Preparations,Properties And Analytical Applications Of Noble Metal Iridium Nanomaterials

Nanomaterial is one of the most important material in the filed of material science,they attact considerable research interest due to their anomalous physical and chemical properties.As a significant component of nanomaterials,noble metal nanomaterials exhibit distinctive optical,electric,magnetic and catalytic properties because of the strong bond,high bulk density and coordinate number of noble metal atom.Iridium（Ir）has high melting point,good stability and corrosion resistance,high catalytic activity and selectivity,thus Ir nanomaterials have become the important materials for national defense and modern industry.Especially for their highly efficient catalytic activity,they have been used for the aerospace,tail gas purification,fine chemicals and so on.In addition,Ir nanomaterials should possess unique optical,electric and catalytic properties and potential application value due to lots of grain boundary structures existing in the Ir nanomaterials.The theme of this paper is to research the enzyme-like activity,surface-enhanced Raman scattering（SERS）activity and fluorescence of the Ir nanomaterials.The Ir nanoparticles（Ir NPs）with enzyme-like and SERS activities were prepared using tannin acid and sodium citrate as stabilizers,and fluorescence Ir nanoclusters（Ir NCs）were synthesized in N,N-dimethylformamide（DMF）media.The reasons why the Ir NPs possess enzyme-like and SERS activities,and Ir NCs have red-edge effect were analyzed.Their potential application values were explored in analytical chemistry field.The detail research contents are as follows:The peroxidase-like activity of the Ir NPs was studied.Tannin acid is one kind of typical green amphiphilic molecule.The multiple-OH sites of tannic acid can weakly stabilized Ir NPs,which is important for mimics to exert the catalytic performance.The good water-solubility is beneficial for peroxidase mimics using in aqueous media.Thus,the Ir NPs were prepared using tannin acid as stabilizer under the reduction of Na BH4.The Ir NPs can catalyze H₂O₂ oxidizing 3,3,5,5-tetramathylbenzidine（TMB）to form blue-colored production ox TMB,showing their peroxidase-like activity.Kinetic analysis suggests that the catalytic behavior is in accordance with the typical Michaelis-Menten kinetics and the catalytic reaction follows the ping-pong mechanism.Electron Spin Resonance（ESR）experiment and the reaction rates of the Ir NPs catalyzing different peroxides oxidizing TMB suggest an electron transfer mechanism is involved in the system of TMB-H₂O₂-Ir NPs.By using TMB as the colorimetric substrate,H₂O₂ can be rapidly determined and this method is extended for the determination of xanthine based on its production of H₂O₂ in the presence of xanthine oxidase.In the absence of H₂O₂,the Ir NPs can also catalyze dissolved oxygen（DO）oxidizing TMB to form ox TMB,although more Ir NPs and longer reaction time are needed,suggesting Ir NPs possess oxidase-like activity.The activity is closely related to the stabilizer and the crystal structure of the Ir NPs.Thus,Ir NPs were synthesized by using sodium citrate as stabilizer and Na BH4 as the reductant.The Ir NPs possess {200},{220} and {311} high-index facets and exhibit excellent oxidase-like property.Under the catalytic action of the Ir NPs,TMB can be oxidized by DO to form oxTMB under ambient conditions without extra energy input.The catalytic mechanism can be ascribed to the production of superoxide anion radical（O2ˉ?）.Based on this chromogenic reaction,one experiment setup for DO real-time determination was designed.This analytical method exhibits a wide calibration range from 12.5 to 257.5 μM with a limit of detection（LOD）as low as 4.7 μM.Compared with other methods,the Ir NPs chemosensor shows higher precision and much faster response time.Except for the oxidase-like activity,the Ir NPs stabilizd by sodium citrate can also exhibit good SERS activity.When the Ir NPs are purified and concentrated by ethanol,they grow into 60 nm NPs due to the aggregation and self-assembly actions.The Ir NPs thin film was prepared by dropping the concentrated Ir NPs on the surface of silicon wafer,and the Ir NPs were well packed from the scanning electron microscopy（SEM）.The free electrons of the Ir NPs occur localized surface plasmon resonance（LSPR）and produce ultrastrong electromagnetic field when the incident light（electromagnetic wave）irradiates onto the surface of the Ir NPs.Thus the Ir NPs exhibit strong SERS activity.The average enhancement factor（EF）at the 1512 cm￣1 peak was calculated to be 3.5×105 using Rhodamine 6G（R6G）as the probe molecule.The excellent SERS performance of the IrNPs was exemplarily applied to the determination of the industrial colorant Sudan Red I.The peak intensity of the Raman band at 1236 cm￣1 is linearly related to the concentration of Sudan Red I.Therefore,Sudan Red I can be determined in the concentration range of 2 nM-8 μM with LOD of 0.6 nM.Ir NCs consist of several Ir atoms.Their size is smaller than Ir NPs and comparable to the Fermi wavelength of electron.The spatial confinement of free electrons in Ir NCs results in electronic transitions,thus the molecular-like properties appear,such as fluorescence.Ir NCs were facilely prepared in DMF solution without using any other reagents.According to matrix-assisted laser desorption ionization time-of-flight mass spectrometry（MALDI-TOF MS）,the Ir NCs consist of 2 to 7 Ir atoms.High resolution transmission electron microscopy（HRTEM）showed that the Ir NCs are monodispersed with an average size of 0.9 ± 0.2 nm.They have excellent solubility in different polar solvents and are stable in these solvents at least for 6 months.Under the excitation with 365 nm light,they emit strong bluish green fluorescence,and the emission spectra typically depend upon the excitation wavelength.Their fluorescence lifetime was measured to be 2.19 ns and the quantum yield was calculated to be 8.29%.In the presence of the Cr（VI）,the fluorescence of Ir NCs is quenched,and the fluorescence wavelength red-shift gradually with the increasing of Cr（VI）concentration.According to the effect of Cr（VI）on the UV-vis absorbance spectra of Ir NCs and the quenching behavior under different reaction temperature,the quenching mode can be ascribed as static quenching.Based on the quenching action,one selective fluorescence Ir NCs sensor was developed for the determination of Cr（VI）with the linear range from 0.1 to 100 μM and LOD of 25 nM.The emission of Ir NCs is dependent on the excitation wavelength,which is called “red-edge effect”.This phenomenon is closely related to the preparation approach and the microstructure of Ir NCs.The mechanism of the Ir NCs generating red-edge effect was analyzed by comparing the microstructures and fluorescence properties of Ir NCs,gold nanoclusters（Au NCs）,silver nanoclusters（Ag NCs）and copper nanoclusters（Cu NCs）.DMF is decomposed into CO and dimethylamine with reducibility,which can react with Ir3+ to from carbonyl clusters.The electron-rich atoms of N and O can transfer electron to the Ir NCs and form ligand-to-metal charge transfer（LMCT）complex.The LMCT complex can interact with surrounding dipoles and perturb the ground state energy of the Ir NCs and thus the red-edge effect appears.The quenching modes of Cr（VI）on four metal nanoclusters are different due to their different microstructures.Cr（VI）can react with the Ir NCs and form the nonfluorescence ground-state complex,thus Cr（VI）quenches the Ir NCs by static mode.The quenching modes of Cr（VI）on Au NCs,Ag NCs and Cu NCs are ascribe to dynamic quenching.The preparations of Ir NPs and Ir NCs,and their enzyme-like activity,SERS activity and fluorescence property researches promote my understanding of anomalous physical and chemical properties of Ir nanomaterials.Meanwhile,this research broadens the application range of Ir nanomaterials in analytical chemistry filed and establishes the foundation for the further study of Ir nanomaterials.