Stimulus-Responsive Au@(MeO₂MA_x-co-OEGMA_y) Nanoparticles Stabilized by Non-DLVO Interactions: Implications of Ionic Strength and Copolymer (x:y) Fraction on Aggregation Kinetics

Functionalized nanoparticles can assist in stabilizing fluid-fluid interfaces; however, developing and applying the appropriate surface modification presents a challenge because successful application of these nanomaterials for biotechnological, food processing, and environmental applications requires their long-term stability in elevated ionic strength media. This work studies stimulus responsive polymeric materials based on random copolymers of di(ethylene glycol) methyl ether methacrylate (x = MeO2MA) and oligo(ethylene glycol) methyl ether methacrylate (y= OEGMA) which, when grafted to gold nanoparticles, show significant, tunable, colloidal stability. The nanoparticles Au@(MeO2MAx-co-OEGMAy) display tunable, reversible aggregation that is highly dependent on the (x:y) ratio and ionic strength. Effects of these parameters on the initial rate constant of aggregation (k11) are studied by time-resolved dynamic light scattering (TR-DLS) experiments. At the same nanoparticle concentration, a strong sensitivity to salt concentration is observed. Over less than 300 mM increase in NaCl concentration, we observed a two-order of magnitude increase in aggregation rate constants, 4.2 × 10(-20) < k11 < 1.8 × 10(-18) m(3)s(-1). Additionally, for the same gold nanoparticles, a higher fraction of OEGMA requires a higher salt concentration to induce aggregation. A linear relationship between the critical NaCl coagulation concentration (CCC) and the copolymer composition is observed. Analysis of the experimental data with an extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory that includes hydration and osmotic forces is used to explain the stability of these systems. We find the hydration pressure, 2.4 < P(h,0) < 7.2 MPa, scales linearly both with the osmotic pressure and the OEGMA monomer concentration (5 < y < 20%). Specific knowledge of P(h,0)(y, C(NaCl)) enables design of both aggregation kinetics and stability as a function of the copolymer ratio and external stimuli.