Polygonal (mainly triangular) silver nanoprisms were prepared by reducing silver perchlorate in formamide in the presence of polyethylene glycol (PEG) at room temperature. The reduction of silver ions by formamide leads to the deposition of arrays of triangular shaped silver nanoparticles on the glass walls of the container, accompanied by evolution of CO,) gas. In the presence of poly(N-vinyl-2-pyrrolidone) (PVP) and PEG (1: 1), both nanospheres and nanoprisms are formed. (c) 2005 Elsevier Inc. All rights reserved.