1-Naphthol as an ESPT fluorescent molecular probe for sensing thermotropic microenvironmental changes of pluronic F127 in aqueous media

Abstract

Thermotropic microenvironmental changes and the level of hydration in different microenvironments of pluronic F127 (PF127), (PEO₁₀₆ PPO₇₀ PEO₁₀₆, average molar mass 13 000) in aqueous media have been studied using 1-naphthol, which is an ESPT fluorescent molecular probe. The appearance of 1-naphthol neutral form fluorescence in aqueous PF127 (10% w/v) solution indicates the ability of 1-naphthol to sense hydrophobic domains in micellar aggregations. There is a marked enhancement of the neutral form fluorescence at and above the gelation temperature (20 °C), which shows that the probe can accurately sense the sol–gel transition. In the temperature range of 10–40 °C, with increase in temperature there is a progressive enhancement of the neutral form fluorescence and the blue shift of the neutral and anionic form fluorescence; a decrease in the deprotonation rate constant (k_pt) indicates that the water–polymer interfacial region is progressively dehydrated. Because k_pt is related to the availability of proton-accepting water in the microenvironment of 1-naphthol, the reduction of k_pt indicates progressive dehydration. The thermotropic response of the I₁/I₃ vibronic band ratio of pyrene-1-butyric acid fluorescence shows a progressive increase in the non-polarity of the interfacial domain with increasing temperature. The increase in non-polarity and the decrease of the hydration level are strongly correlated.