Study On Electrochemical Sensing Of Heavy Metal Ions Based On Organic Framework Materials

The electrochemical sensor is a kind of device which uses the electrochemical principle to connect the concentration change of the tested components with the electrochemical signal,so as to provide real-time information of the chemical components in the detected system.When the electrochemical method is used to detect the tested substance,it has the advantages of relatively simple detection process,fast detection speed,high sensitivity and low cost,so it has become a research hotspot in the field of analysis.Heavy metals are highly toxic,and even a small amount of exposure to the biosphere will cause serious harm to human health and other organisms.Therefore,it is particularly important to control the environmental pollution caused by heavy metal ions and improve the water quality in modern society.Therefore,simple and highly sensitive detection of heavy metal ions and removal of heavy metal ions from pollutants are very important.In this paper,a series of electrochemical sensors have been developed by selecting environmentally friendly biomass carbon materials,metal-organic framework materials（MOF）and covalent organic framework materials（COF）,which can be effectively used for the detection and adsorption of heavy metal ions.The specific work is as follows:1.Herein,mercaptan functionalized metal-organic frameworks（MOFs）were prepared by the coordination between Zr（Ⅳ）and 2,5-dimercaptoterephthalic acid（Zr-DMBD MOFs）and attached to three-dimensional macroporous carbon from kenaf stem（3D-KSC）to form a novel nanocomposite for removal and electrochemical detection of Hg（Ⅱ）.The sensitivity of Hg（Ⅱ）detection was 324.58μAμM^-1cm^-2,the linear range was 0.25μM-3.5μM and the detection limit was 0.083μM.The good performances were because lots of Zr-DMBD MOFs were uniformly dispersed on 3D-KSC and the porous Zr-DMBD MOFs had a large number of mercaptan groups to enrich Hg（Ⅱ）significantly on the electrode.The detection of Hg（Ⅱ）would not be affected by other co-existing heavy metal ions,and the Zr-DMBD MOFs/3D-KSC also showed excellent stability and reproducibility.Furthermore,the Zr-DMBD MOFs/3D-KSC could effectively remove Hg（Ⅱ）in real wastewater.The residues were lower than national industrial wastewater discharge standard（GB30770-2014）.The results showed that Zr-DMBD MOFs/3D-KSC nanocomposites could be applied for the removal and detection of Hg（Ⅱ）in real water samples.2.In this paper,the metal-organic skeleton（MOFs）UiO-66-NH₂ was synthesized and modified,and without affecting the original skeleton structure,the isothiocyanate functional group was introduced into the skeleton to prepare UiO-66-NHC（S）NHMe,and attached to kenaf stem-based three-dimensional macroporous carbon（3D-KSC）to form a new type of nanocomposites.It is found that the content of MOFs on 3D-KSC can be effectively controlled by adjusting the monomer concentration.Each structural unit of UiO-66-NHC（S）NHMe has three adsorption sites（1 sulfur atom and 2 nitrogen atoms）for heavy metal ion（HMIs）and regular pores to promote the transfer of HMIs.Therefore,UiO-66-NHC（S）NHMe/3D-KSC nanocomposites can be used for the removal of Hg（Ⅱ）and electrochemical detection of Cd（Ⅱ）,Pb（Ⅱ）,Cu（Ⅱ）,Hg（Ⅱ）at the same time.The detection limits of Cd（Ⅱ）,Pb（Ⅱ）,Cu（Ⅱ）and Hg（Ⅱ）were 0.012μM,0.011μM,0.013μM,0.0094μM,respectively.The sensitivities are 887.6μAμM^-1cm^-2,992.5μAμM^-1cm^-2,879.1μAμM^-1cm^-2 and 1160.7μAμM^-1cm^-2,respectively.UiO-66-NHC（S）NHMe/3D-KSC also shows good stability and reproducibility as well as strong anti-interference ability.In addition,UiO-66-NHC（S）NHMe/3D-KSC can effectively remove Hg（Ⅱ）,residues in actual wastewater,which is lower than the national industrial wastewater discharge standard（GB30770-2014）.The results show that UiO-66-NHC（S）NHMe/3D-KSC has great potential in adsorption and detection of HMIs.3.For the first time,a COF material(COF_MELE-BTDD)with multiple adsorption sites of heavy metal ions was synthesized and applied to the study of electrochemical sensing and adsorption of Hg（Ⅱ）.The layered COF_MELE-BTDD material was prepared by the dehydration condensation of Melem（MELE）and4,4’-（benzo[c][1,2,5]thiadiazole-4,7-diyl）dibenzaldehyde（BTDD）.The material is a two-dimensional crystal nanosheet with highly ordered conjugated structure and the pore size is 9.65 nm.The glassy carbon electrode modified with this material was used as a new type of electrochemical sensor for the determination of Cd（Ⅱ）,Pb（Ⅱ）,Cu（Ⅱ）,Hg（Ⅱ）in aqueous medium.By complexing with amino groups,metal ions are enriched on the surface of COF_MELE-BTDD and detected by square wave anodic stripping voltammetry.Due to the unique structural characteristics of COF,the prepared sensor has fast electron transfer rate and excellent metal adsorption capacity.In the whole process of analysis,the effects of various experimental parameters（COF content,p H value of electrolyte,enrichment potential and enrichment time）on the peak current were investigated.Under the optimum conditions,the concentration of,Cd（Ⅱ）,Pb（Ⅱ）,Cu（Ⅱ）,Hg（Ⅱ）has a good linear relationship with the peak area in the range of 0-4.00μM.the detection limits of,Cd（Ⅱ）,Pb（Ⅱ）,Cu（Ⅱ）,Hg（Ⅱ）are 0.0047μM,0.0012μM,0.0011μM and 0.0011μM,respectively,and the sensitivities are105.0μAμM^-1cm^-2,405.5μAμM^-1cm^-2,439.0μAμM^-1cm^-2 and 465.1μAμM^-1cm^-2,respectively.COF_MELE-BTDD/GCE also shows good stability,reproducibility and strong anti-interference ability.In addition,COF_MELE-BTDD can effectively remove Hg（Ⅱ）,from water samples.The results show that COF_MELE-BTDD has great potential in adsorption and detection of heavy metal ions.This method not only shows the feasibility of the sensor based on COF for the detection of trace metal ions,but also broadens the application range of hybrid materials based on COF in electroanalytical chemistry.