Synthesis of silver nanoparticles with thermo-controllable optical properties

Silver nanoparticles (SNPs) coated with a thermoresponsive polymer, poly(N-isopropylacrylamide)-g-poly-(ethylene oxide) (PNIPAM-g-PEO), have been prepared by the reduction of silver ions with NaH₄B in aqueous solutions. The obtained SNPs were very stable at room temperature because of the extended coil conformation of the PNIPAM-g-PEO chain. At higher temperatures (such as 40°C) above the phase transition of PNIPAM-g-PEO, only minute aggregation between the SNPs was observed, indicating that the PEO chains still retained the ability to stabilise the SNPs. Moreover, the thermoresponsive properties of SNPs coated with PNIPAM-g-PEO were further investigated as a function of the temperature by UV°vis spectroscopy, dynamic light scattering and transmission electron microscopy. The switchable optical and size response of the SNPs triggered by temperature change was observed. The absorbing peak and size of the SNPs was blue shift from 395.0 to 382.5°nm and increased from 29.4 to 33.5°nm at elevating the temperature from 25 to 40°C, respectively. The result was attributed to the phase transition of PNIPAM-g-PEO adsorbing on the surface of the SNPs as a function of temperature.