Macromolecules, Vol.44, No.20, 8207-8214, 2011
Micellar Nanoparticles of Coil-Rod-Coil Triblock Copolymers for Highly Sensitive and Ratiometric Fluorescent Detection of Fluoride Ions
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 similar to 2.92-fold decrease of fluorescence intensity ratio, I(515)/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 similar to 8.75-fold decrease in the emission intensity ratio, I(515)/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 degrees C, the detection limit of F(-) ions can be down to similar to 4.78 mu M (similar to 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.