We have synthesized the alkyl-passivated Si nanoparticles by a solution route, and have carried out the various spectroscopic studies in order to investigate their intrinsic optical properties, electronic structures, and surface chemistry. Photoluminescence (PL) spectra of the n-butyl-passivated Si nanoparticles with mean diameter less than 2 nm exhibit the strong ultraviolet-blue emission. Moreover, it is found that their valence-band maximum energies directly estimated from the synchrotron-radiation valence-band photoemission spectra correspond to the resonance features in the PL excitation spectra. Therefore, it is concluded that the PL from the present n-butyl-passivated Si nanoparticles originates from the electron-hole pair recombination between the modified valence-band and conduction-band due to the quantum size effect. From the synchrotron-radiation Si 2p core-level photoemission spectra, their interfacial electronic structures of these Si nanoparticles have also been characterized. From these results, we discuss the detailed electronic and chemical properties of alkyl-passivated Si nanoparticles.