Electrochemical Biosensors Based On G-C₃N₄ And Its Nanocomposites

The sensitive detection of small biological molecules closely related to life activities is of great significance in the early diagnosis and treatment of diseases.Electrochemical sensors have been widely used in biomolecular detection because of their high sensitivity and fast analysis speed.The nano-sized graphite phase carbon nitride（g-C₃N₄）possesses a unique band structure,excellent stability and excellent biocompatibility,and can provide a sensitive sensing platform for electrochemical detection of small biological molecules.Those g-C₃N₄ prepared by traditional thermal polymerization methods show smaller surface area and poor dispersion.To improve the electrochemical activity of g-C₃N₄,in this dissertation,it is proposed three different,simple synthesis methods to prepare g-C₃N₄ and its nanocomposites.Moreover,they are applied to the electrochemical sensors for ascorbic acid,dopamine and tyrosine,respectively.The main content of this dissertation is as following:（1）A green gas phase exfoliate method was used to exfoliate the bulk g-C₃N₄ to prepare an ultrathin（UT）g-C₃N₄ with an average thickness of 2.8 nm,a specific surface area of 136.9 cm².g^-1 and high electrocatalytic activity.On the basis of the UT-g-C₃N₄modifiedglassycarbonelectrode（GCE）,themodifiedelectrode poly-L-Cys/UT-g-C₃N₄/GCE was synthesized by polymerizing L-cysteine.Cyclic voltammetry was used to investigate the electrochemical behavior of ascorbic acid on poly-L-Cys/UT-g-C₃N₄/GCE,and the number of polymerization cycles,buffer solution pH and sweep rate were optimized.Under the optimal conditions,the chronoamperometric method（i-t）was used to study the relationship between peak current and concentration of ascorbic acid oxidation.The amperometric response current of ascorbic acid and its concentration shows a good linear relationship ranged from 0.25μmol L^-1 to 150μmol L^-1with a minimum detection limit of 0.06μmol·L^-1（S/N=3）.And this electrochemical sensor was applied to the analysis of vitamin C in tablets,and the recovery is about 93.5%to 97.2%.（2）Sheet/hollow rod shaped g-C₃N₄/ZnO nanocomposites were successfully prepared by microwave method using zinc acetate,hexamethylenetetramine and g-C₃N₄ as precursors.The crystal structure,morphology and electronic structure of the nanocomposite were characterized by X-ray diffraction,scanning electron microscopy and UV-vis diffuse-reflectance spectroscopy,and other modern testing techniques.A g-C₃N₄/ZnO/GCE electrochemical sensor with high electrocatalytic activity for dopamine was constructed.The electrochemical behaviors of dopamine on g-C₃N₄/ZnO/GCE were investigated by i-t curve method in 0.1 mol·L^-1 Tris-HCl（pH=7.3）buffer solution at a working potential of 0.3 V.The results showed that when the concentrations of dopamine were in the range of 0.01μmol·L^-1 to 150μmol·L^-1,the amperometric response current was in good linear relationship with the concentration,with a detection limit of 5 nmol·L^-1（S/N=3）.The fabricated sensor was applied to the analysis of dopamine hydrochloride injection samples.It was found that the recoveries were 90.1%⁹2.6%,indicating that the established g-C₃N₄/ZnO/GCE electrochemical sensing could be applied to the analysis of actual samples.（3）Two-dimensional ultrathin UT-g-C₃N₄ with average thickness of 2.3 nm and specific surface area of 192.9 m²·g^-1 was prepared by thermal polymerization of melamine.Using AgNO₃ as precursor,UT-g-C₃N₄/Ag nanocomposites with 7 wt%weight percentage of Ag were prepared by photo-assisted reduction method.Electrochemical impedance spectroscopy results show that the prepared UT-g-C₃N₄/Ag has a faster electron transfer rate than the bulk g-C₃N₄ and UT-g-C₃N₄.The optimal conditions for the electrocatalytic oxidation of L-tyrosine on UT-g-C₃N₄/Ag/GCE were systematically studied.In phosphate buffer solution（pH=6.0）,the modified electrode showed excellent analytical performance for the electrochemical determination of L-tyrosine.The amperometric response currents possess a linear relationship with the concentration of L-tyrosine in the range of 1μmol·L^-1to 150μmol·L^-1 and the detection limit is 0.14μmol·L^-1（S/N=3）.In addition,the modified electrode was used to determine L-tyrosine tablets,the recovery rate was 97.2%to 99.3%.Based on the UT-g-C₃N₄/Ag nanocomposite,a rapid and sensitive detection method of L-tyrosine was established.