Fully Reversible Quantitative Phase Transfer of Gold Nanoparticles Using Bifunctional PNIPAM Ligands

Ligand exchange with end-functionalized polymers is often applied to render nanoparticles with enhanced colloidal stability, to change the solubility in various environments, and/or to introduce new functionalities. Here we show that exchange of citrate molecules with α-trithiocarbonate-ω-carboxyl-terminated poly(N-isopropylacrylamide) can successfully stabilize spherical gold particles of different diameters ranging from 15 to 53 nm. This is verified by transmission electron microscopy, dynamic light scattering, and extinction spectroscopy. We show that the polymer-decorated nanoparticles respond to temperature and pH allowing access to control interparticle interactions. In a range of pH slightly below the pK_a of the terminal carboxyl groups, phase transfer of the particles from water to chloroform can be mediated by increasing the dispersion temperature above the lower critical solution temperature of poly(N-isopropylacrylamide). Upon cooling, fully reversible phase transfer to the water phase is observed. Extinction spectroscopy reveals phase transfer efficiencies close to 100% for every system under investigation.