Fluorescence Enhancement Of Graphite Carbon Nitride And Its Application In The Field Of Fluorescence Sensing

.People pay more attention to environmental protection and physical health.This requires the detection of trace elements in environmental detection,food safety,body trace element monitoring and other fields,so the sensitivity requirements for various detection technologies are very high,and fluorescent sensing technology is a very important and feasible detection method.Although fluorescence technology plays an increasingly important role in the fields of chemical sensing probe and optical bioassay,most fluorescent sensing materials have potential toxicity to the environment and organisms,which limits their wide application.Graphite carbon nitride(g-C3N4)is a fluorescent material with good photochemical stability,non-toxic,environmental protection,and biocompatibility,but its luminescence mechanism is relatively vague,fluorescence intensity is relatively weak,and low water solubility,these shortcomings limit its in-depth application in highly sensitive fluorescence sensing technology.The work of this thesis mainly improves the fluorescence performance of g-C3N4,enhances its luminous intensity and water solubility,and further explores its application in the field of highly sensitive fluorescence sensing.The main work content is as follows:1.Using the improved Hummers method to oxidize g-C3N4 into nano-particle graphite oxide carbon nitride(nano-CNO).The introduction of carboxyl groups through oxidation enhances the nano-CNO’s luminous intensity three times that of the original g-C3N4;and improves the water solubility of graphite carbon nitride.The obtained nano-CNO powder has good photobleaching resistance and acid and alkali resistance.Based on the density functional theory,we further proposed a possible mechanism of the enhanced fluorescence intensity and the blue shift of fluorescence through theoretical calculations.Calculations indicate that the introduction of carboxyl groups increases the symmetry matching of the electronic composition of HOMO and LUMO of nano-CNO,which may increase the probability of electronic transitions,thereby causing fluorescence enhancement.Moreover,the experimental and calculation results show that the introduction of carboxyl groups increases the nano-CNO band gap,which may cause fluorescence blue shift;in addition,the reduction of nano-CNO size may also be one of the factors of fluorescence blue shift.2.Exploring the application of the prepared nano-CNO powder in the field of fluorescence sensing.The nano-CNO suspension showed good selectivity to I-and Cr(VI),and could control the "ON" and "OFF" of the fluorescence of the nano-CNO suspension mixed with I’ through adjusting pH value.Using the fluorescence titration method and the linear relationship fitted by the Stern-Volmer equation,based the 3αIUPAC standard,the detection limit of nano-CNO for I-and Cr(Ⅵ)are estimated to be 12.4nM and 9.6nM respectively,and the detection limit of Cr(Ⅵ)quenching nano-CNO suspension for ascorbic acid(AA)is 0.23pM.In addition,the quenching mechanism of I-on nano-CNO suspension was analyzed through theoretical calculation.It may be due to the formation of hydrogen bond between I-with heavy atom effect and the carboxyl group in nano-CNO molecule,which destroys the symmetry matching of the electronic composition of HOMO and LUMO of nano-CNO.Combined with the experiment,the quenching mechanism of Cr(Ⅵ)on nano-CNO suspension includes fluorescence internal filtering effect and dynamic quenching caused by collision.