A polymerizable Aggregation Induced Emission (AIE)-active dye with remarkable pH fluorescence switching based on benzothiazole and its application in biological imaging

Abstract

In recent years, Aggregation Induced Emission (AIE)-active fluorescent materials have been attracting considerable attention because they play an important role in clinical medicine, photoelectricity, environmental science, biological imaging, and sensors. In this contribution, a novel tetraphenylethylene (TPE) derivative of 2-(4-(1,2,2-triphenylvinyl)benzene) benzothiazole methacrylate (TPBMA) monomer was synthesized by a Suzuki coupling and an acylation reaction using TPE and benzothiazole as structural units, whose maximum emission wavelength respectively appeared at 506 nm in neutral medium and 579 nm in acidic medium with excellent pH sensitivity, and it could change repeatedly at 500 nm and 579 nm under HCl/NH3 fuming, indicating the reversible fluorescence conversion property. As compared with the TPB dye, the emission wavelength of TPBMA spectra has an obvious red-shift, which is consistent with the transition energy level difference (△E) of highest occupied molecular orbital (HOMO) and lowest unoccupied orbital (LUMO) situation by quantum chemistry calculation. Subsequently, the copolymers PEG-TB were successfully prepared by RAFT polymerization of TPBMA and poly (ethylene glycol) monomethacrylate (PEGMA), and their molecular weight (Mw) was about 2.12 × 104 with a narrow polydispersity index (PDI). From the 1H NMR analysis, the actual fraction of TPBMA in the PEG-TB copolymers increased to 22.6% from 17.4% when the feed ratio of TBMTA changed to 25.0% from 20.0%. The obtained PEG-TB could self-assemble into fluorescent organic nanoparticles (FONs) with 100–150 nm diameter in aqueous solution, and their fluorescence intensity increased gradually with the increase of water fraction in the THF solution, indicating the obvious AIE behavior. Moreover, the as-prepared PEG-TB FONs exhibited the prospective potential in the field of biological imaging due to their low cytotoxicity, excellent biocompatibility and easy cells absorption.