Synthesis Of Nitrogen Doped Carbon Dots And Study On The Detection Performance For Tetracycline And Zinc(Ⅱ)

Carbon quantum dots（CQDs）,as a new zero-dimensional material,it has become a feasible substitute for traditional semiconductor quantum dots and metal quantum dots at present.CQDs not only have excellent optical properties,but also have low toxicity,good water solubility and biocompatibility,which can make up for the shortcomings of traditional semiconductor quantum dots and metal quantum dots to the greatest extent.Based on their excellent photoelectric properties,CQDs have been widely used in different countries and fields,such as:antibacterial materials,pollutant degradation,biological imaging,drug delivery,photocatalysts,etc.It is necessary to realize the emission of CQDs in long wavelength region based on the emission wavelength in blue region and tunability of CQDs.Generally speaking,long wavelength emission depends on sp² domain and surface defect abundance of carbon quantum dots.The sp² domain largely depends on the carbon precursors and the carbonization process,while the formation of surface defects is more inclined to the post-modification or the introduction of heteroatoms.Inspired by this,we have gradually realized the preparation of long wavelength emission fluorescent carbon points through the synergistic effect of sp² domain and heteratom introduction.Specific work is as follows:（1）Nitrogen-doped graphene oxide quantum dots（N-GOQDs）were prepared by using graphene oxide as carbon material precursor and ammonia as nitrogen source.The synthesized N-GOQDs exhibit excitation dependence in the excitation range of380 nm-440 nm,and have the best emission peak at 483 nm under the excitation wavelength of 420 nm.At the same time,the quantum dots showed excellent fluorescence properties,water solubility and high quantum yield（53.53%）.In the process of exploring its practical application,we found that N-GOQDs showed fluorescence enhancement for tetracycline,so N-GOQDs was used to detect tetracycline in alkaline environment.It was proved by experiment N-GOQDs has good selectivity and sensitivity for tetracycline detection,and the detection limit is5.33μM by linear calculation.（2）In the first part,we find that the introduction of sp² and nitrogen heteroatoms have certain effects on the optical properties of carbon quantum dots.Therefore,in the second part of the work,we selected o-phenylenediamine（OPD）as the carbon precursor and ethylenediamine（EDA）as the nitrogen source to obtain green luminous carbon quantum dots（GN-CQDs）.The fluorescence spectrum analysis of GN-CQDs shows that under the optimal excitation wavelength of 400 nm,the best emission intensity is shown at 515 nm.In addition,GN-CQDs exhibits a small excitation dependence in the excitation range of 360 nm-440 nm,the reason is related to the electron transition and probability in GN-CQDs.We used GN-CQDs for selective recognition of Zn²⁺:Zn²⁺causes different degrees of fluorescence quenching phenomenon on GN-CQDs through the change of concentration,and analyzed that the possible reason lies in the effective chelation between Zn²⁺and the heteroatoms on the surface of GN-CQDs.The detection limit of Zn²⁺was 0.44μM by linear calculation.（3）In the second part of work,we found that o-phenylenediamine（OPD）not only has conjugate structure,but also can realize self-doping through amino group.Therefore,in the third part of work,we used the o-phenylenediamine（OPD）in the second part of work as carbon precursor and heteroatomic dopant at the same time.In order to compare the effect of sp² domain on the emission wavelength of carbon quantum dots,we use p-phenyldiformaldehyde instead of ethylenediamine,compounds with larger sp² domain were obtained by Schiff base reaction.Using the final reaction system as a precursor solution,yellow luminescent carbon quantum dots（YN-CQDs）were obtained by one step hydrothermal treatment.YN-CQDs exhibit non-excitation dependence in the excitation range from 420 nm to 510 nm,and at the optimal excitation wavelength of 470 nm,the optimal emission peak is located at 560nm.In addition,we found that YN-CQDs selectively exhibited fluorescence quenching to tetracycline antibiotics under weakly acidic conditions.Therefore,we used YN-CQDs as an independent fluorescence sensing platform for the detection of tetracycline（TC）.The minimum detection limit of TC was calculated by linear relationship as 1.99μM.By analyzing the UV-vis absorption spectra of TC and the fluorescence spectrum of YN-CQDs,it is found that the fluorescence quenching phenomenon may be caused by the effective chelation of TC and YN-CQDs surface heteroatoms.