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-<i>co</i>-NBDAE)-<i>b</i>-PF-<i>b</i>-P(MMA-<i>co</i>-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, <i>I</i>₅₁₅/<i>I</i>₄₁₇, 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, <i>I</i>₅₁₅/<i>I</i>₄₁₇, 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.