Electrochemical Behavior of Dopamine and Ascorbic Acid at Osmium(II) Complex Cationic Monolayer Modified Gold Electrodes

Abstract

IntroductionAs an important neurotransmitter in the mammalian central nervous system, DA has received much interest. 1,2 However, the electrochemical detection of DA at the conventional electrodes has suffered from a high concentration of AA present in mammalian nerve and brain tissues because of close oxidation potentials and the reaction between AA and the oxidized form of DA.3 In order to improve the selectivity and sensitivity, various modified electrodes, 4-11 especially based on self-assembly technology, 12 have been developed for determining DA. [13][14][15][16][17][18] Since DA and AA carry different charges in neutral solutions, negatively charged self-assembled monolayers of ω-mercapto carboxylic acids on gold electrodes were reported as a means to induce an electrochemical difference between DA and AA based on electrostatic interactions. 19 On the other hand, gold electrodes modified by positively charged self-assembled monolayers of 2,2′-dithiobisethaneamine and 6,6′-dithiobishexaneamine 20 and dinickel(II) (2,2′-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl Experimental Chemicals and materials[Os(bpy)2(bpy-(CH2)13SH)](PF6)2 (bpy = 2,2′-bipyridine) was synthesized and characterized as reported. 28 Fresh phosphate buffer solutions (PBS) of DA (Sigma) and AA (Aldrich) were prepared before each measurement. PBS was prepared by mixing stock solutions of 0.2 M Na2HPO4 and 0.2 M NaH2PO4 and diluting with water. Triply distilled water was used in all runs. Preparation of an [Os(bpy)2(bpy (CH2)13SH)] 2+ /Au electrode and an [Os(bpy)2(bpy-(CH2)13SH)] 2+ + CH3(CH2)11SH/Au electrodeA polycrystalline gold disc electrode (0.2 cm 2 ) was pretreated in a piranha solution for 15 min and sonicated for several minutes. It was then washed with water and ethanol. After being dried with a stream of high-purity nitrogen gas, the electrode was soaked into a methanol solution of 1 mM [Os(bpy)2(bpy-(CH2)13SH)] 2+ for 24 h. Thus, a gold electrode modified with [Os(bpy)2(bpy-(CH2)13SH)] 2+ self-assembled monolayer ([Os(bpy)2(bpy-(CH2)13SH)] 2+ /Au) was prepared. It was used after being thoroughly rinsed by water and ethanol and dried with nitrogen gas. The [Os(bpy)2(bpy-(CH2)13SH)] 2+ + CH3(CH2)11SH/Au electrode was prepared by soaking the gold electrode into a methanol solution consisting of 1 mM [Os-(bpy)2(bpy-(CH2)13SH)] 2+ and 0.5 mM CH3(CH2)11SH for 24 h. Apparatus and proceduresElectrochemical experiments were carried out using a BAS 100B/W electrochemical analyzer. The electrooxidation of dopamine (DA) and ascorbic acid (AA) was studied using gold electrodes modified by a cationic self-assembled monolayer of [Os(bpy)2(bpy-(CH2)13SH)] 2+ by cyclic voltammetry.At an [Os(bpy)2(bpy-(CH2)13SH)] 2+ /Au electrode, the oxidation peak of DA shifted to a much more positive potential as compared with that of a bare gold electrode, while the oxidation peak potential of AA showed a slightly negative shift due to their different electrostatic interactions with the cationic monolayer. Thus, a sufficient potential difference was achieved ...

A Chitosan-Multiwall Carbon Nanotube Modified Electrode for Simultaneous Detection of Dopamine and Ascorbic Acid

“…23,24 Chitosan would electrostaticly attract AA, which promoted the access of AA to the electrode, and further catalyzed the oxidation of AA, resulting in that the Epa of AA shifted negatively to -0.20 V. Although there was electrostatic repulsion between DA and chitosan, its blocking effect on the oxidation of DA was very faint because the -NH 3+ concentration was low in a pH 7.2 solution. Also, the H-bond interaction between DA and chitosan was more dominant than the electrostatic repulsion effect, so DA was still catalyzed by the modifier.…”

Enhanced electrochemical response in oxidative differential pulse voltammetry of dopamine in the presence of ascorbic acid at carboxyl-terminated boron-doped diamond electrodes

“…The first problem encountered with the simultaneous presence of DA and AA is well known: both can be oxidized at very similar potentials at most solid electrode materials, causing an overlapping voltammetric response. In order to address this problem, a number of approaches have been developed involving surface modification of electrode materials with polymer films [3][4][5][6][7] and monolayers [8][9][10][11][12][13][14][15], principally with negatively charged functional groups, in order to electrostatically attract the cationic DA while repelling the anionic AA. The simultaneous determination of DA and AA in mixed solutions has been demonstrated using such modified electrode materials.…”