Modification Of Graphitic Carbon Nitride Quantum Dots And Their Application In Fluorescence Sensor

In recent years,incidents that endanger human health due to environmental pollution and excessive food additives have been widely reported.Therefore,it is very important to accurately quantify common pollutants in the environment and some food additives.At present,the commonly used detection methods include chromatography,capillary electrophoresis,electrochemical analysis,and fluorescence analysis.Compared with other methods,fluorescence analysis has the advantages of low cost,simple operation,high detection sensitivity,strong selectivity,and simple sample preparation,which has attracted extensive attention of researchers.In this paper,a series of fluorescence sensors were constructed for different analytes and applied to the detection of real samples.The details are as follows:1.Blue fluorescent carbon nitride quantum dots（g-C₃N₄QDs）were synthesized by a simple one-pot solvothermal method using melamine as carbon and nitrogen sources,and successfully applied to the detection of tetracycline（TC）in water samples.To explore the structure,morphology,and optical properties of g-C₃N₄QDs,instruments such as X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,fourier transform infrared spectroscopy（FT-IR）,and fluorescence spectroscopy（FL）were employed.The fluorescence intensity of g-C₃N₄QDs decreased with increasing TC concentration（excitation/emission maxima at 325/394nm）,which was caused by the inner filter effect.The fluorescence emitted by g-C₃N₄QDs was absorbed by TC,which resulted in fluorescence quenching of g-C₃N₄QDs.Good linearity was obtained over the TC concentration range of0.23-11.26μM and 11.26-202.7μM,with a limit of detection（LOD）of 0.19μM.The results showed that the detection method had high sensitivity,good selectivity and wide detection range,and could be applied to the detection of TC concentration in real water samples.2.Graphitic carbon nitride quantum dots（P-CNQDs）were decorated with phosphorus as a doping element.Elemental doping can improve the optical properties of fluorescent nanomaterials,and since phosphorus is a non-metallic element with more electron d orbitals,it is easier to donate electrons.Similarly,uniform and narrowly distributed P-CNQDs with an average size of 2.5 nm were prepared by chemical oxidation and hydrothermal methods using melamine as carbon and nitrogen sources,and diammonium hydrogen phosphate as phosphorus source.Ag⁺quenched the blue fluorescence of P-CNQDs in the form of charge transfer through a dynamic quenching process.P-CNQDs sensor could detect Ag⁺with two-segment linear relationships in the concentration range of 0.01-5 and 5-100μM and with an LOD of0.0171μM.The results showed that the sensor could successfully monitor Ag⁺in river water.3.We found that adding ciprofloxacin（CIP）to the solution of Ag⁺-quenched P-CNQDs restored blue fluorescence and switched from“off”to“on”state.Therefore,quantitative detection of CIP in the form of fluorescence-enhanced responses of P-CNQDs was feasible.The mechanism of CIP-enhanced fluorescence of P-CNQDs was attributed to the formation of hydrogen bonds and the formation of complexes with Ag⁺.The CIP concentration had a good linear relationship in the range of 0.002-2μM,and the LOD value was 1.12×10^-3μM.The sensor was successfully applied to detect CIP in river water,and the recovery rate of standard addition was98.28-105.99%.4.Riboflavin（RF）was detected based on the mechanism of fluorescence resonance energy transfer（FRET）.FRET occurs between two luminescent chromophores and can greatly improve the selectivity and sensitivity of fluorescence analysis.With the increase of RF concentration,the fluorescence intensity of the emission peak of P-CNQDs-CIP at 430 nm decreased,and the fluorescence peak of RF at 525 nm increased regularly,the ratio of the fluorescence intensity of 525 nm and 430 nm exhibited a good linear relationship between 0-60μM of RF,and the detection limit of RF（0.021μM）was quite low compared to the reported method.And the mechanism of FRET from P-CNQDs-CIP to RF was discussed based on the aspects of spectral observation,thermodynamic parameters and fluorescence lifetime changes.This method can be successfully applied used to quantify RF in complex system vitamin B2 tablets.