Fabrication of Photoswitchable and Thermotunable Multicolor Fluorescent Hybrid Silica Nanoparticles Coated with Dye-Labeled Poly(N-isopropylacrylamide) Brushes

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Abstract

We report on the fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes with inner and outer layers selectively labeled with fluorescence resonance energy transfer (FRET) donors, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and photoswitchable acceptors, 1′-(2-methacryloxyethyl)-3′,3′-dimethyl-6-nitro-spiro(2H-1-benzo-pyran-2,2′-indoline) (SPMA), respectively, via surface-initiated sequential atom transfer radical polymerization (ATRP). P(NIPAM-co-NBDAE)-b-P(NIPAM-co-SPMA) brushes at the surface of silica core exhibit collapse in the broad temperature range of 20−37 °C. UV irradiation of the aqueous dispersion of hybrid silica nanoparticles induces the transformation of SPMA moieties in the outer layer of polymer brushes from nonfluorescent spiropyran (SP) form to fluorescent merocyanine (MC) form, leading to occurrence of the FRET process between NBDAE and SPMA residues. Most importantly, the FRET efficiency can be facilely tuned via thermo-induced collapse/swelling of P(NIPAM-co-NBDAE)-b-P(NIPAM-co-SPMA) brushes by changing the relative distance between donor and acceptor species located within the inner and outer layers of polymer brushes, respectively. Thus, hybrid silica nanoparticles coated with P(NIPAM-co-NBDAE)-b-P(NIPAM-co-SPMA) brushes can serve as a sensitive ratiometric fluorescent thermometer. On the other hand, when the hybrid nanoparticle dispersion was irradiated with visible light again after UV irradiation, the MC form of SPMA moieties reverts back to the nonfluorescent SP form, leading to the turn-off of FRET process. Overall, aqueous dispersion of this novel type of hybrid silica nanoparticle is capable of emitting multicolor fluorescence, which can be facilely tuned by UV irradiation, visible light, and temperatures or a proper combination of these factors.

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