In situ synthesis of fine thallium and cadmium particles has been carried out by gamma-irradiation at room temperature in a preorganized polyacrylamide gel. It appears that Cd and Tl ions are chelated by the polymer matrix through physical entrapment as well as by weak chemical complexation. The metallic nanoparticles of Cd or Tl are well dispersed in the polymer matrix. The nanoparticles are stable in air and transfer electrons to methyl viologen (paraquat; 1,1'-dimethyl-4,4'-dipyridinium dichloride; MV2+(Cl-)(2)). The radical, MV+., formed upon electron transfer, also gets trapped in the polymer matrix and shows high stability even in the presence of air. (C) 2003 Elsevier Inc. All rights reserved.