Probing mobility and structural inhomogeneities in grafted hydrogel films by fluorescence correlation spectroscopy

We employed fluorescence correlation spectroscopy to investigate the effect of crosslinking density, annealing in the dry state, temperature, and solvent quality on the one-dimensional swelling, permeability, and mobility of tracer molecules in thermoresponsive hydrogel films. These consist of a carboxylated poly(N-isopropylacrylamide) derivative (PNIPAAm) covalently anchored to glass substrates. Upon increasing the temperature beyond the collapse transition at about 32 °C, the gels shrunk from the swollen to a collapsed state. A molecular dye (Alexa 647) and green fluorescent protein were chosen as tracers as they display only weak interaction with the carboxylated PNIPAAm. At large swelling ratios (low temperatures) the hydrogel layers are spatially homogeneous and both tracers show single Fickian diffusion. Diffusion coefficients scale with the PNIPAAm volume fraction. Upon temperature increase a qualitatively different behavior is observed already in the pretransition region (25–32 °C) concurrently with moderate swelling ratios (<4). This is manifested by an additional, faster Fickian diffusion and structural inhomogeneities, which are also found by optical waveguide mode spectroscopy. Above the collapse transition all diffusants are expelled from the hydrogels at a limiting swelling ratio ∼1.5. Subtle differences in the solvent quality influence the diffusion of tracers in the PNIPAAm hydrogel films. In the transition temperature range structural inhomogeneities at the nanoscale appeared.