Mixed Oxidant Synthesizing Carbon Quantum Dots And The Determination Of Hg²⁺ As Fluorescent Probe

In recent years, because of their excellent fluorescent properties， metal quantumdots have been widely employed in biology, medicine and other fields. We found thatheavy metal ions in the quantum dots had certain cytotoxicity to cells for further study,which may influence the activity of cells and biological macromolecules. At the sametime quantum dots also had a relative large particle size and light blinkingphenomenon, which made their application restricted for a certain degree, especiallythe application on biology. However, carbon quantum dots could overcome theseshortcomings and deficiencies of the traditional metal quantum dots.Fluorescent carbon quantum dots as another newly developed novel carbonnanomaterials for few years, not only did they have the structural characteristics ofcarbon nanomaterials, but also they had the advantages that traditional quantum dotshad, which also owned more superior characteristics compared with traditionalquantum dots. Carbon dots overcame the disadvantages of organic dyes and hadsimilar fluorescent properties which traditional metal quantum dots possessed, theyalso owned unique advantages, such as: low molecular weight and small particle size,high fluorescent stability, no fluorescent light flash, wide and continuous excitationspectrum, tunable emission wavelength, good biocompatibility and low toxicity.Overcoming some shortcomings of the traditional quantum dots and easily realizingsurface functionalization，fluorescent carbon dots were considered to be the most idealsubstitute for quantum dots researching in biological labeling and fluorescentimaging.At present, the preparation and application of carbon dots was still in the initialstage. The fluorescent quantum yields of the prepared carbon dots have not reachedthe level of traditional metal quantum dots. Therefore, it was particularly important toresearch the new method to synthesize the high fluorescent quantum yields carbon dots and their new application. We did research mainly in the preparation andapplication of the fluorescent carbon dots these two aspects in this article. On the onehand, we explored a new method to prepare excellent fluorescent carbon dots withmixed oxidant and modified the surface of synthesized carbon dots. This method waseconomical and practical. On the other hand, the surface carboxylated fluorescentcarbon dots was applied to the detection of mercury ion as a fluorescent probe. Thefluorescent quenching mechanisms of carbon dots with metal ions were investigated.The main research work was as follows:1. Based on the commercial activated carbon as carbon source, we havesynthesized the water soluble fluorescent carbon dots applying nitric acid andhydrogen peroxide mixed oxidant for the first time. Carbon dots possessed a strongerand more stable fluorescence after their surface passivated by ethylenediamine, whichcould be preserved for a long time. Under the optimum conditions, the particle size ofprepared carbon dots was about5nm. They were easily soluble in water but insolublein organic reagent because their surface would contain abundant hydrophilic groups,such as hydroxyl, carbonyl, carboxyl groups and amino on the modified surface. Inthe study of effects of fluorescent intensity of carbon dots by some outside factors, wefound that fluorescent intensity of bare carbon dots was maximum when pH was at6（pH at7for passivated carbon dots）. A number of common metal ions did notinfluence the fluorescent intensity of carbon dots whether they passivated nor notexcept for Cu²⁺and Hg²⁺.2. In this study a new method to measure the Hg²⁺with carbon dots asfluorescent probes were established.400ng/mL for bare carbon dots （300ng/mL forthe passivated carbon dots） reserved solution were mixed with a certain amount ofHg²⁺working solution in phosphate buffer solution （PBS） at pH6（pH6.5for thepassivated carbon dots） at room temperature for25minutes （20minutes for thepassivated carbon dots）. Under the conditions of the excitation wavelength at365nmand fluorescent emission wavelength at374nm, when the proper concentration ofHg²⁺was measured beween0.5ng/mL and16ng/mL, the linear regression equationwas I0/I=0.180c+1.039. The linear correlation coefficient R2was0.999（R=0.9995）with the detection limit of0.2ng/mL and the relative standard deviation of1.6%forRSD. The recovery of measuring tap water was95-105%by the new method. Thecontent of Hg²⁺in lake were0.694ng/mL and0.712ng/mL respectively with the useof atomic fluorescent spectrometry and fluorescent probe. The relative standarddeviation of two methods was1.5%and1.9%respectively. There was no significant differences between these two methods, thus the new method could be employed inthe determination of the actual samples.3. The fluorescent quenching mechanisms of carbon dots with Hg²⁺wereinvestigated. The results suggested that: Bare carbon dots’ quenching mechanismswere more complex, so we hypothesized that the outcome may be affected by severalquenching forms. Meanwhile, the fluorescent quenching mechanisms of passivatedcarbon dots were relatively simple, belonging to static quenching in line withStern-Volmer equation, with the quenching constants of Hg²⁺and passivated carbondots Ksv=1.8x1011ng/mL.