Tris2,2 0 -bipyridyl)dichloro-rutheniumII) modi®ed carbon paste electrodes for electrocatalytic detection of DNA $ Nagwa Abo El-Maali 1 , Joseph Wang * Department of Chemistry, New Mexico State University, Las Cruces, NM 88003, USA Abstract Tris2,2 0 -bipyridyl)dichloro-rutheniumII) TBRu) containing carbon paste electrodes CPEs) exhibiting electrocatalytic response toward nucleic acids are described. The electrocatalytic action of the TBRu modi®er results in a dramatic enhancement of guanine oxidation response of ssDNA. The in¯uence of the paste composition, operating potential, DNA concentration, and other variables is described. The analytical advantages accrued from the electrocatalytic action of TBRu are demonstrated using ¯ow injection amperometry FIA) and square-wave voltammetry SWV). Detection limits of 24 and 500 ppb are obtained using the FIA and SWVoperations, respectively. The FIA response is highly linear R 2  0:999) and reproducible R.S.D. of 1.65%, n  25). The attractive behavior of the carbon paste electrocatalytic detector holds promise for on-line monitoring of nucleic acids. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon paste electrodes CPEs); Electrocatalytic detection; ssDNA; Guanine 1. Introduction Advanced electrochemical techniques represent versatile and sensitive tools for detecting low levels of nucleic acids, and for investigating the structure and interactions of DNA [1,2]. Such electroanalytical methods rely on the reduction or oxidation signals of nucleic acids at mercury or carbon electrodes, associated with the electroactivity of various nucleobases. Coupling these redox processes with the inter- facial accumulation of DNA or RNA has led to highly sensitive adsorptive stripping protocols [1,3]. The electro- oxidation of the sugar backbone has also been exploited for detecting nucleic acids in connection to copper electrodes, alkaline medium, and a sinusoidal ac) voltammetric opera- tion [4]. Chemically modi®ed electrodes CME) have rarely been used for measuring nucleic acids despite their wide- spread use in electroanalysis [5]. This article describes the use of modi®ed carbon paste electrodes CPEs) for electrocatalytic detection of nucleic acids. Mixed CPEs, made with carbon pastes that are doped with a modifying moiety, have received increasing attention in recent years [6]. Such bulk modi®ed electrodes have been popular due to the ease of surface modi®cation, renewal, or miniaturization, and overall versatility and attractive perfor- mance. While unmodi®ed CPEs have been widely used for strippinganalysisofnucleicacids[3],therearenoreportson the use of modi®ed carbon pastes for enhancing the voltam- metricresponseofDNAorRNA.Modi®cationofcarbonpaste with the Tris2,2 0 -bipyridyl)dichloro-rutheniumII) TBRu) hexahydrate catalyst is shown in the following sections to greatly enhance the electrochemical detection of nucleic acids. Poly[Ru4-vinyl-4 0 -methyl-2,2 0 -bipyridine) 3 2 ]- and Poly[Ru4-vinyl-4 0 -methyl-2,2 0 -bipyridine) 3 2 p-vinyl ben- zoic acid] modi®ed glassy carbon electrodes were used for electrocatalyticdetectionofDNAinconnectiontotheaccel- erated oxidation of the guanine nucleobase [7]. Soluble Rubpy) 3 2 complexeshavebeenusedtocatalyzetheoxida- tion of the guanine residues of DNA molecule [8]. 2. Experimental 2.1. Apparatus Flow injection analysis was performed with a carrier reservoir with normal gravity ¯ow, a Rainin Model 5041 injection valve 20 ml loop), interconnecting PTFE tubing, and the BAS LC-4C Amperometric Detector. Cyclic voltammetric experiments were carried out in a 10 ml cell Model CV-27 BAS Voltammograph). Sensors and Actuators B 76 2001) 211±214 $ This work has been accepted for presentation at the 8th International Meeting on Chemical Sensors, Basel, Sweitzerland, 2000. * Corresponding author. E-mail address: joewang@nmsu.edu J. Wang). 1 Permanent address: Department of Chemistry, Faculty of Science, Assiut University, 71516 Assiut, Egypt. 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00643-8