Local Water Content in Polymer Gels Measured with Super-Resolved Fluorescence Lifetime Imaging

Water molecules play an important role in the structure, function, and dynamics of (bio-) materials. A direct access to the number of water molecules in nanoscopic volumes can thus give new molecular insights into materials and allow for fine-tuning their properties in sophisticated applications. The determination of the local water content has become possible by the finding that H2O quenches the fluorescence of red-emitting dyes. Since deuterated water, D2O, does not induce significant fluorescence quenching, fluorescence lifetime measurements performed in different H2O/D2O-ratios yield the local water concentration. We combined this effect with the recently developed fluorescence lifetime single molecule localization microscopy imaging (FL-SMLM) in order to nanoscopically determine the local water content in microgels, i.e. soft hydrogel particles consisting of a cross-linked polymer swollen in water. The change in water content of thermo-responsive microgels when changing from their swollen state at room temperature to a collapsed state at elevated temperature could be analyzed. A clear decrease in water content was found that was, to our surprise, rather uniform throughout the entire microgel volume. Only a slightly higher water content around the dye was found in the periphery with respect to the center of the swollen microgels. Aqueous polymer systems play a key role in many applications and technological innovations. The fluorescence lifetime of a dye provides access to the number of water molecules in its vicinity, allowing the local water content of aqueous polymer networks to be studied with super-resolved fluorescence imaging. This method was applied to thermoresponsive microgels to study the water content in their swollen and collapsed state. image