Microdomain pH gradient and kinetics inside composite polymeric membranes of pH and glucose sensitivity

Purpose: Polymeric membranes containing pH-sensitive nano-hydrogels and glucose oxidase were found to exhibit glucose-responsive insulin release. To verify that this glucose-responsiveness stemmed from the decrease in the internal pH of the membranes, we determined the spatial and temporal pH profiles inside the composite membranes in situ for the first time. Materials and Methods: A pH-sensitive fluorescent dye and an inert internal reference was incorporated in the membranes consisting of poly(N-isopropylacrylamide-co-methacrylic acid) nanoparticles in a polymer matrix, with or without glucose oxidase and catalase. The fluorescence intensity versus time was measured by laser scanning confocal microscopy. The intensity ratios of the two fluorescent dyes were used to determine the internal pH profiles of the membranes in buffer solutions of various pH or glucose levels. Results: The internal pH was found to be lower than the external pH of buffer solutions. The pH decreased with an increase in glucose concentration, incubation time and the distance towards the center of the membranes due to the relative rates of glucose oxidation and solute diffusion. Conclusions: The results provided direct experimental evidence of acidic internal pH that inversely related to external glucose concentration in an external medium of constant neutral pH.