Raman Spectroscopy Substrate Optimization and Method Development for the Detection and Analysis of Cyclic Organic Molecules and Materials

Organic molecules are ubiquitous species, with approximately 9 million compounds classified as organic molecules, and more being synthesized in laboratories around the world each day. Of these molecules, cyclic organic molecules are especially important in biological and industrial settings. Detection of these molecules is of paramount interest in several fields, including biosensing, environmental, and process monitoring. Additionally, materials are made of cyclic organic molecules, and they can be characterized using a variety of techniques. Raman spectroscopy is an attractive method for both the detection of molecules and characterization of materials. Raman scattering is a phenomenon in which light is inelastically scattered by a molecule upon interaction with monochromatic light. The inelastically scattered photons differ in energy from the incident photons by the magnitude of a molecular vibrational mode frequency. The resulting spectrum is considered a molecular “fingerprint,” since each molecule gives a unique Raman spectrum based on the constituent atoms and bonding environments. This excellent specificity allows Raman scattering to be used for a variety of detection and characterization processes. Raman scattering is an inherently weak phenomenon, but this weakness can be overcome by placing the molecule of interest within 1 – 2 nm of a plasmonic metal surface. The incident light causes the conduction band electrons of the metal to oscillate, which results in an increased electric field at the surface due to the localized surface plasmon resonance (LSPR) effect, which then increases the intensity of the Raman scattered light. The use of these plasmonic metal surfaces for amplified Raman scattering is called surface enhanced Raman spectroscopy (SERS). This surface enhancement allows for trace levels of analyte molecules to be detected.Experiments herein were conducted to detect cortisol at physiological concentrations using silver colloidal nanoparticles. A detection limit of 177 nM was established, which falls at the low end of the physiological concentration. Additionally, cyclic volatile organic compounds were detected in the gas phase using a multi-dimensional SERS substrate, which shows detection of benzenethiol at the parts-per-million level. Lastly, normal Raman spectroscopy is used to determine the surface characteristics and interior character of a series of oxidized polyacrylonitrile-based carbon fibers.