Salt Effect on the Heat-Induced Association Behavior of Gold Nanoparticles Coated with Poly(<i>N</i>-isopropylacrylamide) Prepared via Reversible Addition−Fragmentation Chain Transfer (RAFT) Radical Polymerization

Poly(N-isopropylacrylamide) (PNIPAM) with a narrow molecular weight distribution was prepared by reversible addition−fragmentation chain transfer (RAFT) radical polymerization. A dithioester group at the chain end of PNIPAM thus prepared was cleaved by treating with 2-ethanolamine to provide thiol-terminated PNIPAM with which gold nanoparticles were coated via reactions of the terminal thiol with gold. The thermoresponsive nature of the maximum wavelength of the surface plasmon band and hydrodynamic radius (Rh) for the PNIPAM-coated gold nanoparticles were found to be sensitively affected by added salt. In pure water, Rh for the PNIPAM-coated gold nanoparticles at 40 °C (>lower critical solution temperature (LCST)) was smaller than that at 25 °C (Rh for the PNIPAM-coated gold nanoparticles at 40 °C was significantly larger than that at 25 °C. Heat-induced association and dissociation for the PNIPAM-coated gold nanoparticles were completely reversible in 50 mM NaCl aqueous solutions, which is responsible for the reversible thermoresponsive color change