Iron-based Carbon Nanozymes Derived From MOFs And Their Analytical Applications

Nanozymes are nanomaterials with enzyme-like activities.Recently,nanozymes have shown a wide range of application potentials in the fields of sensing,clinical diagnosis,and tumor therapy,because of their extraordinary merits over natural enzymes,such as low cost,good stability,flexible structural design,and easy functionalization.Although the research of nanozymes has made great progress,compared with natural enzymes,most of the nanozymes still face problems such as relatively low catalytic activity,single application mode,and unclear catalytic mechanism.Therefore,the design and synthesis of nanozymes with high catalytic activity and multi-functional application characteristics is the focus of research in this field,and it is of great significance.Metal-organic frameworks(MOFs)are a kind of porous crystalline material formed by self-assembly of metals or metal nanoclusters and organic ligands,which are characterized by large surface area,highly ordered pore structure,unsaturated metal nodes and adjustable structure.Recently,MOFs are considered as ideal precursors or templates for constructing carbon-based nanomaterials with adjustable structure and composition.On the one hand,the derivatization of MOFs can maintain its structural characteristics(such as large specific surface area and porous structure);on the other hand,the carbon materials obtained after derivatization of MOFs are conducive to accelerating electron conduction,improving catalytic activity,as well as solving the inherent chemical instability of MOFs.Therefore,the new carbon-based nanomaterials derived from MOFs are expected to become the research focus of next-generation nanozymes,opening up new ideas for the development of nanozymes.In this thesis,several iron-based carbon nanozymes derived from MOFs are designed and synthesized through chemical doping,anion exchange,morphology and structure control strategies.Combined with a variety of characterization techniques(such as:X-ray powder diffraction(XRD),X-ray photoelectron spectroscopy(XPS),transmission electron microscope(TEM),Raman spectroscopy(Raman)and X-ray absorption spectroscopy(XAS),etc.),we sdudy the relationship between the structure and activity of nanozymes,and further discuss its catalytic mechanism through density functional theory(DFT)calculations.The constructed iron-based carbon nanozymes possess the characteristics of high catalytic activity and good stability.They have been successfully applied to highly sensitive chemical/biological sensing and environmental pollutant removal,realizing the multifunctional application of nanoenzymes.The specific research content is as follows:1.N-rich ZIF-8 is selected as a precursor,then derived into N-doped porous carbon material(NC-800)by high temperature carbonization at 800?.Next,Fe/NC-800 hybrid is prepared by uniform dispersion of in situ formed FeNPs onto NC-800.The synthesized Fe/NC-800 possess the following functions:(1)NC-800 has a large specific surface area(952.4 m²/g),which can enrich a large amount of O₂.Besides,the N-rich feature will cause uneven charge distribution of carbon atoms,which helps to capture O₂ and transfer it to the catalytic sites.(2)The uniformly dispersed FeNPs provide adequate active sites to transform O₂ into ROS.Experimental results revealed that Fe/NC-800 can directly catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine(TMB)to blue oxidation TMB(ox TMB)due to the synergistic effects between FeNPs and NC-800,exhibiting eximious oxidase-like activity,but sole FeNPs and NC-800 had no oxidase-mimicking activity.Giving that dopamine(DA)can compete with FeNC-800 to consume O₂,reducing the amount of ROS in the system.A highly sensitive and selective sensor for visual detection of DA can be constructed.The method is further applied to detect DA in serum and dopamine injection samples,and the recoveries are in the range of 99.7-103.5%and 99.5-103.3%,respectively,demonstrating that the results are satisfactory.2.The above studies reveal that Feand N elements play an important role in increasing oxidase activity,but the FeNPs distribution on NC-800 is not uniform enough.Therefore,a chemical doping method is used to synthesize Fe-Zn ZIFs precursors with even distribution of Feelements by directly introducing a small amount of Fe³⁺into ZIF-8,where both Fe³⁺and Zn²⁺are coordinated with four N atoms.Then the Fe-N/C catalysts containing single-atom Fe-N₄ sites are prepared via a simple one-step carbonization of the Fe-Zn ZIFs.By adjusting the molar ratio of methanol to metal salt,the size-controllable Fe-Zn ZIFs precursor can be obtained.Correspondingly,Fe-N/C single-atom catalysts with different sizes can be prepared.Experiments reveal that the prepared Fe-N/C possess an excellent oxidase-like activity,and their catalytic activities are closely related to the size of Fe-N/C.Based on the reducibility of ascorbic acid(AA),a colorimetric sensing platform can be constructed to detect AA.Besides,in the presence of alkaline phosphatase(ALP),ascorbic acid2-phosphate(AAP)is hydrolyzed to produce AA.Therefore,when coupled ALP/AAP with Fe-N/C/TMB,a novel enzyme-nanozyme cascade reaction system can be constructed for ALP detection.This work not only provides an ideal strategy for rationally designing transition metal-N/C single-atom nanozymes(SAzymes)with high catalytic activity,but also expands nanozymes to biological enzyme activity screening applications.3.Previous studies have found that SAzymes possess excellent enzyme-like catalytic activity.However,single-atom metals are thermodynamically unstable,high concentrations of metals will migrate and aggregate into metal nanoclusters/particles during the pyrolysis process,resulting in low metal loadings of SAzymes.In order to increase the metal content of SAzymes and expose more active sites,we design and prepare a mesoporous FeBi-NC SAzyme with dual catalytic sites by pyrolysis of a 2D Fe-Bi bimetallic organic framework(FeBi-MOF).X-ray absorption spectra proved that FeBi-NC SAzymes contain both Fe-N₄ and Bi-N₄ single-atom sites.The FeBi-NC SAzyme possess ultrahigh single atoms loadings of Fe(2.61 wt%)and Bi(8.01 wt%),which can efficiently activate O₂ to generate ROS for the oxidation of TMB,revealing an excellent oxidase-like activity.Compared with the Fe-NC and Bi-NC SAzymes with single active site,the FeBi-NC SAzyme with dual sites displayed 5.9-and 9.8-fold oxidase-like activity enhancement,respectively.The mass-normalized constant K_w of FeBi-NC SAzymes is several to 100 times higher than those of other reported SAzymes.When integrating with the acetylcholinesterase/acetylthiocholine(ACh E/ATCh),a cascade enzyme-nanozyme system was constructed for selective and sensitive screening of ACh E activity with an extremely low detection limit of 1×10^-4m U m L^-1.In addition,FeBi-NC SAzymes can effectively activate PMS to produce ¹O₂,·OH and SO₄^·-,and achieve high efficiency(>95%)and fast(<2 min)removal of rhodamine B,methylene blue and other dyes.Density functional theory(DFT)calculations showed that both Fe-N₄ and Bi-N₄ in FeBi-NC displayed a strong binding energy and electron donating capability for PMS,promoting PMS activation to generate highly active·OH and SO₄^·-radicals for dyes degradation.Combining theoretical and experimental results,the high catalytic activity of FeBi-NC SAzymes is attributed to the synergistic effect between Fe-N₄ and Bi-N₄ sites.4.This chapter studies the relationship between nanoenzyme structure and activity.ZIF-67 is used as the starting template to prepare Co₃O₄@Co-Feoxide double-shelled nanocages(DSNCs)through anion-exchange combined with low-temperature pyrolysis.Co₃O₄@Co-Feoxide DSNCs maximize the advantages of hollow nanostructures,acting as both nanoreactors and substrate channels to simulate the structural characteristics of natural enzymes.In addition,Co₃O₄ single-shell nanocages(SSNCs)and Co-FeOxide SSNCs are prepared as contrast materials.The results show that all the three catalysts are of specific peroxidase-like activity,but the Co₃O₄@Co-Feoxide DSNCs is the best one.The K_m value of Co₃O₄@Co-Feoxide DSNCs for H₂O₂ is 1.38 and 3.33-fold lower than those of single-shelled Co-Feoxide and Co₃O₄ SSNCs,respectively,indicating that Co₃O₄@Co-Feoxide DSNCs has a stronger affinity with H₂O₂.Based on the highly specific peroxidase-like activity of Co₃O₄@Co-FeOxide DSNC,a highly sensitive sensing platform for H₂O₂ detection was successfully constructed.When coupled with ACh E/ATCh cascade enzymatic reaction,we construct an ultra-sensitive colorimetric platform for ACh E activity screening.Importantly,Co₃O₄@Co-Feoxide DSNCs can also effectively activate PMS to produce·OH and SO₄^·-,degrading 99.1%acid fuchsin(AF)within 20 min,and can still remove 92.3%AF after 10 times of reuse,exhibiting a good repeatability.