The Synthesis,Characterization And Biological Application Of Photoluminescent Gold Nanoclusters

When gold nanoparticle size becomes comparable to Fermi wavelength(～0.5 nm)of an electron at the Fermi energy,the phenomenon of surface plasmon resonance will disappear,and it will also exhibit photoluminescence.The ultra-small gold nanoparticles with the above properties are called gold nanoclusters(AuNCs).AuNCs have the advantages of low biotoxicity,high chemical stability,resistance to photobleaching,and large stokes.Therefore,they have great application value in the field of biomedical imaging.However,as a fluorescent probe,AuNCs still have problems,such as complex preparation paths,unclear luminescence mechanism,relatively few synthesis methods,and low photoluminescence quantum yield(PLQY),which limit the wide application of AuNCs.Aiming to solve the problem mentioned above,four kinds of gold clusters were synthesized by chemical or photochemical methods in the study,including 1-thiol-β-D-lactose coordinated gold clusters(LAC-AuNCs),11-mercaptoundecanoic acid coordinated gold clusters(MUA-AuNCs),16-mercaptohexadecanoic acid coordinated gold clusters(MHAAuNCs)and(5-mercapto-1,3,4-thiadiazol-2-ylthio)acetic acid coordinated gold clusters(TMT-AuNCs).Fluorescence spectrophotometer,ultraviolet-visible spectrophotometer,infrared spectrometer,laser particle size analyzer,thermogravimetric analyzer,multifunctional imaging electron spectrometer,matrix-assisted laser desorption ionization-time of flight mass spectrometer,steady state and transient state fluorescence spectrometer,transmission electron microscope,multiphoton laser scanning microscopy and other instruments and equipment were used to systematically characterize and analyze the structure and optical properties of the four kind of AuNCs.In this study,the glycocluster effect of multivalent lactose constructed on the surface of Lac-AuNCs synthesized by one-step has the function of recognizing CD11b which is a membrane protein on the surface of white blood cells.Lac-AuNCs realized the specific recognition and fluorescence microscopic imaging of the white blood cells separated from the peripheral blood of New Zealand white rabbits.Although Lac-AuNCs possess biological functions,the PLQY of Lac-AuNCs is extremely low.In order to improve the PLQY of AuNCs,the study has carried out the investigation of the emission mechanism of MUA-AuNCs which have unique spectral properties.MUA-AuNCs have a very large stokes shift(>300 nm).In addition,the excitation peak and absorption peak of MUA-AuNCs do not correspond:360 nm and 390 nm have very strong absorption,but when these wavelengths were selected as excitation wavelengths,MUA-AuNCs cannot produce emission.Next,through the oxidation of the ligands to make them partially fall off,which had an impact on the spectral properties of MUA-AuNCs,we proposed a model of emission mechanism,and learned that the gold atoms on the surface of the gold core and ligands play an important role in improving the luminescence quantum yield.Based on the above understanding,MHA with long hydrophobic chain and TMT with the property of photosensitizer and electron-rich groups were selected as ligands.MHA-AuNCs and TMT-AuNCs were synthesized by weak reductant and photocatalytic reduction methods,respectively.Both methods effectively increased the content of Au(Ⅰ)on the surface of gold core,which significantly improved the PLQY of MHA-AuNCs and TMT-AuNCs(17.9%and 19.7%).By chemically coupling the bioactive molecules folate and galactose derivatives with MHA-AuNCs and TMT-AuNCs respectively,the fluorescence microscopic imaging of tumor cells was realized,especially the two-photon excitation fluorescence microscopic imaging significantly improved the signal-to-noise ratio.In view of the current problems faced by the research field of AuNCs,this study has made progress in the simple preparation of functionalized AuNCs,revealing the emission mechanism,establishing a novel photocatalytic reduction synthesis method,and improving the PLQY of thiol-coordinated AuNCs.Both synthesized AuNCs(Lac-AuNCs)by one-step and the AuNCs(MHA-AuNCs and TMT-AuNCs)chemically coupled with bioactive molecules can realize high specificity and sensitivity fluorescence imaging of cells,which lays a solid foundation for the vigorous development of AuNCs in biomedical field.