Aggregation of Gold Nanoparticles Induced by Halide Salts

Introduction Metal nanoparticles exhibit unique optical properties due to their interaction with electromagnetic fields, known as Surface Plasmon Resonance (SPR). This is dependent on the environment as well as their size, shape and aggregation state. Gold nanoparticles (Au NPs) have been proposed for applications in a wide range of fields including sensing, medicine, photonics and many others. Their SPR can be tuned by changing their shape into e.g. nanorods, nanoplates or nanostars. Alternatively, SPR can be tuned by controlling their aggregation state. In this study, we present the controlled aggregation of Au NP prepared by laser ablation in liquids (LAL) by addition of halide salts. Methods Au NPs are prepared by laser ablation in liquid using an ablation wavelength of 1064 nm at 10Hz pulse frequency. The liquid environment is water kept at pH=9 and containing NaCl 0.1M. The aggregation is induced by addition of KBr and monitored by absorption spectroscopy. FDTD simulations are also employed to interpret spectroscopy results. Results and Discussion Fig. 1 shows that by adding KBr a new SPR band is formed and increasing its concentration such band is progressively red-shifted. Simulations suggest that at low KBR concentration (25mM) short NP chains are formed, while longer chains are formed at 30mM. Further increasing KBR concentration produces SPR band that is not related to linear chains, thus we believe that branched or 3D structures are formed, rather than linear chains. We also investigate the effect of temperature on the aggregation process. Fig. 2 shows that for 30mM KBr concentration the aggregation is promoted at lower temperature and prevended at higher temperature, where the new SPR band hardly forms. This is because despite more frequent and energetic collision, at higher temperatures the attachment rate is lower, while at lower temperature the attachment rate is higher and the aggregates that form are more stable.