Responsive polymers-based dual fluorescent chemosensors for Zn2+ ions and temperatures working in purely aqueous...

We report on the fabrication of responsive double hydrophilic block copolymers (DHBCs)-based dual fluorescent chemosensors for Zn(2+) ions and temperatures and investigate the effects of thermo-induced micellization and detection conditions on the probing sensitivity and binding reversibility of Zn(2+) ions. A novel quinoline-based polarity-sensitive and Zn(2+)-recognizing fluorescent monomer (ZQMA, 6) was synthesized at first. Well-defined DHBCs bearing quinoline-based Zn(2+)-recognizing moieties (ZQMA) in the thermoresponsive block, PEG-b-P(MEO(2)MA-co-OEGMA-co-ZQMA), were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization of 2-(2-methoxyethoxy)ethyl methacrylate (MEO(2)MA), oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA), and ZQMA in the presence of PEG-based macroRAFT agent. The OEGMA contents in the thermoresponsive block varied in the range of 0-12.0 mol % to tune their lower critical solution temperatures (LCSTs). At 20 °C, almost nonfluorescent PEG-b-P(MEO(2)MA-co-ZQMA) molecularly dissolved in water and can selectively bind with Zn(2+) ions over other common metal ions, leading to prominent fluorescence enhancement due to the coordination of ZQMA with Zn(2+). At a polymer concentration of 0.2 g/L, the Zn(2+) detection limit can be down to ~3.0 nM. PEG-b-P(MEO(2)MA-co-ZQMA) self-assembles into micelles possessing P(MEO(2)MA-co-ZQMA) cores and well-solvated PEG coronas upon heating to above the LCST, and the fluorescence intensity exhibit ~6.0-fold increase due to the fact that ZQMA moieties are now located in a more hydrophobic microenvironment. Compared to the unimer state at 20 °C, although PEG-b-P(MEO(2)MA-co-ZQMA) micelles possess a slightly decreased detection limit for Zn(2+) (~14 nM), reversible binding between ZQMA moieties and Zn(2+) ions at 37 °C can be achieved, as evidenced by the on/off switching of fluorescence emission via the sequential addition of Zn(2+) and EDTA. In vitro fluorescence imaging studies suggested that the micelles can effectively enter into living cells and sensitively respond to Zn(2+) ions. This work represents the first example of a purely aqueous-based polymeric Zn(2+) sensing system by integrating the well-developed small molecule Zn(2+)-sensing moieties with stimuli-responsive DHBCs.