Preparation Of Carboxyporphyrin MOFs Membrane Electrode And Its Performance In Nitrite Detection

Nitrite is a chemical compound that can be found in food,drinking water and environment.It can cause serious health issues such as reducing the supply of blood oxygen producing carcinogens,leading to spontaneous abortion and birth defects if did not used in controlled quantity.Electrochemical detection is used to detect nitrite（NO₂^-）due to its advantages of fast response,easy operation,high sensitivity,low cost and easy miniaturization.The key to achieving excellent detection performance is done through modifying the electrode through a material with high electrical conductivity and stability.Metal-organic frameworks（MOFs）materials have many advantages such as high porosity and high specific surface area which are effective to enhance the electrochemical response through the efficient mass transfer of analyte.These advantages not only provide excellent electrocatalytic capabilities for MOFs,but also promote the development of MOFs materials for electrochemical sensing.Porphyrin has a large ring conjugated structure formed by 18πelectrons,which is conducive to its electron transmission.The central metal can be used as the active site of nitrite.At the same time,the central metal and node coordination metal can form a bimetallic catalytic synergistic effect.Therefore,using porphyrin as an organic ligand to construct MOFs films is expected to achieve highly sensitive detection of nitrite.The metal as the node is closely related to the detection sensitivity and the stability of the film.According to the theory of Hard-Soft-Acid-Base,carboxylic acids are categorized into hard acids due to its quick reaction with hard base and create a strong chemical bond.Therefore,metals（hard bases）with small volume and high positive charge can be selected to coordinate with them to synthesize carboxyporphyrin MOFs with high stability.This article has been carried out the following two aspects of work:1.It focuses on in situ of carboxyporphyrin MOFs membrane through electrochemical method and determination of nitrite.Based on the theory of Hard-Soft-Acid-Base,the carboxyporphyrin MOFs（Gd-NiTCPPMOFs）materials were prepared in situ by electrochemical cathodic reduction by selecting high positive charge metal gadolinium(Gd³⁺)as its node for coordination with carboxyporphyrin.The controlled growth of carboxyporphyrin MOFs on the surface of glass carbon electrode was achieved by adjusting the supporting electrolyte concentration,the proportion of polar solvent and the reduction voltage.Gd-NiTCPPMOFs films with regular morphology and stable existence in acidic and weakly alkaline environments were obtained as a final product.Electrochemical in-situ growth method improves the binding ability between materials and electrodes.Meanwhile,the synergistic promotion between metal nodes and carboxylporphyrin center metal overcomes the limitation of few catalytic active sites of mono-metallic porphyrin MOFs,which indicates the good catalytic activity of carboxylporphyrin MOFs.At the same time,due to the strong chemical bond formed by Gd³⁺and-COOH,the material has a high stability,realizing the highly sensitive detection of nitrite at pH=3～8.In the current study,various techniques i.e Cyclic voltammetry,chronocurrent method,AC impedance and other electrochemical method were used for the construction of carboxyporphyrin MOFs sensor to test its performance.Under the detection conditions of pH=3 and pH=8,the sensitivity of the electrochemical sensor to NO₂^-is 375.0μA·m M^-1·cm^-2,326.57μA·m M^-1·cm^-2,and the detection limits are0.17μM and 0.20μM,respectively.The material has the ability to detect nitrite in acidic and weakly alkaline environment with a good selectivity and stability.The electrocatalytic mechanism of Gd-NiTCPPMOFs detection was preliminarily explored.2.Preparation of ErGO/Gd-NiTCPPMOFs composite membrane electrode and study on nitrite detection performance.The disadvantage of carboxyl porphyrin MOFs is its poor conductivity,which limited their sensitivity to nitrite detection.The material conductivity and sensitivity can be improved by preparation carboxyporphyrin MOFs and ErGO composite materials though in-situ growth of Gd-NiTCPPMOFs thin films on electrochemically reduced graphene oxide（ErGO）conductive layer.As compared to carboxyporphyrin MOFs,the ErGO/Gd-NiTCPPMOFs composite has a double sensitivity of nitrite detection.The sensitivity of the sensor is 1158.0μA·m M^-1·cm^-2,1104.29μA·m M^-1·cm^-2,and the detection limits are 0.05μM and 0.05μM,respectively,at pH values of 3 and 8.The introduction of electrochemical reduction graphene oxide enhanced the electron transport capacity and improved the nitrite detection performance.