Properties Of Carbon Dots Prepared By Dihydroxynaphthalene And Nitrilotriacetic Acid

Carbon dots（CDs）are zero-dimensional carbon nanoparticles with properties such as simple synthesis,reasonable price,tiny structure,adjustable fluorescence brightness,excellent solubility in various solvents,and excellent photostability.It has been used in various fields such as electrochemistry,photovoltaics,catalysis,fluorescent ink,analytical detection of various molecules,drug delivery and photomedicine.In this experiment,two precursors,nitrilotriacetic acid and 2,7-dihydroxynaphthalene,were selected and synthesized by the solvothermal method using ethanol as the solvent.And they were nitrogen-doped（N-CDs）and nitrogen-sulfur co-doped（N/S-CDs）by urea and thiourea,respectively.The experiments were first conducted to investigate the effect of reaction conditions on the preparation of CDs by varying the reaction time,temperature and mass ratio of reactants with nitrilotriacetic acid as the precursor,and to select the optimal reaction conditions for the investigation of optical properties.Finally,N-CDs with the optimum excitation of 330 nm and emission peak at about 400 nm were successfully synthesized at a mass ratio of 1:2,reaction temperature and reaction time of 180℃and 12 h.N/S-CDs with an optimum excitation of 370 nm and an emission peak at about 430 nm were obtained at a mass ratio of 1:5 and a reaction temperature and time of 180℃ and 12 h.Both showed blue luminescence,and the fluorescence intensity was about five times higher after heteroatom doping compared with CDs synthesized from pure ammonia triacetate.Among them,N/S-CDs are highly selective for Ag⁺and Cu²⁺,and there is a linear relationship between the fluorescence intensity of N/S-CDs and the concentrations of Ag⁺and Cu²⁺in the range of2-20μM.The limits of detection were calculated to be 0.68μM and 1.28μM and the response times were all within 30 seconds,which can be used for the detection of these two metal ions.The mechanism may be that Ag⁺and Cu²⁺combined with-N=C=S on the surface of N/S-CDs,leading to the fluorescence burst of N/S-CDs.Then,using 2,7-dihydroxynaphthalene as the precursor,it was found that the addition of H₂O₂,reaction time and urea content at the time of nitrogen doping had some effects on the optical properties of the products.The results showed that the addition of H₂O₂and the increase of urea content could effectively increase the fluorescence intensity of N-CDs,reaction time has little effect on fluorescence intensity but can lead to emission redshift.When the reaction time was increased from 5 h to 12 h at a raw material quality ratio of 4:1,the N-CDs changed from blue to red/orange luminescence,attributed to a greater degree of sp²conjugation due to the increase in reaction time.The N/S-CDs were found to be able to cause emission red-shift and fluorescence burst with increasing concentration when co-doped with nitrogen and sulfur.It is due to the extremely short distance between individual N/S-CDs and high surface energy that the unstable homogeneous dispersion of N/S-CDs at high concentrations leads to aggregation.Based on the good dispersion of N/S-CDs in water and the high absolute fluorescence quantum yield（QY）value（18%）,an aqueous solution of N/S-CDs that can be used directly for fluorescent inks without other treatments was developed.In order to investigate the potential of CDs in Light-emitting diodes（LED）devices,N/S-CDs and N-CDs synthesized under the condition of 5 h reaction time and 4:16mass ratio of raw materials were compounded with polyvinyl alcohol（PVA）to obtain blue-violet and yellow-green fluorescent films with UV transmittance of more than 75%.