Modulation of the Surface Charge on Polymer-Stabilized Gold Nanoparticles by the Application of an External Stimulus

A new approach to controlling the charge on gold nanoparticle surfaces is described. The method exploits the simultaneous coattachment of both charged and neutral polymers onto gold surfaces. The charged and neutral polymers were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization, and the RAFT end-group functionality was used as the anchor for attachment to gold. The approach described is general and can be applied to a wide range of monomers; those exemplified in the paper are poly(2-aminoethyl methacrylamide) (P(AEA)), poly(acrylic acid) (PAA), and poly(N,N-diemthylaminoethyl acrylate) (P(DMAEA)) together with neutral polymers based on poly(oligoethylene oxide) acrylate (P(OEG-A)). The hybrid polymer-stabilized GNPs thus formed were characterized in solution using dynamic light scattering and zeta potential measurements, transmission electron microscopy, UV−visible spectroscopy, X-ray photoelectron spectroscopy, and attenuated total reflection-Fourier-transform IR spectroscopy. The grafting densities of the polymers on GNPs were measured using thermal gravimetric analyses (TGA), as 0.4 chains/nm² (for PAA), 0.9 chains/nm² (for neutral polymers, such as P(NIPAAm), and 0.6 chain/nm² for the positive charged polymers P(AEA) and P(DMAEA). The directed coassembly of two different polymers (one charged and one noncharged) on the gold nanoparticle surfaces provided a mechanism (dependent on molecular weight) for shielding the surface charge imparted by the charged polymer component, allowing for a range of surface charges on the GNPs from −30 to +39 mV. In further work, the surface-charges were modulated by an external stimulus (temperature). The charge-modulation was controlled by the use of thermosensitive neutral polymers coassembled with charged polymers. The thermosensitive polymers exemplified in this paper are P(oligoethylene oxide acrylate-co-diethylene oxide acrylate) (P(OEG-A-co-DEG-A)) and P(N-isopropyl acrylamide) (P(NIPAAm). The temperature of the aqueous phase (from 15 to 70 °C) was then adjusted to tune the zeta potentials of the hybrid GNPs from +39 or −30 to ∼0 mV. Finally, by manipulating the solution pH, reversible aggregation behavior of the hybrid GNPs could be induced.