The Stability Of Gold Nanowires And Their Application To Surface Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption spectroscopy(SEIRA),as a kind of highly sensitive detection technology of surface enhanced vibration spectrum,through combining the fingerprint of the infrared spectroscopy and surface enhanced means to improve the signal strength,is among the category of highly sensitive and highly specific analytical methods.Surface-enhanced vibration spectroscopy mainly includes surface-enhanced infrared absorption spectra and surface-enhanced Raman spectroscopy.Two different mechanisms are supposed to contribute to the total surface enhancement,that is,an electromagnetic and a chemical effect,with the electromagnetic field enhancement effects as the dominate one.Raman spectra signal enhancement is directly proportional to the fourth power of the surrounding electromagnetic field.Meanwhile,the signal enhancement of the infrared absorption spectrum is proportional to the square of the environmental electromagnetic field intensity.Because of the difference in the order of magnitude for the electromagnetic field enhancement,the surface enhanced Raman spectroscopy has already achieved single molecular level detection in the 1990s,while the surface enhanced infrared absorption spectra can only achieve the single monolayer level detection by means of specially designed spectroscopy,such as near field spectroscopy,up to the moment of writing this thesis.The surface enhanced infrared spectra can be divided into resonant and non-resonant surface enhanced infrared spectra,based upon the signal overlap between the substrate and the vibrational mode.And the resonant surface enhanced infrared spectrum is expected to further increase the enhancement factor of infrared absorption.The existing techniques have mainly focused on top-down methods,such as lithography,to extend the electromagnetic field of substrate into infrared region.The advantage of this method is that it can accurately control the structure,morphology and repeating units of the substrate,but its further development is impeded by the high cost,complex preparation and limited tunability.In this thesis,one-dimensional gold nanowires with infrared surface plasmon resonance were prepared by the bottom-up method through colloidal preparation.In addition,by using oleylamine,which is the surface ligand of gold nanowires in the preparation process,as the probe,we successfully observed the SEIRA phenomenon.The localized infrared absorption enhancement reached to three orders of magnitude.This technique has advantages of low cost,simple process,facile tuning on surface plasmon resonance and decent enhancement factor.The research are summarized as following:One-dimensional gold nanowires are a kind of emerging nanomaterials with unique molecular structures and optoelectrical properties.They have been widely using in a broad spectrum of fields,such as electrochemical,biosensing,and chemical catalysis applications.In Chapter 2,we synthesized one-dimensional gold nanowire materials with different lengths and discussed the quantitative relationship between the length of the prepared gold nanowires and the added reactants.Transmission Electron Microscopy(TEM)analysis shows that the length of gold nanowires varied in a reasonable range,and their diameter was about 1.6 nm.The spectral analysis shows that the gold nanowires have two surface plasmon resonance absorption peaks,the transverse resonance absorption peak is located at around 500 nm,and it remains almost constant,while the longitudinal resonance absorption peak is tunable in infrared region,and the corresponding resonance peak position is positively correlated with the length of the gold nanowires.During the exploration of the stability of the gold nanowires,it was found that the gold nanowires can maintain colloidal stability in the solution,however,if the oleylamine ligand on its surface was replaced by the thiol-based compound,the gold nanowires would break apart,indicating that its stability needs to be improved and warrants further studies.The one-dimensional gold nanowires can be used as a substrate for resonant SEIRA because of the tunable longitudinal surface plasma resonance in the infrared region.In chapter 3,we took out the reaction liquid during the preparation process of gold nanowires,with oleylamine as the signal molecule,enhanced infrared absorption from oleylamine molecules was successfully observed accompanying with the gold nanowire growth.On the ensemble level,the enhancement factor reaches around 27,and this value is actually at the level of around 4.8×103,since only localized ligands directly conjugated to the gold nano wires surface are actually influenced by the highly localized electromagnetic field during the infrared absorption.In addition,through a series of control experiments,we ruled out potential artifacts,such as the variation of oleylamine concentration during synthesis.In conclusion,we confirmed that the enhancement of the infrared signal is induced by the resonant surface plasmon resonance(SPR)and corresponding localized electromagnetic field.