Colloid Chemical Approach to Nanoelectrode Ensembles with Highly

Anal. Chem. 2002, 74, 3599-3604

Colloid Chemical Approach to Nanoelectrode Ensembles with Highly Controllable Active Area Fraction 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

A novel “bottom-up” approach to highly controllable nanoelectrode ensembles (NEEs) has been developed using colloidal nanoparticle self-assembly techniques. This solution-based strategy allows flexible control over nanoelectrode size, shape, and interspacing of the asprepared NEEs. Atomic force microscopy (AFM) was proved to be a powerful tool to monitor the NEE topography, which yields parameters that can be used to calculate the fractional nanoelectrode area of the NEEs. AFM, ac impedance, and cyclic voltammetry studies demonstrate that most of nanoelectrodes on the NEEs (at least by 9-min self-assembly) are not diffusionally isolated under conventional ac frequency range and scan rates. As a result, the NEEs behave as “nanoelectrode-patch” assemblies. Besides, the as-prepared NEEs by different self-assembling times show an adjustable sensitivity to heterogeneous electron-transfer kinetics, which may be helpful to sensor applications. Like these NEEs constructed by other techniques, the present NEEs prepared by chemical self-assembly also exhibit the enhancement of electroanalytical detection limit consistent with NEE theory prediction. Electrodes with microscopic dimensions are attractive tools for undertaking a variety of electrochemical experiments such as in highly resistive media1-3 and the possibility of investigating kinetics of redox processes that is too fast to be studied at electrodes of conventional dimensions.4-6,9d,14 Numberous methods * Corresponding author. Fax: +86-431-5689711. E-mail: [email protected]. (1) Drew, S. M.; Wightman, R. M. J. Electroanal. Chem. 1991, 317, 117-124. (2) Porat, Z.; Crooker, J. C.; Zhang, Y.; Mest, Y. L.; Murray, R. W. Anal. Chem. 1997, 69, 5073-5081. (3) Bento, M. F.; Medeiros, M. J.; Montenegro, M. I.; Beriot, C.; Pletcher, D. J. Electroanal. Chem. 1993, 345, 273-286. (4) (a) Russell, A.; Repka, K.; Dibble, T.; Ghoroghchian, J.; Smith, J. J.; Fleischmann, M.; Pitt, C. H.; Pons, S. Anal. Chem. 1986, 58, 2961-2964. (b) Oyama, N.; Ohsaka, T.; Yamamoto, N.; Matsui, J.; Hatozaki, O. J. Electroanal. Chem. 1989, 265, 297-304. (c) Stafford, L. K.; Weaver, M. J. J. Electroanal. Chem. 1992, 331, 857-876. (5) (a) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Thormann, W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878-1882. (b) Hatazawa, T.; Terrill, R. H.; Murray, R. W. Anal. Chem. 1996, 68, 597-603. (6) Karpinski, Z. J.; Osteryoung, R. A. J. Electroanal. Chem. 1993, 349, 285297. (7) 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. 10.1021/ac025661o CCC: $22.00 Published on Web 06/25/2002

© 2002 American Chemical Society

for preparing microscopic electrodes have been devised, and it is now fairly easy to make electrodes with dimensions on the order of micrometers.7,8 It is not the case for nanoscale electrodes, although a lot of benefits over macrosized electrodes have been reported.9-11 Nanoelectrode ensembles are versatile electroanalytical tools that have applications ranging from in vivo sensors to “smart” electronic noses.11 However, the studies on nanoscale electrodes are still an area of science where theory is ahead of experimental practice due to the difficulties in preparing truly nanoscopic electrodes with well-defined geometry.12,13 Currently, several reports have dealt with the preparation of a single nanoelectrode.9b,c,14 As for nanoelectrode ensembles (NEEs), they are primarily confined in two ways: the “template synthesis” approach developed by Martin et al.15 and the “microphase separation” approach to create molecule-sized defects on a gold surface developed by Crooks et al.16 Recently, a block copolymer selfassembling method for highly ordered, densely packed nanoporous arrays was introduced.17 (8) (a) Stulik, K.; Amatore, C.; Holub, K.; Marecek, V.; Kutner, W. Pure Appl. Chem. 2000 72, 1483-1492. (b) Howell, J. O.; Wightman, R. M. Anal. Chem. 1984, 56, 524-529. (c) Wehmeyer, K. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1989-1993. (d) Amatore, C. A.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1986, 206, 23-36. (e) Schuette, S. A.; McCreery, R. L. Anal. Chem. 1986, 58, 1778-1782. (9) (a) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276-285. (b) Fan, F.-R.; Dwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669-9675. (c) Bard, A. J.; Fan, F.-R. Acc. Chem. Res. 1996, 29, 572-578. (d) Bath, B. D.; Lee, R. D.; White, H. S. Anal. Chem. 1998, 70, 1047-1058. (e) Mirkin, M. V.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97, 7672-7677. (f) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Schulte, A.; Besenhard, J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281-1284. (10) (a) Wightman, R. M. Anal. Chem. 1981, 53, 1125A-1134A. (b) Oldham, K. B. J. Electroanal. Chem. 1992, 323, 53-76. (c) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmit, P. P. Ultramicroelectrodes; Datatech Systems, Inc.: Morgato, NC, 1987; Chapter 3, p 65. (11) (a) Martin, C. R. Science 1994, 266, 1961-1966. (b) Martin, C. R. Acc. Chem. Res. 1995, 28, 61-68. (c) Kobayashi, Y.; Martin, C. R. Anal. Chem. 1999, 71, 3665-3672. (d) Parthasarathy, R. V. Nature 1994, 369, 298301. (e) Martin, C. R.; Mitchell, D. T. Anal. Chem. 1998, 70, 322A-327A. (12) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118-1121. (13) (a) Pendley, B. P.; Abruna, H. D. Anal. Chem. 1990, 62, 782-784. (b) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663-6669. (14) (a) Conyers, J. L.; White, H. S. Anal. Chem. 2000, 72, 4441-4446. (b) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 35593564. (15) (a) Martin, C. R. Chem. Mater. 1996, 8, 1739-1746. (b) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (c) Kobayashi, Y.; Martin, C. R. Anal. Chem. 1999, 71, 3665-3672.

Analytical Chemistry, Vol. 74, No. 15, August 1, 2002 3599

Figure 1. Self-assembly kinetics of gold nanoparticles attached to MPTMS derivatized mica. (A-C) Three typical NEE topographies by different self-assembling times (as noted on the images). AFM tip convolution makes elements larger than their real diameters. (D) Plot of 12-nmdiameter nanoelectrode coverage as a function of self-assembling times. Solid squares are derived from manually number counting in AFM images at different assembling times. The solid line is the nonlinear least-squares best fit to data points.

Colloidal chemical synthesis provides an accurate control over particle size and shape,19,20 and if a proper immobilization approach works, a colloidal particle-based nanoelectrode (or NEEs) with various geometries can be achieved. Creatively, Bard et al. reported the first colloidal chemical approach to the preparation of spherical gold ultramicroelectrodes.18 By virtue of the self(16) (a) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329-1340. (b) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041-10046. (c) Li, S.; Crooks, R. M. Langmuir 1999, 15, 738-741. (d) Chailapakul, O.; Sun, L.; Xu, C. J.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 1245912467. (17) (a) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. Adv. Mater. 2000, 12, 787-791. (b) Jeoung, E.; Galow, T. H.; Schotter, J.; Bal, M.; Ursache, A.; Tuominen, M. T.; Stafford, C. M.; Russell, T. P.; Rotello, V. M. Langmuir 2001, 17, 6396-6398. (18) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Anal. Chem. 1997, 69, 2323-2328. (19) Frens, G. Nature 1973, 241, 20-22. (20) (a) El-sayed, M. A. Acc. Chem. Res. 2001, 34, 257-264. (b) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306-313.

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assembling techniques, they prepared perfectly spherical electrodes, in the size range of 1-30 µm. In this article, colloid Au nanoparticles are used as conductive building blocks, and their assemblies on an insulated plane are proved to behave as nanoelectrode ensembles. Different from the NEEs constructed by other techniques,15,16 the colloid chemical strategy reveals several advantages: (a) it is robust and can survive in many electrochemical reactions; (b) colloid synthesis provides welldefined geometry of nanoelectrodes,19,20 which might be the basis of future nanosheet or nanorod arrays, etc., by using such “bottomup” strategy; (c) time-dependent self-assembly provides an accurate control over interelectrode spacing;23,24 (d) this solutionbased strategy is inexpensive and not confined in substrate size and shape.23,24 (21) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo, M. L.; Zasadzinski, J. A. N. Langmuir 1991, 7, 1051-1054. (22) Hansma, H. G.; Weisenhorn, A. L.; Edmundson, A. B.; Gaub, H. E.; Hansma, P. K. Clin. Chem. 1991, 37, 1497-1503.

EXPERIMENTAL SECTION Reagents. (3-Mercaptopropyl)trimethoxysilane (MPTMS), potassium ferricyanide, potassium ferrocyanide, and sodium perchlorate hydrate were purchased from Aldrich and used without further purification. All other materials were of reagent grade and used as received. Solutions were prepared from ultrapure water purified with Milli-Q plus system (Millipore Co.). Its resistivity was over 18 MΩ cm. Instrumentation. The electrochemical measurements were performed using an EG&G M273A electrochemical system. A Ag/ AgCl electrode was used as the reference electrode, a Pt foil as the counter electrode, and an O-ring with 6-mm inner diameter was used for all electrochemical experiments (geometric area is ∼0.283 cm2). 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 256 lines per image resolution. Construction of NEEs on ITO (Mica) and Their Atomic Force Microscopy (AFM) and Electrochemical Characterization. Spherical colloidal gold nanoparticles were prepared by citrate reduction of HAuCl4 in aqueous solution.19 The average nanoparticle diameter is 12 nm as measured by TEM (not shown here). The concentration of colloidal solution for the NEE construction is ∼3 nM. ITO grass was sonicated in neat acetone, soap solution, and water for ∼15 min each and then immersed in saturated NaOH aqueous solution. After being thoroughly rinsed by water and dried in argon, the ITO was immersed in 2 mM MPTMS ethanol solution for 1 day. After exhaustive rinsing by ethanol, the ITO substrate was heated to 80 °C to remove loosely bound MPTMS molecules (loosely bound MPTMS can lead to nanoparticle aggregate immersing ITO substrate to colloid solution). The preparation of MPTMS-derivatized mica followed the same procedure as the ITO except for an additional hydroxylation process in concentrated H2SO4.21,22 The MPTMS-derivatized mica substrate was then immersed in the above colloid solution for various times, rinsed thoroughly, and stored in water. Prior to AFM imaging, the colloidal Au-derivatized substrate was dried in argon. RESULTS AND DISCUSSION AFM Characterization for Nanoelectrode Number Density. Currently, colloid synthesis techniques provide flexible control19,20 over metal nanoparticle size and geometry such as nanoballs, nanorods, nanosheets, and so on. Combined with such synthesis control, self-assembly chemistry of colloidal metal nanoparticles is expected to lead to the NEEs with variable nanoelectrode element geometry and interspacing. As a standard nanometerscale imaging tool, AFM was used to monitor the topographical characteristics of the NEEs.23,24 Here, spherical colloidal Au nanoparticles with an average diameter of 12 nm were used as examples to illustrate the preparation of the NEEs and control over NEEs parameters as shown in Figure 1. Mica was used for AFM imaging to obtain better images though we demonstrate the attaching efficiency of colloid nanoparticles on both substrates (23) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314-323. (24) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P. and Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148-1153.

Figure 2. Ac impedance plots of MPTMS-derivatized ITO (open circles), the NEEs by 9-min self-assembly (open squares) and the NEEs by 1-day self-assembly (solid squares) in the as-prepared colloid solution. (A) is the plots of imaginary part vs real part, and (B) is the plots of the in-phase impedance Ω vs ω-1/2 after correcting for solution resistance and double-layer capacitance.

is nearly same in separate experiments. Experimental nanoelectrode number density was obtained by manually counting nanoelectrode elements in AFM images, and each solid square in Figure 1D represents an average value of three AFM images at the same NEEs (for clarity, only three typical images are shown here). As described previously,23,24 nonlinear least-squares fitting of AFM data (solid squares in Figure 1D) in different anchoring times leads to a quantitative equation (solid line in Figure 1D): q ) 106.473t1/2, where q is the number of particles reaching a 1-cm2 surface per minute (namely, nanoelectrode number density) and t is self-assembly time (in minutes). This equation shows that self-assembling time scale control determines wholly the number density of nanoelectrodes at the NEEs system. Ac Impedance Studies. Following Finklea’s treatment25 of a pinhole-based microarray electrode, ac impedance was performed to map roughly nanoelectrode distribution on MPTMS. It has to be noted that alternating current impedance spectroscopy provides the most accurate picture of pinhole parameters in an self(25) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667.


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Figure 3. Cyclic voltammograms for 1 mM Fe(CN)63-/4- in 0.1 M KCl aqueous solution obtained using (A) the NEEs by 9-min self-assembly and (B) the NEEs by 1-day self-assembly. The scan rates are 1, 10, 50, 100, and 200 mV/s.

assembly monolayer because dc potential is set at zero overpotential and the ac potential is only a few millivolts in magnitude.25 The ac impedance spectroscopy of MPTMS-modified ITO exhibits two nearly linear domains at high and low frequencies as shown in Figure 2. Following the Finklea’s treatment,25 an approximate estimate of the nearest spacing Ro between the two active sites can be derived from the frequency corresponding to the “knee” frequency of the in-phase plot, i.e., the transition between highand low-frequency domains. The radius of the active area Ra can also be estimated according to active area fraction and Ro value derived from ac impedance spectroscopy. When 3.21 × 1010 nanoelectrodes/cm2 (by 9-min self-assembly) were anchored on the MPTMS, both the electron-transfer resistance and the “knee” frequency of the in-phase plot exhibit evident decrease. Following the above treatment, 1.27 and 6.69 µm were obtained for Ra and Ro, respectively (the nanoelectrode area fraction for the NEEs is obtained from the kinetics curve by AFM measurements in Figure 1D). Why is the Ra value derived from the ac impedance 2 orders larger than the true diameter of nanoelectrodes? Due to frequency limitation, in fact, the faradaic impedance cannot determine the parameters of the nanoelectrode on the order of 1 nm.25 Therefore, the Ra values derived from ac impedance plots are not the radius of a single nanoelectrode but rather the average radius of the patches consisting of closely packed nanoelectrodes. This is consistent with nonuniform distribution of low-coverage nano3602


electrodes on MPTMS shown in AFM images (Figure 1A and B). At this point, the NEEs behave as the “nanoelectrode-patch” assemblies in the ac impedance spectroscopy like “SAM-pinholebased” microelectrode arrays. In contrast, the NEEs by 1-day selfassembly exhibit a smaller heterogeneous electron-transfer resistance and larger rate constant, which is more similar to a macroelectrode. This is consistent with the characteristics of other NEE systems that show a high sensitivity to the kinetics of electron transfer.26 Variable Scan Rate Voltammograms. A proposed condition generally reported for observing independent behavior of the active sites and therefore avoiding a shielding effect in microelectrode array system is that27

d/r > 12

(1)

where d is the interelectrode center-to-center distance and r is the radius of the active site. This condition is often used for the design of a microelectrode array. Take the NEEs by 9-min selfassembly, for example, the number density of 3.21 × 1010 nanoelectrodes/cm2 (by referring to the kinetics curve in Figure (26) (a) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663-6669. (b) Bond, A. M.; Oldham, K. B.; Zoski, C. G. Anal. Chim. Acta 1989, 216, 177-230. (27) Saito, Y. Rev. Plarogr. 1968, 15, 177-189.

Table 1. ∆EPeak Values as a Function of Scan Rate for the Fe(CN)63-/4- Couple for the As-Prepared NEEs by 9 min and 1 day ∆Epeak (mV)

scan rate (mV/s)

9 min

1 day

1 10 50 100 200

80 139 213 275 343

59 65 104 122 146

Figure 4. Log(anodic peak current) vs log(scan rate) for cyclic voltammograms of 1 mM Fe(CN)63-/4- in 0.1 M KCl aqueous solution obtained using the NEEs by 9-min self-assembly (open circles) and the NEEs by 1-day self-assembly (open squares). The solid line with a slope of 0.5 is made artificially in order to make a comparison with the linear diffusion to the macroelectrode.

1D) corresponds to an average interelectrode separation of ∼50 nm. By the above model, most of nanoelectrodes in the NEEs are not diffusionally isolated unless the scan rate is sufficiently large. Therefore, the ideally radial diffusion current responses cannot be attained at conventional scan rates for the NEEs by 9-min self-assembly. The above AFM and ac impedance plot studies demonstrate that the distribution of nanoelectrodes is nonuniform and tends to group; therefore, the as-prepared NEEs behave as “patch” assemblies at conventional scan rates due to diffusional shielding effects. At this case, the NEEs with different area fractions would have different “patch” size and “interpatch” spacing; therefore, transition to quasireversible behavior would be expected to occur at different scan rates. Further supportive evidence can be seen in Figure 3. Figure 3B shows voltammograms at various scan rates for Fe(CN)63-/4at the as-prepared NEEs by 1-day self-assembly. The peak separation (∆Epeak) values are shown in Table 1. This couple shows reversible voltammetry (∆Epeak ) ∼59 mV) at scan rates below 1 mV/s. But the voltammograms become quasireversible at scan

Figure 5. Cyclic voltammograms at 50 mV/s in aqueous Fe(CN)64at (A) a gold macroelectrode in 50 mM KNO3 and (B) the NEEs by 9-min self-assembly in 50 mM KNO3. Fe(CN)64- concentrations are as indicated.

rates above 1 mV/s (Table 1). The voltammograms for Fe(CN)63-/4at the NEEs by 9-min self-assembly are shown in Figure 3A. The corresponding ∆Epeak values are shown in Table 1. At the NEEs by 9-min self-assembly, it is impossible to obtain the reversible case, even at a scan rate of 1 mV/s (Table 1). The effect of quasireversible electrochemistry is also clearly seen in the diminution of the peak currents at the NEEs by 9-min selfassembly (relative to the NEEs by 1-day self-assembly). This kind of diminution in peak current (relative to reversible case) is characteristic of the quasireversible case at other NEE systems.28 (28) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51.


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The above results show that the “transition point” between reversible and quasireversible cases can be easily adjusted by selfassembly kinetics control of nanoparticles. The above voltammetric data were also subjected to semiquantitative analysis by plotting log(ipa) versus log(ν), where ipa is the anodic peak current in the voltammogram and ν the scan rate as shown in Figure 4. As expected for the macroelectrode system, a slope of 0.5 is obtained at conventional scan rates, associated with linear diffusion to the electrode. However, this is not the case for the present NEE system. For intuitionistic comparison with linear diffusion to macroelectrode system, a line with a slope of 0.5 was made artificially as shown in Figure 4 (solid line). It is observed that, at low scan rates, the slope equals 0.5, consistent with linear diffusion. As scan rates are increased, the slope slightly decreases because mixed diffusional fields (linear and nonlinear) are operative. Besides, the decreasing trend of the slope for the NEEs by 9-min self-assembly is more evident than that by 1-day self-assembly. This shows that the NEEs with a smaller area fraction (or number density) need the longer time scale (lower scan rates) to attain the linear diffusion profile. Both the above ∆Epeak and log(ipa) studies demonstrate that the NEEs by different self-assembling times show variable sensitivity to heterogeneous electron-transfer kinetics. The NEEs with a lower number density show a higher sensitivity to electrontransfer kinetics. For example, fractional electrode areas (by referring to Figure 1D) indicate that, for the reversible case, the diffusional flux of species at the elements of the NEEs by 9-min self-assembly will be 6 times higher than at the NEEs by 1-day self-assembly. The higher fluxes at the NEE elements mean that the NEEs by 9-min self-assembly will be more sensitive26 to the kinetics of electron transfer than that by one self-assembly. The adjustable electrochemical interface by nanoelectrode number density is expected to have some advantages in electrochemical sensor applications where the electroactive species with a different electron-transfer rate constant can be differentiated selectively. (At the NEEs, the redox wave for the kinetically slow species might be unobservable in its redox potential region where the (29) (a) Ugo, P.; Moretto, L. M.; Bellomi, S.; Menon, V. P.; Martin, C. R. Anal. Chem. 1996, 68, 4160-4165. (b) Brunetti, B.; Ugo, P.; Moretto, L. M.; Martin, C. R. J. Electroanal. Chem. 2000, 491, 166-174. (30) Bard, A. J.; Rubinstein, I. Electroanalytical Chemistry; Marcel Dekker Inc.: New York, 1996; pp 183-184. (31) Rai-Choudhury, P., Ed. SPIE Handbook of Microlithography, Microchining and Microfabrication; SPIE: Bellingham, WA, 1997; Vol. 1, pp 139-250. (32) (a)Natelson, R. L.; Willet, K. W.; West, L. N.; Pfeiffer, D. Appl. Phys. Lett. 2000, 77, 1991-1999. (b) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718-8729.

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kinetically fast redox species in the same region still show redox waves.) Electroanalytical Detection Limits at the NEEs. It has been proved that the NEEs show enhanced electroanalytical detection limits relative to a conventional macroelectrode.29 The present NEEs follow the same rule. Voltammograms for various concentrations of K4Fe(CN)6 at the NEEs by 9-min self-assembly are shown in Figure 5B. Compared with the similar experiment performed on the gold macroelectrode with the same geometric area (Figure 5A), the NEEs gives a voltammetric detection limit that is at least 20 times lower than that at the macroelectrode. According to the theory of microelectrode array8a under “totaloverlap” diffusional regime, the signal is proportional to the geometric area, while noise is proportional only to fractional electrode area, which improves the signal-to-noise ratio. The active area of the NEEs by 9-min self-assembly is ∼0.0363 cm2 in 1-cm2 geometric area; the theoretical detection limit is ∼28 times larger than macroelectrode. This is consistent with experimental results. CONCLUSIONS Actually, nanoparticles anchored onto a SAM-blocking electrode is equivalent to pinholes or defects of SAM in a heterogeneous electron-transfer process. It is logical to treat nanoparticle ensembles on SAM as nanoelectrode ensembles.30 Remarkably, this colloid chemical approach to the NEEs is simple, inexpensive, and reproducible. Also, this solution-based fabrication strategy was not constrained by substrate size and shape (for example, even the inner surface of a capillary can also be used as a substrate), which could be helpful to sensor device fabrication. Besides, this study indicates that the sensitivity to heterogeneous electrontransfer kinetics is highly adjustable, simply, by time scale control. It is also noted that the anchoring of nanoelectrodes on a mixed monolayer of mercaptosilanes and methyl-terminated alkysilanes could be another strategy to control nanoelectrode number density and spacing; studies in this direction are underway. The selfassembly techniques can also be combined with electron beam lithography31,32 and surface-confined seeding growth,23 which could be an excellent “bottom-up” strategy to the construction of wellordered and patterned nanoelectrode arrays. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China. Received for review March 28, 2002. Accepted May 6, 2002. AC025661O

Colloid Chemical Approach to Nanoelectrode Ensembles with Highly

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