Nanoparticle aggregation controlled by desalting kinetics

TL;DR

This study demonstrates that controlling desalting kinetics allows precise regulation of nanoparticle-polymer cluster formation, size, and stability, revealing a kinetic freeze point influenced by ionic strength changes.

Contribution

It introduces a novel approach to control nanoparticle cluster formation by tuning desalting kinetics, linking ionic strength removal rate to cluster size and stability.

Findings

Cluster size varies from 100 nm to over 1 μm with desalting rate.

Rapid desalting leads to kinetically frozen clusters.

Cluster formation is driven by polymer adsorption-desorption transitions.

Abstract

We report the formation of stable nanoparticle-polymer clusters obtained by electrostatic complexation. The nanoparticles placed under scrutiny are nanoceria (CeO2) coated by short poly(acrylic acid) moieties, whereas the polymers are 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 in 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 controls the kinetics of formation of electrostatic clusters is the rate dIS/dt at which the salt is removed from the solution, where IS denotes the ionic strength. With decreasing IS, the electrostatically screened polymers and nanoparticles system undergo 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 micrometer. At low ionic strength, the clusters were found to be kinetically frozen. It is proposed that the onset of aggregation is driven by the desorption-adsorption transition of the polymers onto the surfaces of the particles.