pH modulated nanoparticle diffusion in silica-polyacrylamide hydrogels

Using single particle tracking (SPT), we investigate nanoparticle (NP) diffusion in polyacrylamide hydrogels containing immobile silica particles (0% - 10% volume). For low concentrations of silica particles, two distinct populations of diffusing NPs are observed, localized and diffusive, whereas only diffusive NPs are found in the neat hydrogel. Primarily diffusive behavior is recovered for high concentrations of silica, attributed to an incomplete formation of the mesh as indicated by rheology. A proposed mechanism for the localized behavior of the NP probes is the pH mediated attraction via hydrogen bonding interaction between the PEG brush grafted to the surface of the NP and the silanol groups on the silica surface. The extent of the PEG-silica interaction and its subsequent impact on NP dynamics is examined at different pH values using quartz crystal microbalance with dissipation (QCM-D). This study provides valuable insight into controlling the diffusion of NPs in hydrogels based on pH mediated interactions with an incorporated tertiary component, with implications in drug delivery and filtration.