pH-Mediated nanoparticle dynamics in hydrogel nanocomposites

The effect of static silica particles on the dynamics of quantum dot (QD) nanoparticles grafted with a poly(ethylene glycol) (PEG) brush in hydrogel nanocomposites is investigated using single particle tracking (SPT). At a low volume fraction of homogeneously dispersed silica (Φ = 0.005), two distinct populations of PEG-QDs are observed, localized and mobile, whereas almost all PEG-QDs are mobile in neat hydrogel (Φ = 0.0). Increasing the silica particle concentration (Φ = 0.01, 0.1) results in an apparent change in the network structure, confounding the impact of silica on PEG-QD dynamics. The localized behavior of PEG-QDs is attributed to pH-mediated attraction between the PEG brush on the probe and surface silanol groups of silica. Using quartz crystal microbalance with dissipation (QCM-D), the extent of this interaction is investigated as a function of pH. At pH 5.8, the PEG brush on the probe can hydrogen bond with the silanol groups on silica, leading to adsorption of PEG-QDs. In contrast, at pH 9.2, silanol groups are deprotonated and PEG-QD is unable to hydrogen bond with silica leading to negligible adsorption. To test the effect of pH, PEG-QD dynamics are further investigated in hydrogel nanocomposites at Φ = 0.005. SPT agrees with the QCM-D results; at pH 5.8, PEG-QDs are localized whereas at pH 9.2 the PEG-QDs are mobile. This study provides insight into controlling probe transport through hydrogel nanocomposites using pH-mediated interactions, with implications for tuning transport of nanoparticles underlying drug delivery and nanofiltration.