Ž . Bioelectrochemistry 53 2001 183–191 www.elsevier.comrlocaterbioelechem Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate Chellappan Retna Raj, Koichi Tokuda, Takeo Ohsaka ) Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, 226-8502, Japan Received 6 September 2000; received in revised form 23 November 2000; accepted 30 November 2000 Abstract Ž . X Ž . X Gold electrodes modified with cationic self-assembled monolayers SAMs of 2,2 -dithiobisethaneamine CYST and 6,6 -di- Ž . Ž . Ž . thiobishexaneamine DTH were used for the simultaneous determination of dopamine DA and ascorbate AA . The cationic SAM Ž . modified electrodes have several advantages over the bare electrode for the oxidation of AA. A very large ;450 mV decrease in the overpotential for the oxidation of AA when compared with the bare electrode has been observed at the cationic monolayer-modified electrode. The electrostatic interaction of negatively charged AA with the monolayer shift the oxidation peak potential of AA to less positive potential and enhances the peak current. On the other hand, the positively charged DA is repelled from the monolayer and the oxidation potential shifts to more positive potential when compared to the bare electrode. The electrochemical oxidation of AA at the Ž . mixed monolayer of CYST and diethyl disulfide DEDS supports the influence of cationic terminal group of the monolayer on the oxidation of AA. Since the oxidation of AA occurs well before the oxidation potential of DA is reached, the homogeneous catalytic oxidation of AA by the oxidized DA has been advantageously eliminated at the monolayer-modified electrode. The cationic self-assembled monolayers successfully detect DA in the presence of high concentration of AA. The sensitivity of the electrode modified with CYST monolayer was found to be 0.036 and 0.021 m ArmM towards AA and DA, respectively. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Cationic self-assembled monolayer; Simultaneous determination; Ascorbate; Dopamine 1. Introduction The design, fabrication and application of novel electro- chemical sensors have been a research of considerable w x interest in the recent years 1,2 . Particularly the develop- ment of voltammetric sensor for the simultaneous determi- Ž . nation of dopamine DA , which is one of the most typical Ž . neurotransmitters, and ascorbate AA in biological sam- w x ples such as brain tissue received much interest 3–8 . Since the electrodes can be made conveniently to sense DA in the mammalian brain, the electrochemical method w x has advantage over other methods 9,10 . However, the major problem with this approach is the presence of high ) Corresponding author. Tel.: q 81-45-924-5404: fax: q 81-45-924- 5489. Ž . E-mail address: ohsaka@echem.titech.ac.jp T. Ohsaka . Ž 3 . w x concentration of AA 10 times higher than DA 11,12 in the brain, which is oxidized in the almost same potential range as DA is done on bare electrodes. Moreover, the bare electrodes very often suffer from the fouling effect due to the accumulation of oxidized product on the elec- trode surface, which results in rather poor selectivity and sensitivity. Another interference in the detection of DA in the presence of AA is the homogeneous catalytic oxidation w x of AA by the oxidized DA 13–16 and this needs to be eliminated for the precise determination of both AA and DA. Since the concentration of AA in the biological samples is very high, both the selectivity and sensitivity are very important in developing a voltammetric sensor for the determination of DA. Different approaches have been attempted to increase the sensitivity and selectivity of the w x electrode process 3–27 . Gonon et al. used electrochemi- cally pretreated carbon fiber electrodes for the detection of neurotransmitters in the presence of AA. Adams et al. 0302-4598r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0302-4598 00 00129-X