Langmuir 2007, 23, 11387-11390
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Gold Nanoparticle-Modified ITO Electrode for Electrogenerated Chemiluminescence: Well-Preserved Transparency and Highly Enhanced Activity Zuofeng Chen and Yanbing Zu* Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China ReceiVed August 7, 2007. In Final Form: September 21, 2007 Although it is desirable to use transparent indium tin oxide (ITO)-coated glass substrates as working electrodes for electrogenerated chemiluminescence (ECL), their applications in ECL studies of the Ru(bpy)32+ (bpy, 2,2′bipyridine)/tri-n-propylamine (TPrA) system have been limited because of the large overpotential of TPrA oxidation and the instability of the ITO surface at high anodic potentials. Here, we describe a simple method to achieve high ECL activity using ITO electrodes modified with gold nanoparticles (GNPs). The GNPs have been capped with fluorosurfactant ligands (i.e., Zonyl FSO). Much more facile TPrA oxidation was achieved by using the modified electrode, and an intense low-oxidation-potential (LOP) ECL signal was observed at ∼0.88 V versus SCE. The electrode transmittance drop upon modification was generally less than 5% over the visible spectrum when small-sized GNPs (∼4 nm) were employed. The well-preserved transparency and highly enhanced activity make the modified electrode promising for ECL studies.
Electrogenerated chemiluminescence (ECL) produces light at an electrode. The last few decades have seen intensive ECL studies on fundamentals and applications, and the ECL of Ru(bpy)32+ (bpy, 2,2′-bipyridine) with tri-n-propylamine (TPrA) as the coreactant has now been widely used in immunoassays, where Ru(bpy)32+ derivatives serve as the labels of large biomolecules.1 The electrochemical reactions near the electrode surface allow for spatial and temporal control of the emission signal. The use of a transparent electrode in ECL may make the configuration design of the electrochemical cell more flexible, for example, the light emission can be transmitted through the electrode to the detector, avoiding the interference from the absorbing species in solution.2-5 Therefore, the development of such electrodes for ECL is very attractive. Indium tin oxide (ITO) thin film coated glass substrates have been used in a wide variety of optoelectronic devices that require good optical transparency over the visible region as well as low electrical resistance, such as flat panels, liquid crystal displays, and organic light-emitting diodes.6-9 The applications of ITO electrodes in ECL studies have also been reported.2-5,10-12 However, it has been found that the activity of ITO toward the oxidation of TPrA (the most commonly used coreactant for Ru(bpy)32+ ECL) is extremely low.5 For the detection of a trace * To whom correspondence should be addressed. Telephone: +852 28598023. Fax: +852 28571586. E-mail:
[email protected]. (1) (a) Bard, A. J., Ed. Electrogenerated Chemiluminescence; Marcel Dekker: New York, 2004. (b) Xu, X.-H. N.; Zu, Y. In New Frontiers in UltrasensitiVe Bioanalysis: AdVanced Analytical Chemistry Applications in Nanobiotechnology, Single Molecule Detection, and Single Cell Analysis; Xu, X.-H. N., Ed.; Wiley: New York, 2007; pp 235-267. (2) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195. (3) Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 384, 57. (4) Popovich, N. D.; Eckhardt, A. E.; Mikulecky, J. C.; Napier, M. E.; Thomas, R. S. Talanta 2002, 56, 821. (5) Wilson, R.; Akhavan-Tafti, H.; DeSilva, R.; Schaap, A. P. Electroanalysis 2001, 13, 1083. (6) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123. (7) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (8) Kim, J. S.; Granstrom, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859. (9) Christou, V.; Etchells, M.; Renault, O.; Dobson, P. J.; Salata, O. V.; Beamson, G.; Egdell, R. G. J. Appl. Phys. 2000, 88, 5180. (10) (a) Zhang, Z.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (b) Bida, M.; Gao, F. G.; Bard, A. J. J. Solid State Electrochem. 2004, 8, 706. (11) Chiang, M. T.; Whang, C. W. J. Chromatogr., A 2001, 934, 59. (12) Sun, X. P.; Du, Y.; Dong, S. J.; Wang, E. K. Anal. Chem. 2005, 77, 8166.
amount of Ru(bpy)32+, the direct oxidation of TPrA at the working electrode is crucial. Because of the sluggish oxidation kinetics of TPrA on ITO, it is hard to achieve reasonably good sensitivity by using the transparent electrode. In addition, anodic potentials beyond 1.0 V versus SCE may induce a corrosive effect in a neutral solution because of an increase in tin oxide, leading to a loss of conductivity of the ITO layer.5,13 In this report, we modified ITO electrodes with gold nanoparticles (GNPs) of different sizes and employed the electrodes in the ECL measurements. Although it is possible to deposit GNPs at an ITO substrate electrochemically or by a seed-mediated growth approach, it has been found that rigorous control of the GNP size is difficult.14,15 Therefore, GNPs were first synthesized by the wet-chemical method and then attached to the ITO substrates via covalent binding. Note that ultrathin gold films (
