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Technical Notes
Nafion-Tris(2-2′-bipyridyl)ruthenium(II) Ultrathin Langmuir-Schaefer Films: Redox Catalysis and Electrochemiluminescent Properties Paolo Bertoncello,*,† Lynn Dennany,‡ Robert J. Forster,‡ and Patrick R. Unwin*,†
Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom, and National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
A simple procedure to incorporate tris(2-2′-bipyridyl)ruthenium(II), [Ru(bpy)3]2+, into Nafion LangmuirSchaefer (LS) films is described. Nafion LS films (tens of nanometers thick) were formed on quartz glass and indium tin oxide (ITO) directly from Nafion-[Ru(bpy)3]2+ Langmuir films assembled at the water-air interface. This procedure allowed the direct incorporation of [Ru(bpy)3]2+ into Nafion films without the need for subsequent loading. UV-vis spectroscopy confirmed the successful incorporation of [Ru(bpy)3]2+ within the LS films and showed that the amount of [Ru(bpy)3]2+ immobilized in this way scaled with film thickness. Voltammetric studies on ITO-modified electrodes confirmed the successful incorporation of [Ru(bpy)3]2+ and demonstrated that [Ru(bpy)3]2+ was retained within the ultrathin films over a long time scale. These electrodes were tested for the electrocatalytic reduction of tripropylamine. Significant catalysis was observed due to the rapid turnover of [Ru(bpy)3]2+/3+ between the electrode surface and outer boundary of the film, as a direct consequence of the ultrathin film dimensions. Concomitant electrochemiluminescence (ECL) was demonstrated highlighting the potential of this material for sensing applications. The demand for inexpensive and selective sensors for analytical applications continues unabated, especially in the areas of clinical diagnostics and environmental monitoring. In this context, electrochemical methods have much to offer, but designing electrode surfaces which offer sensitivity comparable to fluorescence is a key challenge. The use of electrogenerated chemiluminescence (ECL) in analysis has significant practical advantages, such as discrimination against common electrochemical interferences.1 The ECL arising from the oxidation and subsequent reaction of tris(2-2′-bipyridyl)ruthenium(II), [Ru(bpy)3]2+, has been extensively studied due to the excellent stability of [Ru(bpy)3]2+ and * Corresponding authors. E-mail:
[email protected] (P.R.U.);
[email protected] (P.B.). † University of Warwick. ‡ Dublin City University. (1) Rubinstein, I.; Martin, C. R.; Bard, A. J. Anal. Chem. 1983, 55, 1580. 10.1021/ac070811m CCC: $37.00 Published on Web 08/23/2007
© 2007 American Chemical Society
the generation of ECL with a good yield.2-8 Ruthenium(II) complex-based ECL systems have been used for the analysis of a wide range of biochemical and medical analytes, such as alkylamines,9 oxalate,1 and amino acids10 but also proteins11 and pharmaceuticals.12 Despite the evident advantages of ECL detection, applications are limited by the large amount of ECL reagent that needs to be delivered into the reaction cell in order to replace the reagents consumed. In order to develop ECL-based electrochemical sensors, much effort has been focused on the immobilization of [Ru(bpy)3]2+ on electrode surfaces: in particular, LangmuirBlodgett13-15 and self-assembled monolayers16,17 have been widely used. Another relatively simple immobilization method is based on ion-exchange of [Ru(bpy)3]2+ in cation-exchange polymers such as Nafion.18-20 Such ECL sensors can be prepared rather easily, since [Ru(bpy)3]2+ is incorporated into Nafion due to electrostatic interactions with the sulfonated groups of Nafion and hydrophobic interactions between [Ru(bpy)3]2+ and the fluorocarbon backbone of Nafion.18,21 In previous studies, Nafion has typically been deposited onto electrodes in the form of recast films18,22-25 with thicknesses from (2) Gerardi, R. D.; Barnett, N. W.; Lewis, S. W. Anal. Chim. Acta 1999, 378, 1. (3) Knight, A. W. TrAC, Trends Anal. Chem. 1999, 18, 47. (4) Gorman, B. A.; Francis, P. S.; Barnett, N. W. Analyst 2006, 131, 616. (5) Zhang, L.; Dong, S. Anal. Chem. 2006, 78, 5119. (6) Sun, X.; Du, Y.; Dong, S.; Wang, E. Anal. Chem. 2005, 77, 8166. (7) Du, Y.; Qi, B.; Yang, X.; Wang, E. J. Phys. Chem. B 2006, 110, 21662. (8) Choi, H. N.; Cho, S.-H.; Lee, W.-Y. Anal. Chem. 2003, 75, 4250. (9) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865. (10) Brune, S. N.; Bobbit, D. R. Anal. Chem. 1992, 64, 166. (11) Li, F.; Cui, H.; Lin, X.-Q. Anal. Chim. Acta 2002, 471, 187. (12) Waguespack, B. L.; Lillquist, A.; Townley, J. C.; Bobbitt, D. R. Anal. Chim. Acta 2001, 441, 231. (13) Zhang, X.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (14) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781. (15) Miller, C. J.; Bard, A. J. Anal. Chem. 1991, 63, 1707. (16) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195. (17) Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 384, 57. (18) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007. (19) Downey, T. M.; Nieman, T. A. Anal. Chem. 1992, 64, 261. (20) Lee, W.-Y.; Nieman, T. A. Anal. Chem. 1995, 67, 1789. (21) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1982, 104, 4824. (22) Shi, M.; Anson, F. C. Anal. Chem. 1997, 69, 2653. (23) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. Science 1992, 257, 364. (24) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817.
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100 nm to a few micrometers. Such modified electrodes are naturally characterized by rather lengthy response times, due to the low constants of physical diffusion and electron hopping in such media.1,18,25 Recently, one of us has demonstrated the possibility of fabricating ultrathin Nafion films (
