Study On The Efficient Extraction Of Lignin And Its Preparation And Application Of Carbon Quantum Dots

Carbon quantum dots（CQDs）are regarded as a fluorescent material with a size of less than 10 nm which have attracted increasing in ion detection,fluorescent labeling,drug delivery,bioimaging,photocatalysis and luminescent material owing to their intoxicating and favorable properties such as water solubility,fluorescence performance,crystallinity,low toxicity,sensitivity,and easily functional modification compared with traditional heavy metal quantum dots such as cadmium sulfide（Cd S）and organic dyes.At present,most of precursors for the preparation of CQDs are small organic polycyclic aromatic hydrocarbon molecules which are toxic,carcinogenic and also serious environmental pollution.Therefore,a more environmentally friendly,economical and green method to prepare CQDs with high quantum yield is still an urgent problem to be solved.Lignin is the second largest green renewable resource in nature,and its content is slightly second to cellulose.It has natural aromatic structure,high carbon content,non-toxic harmless,and has unparalleled advantages compared with other small organic molecules such as polycyclic aromatic hydrocarbon molecules.The large-scale preparation of lignin-based carbon quantum dots（LCQDs）using lignin as the precursor not only can turn waste into treasure,but also promote the sustainable development of the economy.In this paper,p-toluenesulfonic acid is used to extract lignin from Lingnan bagasse.The lignin is modified by carboxylation,and then combined with citric acid and o-phenylenediamine to form LCQDs through dehydration,polymerization,and carbonization processes.By adjusting the pH value to make them protonated or deprotonated,blue and green fluorescent LCQDs can be obtained and applied to the field of iron ion,nitrite ion detection and cell imaging.Specifically,the main research content includes the following three chapters:（1）Efficient extraction of lignin from bagasse.Using Lingnan bagasse as the raw material and p-toluenesulfonic acid as the pretreatment agent,under green and mild conditions,the three major components can be separated efficiently,and the content of the three major components is:cellulose 38.94%,hemicellulose 26.55%,lignin 31.76%.When the concentration of p-toluenesulfonic acid is 70%,the temperature is 80℃,and the time is between 30-40 min,the reaction effect is the best.At this time,the extraction rate of lignin can reach 90.33%,the weight average molecular weight is 1561 Da,and the distribution is relatively uniform,which lays a good foundation for the subsequent preparation of carbon quantum dots using lignin as a precursor.（2）Preparation and fluorescence properties of LCQDs.By a one-step hydrothermal method,LCQDs are formed through dehydration,polymerization and carbonization processes,and the pH value is adjusted to make them protonated or deprotonated to obtain blue and green fluorescent LCQDs.The quantum yield of LCQDs is 59.32%,the average size is 2.5 nm,and the adjacent lattice spacing is 0.21 nm.The surface of LCQDs is rich in hydroxyl,carboxyl and amino groups,and has good water solubility.In addition,the LCQDs have strong solution stability and are basically not affected by inorganic salt ions.The fluorescence intensity remains at 77.79%after being placed at room temperature for one month.（3）LCQDs are used for ion detection.LCQDs are used as fluorescent probes to achieve specific fluorescence responses to iron ions and nitrite ions,with selectivity of 99.34%and77.38%,respectively.The fluorescence quenching mechanism of iron ions in aqueous solution of LCQDs has been studied.The surface of LCQDs have a large number of oxygen-containing functional groups such as hydroxyl and carboxyl groups,which are easy to chelate or coordinate with Fe³⁺,thereby forming a stable LCQDs-Fe³⁺system.The fluorescence quenching mechanism static quenching;When the concentration of l LCQDs is low（500μg/m L）,the toxicity for cancer cells,ordinary cells and stem cells is very low,and bright blue light is observed under the confocal laser microscope.