Gold Nanoparticles with Poly(N-isopropylacrylamide) Formed via Surface Initiated Atom Transfer Free Radical Polymerization Exhibit Unusually Slow Aggregation Kinetics

Thermoresponsive polymer brushes on 20 nm colloidal gold were formed through atom transfer free radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAm) in aqueous media. In this approach, the "grafting-from" technique was used with atom transfer radical polymerization (ATRP) to grow polymer chains from the surface of gold nanoparticles (similar to 20 nm). "Grafting from" using the ATRP technique enables dense, uniform, and homogeneous coverage of polymer chains on the surface of gold nanoparticles. Other advantages of ATRP are the growth of polymer chains without appreciable chain termination or chain transfer and that the presence of an active initiator site at the end of the growing polymer chain facilitates synthesis of surface grafted block copolymers. In the present work, pNIPAAm was grown from the surface of nanoparticles with the help of 2-bromopropionyl bromide as the initiator. The polymerization reaction was carried out at room temperature under inert atmosphere and aqueous conditions. The system was found to exhibit thermoresponsive behavior above and below the LCST. This behavior could be exploited to develop aggregation based assays for making drug delivery systems, detection assays, and bioseparations. The hybrid polymer-gold nanoparticle system was characterized using optical absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). These analytical techniques confirmed the growth of polymer chains in the reaction scheme yielding the final product and a qualitative estimate of polymer chain thickness grafted onto the surface of the nanoparticles.