Nanoparticle dynamics near the volume-phase transition temperature of N-isopropylacrylamide gels

Nanoparticle (NP) probes were used to characterize the local structure of N-isopropylacrylamide (NIPAAm), a thermoresponsive hydrogel, using single particle tracking (SPT). Swelling ratio, and thus gel network confinement, was varied by tuning polymer and crosslinker concentration. Based on the swelling ratio, the volume-phase transition (VPT) was determined to be near 32 °C. In general, NPs were found to be localized by two barriers. A primary barrier with a localization region of approximately 100 nm was attributed to attractive interactions between the NIPAAm strands and the polyethylene glycol (PEG) brush grafted to the NP. As the polymer and crosslinker concentration were reduced, or temperature approached the VPT, NPs were able to escape the primary barrier and explore a larger secondary localization region (150-300 nm), ascribed to the confinement of the gel network. As temperature was raised above the VPT, however, the increase in confinement due to the collapse of the gel dominated, causing NPs to become localized to a single cage despite the higher temperature. This study of NP dynamics provides insight into controlling the release and loading of drugs in responsive hydrogel systems.