Fluorescence Modulation Of Diamond-Type Carbon Quantum Dots For Selective Detection Of Heavy Metal Ions

In this paper,the carbon dots were prepared mainly by a solvothermal method using different solvents as well as readily available citric acid,thiourea and glucose as raw materials.These raw materials contain different functional groups that can be further introduced into the carbon dots as dopants to achieve fluorescence modulation.Finally,the different carbon dots for selective quenching of metal ions were investigated through a series of tests,which proved that the prepared carbon dots have high quantum yields and can be used for metal ion detection in real environments.Firstly,nitrogen-sulfur co-doped fluorescent carbon dots（N,S-CDs）for Fe³⁺detection were synthesized by a one-step thermal method using citric acid and thiourea as raw materials and ethanol as solvent.The results showed that the N,S-CDs had a high yield of QY53.80%and presented blue fluorescence with an optimal excitation wavelength of 360 nm.Meanwhile,the N,S-CDs showed a high selective burst response to Fe³⁺with a detection limit of 0.2μM at concentrations of 0-100μM,which can be used in real water environments.The quenching effect of the prepared N,S-CDs on Fe³⁺was known to be in accordance with the static burst law by combining UV and IR spectroscopy,which proved that the carbon dots have good advantages in the detection of Fe³⁺.Secondly,fluorescent carbon dots（D-CDs）for Cr⁶⁺detection were synthesized by a one-step thermal method using N,N-dimethylformamide（DMF）as the solvent without changing the carbon and nitrogen-sulfur sources.The addition of DMF solvent can provide amide groups during the reaction,thus increasing the N content in the carbon dots,and the synthesized blue fluorescent carbon dots with QY26.23%exhibited excitation light dependence.The D-CDs showed a highly selective burst response to Cr⁶⁺with a measured detection limit of 0.36μM in the concentration range of 0-60μM,and can also be used as a fluorescent probe for the detection of Cr⁶⁺in real water samples.The UV spectral analysis revealed a new absorption peak at 500 nm,which may be caused by the interaction between Cr⁶⁺and the groups on the surface of D-CDs to form a new complex.Through the infrared spectrogram,it was also confirmed that the addition of Cr⁶⁺complexed with the carbonyl functional group and carboxyl functional group on the surface of D-CDs to form a basal complex,which underwent static quenching,demonstrating the good advantage of D-CDs for Cr⁶⁺detection.Finally,glucose,as a polyhydroxy aldehyde,was chosen as the carbon source instead of the monohydroxy citric acid to synthesize diamond-type carbon quantum dots.and the degree of oxidation was controlled by changing the crystallization temperature.The effect of their structure on the fluorescence properties and the selectivity in the detection of heavy metal ions was studied.The diffraction dots were found to be uniformly distributed and presented on a series of rings by selected area electron diffraction（SAED）.The diffraction rings are consistent with the diffraction of（111）,（220）,（311）,（331）and（400）faces of cubic diamond,where the crystal face spacing obtained in SAED is in good agreement with the fcc crystal lattice constant within the experimental error.This indicates that the synthesized nanoparticles（with a particle size of about 7-8 nm）are not the common cubic diamonds,but have diamond-type carbon quantum dots（n-DQDs）.In addition,the stability tests revealed that they are highly stable in weak acidic systems and have a high selective burst response to Cr⁶⁺at different temperatures.The detection limit of 0.27μM was measured in the concentration range of 0-90μM,demonstrating the excellent optical properties and analytical chemistry of the n-DQDs,which provids more possibilities for their application as multicolor biomarkers in living organisms and in the field of environmental detection.