Anal. Chem. 2009, 81, 8249–8255
Technical Notes Fabrication of TiO2 and Metal Nanoparticle-Microelectrode Arrays by Photolithography and Site-Selective Photocatalytic Deposition Xiaoguang Li, Yang Tian,* Peipei Xia, Yongping Luo, and Qi Rui Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, People’s Republic of China This paper demonstrates a novel and facile technique for the production of microelectrode arrays based on either TiO2 or metal nanoparticles, which combines photolithography and photocatalytic deposition. A procedure that involves photolithographic selective decomposition of superhydrophobic n-octadecyltriethoxysilane (ODS) has been developed to create superhydrophobic/superhydrophilic TiO2 patterns (i.e., microelectrode arrays based on TiO2 nanoparticles). The generated TiO2 patterns can function as molecular microtemplates, in which elevated metal nanoparticle-based microelectrode arrays are produced by photocatalytic deposition to form microelectrode arrays based on metal nanoparticles. Microscopy and atomic force microscopy have demonstrated that the metal nanoparticles grow siteselectively inside the TiO2 microtemplates. This developed approach can be used to create microelectrode arrays composed by either TiO2 nanoparticles or various metal nanoparticles, such as gold, silver, and platinum, with different patterns. The electrochemical behavior of the as-prepared microelectrode arrays has also been characterized by cyclic voltammetry. Microelectrodes, especially microelectrode arrays have been recognized as the important analytical tools, because of remarkable advantages, in comparison to classical macroelectrodes.1,2 * To whom correspondence should be addressed. Tel.: +86-21-65987075. Fax: +86-21-65982287. E-mail:
[email protected]. (1) (a) Daniele, S.; Ciani, I.; Battistel, D. Anal. Chem. 2008, 80, 253–259. (b) Lin, Z. Y.; Takahashi, Y.; Kitagawa, Y.; Umemura, T.; Shiku, H.; Matsue, T. Anal. Chem. 2008, 80, 6830–6833. (c) Lin, Z. Y.; Takahashi, Y.; Murata, T.; Takeda, M.; Ino, K.; Shiku, H.; Matsue, T. Angew. Chem., Int. Ed. Engl. 2009, 48, 2044–2046. (d) Lin, Z. Y.; Takahashi, Y.; Murata, T.; Takeda, M.; Ino, K.; Shiku, H.; Murata, T. Angew. Chem., Int. Ed. Engl. 2009, 48, 2044–2046. (e) Valsesia, A.; Colpo, P.; Mannelli, I.; Mornet, S.; Bretagnol, F.; Ceccone, G.; Rossi, F. Anal. Chem. 2008, 80, 1418–1424. (2) (a) Burmeister, J. J.; Moxon, K.; Gerhardt, G. A. Anal. Chem. 2000, 72, 187–192. (b) Valsesia, A.; Lisboa, P.; Colpo, P.; Rossi, F. Anal. Chem. 2006, 78, 7588–7591. (c) Shi, G.; Sun, Z.; Liu, M.; Zhang, L.; Liu, Y.; Qu, Y.; Jin, L. Anal. Chem. 2007, 79, 3581–3588. 10.1021/ac9009879 CCC: $40.75 2009 American Chemical Society Published on Web 09/08/2009
Their attractive properties include enhanced mass-transport rates, an associated rapid approach to steady-state, improved Faradicto-capacitive current ratios and reduced ohmic potential drops.3,4 With the development of nanoscience and nanotechnology, many microelectrode arrays that are based on nanomaterials have been exploited for biosensors, microreactors, biological applications, and so on. Recently, more and more nanomaterials derived from semiconductors and metals have been reported to be used as excellent electrode materials, because of their striking advantages such as high electrocatalytic activity, good stability, and high biocompatibility.5-7 Direct electron transfer of enzymes and proteins has been widely realized at these nanomaterials,8-15 which can constitute ideal platforms for the electrochemical (3) (a) Davies, T. J.; Compton, R. G. J. Electroanal. Chem. 2005, 585, 63–82. (b) Sandison, M. E.; Anicet, N.; Glidle, A.; Cooper, J. M. Anal. Chem. 2002, 74, 5717–5725. (4) (a) Yang, Q.; Liang, Y.; Zhou, T.; Shi, G.; Jin, L. Electrochem. Commun. 2009, 11, 893–896. (b) Streeter, I.; Fietkau, N.; Campo, J. D.; Mas, R.; Mnoz, F. X.; Compton, R. G. J. Phys. Chem. C 2007, 111, 12058–12066. (5) (a) Tian, Y.; Mao, L.; Okajima, T.; Ohsaka, T. Anal. Chem. 2002, 74, 2428– 2434. (b) Tian, Y.; Mao, L.; Okajima, T.; Ohsaka, T. Anal. Chem. 2004, 76, 4162–4168. (c) Liu, H.; Tian, Y.; Deng, Z. Langmuir 2007, 23, 9487– 9494. (d) Xia, P.; Liu, H.; Tian, Y. Biosen. Bioelectron. 2009, 4, 2470–2474. (e) Zhu, A.; Tian, Y.; Liu, H.; Luo, Y. Biomaterials 2009, 30, 3183–3188. (6) (a) Deng, Z.; Rui, Q.; Yin, X.; Liu, H.; Tian, Y. Anal. Chem. 2008, 80, 5839– 5846. (b) Deng, Z.; Tian, Y.; Yin, X.; Rui, Q.; Liu, H.; Luo, Y. Electrochem. Commun. 2008, 10, 818–820. (c) Deng, Z.; Gong, Y.; Luo, Y.; Tian, Y. Biosen. Bioelectron. 2009, 24, 2465–2469. (d) Luo, Y.; Tian, Y.; Zhu, A.; Rui, Q.; Liu, H. Electrochem. Commun. 2009, 11, 174–176. (e) Luo, Y.; Liu, H.; Rui, Q.; Tian, Y. Anal. Chem. 2009, 81, 3035–3041. (f) Luo, Y.; Tian, Y.; Rui, Q. Chem. Commun. 2009, 3014–3017. (7) (a) Bhaskara, V. C.; Hongyun, L.; Vigneshwaran, M.; Fotios, P.; James, F. R. Electrochem. Commun. 2009, 11, 819–822. (b) Weizmann, Y.; Patolsky, F.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 3452–3454. (c) Niazov, T.; Shlyahovsky, B.; Willner, I. J. Am. Chem. Soc. 2007, 129, 6374–6375. (d) Marritt, S. J.; Kemp, G. L.; Xiaoe, L.; Durrant, J. R.; Cheesman, M. R.; Butt, J. N. J. Am. Chem. Soc. 2008, 130, 8588–8589. (8) Bhirde, A.; Patel, V.; Gavard, J.; Zhang, G.; Sousa, A. A.; Masedunskas, A.; Leapman, R. D.; Weigert, R.; Gutkind, J. S.; Rusling, J. F. ACS Nano 2009, 3, 307–316. (9) Kim, S. N.; Rusling, J. F.; Papadimitrakopolous, F. Adv. Mater. 2007, 19, 3214–3228. (10) Baron, R.; Zayats, M.; Willner, I. Anal. Chem. 2005, 77, 1566–1571. (b) Riskin, M.; Basnar, B.; Chegel, V. I.; Katz, E.; Willner, I.; Shi, F.; Zhang, X. J. Am. Chem. Soc. 2006, 128, 1253–1260. (11) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21–25.
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
8249
detection of biological species. However, it is still challenging work to develop a facile, low-cost, and environmentally friendly method for the creation of microelectrode arrays based on semiconductor and metal nanomaterials. This paper reports a novel and facile route through photolithography and photocatalytic deposition, for the fabrication of large-area microelectrode arrays based on TiO2 and metal nanoparticles. Microelectrode arrays of TiO2 nanoparticles that feature superhydrophobic/superhydrophilic surfaces have been created by the selective photodecomposition of superhydrophobic n-octadecyltriethoxysilane (ODS)-modified TiO2 film. Ultraviolet (UV) irradiation going through a photomask is conducted from the back of the film. The superhydrophilic regions then have been used in the site-selective photocatalytic growth of metal nanoparticles to generate elevated metal nanoparticle-based microelectrode arrays. This approach readily allows the production of microelectrode arrays based on either TiO2 or a variety of metal (such as gold, silver, and platinum) nanoparticles, with sizes down to several micrometers. The advantages of the developed process, which was performed under ambient “air-phase” conditions, include easy-operating, low-cost, template-free, and nonenvironmental harming. EXPERIMENTAL SECTION Chemicals and Materials. n-Octadecyltriethoxysilane (ODS) was obtained from Alfa Aesar. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4, 99.0%) was purchased from Aldrich (St. Louis, MO). Hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6, 98.5%) and fluorescein sodium (Uranine) were obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). AgNO3 was received from Shanghai Chemical Reagents (Shanghai, PRC). The solution used in the work was freshly prepared using Milli-Q water. Indium tin oxide (ITO)-coated glass plates with a square resistance of 10-20 Ω/cm2 were obtained from Nanbo (Shenzhen, PRC). Instruments and Measurements. The micropatterns were characterized via microscopy (Model XTL-3400XE, CANY, Shanghai, PRC), fluorescent microscopy (Model Eclipse TE300 Quantum inverted microscope, Nikon), and atomic force microsocpy (AFM) (Model AJ-III, Aijian Nano-Science Development Co., Ltd., Shanghai, PRC). A Hitachi Model S-4500 (Tokyo, Japan) scanning electron microscopy (SEM) system and a Model Quantum-2000 Scanning ESCA microprobe (ULVAC-PHI, Inc.) with Mg KR radiation were employed to record the images of TiO2 and metal nanoparticles. The water contact angle was measured with a contact angle meter (Model JC 2000A, Shanghai, PRC). X-ray photoelectron spectroscopy (XPS) (Model PHI 5000) was used to track the modification and decomposition of ODS, as well as the growth of metal nanoparticles. A CHI 660 electrochemical workstation (CH Instruments, Shanghai, PRC) was used in all electrochemical measurements, which were performed in a conventional three-electrode electrochemical cell. The (12) Weizmann, Y.; Patolsky, F.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 3452–3454. (13) Krishnan, S.; Hvastkovs, E. G.; Bajrami, B.; Choudhary, D.; Schenkman, J. B.; Rusling, J. F. Anal. Chem. 2008, 80, 5279–5285. (14) Krishnan, S.; Hvastkovs, E. G.; Bajrami, B.; Jansson, I.; Schenkman, J. B.; Rusling, J. F. Chem. Commun. 2007, 1713–1715. (15) Hoover, D. K.; Chan, E. W. L.; Yousaf, M. N. J. Am. Chem. Soc. 2008, 130, 3280–3281.
8250
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
Figure 1. Fabrication procedure of microelectrode arrays: (a) superhydrophobic ODS-modified TiO2 film, (b) TiO2 nanoparticlebased microelectrode arrays after photolithography with a photomask, and (c) metal nanoparticle-based microelectrode arrays after siteselective photocatalytic deposition of metal nanoparticles.
reference electrode was a KCl-saturated Ag|AgCl electrode, and the counter electrode was platinum wire. Preparation of TiO2 Films. ITO-coated glass plates were thoroughly cleaned by sonicating for 30 min in the following solvents successively: soapy water, neat ethanol, 1 M NaOH, and water. A TiO2 film was prepared as follows: an ITO-coated glass plate was covered with a nanoporous TiO2 film prepared from an anatase TiO2 sol (diameter of 20-50 nm TiO2 nanoparticles, Ishihara Sangyo Kaisha) by spin-coating (sintered at 723 K for 30 min). RESULTS AND DISCUSSION Fabrication of TiO2 Nanoparticle-Based Microelectrode Arrays. The procedure for the fabrication of TiO2 and metal nanoparticle-based microelectrode arrays is outlined in Figure 1. First, ODS-modified TiO2 film was obtained by immersing a TiO2 film into ODS ethanol solution for 24 h. The presence of ODS molecules onto the TiO2 film was directly verified using X-ray photoelectron spectroscopy (XPS). As shown in Figure 2, a new Si 2s peak at 154.3 eV was generated, but Ti 2p peaks at 464.2 and 458.6 eV were decreased to almost background,16,17 (16) Hong, J.; Porter, D. W.; Sreenivasan, R.; McIntyre, P. C.; Bent, S. F. Langmuir 2007, 23, 1160–1165. (17) (a) Akamatsu, K.; Kimura, A.; Matsubara, H.; Ikeda, S.; Nawafune, H. Langmuir 2005, 21, 8099–8102. (b) Blackstock, J. J.; Donley, C. L.; Stickle, W. F.; Ohlberg, D. A. A.; Yang, J. J.; Stewart, D. R.; Williams, R. S. J. Am. Chem. Soc. 2008, 130, 4041–4047.
Figure 2. X-ray photoelectron spectroscopy (XPS) spectra of (A) Si 2s and (B) Ti 2p obtained at the bare TiO2 film (spectrum a), ODS-modified TiO2 film (spectrum b), and ODS-modified TiO2 film (spectrum c) after irradiation under UV light (50 mW/cm2) for 30 min.
Figure 3. Cyclic voltammograms (CVs) for 1 mM K3Fe(CN)6 obtained at the ITO substrate (trace a), the bare TiO2 film (trace b), the ODS-modified TiO2 film (trace c), and TiO2 nanoparticle-based microdisk arrays (trace d) with a radius of 6 µm in 1 M KCl solution at a scan rate of 5 mV/s.
indicating that ODS was bound to the TiO2 surface. As expected, the film showed a superhydrophobic surface with a water contact angle of 155° ± 3° (Figure 1a) and electrochemical passivation. From Figure 3, we can see that well-defined electrochemical redox peaks of K3Fe(CN)6 were obtained at the ITO substrate (see Figure 3a) and the bare TiO2 surface (see Figure 3b); however, no electrochemical response was observed at the ODS-modified TiO2 film (see Figure 3c). These results reveal that the modification of ODS passivated the electrochemical activity of the TiO2 surface. After finishing the preparation of the superhydrophobic TiO2 film, the back of this film was covered with a photolitho-
graphic mask and exposed to UV irradiation. To optimize the intensity of UV light and choose the proper irradiation time, XPS and contact angle analysis were applied to direct the decomposition of ODS molecules. After being irradiated by 50 mW/cm2 UV light for 30 min, the light transmission regions of the ODS-modified TiO2 surface exhibited superhydrophilic properties (see Figure 1b). Meanwhile, as demonstrated in Figure 2, the XPS peak of Si 2s decreased to the background,16 whereas the peak ascribed to Ti 2p was restored after UV irradiation for 30 min,17 which indicated the decomposition of ODS. As a consequence, the superhydrophobic/ superhydrophilic TiO2 patterns were obtained as depicted in Figure 1b. The designed photomask, having light transmission regions of 6-µm-radius disks and 10-µm-width bands were respectively placed on the back of ODS-modified TiO2 film for UV irradiation to generate the superhydrophobic/superhydrophilic patterns. Figure 4 shows fluorescence micrographs of TiO2 patterns with a 6-µm-radius disk (Figure 4a) and 10 µm-width band (Figure 4b), indicating that the patterned TiO2 films retain the dimensions of the photomasks without noticeable spreading. SEM images of the TiO2 film were also given in Figures 4c and 4d. The diameter of TiO2 nanoparticles dispersed onto ITO substrate was in the range of 20-50 nm, and the thickness of TiO2 film is ∼3 µm. One marked property of the superhydrophobic|superhydrophilic surface is its very large electrochemical activity contrast, which can be employed to form TiO2 nanoparticle-based microelectrode arrays. As shown in Figure 3d, the redox activity was restored and the steady-state currents were obtained at patterned TiO2 nanoparticles surface with radius of 6 µm, indicating that, as expected, TiO2 nanoparticle-based microelectrode arrays were successfully fabricated by the developed methodology. Photocatalytic Deposition of Metal Nanoparticle-Based Microelectrode Arrays. The superhydrophobic/superhydrophilic TiO2 surface, which functions as a molecular microtemplate, can be employed to selectively fill superhydrophilic regions with water-soluble materials. In the case of the formation of gold nanoparticle-based patterns, a solution of 0.1 M HAuCl4 Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
8251
Figure 4. (a, b) Fluorescence micrographs of TiO2 nanoparticlebased microelectrode arrays (a) with a disk radius of 6 µm and (b) with a bandwidth of 10 µm. (c, d) SEM images of TiO2 film. (Panel d shows a cross-sectional image.)
was introduced dropwise on the superhydrophobic/superhydrophilic surface. UV light then was going through the photomask and was irradiated onto the back of the ITO substrate to reduce metal nanoparticles from solution, because TiO2 has well-studied photocatalytic properties, which makes it an excellent choice for reducing metal ions from solution by UV illumination,22,23 as shown in eqs 1 and 2. TiO2 + hν(UV) f e-(CB) + h+(VB)
(1)
Au3+ + 3e- f Au
(2)
XPS and ultraviolet-visible-light (UV-vis) spectroscopy were applied to confirm the deposition of gold nanoparticles. As depicted in Figure 5A, the XPS peaks were observed at 87.9 eV (Au 4f5/2) and 84.3 eV (Au 4f7/2), which are ascribed to metallic gold.24,25 As shown in Figure 5B, the UV-vis spectrum of gold nanoparticles deposited onto the TiO2 film exhibits a peak at ∼550 nm, which should be attributed to the surface plasmon resonance of gold nanoparticles. The size of gold nanoparticles (18) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley: New York, 2001; pp 173-174. (b) Liu, X.; Baronian, K. H. R.; Downard, A. J. Anal. Chem. 2008, 80, 8835–8839. (c) Lee, H. J.; Beriet, C.; Ferrigno, R.; Girault, H. H. J. Electroanal. Chem. 2001, 502, 138–145. (19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley: New York, 2001; pp 786-788. (20) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969; pp 220-222. (21) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley: New York, 2001; pp 174-185. (b) Berduque, A.; Lanyon, Y. H.; Beni, V.; Herzog, G.; Watson, Y. E.; Rodgers, K.; Stam, F.; Alderman, J.; Arrigan, D. W. M. Talanta 2007, 71, 1022– 1030. (22) Corain, B.; Schmid, G.; Toshima, N. Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control, 1st ed.; Elsevier: Amsterdam, 2008; pp 263-267. (23) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Langmuir 2003, 19, 469–474. (24) Casaletto, M. P.; Longo, A.; Martorana, A.; Prestianni, A.; Venezia, A. M. Surf. Interface Anal. 2006, 38, 215–218. (25) Yu, J. C.; Wang, X.; Wu, L.; Ho, W.; Zhang, L.; Zhou, G. Adv. Funct. Mater. 2004, 14, 1178–1183.
8252
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
themselves is ∼25 nm (see inset in Figure 5B). Photocatalytic deposition occurred site-selectively in the superhydrophilic regions, thus producing patterns of metal nanoparticles with welldefined structures. Figure 6a and 6b present microdisk patterns of gold nanoparticles produced from the 6-µm-radius superhydrophobic/superhydrophilic TiO2 microtemplates. From microscopic (Figure 6a) and AFM (Figure 6b) images, elevated patterns of gold nanoparticles with 6-µm-radius disks were clearly observed. The height of the elevated gold nanoparticles is ∼750 nm. Meanwhile, the patterns of gold nanoparticles with 10-µmwidth bands are also shown in Figure 6c and Figure 6d. The size of these patterns is in good agreement with the respective TiO2 nanoparticle-based patterns, suggesting that TiO2 patterns have been successfully employed as microtemplates for production of metal nanoparticle-based patterns. For the patterned surface, a large wettability contrast and electrochemical activity contrast are found, which could be used to further applications in microelectrode arrays. Electrochemical Characterization. Voltammetric studies with a model electroactive species have frequently been used to confirm microelectrode dimensions where assumptions are made about the geometry of the microsized electrode, as well as to investigate the diffusion characteristics of the probe species to the electrode surface. Figure 7A shows representative cyclic voltammetric data for 1 mM K3Fe(CN)6 obtained at TiO2 nanoparticle-based microdisk arrays with various radii. It can be seen that the shape of each voltammogram is strongly dependent on the individual microelectrode radius. Typical voltammogram with redox peaks was observed at the bare TiO2 surface, whereas very well-defined steady-state responses were obtained at TiO2-based microelectrode arrays with radii of 6 and 8 µm. Cyclic voltammograms (CVs) were increasingly peaked, indicating an increasing contribution of linear diffusion, as the individual microelectrode radius was enlarged from 6 µm to 12.5 µm. More interestingly, the microarrays with a radius of 6 µm generated the largest current densitys4 times greater than that produced by the 12.5-µm-radius arrayssrevealing the enhanced sensitivity of microelectrode arrays. Because the height of ODS molecules (on the order of tens of angstroms) modified onto TiO2 film could be negligible, compared to the roughness of nanostructured TiO2 surface and the microsized patterns, the TiO2 nanoparticle-based recessed microdisk arrays could be considered as coplanar ones. Thus, the Ilim value is given by18 iL ) 4nNFrDC
(3)
where n is the number of electrons, N the number of microdisks in the array, F the Faraday constant, D the diffusion coefficient of K3Fe(CN)6 (an average value of 7.6 × 10-6 cm2/s was assumed19,20), r the radius of an individual microdisk, and C the concentration of K3Fe(CN)6. The radii calculated by the coplanar microdisk arrays model are summarized in Table 1, which agree well with those measured by the microscopic images. On the other hand, CVs for 1 mM K3Fe(CN)6 in 1 M KCl obtained at TiO2 nanoparticle-based microband arrays with various widths are also given in Figure 7B. Only quasi-steadystate responses were observed at microband arrays, even at a
Figure 5. (A) X-ray photoelectron spectra of Au 4f obtained at the bare TiO2 film (spectrum a), ODS-modified TiO2 film (spectrum b), ODSmodified TiO2 film after photolithography (spectrum c) and (d) gold nanoparticles deposited onto TiO2 film (spectrum d). (B) UV-vis spectrum of gold nanoparticles deposited onto TiO2 film; inset shows an SEM image of gold nanoparticles.
Figure 6. (a, c) Microscopic images and (b, d) AFM images of elevated gold nanoparticle-based microarray electrodes (a, b) with disk radius of 6 µm and (c, d) with bandwidth of 10 µm.
width of 10 µm. This observation may be due to the twodimensional (2D) diffusion model of band electrodes behaving much like the hemicylindrical system and the partial overlapping of the diffusion fields, which resulted from the relatively short distance between each microelectrode. Similar to the
electrochemical results obtained at microdisk arrays, CVs were also increasingly peaked with the increasing individual microband width. The microarrays with a width of 10 µm produced the largest current density (twice as large as the smallest one, which was produced by the arrays with a width of 100 µm. Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
8253
Figure 7. Cyclic voltammograms (CVs) for 1 mM K3Fe(CN)6 obtained at TiO2 nanoparticle-based (A) microdisk electrode arrays with a radius of 6 µm (trace a), 8 µm (trace b), 10 µm (trace c), 12.5 µm (trace d), and bare TiO2 film (trace e) and (B) microband electrode arrays with a width of 10 µm (trace a), 50 µm (trace b), 100 µm (trace c) and bare TiO2 film (trace d) in 1 M KCl solution at a scan rate of 5 mV/s.
Figure 8. CVs for 1 mM K3Fe(CN)6 obtained at elevated gold nanoparticle-based (A) microdisk arrays with a radius of 6 µm (trace a), 8 µm (trace b), 10 µm (trace c), 12.5 µm (trace d), and bare gold nanoparticles film (trace e) and (B) microband arrays with width of 10 µm (trace a), 50 µm (trace b), 100 µm (trace c), and bare gold nanoparticles film (trace d) in 1 M KCl solution at a scan rate of 5 mV/s.
Table 1. Values for the Geometrical and Electrochemical Parameters of the TiO2 Nanoparticle-Based Microelectrode Arrays
gas constant, T the temperature, v the scan rate, and x the number of arrays. According to eq 4, the widths of microband electrodes were calculated using the Ilim values from the CVs and are also summarized in Table 1. The calculated values are a slightly larger than those estimated from microscopic data. This is possibly due to the overlapping of diffusion regions and derivates resulted from the measurements of limited currents, which are only quasi-steady-state signals. The electrochemical characteristics of the elevated gold nanoparticle-based microelectrode arrays were then evaluated and compared with a uniform surface of gold nanoparticles using the same experimental conditions as demonstrated previously. Figure 8 shows that, similar to the characteristics of TiO2 nanoparticle-based microelectrode arrays, the shape of each voltammogram obtained at gold nanoparticle-based microelectrode arrays is strongly dependent on the individual microelectrode size. Sigmoidal-shaped voltammograms were observed at elevated microdisk electrode arrays with radii of 6 and 8 µm. CVs were increasingly peaked with the increasing size due to the decreasing contribution of spherical diffusion. In addition, the smaller the size of individual microelectrode, the larger the current density. This confirms that the enhanced analytical performance can also be realized at this elevated microelectrode array, based on gold nanoparticles. From the results previously demonstrated, it is evident that the methodology established has the potential for the development of new analytical devices.
Set A: Microdisk Electrode Arrays geometrical radius (µm) 12.5 10 8 6
number of arrays, n 500 800 800 500
limiting current (A) 1.178 × 10-4 1.435 × 10-4 1.167 × 10-4 7.736 × 10-5
current density (mA/cm2) 0.291 0.466 0.727 1.150
experimental radius (µm) 14 ± 3 11 ± 3 9±2 8±2
Set B: Microband Electrode Arrays geometrical bandwidth (µm) 100 50 10
number of bands, x 5 10 50
limiting current (A) 1.803 × 10-4 2.582 × 10-4 3.579 × 10-4
current density (mA/cm2) 0.351 0.512 0.705
experimental bandwidth (µm) 165 ± 3 72 ± 3 16 ± 2
According to the model of coplanar microband arrays, the Ilim term is given by21
iL )
[
]
2nπFCDl x ln(64Dt/w2)
(4)
where n is the number of electrons, F the Faraday constant, D the diffusion coefficient of K3Fe(CN)6, C the bulk concentration of K3Fe(CN)6, l the microband length, w the microband width, and t the time (which is given as t ) RT/(Fv), where R is the 8254
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
CONCLUSION A facile and efficient approach has been developed to create large-area microelectrode arrays based on either TiO2 or metal nanoparticles. TiO2 nanoparticle-based microelectrode arrays, which also function as superhydrophobic/superhydrophilic molecular microtemplates, have been produced using the photolithographic method. Site-selective photocatalytic deposition has been employed to generate metal nanoparticles into the superhydrophilic regions, as a result, wellstructured microelectrode arrays based on metal nanoparticles have been produced. An apparent advantage of the method is that all experiments can be performed on a laboratory benchtop scale with commercially available and relatively inexpensive materials. In addition to the enhanced analytical performance of the as-prepared microelectrode arrays, the control of stuctural patterns over large areas has demonstrated that this technique has provided a very useful platform for biosensors, fluid microchips, and microreactors,
as well as catalytic applications. Continuous work is underway in our laboratory. Finally, this methodology should also be applicable for fabricating microelectrode arrays of other metal nanoparticles such as copper, palladium, nickel, and so on. ACKNOWLEDGMENT This work was financially supported by the Program for New Century Excellent Talents in University (No. NCET-06-0380) and by the Scientific Research Foundation for the Returned Overseas Chinese Scholars from State Education Ministry, China, and the project sponsored by Nanometer Science Foundation of Shanghai (No. 0952 nm04900). Tongji University is also greatly acknowledged.
Received for review May 7, 2009. Accepted August 2, 2009. AC9009879
Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
8255
