Colloidal stability of zwitterionic polymer-grafted gold nanoparticles in water.

We investigate the colloidal stability of gold nanoparticles (AuNPs) coated with zwitterionic sulfobetaine polymers in aqueous solution. Zwitterionic polymers with different molar masses, synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N,N'-dimethyl(methacrylamido propyl)ammonium propanesulfonate (SPP) exhibit a well known Upper Critical Solution Temperature (UCST) in water, i.e., phase separate at low temperature. The colloidal stability of gold nanoparticles grafted with PSPP was studied as a function of the temperature. The effects of the molar mass of the grafted polymers, the salt concentration, and the presence of free polymer chains in solution were investigated. UV-vis spectroscopy and dynamic light scattering measurements show that whatever the molar mass of the grafted polymer, the nanoparticles never aggregate at low temperature in pure water. However, a reversible thermal-driven aggregation process of the gold nanoparticles is observed in presence of free polymer chains in solution and explained by a depletion process.