Preparation Of Nitrogen-doped Carbon Quantum Dots And Their Application In Multifunctional Fluorescent Probes

Carbon quantum dots(CDs)are a new type of carbon-based fluorescent material with a particle size of less than 10 nm and near-spherical shape.Since their discovery,the CDs have exhibited huge application potential in many fields including photocatalysis,bioimaging,drag delivery,optoelectronic device and fluorescence detection.So far,some traditional fluorescent materials such as fluorescent dye,semiconductor quantum dots and noble metallic nanoclusters have been used for fluorescence detection.In comparison with fluorescent dye,the CDs show better biocompatibility and photostability.The CDs possess lower toxicity and higher chemical inertness compared with semiconductor quantum dots.In addition,the cost and preparation of CDs are much lower and easier than that of noble metallic nanoclusters.Thus,The CDs-based fluorescent probes are attracting more and more attention in recent years.However,the researches about CDs-based multifunctional fluorescent probes are few.Moreover,the CDs-based multifunctional proportional fluorescent probes have seldom been reported yet.To further explore and widen the application of CDs in fluorescence detection is meaningful.Herein,we synthesized two kinds of N-doped CDs(NCDs)by selecting different precursors and methods.Based on the synthesized NCDs,two multifunctional fluorescent probes(conventional and proportional)were constructed to realize the fluorescent detection of specific ions and chemical substances with high sensitivity and selectivity.The specific research content was listed as follows.(1)Using ethanolamine as carbon source and 1-carboxyethyl-3-methylimidazolium chloride as dopant,the NCDs emitted blue fluorescence under ultraviolet light irradiation(365 nm)were synthesized by the hydrothermal method.The morphology,structure and surface groups of NCDs were characterized by TEM,XRD,XPS and FTIR technologies.The results demonstrated that the NCDs with an average particle size of 3.4 nm were near-spherical,almost amorphous and abundant in amino and oxygen-containing groups.Hence,the NCDs exhibited good dispersion stability in water.The fluorescence spectra illustrated that the emission wavelength and intensity of NCDs aqueous dispersion was dependent on excitation wavelength.The maximum excitation and emission peak is at 350 nm and 443 nm,respectively.The fluorescence quantum yield of NCDs was 24.7%under 350 nm excitation.The fluorescent properties of NCDs were stable under wide pH range of 4-11 and high salinity and hence could be a kind of ideal material for constructing fluorescent probe.The fluorescence of NCDs was greatly quenched when Hg2+ and Cu2+were added.The NCDs-based multifunctional probe towards Hg2+and Cu2+was constructed using S2O32-and P2O74-as masking agent,respectively.The fluorescence quenching mechanism was photo-induced electron transfer.The linear relationships between the concentration(c)of Hg2+and the fluorescence quenching efficiency of NCDs were in the ranges of 0-10 μM and 10-50 μM.The linear ranges for Cu2+probe were 0-2.5 μM and 2.5-40 μM.Based on the linear range,the limit of detection(LOD)for Hg2+and Cu2+was 0.075 and 0.125 μM,respectively.In addition,addition of S2O32-into NCDs-Hg2+system resulted in the fluorescence recovery of NCDs.Then the NCDs-based S2O32-probe was built with linear ranges of 0-20 μM and 20-80 μM and LOD of 1.17μM.All of above probes with favorable selectivity could realize the accurate detection of specific ions in tap water samples.(2)Using p-phenylenediamine as the carbon source and toluene as solvent,the NCDs emitted red fluorescence under ultraviolet light irradiation were synthesized by the solvothermal method.The characterization results demonstrated that NCDs with an average particle size of 5.0 nm and relatively high N content were near-spherical,almost amorphous and abundant in amino and oxygen-containing groups.The emission wavelength of NCDs aqueous dispersion was independent of excitation wavelength.The fluorescence quantum yield of NCDs was 9.6%,18.2%and 26.8%under 360,440 and 500 nm excitation,respectively.The fluorescent properties of NCDs were stable under wide pH range of 5-11 and high salinity.Hence,the NCDs was proper fluorescent materials for building fluorescent probe.Under 440 nm excitation,when the Zn2+was added into the NCDs-quercetin system,the complex of Zn2+-quercetin emitted green-blue fluorescence(480 nm)was formed.Meanwhile,the red fluorescence at 610 nm of NCDs was greatly quenched.Then the NCDs-based Zn2+pproportional fluorescent probe was constructed.The czn2+and fluorescence intensity ratio(1480/1610)illustrated a good linear relationship in the range of 0-30 μM and the LOD was calculating as 0.143 μM.It is well known that the ATP has stronger affinity towards Zn2+than quercetin.Adding of ATP into NCDs-Zn2+-quercetin system caused the fluorescence recovery of NCDs and fluorescence quenching of Zn2+-quercetin complex.Based on this,the NCDs-based ATP proportional fluorescent probe was built with direction ranges of 0-10 μM and 10-35 μM and LOD of 0.69 μM.Under 360 nm excitation,addition of Al3+into NCDs-quercetin system resulted in the formation of Al3+-quercetin emitted green-blue fluorescence(480 nm).While the fluorescence intensity of NCDs was almost constant.Hence,the NCDs-based A13+proportional fluorescent probe was constructed with the direction range of 0-35 μM and LOD of 0.09 μM.All of the above probes had good selectivity and the Al3+probe realized the accurate detection of Al3+in tap water sample.