Raman sensing and imaging by interference and surface enhancement processes

Invited lecture given at the 19th Conference on Dynamical Processes in Excited States of Solids, held in Paris (France) on July 17-22th, 2016.Raman spectroscopy meets the required specificity criterion required for the detection of low concentrations of different agents in complex media, such as protein biomarkers in blood, since the vibrational spectrum of every component is a specific signature that can be used for its identification. Moreover, compared to magnetic resonance, Raman imaging has the ability of visualizing morphological details in cells and tissues on a much higher spatial resolution. Also, no external markers are required, it has a sub-micron resolution and quite good penetration depth. Nevertheless, Raman spectroscopy is strongly limited by its sensitivity. For the last years, a great effort is underway to increase Raman intensity mainly by localized surface plasmon resonance from metal nanoparticles (NPs). These SERS platforms still deals with problems such as the NPs stability, their interaction with the analyte and the adsorption, distribution and arrangement of the probed molecules on the substrate. Another enhancement process is the use of excitation wavelengths in resonant conditions for the sensed molecule, in general using ultra-violet lasers. Our aim is the fabrication of different architectures of SERS substrates that combine different Raman enhancement mechanisms in one multilayered hybrid system based on graphene. Graphene has several roles to play in optical sensing since it provides a biocompatible surface adequate for many organic and bio materials and also a protection of the metal NPs increasing the stability of the system. We will present our approaches to the different Raman enhancement processes. Graphene protected ruthenium ultrathin films with controlled size and shape of the particles present different characteristics of the Rulasmon resonance adequate for UV-SERS. We will show that the intensification mechanism of Raman signal due to constructive interference produced in adequately designed dielectric films is very promising. Also we have explored the limit of ultrasmall Ag nanoparticles (4 nm) to study their interaction with graphene and its SERS amplification capabilities. The results of these approaches pave the way to the design and fabrication of extremely sensitive graphene based bio-compatible platforms by combining the different Raman enhancement processes