Preparation And Optical Modulation Of Carbon Dots And Application In Fluorescent Sensing

As a novel carbon-based nanomaterials,carbon dots（Cdots）have been proposed as advanced materials for potential applications in the fields such as fluorescence sensing,biomedicine,photovoltaics and display devices,etc,due to their excellent optical properties,good bio-compatibility,environmental friendliness and easy preparation.Therefore,the investigation of Cdots has quickly become a hot and fast-growing field of research in physics,chemistry,biology,medicine,material science and in other interdisciplinary areas.Although the optical properties and application technologies of Cdots have been widely studied and developed,due to the complexity of the structure and composition of Cdots,there are still a lot of key scientific problems to be solved in the aspects of the controllable preparation,performance characterization,physical property regulate and the exploration of emerging applications from Cdots,to mention but a few.To address some of these issues,in this thesis I apply the biomass wastes and the analytically pure organic compounds as carbon source,the low cost and tractable experimental approaches for material preparation are developed.The fluorescence properties of Cdots were optimized and regulated by modification of different surface functional groups or doping of different elements.Based on the excellent fluorescence properties of Cdots prepared in this work,the selective detection of Fe³⁺and Cr³⁺,the characteristic fluorescence response to p H values,and the TaqMan probe of Cdots are realized and studied systematically.The main results and outcomes from this work are summarized as follows:1.Preparation of Cdots using biomass wastes as carbon sources.The approaches for preparation of Cdots from tobacco wastewater,soybean wastewater and wine lees are developed respectively.The corresponding fluorescence properties of these Cdots are studied in details.It is found that:（1）blue-light fluorescent Cdots can be prepared from tobacco wastewater.（2）blue-and green-light fluorescent Cdots can be prepared from soybean wastewater and its concentrated solution,respectively.Different nitrogen contents and nitrogen-containing chemical bonds in the two kinds of Cdots can induce different photoluminescence behaviors.（3）red-light fluorescent Cdots with narrow half maximum width（25 nm）can be prepared from wine lees.The surface functional groups play a leading role in the photoluminescence（PL）of these Cdots.It is demonstrated that the wastes from industrial and agricultural productions can be applied as carbon sources to prepare fluorescent Cdots.Thus,we can turn waste into treasure,which provides a scheme and technical way to solve at least partly the environmental pollution problem of waste in industrial and agricultural production.2.Study on the mechanism of fluorescence modulation of Cdots.In order to understand the structure and fluorescence properties of Cdots prepared by different techniques using different materials,and to explore the relationship between the structure of Cdots and fluorescence properties,in this thesis,the analytical reagents including citric acid,L-glutamic acid,urea and methionine are used as carbon sources to prepare Cdots.The Cdots modified by nitrogen-,sulfur-and phosphorus-containing functional groups are obtained respectively.Moreover,the nitrogen doped and nitrogen-sulfur co-doped Cdots are aslo obtained experimentally.The modulation mechanisms of surface functional groups and of doping on the fluorescence properties of Cdots are studied and discussed.The main results obtained from these studies are as follows.（1）The hydroxyl group attached to the edges of the sp²conjugatedπdomain can significantly increase the fluorescence intensity of the Cdots,while the carboxyl groups adsorbed on the surface/edge of the Cdots can form the non-radiative recombination centers in the Cdot-based luminescent system,which weakens the fluorescence of Cdots.（2）In comparison to nitrogen-or phosphorus-containing functional groups,the surface modification of Cdots with sulfur-containing functional groups can significantly enhance the Fluorescence quantum yield（FLQY）of Cdots.（3）The impurities can be substitutional atoms of the carbon structure in the sp²conjugatedπdomain,or can be adsorbed on the edge of the sp³carbon orbits to change the electronic energy structure of the Cdots,so as to regulate the fluorescence of the Cdots.（4）Compared to undoped Cdots,the band gaps of nitrogen-doped and nitrogen-sulfur co-doped Cdots are narrower and,therefore,their characteristic absorption peaks and fluorescence emission peaks are red shifted to different degrees.Most importantly,they have relatively higher FLQY.（5）Compared to nitrogen doped,nitrogen-sulfur co-doping can significantly improve the FLQY of Cdots.This study provides an effective experimental method and technical way to improve the fluorescent emission and to modulate the color of the PL emission from Cdots.3.The application of Cdots for fluorescent sensing.（1）The fluorescent Cdots prepared from tobacco wastewater are used to realize the selective detection of metal Fe³⁺.The Cdots have a good fluorescence linear response in the wide range of Fe³⁺concentration from 0.1μM to 700μM.The fluorescence quenching effect is mainly attributed to the dynamic quenching process.（2）The fluorescent Cdots prepared from wine lees are used for selective detection of metal Cr³⁺.The Cdots have good PL linear response in the concentration range of 0.1-0.6μM and 1-80μM.The fluorescence quenching effect are resulted from the coordination and dynamic quenching mechanism.（3）Based on the protonation and deprotonation of carboxyl groups on the surface of Cdots,the Cdots prepared using wine lees as carbon source have strong p H sensitivity.The corresponding fluorescence intensity has a linear relationship with the p H values（4～12）.This work provides a new way for the detection of p H value,Fe³⁺and Cr³⁺in solution for environmental protection and monitoring.4.The effect of TaqMan probe on the fluorescence properties of Cdots.The result shows that TaqMan probe can inhibit the fluorescence of Cdots.In this study,the mechanism of the effect of TaqMan probe on the fluorescence properties of Cdots is proposed and examined.（1）There is a process of energy resonant transfer between the Cdots and MGB-NFQ labeled on TaqMan probe.（2）The PL emission frequency from Cdots can be reduced by the decrease in oxygen-containing functional groups and nitrogen content on the surface of Cdots.This work provides an experimental basis for the application of Cdots in the study of TaqMan probe.