Analyte-Reactive Amphiphilic Thermoresponsive Diblock Copolymer Micelles-Based Multifunctional Ratiometric Fluorescent Chemosensors

We report on the fabrication of amphiphilic thermoresponsive diblock copolymer micelle-based multifunctional ratiometric fluorescent chemosensors for metal ions (Hg2+ and Cu2+), pH, and temperatures. A fluorescence resonance energy transfer (FRET) pair consisting of 4-(2-acryloyloxyethylarnino)-7-nitro-2,1,3-benzoxadiazole (NBDAE) donor and rhodamine B-based potential acceptor (RhBHA) in the spirolactam form with pH and Hg2+ (Cu2+)-reactive characteristics were respectively copolymerized into the hydrophobic PS and thermoresponsive PNIPAM block of P(St-co-NBDAE)-b-P(NIPAM-co-RhBHA) amphiphilic diblock copolymers, where PS and PNIPAM represent polystyrene and poly(N-isopropylacrylamide). In aqueous solution, the FRET pair-labeled diblock copolymer self-assembles into nanosized micelles with NBDAE moieties located in the micellar cores and RhBHA in the thermoresponsive coronas. Because of that Hg2+ ions and acidic pH can induce the transformation of RhBHA from the nonfluorescent spirolactam form to highly fluorescent acyclic form, and the FRET process between NBDAE and RhBHA moieties, located respectively within micellar cores and coronas, can be effectively switched on. Thus, these nanosized micelles can serve as excellent ratiometric fluorescent probes for Hg2+ ions and pH, accompanied by fluorometric transition from green to orange and colorimetric change from almost colorless to pink. At a micellar concentration of 0.05 g/L and 25 degrees C, the detection limit of Hg2+ ions can be down to similar to 14.8 ppb. On the other hand, Cu2+ ions can quantitatively induce the ring-opening of RhBHA moieties and afford nonfluorescent residues, which can effectively quench the NBDAE emission. On the basis of the relative changes in NBDAE emission intensities, the Cu2+ detection limit can be down to similar to 4.3 ppb. Most importantly, the spatial distance of the FRET pair can be facilely tuned via thermo-induced collapse of PNIPAM micellar coronas, which dramatically increase the FRET efficiency and enhance the pH detection sensitivity. This work represents a proof-of-concept example of amphiphilic block copolymer micelles-based multifunctional ratiometric fluorescent probes for two types of metal ions (Hg2+ and Cu2+), pH, and temperatures, which augurs well for their potential applications as nanocarriers with integrated functions such as imaging, sensing, and controlled-release of therapeutics.