Electrochemical Sensing And Miniaturized Device Construction For Ultra-trace Heavy Metals Based On MOFs

Heavy metals are non-biodegradable and discharged into the environment through industrial waste,and eventually enriched in the human body along the food chain even at trace/ultra-trace levels,threatening public health.Therefore,the development of sensitive and rapid detection methods of heavy metals to achieve simultaneous detection of multiple heavy metals,or multi-model analysis of one type of heavy metal,as well as point-of-care analysis applicable to different detection scenarios,is essential to related environmental pollution monitoring and prevention of food safety problems.Electrochemical methods,due to the advantages of high sensitivity,fast response,low cost,designable and miniaturized analysis device,and the ability of multi-model analysis in conjunction with other methods for various application scenarios,have attracted wide attention to heavy metal detection.While some of the measures not adopted for electrochemical sensing have limited the application to on-site detection,for example,the detection sensitivity of electrochemical sensor still needs to be improved,the microminiaturization and portable design of analysis device are urgently needed to be studied to better adapt to practical application scenarios,and the reliability of qualitative analysis and detection concentration range is limited.Metal-organic frameworks（MOFs）were assembled by metal ions or metal oxide clusters and organic ligands through coordination bonding with infinite frame structure,adjustable structure,high porosity and rich active sites,which showed good prospects in adsorption,catalysis and electrochemical sensing applications.In this paper,amino functionalized MOFs have been synthesized and modified on electrode to improve the specific response of target analytes,as well as for method development and device construction of ultra-sensitive electrochemical sensors for heavy metal detection.The main research content is as follows:（1）Bismuth MOF based electrochemical sensor for simultaneous detection of Pb and CdTraditional bismuth film electrode has been widely used for heavy metal detection,while oxidation reaction and alloy formation with detected heavy metal ions lead to the inevitable loss of the bismuth film.In this work,an amino-functionalized bismuth metal-organic framework（Bi-BDC-NH₂）composited with nitrogen-doped mesoporous carbon nanosphere（NMCS）was prepared for simultaneous determination of Pb and Cd.Compared with the traditional bismuth film electrode,Bi-BDC-NH₂@NMCS modified electrode possessed porous structure,large specific area and exposed Bi sites,which could co-promote the adsorption of metal ions and electron transfer with NMCS on the electrode surface.The developed method showed superior stripping performance with detection limits of 0.36 ng/m L for Pb and 0.44 ng/m L for Cd,respectively,and a linear range of 1.0 to 1500 ng/m L.It was applied to detect Pb and Cd in liver,fish and shrimp samples with recoveries of 93.6%～99.7%and 94.4%～103.2%,respectively.（2）Construction of miniaturized electrochemical device for Pb and Cd detectionThe miniaturization of the electrochemical device and the integration of the three-electrode system are preliminary to meet the requirements of portable,rapid and low-cost detection.In this work,a carbon paper electrode,with large porosity and specific surface area,has been used as a novel electrode substrate,combined with the strategy of laminated assembly of flat electrodes and microelectrolytic cells,to form a stack-up electrochemical device（SED）.In addition,the amino-functionalized MOF and electro-deposited gold nanoparticles were co-modified on CPE and simultaneously detected Pb and Cd at ultra-trace level.The proposed method could simultaneously determinate Pb and Cd via anodic stripping voltammetry with the detection limit of 0.07 ng/m L and 0.11 ng/m L,and was applied in real food samples（drinking water,juice,tea,grain,fruits,vegetables,liver and aquatic products）with the recovery of 91.2%～103.4%and 93.7%～111.2%,respectively.（3）MOF nanozyme based colorimetric-electrochemical dual-model sensing of CrSingle-model analysis is prone to fluctuations in test data due to external factors.In order to improve the accuracy,reliability and diversity of detection by self-correction,the colorimetric electrochemical dual-model sensing method based on MOF nanozyme was explored.In this work,3D ball-flower shaped bismuth oxide formate（Bi OCOOH）was used as a precursor and template to construct the metal-organic framework nanozyme Bi O-BDC-NH₂,which possesses intrinsic peroxidase-mimic activity to efficiently catalyze TMB in the presence of H₂O₂ for visual colorimetric sensing model（LOD of Cr:0.44 ng/m L）.Based on the catalytic activity of Bi O-BDC-NH₂ nanozyme and the method of constructing an enzyme-based electrochemical sensor,the colorimetric model has been converted into an electrochemical model,so as to realize dual-model Cr detection and further improve the detection limit to ultra-trace level（LOD:9.00 pg/m L）.The catalytic activity and kinetics of Bi O-BDC-NH₂ nanozyme were investigated,and the mechanism of Cr influence on the catalytic system was revealed.Then,method verification was carried out for the developed colorimetric-electrochemical dual-mode sensing method,and application to Cr detection in water samples（drinking water,tap water and lake water）,recoveris of colorimetric and electrochemical models were 93.8%～104.1%and 91.5%～105.4%,respectively.