Raman Spectroscopic Probing of the Surface Potential of Metal Nanostructures and its Application to Sensors

Abstract

Chapter 1 describes the general introduction of this thesis. Especially, a brief overview of surface-enhanced Raman scattering (SERS) is presented. In chapter 2, a novel fabrication method of gold and silver nanoparticles films on planar and curved surfaces of glass is described. First, branched poly(ethylenimine) (PEI) was discovered to be an efficient agent for the preparation of stabilized Au nanoparticles at room temperature. The PEI-capped Au nanoparticles prepared in aqueous phase were assembled into 2D arrays, simply by the addition of benzenethiol (BT), not only at the aqueous-toluene interface but also onto the glasses including the inner walls of capillary tubes, the dielectric beads, and even the cotton fabrics. The Au-coated film thus created was highly SERS-active, showing intense peaks of BT. BT could be removed from Au while maintaining the initial SERS activity by treating with a borohydride solution. The BT desorbed Au-coated capillary was used in the study of the relative adsorption of various thiols on Au by SERS. Similary to Au, PEI-capped Ag nanoparticles were produced when a mixture of PEI and AgNO3 solutions was heated. A robust 2D Ag film was formed, by the use of BT and toluene, not only on the planar glass but also on the inner surface of the glass capillary. The silver-coated glass capillary was also highly SERS active, showing intense peaks of BT. BT was removable from the Ag surface by borohydride, maintaining the initial SERS activity. Since PEI was a cationic polyelectrolyte, PEI-capped Ag nanoparticle film was usable in quantification of charged dye molecules, such as sulforhodamine B (SRB), and rhodamine-123 (R123) by the help of SERS and metal-enhanced fluorescence (MEF). In chapter 3, the SERS of organic isocyanide or cyanide and their application in chemical sensors are presented. First, 2,6-dimethylphenylisocyanide (2,6-DMPI) was adsorbed onto a PEI-capped Au film, and its Raman spectrum was taken in various conditions. When the solution pH was varied, the NC stretching peak of 2,6-DMPI was shifted sharply around pH 8.5, close to the pKa value of the primary amine of PEI. When a negatively charged poly(acrylic acid) (PAA) was deposited onto the PEI, the NC peak shift occured around pH 6.5, close to the average pKa value of PEI and PAA. When additional PEI was deposited onto the PAA, the peak shift of the NC stretching band occured once again around pH 8.5, indicative of the stronger interaction of upper two polyelectrolyte layers. These observation reflected the fact that the surface potential of Au nanoparticles was alternating by the deposition of PEI and PAA. Consulting the potential-dependent SERS data, the surface potential of Au nanoparticles attained a plateau value, that is, -0.41 V versus a saturated Ag/AgCl electrode, after the deposition of five bilayers of PAA and PEI. In ambient condition, the SERS peak of 2,6-DMPI was also affected by volatile organic compounds (VOCs). The NC stretching peak of 2,6-DMPI on Au was either blue- or red-shifted upon the exposure of VOCs. This could be understood by presuming that VOCs were affecting the surface potential of metal nanoparticles. We demonstrated further that the cyanide anion is a far better adsorbate than are isocyanides for the detection of VOCs. This was because the CN stretching frequency varied by nearly twice that of isocyanide in response to an external electric field. Even hazardous metal ions could then be detected since transition metal ions were able to bind to the pendent nitrogen lone pair of electrons of CN on Au, resulting in the shift of the CN stretching band.