SERS on Functionalized Silver Nanostructures: Towards the Detection of Single Molecules in Hot Spots

The optical properties of metallic Nanoparticles have a big scientific and technological importance. Such properties are determined by the Localized Surface Plasmons (LSP) the nanoparticles (NPs) support, which strongly depend on the NPs size and shape. The resonant excitation of the LSP leads to enormous enhancement of the electromagnetic field in close proximity of the NPs which originates a very important enhancement in the cross sections of SERS technique (Surface Enhanced Raman Scattering). This is the reason of the high sensitivity of this technique which, together with its ability to provide accurate structural information, makes it suitable to be employed as a molecular sensing technique. The possibility of controlling the LSP, tailoring the NPs morphology, pushes ahead the investigation of new methods of production of metallic NPs. Besides, they can be functionalized to improve their properties and increase their selectivity, consequently enlarging their applications. In general the molecules active in SERS show some affinity for the metal resulting in the necessary approach to the surface. However many other molecules, whose trace detection is of great interest, do not present this affinity and their SERS signal is not detectable. The important environmental contaminants polycyclic aromatic hydrocarbons (PAHs) belong to the last group of molecules. Nonetheless it has been shown that by modifying properly the chemical properties of the metal surface it is possible to augment drastically the approaching of these pollutants to the metallic substrate 1-3 , then their SERS sensing being feasible.