Preparation Of Doped Carbon Quantum Dots And Their Application In Fluorescence Sensing

Carbon quantum dots(CDs)are a new type of fluorescent nanomaterials with a size less than 10 nm.They have many excellent properties,including photostability,biocompatibility and water solubility,and have been widely used in biosensing,bioimaging,drug delivery and other fields.However,CDs prepared by traditional methods still have many shortcomings,mainly manifested in low quantum yield,short emission wavelength and monochromatic fluorescence.In order to overcome these shortcomings,people use surface passivation and heteroatom doping to change the optical properties of CDs.Compared with the cumbersome surface passivation process,the simple and non-toxic heteroatom doping method is widely used to improve the quantum yield,water solubility,and fluorescence properties of CDs.According to the number of heteroatoms,heteroatom-doped CDs can be divided into single-heteroatom-doped CDs and multi-heteroatom co-doped CDs.The latter produce a unique electronic structure due to the synergistic effect between a variety of heteroatoms,which significantly improves the fluorescence quantum yield and intrinsic performance of CDs.In recent years,due to the excellent fluorescence properties of heteroatom-doped CDs,they are often used as a combination of fluorescent probes and nanomaterial-based quenchers for the design of promising sensors.Nano materials or semiconductors,such as graphene oxide,molybdenum disulfide,gold nanoparticles,silver nanoparticles and manganese dioxide nanosheets are all good fluorescence quenchers,which have been used to improve fluorescence sensors to sensitively identify various analytes.Among them,gold nanoparticles and manganese dioxide nanosheets have high extinction coefficients and broad absorption spectra,and are often used to construct sensors based on fluorescence resonance energy transfer(FRET)or internal filtration effect(IFE).In this paper,we synthesized two kinds of CDs co-doped with heteroatoms.Based on the fluorescence properties of CDs,we designed two kinds of fluorescence sensors for the field of biological analysis,and realized the detection of biological samples.The main content of the paper includes the following three parts:Chapter 1:Ultrasensitive“off-on-off”fluorescent sensor for protamine and trypsin detection based on inner-filter effect between AuNPs and N,S-CDsIn this chapter,we constructed an“off-on-off”fluorescent sensor for sensitive and selective detection of protamine and trypsin based on the internal filtration effect(IFE)between AuNPs and N,S-CDs.In this experiment,N,S-CDs are used as phosphors,and AuNPs are fluorescent absorbers.The latter use IFE to quench the fluorescence of N,S-CDs.After adding protamine,protamine acts as a cationic peptide and interacts electrostatically with negatively charged AuNPs,which leads to the aggregation of AuNPs and restores the fluorescence of N,S-CDs.The addition of trypsin can specifically hydrolyze protamine,causing the depolymerization of AuNPs and quenching the fluorescence of N,S-CDs again.Under optimized conditions,the sensor can be used to quantitatively detect protamine and trypsin,and the detection limits are 4.7 ng/m L and4.3 ng/m L,respectively.At the same time,the sensor has been successfully used for the determination of protamine and trypsin content in human urine samples,which has certain clinical significance.Chapter 2:Synthesis,characterization and properties of carbon dots co-doped with nitrogen and phosphorusIn this chapter,we used m-phenylenediamine and D(+)-galactose as precursor substances through a simple hydrothermal method to prepare eight different CDs with and without acid.Among them,N,P-CDs prepared with phosphoric acid as the phosphorus atom dopant have excellent luminous efficiency,and their fluorescence quantum yield(QY)is as high as 45.6%.The prepared N,P-CDs are single dispersed spherical particles with high QY,good water solubility,light stability,p H sensitivity and biocompatibility.Therefore,they have great application potential in the fields of biosensing and bioimaging.Chapter 3:Based on fluorescence resonance energy transfer between N,P-CDs and Mn O₂ nanosheets for monitoring biocatalytic conversionIn this chapter,we combine the N,P-CDs prepared in Chapter 2 with Mn O₂nanosheets to construct N,P-CDs-Mn O₂ nanocomposites.Mn O₂ nanosheets can quench the fluorescence of N,P-CDs through fluorescence resonance energy transfer.When the system contains H₂O₂,Mn O₂ nanosheets are easily degraded into Mn²⁺,accompanied by the recovery of the fluorescence of N,P-CDs.Based on this,we used the nanocomposite for the determination of H₂O₂ related analytes and monitoring of biocatalytic reactions,including glucose oxidase catalyzed oxidation of glucose,sarcosine oxidase catalyzed oxidation of sarcosine,xanthine oxidase catalyzed oxidation of xanthine,and?-galactosidase/glucose oxidase mediated lactose double enzyme reaction.Based on the interaction between the substrate and the oxidase,we rationally designed the“AND”and“OR”logic gates,which have application value for the screening or diagnosis of metabolic diseases in the future.At the same time,the sensor can also enter the cell to achieve intracellular imaging.