Direct laser-assisted synthesis of localized gold nanoparticles from both both Au(III) and Au(I) precursors within a silica monolith

This work presents a solvent-free and laser-assisted growth of gold nanoparticles (Au-NPs) within silica monoliths using both Au(III) and Au(I) precursors. The novelty of the synthesis method is that Au-NPs of about 20 nm in diameter were obtained well dispersed in the matrix with no need of either reducing or capping agents. Moreover, the laser-assisted synthetic procedure here described made it possible to obtain reproducible 2D and 3D patterns of Au-NPs. For this purpose, suitable Au(I) and Au(III) precursors, soluble in dichloromethane, were easily prepared following a well-known procedure. The mesoporous silica matrix was first loaded with the precursors via a simple impregnation and then irradiated using either a continuous laser (λ= 266 or 532 nm) or a pulsed laser (λ=800 nm; pulse: 120 fs; repetition rate: 1KHz). In all cases, a photothermal gold reduction was observed. The Au-NPs have been characterized using UV-vis absorption spectroscopy, x-ray diffraction and Transmission Electron Microscopy. Finally it is shown that the excess gold precursors can be removed after the Au-NP synthesis by a simple washing of the monolith with a few immersions in the pure solvent. The stability of the Au-NPs was further tested by a series of heat-treatments up to 500°C, showing that the silica monolith acts as an effective support to prevent the agglomeration of the nanoparticles.