Surface Enhanced Raman Mechanism And Trace Analysis Of Uranyl On Silver Substrates

There is an urgent demand for rapid and sensitive analysis of uranyl ion and uranyl complexes in the nuclear emergency field.However,it's difficult for the current analysis methods to provide the content of uranyl ion and the structural information of uranyl complexes simultaneously.Surface-enhanced Raman scattering(SERS)technique is able to reach the single molecule detection and provide the information of molecule structure as well.Hence,SERS may be used for trace analysis of uranyl ion and uranyl complexes.The previous studies about the SERS of uranyl compounds mainly focused on the preparation of Ag substrates with strong electromagnetic enhancement effect to significantly enhance the Raman intensity,and thus to detect uranyl ion in trace level.However,the structural identification of uranyl complexes based on SERS technique was still quite deficient,and SERS mechanisms needed to be further promoted.Most of the reported Ag substrates suffered a poor stability in ambient atmosphere,leading poor reusability.Besides,the capability for quantitative analysis of uranyl ion needed to be further strengthened.Therefore,several Ag-based SERS substrates have been prepared in this thesis,and hence to explain the electromagnetic enhancement and charge-transfer enhancement mechanisms for the uranyl ion adsorbed on the Ag-based substrates.On this basis,the stability and reusability of these substrates,the reproducibility of SERS spectra,the capability for quantitative analysis of uranyl ion,and the relations between the SERS spectra and the structure of uranyl complexes were fully evaluated in this thesis.The main conclusions are as follows.1.The well-distributed silver nanoparticles were prepared through the reduction of silver ions by sodium citrate,and then the uniform and compact SERS substrate was obtained through self-assembly of silver nanoparticles on an APTMS-modified silicon wafer.For uranyl ion adsorbed on this silver substrate,the v1 symmetric stretch vibration frequency of O=U=O shifted from 871 cm-1 to 720 cm-1 and 826 cm-1 were derived from the photo induced charge-transfer and ground state charge-transfer respectively,which was first proposed in this studuy.Besides,the SERS spectra of uranyl ion exhibited an excellent reproducibility,and both the electromagnetic enhancement and charge-transfer enhancement were responsible for the Raman enhancement of uranyl ion.The SERS technique based on this substrate could realize the quantitative analysis of uranyl ion with a detection limit of 10-7 mol·L-1,at the same level as that reported in the previous studies.2.The silver nanorods array was fabricated by electron beam evaporation deposition,and the HfO2 ultrathin film was coated on the silver nanorods array surface by atomic layer deposition.As the HfO2 ultrathin film prevented the charge-transfer process between uranyl ion and the silver substrate,the SERS of uranyl ion on the HfO2-wrapped silver nanorods array was only derived from the electromagnetic enhancement mechanism.Besides,the stability and reusability of the silver nanorods array were improved after wrapped by the HfO2 ultrathin film,and the SERS spectra of uranyl ion showed an excellent reproducibility.The apparent Raman enhancement factor was as high as 106,and the SERS technique based on this substrate could realize the trace analysis of uranyl ion with a detection limit of 3.08×10-8 mol·L-1 which was lower than that reported in the previous literatures.Most importantly,the SERS peaks of different uranyl hydrolysates showed obvious differences,providing references for their structural identification;the SERS peaks of UO2(CO3)34-were quite similar with its normal Raman,hence the approximate in situ SERS of UO2(CO3)34-was achieved on this substrate,which was conducive to its structural identification.Based on the reusable HfO2-wrapped silver nanorods array substrate,this study first proposed a SERS technique that provided the content of uranyl ion and the structural information of uranyl complexes simultaneously,indicating a favor practical application potential.3.The Ag2O aggregates substrate was prepared by the chemical precipitation method.The SERS of uranyl ion on the Ag2O aggregates was derived from the photoinduced charge-transfer enhancement mechanism.Specifically,the laser induced the electrons transfer from the valence band of Ag2O to the LUMO of uranyl ion,resulting in the repulsion of the axial oxygen atom of O=U=O and thus to increase its polarizability and acquire much higher intense Raman mode(the apparent Raman enhancement factor was as high as 105).The Ag2O aggregates possessed an excellent adsorption affinity to uranyl ion,hence the concentration of 10-8 mol·L-1 or even lower can be detected through the full enrichment of uranyl ion on the Ag2O aggregates.Besides,compared with the reported silver-based SERS substrates,the Ag2O aggregates substrate possessed some special advantages in trace analysis of uranyl ion:(1)the Ag2O aggregates substrate was easily to be prepared and exhibited an excellent stability in ambient atmosphere;(2)the Raman background signals of Ag2O aggregates were negligible,improving the analysis accuracy;(3)the SERS technique based on the Ag2O aggregates substrate could selectively detect uranyl ion without the interferences of many other molecules and ions,which was conducive to the trace analysis of uranyl ion in the complex aqueous solutions;(4)the SERS spectra of different uranyl complexes showed significant differences,providing references for the structural identification of uranyl complexes.The photoinduced charge-transfer enhancement mechanism for the uranyl ion adsorbed on the Ag20 aggregates was firstly proposed in this study,which further proved that the photo induced charge-transfer effect was also happened between uranyl ion and the Ag substrate and contributed much to the Raman enhancement of uranyl ion.In this thesis,SERS technique for on-site analysis of uranyl compounds in the nuclear emergency field has been developed for the first time.This method possesses the ability of high sensitivity and rapid analysis,which meets the requirements of analysis techniques in the nuclear emergency field.In addition,SERS studies of other actinyl compounds can also be carried out on this research platform,the results of which may be used for the rapid on-site analysis of actinyl compounds.