Surface Processes On Gold And Silver Nanoparticles Studied With United Multi-spectral Analysis

Metallic nanoparticles,especially gold or silver nanoparticles,have broad application prospects in many fields due to their unique physical,chemical and biological properties.In situ and real-time probing the surface processes of metallic nanoparticles,especially metallic nanoparticles buried in solutions,is crucial for understanding the various surface processes occured on metal nanoparticles and designing nanoparticles with special functions.With the development of nanoscience and instrument technology,a variety of detection methods have been used for surface study of metallic nanoparticles.Among these methods,second-harmonic generation（SHG）has the characteristics of interface selectivity and sensitivity,thus has been widely used to detect various surface behaviors on colloidal metallic nanoparticles.This dissertation mainly employed SHG and/or two-photon flourescence/luminescence（TPF/TPL）spectroscopies,assisted with some other commonly used characterization methods including ultraviolet-visible absorption spectroscopy（UV-vis）,dynamic light scattering（DLS）,etc.,to probe some surface processes at colloidal gold and silver nanoparticles.This dissertation expanded the application of SHG and two-photon fluorescence technologies in the study of surface processes at metallic nanoparticles.It is also showed that more abundant surface information of nanoparticles could be obtained by the multi-spectral analysis methods.Firstly,the photocatalytic oxidation and reduction processes of silver on the surface of colloidal silver nanoparticles（AgNPs）were probed using the combined analysis of the SHG and TPL spectroscopy,assisted with UV-vis,DLS,and TEM.It was found that the oxidation of silver on the surface of AgNPs was accelerated upon femtosecond laser irradiation,resulting in a decrease in the SHG and TPL emissions from the AgNPs.Under high-power laser irradiation,the photon-induced reduction of oxidized silver on AgNPs and the formation of surface defects were also revealed by the changes in the SHG and TPL signals.Although significant changes in the size and surface morphology of AgNPs were not observed by the DLS and TEM measurements,the verification of a variety of comparative experiments showed that the notable changes of SHG and TPL signals can sensitively detect slight structural changes on the surface of metallic nanoparticles.Then,the interaction between gold nanoparticles（AuNPs）with different sizes and fluorescein isothiocyanate（FITC）was investigated using spectroscopic techniques including fluorescence spectroscopy,TPF and UV-vis spectroscopy.It was found that AuNPs with relatively large sizes quenched the fluorescence of FITC（15 and 37 nm AuNPs quenched～67%and～36%fluorescence of FITC molecules,respectively）,while the smaller AuNPs（1.6 nm）had no such effect.In addition to this difference that may be caused by fluorescence resonance energy transfer,it was also found that the amount of fluorescence quenching by large-sized AuNPs was significantly greater than the amount of FITC molecules chemically adsorbed on the AuNPs（15 and 37 nm AuNPs adsorbed～19%and～11%FITC,respectively）.This phenomenon indicated that aggregation-induced fluorescence quenching also contributed to the attenuation of fluorescence.Based on this observation,a surface curvature-dependent adsorption and aggregation behavior of FITC molecules on AuNPs surface was proposed,and the time-dependent two-photon fluorescence measurement proved this speculation.By revealing the adsorption and aggregation behavior of FITC molecules on the surface of AuNPs,this work demonstrated the feasibility of using two-photon fluorescence and other multispectral combined analysis methods in probing the interaction process between metallic nanoparticles and dye molecules.Finally,the SHG,TPL and UV-vis spectra were employed to study the model reactions of AuNPs catalyzed reduction of 4-nitrophenol and 4-nitrothiophenol molecules by Na BH₄.It was found that the active hydrogen produced by the hydrolysis of Na BH₄ could substantially replace other molecules on the surface of AuNPs,causing the increase in SHG and TPL signals scattered by AuNPs.UV-vis spectroscopy measurement revealed that when Na BH₄ was added after the nitro compound,molecular reaction degree was 100%,there was an induction time of～500 s for the reduction of 4-nitrothiophenol but not for the reduction of4-nitrophenol.The combined multi-spectral analysis and comparative experiments confirmed that the induction time of the catalytic reduction of 4-nitrothiophenol was derived from the inhibition of adsorbed 4-nitrothiophenol on AuNPs to the adsorption of active hydrogen on surface.This study demonstrated the effectiveness of the combined analysis of SHG and TPL spectroscopies in detecting chemical reactions on the surface of metallic nanoparticles.