Synthesis And Fluorescence Sensing Applications Of Carbon Dots And Their Composites

Carbon dots are a class of zero-dimensional carbon materials with fluorescent properties.As an emerging fluorescent carbon material,carbon dots have attracted wide attention.Compared with conventional fluorescent materials(such as quantum dots and organic dyes),carbon dots have many unique properties such as fluorescence tunability,good biocompatibility,simple preparation process and strong resistance to photobleaching.In recent years,the application of carbon dots in fluorescence sensing has been a popular topic and demonstrated superior performance such as high sensitivity,good selectivity and easy to use,which is expected to provide real-time visualization of target analyte tracking.Therefore,it is attractive to design and synthesize carbon dots with specific recognition sites for fluorescence analysis of ions/molecules.In addition,it is an effective strategy to construct carbon dots-based composites combined with other functional materials for some special application environments,which will be helpful for expanding their sensing application range as well.Therefore,to meet the requirements of fluorescent materials with different properties in fluorescence sensing,it is of great significance to design and synthesize carbon dots and their composites with different surface functional groups,good fluorescence stability,and higher quantum yield.In this thesis,a variety of carbon dots and their composites are designed and synthesized.Their structures and properties are well characterized in detail.These carbon dots or their composites are used for ion/molecule fluorescence analysis or latent fingerprint development,respectively.The main points of this thesis are outlines as follows:The first chapter introduces the research background of this thesis,including the definition of carbon dots,synthesis strategy,luminescence mechanism,mechanism of carbon dots used in fluorescence sensing,and the main analytes of carbon dots in fluorescence sensing.The advantages and disadvantages of the current methods and the problems need to be solved are discussed.Then,the research significance of this thesis is proposed.The second chapter presents a green and low-temperature strategy for the preparation of N-doped carbon dots(N-CDs)and their application in fluorescence sensing Fe3+.Ascorbic acid and arginine served as carbon and nitrogen sources,respectively,and water was used as the reaction medium.Nitrogen-doped carbon dots(N-CDs)can be obtained via reacting at 60? without the presence of any strong acids/bases or organic solvents.Thus,it can be considered as a green synthesis process.The research results show that the average particle size of N-CDs is 20.3 nm.These N-CDs have good water dispersibility.In the pH range from 3 to 11,the fluorescence emission intensity of N-CDs remains basically unchanged,indicating that they have stable fluorescence emission.By examining the effect of various metal ions on the fluorescence intensity of N-CDs,we find that only Fe3+can effectively quench the fluorescence of N-CDs,and the response process is very fast(<30s).The standard curve for Fe3+ detection can be obtained in a range of 10-200 ?M.Based on 3?/Slope,the limit of detection is as low as 2.2 ?M.The third chapter presents one-pot synthesis of carbon dots via hydrothermal reaction and their application for fluorescence sensing Fe3+.To obtain nitrogen-doped carbon dots(N-CDs)for sensing Fe3+under acidic conditions,robust and highly fluorescent N-CDs were prepared by one-pot hydrothermal treatment of dopamine and ethylenediamine.The N-CDs have a relatively uniform size of approximately 4.9 nm and good water dispersibility.The N-CDs exhibit strong visible fluorescence emission(ca.497 nm),which is high stability at low pH(i.e.2-3).The FTIR spectra and the XPS analyses indicated that primary amines and distinctive catechol groups exist on the surface of the N-CDs.The FTIR spectra further prove that the catechol groups on their surfaces can be oxidized to the quinone species by Fe3+ contributing to fluorescence response to Fe3+.Thus,the N-CDs as fluorescence probes allowed the detection of Fe3+under acidic conditions,which has been verified in this work.Based on 36/Slope,the limit of detection was estimated to be about 10.8 ?M.The fourth chapter presents an environmentally friendly synthetic route for CDs/ZIF-8 fluorescent powders and their application in latent fingerprint imaging.It was found that the carbon dots can greatly promote the growth of ZIF-8 crystals.The CDs/ZIF-8 fluorescent powders generated at low 2-methylimidazole/Zn2+molar ratio(10:1)in aqueous solution at room temperature.Different morphologies and sizes of the CDs/ZIF-8 fluorescent powders can be easily obtained by adjusting the 2-methylimidazole/Zn2+ molar ratio.The as-prepared CDs/ZIF-8 fluorescent powders can afford multi-colored and high contrast imaging for latent fingerprints on different substrates,showing great potential for latent fingerprint detection.The fifth chapter presents a fast and novel strategy for the preparation of CDs/ZIF-8/MnO2 and their application in fluorescence sensing ascorbic acid.The CDs/ZIF-8 hybrid nanocomposite was synthesized via a rapid method within 5 min at room temperature.Then,a sensitive fluorescent nano-sensor based on MnO2 quenching fluorescent carbon dots was facilely developed by MnO2 in situ formed in the pores of CDs/ZIF-8 hybrid nanocomposite.This step involves a redox reaction between the CDs/ZIF-8 hybrid nanocomposite and KMnO4,leading the MnO2 form into the ZIF-8 framework.The resulting fluorescent nano-sensor exhibited a relatively uniform particle size of about 100 nm,good water-dispersibility and a highly selective fluorescence response toward ascorbic acid with a detection limit as low as 83 nM and a linear range of 0.5-8 ?M.