Robust and smart gold nanoparticles: one-step synthesis, tunable optical property, and switchable catalytic activity

We have developed a one-step method to prepare robust and smart gold nanoparticles (AuNPs) utilizing poly(N-isopropylacrylamide) (PNIPAAm)-grafted dextran (DexPNI) via a grafting-onto'' approach. Using NaBH4 as a reducing agent, and DexPNI as a nucleating and stabilizing agent, the AuNPs can be obtained at ambient temperature within a short reaction time. Different features of AuNPs, such as their morphologies, optical properties and temperature-responsive behaviours are found to be dependent on the dosage of the gold precursor. The DexPNI-stabilized AuNPs not only show excellent stability against heat, high salt concentration and extreme pH, but could also be readily redispersed after freeze-drying. UV measurements and kinetic analysis of the AuNPs-catalyzed reduction of 4-nitrophenol at different temperatures illuminated that the optical properties and catalytic activity of AuNPs are closely related to temperature. Compared with previously reported diffusion-regulated nanoreacters, our AuNPs provide a unique opportunity to control the reaction rate in an inverse temperature-responsive manner at 32-35 degrees C, and further switch off the reaction at temperatures higher than 35 degrees C. These temperature-responsive properties could be used to create a smart sensor and catalyst, acting as an optical temperature alarm and recloser when the reaction temperature accidentally goes beyond the limit value. By using monomers and colloidal precursors other than NIPAAm and HAuCl4, it will be straightforward to extend the present route to build a variety of robust and smart nanomaterials with unprecedented properties.