Spectroscopy and Sensing

Abstract

The main part of this chapter describes different techniques for spectro- scopic investigations of localized plasmon resonances in single metal nanopar- ticles, with a view to applications in sensing. The basic principle of single- particle sensors is the exploitation of the fact that the spectral position of their resonances depends on the dielectric environment within the electromagnetic near field. Applied to biological sensing, adsorption of molecules on a func- tionalized metal surface leads to spectral changes of the sustained plasmon modes. Due to the very localized nature and therefore high energy concentra- tion in the near field of surface plasmons, even molecular monolayers can lead to discernible spectral changes. This high sensitivity has allowed surface plas- mon sensors to become established as an analytical sensing technology over the last two decades. The most important challenge encountered in almost any biosensor design is that of ensuring selectivity. In the case of surface-plasmon-based sensors, this is achieved via functionalization of the metallic surface to ensure only se- lective binding of the agent to be sensed. We will not focus on this aspect of sensor design here, but only mention that the surface chemistry of gold de- serves special attention due to the relative ease of establishing sulfur bonds be- tween gold atoms and organic molecules. Therefore, gold has emerged as the metal of choice for almost all practical optical sensing applications, including those based on surface plasmons. An important consequence is that due to the permittivity of gold, sensing is usually limited to the visible and near-infrared part of the spectrum.