A new method to determine the number concentration of gold nanoparticles in colloidal solutions is based on silver electrodeposition on the particles adsorbed on a carbon paste electrode. When colloidal gold was attached to a carbon paste electrode surface and then oxidised in a suitable media, the overpotential for cathodic deposition of silver was lowered relative to that of the carbon paste. Thus a range of potentials exists where the direct reduction of silver ions on the carbon paste surface could be avoided whilst the process did proceed on the surface of the adsorbed gold particles. A pretreated carbon paste electrode was immersed in colloidal gold solutions for 10 min. The modified electrode was then activated in a suitable media (0.1 M NaOH and 0.1 M H2SO4) to form oxides on the surface of gold particles. Then, silver electrodeposition at - 0.12 V for 45 s followed by anodic stripping voltammetry were carried out in aqueous 1.0 M NH3-2.0 x 10(-4) M AgNO3. A linear relationship was found between the area of the anodic stripping peak and the number of colloidal gold particles cm(-3) (from 3.62 x 10(7) to 9.06 x 10(8) particles cm(-3)) and a detection limit of 1.25 x 10(7) gold particles cm(-3) were obtained. In addition, colloids of gold with molecules of protein (albumin) adsorbed on their particles could be determined. In this last case, a detection limit of 1.80 x 10(7) gold particles cm(-3) was obtained. (C) 2000 Elsevier Science Ltd. All rights reserved.