Electrocatalytic Oxidation and Sensitive Detection of Cysteine on a Lead Ruthenate Pyrochlore Modified Electrode Jyh-Myng Zen,* Annamalai Senthil Kumar, and Jyh-Cheng Chen Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan Electrocatalytic oxidation of cysteine (CySH) at Nafion/ lead ruthenate pyrochlore ( Py) chemically modified elec- trodes was thoroughly studied. Electrochemical ac im- pedance spectroscopy analysis indicated the formation of Py microparticles in the interfacial galleries of Nafion. Experiments with benchmark systems of Fe(CN) 6 3 -/4 - and Ru(bpy) 2 +/3 + reveal the suppression of Nafion’s anionic character after the in situ precipitation of Py. Michaleis-Menten-type kinetics with the rate determina- tion step of CyS-Py-Ru VI f Py-Ru IV + CyS-SCy was proposed for this catalytic oxidation. The electrocatalytic behavior is further developed as a sensitive detection scheme for CySH by square-wave voltammetry (SWV) and flow injection analysis (FIA). Under the optimized condi- tions, the calibration curve is linear up to 5 6 0 μM with a detection limit (signal/ noise 3) of 1.91 μM in SWV. The detection limit can be improved to 1.70 nM (i.e., 24.22 ng in a 2 0 -μL sample loop) in FIA. This is the lowest value ever reported for direct CySH determination without preliminary accumulation. The sulfur-containing molecule cysteine (CySH) plays a crucial role in biological systems, especially in folding and defolding mechanisms. 1 Because CySH possesses a very low molar extinc- tion coefficient, a spectroscopic method is suitable for its detection only with derivatization via the sulfhydryl functionality. 2 Compared to other options, electroanalysis has the advantage of simplicity and high sensitivity. Several electrochemical systems, such as Nafion/ Os(bpy) 3 2+ , polycrystalline gold, vitamin B 12 -adsorbed graphite, phthalocyanine (Pc) complexes of Co and Mo, water- soluble Fe and Mn porphyrins, and Ni-Pc immobilized silica gel- modified TiO 2 (ST-NiTsPc) electrodes, were reported for CySH detection. 3-10 Unfortunately, most electrodes contain certain disadvantages to extend them into real application. For example, the Nafion/ Os(bpy) 3 2+ electrode showed considerable leaching of Os(bpy) 3 2+ even after it was stabilized in Nafion film. 3 The oxidation process of CySH on polycrystalline gold electrode displayed complicated kinetics and the irreversible adsorption behavior rendered the routine analysis difficult. 4 Although the Pc complexes possess substantial catalytic activity, there is a solubility problem in an acidic environment. The ST-NiTsPc/ carbon paste electrode was reported recently to overcome these problems except that the detection range (1-7 mM) is not sensitive enough for real-sample analysis and the interference from other biological chemicals is considerably high. 10 Surprisingly, metallic oxide electrodes have hardly been used for this purpose, although they possess redox groups with tunable oxidation state and large surface area. Ruthenium dioxide (RuO 2 ) has been reported to have excellent electrocatalytic activity toward a number of organic compounds through mediation by Ru(VII)/ Ru(VI), Ru(VI)/ Ru(IV), or Ru(IV)/ Ru(III) redox couples. 11-21 Nevertheless, the high-temperature pyrolysis (300-700 °C) route in preparation and large double-layer charging effect make RuO 2 less favorable for analytical applications. 19-21 To overcome the main drawbacks of conventional RuO 2 electrodes, we disclosed a Nafion/ lead ruthenate pyrochlore ( Py) chemically modified elec- trode (designated as NPyCME) in electrocatalytic application with excellent sensitivity. 22-34 In this paper, we report the detail and * To whom correspondence should be addressed. Fax: 886-4-2862547. E-mail: jmzen@ dragon.nchu.edu.tw. (1) Voet, D.; Voet, J. G. Biochemistry, 2nd ed.; John Wiley & Sons: New York, 1995; p 1263. (2) Chwatko, G.; Bald, E. Talanta 2000 , 52, 509-515. (3) Chen, X.; Xia, B.; He, P. J. Electroanal. Chem. 1990 , 281, 185-198. 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Chem. 2001, 73, 1169-1175 10.1021/ac0010781 CCC: $20.00 © 2001 American Chemical Society Analytical Chemistry, Vol. 73, No. 6, March 15, 2001 1169 Published on Web 02/15/2001