Graft polymerization has been used, for the first time, to prepare a dense conductive polymer coating on free-standing luminescent silicon nanoparticles. The silicon nanoparticles maintained their photoluminescence and crystallinity after surface modification. The nanoparticles were first surface hydroxylated and then reacted with (3-bromopropyl)trichlorosilane to form a dense bromopropylsilane monolayer. This was further reacted with aniline, which displaced the bromine atoms. The surface-bound aniline molecules were then used as active sites for the graft polymerization of polyaniline (PANI). The composition, structure, morphology, and other physical properties of the PANI-capped Si nanoparticles were examined by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy. The silane self-assembled monolayer effectively protected the silicon particles against photoluminescence quenching and degradation in basic solutions that rapidly quench the photoluminescence of unprotected particles. The PANI coating further enhanced this protection, even in its nonconducting emeraldine base state. The electrical conductivity of the HCl-doped (emeraldine salt) PANI-capped Si nanocomposite exceeded 10(-2) S/cm, which is 6 orders of magnitude higher than that of the bare Si nanoparticles. However, there was negligible change in the photoluminescence spectrum or lifetime upon addition of the PANI layer, suggesting that the charge carriers responsible for the luminescence remained confined within the Si nanoparticles.