Silver Colliods And Silver Film Preparation And SERS Effect

Surface-Enhanced Raman Spectroscopy (SERS), due to its high sensitivity andlittle interference from water, has served as a valuable tool for those researches,such as surface complexes, surface state of adsorption interface, orientation andconfiguration of biomolecules, analysis of trace organic compounds and drugs, andanalysis of intermediate and final products structures of photochemical reactions.SERS substrate preparation is the most important areas of research in SERS. It hasbecome the pursuit and aim of researchers on finding ways to prepare a highlystable, highly repeatable, highly universal, highly active, simple and inexpensivesubstrate. The main goals in this project are to develop new substrates of nano-silver colloid and film, and to study its properties systematically.Raman spectra of several probe molecules, such as chloramphenicol, methylorange, glycine, glycine anhydride, were computed firstly using quantummechanical ab initio theory and Gaussian’98package. The assignments of Ramanspectrum and SERS of those probe molecules were calculated to obtain informationabout absorption mode and existence forms of probe molecules on surface of nano-silver.Positive and negative nano-silver were prepared and investigated. Sodiumcitrate was used to reduce silver nitrate and obtained the light yellow colored nano-silver colloids; the electrophoresis experiments show that the resulting nano-silveris with positive charge. The absorption spectrum and TEM imaging show that theaverage diameter of the particles is5nm and maximum absorption peaks was398nm for positive nano-silver particles. Tannic acid was used to reduce silver nitrateand obtained the reddish-brown colored nano-silver colloids; the electrophoresisexperiments show that the resulting nano-silver is with negative charge. Theabsorption spectrum and TEM imaging show that the average diameter of theparticles is10nm and maximum absorption peaks is located at418nm. Anionic,cationic, neutral, zwitterionic molecules were used probe molecules to obtain SERSspectrum. The results showed that the positive nano-silver is suitable as a substratefor anionic molecules, and the negative nano-silver is suitable as a substrate forcationic molecules. The negative silver colloids kept at room temperature for two years (for short, old NCS) were compared with the new NCS for absorption spectra,TEM imaging, and SERS spectra. It is found that the NCS, over time, have betterstability and repeatability than the silver colloids prepared by traditional method.The two positive nano-silvers by the same preparation method at different timewere observed by TEM, and it is found that it has good repeatability. Comparedwith the traditionally prepared nano-silver, the positive nano-silver has strongerSERS for probe molecules. The SERS spectrum of glycine anhydride and glycineon positive and negative nano-silver were assigned with the help of the theory ofcalculation of their Raman spectra. The possible adsorption modes of molecules onnano-silver particles were deduced.Nano-silver colloids are prepared by using a microwave heating method. Thecolor of nano-silver changes from light yellow to dark gray when the heat increases.Electrophoresis experiments, absorption spectrum, and electron microscopicobservations show that one can control the nano-silver surface charge, theabsorption peak position, and the morphology of nano-silver by controlling theheating time and heating method,. Most nano-silver prepared by this method has astrong SERS activity. On the nano-silver, Raman spectra of chloramphenicol with adetection limit of10-5M has been obtained.Three kinds of nano-silver colloids were obtained through electrolysis by usingthe mixture solution of polyvinyl alcohol and silver nitrate as electrolyte and themixture solution of sodium citrate and silver nitrate as electrolyte. The resultantlight yellow colored silver colloid turned to brownish yellow after sitting for one ortwo days. Through studying of electrophoresis experiment, absorption spectra, TEMobservation and SERS spectra, it is found that nano-silver surface charge is positiveand absorption peaks are at404,421and434nm (polyvinyl alcohol) respectively.Under electron microscope the nano-silver prepared with polyvinyl alcohol aselectrolyte appeared to be polygon particles, and nano-silver prepared with sodiumcitrate as electrolyte appeared to be spherical particles. Through SERS research, itis found that the nano-silver prepared with the above two types of electrolytes hasstrong SERS activity. Furthermore, the nano-silver colloid that used silver nitratemixing with polyvinyl alcohol solution as electrolyte has the strongest SERSactivity among all the tested molecules. The SERS of methyl orange was obtainedon the nano-silver colloids; it could not have been obtained on the colloids prepared by electrolysis of silver rod using sodium citrate solution and on the silver colloidsprepared by traditional means..A nanoscale silver film was made by electrolysis method, and was studied byscanning electronic microscopy (SEM). The nano-silver particle size is70nm.SERS spectra of chloramphenicol, methyl orange, melamine, viruses, Xanthomonasoryzae and Oryzicola have been obtained using the nanoscale silver film andportable Raman spectrometer. These SERS spectrum have very good stability andrepeatability. The detection limits of Chloramphenicol and methyl orange are in thesame order as using Micro-Raman spectrometer.