Application Of Metal-organic Frameworks In Electrochemistry: High Sensitivity Electrochemical Sensor

Hydroquinone（HQ）and catechol（CT）are two isomers of dihydroxybenzene,widely used in cosmetics,dyes,rubber,pharmaceuticals,and other industrial applications.They are highly toxic and resistant to degradation,seriously endangering the ecological environment and human health.They can invade cells through the human skin,respiratory tract,or digestive tract,and many national environmental protection organizations have listed them as environmental pollutants.Therefore,it is urgent to develop a simple,rapid,sensitive,accurate,and low-cost analytical method to determine the isomers of dihydroxybenzene.Sodium nitrite（NaNO₂）is a common food additive,but its high content(>250 mg^-1)will be harmful to human health.For example,sodium nitrite can promote methemoglobin formation to prevent oxygen delivery in the body and lead to hypoxia.In the gastrointestinal tract,nitrite is easily absorbed,forming highly carcinogenic nitrosamines under acid conditions.Therefore,a highly sensitive,simple,and rapid method for detecting NaNO₂is of great importance to us.Among many analytical methods,researchers widely prefer electrochemical analysis methods with the advantages of simplicity,fast,high sensitivity,and low cost.However,HQ and CT are similar in structure and properties,usually coexist,and show overlapping voltammetric peaks on traditional electrodes,which makes it difficult to separate and measure analytes at the same time.And sodium nitrite shows high oxidation potential or slow oxidation kinetics on the bare electrode,which hinders its accurate and rapid determination.This prompted us to explore an optimal platform to improve the sensitivity and selectivity of the electrode,while conveniently monitoring the isomer of dihydroxybenzene or nitrite.Metal-organic frameworks（MOFs）are a new type of porous material with an ordered structure composed of metal nodes and polydentate organic linkers in three-dimensional space.Because of its high specific surface area,adjustable pore size,customizable function,high thermal stability,and other characteristics,as well as the usual cheap raw materials,it has been widely used in various fields.However,the poor electronic conductivity has become the kinetic bottleneck hindering the electrocatalytic performance of most MOFs.Researchers have proposed many strategies to improve the conductivity of MOFs and maximize their unique pore structure.In this paper,the post-synthetic modification（PSM）strategy and/or composite with carbon materials strategy are used to improve the conductivity of MOFs and apply them in the field of electrochemical sensing.These main research contents are as follows:1.UiO-bpydc-Cu/MWCNTs/GCE simultaneously detecting hydroquinone and catecholThrough the solvothermal method,we effectively coordinated the organic ligand 2,2’-bipyridyl-5,5’-dicarboxylic acid（H₂bpydc）with zirconium chloride（Zr Cl₄）to form a Zr-based metal-organic framework material UiO-bpydc.Post-synthetic copper chelating of bpy sites in the MOFs was performed to achieve Cu-functionalized UiO-type frameworks（UiO-bpydc-Cu）.The UiO-bpydc-Cu was uniformly dispersed in the suspension of multi-walled carbon nanotubes（MWCNTs）by ultrasonic dispersion method,and finally obtained UiO-bpydc-Cu/MWCNTs composite material.The composite electrode UiO-bpydc-Cu/MWCNTs/GCE was prepared by coating the mixed suspension droplets on the glassy carbon electrode（GCE）.Due to the synergistic effect,the composite electrode showed a significant voltammetric response to the electro-oxidation and reduction of HQ and CT.We established a fast and sensitive method for the simultaneous detection of HQ and CT by differential pulse voltammetry（DPV）.Experiments show that the linear response range of HQ and CT is 0.5–565μM and 1–1350μM,respectively.The detection limit is low,0.361μM and 0.245μM.The proposed electrochemical sensor has high sensitivity,good anti-interference,outstanding reproducibility,and excellent stability.What’s more,it can also be used to detect HQ and CT in actual samples.2.Ni-HHTP@MWCNTs/GCE highly sensitive detecting sodium nitriteConductive linking ligands 2,3,6,7,10,11-hexahydroxytriphenylene（HHTP）,nickel acetate tetrahydrate Ni（OAc）₂·4H₂O and multi-walled carbon nanotubes（MWCNTs）synthesize Ni-HHTP@MWCNTs composites in situ by hydrothermal method.The composite electrode was prepared by the coating method.The redox characteristics of the electrode were characterized by electrochemical impedance spectroscopy（EIS）.The synergistic combination of Ni-HHTP and MWCNTs enables the electrode to possess fast electronic conductivity and generate sensitive electrochemical signals for sodium nitrite（NaNO₂）.By means of a controlled variable approach,the optimal test conditions for a highly sensitive response to the analyte are determined.DPV studies show that the composite electrode has a wide linear response range of 1-10000μM to NaNO₂,the detection limit is as low as 0.95μM,and the sensitivity is increased to 0.96 m A·m M^-1·cm^-2,and it shows good reproducibility and stability performance.In addition,to develop the practical application potential of this composite material in electrochemical sensing,we detected NaNO₂ in actual water samples with a recovery rate of 97.2%-103.7%,which shows an important value in electrochemical detection.We also further explored the mechanism of the specific response process of the modified electrode to the analyte under optimal test conditions,preliminary exploration was conducted on new electrochemical sensors for specific sensing and high-precision detection.