Direct synthesis of monodispersed thiol-functionalized nanoporous silica spheres and their application to a colloidal crystal embedded with gold nanoparticles

Highly monodispersed thiol-functionalized nanoporous silica spheres with diameters in the submicron range were successfully synthesized via a surfactant-directed co-condensation of tetramethylorthosilicate (TMOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) in a very dilute alkaline methanol–water mixture. The incorporation of the thiol groups induced no drastic change in particle morphology in our reaction system, and the uniform spherical shape and the ordered hexagonal porous structure were simultaneously achieved when the molar ratio of MPTMS in the silica source was below 0.5. The inclusion of thiol groups into ordered nanoporous spheres was confirmed with EDX analysis, Raman spectroscopy, 29Si MAS NMR, and sorption measurements. By using the thiol groups as the adsorption sites of a gold precursor, the thiol-functionalized spheres were converted into monodispersed gold-nanoparticle-embedded nanoporous spheres. The resultant spheres were readily self-assembled into a close-packed colloidal crystalline array, which exhibited an optical stop band corresponding to Bragg diffraction as well as a surface-plasmon-derived adsorption peak. The current work provides a versatile route to multi-functional colloidal crystals with hierarchical porosity from modified porous spheres.