Sensors for the Detection and Control of Specific Substances for Broad Spectrum Applications: Security, Bio-medical, Agriculture, Quality Control, Water Quality, etc.

The present project aims to develop substrates using spectroscopy enhanced Raman (SERS) that are versatile in presentation-paper, 3D printing, silicon solid supports, metallic Al, Au, Ag, AuAg, in colloidal presentation, etc., and optimized by type of NPs used and towards the molecules to which they are focused, i.e., problems to attack in the areas of health, safety or agri-food, which help to simplify the detection of problem molecules, their precursors and their derivatives, in addition to their own pesticides in the use of regional crops, problems to be attacked in the areas of health, safety or agri-food, which help to simplify the detection of problem molecules, their precursors and their derivatives, as well as pesticides in the use of regional crops, such as chili, lettuce, oranges, corn, etc., which farmers are known to be interested in monitoring due to the economic repercussions they suffer when exceeding the minimum permitted levels, to mention two fields of high interest at regional and national level due to their social and economic repercussions.

Briefly from the theory, Raman scattering consists of inelastic light scattered by molecular systems, which has structure-specific energy and scattering intensity proportional to the density of the molecules under study. Moreover, the fact that this Raman scattering phenomenon can be generated directly in gaseous, solid and liquid samples makes the Raman spectroscopy technique a versatile and non-destructive tool for qualitative and quantitative chemical analysis. However, despite these advantages, the application of Raman spectroscopy in the detection of real problem molecules is limited since normal Raman scattering is intrinsically weak and occurs in only one out of every 106-108 photons scattered. Surface-enhanced Raman scattering is a phenomenon associated with significant amplification of Raman signals from analytes located near the surface of nanostructured materials that enhance or augment the signal.

This enhancement in Raman signals observed by SERS has been attributed mainly to two factors, the electromagnetic and the chemical, the former being widely recognized as the dominant factor. More specifically at this point, the enhancement stemming from the electromagnetic factor arises from the amplification of Raman scattering mediated by localized electromagnetic fields at the surface of the nanostructured material consisting of or containing the substrate (known this coupling as localized surface plasmon resonance, or LSPR). This is achieved by a synergy between the problem molecule attached to or near a metallic nanostructure and a constructive coupling between the frequency of the external laser radiation and the oscillation of the surface electrons of the nanostructures. On the other hand, the chemical factor comes from the charge transfer between the analyte molecule and the substrate enhancer which leads to a change in the polarizability of the molecule, resulting in the enhancement of its Raman scattering. The combination of both enhancement pathways provides us with a significant SERS signal with increased sensitivity that can reach individual molecules.

It is evident from this theoretical description that the most important element of the SERS technique is the substrate and, consequently, the history of SERS has largely been one of the development of signal-enhancing materials, and over the last two decades, with the particular use of substrates with plasmonic nanostructured systems that provide an intense enhancement in electromagnetic response. In general, the inherent plasmonic properties of anisotropic nanoparticles with various shapes or habits (nanostars, nanorods, urchin-like, deposited on graphene oxide, etc.) of Ag and Au, are the most efficient, and are therefore the most widely used nanomaterials for SERS, but they are also the most expensive components of the technique because they are typically single-use.

To finish this part, I will mention that Raman spectroscopy systems use lasers with wavelengths at 520 nm, 630 nm, 785 nm and for more complicated samples that show fluorescence, a laser at 1064 nm is used.