Surface-Enhanced Fluorescence Effect On Assembled Structure Of Silver Nanoparticles

Fluorescence detection is a useful technique for biological sciences, biotechnology and medical diagnostics. In all these applications there is a need for increased sensitivity to allow detection of a small number of target molecules. These methods include surface-enhanced fluorescence on metal nanostructures (SEF). And more and more attention has been attracted to metallic nanoscale particles. How to develop sensitive detection techniques with these metal particles is of high interest. Their origin is attributed to the collective oscillation of the free conduction electrons induced by an interacting electromagnetic field. These resonances are also denoted as surface plasmons.We now know that the fluorescence can be enhanced to (10-103)-fold by localizing fluorophore near metal nanoparticles, and the enhancement is suggested to occur via a coupling interaction of the fluorophore and the electric field around the metal particle, which can be induced by incident light.The main research works and the conclusions are as follows:Homogeneous silver nanoparticles were prepared by reducing saturated silver oxide solution with hydrogen gas. By self-assembled onto the polyvinyl-pyridine modified glass slide, monolayer silver nanoparticle arrays were obtained. UV-visiable absorption spectra of the assemblies demonstrated that the interaction between silver nanoparticles resulted in large shifts of dipolar surface plasmon resonance, whereas only slight effect was observed for quadrupolar surface plasmon resonance. It illustrates there may exist somewhat close relativity between enhancement of electrict field in nearby particles and dipolar interaction. Fluorescence intensity could be affected by the diameter of the silver nanoparticles, which has significant quadrupolar surface plasmon resonance in the absorption spectra, SEF could correspond be gotten on the silver nanoparticles with larger diameter. To observe large electromagnetic(EM) enhancement of fluorescence it is desirable to excite the Rodamine B by light the energy of which is close to surface plasmon resonance from silver absorption spectra, the optimum excite wavelength in our experiments is 470nm. Fluorescence intensity is also distinctly with the assembly density of the silver nanoparticles. When the assembly density of the silver nanoparticles is small, that is the distance of the silver nanoaparticles is large, it is hard for the silver nanoparticles to produce strong electromagnetic coupling; however when the assembly density of the silver nanoparticles is over large, the silver nanoparticles becomes aggregating, which results in large probability of collision between the silver nanoparticles and the fluorescence molecules. At the same time the aggregation of the silver nanoparticles could ward off part of the fluorescence, so SEF could not be produced in this situation. Based on the experiments, we could get the obvious SEF on the assembly structure of large silver nanparticles with the assembly density of 1.00.In addition fluorescence intensity could be affected by the distance between the fluorescent molecule and the silver nanoparticles. To get different distance between the fluorescent molecule and the silver nanoparticles, the mercaptans molecules were adsorbed on the silver nanoparticles. The study found that the fluorescence intensity increased once again when the n-Dodecanethiol molecule was adsorbed on the silver nanoparticles assembly with assembly density of 0.50.We study the SEF of the Rodamine 6G in the same system to verify the impact of factors of surface-enhanced fluorescence: the assembly density of the silver nanoparticles and the distance of the assembly structure of silver nanoparticles and the fluorescence molecule.