The Mechanism Of Highly Fluorescent N,S Co-Doped Carbon Dots And Ti₃C₂ Based Quantum Dots And Their Application In Biosensing

The rapidly development of graphene materials has promoted the improvement of new two-dimensional(2D)inorganic layered materials,such as phosphene,transition metal carbon and/or nitride(MXenes)and transition metal dihalides(TMDs),to compensate for the defects of existing materials or providing outstanding properties that can't find in other materials.When these bulk inorganic layered materials are converted into a zero-dimensional(0D),that is two-dimensional quantum dots(referred to as2D-QDs),(the lateral size range is 1-100nm,and it has been reported that most are<10nm),by chemical etching,Li⁺/K⁺intercalation,or ultrasonic processing,quantum confinement effects can remarkably enhance existing or provide novel physical properties,such as better dispersion,easier to functionalization or doping and more novel fluorescence,while remains some inherent properties of the original buck counterpart.The unique and excellent properties of 2D-QDs make them a suitable candidate for many interesting research fields,including biological imaging,sensing,supercapacitors,photocatalysis,light-emitting diodes(LEDs)and batteries.In this paper,red light emitting carbon dots co-doped with nitrogen and sulfur(N,S-CDs)and green fluorescent MXenes-based quantum dots co-doped with nitrogen and phosphorus(N,P-MQDs)were prepared by one-step solvothermal and hydrothermal methods,respectively.The synthesis conditions of the two kind of quantum dots were optimized.Besides,the optical properties and basic properties of the morphology were also analyzed and investigated.Moreover,the density functional function theory(DFT)was used to deeply explore the fluorescence shift and PLQY enhancement mechanism.Taking advantages of biocompatibility and low toxicity,N,P-MQDs were further successfully used in sensing and bioimaging.The main contents are as follows:1.N,S-CDs with PLQY up to 29.7%were synthesized by one-step solvothermal method with o-phenylenediamine as carbon source,sulfamide as nitrogen and sulfur source,and ethanol as solvent.The synthesis conditions were optimized.TEM and AFM results demonstrate the successful synthesis of monodisperse graphene-like QDs with uniform lateral distribution and narrow height distribution.XPS and FTIR characterization results show that the incorporation of nitrogen and sulfur increases the degree of polymerization of aromatic rings in OPD,especially the degree of polymerization of N atoms,and the carbonyl groups on the surface of N,S-CDs tend to result in low PLQY.By contrast,the sp² graphite carbon structure of the aromatic domain can effectively improve PLQY.HRTEM,XRD and Raman spectra prove the existence of graphene-like structures.Due to their uniform size and similar surface states,N,S-CDs exhibit excellent excitation-independent PL lines at longer excitation wavelengths.In addition,the synthesized N,S-CDs are used to prepare highly stable WLEDs and ultra-sensitive fluorescent detection agents for water content in ethanol.2.Using Ti₃Al C₂ as raw material,unprecedented nitrogen and phosphorus co-doped Ti₃C₂-MXene quantum dots(N,P-MQDs),with the highest photoluminescence quantum yield(PLQY)and excellent green fluorescence emission,are successfully developed in this study.The PLQY of N,P-MQDs is as high as 20.1%.In addition,the mechanism behind its PLQY fluorescence shift and enhanced PLQY was explored and studied.Detailed structural and chemical studies,as well as a variety of comprehensive characterization and well-resolved density functional theory(DFT)calculations were utilized in this study.It is found that the synergetic effect between nitrogen and phosphorus doped in N,P-MQDs is proposed to create electron consumption and charge transfer in a more effective way.Due to its advantages such as high light stability and excellent fluorescence,N,P-MQDs are used as a fluorescence detection platform for macrophage labeling and life-related Cu²⁺ion detection,which demonstrates the potential of N,P-MQDs for biological imaging,selective nanoscale sensing and as a reusable super-concentrated fluorescent probe.