Controllable Emission Via Tuning The Size Of Fluorescent Probes Formed By Amphiphilic Copolymers

In the past two decades,the molecules with aggregation-induced emission(AIE)effect were widely applied in cell imaging.These AIE molecules solve the problem that the fluorescence of conventional fluorophores will decreases or even quenches after aggregation However,long conjugated backbones in AIE molecules lead to poor water-solubility,which limits the applications of cell imaging.Hence,one effective solution is grafting AIE molecules into polymers.By this method,the resulting polymers own AIE effect and increase the fluorescence emission.Meanwhile,the waster-solubility and biocompatibility can be improved by introducing water-soluble and biocompatible groups.The obtained polymers by this strategy are also called amphiphilic fluorescent copolymers(AFP).Nowadays,according to literatures,the synthesis of AFPs can be divided into two categories:direct polymerization and postfunctionalization.Direct polymerization means that the polymers were obtained by polymerizing AIE monomers with hydrophilic monomers directly;in postfunctionalization,a hydrophilic polymer chain was prepared first and then AIE groups were grafted to the polymer via high active reactions.However,both of two methods are hard to systematically control the ratio of hydrophilic groups and fluorescent groups,which restricts researches on the relation between fluorescent property and size of AFP assemblies.The size and fluorescent property of AFP assemblies are critical to cell imaging because the size determines the cell uptake while fluorescent property determines the imaging resolution.In our previous research,post-polymerization modification(PPM)method was adopted to prepare AFP,where the ratio of hydrophilic and fluorescent units is 4:1.Briefly,we firstly synthesized high active poly(pentafluorophenyl acrylate).Afterwards the hydrophilic monomer(denoted by PEG-NH2)and the AIE monomer(tetraphenylethylene derivative,denoted by TPE-NH2)were employed to substitute the pentafluorophenyl units to acquire AFP.Then the obtained AFP was applied in labelling HeLa cells.As an extension of above-mentioned research,in this study,a series of copolymers with same degree of polymerization(DP=50)were prepared via PPM by regulating the feed ratio of PEG-NH2 and TPE-NH2.Then the resulting copolymers were employed to investigate the relation between fluorescence and assemblies size.The ratios of PEG and TPE groups in copolymers are 45:5,43:7,42:8,38:12,and 34:14,respectively.Afterwards,the copolymers self-assembled into nanoparticles in aqueous solution.We found that the nanoparticles owned excellent dispersity in water.Transmission electron microscope and dynamic laser scattering are adopted to determine the morphology and size of nanoparticles.The result shows that the decline of PEG group brought about the decrease of particle size,indicating the successful size control by utilizing PPM method.Meanwhile,the particle size decreased from 305 nm to 181 nm along with the quantum yields increasing from 1.2%to 11.7%.What caused this phenomenon was that the size decrease results in the tighter packing of TPE units,which restrict the motion of TPE units and reduce the energy consumption.Furthermore,size change has little effect on cell viability of HeLa cells,after taking the biggest and smallest particles as contrast.Both of them can be applied in cell imaging and provide high contrast fluorescent images.