Aqueous Synthesis,property Investigation,and Application Of Biocompatible Fluorescent Non-cadmium QDs

In this thesis,we focus on the study of multicomponent non-cadmium（Cd）quantum dots（QDs）with high photoluminescence efficiency via aqueous synthesis.By tuning the chemical composition and optimizing the type of surface ligands of QDs,near-infrared emitting QDs with high fluorescence efficiency are successfully prepared.Upon further ZnS shell coating,QDs possess an improved fluorescence intensity and terrific long-term fluorescent stability.Based on the cation exchange strategy,a novel fluorescent sensor was constructed to detect the heavy metal Cu²⁺rapidly and mildly.In addition,QDs with high quantum yield（QY）and ultralong lifetime were obtained by optimizing surface ligands,which are ideal materials for long-term and real-time bioimaging.Firstly,the near-infrared emitting Cu-In-S（CIS）QDs were prepared by using the ligand to properly balance the reactivity of the precursor and by changing the ratio of Cu/In element.The temperature-dependent optical properties of QDs were studied by steady-state and transient fluorescence spectra.The results show that the band gap between the conduction band and Cu⁺level would be increased by the extremely high copper deficiency,which leads to the emergence of high-energy emission.Therefore,the QDs show dual-emission.As the content of copper increases,the band gap of QDs gradually decreases,and the dual-emission gradually evolves into a single emission in the near-infrared region.In addition,the change of the stoichiometric ratio has no effect on the peak position of high-energy emission.The core-shell Cu-In-S/ZnS QDs were obtained after the ZnS shell was coated.The fluorescence efficiency of the core-shell QDs was increased by an order of magnitude,reaching 13.8%,and the stability was also greatly improved.Meanwhile,the near-infrared emitting QDs have low cytotoxicity,showing great potential in the field of in vivo imaging.Secondly,on the basis of the above study,Zn²⁺was introduced into the CIS QDs to obtain quaternary Cu-In-Zn-S（CIZS）QDs with high quantum efficiency and high stability.QDs with tunable emission in a range of 535-645 nm and narrow full width at half-maximum（FWHM）have been successfully prepared through one-pot aqueous synthesis.The results of transient fluorescence spectra show that the fluorescence of CIZS QDs comes from eigenstate exciton luminescence and defect-related luminescence.The increase of zinc content can effectively reduce the surface defects of QDs,thus reducing the occurrence of non-radiative transition,and improving the luminescence efficiency.Upon further ZnS shell coating,the water-soluble QDs possess an improved absolute QY up to 30.8%with nearly no blue-shifted emission.Based on the cation exchange strategy,CIZS/ZnS QDs were used as the template to construct a new fluorescent sensor by utilizing the change of intrinsic optical properties of the host QDs,and the resulting CIZS/ZnS QDs exhibit sensitive response to Cu²⁺in a mild and rapid pathway.Moreover,the sensor shows high selectivity towards Cu²⁺and comparable accuracy to ICP-AES method,as well as good performance in real sample detection,which brings new perspective for the facile design of chemosensors.Finally,along with the target of obtaining strongly emitting non-cadmium QDs,we further explored the influence of surface ligands on the optical properties of silver indium sulfide（AIS）QDs,and a general strategy for achieving high fluorescence efficiency was developed.The AIS QDs co-stabilized by glutathione（GSH）and polypropylene imide（PEI）have been successfully prepared with an absolute PL QY up to 37.2%,which was much higher than that of the QDs modified solely by PEI（4.97%）.It has been demonstrated by both spectroscopic and structural characterizations that GSH as co-ligand bearing electron-rich groups has higher surface coating density onto the QDs,and can effectively passivate the surface trap centers which leads to non-radiative emission,thus greatly enhancing the PL intensity.In addition,PEI is indispensable for achieving the strongly emitting AIS QDs by better balancing the precursor reactivity.Moreover,the obtained QDs present ultralong lifetime of 3.69μs,excellent fluorescent stability,salt tolerance and low cytotoxicity,which enables the successful long-term imaging of HeLa cells.In summary,this thesis provides a green and reliable approach for synthesizing biocompatible multicomponent Cd-free QDs through one-pot aqueous synthesis,which is highly desirable for their practical applications.