Surface Regulation And Appliations Of Ir Nanoparticles Simulating Enzyme Properties

The nanomatericals with enzyme-like activities have attracted extensive research interest because of their advantages of convenient preparation,size/surface tunable catalytic activity and excellent stability.With seven electrons in 5d orbital,metal Ir can adsorb some small molecules and react with them to form intermediate compounds to exhibit the catalytic activity.As known,Ir nanomaterials can not only adsorb C₂H₄,CO,C₂H₂,O₂and H₂ but also catalyze the reactions of oxidation,hydrogenation,cyclization,isomerization,dehydrogenation,pyrolysis,and hydrolysis.Therefore,iridium nanomaterials have a broad application prospect in the field of catalysis.Particularly,amounts of noble metal nanoparticles have been developed as effective catalysts for the selective oxidation of alcohols.Early study has reported that the size of iridium nanoparticles was smaller than other metal nanoparticles under the same synthetic conditions,which should be attributed to the slower nucleation and growth of iridium.This prompted us to consider the preparation of iridium nanomaterials acting as nanozymes.According to the report,surface modification of nanomaterials can affect the surface chemical properties of metal and metal oxide nanozymes,and thus achieve the regulation of catalytic performance of catalysts.Herein,a series of IrNPs was synthesized and their properties as well as applications were comprehensive studied.Additionally,the mechanisms behind their enzymatic behaviors were revealed based on the theoretical calculations in this paper.1.An enzyme-like nanoplatform obtained from sodium citrate-modified iridium nanoparticles（Cit-IrNPs）was presented,which exhibits excellent peroxidase,catalase and oxidase-like activities owing to the large accessible surface area and high-index facets of the mono-dispersed nanoparticles.The morphology and structures of nanoparticles were comprehensively characterized by TEM,XRD,FT-IR and XPS.The surface chemistry of Cit-IrNPs reveals that the oxidase-like activity can be ascribed to the formation of O₂^·-from the activation of dissolved oxygen on the high-index facets of IrNPs.The oxidase-like activity of Cit-IrNPs was further manifested by the selectively oxidation of aromatic alcohols to corresponding aldehydes under mild conditions with over 90%conversion rate.The kinetic analysis of the enzyme mimic suggests the reaction follows a classical Michaelis-Menten model.The enzyme mimic also shows a good stability,as no obvious decrease in catalytic activity was observed after recycled use for 6 times.Furthermore,DFT calculation was employed to elucidate the reaction mechanism and it was found that the alcohol is initially bond to Ir（0）and subsequently forms Ir^δ+-alkoxide species and the carbonyl product.Meanwhile,the Ir^δ+-hydride species could reductively eliminate O₂^·-back to Ir（0）.To the best of our knowledge,this is the first report of metal nanomaterials which can effectively transform aromatic alcohols to corresponding aldehydes at ambient conditions without need of external energy input.2.Selective removal of an organic compound in the coexistence of other constituents is a great challenge in separation and purification processes.In this work,bovine serum albumin（BSA）stabilized iridium oxide nanoparticles（Ir Ox NPs）were prepared vie a facile one-step precipitation method.The resultant BSA-Ir Ox NPs were comprehensively characterized with TEM,XRD,XPS,UV-vis,FT-IR,fluorescence spectroscopy as well as circular dichroism spectrometry.It was found that the nanoparticles with an average diameter of 3.6 nm are embedded in the aggregated protein matrix and the structure of coating agent maintained well on the surface of nanoparticles.As prepared nanozymes（BSA-Ir Ox NPs）exhibit strong peroxidase-like activity,which can selectively catalyze the degradation of cationic compounds by H₂O₂ in the coexistence of other inorganic or organic substances at room temperature.Interestingly,the degradation of amino acids could be precisely controlled by adjusting the p H up or down to their isoelectric points（p I）.The catalytic selectivity of BSA-Ir Ox NPs could be ascribed to the anchoring effect between the amidogen containing molecules and BSA through the electrostatic adsorption.The nanozyme also exhibits superb reusability,as it can be readily recycled from solution by static settlement or centrifuge.Therefore,the BSA-Ir Ox NPs have great potential in the selective removal of cationic compounds and amino acids in a complexed matrix.3.A new kind of IrNPs stabilized withβ-cyclodextrin（CD）was prepared in this work,which show oxidase,catalase and peroxidase-like activities.With the help of characterizations,we suggestβ-CD was successfully coordinated with the nanoparticles via O-Ir conjunction through the carboxyl groups of CD.A fluorescence based cholesterol detection method has been developed using competitive host-guest interaction betweenβ-CD with Rhodamine B（Rh B）and cholesterol.Fluorescence of CD-IrNPs incorporated Rh B is quenched by nanoparticles but is then‘turned on’by cholesterol as it replaces Rh B from theβ-CD host.4.Iridium nanoparticles（IrNPs）have shown intriguing enzyme-like activities,however,the mechanism behind their versatile enzymatic behaviors is still elusive.In this work,it was found that the catalytic properties of IrNPs should be driven from the lattice planes.All the IrNPs could catalyze the degradation of H₂O₂ due to the presence of Ir（111）plane while only the IrNPs with Ir（220）plane exhibit oxidase-like activity.A new mechanism for the catalase-like activity of IrNPs was proposed.The adsorption of H₂O₂ on IrNPs induces the breakdown of its O-O bond to form two·OH radicals.The produced·OH will then couple with another·OH or H₂O₂,forming the key intermediate species（·O or·OOH）before the production of O₂.According to the energy diagrams of H₂O₂ decomposition,we suggest the catalase-like activity is an intrinsic property of Ir（111）plane and is independent of other crystal faces or coating agents.In addition,the radicals（·OH and·OOH）generated on different Ir crystal planes show varied stability and reactivity,which is accountable for the diverse peroxidase-like activities of IrNPs.Finally,the oxidase-like property is derived from the Ir（220）plane,where the chemisorbed O₂ could accept one electron to generate superoxide species.The mechanism revealed in the present work should be helpful for understanding the catalytic role of lattice planes of IrNPs,which may also be applicable to other metal nanozymes.