Preparation Of Functionalized MOFs And Its Application In Electrochemical Sensing Of Heavy Metal Ions

Heavy metal ions(HMIs)such as chromium,lead,and mercury are highly toxic and pose a serious risk to the environment and human health.With the rapid development of modern society,the problem of heavy metal pollution has become increasingly serious.Therefore,the development of HMI detection technology has important practical significance.Metal-organic frameworks(MOFs),also known as porous coordination polymers(PCPs),are a new type of hybrid crystal material composed of inorganic and organic units.Due to the advantages of large specific surface area,ultra-high porosity,large pore volume,designable framework/pore structure and open active sites,MOF materials have shown promising applications in gas/energy storage,adsorption,separation and sensing.In the field of HMI sensing,MOF materials also show unique advantages.Optical HMI sensors based on luminescent MOF have received a lot of attention from researchers.However,due to the poor conductivity and stability of MOF materials,their application in HMI electrochemical sensing is limited.By coupling MOF materials with conductive materials such as carbon materials and conductive polymers can improve the electrochemical properties of the composites and expand their applications in the field of electrochemical sensing.Based on the above research background,a variety of MOF materials with good water and thermal stability have been prepared in this paper.They are functionally modified by means of post-synthesis modification and material hybridization to improve their electrochemical performance.Based on these functionalized MOF materials,novel electrochemical sensing platforms and sensing strategies were constructed and proposed to achieve simple,sensitive and reliable electrochemical detection of various HMIs in aqueous solutions.Details of the research are as follows.(1)The nickel-based MOF,NH₂-Ni-MOF,was synthesized by solvothermal method,and it was functionally modified with ferrocene(Fc)to prepare Fc-NH₂-Ni-MOF,which was used for the simultaneous determination of multiple HMIs.The modification of MOF by Fc not only enhances the conductivity of the MOF material,but also provides an internal reference signal for the ratiometric analysis.Due to the excellent properties of Fc-NH₂-Ni-MOF,the prepared ratiometric electrochemical sensing platform has a wide linear response range(0.001?M-2.0?M for Pb²⁺and 0.01?M-2.0?M for Cu²⁺and Cd²⁺)and high sensitivity(the detection limits of 6.3 n M,0.2 n M and 7.1 n M for Cu²⁺,Pb²⁺and Cd²⁺,respectively).In addition,compared with non-ratiometric measurement strategies,the reproducibility of ratiometric detection has been significantly improved.(2)A zirconium-based MOF material,Ui O-bpy,was prepared by solvothermal method and compounded with electrochemically reduced graphene oxide(r GO)to obtain r GO/Ui O-bpy composite material,which was used for the electrochemical determination of Pb²⁺.The composite materials were characterized by X-ray diffraction,Raman spectroscopy,Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.It was found that the signal of Ui O-bpy showed an inverse response to the concentration of Pb²⁺.Based on this,we developed a novel on-off ratiometric sensing strategy for Pb²⁺detection.The results showed that the oxidation peak current of Pb²⁺showed a good linear relationship with the concentration in the detection range of0.0010-0.10?M and 0.10-0.80?M,and the detection limit of Pb²⁺was as low as 0.1 n M.In addition,it also shows a bright application prospect in the actual sample detection.(3)H-C-ZIF-8@Ui O-66-NH₂composite was prepared by growing zirconium-based MOF material,Ui O-66-NH₂,on large-size hollow porous carbon polyhedra(H-C-ZIF-8).Based on the H-C-ZIF-8@Ui O-66-NH₂composite,an electrochemical sensing platform for Pb²⁺determination was constructed.Through the a two-solvent-mediated overgrowth and a subsequent calcination process,the obtained H-C-ZIF-8 has a polyhedral morphology,well-defined hollow structure and good conductivity.The Ui O-66-NH₂grown on its surface has a large specific surface area and abundant amino groups.The synergistic effect of H-C-ZIF-8 and Ui O-66-NH₂can effectively improve the adsorption and pre-concentration capacity of H-C-ZIF-8@Ui O-66-NH₂for Pb²⁺.Therefore,the electrochemical sensing performance was greatly enhanced.The results show that the electrochemical sensor based on H-C-ZIF-8@Ui O-66-NH₂can achieve a wide detection linear range of 0.1-500 n M and a low detection limit of 0.049 n M for Pb²⁺with good selectivity,high stability and reproducibility.In addition,the electrochemical sensor can be used for the determination of Pb²⁺in real water samples with satisfactory results.