Sulfur And Nitrogen Co-doped MOFs Derivative Oxidase Mimics And Their Analytical Applications

Nanozymes are nanomaterials with enzyme-like properties.Compared with natural enzymes,nanozymes have the advantages of low cost,good stability and easy preparation,etc.,and have attracted wide attention in the fields of molecular detection,tumor therapy and environmental protection.At present,the main reported nanozymes materials are noble metal-based nanozymes,transition metal-based nanozymes,carbon-based nanozymes,etc.Among them,carbon-based nanozymes have the advantages of low cost and good biocompatibility,so they are currently the research hotspot in the field of nanozymes.However,the limited catalytic capacity of individual carbon-based nanomaterials leads to limited applications.Doping is one of the effective ways to improve the catalytic activity of carbon-based nanozymes,especially metal-doped carbon-based nanozymes,which can improve their enzyme-like catalytic performance by using metal sites to simulate the catalytic active sites of natural enzymes.At the same time,the electronegativity difference of heteroatoms（N,S,etc.）can be used to adjust the electronic structure of the central metal atoms,which can further enhance the catalytic performance of nanozymes.In addition,as a porous material,metal-organic frameworks（MOFs）have the advantages of high surface area and porosity control.Nitrogen-doped carbon-based materials obtained by pyrolysis using MOFs as precursor can not only maintain the original advantages of MOFs,but also improve the material properties.Therefore,it is expected to prepare carbon-based nanozymes with efficient activity through MOFs derivations combined with heteroatom doping strategy.Herein,two metals（Co,Fe）and S,N co-doped carbon-based materials are synthesized by using the structural characteristics of MOFs derivatives and the S,N co-doped strategy.The structure and composition of these materials are characterized and measured by various analytical methods（such as XRD,XPS,SEM,TEM,Raman and BET,etc.）.The properties of nanozymes and their influencing factors are studied in details.According to their oxidase-like properties,the analytical methods for the detection of heavy metal ions are established,which are successfully applied to the detection of heavy metal ions in environmental samples.The main research contents are as follows:（1）Co,N,S co-doped hollow carbon oxidase nanozyme for the detection of Hg²⁺and Fe³⁺ions.Through a simple doping strategy,using zeolitic imidazolate framework（ZIF-67）and thiomalic acid（TA）as precursor and sulfur source respectively,the Co,N,S co-doped hollow carbon material,i.e.Co-N/S-HCN was fabricated by one-step high temperature carbonization.The results of electron microscopy showed that the shape of Co-N/S-HCN was hollow dodecahedron.Raman characterization shows that Co-N/S-HCN has defective carbon and graphitic carbon structures.The electrochemical test shows that doping of S reduces the charge transfer resistance and accelerates the electron transfer rate,which is conducive to improving the catalytic activity.The Co-N/S-HCN shows superior oxidase-like activity and can convert colorless 3,3’,5,5’-tetramethylbenzidine（TMB）,o-phenylenediamine（OPD）and2,2’-azo-bis（3-ethylbenzothiazolin-6-sulfonic acid）diammonium salt（ABTS）into visible color substrates without hydrogen peroxide.The oxidase-like activity of Co-N/S-HCN was 2.1 times that of Co-N-HCN.The free radical scavenging experiments revealed the generation of O₂^·-,·OH and ¹O₂ during catalytic TMB oxidation by Co-N/S-HCN.The Co-N/S-HCN oxidase-like nanozyme exhibited good affinity to TMB with a K_m value of 0.25 m M and V_maxof 4.19×10^-7 M s^-1.In terms of Soft-Hard Acid-Base principle,Hg²⁺combines easily with the sulfur.Thus,the introduction of Hg²⁺ions can restrain TMB oxidation catalyzed by Co-N/S-HCN,thereby inhibiting the color reaction.In addition,the addition of 8-hydroxyquinoline can restrain TMB oxidation catalyzed by Co-N/S-HCN,resulting in a decrease in absorbance at 652 nm.However,the color reaction recovers in the presence of Fe³⁺,resulting in an increase in absorbance at 652 nm.In terms of above findings,a colorimetric sensing platform for detecting Hg²⁺and Fe³⁺ions are established and successfully used for detecting Hg²⁺in water samples and Fe³⁺in liquors.This work offers a new way to prepare nanozymes with efficient oxidase-like activity and is of great significance for detecting heavy metal ions in environmental and food samples.（2）S-N co-doped carbon coated Fe nanomaterial oxidase nanozyme for the detection of Hg²⁺ions.S-N co-doped carbon coated Fe nanomaterials,i.e.Fe-SNC was prepared by utilizing the structural characteristics of MOFs-derived porous carbon materials,combined with heteroatom doping strategy.The results of electron microscopy showed that Fe-SNC was hollow dodecahedron with an average particle size of 120 nm.The results of nitrogen adsorption-desorption isotherm showed that the BET specific surface area and pore volume were 724.92 m²/g and 1.25 cm³/g,respectively.The large specific surface area and pore volume are conducive to providing sufficient pathways for electron transfer and target adsorption.The results of XPS analysis showed that Fe-SNC contains abundant Fe-N_x sites,and the doping of S induces defects on the carbon support,which significantly improves the catalytic activity.Fe-SNC has excellent oxidase-like activity,which can directly catalyze the oxidation of color substrate TMB.Compared with undoped Fe-NC and SNC,the oxidase-like activity of Fe-SNC was increased 3.8 times and 21.9 times,respectively.Fe-SNC has good affinity for TMB,K_m value is 0.21 m M,and the maximum reaction rate V_maxvalue is 5.39×10^-8 M s^-1.According to the specific interaction between Hg²⁺and S,a colorimetric analysis method was established for the detection of Hg²⁺ions.The linear range of Hg²⁺ions was 10 n M-10μM,and the limit of detection（LOD）was as low as 2 n M.In addition,an analysis method based on smart phone was designed to realize the visual detection of Hg²⁺ions.The proposed method was applied to the detection of Hg²⁺ions in actual water samples,and satisfactory results were obtained.The above results show that this method has potential application value in the field of environmental detection.