The Studies On Optical Properties And Preparation Of Phenylalanine Modified Carbon Quantum Dots

Carbon quantum dots（CDs）are a new class of zero-dimensional luminescent carbon nanomaterials,which usually refer to carbon nanoparticles with different passivation degrees on their surfaces.The photoluminescent carbon quantum dots have two significant characteristics:first,the carbon nanoparticles are small（less than 10 nm）;Second,the surface of particles can interact with other molecules through covalent bonds,non-covalent bonds or chemisorption.In addition,CDs have excellent properties such as good biocompatibility,photostability,selectivity and high sensitivity,tunable fluorescence emission and excitation,and have a wide range of applications in cell imaging,fluorescence detection,photocatalysis,photovoltaic devices and photodetectors.In this thesis,nitrogen-and sulfur-doped carbon quantum dots（t-CDs）were prepared by a solvothermal method.t-CDs have dual-emission red fluorescence and are capable of selective detection of Hg²⁺;In addition,t-CDs were modified using phenylalanine（Phe）to improve the fluorescence quantum efficiency of t-CDs,while the Phe-modified carbon quantum dots were used as fluorescent probes for imaging in cells.The details of the study are as follows:（1）Nitrogen and sulfur-doped carbon quantum dots（t-CDs）were prepared by a solvothermal method using thiourea and citric acid as reactants.The morphology and structure of t-CDs were characterized using transmission electron microscopy,atomic force microscopy,X-ray photoelectron spectroscopy,Raman spectroscopy detector,X-ray photoelectron diffraction,and Fourier transform infrared spectroscopy.The results showed that the prepared carbon quantum dots were uniform in size with an average particle size of4.5 nm.The lattice spacing of t-CDs is 0.21 nm,which corresponds to the（100）crystal plane of graphene.Most of the carbon elements exist in the form of sp² hybridized carbon,and the surface of carbon quantum dots is rich in functional groups such as-NH₂,-COOH and-SH.The optical properties were investigated using steady-state fluorescence spectroscopy,The emission peaks of t-CDs were at 510 nm and 630 nm,exhibiting double-emission red fluorescence with a fluorescence quantum efficiency of 5.7%in water,Under basic conditions t-CDs can enhance fluorescence due to protonation in the monodisperse state.In addition,t-CDs can selectively detect Hg²⁺,resulting in significant quenching of fluorescence.（2）Phe was selected for the modification of t-CDs.The fluorescence quantum efficiency of t-CDs in water was increased to 20.8%by controlling the reaction temperature,reaction time,pH value,and Phe concentration.The results of the morphology and structure show that t-CDs-Phe has a similar morphology to t-CDs,with an increased N element content and a blue shift of the fluorescence emission peak position to561 nm.we analyzed the fluorescence lifetime decay of t-CDs-Phe,and the average lifetime of t-CDs-Phe increased to 5.4 ns compared to t-CDs.The t-CDs-Phe also has good two-photon excitation fluorescence emission properties with a two-photon absorption cross section of 6.0×10^-47 cm⁴ s¹ photon^-1 under 800 nm light excitation.Two-photon cell imaging with t-CDs-Phe was achieved by imaging HeLa cells with intense red fluorescence when the excitation wavelength was 800 nm.We prepared t-CDs by bottom-up solvothermal method,and modified t-CDs by Phe.The optical characteristics of single-and two-photon excitation fluorescence of t-CDs as well as t-CDs-Phe were studied,and two-photon cellular imaging of t-CDs-Phe was realized.It shows great potential for applications in near-infrared excitation fluorescence imaging.