Preparation,Emission Mechanism And Applications In Fluorescence Sensing Of Multi-Color Tunable Carbon Dots

Fluorescent carbon dot is one of the most closely concerned and interesting members of carbon based luminescent nanomaterials in recent years.Due to their advantages of low toxicity,good biocompatibility and diversity of luminous color,carbon dots have shown great application potential in the fields of biological imaging and medical diagnosis,construction of photoelectric device,information encryption and anti-counterfeiting,sensing and detection,etc.At present,carbon dots with excitation-dependent or excitation-independent fluorescence emission characteristics can be effectively prepared by top-down and bottom-up synthesis methods.Although the optical properties and applications of these carbon dots have been widely developed,there are still many problems to be solved in the field of zero-dimensional carbon-based fluorescent nanomaterials.In aspect of synthesis optimization,in addition to inorganic carbon materials,biomass materials and a few organic molecules as carbon source precursors,there is still a lack of synthetic strategies for preparing carbon dots with excellent luminescent properties by using other types of organic substrates as reaction materials.In terms of mechanism research,due to the complexity of carbon dots'structure and composition,most of the studies on its luminescence mechanism can only verify the optical properties of its ensembles by means of relatively simple structure-property characterization or theoretical simulation methods.However,the origin of their fluorescence behaviors is rarely reported from the perspective of microscopic single particle level,which makes its application in the fields related to single particle imaging and tracking limited.In terms of application development,it is still very challenging to realize fluorescence sensing and detection applications with multi-functional,multi-purpose and multi-modal characteristics of carbon dots.Therefore,in this paper,we studied the use of chloroform as a novel carbon source precursor,through the combination with different amines,a series of controllable preparation of carbon dots with excellent luminescent properties.Combined with single particle fluorescence imaging and single particle spectroscopy,the luminescence mechanism of carbon dots were further investigated.Finally,we expanded the multi-functional,multi-purpose,and multi-modal demonstration of these carbon dots for fluorescence sensing and detection in hazardous explosives and metal ions.In the first chapter,we introduce the development history,main preparation methods,luminescence types,emission mechanism and application trend of fluorescent carbon dots,and further elaborate the main topics and design ideas of this paper.In chapter two,single particle fluorescence imaging and single particle spectroscopy were utilized to further study the single-particle luminescence behavior of F-C dots prepared by using chloroform and diethylamine as raw materials in our group's previous report.The co-localization single particle fluorescence imaging and spectral test results of F-C dots show that a single F-C dots can exhibit a wide range of fluorescence emission from blue to red light,but the emission intensity varies to a certain extent in different luminous regions between different particles.Therefore,we believe that a single F-C dots can have continuously adjustable multi-chromatic emission properties.This phenomenon is obviously different from other reported carbon dots which the excitation-dependent fluorescence behavior is caused by the heterogeneity of size and structure.Next,by comparing the fluorescence emission properties of F-C dots with the small molecular by-products produced by the reaction,the origin of such multi-color emission carbon dots at the single particle level are more understood.It can be found that the emission peaks of many molecular by-products generated by the reaction can also cover the entire visible region.Moreover,the fluorescence emission peak of these molecular by-products is close to that of F-C dots at same excitations,but they have different fluorescence lifetimes.Combined with the previous analysis of its macroscopic properties and structure,we believe that the multi-color fluorescence behavior of F-C dots results from the hybridization of various luminescent molecular states on its surface and many graphitized fragments with fluorescence emission properties in the carbon core.This study will lay a foundation for the development of single-particle imaging and tracking technology.In the third chapter,we improve the synthesis route of fluorescent carbon dots,with chloroform and o-phenylendiamine as raw materials,via a simple and convenient solvothermal strategy for preparing a class of excitation-independent yellow-green emission carbon dots with low biotoxicity and good photostability.Subsequently,we found that the carbon dots can be applied to detect 2,4,6-trinitrophenol?TNP?selectively and sensitively with a minimum detection limit of 2?M,compared to other kinds of multi-nitro substituted dangerous explosives.Through a series of spectral analysis and theoretical calculation,it is proved that the mechanism of selective detection is attributed to electron transfer assisted by hydrogen-bond interaction and electron transfer assisted by proton transfer.At last,by developing a carbon dots-based test paper,we also achieved the results of highly selective fluorescence detection of TNP in the solid phase.In the last chapter,we introduce heteroatom doping strategies,further optimize the synthesis method of fluorescent carbon dots in the above chapter,with chloroform,o-phenylendiamine and dithiothreitol as raw materials,via a facile solvothermal process for synthesizing N,S co-doped longer wavelength red emission carbon dots with²3%high fluorescence quantum yield.In addition to the selective detection of Cu²⁺by these carbon dots in the traditional fluorescence mode,we have also developed fingerprint recognition of Cu²⁺by UV-vis absorption spectrum,which greatly improves the accuracy of detection results.Subsequently,the reason for this selective recognition and detection of Cu²⁺was considered to be influenced by the quenching mechanism assisted by coordination.Moreover,the use of carbon dots-based detection paper in the absorption and emission dual mode can also achieve the visual selective recognition of Cu²⁺,which provides a reference for the future development of monitoring methods of trace metal elements.