Covalent modification of carbon electrodes for voltammetric differentiation of dopamine and ascorbic acid

Conventional-size glassy carbon electrodes and glassy carbon fibre microelectrodes have been covalently modified with P-phenylacetate groups via a diazonium salt coupling procedure. Using catecholates of different charge, changes in electrode processes and accumulation or exclusion of analyte at the electrode surface are shown to be controlled by electrostatic interactions with surface groups. The modified electrodes have been assessed for the voltammetric differentiation of dopamine (DA) and ascorbic acid (AA). At the macroelectrode, the differential pulse peak current for oxidation of DA is significantly enhanced after modification of the electrode and that for AA is drastically decreased. The detection limit for DA at the modified electrode (measured as 3σ for 0.2 μM DA) is 90 nM, and the linear working range is 0.45–32.0 μM. Under optimized chronoamperometric conditions, the relative sensitivity for equimolar AA: DA is 0.43%. Modification of glassy carbon fibre microelectrodes leads to larger cyclic voltammetric peak currents for DA at fast scan rates and gives excellent resolution between the DA and AA signals in the differential pulse mode.