Micellar Nanoparticles of Coil–Rod–Coil Triblock Copolymers for Highly Sensitive and Ratiometric Fluorescent Detection of Fluoride Ions

Abstract

We report on the fabrication of a novel type of ratiometric fluorescent polymeric probes for fluoride ions (F–) based on self-assembled micellar nanoparticles of P(MMA-co-NBDAE)-b-PF-b-P(MMA-co-NBDAE) coil–rod–coil triblock copolymer, where MMA, NBDAE, and PF are methyl methacrylate, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole, and polyfluorene, respectively. Blue-emitting conjugated PF block and green-emitting NBDAE moieties with F– turn-off characteristics within the PMMA block serve as fluorescence resonance energy transfer (FRET) donors and switchable acceptors, respectively. For coil–rod–coil triblock copolymer in a good solvent such as THF, the blue emission of PF block dominates due to unimolecularly dissolved state associated with ineffective FRET process. The addition of F– ions only leads to ∼2.92-fold decrease of fluorescence intensity ratio, I515/I417, of characteristic NBDAE and PF emission bands. In acetone, the triblock copolymer spontaneously self-assembles into micelles possessing PF cores and NBDAE-labeled PMMA coronas. In the absence of F– ions, effective FRET processes between micellar cores and coronas occurs, resulting in prominently enhanced NBDAE emission. Upon addition of F– ions, the quenching of NBDAE emission bands leads to ∼8.75-fold decrease in the emission intensity ratio, I515/I417, which is also accompanied by naked eye-discernible fluorometric transition from cyan to blue emissions and colorimetric transition from green to yellowish. At a micellar concentration of 0.1 g/L in acetone at 25 °C, the detection limit of F– ions can be down to ∼4.78 μM (∼0.09 ppm). This work presents a new example of polymeric micelles-based optical F– probes and manifests that, upon proper structural design and optimization of spatial distribution of FRET donors and acceptors, self-assembled micelles of coil–rod–coil triblock copolymers serve as better ratiometric fluorescent F– ion sensors possessing visual detection capability, as compared to that of molecularly dissolved chains.