Preparation And Electrochemical Sensing Application Of Copper-based Metal-Organic Frameworks

The applications of copper-based metal-organic frameworks（Cu-MOFs）in electrochemical sensing have been attracting a lot of research interests.The electrochemical sensing performance of Cu-MOFs is highly dependent on their morphologies and ligands,however,the underlying mechanisms are not clear.Therefore,systematic studies are needed on revealing the mechanisms,and thus to improve the catalytic and sensitizing effects of Cu-MOFs.This thesis is mainly focused on the syntheses and electrochemical sensing applications of Cu-MOFs with different shapes and ligands as well as Cu-MOFs/graphene composites.The effects of shape,ligand,and graphene composite on the electrochemical sensing performance have been explored.Several electrochemical sensing platforms with high sensitivity have been constructed.The main contents are as follows:（1）Cu-BTC frameworks have been synthesized through room temperature standing.In the synthesis,Cu（NO₃）·3H₂O and 1,3,5-benzenetricarboxylic acid（H₃BTC）are used as the precursors,and triethylamine is utilized as the alkali source.With the increasing concentration of triethylamine,the morphology of Cu-BTC frameworks evolves from bulk structure to uniform nanoparticles（average diameter of 30 nm）,and then to agglomerations made of larger nanoparticles.It is found that Cu-BTC frameworks with different shapes show distinct active response areas,electron transfer rates,and accumulation capabilities,and thus different responses to xanthine（XA）and hypoxanthine（HXA）,respectively.By manipulating the nucleation rate,Cu-BTC frameworks with different shapes have been successfully synthesized,which show distinct electrochemical sensing performances.Based on the signal amplification strategy adjusted by triethylamine,a highly sensitive electrochemical method has been developed for the detection of XA and HXA simultaneously.The limits of detection for XA and HXA are 0.89 and 2.1 n M(10^–9mol L^–1)with the linear ranges of 5.0–8000.0 n M and 10.0–10000.0 n M,respectively.Furthermore,the freshness of fish has been evaluated rapidly by determining the concentrations of XA and HXA based on the electrochemical method,which is consistent to the results of high-performance liquid chromatography（HPLC）.（2）Malachite green（MG）has been widely used as an antibacterial drug in aquaculture.Rapid detection of MG in aquaculture environment is critical to fast evaluation of whether MG has been abused.In this section,the electrochemical behaviors of MG have been investigated using rotating disk electrode,electrochemical impedance spectroscopy,and double potential step chronocoulometry techniques.It is found that Cu-BTC frameworks could not only accelerate the electron transfer of MG,but also show excellent accumulation capability of MG.Therefore,the oxidation signal of MG has been significantly enhanced,leading to a notable improvement of detection sensitivity of MG.The detection conditions are optimized based on response surface method and an electrochemical method has been established for the rapid detection of trace MG.The limit of detection for MG is 0.67 n M and the linear range is 2.0-500.0 n M.Other commonly used antibacterial drugs including500.0 n M erythromycin,chloramphenicol,oxytetracycline,furazolidone,and nitrofurazone show no interference with the detection of MG at a level of 50.0 n M.The electrochemical method has been successfully applied to the analysis of real water samples and the results are consistent to that of HPLC with a relative error of less than 5%.（3）The detection of MG residue in aquatic products is even more important than that in aquaculture environment.However,the potential interferences of coexisting XA and HXA in aquatic products should be considered.In this section,the catalytic activities and sensitizing effects of MOFs have been successfully adjusted by controlling the types of ligands and metal centers,and thus leading to the detection of XA and HXA at high concentrations,as well as trace MG,simultaneously.Three different MOFs,namely porous Cu-2MI（Cu-2-methylimidazole）nanosheets,bulk Cu-BTC,and dodecahedral Zn-2MI,have been synthesized using 2-MI,H₃BTC,Cu（NO₃）₂·3H₂O,and Zn（NO₃）₂·6H₂O as the precursors.Among which,Cu-2MI nanosheets show higher electrochemical active response areas and electron transfer rates,which is likely due to their higher conductivity,more exposed and accessible active sites,as well as lower diffusion barrier.Density functional theory calculations and rotating disk electrode measurements reveal that Cu-2MI show stronger adsorption of XA,HXA,and MG,and higher catalytic activities toward their oxidations.Therefore,the potentials of these oxidation peaks become more distinct and the oxidation currents are higher.Using Cu-2MI as the sensing materials,an electrochemical sensing platform has been developed for the detection of XA,MG,and HXA simultaneously.The limits of detection for XA,MG,and HXA are 8.0,1.6,and 16.5 n M,respectively.Based on the electrochemical sensing platform,the concentrations of XA,HXA,and MG in fish are determined to be in the ranges of 13.3–20.8,8.78–15.0 and 0.0251–0.100μg g^–1.The results are consistent to that determined by HPLC and the relative error is less than8.0%.（4）Graphene supported Cu-2MI composites（Cu-2MI@GS）are successfully synthesized by sequentially adding aqueous Cu（NO₃）·3H₂O and 2-MI solutions into graphene nanosheets suspensions（the volume ratio of methanol and H₂O is 1:1）at room temperature for 1-min reaction.The effects of Cu²⁺and 2-MI concentrations on the morphology,composite efficiency,and electrochemical performance of Cu-2MI@GS composites are systematically investigated.Cu-2MI@GS composites show higher electrochemical active response areas and electrocatalytic activity toward the oxidation ofβ-nicotinamide adenine dinucleotide,as well as enhanced signal amplification effect on XA,HXA,and MG.Based on the synergistic effect of the Cu-2MI@GS,an electrochemical sensing platform has been developed for the detection of XA,HXA,and MG,simultaneously.The sensing platform is further utilized for the analysis of XA,HXA,and MG in fish and the recovery is in the range of 95.4–103.6%.