Controlled Synthesis And Surface Enhanced Raman Scattering Spectroscopy Application Of AuNR@Ag Nanostructures

The performance of gold nanorod-silver core-shell（AuNR@Ag）nanoparticles is closely related to their size,morphology and composition.During the preparation of AuNR@Ag nanoparticles by seed growth method,surfactants and ligands affect the deposition behavior of silver on the surface of gold nanorods（AuNRs）.In This paper,by utilizing the method of constructing Au-S bonds on the surface of crystal nuclei,the appropriate ligands are selected for asymmetrically modifying the AuNRs end and side to explore the controllable preparation of AuNR@Ag nanoparticles.The AuNR@Ag nanoparticles with different morphology were selected to self-assemble into two-dimensional membrane through the oil-water interface.The Surface Enhanced Raman Scattering（SERS）spectral properties of monolayer and interlayer of nanoparticles two-dimensional films were studied.This research mainly carried out the following three aspects:1.Adding hydrophilic/hydrophobic ligand molecules with calculated amount to colloidal solution of AuNRs stabilized by Cetyl trimethyl ammonium Chloride（CTAC）to construct Au-S bonds on the surface of single-crystal.By using the modified Au NRs as seeds for growth of silver the deposition behavior of silver atoms on AuNRs and the possible mechanism were explored.The result shown that it is difficult to achieve regular growth induced by hydrophobic modification,but hydrophilic modification can obtain regular structure with the resulting core-shell structure morphology has obviously changes.While the hydrophilic molecule thiol end group-methoxy-PEG（HS-PEG）was added at the full modification amount to modify the surface of AuNRs,capsule-shaped AuNR@Ag nanoparticles are obtained;While the hydrophilic moleculeβ-mercaptoethylamine（2-MEA）was added at the full modification amount to modify the surface of AuNRs,the stable octahedral nanostructures were obtained.The use of fully modified AuNRs as seeds achieved the initial regulation of Ag growth on AuNRs.2.Adding the hydrophilic ligand HS-PEG or small thiol molecules to colloidal solution of AuNRs to modify AuNRs asymmetrically according to the theoretical calculation amount required for the end and side modification.It is expected to use the asymmetrically modified AuNRs as seeds to regulate the formation of AuNR@Ag nanoparticles.While using the AuNRs only modified with HS-PEG at end as seeds,two growth modes of Frank-van der Merwe（F-M）and Stranski-Krastanow（S-K）were obtained;the S-K mode becomes more obvious as the molecular weight of HS-PEG increases,and the final UV-vis absorption peak of the core-shell nanoparticles shows a regular blue shift.The Au NRs of asymmetrically modified HS-PEG are used as seeds to achieve effective regulation of the core-shell nanostructures.While HS-PEG and thiols small molecules modifying the ends and sides of AuNRs,regardless of the order of modification,the core-shell nanostructure with a stable（111）crystal plane were obtained.The functional groups and the length of the hydrocarbon chain of the small thiol molecules have unclear effect on the final morphology.The asymmetric modification of AuNRs by HS-PEG and small thiol molecules to achieve the purpose of regulating the morphology of AuNR@Ag is not fully achieved.The mechanism of action of small thiol molecules and the conditions of asymmetric modification of AuNRs by different ligand molecules need further research.3.Making the prepared AuNR@Ag nanoparticles self-assemble into two-dimensional film through the oil-water interface and exploring the SERS enhancement effect of single layer and interlayer structure of AuNR@Ag nanoparticles two-dimensional film for detection of p-mercaptobenzoic acid（4-MBA）.The obtained two-dimensional monolayer and interlayer structure significantly enhanced the SERS signal of 4-MBA.Selecting laser of 785 nm,laser power of 14 mW and exposure time of 1 s for detection.For the detection of higher concentration 10^-4 M 4-MBA,the enhancement factor of AuNRs is similar to the literature results;The detection signal of single-layer film of the AuNR@Ag cube and truncated cube is more than 10 times higher than that of AuNRs;for the detection of lower concentrations of 10^-7 M 4-MBA,the detection signal of the AuNR@Ag truncated cubic single-layer film is almost 8times than that of the cubic film;The SERS signal of the interlayer structure of AuNR@Ag nanoparticles film is about twice as single-layer film.At the same time,the enhancement effect 10^-4 M Rhodamine 6G（R6G）molecule was investigated.The cubic AuNR@Ag nanoparticles film has a better detection effect,and the interlayer structure obtains an approximately 2.5-fold enhanced signal compared to the monolayer film.