Construction and Performance of Dual-nitrogen Source Co-doped Lignin Carbon Quantum Dots/Organic Solvent Moisture Probes
Water is an essential substance in daily life,however,the presence of water in organic solvents is often considered a safety hazard in industrial production and chemical experiments.In chemical experiments,the moisture in organic solvents can affect product properties,reduce chemical reaction rates,and even alter reaction processes.Therefore,the determination of water content in organic solvents is of great practical significance.The Karl Fischer titration method,as a classic method,is widely used for the determination of water in organic solvents;however,this method has defects such as high reagent toxicity,complex operation,and inability to achieve real-time monitoring.Both gas chromatography and nuclear magnetic resonance can be used to detect water in organic solvents,with the advantages of fast detection speed,wide application range,and high detection sensitivity.However,there are also problems such as expensive instruments,long time consumption,and the need for professional personnel to operate.In recent years,some have proposed using fluorescent probes for the detection of water in organic solvents.However,the current synthesis process of fluorescent substances is complex and costly,which is not conducive to large-scale applications.Carbon Quantum Dots(CQDs),as a new type of fluorescent nanomaterial,have good optical stability,low toxicity,simple synthesis process,low cost,and are easy to functionalize.Therefore,they have received increasing attention in organic solvent water detection.At present,CQDs synthesized using chemical small molecules as carbon sources have potential applications in organic solvent moisture detection.However,compared with biomass materials,chemical small molecules are more expensive and require the use of strong acids such as HNO3 and H2SO4 during the transition to CQDs.The resulting CQDs have potential toxicity,poor environmental friendliness,and are not conducive to large-scale applications.In addition,most reported CQDs/organic solvent fluorescent probes can only detect water content within a narrow range and cannot achieve good visual detection.Therefore,it is of great significance to synthesize CQDs with good biocompatibility using green chemical methods,using biomass with a wide range of sources and low prices as a carbon source,and using them to construct fluorescent probes for organic solvent moisture detection with good visual detection effects and a wide range of water content detection.As the second largest natural biomass resource in nature,after cellulose,lignin has a high carbon content,rich oxygen-containing functional groups,and a complex aromatic framework structure,which helps to form surface functionalized conjugated graphite based CQDs.However,the value of lignin has not been widely utilized,and the fluorescence quantum yield of lignin CQDs in previous studies is relatively low,and it has not yet been seen to be applied for water detection in organic solvents.This article uses lignin as the carbon source,hydrochloric acid as the depolymerizing agent,and o-phenylenediamine and urea as nitrogen dopants to obtain CQDs through a one pot hydrothermal method.The CQDs exhibit a significant spherical shape with a lattice stripe spacing of about 0.21 nm and an average particle size of about 6.49 nm.The carbon core contains a graphite like sp2 carbon structure,and the surface contains functional groups such as C-C/C=C,C-O,N-H,C=O,C-N,O-H,C-H,etc.The fluorescence quantum yield is 2.05％.Its aqueous solution exhibits significant absorption in the ultraviolet region and strong solvent dependence.The research results on fluorescence stability indicate that the CQDs have good light bleaching resistance and photoluminescence durability,which is beneficial for the construction and application of organic solvent water fluorescent probes.When used to detect three commonly used organic solvents in the laboratory(ethanol,N,N-dimethylformamide,and tetrahydro furan),as the water content in the organic solvents increases,the emission spectrum shows a red shift and its fluorescence intensity decreases.This may be attributed to the further increase in water content in organic solvents,which increases the polarity of CQDs/organic solvent water fluorescent probes.More hydrogen bonds are formed between water and CQDs,causing adjacent CQDs particles to aggregate with each other,increasing the interaction and resonance energy transfer of π-7r bonds in the CQDs structure.This not only reduces the emission energy level of CQDs,but also increases their probability of non radiative jumps,causing further shift in the fluorescence emission peak position of CQDs probes,resulting in multicolored luminescence.In addition,in the constructed dual nitrogen source co doped lignin CQDs/organic solvent water fluorescence probe,there is a good linear relationship between the fluorescence intensity of the carbon quantum dots and the water content of the organic solvent.The detection limits for water in ethanol,N,N-dimethylformamide,and tetrahydrofuran are 0.079％,0.075％,and 0.080％,respectively,and visual water detection has been achieved.The research results can provide theoretical and experimental references for the detection of trace amounts of water in organic solvents.
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