Preparation of AIE Polymer Dots (Pdots) Based on Poly(N-vinyl-2-pyrrolidone)-Eu(III) Complex and Dual-color Live Cell Imaging

In recent years, polymer dots (Pdots) have been developed as an excellent organic fluorescent nanoparticles due to its excellent optical properties, diverse structures, easy surface modification and good biocompatibility. So, they have important application potential in biological imaging, sensing and detection, drug delivery and therapeutic diagnosis. However, the fluorescence quenching of semiconducting Pdots with large conjugated structure due to aggregation-caused quenching (ACQ) effect limits its applications for bioimaging in aggregated states. The ACQ phenomenon of Pdots could been eliminated by introducing aggregation-induced emission (AIE)-active molecules in Pdots. In this paper, a kind of responsive AIE-active Pdots, which were composed of tetraphenylethylene (TPE) with blue fluorescent light emission and poly(N-vinyl-2-pyrrolidone)-Eu(III) complex (PVP-Eu(III)) with red fluorescent light emission, were constructed. Firstly, a TPE derivative initiator (TPE-tetraAZO) containing four arms was synthesized by using 4,4'-azobis-(4-cyanovaleric acid) to modify TPE, and a multi-stimuli-responsive amphiphilic polymer of tetraphenylethene-graft-poly(N-vinyl-2-pyrrolidone) (TPE-tetraPVP) was then successfully synthesized by using TPE-tetraAZO as initiator. Finally, the complex TPE-tetraPVP-Eu(III) with AIE characteristic and dual fluorescence was obtained through the coordination between TPE-tetraPVP and rare earth element Eu(III). The amphiphilic 4-arm star polymer TPE-tetraPVP-Eu(III) formed Pdots consisted of hydrophobic AlEgens TPE core and hydrophilic PVP shell by a self-assembling process. The morphology and particle size of Pdots were investigated by transmission electron microscope (TEM). Results showed that Pdots was a relatively uniform diameter around 20 nm and exhibited regular sphere morphology. The results of fluorescence experiments showed that TPE-tetraPVP-Eu(III) Pdots had two emission bands centered at about 435 (blue) and 615 nm (red) with a wavelength difference of 180 nm, which were obtained under optimum excitation at 360 and 395 nm, respectively. Among them, the blue emission showed typical AIE property. Moreover, the lower critical solution temperature (LCST) of TPE-tetraPVP-Eu(III) in aqueous solution was about 37 V, which was close to normal body temperature. Meanwhile, at different temperatures from 10 to 60 V, photoluminescence (PL) intensities of TPE-tetraPVP-Eu(III) Pdots firstly decreased with increasing temperature from 10 to 36 V, and then increased with increasing temperature from 37 to 60 V . It was interesting that the fluorescent response of Pdots could be caused by the phase transfer of PVP. Besides, the PL intensity of Pdots in aqueous solution changed at different pH. Therefore, TPE-tetraPVP-Eu(III) Pdots might be used as multi-functional and smart fluorescent sensors. Furthermore, the results of cellular imaging indicated the efficient cellular uptake and low cytotoxicity of Pdots in HeLa, HepG2 and A549 cells. And, the photoswitchable dual-emission could be easily realized in three cells by simply tuning the excitation wavelength. Thus, the non-conjugated Pdots is an ideal dual-color live cell imaging probe.