Preparation And Fluorescence Imaging Properties Of Ag₂Te Qunantum Dots In Near-Infrared Ⅱ Region

Quantum dots have excellent optical properties,including robust photolumines-cence,good photostability,tunable emission spectra,and broad absorption spectra that outperform the conventional organic fluorophores,and are widely used in the biomedical field.Near-infrared fluorescence possesses much higher tissue penetration capability than visible light and is superior in in vivo optical imaging.Compared with NIR I fluorescence,NIR II fluorescence has higher spatiotemporal resolution,signal-to-noise ratio,and tissue penetration depth due to the substantial reduction of tissue autofluorescence and scattering.At present,NIR II quantum dots are mainly II-VI,IV-VI and I-VI semiconductor materials.The presence of heavy metal elements such as Pb²⁺and Cd²⁺greatly limits the further application of quantum dots in the biomedical field.Silver chalcogenide quantum dots are the most promising near-infrared quantum dots with low toxicity to date.Compared with silver sulfide（Ag₂S）and silver selenide（Ag₂Se）,silver telluride（Ag₂Te）has a narrower direct bandgap（0.06 e V）and lower solubility(K_sp=2×10^-72)without highly toxic heavy metal ions,and are expected to be ideal nanoprobes for bioimaging in the second near-infrared window.However,the low photoluminescence quantum yield（PLQY）and poor stability limit the wide application of Ag Te quantum dots.Therefore,in this paper,cubic PbTe quantum dots were first synthesized,and then using cubic PbTe quantum dots as raw materials,a new thiolate etching method was used to synthesize uniform Ag₂Te quantum dots emmiting in the second near-infrared window,and finally through water-soluble modification and coupling PEG to construct an NIRⅡfluorescent probe.The probe has good photostability,high quantum yield and good biocompatibility,and is used for high-resolution in vivo imaging.The specific research contents are as follows:1.The optimal synthesis conditions of PbTe quantum dots were explored,and the effects of reaction time and reaction temperature on the properties of the synthesized products were studied,and the optimal synthesis conditions of PbTe quantum dots were obtained.High quality cubic PbTe quantum dots were prepared by reacting at 170°C for 10 min,which laid a solid foundation for the subsequent thiolate etching reaction.2.The emission wavelength of Ag₂Te quantum dots can be adjusted in the range of 1100 nm-1300 nm by increasing the reaction temperature or the ratio of ligands.The analysis results of elemental composition and optical properties show that there are no residual Pb atoms in the Ag₂Te quantum dots.The formation mechanism of Ag₂Te quantum dots was studied.The characterization results of ¹H NMR,transmission electron microscopy and fluorescence spectroscopy were studied.Taking advantage of the activity of thiol ligand 1-dodecanethiol（DT）,Ag（I）-thiolate could trigger the association of Ag（I）with Te on the surface of cubic PbTe QDs and subsequent decomposition of PbTe QDs,resulting in a ripening process for the formation of uniform Ag₂Te nanocrystals.3.A NIRⅡfluorescent probe was constructed by OPA modification and PEG coupling of Ag₂Te quantum dots emmiting at 1300 nm.The results of fluorescence spectrum and cytotoxicity experiments show that the Ag₂Te fluorescent probe has good photostability,high quantum yield and good biocompatibility.Furthermore,the Ag₂Te fluorescent probe enables high-resolution in vivo fluorescence imaging of microvessels（71μm）.In summary,the thiolate etching method can synthesize uniform Ag₂Te quantum dots emmiting in the second near-infrared window.This work not only provides a new tool for deep tissue fluorescence imaging,but also proposes a facile route for the preparation of metal telluride nanocrystals.