Electroanalysis of ascorbate and dopamine at a gold electrode modified with a positively charged self-assembled monolayer

The self-assembled monolayer (SAM) of a di-positive nickel macrocycle (1) (dinickel(II) (2,2′-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate) has been utilized for the electroanalysis of ascorbate (AA) and dopamine (DA). A very large decrease (∼450 mV) in the overpotential for the oxidation of AA has been observed at the SAM-1 modified gold electrode. The electrostatic interaction of negatively charged AA with the di-positive SAM-1 facilitates the oxidation of AA at the monolayer-modified electrode. The electrostatic interaction of AA with the SAM-1 has been verified with the mixed monolayers of 1 and diethyl disulfide (DEDS). The peak potential for the oxidation of AA shifts towards more positive potential and the oxidation peak current decreases as the fraction of DEDS increases. The oxidation of DA at the monolayer-modified electrode is less favorable owing to the electrostatic repulsion between DA and the di-positive monolayer. The SAM-1 modified gold electrode resolves well the voltammetric peaks of these analytes, which are indistinguishable at the bare electrode. The catalytic oxidation of AA by oxidized DA has been successfully eliminated at the monolayer-modified electrode. The SAM-1 electrode has been used for the simultaneous determination of AA and DA. The results obtained at the SAM-1 electrode are compared with those at the gold electrode modified with a mono-positive monolayer of cystamine (CYST).