Reactive fluorescence turn-on probes for fluoride ions in purely aqueous media fabricated from functionalized responsive block copolymers

We report on the fabrication of a novel type of responsive double hydrophilic block copolymer (DHBC)-based highly selective and sensitive fluorescence turn-on reactive probes for fluoride ions (F -) working in purely aqueous media by exploiting F --induced cyclization reaction of nonfluorescent moieties to induce the formation of fluorescent coumarin moieties, as inspired by the previous work of the Swager research group (Angew. Chem. Int. Ed. 2003, 42, 4803). Diblock copolymers bearing F--reactive moieties (SiCouMA) in the thermoresponsive block, PEO-b-P(MEO2MA-co-OEGMA-co-SiCouMA), were synthesized at first via reversible addition-fragmentation chain transfer (RAFT) technique followed by postmodification, where PEO, MEO2MA, and OEGMA are poly(ethylene glycol), di(ethylene glycol) monomethyl ether methacrylate, and oligo(ethylene glycol) monomethyl ether methacrylate, respectively. As-synthesized diblock copolymers molecularly dissolve in water at room temperature and self-assemble into micellar nanoparticles above the critical micellization temperature (33 ?C). In the presence of F- ions, deprotection of nonfluorescent SiCouMA moieties followed by spontaneous cyclization reaction leads to the formation of highly fluorescent coumarin residues (CouMA). Thus, PEO-b-P(MEO2MA-co-OEGMA-co- SiCouMA) diblock copolymers can serve as highly efficient and selective fluorescence turn-on reaction probes for F- ions in aqueous media. In the range of 0-1600 equiv of F- ions, diblock unimers and micellar solutions at 20 and 40 ?C exhibit 88-fold and 30-fold increase in fluorescence emission intensity (20 min incubation time), respectively. The detection limits were determined to be 0.065 and 0.05 ppm for diblock unimers and micelles, respectively. Most importantly, in the low F- concentration range, excellent linear correlation between F- concentration and emission intensity was observed (0-15 ppm for unimers at 20 ?C and 0-8 ppm for micelles at 40 ?C). Interestingly, upon complete transformation of nonfluorescent SiCouMA moieties into fluorescent CouMA, the emission intensity of diblock copolymer solution decreases linearly with temperatures in the range of 20-60 ?C, suggesting its further application as fluorometric temperature sensors. To the best of our knowledge, this work represents the first example of F--reactive polymeric probes working in purely aqueous media, which are capable of highly sensitive and selective fluorescent F- sensing in the form of both unimers and micellar nanoparticles.