Carbon Dots-based Fluorescent Probes For Trace Hg²⁺ Detection In Aqueous Solutions

Carbon quantum dots?CDs?represent an emerging class of fluorescent carbon nanomaterials in recent years.Compared to traditional semiconductor quantum dots and organic dyes,CDs are superior in terms of high resistance to photobleaching,good biocompatibility,easy functionalization and so on.As a result,much attention has also been paid on their potential applications in bioimaging,biological labeling and ions sensing.In this paper,we mainly described the preparation and application of CDs,two kinds of fluorescent carbon quantum dots were synthesized.The morphology structure and optical properties of CDs were characterized.Based on the mechanism of fluorescence resonance energy transfer?FRET?,the functionalized CDs were applied to the detection of trace Hg²⁺in water samples.The main contents of the dissertation consist of the following four parts:1.The first section of this article provided a brief introduction to the CDs,including the synthetic methods and photoluminescence properties.Then the functionlization methods and application of CDs were reviewed.2.Section two,the amino-functionalized carbon quantum dots?N-CDs?with good water-solubility was prepared by a facile and green hydrothermal method.Anhydrous citric acid,NaBH₄ and ammonia were used as carbon source,reducing agent and passivating agent,respectively.The synthesis procedure of N-CDs can be divided into three steps:carbonization,reduction and passivation.The following experimental conditions in the last two steps were found to give the best results:?a?0.25 g of NaBH₄ and 2 h of reduction time,?b?passivation at180^oC for 5 h,and 1:7 of the molar ratio between citric acid and ammonia.TEM image shows that the N-CDs are spherical with an average diameter about 3-5 nm.The fluorescent quantum yield is about 36%using quinine suifate as the reference.As the fluorescence intensity can be quenched by Hg²⁺selectively,the N-CDs can be used as an effective fluorescent sensing probe for the quantitative detection of trace Hg²⁺in aqueous solutions with a detection limit of 20 nmol L^-1.A good linear correlation can be obtained with the concentrations of Hg²⁺ranging from 0.1?mol L^-1 to 1.2?mol L^-1.The recoveries of spiked Hg²⁺in tap water sample ranged from 83.1 to98.0%.We also use HG-AFS to analyze the same sample for comparison.Thus,the prepared N-CDs could be successfully uesd for the daily monitoring of Hg²⁺in water environment.3.Section three,to further improve the detection sensitivity,the N-droped carbon dots?NCDs?was prepared by a facile one-step hydrothermal method.Citric acid and urea were used as the carbon source and nitrogen source,respectively.The optimized synthetic conditions were described as follows:?a?a mass ratio of 1:1 between citric acid and urea,?b?a reaction temperature of 160^oC,?c?a reaction time of 4 hours.The TEM images illustrate that the NCDs with an average diameter about 2.4 nm.The fluorescence quantum yield is 42.5%using quinine sulfate as the reference.Similarly,the fluorescence intensity of NCDs can also be quenched by Hg²⁺selectively.A good linear correlation can be obtained with the concentrations of Hg²⁺ranging from 0.05?mol L^-1 to 5?mol L^-1.The recoveries of spiked Hg²⁺in tap water and lake water samples ranged from 81.7 to 98.2%with a detection limit of 7.3 nmol L^-1.All the results showed the developed NCDs-based sensor presents a promising sensing platform for the detection of Hg²⁺in freshwater including drinking water.4.Section four,based on the ability of oligodeoxyribonucleotide?ssDNA?rich in thymine?T?for recognizing Hg²⁺via the specific formation of a Hg²⁺–mediated base pair(T–Hg²⁺–T),we designed a simple,highly sensitive fluorometric Hg²⁺sensor based on NCDs-labeled ssDNA.The NCDs-ssDNA were prepared by using EDC/NHS for coupling ssDNA to NCDs.The fluorescence signal of NCDs can be quenched by the FRET between NCDs and GO.In the present of Hg²⁺,the thymine in ssDNA can combined with Hg²⁺to form a stable complex resulted an increasement in fluorescence intensity.The experimental results showed a good linear correlation with the concentration of Hg²⁺ranging from 0–9 nmol L^-1 with a detection limit of 0.24 nmol L^-1.The recoveries of spiked Hg²⁺in tap water ranged from 80.3 to 91.8%.