Tailoring the Phase Transition and Luminescence Behaviors of a Poly(ionic liquid) to Ensure Visual Temperature Sensing

The thermo-responsive and fluorescent nature of a poly(ionic liquid) (PIL) is tailored by reversible addition-fragmentation transfer (RAFT) copolymerization of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) and ionic liquid tetrabutylphosphonium styrenesulfonate ([P-4444][SS]). The PIL is shortly termed as poly([P-4444][SS]-DDMAT). In this strategy, DDMAT plays the following multiple functions: (1) it serves as an RAFT reagent to ensure precise control of the molecular weight of PIL; (2) its dodecyl chain is grafted at the chain end of PIL to reduce the cloud point temperature (T-cp); and (3) it regulates the fluorescence properties of PIL to induce a redshift of the emission. The T-cp of PIL decreases with the increment of its molecular weight and concentration, while it increases with the enhancement in pH and salt level. PIL displays blue-green fluorescence (lambda(ex)/lambda(em) = 392/478 nm) in an aqueous medium with a quantum yield (Phi) of 0.86% and a lifetime (tau) of 1.09 ns. It also shows thermo-responsive fluorescence activity. Its phase transition temperature from clear to turbid is tailored into a range covering the physiological temperature. The variation of temperature within a certain range is made to be visually visible based on the degree of turbidity, which well ensures skin surface temperature sensing and facilitates preliminary diagnosis of fever.