A biosensor electrode possessing highly sensitive and selective determination of dopamine (DA) is fabricated. This electrode, a silver (Ag) thin film on indium-tin-oxide glass, is treated with a silver sulfide (Ag2S) film using electrochemical deposition. Active Ag ion is easier to form on Ag2S than on pristine Ag, which prefers to attract ascorbic acid (AA). The Ag2S layer reduces the oxidation potential of AA due to the electrostatic interaction, which results in well-separation of mixed oxidation responses to both of DA and AA. Besides, the Ag2S-modified electrode exhibits dramatic electrocatalytic effect on the oxidation of DA in the presence of AA. In 0.1 M phosphate buffer solution at pH similar to 7.0, the differential pulse voltammetric peak intensity linearly correlates with DA concentration in two regions, viz. 1.0-10, and 10-100 mu M, with correlation coefficient of 0.998 and 0.995, respectively. The lowest concentration limit of 1.0 mu M DA can be detected. The interference of AA effectively diminishes in the mixed solution. These features make the Ag2S significant for selective and sensitive measurement of DA in the presence of excess AA. (C) 2010 Elsevier Ltd. All rights reserved.