Preparation Of Graphene Quantum Dots And Its Applications In Detection Of Hg²⁺

Fast development of economy and rapid increase of population have led to severe water pollution, especially for heavy metal pollution of water. Mercury ion (Hg2+) was one of the most representatives in heavy metal pollutants in water. Trace Hg2+ could cause serious toxicological damage, and then produced a huge threat to ecological environment and public health. Therefore, facile monitoring of Hg2+ is very important in the fields of environmental monitoring, food safety and toxicity assessment. In this paper, a facile fluorescence sensor based on graphene quantum dots with excellent luminescence properties was developed for fast, sensitive detection of Hg2+ in water and living cells. The main contents were as follows:Graphene quantum dots (GQDs) were prepared by a hydrothermal method using graphene oxide as original material. The GQDs were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM) and nano-particle size analyzer. The results showed that the GQDs had an average diameter of 5～10 nm and a topographic height of～0.8 nm, suggesting that the GQDs consisted of single graphene layer. The prepared conditions were studied. Under the optimized conditions, the GQDs emitted the strongest blue fluorescence at 425 nm using the excitation light of 320 nm. The GQDs showed strong anti-photobleach ability and stability and thus was ideal optical sensing material.The GQDs were modified by a DNA aptamer which could specifically recognize Hg2+ (5’-(NH2C6)TTCTTTCTTCCCTTGTTTGTT-3’)-Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and Zeta potential analyzer were used to characterize the prepared DNA-GQDs. In the presence of Hg2+, the fluorescence emission of the DNA-GQDs rapidly decreased due to the strong coordination of the thymine bases for Hg2+. The fluorescence emission intensity decreased～90% of the initial intensity of DNA-GQDs when 10 μmol/L Hg2+ was added at pH 7.5 within 1 min. A linear calibration curve between F/F0 of DNA-GQDs and the Hg2+ concentrations from 1 nM to 10 μM was achieved, where F0 and F are the fluorescence emission intensities of DNA-GQDs at 460 nm in the absence and presence of Hg2+. The lowest detection limit (LOD) was 0.25 nmol/L. Further, addition of S2-could restore the fluorescence of the DNA-GQDs due to the strong coordination between S2-and Hg2+. Therefore, the present sensor also could be used for the detection of S2-,with a linear range of 0.1～20 μmol/L.The DNA-GQDs were used for the detection of Hg2+ in Hela cells by laser confocal microscope. The images showed that the fluorescence intensity decreased with the mercury ions concentration. The blue fluorescence of GQDs completely disappeared when Hg2+ concentration reached 20 μmol/L. Biocompatibility was considered for applications in living cells. MTT (3-(4,5 dimethylthiazol)-2,5-diphenyltetrazolium bromide) experiments showed that the DNA-GQDs had no effect on cell viability when the concentration was 50 μg/mL, and the cell vitality still remained 75% when the concentration increased to 250 μg/mL, indicating that the DNA-GQDs had good biocompatibility.This present fluorescence sensor for the detection of Hg2+ in water and living cells will have potential and broad applications in biological imaging and environmental monitoring.