Study On The Surface-enhanced Fluorescence Effect Of Silica-coated Single-particle Core-shell Nanostructures

Compared with traditional colorimetry and spectrophotometry,fluorescence spectroscopy has high selectivity and sensitivity,therefor,it is widely used in life science,material science,food safety,public security information,environmental detection,disease diagnosis and other fields.Many fluorescent substances emit very weak fluorescent signals which are difficult to observe due to high non-radiative decay rate.Investigation on the gene expression,enzyme catalysis,protein synthesis and movement has great significance to life science.Many RNA,DNA and some proteins have their own fluorescence emission,but their non-radiative rate is too high to hardly observed fluorescence signal.Thus how to enhance the emission intensity of fluorescent substances has been a key issue in the current research of fluorescence spectroscopy.Scientific research shows that metal nanoparticles can generate surface plasmon resonance under the illumination of light,which can greatly enhance the local electromagnetic field around the surface of the nanoparticle,thereby enhancing the fluorescent radiation signal.Au-Ag nanoparticles have excellent optical properties,such as LSPR peaks can be well regulated in the range of visible spectrum and near-infrared spectrum by preparing Au-Ag alloy particles with different composition,morphology and distribution.However,in order to obtain a metal-enhanced fluorescence effect,a suitable distance between the fluorescent molecules and the metal surface is required.Owing to this reason,they are widely used in surface enhanced spectroscopy.However,a suitable distance between the fluorescent molecules and the metal surface is required for obtaining a metal-enhanced fluorescence effect.Thus how to achieve precise regulation of metal and molecular spacing,greatly enhance the fluorescence emission intensity and improve the stability and repeatability of fluorescence signals,is a problem that needs to be solved.Genreally fluorescence measurement is based on self-assembly of nanoparticles or metal nano-sol system,which collects signals from a large number of metal particles,but the interaction between particles could affect the stability and repeatability of the measurement.In current thesis fluorescence spectra of single Au-Ag alloy@SiO2-dye and Au-Ag alloy@SiO2@dye particle are collected by using in situ labeling and confocal fluorescent microscopy technique.The effects of different molecular distribution that include adsorption and doping on fluorescence enhancement are investigated.The fluorescence enhancement effect of these systems are also studied with different shell thickness as well as the separation between the metal particle and dye molecules.Moreover,we also have researched the effects of excitation coupling efficiency on fluorescence enhancement.The main research work is as follows:1.Au-Ag alloy nanoparticles are prepared by using method of sodium citrate reduction under condations of 100? in water with stirring vigorously.The Au-Ag alloy nanoparticles with different proportions can be obtained by adjusting the added proportion of silver nitrate and chloroauric acid.Au-Ag alloy nanoparticles were coated with SiO2 by sol-gel method and the silica shell of different thickness was changed by controlling the reaction time and adding different amounts of TEOS.By absorbing fluorescent molecules on the surface of Au-Ag alloy@SiO2 NPs,one obtains Au-Ag alloy@SiO2@dye nanoparticles.2.Au-Ag alloy@SiO2-dye NPs was synthesized successfully by using sol-gel method and one-step embedding method with different fluorescent molecules.In situ labeling technique and confocal fluorescent microscopy were we used to determine location of particle and collect it's in-situ spectra signals,respectively.We investigated the effect of different thickness of silica shell on surface enhanced fluorescence.The result shows that the best fluorescence enhancement is obtained when the SiO2-dye shell is about 15 nm.When the thickness of SiO2-dye shell is about 25 nm,the fluorescence intensity is decreases.Besides,by selecting different fluorescent molecules,the effect of excitation coupling efficiency on surface enhanced fluorescence was also studied.3.In addition,we synthesed Au-Ag alloy@SiO2@dye NPs with a gold-silver ratio of 3:1 and Au-Ag alloy@SiO2@dye NPs with a gold-silver ratio of 1:3.By measuring the single-particle fluorescence intensity of the Au-Ag alloy@SiO2@dye core-shell structure with different ratios of gold and silver,we investigated different degrees of coupling between surface plasmons and excitation light.Moreover the fluorescence enhancement effect with different shell thickness is studied.