Enhanced Luminescence Properties And Applications Of Graphene Quantum Dots By Amino Acid Modification And Special Atomic Doping

Graphene quantum dots（GQDs）is a new type of quantum dots,which has increasingly attracted the attention of scientists due to its many excellent properties and wide application prospects.Compared with traditional graphene,graphene quantum dots not only has the special physical and chemical properties of graphene,but also the advantages of low cytotoxicity,high biocompatibility and fluorescence coordination.Compared with traditional semiconductor quantum dots,it avoids the potential toxicity threat of heavy metals and can be used in biomedicine、sensors and other fields.However,there are still some problems in the preparation of quantum dots,such as uneven size and low fluorescence quantum efficiency,which greatly limit its application in biological cell imaging.Therefore,the preparation of GQDs with high fluorescence quantum efficiency still needs our continuous exploration.Based on current research status,firstly,graphene quantum dots were prepared by one-step solvothermal method.The prepared graphene quantum dots has the characteristics of good water solubility and low cytotoxicity,and can be well applied to the fluorescence imaging of biological cells.In addition,the prepared GQDs interacted with three aromatic hydrocarbon amino acids（tryptophan,tyrosine and phenylalanine）with similar structure by stirring and heating,which realized the red shift of luminescence peak and the significant improvement of fluorescence quantum efficiency.Finally,nitrogen doped quantum dots can improve the fluorescence quantum efficiency of the prepared N-GQDs and be used for two-photon cell imaging.The specific working procedures are as follows:（1）The traditional Hummer method was improved.Graphene oxide solid with high conversion and purity was prepared by changing the proportion of concentrated sulfuric acid,potassium permanganate and hydrogen peroxide.The green emitting water-soluble graphene quantum dots were prepared by tip ultrasound of graphene oxide with cell fragmentation instrument.The average particle size of the obtained luminescent quantum dots is 6.3 nm,which has good dispersion in water,has a single fluorescence band,the peak position is 510 nm and the half height width is 75 nm;The fluorescence quantum efficiency is up to 14.9%in water.Three amino acids（tryptophan,tyrosine and phenylalanine）were used to modify GQDs.By controlling the reaction temperature,reaction time,p H value and amount of amino acids,the fluorescence quantum efficiency of GQDs in water was increased to 22.4%.The GQDs were used to image He La cells.When the excitation wavelength was 488 nm,it emitted strong green fluorescence.（2）Nitrogen doped graphene quantum dots were prepared by using high-purity graphene oxide, ethylenediamine（EDA）and hydrogen peroxide（H₂O₂）as reactants.TEM results showed that the average particle size of N-GQDs was 6 nm and the fluorescence quantum yield in water was 41%.Fourier transform infrared spectroscopy（FTIR）showed that the surface of N-GQDs contained-NH₂,-COOH and other groups.At the same time,the fluorescence lifetime of N-GQDs was analyzed.The results showed that compared with N-GQDs,the proportion of fast life attenuation channels of N-GQDs was reduced and the proportion of slow life attenuation channels was increased.Nitrogen doping regulates the nonradiative and radiative relaxation processes of excited electrons and improves the fluorescence quantum efficiency.In addition,N-GQDs had a large two-photon absorption cross section of 5.7×10^-47 cm⁴ s¹ photon^-1).Under800 nm light excitation,N-GQDs was applied to He La cell imaging to realize biological imaging under near-infrared excitation.To sum up,in this paper,we prepared high-purity graphene oxide by the improved Hummer method.Based on this,we prepared graphene quantum dots in the first step,and further improved the fluorescence quantum efficiency by amino acid modification,so as to be applied to single photon cell imaging.The second step is to prepare nitrogen doped graphene quantum dots directly by nitrogen doping of quantum dots,which significantly improves the fluorescence quantum efficiency.It can be applied not only in single photon cell imaging,but also in the field of two-photon cell imaging.