Preparation Of Two-dimensional Ag-Au Bimetallic Composite Films With Tunable LSPR Properties And Their SERS Properties

Surface-enhanced Raman scattering(SERS),resulted from the local surface plasmon resonance(LSPR)effect,which is derived from the rough surface of nano-metals(mainly Ag,Au,Cu,etc.),have been widely concerned in the fields of analysis and detection.The metal nanomaterials with LSPR effect can be used as SERS active substrates.The LSPR property of metal nanomaterials as active substrate was the key to SERS enhanced effect,and the construction of SERS active substrate with high sensitivity,good stability,reusability and even Raman signal has become an important issue for researchers.Compared with the particle structure,metallic nanofilms with two-dimensional structure can provide metal active sites with high density concentration through the synergistic effect and aggregation effect among particles,and thus significantly improve their optical,electrical and catalytic activities.Moreover,the metallic nanofilms can exhibit the regulated LSPR performance through adjusting the two-dimensional(2D)colloidal crystal structure parameters and the nanometer metal morphology.2D metallic nanofilms are widely used in optical sensing,catalysis,bio-medicine,optical devices,and environmental monitoring.Bimetallic nanostructures with Ag and Au can provide an excellent long-term stability.Moreover,the synergistic effect of plasmon resonance coupling between Ag and Au also endows the bimetallic nanostructures an enhanced LSPR activity.In addition,bovine serum albumin(BSA)has rich disulfide bonds,which can easily adsorb metal nanoparticles and stabilize the nanometal particles.And,their skeleton structure can regulate the morphology and surface properties of metallic nanoparticle and provide a good biocompatible microenvironment Meanwhile,the weak reducing ability of BSA also can regulate the morphological structures of the prepared metallic nanoparticles.Based on the above research background,this thesis proposes a tehnique for construction of bimetallic composite nanofilm based on two-dimensional colloidal crystal templates.In this work,two-dimensional structured Ag nanofilm was obtained by an ion sputtering method,which can easily realize the large-area deposition.Through nanometer Ag is partially converted into Au,a controllable two-dimensional Ag-Au composite bimetallic nanofilm with regulated LSPR performance was obtained by a facile galvanic replacement reaction(GRR).Ag-Au bimetallic composite films with different compositions and complex morphology were prepared by using the weak reductibility of BSA and the stabilization of the skeleton structure.Therefore,we aim to construct SERS active material with high sensitivity,high reusability and high stability.This thesis mainly consists of the following three parts:(1)For constructing a two-dimensional structure with uniform SERS activity,this study proposed a preparation method for the 2D Ag nanostructure to get the regulated LSPR property.In this part,2D polystyrene@polyacrylic acid(PS@PAA)colloidal crystal structure which assembled by gas-liquid interface tehnique,was used as structural template.Then,Ag nanofilm was obtained by Ag ion sputtering on the 2D colloidal crystal structure.The size and the thickness of Ag layer were adjusted by the ion sputtering time.After etching in the presence of H2O2 solution;the optical absorption characteristics of 2D Ag nanofilm was investigated.The results showed that the absorption peak of PS@PAA-Ag nanofilm was red shifted from 423 nm to 438 nm,with the increased size and thickness of Ag layer,In addition,Ag nanoparticle layer etched by H2O2 formed "nanometer cap" and "isolated island"structure.With the increase of H2O2 dosage,the absorption peak of the prepared composite film shifted from 532 nm to 564 nm.(2)In order to construct a 2D structure SERS active substrate with high stability,this study proposes a method for preparing a 2D Ag-Au composite nanofilm.The bimetallic nanofilm was prepared via a galvanic replacement method(GRR)by using Ag nanofilm as a sacrificial template.The various bimetallic nanofilms with different metal contents and surface microstructures were obtained by changing the concentration of the precursor salt solution HAuCl4.The results showed that the increased Au concentration induced the maximum absorption peak of the bimetallic nanofilm obviously red-shifted from 585 to 611 nm.Meanwhile,the optical absorption of Ag-Au nanofilm were also investigated after etching in the presence of H2O2 solution,Ag-Au semishells with porous structure was formed on the surface of the2D colloidal crystal,absorption peak blue-shifted from592 nm to 582 nm.(3)In order to enhance the stability and biocompatibility of 2D Ag-Au bimetallic composite nanofilm,this study proposes a method that adsorbing BSA on the surface of Ag-Au nanofilm to stabilize the bimetallic nanoparticles,which also can regulate the preparation process of Ag-Au nanofilm due to its skeleton structure and weak reducibility.The results showed that the BSA film can enhance hydrophilicity of the Ag-Au bimetallic nanofilm.Compared with unfunctional Ag-Au nanofilm,the existence of BSA layer made the metal structure of Ag-Au nanopartilces transfer from pore to irregular structure.With the increase of BSA concentration,the average size of irregular Ag-Au nanoparticles was also increased.In this study,4-mercaptobenzoic acid(4-MBA)was used as probe molecule to compare the SERS properties of three kinds of the prepared 2D metal composite nanfilm materials as active substrate materials.It was shows that the detection limit of PS-PAA-Ag nanofilm for 4-MBA molecules is 10-8 M,but the stability is not strong.PS@PAA-AgAu bimetallic composite nanofilm has a higher stability,but the sensitivity is lower.PS@PAA-Ag(BSA)Au bimetallic composite film has high sensitivity and good stability.After storage for 20 days,the Raman characteristic peak strength of 4-MBA molecule can still maintain around 91%.This research is focused on the study of the regulated LSPR and SESR properties of Ag-Au bimetallic nanofilms with 2D colloidal crystal structure,which has an important application of optical sensing,catalysis,and photo-thermal conversion.