Gold nanoparticles have been prepared by UV photoactivation in the presence of a biopolymer, sodium alginate. The particles are characterized by UV-vis spectra and TEM studies. Both particle size and the UV-visible absorption peak are dependent Oil the sodium alginate concentration. The effects of various other parameters such as change of light Source, cell material of the reaction chamber, heating effect, irradiation time. and HAuCl4 concentration are studied. The particles are spherical and in situ stabilized by the biopolymer. The method is very simple and reproducible. (c) 2005 Elsevier Inc. All rights reserved.