Salt effect on the heat-induced association behavior of gold nanoparticles coated with poly(N-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 degrees C (>lower critical solution temperature (LCST)) was smaller than that at 25 degrees C (<LCST) due to the shrinkage of the PNIPAM chains on the gold surface. The maximum wavelength of the plasmon band in pure water was almost independent of the temperature, the color of the solution remaining pink regardless of temperature. On the other hand, in the presence of 50 mM NaCl, the plasmon band at 40 degrees C was red shifted compared with that at 25 degrees C because of interparticle associations, the pink color at 25 degrees C changing to blue-purple at 40 degrees C. Furthermore, Rh for the PNIPAM-coated gold nanoparticles at 40 degrees C was significantly larger than that at 25 degrees 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.