A new voltammetric method for a direct determination of gold nanoparticles, based on adsorption and electrochemical detection of colloidal gold, is described. In this protocol, the absorption of gold nanoparticles onto the rough surface of graphite-epoxy composite electrode is followed by their electrochemical oxidation in 0.1 M HCl medium at a potential of +1.25 V. The resulting tetrachloroaurate ions generated near the electrode surface are detected by differential pulse voltammetry (DPV). The DPV response is linear in the range from 4.7 x 10(8) to 4.7 x 10(11) nanoparticles cm(-3) with a limit of detection of 1.8 x 10(8) gold nanoparticles cm(-3). The surface characteristics of the composite electrode are investigated and the parameters that affect the complete analytical detection process of gold nanoparticles are optimized. (c) 2005 Elsevier Ltd. All rights reserved.