The Synthesis And Anaiytical Applications Of Nitrogen-doped Carbon Quantum Dots

Fluorescent carbon quantum dots?CQDs?have received many attentions due to their excellentoptical properties?high stability,good biocompatibility,easy surface functionalization?.Fluorescent carbon spots have broad prospects in many fields,such as fluorescent probes,biosensors,photocatalysis,optical photoluminescence and energy conversion,and so on.There are a variety of methods to prepare CQDs,among which hydrothermal treatment and microwave-assisted method are convenient and efficient,so they have become the most commonly used preparation methods,and a series of materials were used as carbon source to synthesize CQDs.In this thesis,three CQDs with different luminescent properties were synthesized through hydrothermal or microwave-assisted method by using peptone,wild jujube and maleic acid as the carbon sources.And the as-prepared CQDs are employed as fluorescent probes for highly sensitive detection of oxalic acid?OA?,Hg?II?and p-nitrophone?4-NP?,respectively.Blue fluorescent carbon quantum dots are prepared by microwave assisted pyrolysis usingpeptone as carbon source and ethylenediamine as surface passivation agent.The as-prepared carbon quantum dots with a high quantum yield?21.3%?.The fluorescence intensity of carbon quantum dots is obviously decreased by adding Co²⁺.Adding oxalic acid,the fluorescence of the CQDs-Co²⁺system recovered to a certain extent.Based on that,the prepared carbon quantum dots are used to detect oxalic acid concentration.The detection limit was 288?g/L.The quenching mechanism is that electron in excited CQDs transfer to Co²⁺,resulting in energy loss and strong fluorescence quenching of CQDs.The coordination of Co²⁺and OA decreases the concentration of free Co²⁺,resulting in weak quenching of CQDs fluorescence.Highly luminescent nitrogen-doped carbon quantum dots?N-CQDs?were synthesized throughhydrothermal method by using wild jujube and DL-tryptophan as raw materials.The as-prepared N-CQDs show high quantum yield of 16.9%.Hg²⁺quenches the fluorescence of carbon quantum dots,and the addition of oxalic acid restored the fluorescence of CQDs-Hg²⁺system.The CQDs-Hg²⁺system can be conveniently used as a fluorescent sensor for highly selective and sensitive determination of oxalic acid with a low detection limit of 57?g/L and a wide linear detection range of 0.1²0 mg/L.The quenching mechanism is that electron of excited CQDs transfer to Hg²⁺,resulting in dynamic quenching of CQDs fluorescence.The strong interaction between Hg²⁺and OA decreases the concentration of free Hg²⁺,resulting in weakened fluorescence quenching of CQDs.Based on the above-mentioned result,the as-prepared carbon quantum dots can be used asfluorescent probes for highly sensitive detection of Hg²⁺.The detection range of 1⁵0 nmol/L with a detection limit of 0.26 nmol/L?0.05?g/L?Hg²⁺.The proposed method can be used for the determination of Hg²⁺in real water samples.A hydrothermal method for synthesis of highly photoluminescent N-CQDs using maleic acid asthe carbon source and melamine as the surface passivation reagent was reported.The fluorescence intensity of the CQDs was found to be effectively quenched by p-nitrophone?4-NP?.Photoluminescent quenching mechanism is ascribed to a combination of inner filter effect and static quenching effect.The sensor presented a good linearity with 4-NP concentration in the range of 0.05⁴?g/mL.The detection limit of 17?g/L 4-NP could be obtained,which is lower than 60?g/L allowed by the U.S.Environmental Protection Agency for drinking water.The probes can be successfully applied to the determination of 4-NP in environmental water samples.The high selectivity and sensitivity of the probes offer this probe a promising platform for environmental sensing applications.