Nanoparticle Aggregation Controlled by Desalting Kinetics

We report the formation of stable nanoparticle−polymer clusters obtained by electrostatic complexation. The nanoparticles placed under scrutiny were nanoceria (CeO2) coated with short poly(acrylic acid) moieties, whereas the polymers were cationic-neutral block copolymers. The cluster formation was monitored using different formulation pathways, including direct mixing, dialysis, dilution and quenching. In the first process, the hybrids were obtained by mixing stock solutions of polymers and nanoparticles. Dialysis and dilution were based on controlled desalting kinetics according to methods developed in molecular biology. The fourth process consisted of a rapid dilution of the salted dispersions and as such it was regarded as a quench of the cluster kinetics. We have found that one key parameter that controlled the kinetics of formation of electrostatic clusters was the rate dI S/dt at which the salt was removed from the solution, where I S denotes the ionic strength. With decreasing I S, the electrostatically screened polymers and nanoparticles system underwent an abrupt transition between an unassociated and a clustered state. By tuning the desalting kinetics, the size of the clusters was varied from 100 nm to over 1 μm. At low ionic strength, the clusters were found to be kinetically frozen. It is proposed that the onset of aggregation was driven by the desorption-adsorption transition of the polymers onto the surfaces of the particles.