Green Synthesis And Analytical Application Of Two Kinds Of Carbon Nanodots

As a kind of novel carbon nanomaterials, carbon dots or carbon quantum dots which consists of grapheme quantum dots, carbon nanodots and polymer quantum dots, featured spherical or near spherical structure and many unique photoluminescent properties, including stable photoluminescence, no photobleaching phenomenon, turnable fluorescence emission, low toxicity and benign biocompatibility. So far, carbon nanodots have been successfully applied in the fields as bioimaging, chemical sensing, nanodrug, photocatalysis and so on. With respect to the green synthesis and analytical application of carbon dots, this thesis has conducted following researches:Firstly, we developed two synthesis methods of carbon dots. One is the microwave assisted one-step green synthesis of photoluminescent carbon dots from serine, PEG600 and glycerine, while the other is simple hydrothermal preparation of carbon nanodots from citric acid and ethylenediamine. We have conducted the surface and structure characterization of these two kinds of carbon dots and explored their luminescence properties. Series of experiments have proved that carbon dots obtained by microwave assisted method appear to be 2-3 nm near-spherical particles well-dispersed in water, showing good stability, while carbon nanodots prepared from hydrothermal method show rather bigger diameter around 20-30 nm. Both of two carbon nanodots were proved to be consisted of three elements: carbon, oxygen and nitrogen. Hydroxyl, carboxyl and amino groups were found on the surface of two kinds of carbon dots. Experiments have proved that carbon dots obtained by microwave assisted method contains more carbon than the carbon nanodots prepared from hydrothermal method, while the latter one contains more carboxyl than the former one. Both of them performed excellent luminescence properties. We have explored the influence of concentration of carbon dots solution, p H and the shelf time on carbon dots' fluorescence emission. Additionally, we investigated the emission mechanism the carbon dots and inferred that the bright emission may be ascribed to the recombination of electron–hole pairs in the strongly localized ? and? * electronic levels of the sp2 sites.Strong CL signal was generated when carbon nanodots were mixed with KMn O4 and Na2SO3 in acid aqueous medium. Thus, we established a novel detecting method for sulfite. Under the optimum experimental conditions, the CL intensity was linearly related to the concentration of SO32- from 1.0×10-4 to 1.0×10-3 mol L-1(r = 0.9906). The method was applied to the determination of sulfite in tremella fuciformis and the recovery ratio is 78%~88.3%.A new fluorescence and colorimetric probe for Hg2+ ions detection is developed based on a one-step hydrothermal method synthesized carbon nanodots. The fluorescence of the carbon dots in p H 5.0 phosphate buffer solution can be dramatically quenched by Hg2+, whereasnearlyunaffectedby other metalions.Based on this, a sensitive and selective method for Hg2+ detection has been established. Under the optimum conditions, a good linear relation exists between the quenching efficiency(F0/F) and the concentration of Hg2+ in the range of 7.0×10-9~7.0×10-7 mol L-1 with a detection limit(3?) of 5.5 ×10-10 mol L-1. Additionally, we have also found that UV-Vis absorbance at 302 nm of carbon dots also shows significant linear correlation with the concentration of Hg2+, which indicating that as-prepared carbon-dots could act as a colorimetric probe of Hg2+ simultaneously. Linear range of this colorimetric probe is 1.0×10-9~7.0×10-7 mol L-1 and the detection limits is 5.7×10-10 mol L-1. This Hg2+ probe gives excellent performance in real tap water samples analysis, suggesting promising application in the future.Further more, we investigated the chemiluminescence mechanism and fluorescence quenching mechanism of carbon nanodots. We assumed that carbon nanodots have played the role of sensitizer in the chemiluminescence reaction. Comparing to traditional reaction system of Na2SO3-KMn O4, due to its unique surface properties, carbon nanodots can absorb energy in the reaction and give out bright chemiluminescence efficiently. Fluorescence quenching reaction between carbon nanodots and Hg2+ may be classified to the static quenching. And the binding constant between them was K=1.29×108 L mol-1.