Ultrasensitive ratiometric fluorescent pH and temperature probes constructed from dye-labeled thermoresponsive double hydrophilic block copolymers

Abstract

We report on the fabrication of highly sensitive ratiometric fluorescent pH and temperature probes based on thermoresponsive double hydrophilic block copolymers (DHBCs) with the two blocks labeled with two types of dyes possessing different pH-switchable emission characteristics. P(NIPAM-co-FITC)-b-P(OEGMA-co-RhBAM) DHBCs were synthesized via consecutive reversible addition–fragmentation chain transfer (RAFT) polymerizations in combination with post-modifications, where NIPAM, OEGMA, FITC, and RhBAM are N-isopropylacrylamide, oligo(ethylene glycol) monomethyl ether methacrylate, fluorescein isothiocyanate, and rhodamine B-based derivatives, respectively. Due to that FITC and RhBAM moieties exhibit prominent decrease and increase in emission intensities with decreasing solution pH, respectively, intensity ratios of characteristic RhBAM and FITC emission bands, I₅₈₂/I₅₂₂, of P(NIPAM-co-FITC)-b-P(OEGMA-co-RhBAM) unimers at 25 °C exhibit ∼39-fold changes in the range of pH 2–10. At elevated temperatures, thermo-induced formation of PNIPAM-core micelles enables effective fluorescence resonance energy transfer (FRET) between FITC and RhBAM moieties respectively located within micellar cores and coronas, and I₅₈₂/I₅₂₂ exhibits ∼52.5-fold changes in the same pH range. The reported dually modulated multicolor-emitting P(NIPAM-co-FITC)-b-P(OEGMA-co-RhBAM) DHBCs are capable of ultrasensitive fluorometric detection of solution pH and temperature in a ratiometric manner, which augurs well for their practical applications in sensing, imaging, and the fabrication of new generation of theranostic systems.