Preparation Of Full-color-emissive Carbon Quantum Dots And Studies On The Tunability Of Fluorescence Colors

In recent years,carbon quantum dots(CQDs),as a new type of carbon nano-luminescent materials,have been widely used as one of the potential replacement materials for fluorescent dyes due to their unique optical effects,excellent surface modification,and biocompatibility.Although the blue and green CQDs currently prepared have the advantages of high quantum yield,high stability and good fluorescence response.There are still complex regulatory processes and mechanisms for full-color-emissive CQDs.This paper mainly established a simple process route for the preparation of full-color-emissive CQDs,and further improved the optical performance and optical stability of the very short wavelength CQDs(purple CQDs),and explored the application of purple CQDs in p H sensors.The specific research contents and conclusions are as follows:(1)Preparation of full-color-emissive CQDs:Using CA and OPD as raw materials,a simple one-step hydrothermal method was used to obtain the full-color-emissive CQDs(from orange fluorescence to purple fluorescence)by temperature control.The orange CQDs prepared at80°C and the yellow CQDs prepared at 120°C can still maintain more than 90%fluorescence intensity and has excellent photobleaching resistance after 6 h UV irradiation.At a high ion concentration of 3 M,the fluorescence intensity of cyan CQDs prepared at 200°C is still above95%,while the fluorescence intensity of purple CQDs prepared at 240°C is also above 75%.And their ionic stability is obviously higher than other CQDs.The blue CQDs prepared by 160°C has both ionic stability and photobleaching resistance,and the fluorescence intensity remains above 90%under long-term UV irradiation and high ion concentration of 3 M Na Cl,showing excellent optical stability.(2)The regulation of fluorescence colors and their mechanism:Through analysis,it was found that the C=O stretching vibration of full-color-emissive CQDs gradually strengthened,indicating that the oxidation degree of its surface increased.By the fitting and calculation of the high-resolution XPS,it was found that the sp~2/sp~3 hybridization factor decreased from 1.627 to 0.263with the increase of temperature.It was proposed that the increase of surface oxidation degree and the decrease of sp~2/sp~3 hybridization factor both led to the increase of band gap,resulting in the blue shift of CQDs fluorescence emission peak and color change.(3)The preparation and properties study of very short wavelength purple CQDs:Using CA as carbon source,purple CQDs with high fluorescence intensity and stability were prepared by one-step hydrothermal method at 120^°C by mixing N doping with OPD and MPD.The quantum yield of the purple CQDs was as high as 5.3%,and its fluorescence intensity can still remain above 80%under high ion concentration and longtime UV irradiation,showing strong fluorescence stability.The fluorescence spectrograms confirmed that the fluorescence intensity of purple CQDs can be changed by regulating the addition amount of MPD.The functional groups and surface elements of the purple CQDs were analyzed by XPS and FTIR.It was proved that the purple CQDs has-COOH,-NH₂?-OH and other functional groups.The contents of sp~2C and sp~3C were calculated and found to be related to the ratio of raw materials.Thus,the possible mechanism of the hydrothermal reaction was proposed.It is believed that the addition of MPD may change the proportion of zigzag and armchair structure in the surface edge of CQDs,which might be the main reason for the change of fluorescence intensity.(4)Studies on the acid-base chromic property of purple CQDs:Under the conditions of p H=1?3 and p H=10?13,the fluorescence color of purple CQDs will change from purple to green,showing excellent acid-base chromic properties.After five acid-base cycles,purple CQDs still had the property of color mutation,and there was no obvious disturbance of fluorescence intensity,which proved that the CQDs had good acid-base chromic reversibility and repeatability.This work successfully prepared full-color-emissive CQDs by using a simple hydrothermal method,and obtained purple CQDs with high fluorescence intensity and high stability by the method of mixed N doping.These preparation methods provide reference for color regulation of CQDs.Their excellent optical properties and special acid-base chromic properties will also expand the application of CQDs in biological p H sensors and other fields.