode area fraction dependent. The selfassembling kinetics of gold nanoparticles on the MPTMSinsulating surface provides good control over the active nanoelectrode area fraction when gold nanoparticles distributed on an insulating plane were taken as nanoelectrode arrays (this kind of reasonability has been described in the text). The active nanoelectrode area fraction is determined by nanoparticle size and number density. In fact, these characteristic parameters of nanoelectrode arrays can be finely tuned by combined controls of the synthesis of colloid, self-assembly, and surface-confined seeding growth in Au3+/NH2OH. That is, gold nanoparticles behave as tuners of the electrochemical properties of the electrode interface. LA026022B (34) Conyers, J. L.; White, H. S. Anal. Chem. 2000, 72, 4441. (35) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 3559. (36) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Anal. Chem. 1997, 69, 2323. (37) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718. (38) Drew, S. M.; Wightman, R. M. J. Electroanal. Chem. 1991, 317, 117. (39) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Thorman, W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878.