Charge transfer dye-based PHEMA hydrogel sensor: Its fluorescence responses to pH, metal ion, and humidity

Poly(2-hydroxyethylmethacrylate) (PHEMA) is one of the most well known synthetic hydrogels. In this study, PHEMA gel containing a charge transfer dye was synthesized using a copolymerization method. This polymer gel swelled appropriately in water with a swelling ratio (Q) of approximately 1.0, and swelled significantly in aprotic polar solvents, such as DMF and DMSO, with Q values of >8.0, whereas it never swelled in nonpolar solvents. The fluorescence (FL) emission band of the hydrogel polymer showed a red shift exclusively in water, whereas it showed a blue shift in the other solvents tested. The emission band also shifted to shorter wavelengths with increasing proton activity (with decrease in pH) or iron cation concentration, and the FL intensity increased gradually. The FL intensity also decreased significantly with increasing absolute humidity. Graphical Abstract: A charge transfer dye-based PHEMA gel was synthesized using a copolymerization method. The FL emission band of the hydrogel polymer showed a red shift exclusively in water. The emission band shifted to shorter wavelengths with increasing in proton activity or iron cation concentration and the FL intensity gradually increased. The FL intensity deceased significantly with increasing absolute humidity.[Figure not available: see fulltext.]