Molybdenum Based Transition Metal Oxide Nanozymes And Their Analytical Applications

Natural enzymes are important components of living systems,with high catalytic activity and specificity,but there are problems such as high costs and difficulties in preparation,which limit their wide application.To solve these problems,nanozymes have been created;they are a class of nanomaterials with natural enzyme-like activity,with low preparation cost,adjustable component structure and good stability,and have been applied to such fields as colorimetric sensing,tumor prevention and control,as well as environmental pollution treatment.At present,the main problems faced by nanozyme research include:1)the enzyme-like catalytic activity of nanozyme is still low compared with natural enzymes;2)the variety of nanozyme is limited.Therefore,research around highly active nanozymes,novel nanozymes and their applications are the hot spot in the field of nanozymes.Transition metal oxides,with their rich redox chemical,optical and electrical properties,have received much attention in the study of nanozymes.Several transition metal oxide nanozymes have been reported,but less research has been done on molybdenum-based transition metal oxide nanozymes.Molybdenum is a transition metal with multiple valence states and therefore,when it forms composite oxides with other transition metals,composites with rich redox chemistry can be expected,while the synergistic effect of the two metals in the composite oxide can enhance the material properties and help to obtain novel highly active nanozymes.To this end,in this thesis,two molybdenum-based transition metal oxide nanozymes were synthesized through strategies such as modulation of bimetallic components and the construction of oxygen vacancies.Their mimetic enzymatic properties were investigated and found to mimic oxidases and laccase,and they were used for the colorimetric detection of reducing substances（e.g.sulfites）and small molecules（e.g.catechins）in food samples,respectively.The results of the related studies have contributed to the construction of highly active nanozymes and enrichment of nanozyme species.The main research contents and results are as follows:（1）Oxygen vacancies rich Co-Mo oxide microspheres as oxidase mimetic for colorimetric detection of sulfiteIn this work,we firstly pyrolyzed CoMo layered hydroxide precursors at 300℃ in an air atmosphere and then prepared Co-Mo bimetallic oxide nanozymes（CoMo-300r）with oxidase-like properties using a NaBH₄ reduction strategy,and optimized the synthesis conditions.Scanning electron microscopy（SEM）images showed that CoMo-300r exhibited a three-dimensional flower-like microsphere morphology.X-ray diffraction（XRD）showed that CoMo-300r was composed of two oxide components,CoMoO₄ and Co₃O₄.N₂ adsorption-desorption experiments showed that both CoMo-300r and CoMo-300 without NaBH₄ treatment were mesoporous structural materials,and the specific surface area of CoMo-300r obtained after NaBH₄ reduction increased significantly.The results of X-ray photoelectron spectroscopy（XPS）and electron paramagnetic resonance spectroscopy（EPR）showed that the reduction of NaBH₄ led to the formation of abundant oxygen vacancies（O_v）and reduced metal ions on the surface of the material,which promoted the kinetics of the redox reaction.The results of enzyme-like catalysis experiments show that CoMo-300r has excellent oxidase-like activity and can catalyze the oxidation of colorless 3,3’,5,5’-tetramethylbenzidine（TMB）to a blue product（ox TMB）by dissolved oxygen,and the enzyme-like catalysis performance of CoMo-300r is 2.6 times higher than that of CoMo-300.The kinetic experiments showed that CoMo-300r had a high affinity for TMB with a Michaelis constant（K_m）of 0.0428 m M and a maximum reaction rate(V_max)of 8.72×10^-8 M·s^-1.Mechanistic investigation experiments demonstrated that the reactive oxygen radicals（ROS）in the CoMo-300r/TMB reaction system are hydroxyl radicals（·OH）,superoxide anion(O₂^·-)and singlet oxygen（¹O₂）.Kinetic experiments showed that sulfite could consume ROS and thus inhibit the oxidation of TMB.Accordingly,a colorimetric method for the determination of SO₃^2-was developed,with a linear range of 0.2-40μM and a detection limit of 0.09μM.The method has good selectivity for the determination of sulfite,and the method was applied to the determination of sulfite in white wine as well as satisfactory results were obtained.This study provides a new method for the preparation of highly active oxide mimetic enzymes and their use in food analysis.（2）Laccase-like properties of Mn-Mo bimetallic oxides and their application in the colorimetric detection of catechinsIn order to extend the simulated enzyme properties of transition metal oxide nanozymes and their applications in food sample detection,in this part of work,Mn-Mo precursors were synthesized by a simple hydrothermal method by regulating the raw material ratio of Mn and Mo and calcination temperature,and then Mn-Mo bimetallic oxide nanozymes（MMOs）were successfully constructed by air calcination.SEM results showed that the MMOs exhibit irregular bulk morphology.XRD characterization showed that the MMOs are mixed metal oxides composed of Mn₂O₃,Mn O₂,and Mn MoO₄.XPS analyses showed that the MMOs material contains different Mn species such as Mn²⁺,Mn³⁺,and Mn⁴⁺,indicating that the introduction of molybdenum atoms drives multiple valence states of manganese and promotes redox cycling of manganese ions in different valence states.N₂ adsorption-desorption experiments have shown that air calcination results in an increase in the specific surface area of the material,which helps to provide free transport channels for reactants and products and exposes more accessible active sites.The results of the enzyme-like catalysis experiments showed that MMOs were able to catalyze 2,4-dichlorophenol（2,4-DP）with high efficiency and that the oxidation product reacted with 4-aminoantipyrine（4-AP）to produce a red product,indicating that MMOs have laccase-like catalytic activity and the catalytic activity was 2.2 times higher than that of natural laccase at the same mass concentration.In addition,O₂ is directly involved in the catalytic reaction,and no H₂O₂ is produced during the process.Catalytic kinetic experiments showed that MMOs have a K_m value of 0.43 m M and a V_max value of 0.12m M·min^-1,indicating that MMOs have a higher affinity and catalytic reaction rate for the substrate 2,4-DP than natural laccase.Compared to natural laccase,MMOs have excellent stability and reusability at extreme pH,high temperature,high salinity and long-term storage conditions.Since catechins can be oxidized by MMOs to form yellow products,a highly sensitive colorimetric method for the determination of catechins was constructed based on the laccase-like nature of MMOs.The linear detection range of the method is 1-340μg·m L^-1 and the detection limit is 0.27μg·m L^-1.The method has good selectivity for the determination of catechins,and the method was applied to the determination of catechins in green tea beverages with satisfactory results.