Heteroaggregation of Neutral and Charged Nanoparticles: A Potential Method of Making Core–Shell Nanohybrids through Self-Assembly

Core–shell nanohybrids have wide applications in catalysis, drug delivery, bioimaging, environmental remediation, and so forth. Previous methods of synthesizing core–shell nanohybrids mostly involved extensive heating or stirring. In this study, core–shell nanohybrids were self-assembled through spontaneous heteroaggregation between model neutral nanoparticles (NPs) (i.e., hematite NPs or HemNPs) and model charged NPs (i.e., carboxylated polystyrene NPs or PSNPs). The heteroaggregation process does not require heating or stirring and thus can be much more energy-efficient than previous methods if successfully applied. The pH of the dispersant solution at 1 mM NaCl was adjusted to 6.3 so that HemNPs were neutral and underwent favorable homoaggregation, whereas PSNPs were negatively charged and underwent no homoaggregation. When the two types of particles were mixed, homoaggregation of HemNPs and heteroaggregation between HemNPs and PSNPs took place simultaneously, forming HemNPs–PSNP heteroaggregates. The hydrodynamic diameter of heteroaggregates increased initially and became stable afterward. The transmission electron microscopy images of heteroaggregates sampled in the stable stage show that HemNPs and PSNPs had self-assembled into core–shell structures in which HemNPs were the cores and PSNPs were the shells. The size of core–shell nanohybrids can be controlled by varying the concentration ratio of HemNPs to PSNPs. The elevated concentration of PSNPs reduced the size of nanohybrids because the abundant PSNPs could quickly surround HemNPs and form the shells, diminishing the chance for HemNPs to grow to larger cores through homoaggregation. The increase in the size of PSNPs resulted in the lower number concentration of PSNPs needed for forming stable nanohybrids and the larger size of nanohybrids.