Simple and Efficient Fabrication of Dimers of Metal Colloidal Particles for Surface-Enhanced Raman Scattering and their Applications

Abstract

Since the advent of surface-enhanced Raman scattering (SERS), researchers have been trying to fabricate highly sensitive, easy to make and stable SERS substrates. This thesis introduces a simple and efficient method to fabricate SERS substrates consisting mainly of dimers of Ag or Au colloidal particles by controlling the surface charge of the particles. The surface charge was controlled from the pristine colloidal solutions by centrifuging. The SERS substrates were fabricated by a three-step immobilization method as follows: immobilization of the colloidal particles, adsorption of target molecules on the immobilized colloidal particles, and second immobilization. The morphology of the SERS substrates was examined by scanning electron spectroscopy and characterized by UV-vis spectroscopy. For the optimized SERS substrates based on Ag sols, the average enhancement factor was calculated to be ~6.1×107. The size effect of colloidal silver particles to SERS enhancements was studied. The silver nanoparticles (NPs) were synthesized through a seed-mediated method. Silver seeds whose size was ranged from approximately 5 to 10 nm were prepared by adding a sodium borohydride solution into a silver nitrate solution containing sodium citrate. The isotropic growth of Ag seed NPs was initiated by adding growth solutions containing appropriate amounts of silver nitrate and sodium ascorbate as a reducing agent into the seed solution. By adjusting the volume ratio of the seed solution to the growth solution, the size of silver NPs was controlled. Using this method, four silver colloidal solutions containing silver NPs whose average diameters were 21, 25, 28, and 31 nm were prepared. The size and shape of silver NPs were examined by transmission electron microscopy. The surface plasmon absorption band due to the longitudinal mode was red-shifted with increasing the average diameter of Ag NPs. The SERS intensity measured by excitation with 514.5 nm laser light was critically affected by the diameter of Ag NPs. The highest SERS intensity was measured from the substrate whose absorption maximum due to the longitudinal mode was near 519 nm. The SERS substrate made of 28 nm Ag NPs showed higher enhancement than that made of 21, 25 or 31 nm Ag NPs. The percentage of intensity contributed by the molecules located at the junctions to the total SERS intensity was calculated to be ~85% from the enhancement difference between two kinds of substrates prepared by three- and four-step methods. The four-step method is as follow: immobilization of the colloidal particles, adsorption of aniline on the immobilized nanoparticles, second immobilization, and adsorption of target molecules on the nanoparticles. The enhancement at the junctions, which acted as the hotspots, was found to be ~3×109. Furthermore, the substrate prepared by adsorbing aniline (four-step method) worked as a universal SERS substrate. When target molecular solutions (adenosine, guanine, glycine, rhodamine 6G, benzoic acid, 4-aminothiophenol) were dropped in arrays on the substrate and then dried, a good SERS spectrum was observed from each spot, with an enhancement factor of ~9.1×106.