Preparation And Analytical Applications Of The Electrochemical Sensor Based On Graphene Quantum Dots

Carbon nanomaterials have attracted much attention as a new type of material in modern society.Graphene quantum dots are zero dimension carbon nanomaterials.Graphene quantum dots have many excellent properties,such as large specific surface area,unique electro-optical properties,low toxicity,excellent biocompatibility and low cost.Graphene quantum dots have important potential applications in the fields of biology,medicine,materials,novel semiconductor devices and so on.Graphene quantum dots can be used as excellent materials in electrochemical sensors.In the past decades,antibiotics have been widely used in agriculture and aquaculture and the abuse of antibiotics has become more and more serious in those areas.Some of the residues of animal-derived antibiotics may be poisonous to the living creatures or may lead to allergic hypersensitivity reactions in human consumption process.Levofloxacin?LV?is diffusely used as the third generation of fluoroquinolone because of its broad spectrum antimicrobial activity.As the active l-isomerisolated from the racemic ofloxacin,LV is applied in biological medicine for the therapy of the infections of the respiration system,skin and soft tissues.Because LV is active against infections caused by gram-negative,gram-positive,intracellular as well as anaerobic bacteria,it has been widely used in human and veterinary medicines.Therefore,the detection of levofloxacin residue is a very great significance.Protein is an important macromolecule material that constitutes the cells and tissues of humans.It is the material basis that plays important roles such as transport,catalysis,immunization and adjustment function in the various life activities.Carcinoembryonic antigen?CEA?is a kind of glycoprotein formed by the colorectal cancer tissue,as antigen can cause the patient's immune response.CEA can reflect the existence of a wide variety of tumors.It can judge the curative effect of colorectal cancer,breast cancer and lung cancer,and is one of the better tumor markers for illness development,monitoring and prognosis estimation.Therefore,developing a convenient,rapid and sensitive analytical method for CEA in serum is of great importance in early diagnosis and medical treatment of cancer.In this paper,an electrochemical sensor for the detection of levofloxacin and carcinoembryonic antigen was constructed by using the graphene quantum dots composite film to amplify the electrochemical signal.The main contents of the paper include the following aspects:?1?A strategy was developed for the voltammetric determination of the antibiotic drug levofloxacin?LV?based on a glassy carbon electrtode modified with a composite consisting of poly?o-aminophenol?and graphene quantum dots?PoAP/GQD?that was fabricated by electropolymerization.The PoAP/GQD composite provides a large surface area and sensing interface and simzultaneously strongly promotes the oxidation current of LV.Under optimal conditions,the modified GCE displays an oxidation peak current best measured at a working voltage of 1.05 V?vs.SCE?that is linearly related to the levofloxacin concentration in the range from 0.05 to 100?M,and the detection limit is 10 nM?at an S/N of 3?.The method was applied to the determination of levofloxacin in spiked milk samples where is gave recoveries between 96.0 and 101.0%.?2?In this work,we have developed an electrochemical aptasensor for high-sensitivity determination of carcinoembryonic antigen?CEA?based on lead ion(Pb²⁺)-dependent DNAzyme-assisted signal amplification and graphene quantum dots-ionic liquid-nafion?GQD-IL-NF?composite film.We designed hairpin DNA containing CEA-specific aptamers and DNAzyme chains.In the presence of CEA,hairpin DNA recognizes the target and performs a DNAzyme-assisted signal amplification reaction to yield a large number of single-stranded DNA.The GQD-IL-NF composite film was immobilized on the glassy carbon electrode for the sake of interacting with single-stranded DNA through noncovalent?-?stacking interaction.Therefore,the methylene blue-labeled substrate DNA?MB-substrate?was fixed on the electrode and exhibited an initial electrochemical signal.Under optimal conditions,the response current change is proportional to the concentration of CEA,demonstrating a wide linear range from 0.5 fg mL^-1 to 0.5 ng mL^-1,with a low detection limit of 0.34 fg mL^-1.Furthermore,the proposed aptasensor was successfully applied in determining CEA in serum samples,showing its superior prospects in clinical diagnosis.