This paper reports a systematic investigation of the attachment of different sized gold nanoparticles (3.5-32nm) to the polyethyleneimine (PEI) and 3-aminopropyltrimethoxysilane (APTMS)-functionalized, respectively, surfaces of different sized silica nanoparticles (60-340 nm) at room temperature. Functionalization of the surfaces of silica nanoparticles with PEI or APTMS has a profound influence on the attachment gold nanoparticles. While PEI-functionalized silica surfaces bound the midnanometer gold nanoparticles (13-32nm), APTMS-functionalized silica surfaces did not. A series of different silica-gold core-shell composites were characterized using transmission electron microscopy (TEM) and UV-vis absorbance spectra. (c) 2007 Elsevier B.V. All rights reserved.