Bimetal Nanozymes Derived From MOFs And Their Analytical Applications

Due to its potentiality to replace natural enzymes,nanozymes have been greatly developed in recent years.As an emerging material,metal-organic frameworks（MOFs）have been applied in fields of nanozymes owing to their high specific surface area,abundant pore structure and easy modification.The metal sites and unique framework structures of MOFs can well simulate the catalytic centers and coordination environment of natural enzymes.In addition,MOFs derivatives with fine structures can facilely obtained by further proceeding different types of MOF precursors,which can be used to fabricate new nanozymes.However,there are some shortcomings of MOFs nanozymes,such as limited enzyme-like types and low activity.To address these issues,we propose to design bimetallic MOFs by doping transition metals such as Fe,Co,Mo and W.Furthermore,bimetallic nanozymes derived from MOFs can be fabricated by a simple pyrolysis process.The research contents include:（1）Fabrication of bimetal MOFs nanozymes by transition metals doping and research on their enzyme-like activities.（2）Fabrication of bimetal MOFs derivatives and research on their enzyme-like activities.（3）Research on the regulatory mechanism for the activity of MOFs-derived bimetal nanozymes and their enzyme-like catalytic mechanism.（4）Construction of a series of colorimetric methods based on the MOFs-derived bimetal nanozymes to detect environmental pollutants and biomolecules.The dissertation consists of the following five chapters.Chapter 1:This chapter mainly reviews the literatures,which focus on the research progress of MOFs-based nanozymes.First,researches on the pristine MOFs,MOF/natural enzyme hybrids and MOFs derivatives with different enzyme-like activities are summarized.Their developments and analytic applications are introduced.Different types of these nanozymes are classified as peroxidase,oxidase,superoxide dismutase,multifunctional enzymes and other enzyme mimics.Second,we conclude the strategies for improving the limited catalytic performance of MOFs-based nanozymes,including regulation of the size or morphology,surface modification,metal doping,design of organic ligands,etc.Among them,metal doping has been recognized as a simple and effective strategy.Therefore,the dissertation aims to increase the catalytic activity of MOFs-based nanozymes,explore newly MOFs-derived bimetal nanozymes and put forward the research ideas of this dissertation.Chapter 2:In this chapter,tremella-like Ni Fe MOF with ultrathin nanosheets was synthesized by a rapid microwave-assisted method through the self-assembly of Ni²⁺and Fe²⁺with NH₂-BDC.Fe-doping was proved as an effective strategy to enhance the peroxidase-like activity of Ni Fe MOF.The characteristics such as structural and surface feature,and textural properties of the as-prepared MOFs were investigated.As expected,the as-obtained binary Ni Fe MOFs exhibited 2.5 to 2.8-fold increase in peroxidase-like activity compared to the single Ni MOF,which efficiently catalyzed the TMB oxidation by H₂O₂.The calculated TMB oxidation rate and H₂O₂ reduction rate catalyzed by unit mass concentration of Ni Fe MOF were 30.6-fold and 1.3-fold by those of Ni MOF,respectively.Density functional theory（DFT）study proved that the doped-Fe is more active than Ni for the reduction of H₂O₂ and the adsorption of reactive oxygen species on Ni Fe MOF.Combined the more exposed active sites by the special nanosheet structure with the larger surface area,Ni Fe MOF ultimately exhibited its boosted peroxidase-like activity.The excellent catalytic performance permitted Ni Fe MOF to sensitive respond H₂O₂.Based on the redox reaction between H₂O₂ and S^2-,TMB/H₂O₂/Ni Fe MOF system was proposed for the determination of S^2-ions with a linear range of 0.5-60μM and a low detection limit of 28 n M（3σ）.This method is expected to be used for the detection of S^2-in environmental water samples.Chapter 3:In this chapter,ferrocenecarboxylic acid（Fc-COOH）was used as the dopant and Co PTA was chose as the pristine MOF to design bimetal MOFs with enhanced enzyme-like activity through a hydrothermal method.Because Fc-COOH can work as both the Fe center and–COOH group donors.The added ligand can partly replace the original one to form missing-linker MOFs.Finally,more unsaturated sites are formed in the MOFs and their catalytic ability can be promoted.XRD,IR,SEM and TEM were used to prove the successful formation of Co PTA/Fc-9 compound.Fc-COOH doping affected the morphology and particle size of Co PTA.Also the crystalline of Co PTA was affected by Fc-COOH doping,which caused more active sites expose on the compound.The enlarged surface area also contributed to the enhanced catalytic activity of Co PTA/Fc-9 and the increased pore volume facilitated the mass transfer in the catalytic progress.Based on the TMB-H₂O₂ system as a model,the high peroxidase-like activity of Co PTA/Fc-9 was demonstrated.The activity of Co PTA/Fc-9increased by 8.6-fold and 3.7-fold than the single Co PTA and Co Fc,respectively.The K_m values of Co PTA/Fc-9 for TMB and H₂O₂ were 0.046 m M and 0.298 m M,respectively,which were 9.4-fold and 12.4-fold lower than those of HRP,suggesting Co PTA/Fc-9 was an excellent peroxidase mimic.Hence,we used TMB-Co PTA/Fc-9system to linear respond 0.05-100μM H₂O₂.Sarcosine oxidase（SOX）can catalyze the reaction of sarcosine and O₂ to produce H₂O₂.Thus,the proposed system was further used for the sensitive detection of sarcosine in the presence of SOX,with a linear range of 0.05-60μM and a LOD of 44 n M（3σ）.Besides,we investigated the potential application of Co PTA/Fc-9 in the removal of environmental pollutants.Organic dyes were chosen as models.As expected,the results shew that the Co PTA/Fc-9 could effective active potassium peroxymonosulfate（PMS）to totally degrade dyes within 10-15 min.Under the same conditions,the degradation efficiency followed the order of malachite green>crystal violet>rhodamine B>methylene blue.Chapter 4:In this chapter,Co/Mn oxides composite was obtained by directly pyrolying cobalt doped Mn-BTC precursor,which was a new oxidase mimetic for colorimetric sensing of acid phosphatase（ACP）.Co-doping was proved to effectively promote the oxidase-like activity of manganese oxides.Because MOFs are self-templated,metal-based nanomaterials with unique MOFs structure can be obtained by a simple pyrolysis derivatization of MOFs,which has potential enzyme-like activity.By tuning the molar ratios of cobalt ions to Mn-BTC precursor,four bimetal Co/Mn-BTCs and their derivatives were obtained.These Co/Mn oxides showed 1.4-fold to 2.6-fold higher oxidase-like activity than that of Mn₂O₃ derived from Mn-BTC,indicating that Co-doping is an effective way for improving and tuning oxidase-like activity of manganese oxides.The rambutan-like Co/Mn oxides with Co/Mn molar ratio of 0.33（denoted CMO-0.33）in the MOFs precursors exhibited the highest oxidase-like activity.The estimated Michaelis constant（K_m）for TMB was 2.26-fold lower than that of Mn₂O₃,indicating its stronger affinity to TMB.Based on the inhibitory effect of ascorbic acid（AA）on the TMB-CMO-0.33 system,a new colorimetric strategy was paved for screening ACP activity when coupled with L-ascorbic acid 2-phosphate trisodium salt（AAP）because ACP could catalyze AAP hydrolysis to generate AA.The proposed colorimetric biosensor allows detection of ACP in the range 0.02-1.0 U/L with a limit of detection of 8.2 m U/L（3σ）.Its potential utilization in ACP assay in human serum samples was successfully demonstrated.Chapter 5:In this chapter,the typical cobalt-based MOF（ZIF-67）was chosen as the parent MOFs and the transition metal Mo or W as the dopant to design hollow Co₃O₄/MO₃（M=Mo,W）mixed-metal oxides by subsequent pyrolysis of binary MOFs under an air atmosphere at 450°C.The hollow Co₃O₄/MO₃（M=Mo,W）mixed-metal oxides displayed tunable oxidase-like and peroxidase-like activities,which were able to efficiently catalyze the oxidation of TMB in the absence or presence of H₂O₂.Relative to that of the un-doped Co₃O₄,the oxidase mimic activity of the Mo-doped Co₃O₄increased to 1.3 to 2.1-fold,while its peroxidase mimic activity increased to 7.1 to19.9-fold,depending on different Mo doping amounts.The oxidase mimic activity of the W-doped Co₃O₄ increased to 2.1 to 2.3-fold,while its peroxidase mimic activity increased to 4.8 to 5.9-fold,depending on the different W doping amounts.The Mo-and W-doped Co₃O₄ compounds exhibited both higher O₂ and H₂O₂ activating capability,and their H₂O₂ activating capacities were superior to the O₂ activating capability.The discrepant peroxidase-like natures of Mo-and W-doped Co₃O₄ compound are likely attributed to their different catalytic mechanisms.The peroxidase-like activity of Mo-doped Co₃O₄ is highly related to the·OH radical,while that of W-doped Co₃O₄ is likely ascribed to the electron transfer between TMB and H₂O₂.The K_m values of Co₃O₄/Mo O₃ for TMB and H₂O₂ were 0.0352 m M and 0.134 m M,which were 3.2-and1.9-fold lower than that of pure Co₃O₄,respectively.A Co₃O₄/Mo O₃-based colorimetric platform was developed for the determination of H₂O₂ in the 0.1–200μM range,with a limit of detection of 0.08μM（3σ）.Based on the thiocholine（TCh）inhibition of the excellent peroxidase-like activity of Co₃O₄/Mo O₃ and the TCh generation via acetylcholinesterase（ACh E）catalyzed hydrolysis of acetylthiocholine chloride（ATCh）,the colorimetric platform was extended to detect ACh E activity and its inhibitor.