Synthesis and Physical Stability of Novel Au-Ag@SiO2 Alloy Nanoparticles

The present study describes the synthesis of nanoparticles of silver-gold alloys and with their electrokinetic and spectroscopic characterisation. The synthesis was made in two steps. In the first step silver nanoparticles coated with silica (Ag@SiO2) were synthesised using a novel method assisted by laser ablation. The second step consisted on the introduction of KAuCl4 in the colloidal solution of Ag@ SiO2 nanoparticles in order to obtain silica-coated silver-gold alloy nanoparticles. The changes of colour and mean diameter of Ag@SiO2 nanoparticles caused by the introduction of the gold salt were found dependent on its concentration. Upon increasing [KAuCl4] the diameter of nanoparticles diminished and the monodispersity increased. The changes in the interparticle interaction potential as a function of [KAuCl4] and time were analysed using -potential values, calculated from their electrophoretic mobilities on the bases of Derjaguin-Laudau-Verwey-Overbeck (DLVO, UTDLVO) theory. The introduction of KAuCl4 produced a higher stability of the colloid and suggests an increase of the interaction energy barrier. However, as time after synthesis increases the barrier slightly decreases and stabilises at a plateau value. The absorbance measurements (Localised Surface Plasmon Resonance, LSPR) were studied as a function of [KAuCl4] and time. With increasing [KAuCl4] the main absorption band diminishes and red-shifts and a new broad band appears. For each value of [KAuCl4], upon increasing time, the two characteristic bands fuse into one and max diminishes in a linear fashion. Altogether, these data suggest that the co-reduced solutions of Ag and Au3 salts at long time consists of alloy Ag-Au nanoparticles, and not a mixture of Ag and Au nanoparticles.