Preparation Of Ag-Au/Si-NPA And Its Surface Enhanced Raman Scattering Effect

Raman scattering, which is the interaction between the molecules and theincident light, can provide the unique structure information of the molecules, so it iswidely used in many fields. However, the Raman scattering cross section of themolecular is so small that the molecular often has very week Raman signal, and thisrestricts the application of Raman technique in specific fields. Surface EnhancedRaman Scattering (SERS) can largely enhance the Raman signal of the moleculeswhich adsorb on the roughed surface of the nobel metal, which makes it possiblethat Raman technique used in more fields, particularly, in biomedical and singlemolecule detection. The substrate with high sensitivity, powerful ability ofenhancement and stability is the base of application of SERS in all kinds of fields.On account of Ag/Au bimetallic nanostructures conbine the optical enhancedproperties of Ag and the surface stable properties of Au, moreover it can largelyenhance the intensity of Raman,so it attracts more attention how to fabricate theAg/Au bimetallic nanostructure SERS active substrates with regular structure.In this article, with the Silver/Silicon Nanoporous Pillar Arrays (Ag/Si-NPA) astemplate, a novel SERS active substrate with regular structure was fabricated byGalvanic Replacement Reaction (GRR).The SERS active substrates with differentmorphologies were fabricated by controlling the concentration of HAuCl4solutionand reaction time. We compared the intensity of SERS spectrum, which collectedfrom10-9mol/L solution of R6G adsorbed on different substrates, to obtain the mostoptimized fabrication condition. And taking R6G as detection molecular,theenhanced mechanism of the optimized was explained. The SERS properties ofoptimized Ag-Au/Si-NPA active substrate were studied, with Rhodamine6G andRhodamine B were taken as detection molecules. The results indicated thatAg-Au/Si-NPA active substrate,with Ag/Si-NPA as templates，had many excellentcharacters, such as high sensitivity, high ability of enhancement, well stability and the ability of single molecule detection, and so on. In this article, the main researchresults are listed as the following:1. With Ag/Si-NPA as templates, the Ag-Au/Si-NPA active substrates withregular structure were fabricated by galvanic replacement reaction and the fabricatecondition was optimized. The Ag-Au/Si-NPA with different morphologies werefabricated by controlling the concentration of HAuCl4solution and the reaction time.Comparing the sensitivity of SERS spectrum of different substrates adsorbed10-9mol/L R6G solution, we obtained the best fabricate condition, namely, theAg/Si-NPA as template,which were prepared by immersing the24hours agedSi-NPA in10-2mol/L AgNO3for5min, the concentration of the HAuCl4was10-3mol/L, and reaction time was5min. Ag-Au/Si-NPA active substrates wereimmered into the10-1mol/L KCl solution for20min to remove the amorphouscarbon impurity, which adsorbed on the surface of substrate.2. The structure, morphology and element of the optimized Ag-Au/Si-NPAwere characted, and the mechanism of the enhancement was explained. The resultsshowed that the Ag-Au/Si-NPA kept the two kinds of structures of Ag/Si-NPAtemplate，namely，the small nanocrystallities covered over Si-NPA completely andthe large nanoparticles formed the “hand chain” surrounding the Si-NPA. However,the substrate experienced new changes, as the depositing of the Au nanoparticles onthe Ag/Si-NPA, on the surface of “hand chain” appered the protuberance. R6G wasdetected and the mechanism of its enhancement was analyzed. Comparing with theSERS spectrum of R6G adsorbed on the Ag/Si-NPA, the ability of enhancement ofAg-Au/Si-NPA was significantly improved, which was attributed to three reasons:①The depositing of Au nanoparticles on Ag/Si-NPA increased the adsorbedspecific surface aera of molecules;②The electromagnetic field coupling amongthe Au nanoparticles on the surface of the Ag/Si-NPA as well as between Aunanoparticles and Ag nanostructure,which formed the SERS “hot dot”;③Chargetransferring between Ag and Au nanoparticles resulted the electron enriching aroundthe Au nanoparticles, which made the interaction between Ag-Au/Si-NPA activesubstrate and R6G with positive electricity more strong. 3. The surface enhanced Raman scattering effect of Ag-Au/Si-NPA activesubstrate were studied. The results showed that: the Ag-Au/Si-NPA active substratehad high sensitivity. To get the sensitivity of Ag-Au/Si-NPA active substrate, theR6G molecules were detected and the detection concentration limit of R6G was aslow as10-17mol/L.We estimated the enhancement factor of Ag-Au/Si-NPA activesubstrate. As the morphologies of Ag-Au/Si-NPA active subsrates resembling theSi-NPA, so we compared the SERS intendities of R6G adsorbed on Si-NPA andAg-Au/Si-NPA, and the enhacement factor of Ag-Au/Si-NPA substrate was about5107,which was enough to realize the single molecule detection. Usingtwo-analyte technique, we characterized the ability of single molecule detection ofAg-Au/Si-NPA active substrate. Collecting the SERS spectrum of mixture resolutionwith isoconcentration and isovolume of R6G and Rhodamine B, the SERS spectrumindicated three different kinds of spectrum, namely, unique SERS spectrum of R6G,unique SERS spectrum of Rhodamine B and the SERS spectrum of both, whichproved the Ag-Au/Si-NPA active substrate have the power for single moleculedetection. Ag-Au/Si-NPA active substrate had well stability, the SERS spectrumintensity of R6G adsorbed on the Ag-Au/Si-NPA only dropped slightly after wasplaced for60days in air.