Preparation Of Nitrogen-doped Graphene Quantum Dots And Their Detection Of Risky Substances In Foods

In recent years,graphene quantum dots（GQDs）have attracted great interest from domestic and international researchers.Fluorescent carbon nanomaterials fluorescent quantum dots（GQDs）have obvious advantages over metal quantum dots due to their special physical and chemical properties,such as high emission quantum yield,good solubility,low toxicity,high molar extinction coefficient,and better biocompatibility,and have broad application prospects in the fields of sensing,biomedicine,energy batteries and photocatalysis.However,GQDs also have deficiencies such as low fluorescence efficiency,poor stability,and fewer active sites.To improve the performance of GQDs,surface modification or elemental doping of heteroatoms（such as N,S and P,etc.）can be used.Functionalization and doping of nitrogen are considered to be more effective in changing the intrinsic properties of GQDs and can provide more active centers,thus further improving the performance of GQDs.Since conventional metal quantum dot materials are toxic and polluting to the environment,and relatively expensive.Therefore,GQDs are a relatively green,low-cost and non-toxic type of quantum dots.There are many methods to prepare GQDs,and the traditional methods mainly include chemical methods,ultrasound-assisted methods,plasma-assisted methods,and laser-assisted methods.The carbon source materials for these methods use more expensive carbon nanotubes and carbon nanofibers.However,lower cost and green carbon sources have been a hot topic of research by scientists,especially bio-carbon-based raw materials,which have been used so far for different bio-carbon-based materials that are environmentally friendly,green,low cost,high carbon content,and rich in functional groups.In this thesis work,firstly,green natural carbon-based materials are used for non-polluting and convenient synthesis of GQDs as well as N-GQDs,several fluorescent probe sensors are constructed with N-GQDs as well as their composite fluorescent probes,and this fluorescent probe sensors are investigated for the detection of risky substances in food,such as antibiotics,hormones and pesticides.1.We prepared nitrogen-doped graphene quantum dots（N-GQDs）by a green,simple,and hydrothermal method without other contamination products using the natural products potato straight-chain starch and urea as precursors.The obtained N-GQDs have a uniform morphological distribution,an average particle size of about 5 nm,and exhibit blue fluorescence under UV lamp excitation.Based on the fluorescence burst mechanism of the internal filtration effect（IFE）of TC on N-GQDs,the fluorescence sensor of N-GQDs was investigated for the detection of tetracycline（TC）.A good linear relationship was established between the relative fluorescence intensity F/F0 of the fluorescence system and the logarithm of TC concentration（R²=0.9930）,and the limit of detection（LOD）was 9.735×10^-13 M.The N-GQDs showed good selectivity and high sensitivity for the detection of TC,and was used for the determination of TC in real foods（whole milk,skim milk,honey）with the detection limits of 3.750×10^-11～2.075×10^-9 M with a wide linear range and the recoveries of 93.80～109.20%,showing the promise of the N-GQDs fluorescent sensors in this chapter for the detection of TCs and other risk substances.2.We use cationic etherified starch as raw material to prepare nitrogen-doped graphene quantum dots（N-GQDs）directly by a one-step method,which has the advantages of being simple,green and non-polluting.Compared with the direct nitrogen doping method using nitrogen source and graphene quantum dots（GQDs）,the advantages of using cationic etherified starch for the direct synthesis of N-GQDs are that the method does not require the use of other nitrogen sources and carbon substrates;due to the large amount of amino groups in cationic etherified starch,the amino groups are directly linked to the starch molecules through chemical bonds,thus allowing the direct synthesis of well-doped N-GQDs and The loss of nitrogen during the synthesis of N-GQDs can be reduced.In addition,based on the static burst mechanism between CLB and N-GQDs,the N-GQDs fluorescent probe can be used for the detection of CLB with good linearity（R²=0.9879）in the concentration range of(5×10^-10～5×10^-6M)and the limit of detection（LOD）of 2.083×10^-13 M.Meanwhile,we used the N-GQDs fluorescent probe for the recoveries ranged from 98.4 to 108.8%with the relative standard deviations（RSDs）of 6.9～10.32%for the detection of CLB in chicken and beef.3.We used a planetary ball mill to prepare cationic starch from potato starch after ball mill mechanical treatment in 2,3-epoxypropyltrimethylammonium chloride（GTA）,and investigated the effect of ball mill mechanical activation treatment on the structure and properties of potato starch.The test results showed that the internal structure of potato starch granules was significantly changed,and the ball mill treatment induced spatial disorder of branched and straight chain starch,resulting in an increase of the disrupted area of the crystalline region in the starch granules,and the chemical reactivity of potato starch granules was improved due to this change,which was beneficial to the synthesis of cationic starch and the preparation of N-GQDs.We used the above-mentioned cationic starch as the carbon source to synthesize nitrogen-doped graphene quantum dots（N-GQDs）directly in a one-step method,and the obtained N-GQDs had a uniform size distribution with a particle size of about 4.5 nm,and fluoresced blue under UV excitation.Based on the aggregation of Fe³⁺with the surface groups of N-GQDs,Fe³⁺can effectively quench the fluorescence of N-GQDs,and the organophosphorus pesticide can inhibit the activity of acetylcholinesterase（Ach E）and thus the decomposition of acetylcholine（Ach）,which leads to the reduction of H₂O₂ production.The process of oxidation of Fe²⁺into Fe³⁺was inhibited and the concentration of Fe³⁺in solution was lower than that in the absence of organophosphorus pesticides,resulting in a gradual enhancement of the fluorescence of N-GQDs.Based on the above principle,we designed a fluorescent"off-on"N-GQDs fluorescent probe sensor for the detection of chlorpyrifos（Cop）with good linearity（R²=0.9854）in the concentration range of 5×10^-9～5×10^-7 M.The sensor was also applied to the detection of Cop in real samples.The method was applied to the determination of Cop residues in real samples with satisfactory recoveries of 95.2～103.0%and relative standard deviations（RSDs）in the range of 5.16～10.03%,indicating the applicability and accuracy of the methods in this chapter for the application of detection of risk substances in foods.