The Preparation Of Carbon Quantum Dots And Their Fluorescence Detection Toward Metal Ions

Heavy metal pollution is closely related to the environment and human health. Heavy metals can accumulated in the living tissue by food chain or direct contact with human body, causing serious diseases in nervous, digestive, or respiratory system, and severe cases can even cause death. Fortunately, with improving social health consciousness, the exploration of effective heavy metal detection technology has flourished. So far, many sensitive detection methods for heavy metal ions emerged, such as atomic absorption spectrometry, atomic fluorescence emission, inductively coupled plasma mass spectrometry and fluorescence method. Although atomic absorption spectrometry, atomic emission spectrometry and inductively coupled plasma mass spectrometry possess high detection sensitivity, these devices are expensive with poor portability, which limit their application in real-time and rapid quantitative and qualitative detection for heavy metal pollution. In recent years, fluorescent method catches the researchers' eyes for its merits such as high speed of detection, simple operation, inexpensive equipment, and portability. Fluorescent probes can be divided into organic fluorescent probe, and fluorescent quantum dots probe(semiconductor/element quantum dots). The stability of organic fluorescent probes are often poor, which are prone to photo-bleaching; semiconductor quantum dots are mostly toxic and oil-soluble, which need further modification to improve their water solubility. In contrast, carbon quantum dots possess advantages of good water solubility, excellent light stability, facile green synthesis, and easy modification, which endow them great values in rapid qualitative and quantitative detection of heavy metal ions in aqueous solution. Based on the fluorescence carbon quantum dots and their surface modification, three fluorescent sensors for mercury ion and iron ion were designed, and their fluorescence detection performance was investigated in detail. The main content of the dissertation can be divided into the following sections:(1) Using sodium citrate and ammonium bicarbonate as raw material, carboxyl-modified carbon quantum dots(CQDs) were obtained by hydrothermal method. L-tryptophan was modified onto carbon quantum dots surface(L-CQDs) after amidation reaction between the amino groups of L-tryptophan and the carboxyl groups of the carbon quantum dots. The obtained L-CQDs possessed a uniform particle size of about 5 nm with a quantum yield of 0.25. Their optimal excitation wavelength was 350 nm, maximum emission wavelength was 440 nm. L-CQDs showed a single selectivity for mercury ions. In the absence of mercury ions, L-CQDs can exhibit a bright blue fluorescence emission, which could be gradually decreased by increasing amount of mercury ions. When the concentration of mercury ions reached to 10 ?M, the fluorescence emission intensity of L-CQDs can be totally quenched. The detection limit is calculated to be 11 nM.(2) Using glucose and urea as raw materials, amino modified carbon quantum dots(NQDs) were prepared by hydrothermal method. Its optimal excitation wavelength was 360 nm, the maximum emission wavelength was 450 nm, and the quantum yield was about 0.15. NQDs showed a single selectivity for mercury ions. In the absence of mercury ions, NQDs can exhibit a bright blue fluorescence emission, which could be gradually decreased by increasing amount of mercury ions. When the concentration of mercury ions reached to 0.2 mM, the fluorescence emission intensity of NQDs can be totally quenched. The detection limit is calculated to be 0.175 ?M.(3) Using chitosan and citric acid as raw material, amino and carboxyl co-modified carbon quantum dots(CNQDs) were prepared by hydrothermal method. Its optimal excitation wavelength was 360 nm, the maximum emission wavelength was 450 nm, and the quantum yield was about 0.11. CNQDs showed a single selectivity for iron ions. In the absence of iron ions, CNQDs can exhibit a bright blue fluorescence emission, which could be gradually decreased by increasing amount of iron ions. When the concentration of iron ions reached to 2.4 mM, the fluorescence emission intensity of CNQDs can be totally quenched. The detection limit is calculated to be 1.68 ?M.