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The surface of boron-doped diamond (BDD) electrode is modified by the polymer film for the first time. The cationic polymer film of N,N-dimethylaniline (DMA) is electrochemically deposited on BDD electrode surface. This polymer (PDMA) film-coated BDD electrode is used as a sensor which selectively detect dopamine (DA) in the presence of ascorbic acid (AA). This electrode also can detect both DA and its metabolite, 3,4-dihydroxy phenyl acetic acid (DOPAC) in the presence of AA in the range of the physiological concentrations of these species. Favorable ionic interaction (i.e., electrostatic attraction) between the PDMA film and AA or DOPAC lowers their oxidation potentials and enhances the current response for AA and DOPAC compared to that at the bare electrode. The PDMA film also shows a hydrophobic interaction with DA and DOPAC. In cyclic voltammetric measurements, the PDMA film-coated electrode can successfully separate the oxidation potentials for AA and DA coexisting in the same solution and the separation is about 200 mV. AA oxidizes at more negative potential than DA. In square-wave voltammetry, the sensitivity of the PDMA film-coated BDD electrode for DA in the presence of higher concentration of AA is higher than that of the PDMA film-coated glassy carbon electrode. The hydrodynamic amperometric experiments confirm that the oxidation of AA is not affected by the oxidized product of DA and vice versa. So, unlike the bare electrode the catalytic oxidation of AA by the oxidized DA is eliminated at the PDMA film-coated BDD electrode. The sensitivities of the modified electrode for AA, DA and DOPAC, which are present in the same solution with their physiological concentration ratios, are calculated to be 0.070, 0.363 and 0.084 mA mM À1 , respectively. The modified electrode exhibits a stable and sensitive response to DA.

The polymer film of N,N-dimethylaniline (DMA) is deposited on the electrochemically pretreated glassy carbon (GC) electrode by continuous electrooxidation of the monomer. This poly N,N-dimethylaniline (PDMA) film-coated electrode can be used as an amperometric sensor of ascorbic acid (AA). The polymer film (thickness (f): 0.3 AE 0.02 mm) having positive charge in its backbone attracts the anionic species AA. Thus, the anodic peak potential (350 mV vs. Ag j AgCl j NaCl (sat) ) for the oxidation of AA at the bare electrode is largely shifted to the negative value (150 mV) at this electrode. The PDMA film-coated electrode is stable in acidic, alkaline and neutral media and can sense AA at different pH×s. The diffusion coefficients of AA in solution (D) and in film (D s ) were estimated by rotating disk electrode voltammetry:, (6.0 AE 0.2) Â 10 À8 and (4.7 AE 0.2) Â 10 À8 cm 2 s À1 for 0.5, 1.5 and 3.0 mM AA, respectively. A permeability of AA through the PDMA film was found to decrease with increasing the concentration of AA in the solution. In the chronoamperometry, the current response for the oxidation of AA at different times elapsed after potential-step application is linearly increased with the increase in AA concentration in a wide range of its concentration from 25 mM to 1.65 mM. In the hydrodynamic amperometry, a successive addition of 10 mM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.178 mA cm À2 mM À1. So, the fouling of the electrode surface caused by the oxidized product of AA is markedly eliminated at this PDMA film-coated electrode. A flow injection analysis based on the present electrode was performed to estimate the concentration of vitamin C in fruit juice.

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