A glassy carbon (GC) electrode was modified with electropolymerized macrocyclic Ni(II) complex, at which dopamine (DA) and ascorbic acid (AA) were clearly resolved below pH 6.0 by cyclic voltammetry. Their charge transfer reaction was examined to elucidate their resolution and the oxidation reaction. DA had two charge transfer reactions below pH 6.0, allowing resolution of DA and AA. DA increased the peak separation between DA and AA, and the GC surface exposed to DA was involved in the DA oxidation that occurred at the Ni(H) complex-modified electrode. DA was determined selectively using the Ni(H) complex-modified electrode and the selective DA accumulation improved the DA response. The linear range was 0.5-50 mu M (R = 0.9989) and the limit of detection (S/N = 3) was 0.15 M. A Ni(II) complex-modified electrode can be used for selective DA detection in an acidic sample, such as urinary DA. (c) 2007 Elsevier B.V. All rights reserved.