Amphiphilic Near-infrared Fluorescent Material And Emission Enhancement With Co-assembly Regulation

Near-infrared(NIR)fluorescence has great potential at bio-imaging due to the better tissue penetration,less photo-damage,and the interference-free wavelength.Inorganic NIR materials such as fluorescent quantum dots,single-walled carbon nanotubes,and fluorescent rare-earth complexes always show notorious cytotoxicity,high cost,small scale of preparation,and difficulty in reproducibility,which limit the application in biological systems.In contrast,organic NIR materials are more suitable for the field of bio-imaging owing to their wider verity,higher molar extinction coefficient,higher quantum yield and excellent biocompatibility.However,most conventional organic fluorophores tend to aggregate in the aqueous environment of biological systems,due to their hydrophobicity caused by conjugated structures.The fluorescence quenching effect has limited its application.As a result,the development of NIR fluorescence probes with aggregation-induced emission(AIE)is more conducive to obtain fluorescence imaging with high-contrast.In this study,we introduced a conjugated structure of anthraquinone and tetraphenylethylene with four pyridinium terminated alkyl chains,and the target molecule(denoted by BTPEAQ-10)was amphiphilic.In the aqueous solution,BTPEAQ-10 not only owns a large Stokes shift(240 nm)but also exhibits AIE effects The aggregates showed a very weak emission at 670 nm.Therefore,we use the method of ionic assembly to enhance its fluorescence intensity by electrostatic interaction.Co-assemblies are formed through introducing sodium dodecyl benzene sulfonate(SDBS).The morphology is transformed from grain-like nanoparticles into larger-sized nanospheres with fluorescence intensity 30-fold greater than that of the neat BTPEAQ-10.After the ionic co-assembly,the radiative decay rates increased rather than the non-radiative decay rates decreased,which leads to an enhancement fluorescence.There are four combination modes from 1:1 to 1:4 of the binding ratio between BTPEAQ-10 and SDBS.Controlling experiments on different surfactants and anionic groups indicate that both the tosyl head and the long alkyl chain play critical roles to the emission enhancement.Both the assembly and co-assembly have good water solubility and cytocompatibility,and were applied in HeLa cell imaging successfully.The co-assemblies showed much better contrast of cell imaging due to the higher emission intensity.This AIE-NIR fluorescent probe provides a powerful tool for observing deep tissue and will make more contributions to life sciences in the future.