Preparation and characterization of silica–gold core–shell nanoparticles

Silica–gold core–shell nanoparticles (NPs) were prepared by gold ion plating on hydrophilic-functionalized silica core NPs using formaldehyde as a reducing reagent. The monodisperse silica particles were first prepared by a sol–gel method, while the ultrafine gold colloids (diameter 1–2 nm) were synthesized by the reduction of chloroauric acid with tetrakis(hydroxymethyl)phosphonium chloride. The growth and attachment of the gold NPs onto the functionalized surface of the silica NPs with average diameter ranging from 40 to 180 nm, using a low-temperature-mediated route, were systematically investigated. The coverage of the gold NPs and clusters on the surface of the silica NPs have been evaluated by means of UV–Vis/near-infrared spectroscopy and transmission electron microscopy. The surface plasmon resonance absorption spectra from 550 to 1,000 nm of the core–shell NPs can be effectively controlled by the surface gold coverage or the silica core NP’s size.