The Application Of Fluorescence Biosensor Based On Graphene Quantum Dots

In the last few years,graphene quantum dots which combined the attractive performance of graphene and quantum dots emerged and drew our attention.The obvious quantum confinement and edge effects of such zero-dimensional nanomaterials allowed them to possess high photoluminescence.Studies also found that graphene quantum dots exhibited low toxicity,robust chemical inertness and good performance in photoluminescence,water solubility and biocompatibility.For those reasons,graphene quantum dots has been widely used for cellular imaging,energy storage,disease diagnosis,drug delivery,electrochemical analysis and fluorescence biosensing since its first synthesis.Compared with conventional organic fluorescent dyes,graphene quantum dots has many distinctive optical properties as fluorescence probe,such as strong signal intensity,high quantum yield,narrow emission peaks and tunable size-dependent photoluminescence.Based on the fluorescence signal of graphene quantum dots,we designed and proposed fluorescent detecting or biosensing methods,which built up certain corresponding relation between the fluorescence signal and the concentration of aimed compound.In this way,the concentration of aimed compound was obtained by detecting the fluorescence intensity.In the chapter 1,we firstly described the basic properties and main prepared methods of graphene quantum dots,and its application in biomedical analysis.Secondly,we introduced the G-quadruplex/hemin DNAzyme with peroxidase-mimicking activities and its substitute for hypothalamic regulatory peptides(HRP).Finally,we made a detailed explanation of the main conception and research meaning of our article.In the chapter 2,a novel and label-free fluorometric assay platform of caffeic acid was constructed.The assay platform was designed based on the peroxidase-mimicking activities of G-quadruplex/hemin DNAzyme.The G-quadruplex/hemin DNAzyme was first synthesized by certain DNA sequence,hemin and potassium chloride.Under the catalysis of the formed DNAzyme,hydrogen peroxide could be decomposed into hydroxyl radicals with strong oxidation properties.Then the caffeic acid would be oxidized by the released hydroxyl radicals,resulting in the corresponding quinones product,caffeic acid-quinone.Normally,caffeic acid has no influence on the fluorescence signal of graphene quantum dots.But when the caffeic acid was oxidized by G-quadruplex/hemin DNAzyme and hydrogen peroxide and produced corresponding quinones,the product caffeic acid-quinone could efficiently quench the fluorescence signal of graphene quantum dots.Under the optimized experimental conditions,the quenched fluorescence intensity was linearly correlated with the concentration of caffeic acid,ranging from 2 ?M to 350 ?M with a detection limit of 200 n M.The proposed method was applied to caffeic acid determination in human serum samples with satisfactory results.In the chapter 3,a simple,efficient and low toxicity biosensing platform of uric acid was constructed.The biosensing platform was designed on the peroxidase-mimicking activities of G-quadruplex/hemin DNAzyme and the introduction of caffeic acid.Uric acid in the human body could be decomposed by uricase and then produced hydrogen peroxide and allantoin.We thus successfully achieved the indirect detection of uric acid by monitoring the concentration of hydrogen peroxide.The G-quadruplex/hemin DNAzyme could act as peroxidase and decompose the hydrogen peroxide into hydroxyl radicals at room temperature.Due to the strong oxidizing of hydroxyl radicals,caffeic acid was converted to corresponding quinone,thus leading to fluorescence quenching of graphene quantum dots.Under the optimized experimental conditions,there is a good linear relationship between the quenched fluorescence intensity and the concentration of uric acid,ranging from 2 ?M to 300 ?M with a detection limit of 500 n M.The applicability of proposed method was further proved with satisfactory results in human serum and urine samples.