Development of Raman Spectroscopic Techniques for the Detection of Biomarkers of Neurological Disease

The goal of the research in this dissertation is to develop detection methods for neurochemicals towards achieving early neurological disease diagnosis. Ideally, the detection methods in physiologically relevant ranges are rapid, selective, label-free, and sensitive. Our approach is Raman spectroscopy, a light scattering technique that provides information about the vibrational modes of a system through interaction with a monochromatic light source. Raman scattering results in a molecular fingerprint for each species, however it is inherently weak. To enhance the Raman signal, we use the localized surface plasmon resonance (LSPR), which arises from the collective oscillation of surface conduction electrons of a metal nanoparticle when excited with a laser, resulting in an enhanced electric field. When a molecule is adsorbed on the nanoparticle surface, the excited Raman scattering is enhanced by the LSPR, and referred to as surface-enhanced Raman spectroscopy (SERS). SERS permits the detection of low concentration analytes. Here we discuss optimization of SERS substrates and excitation wavelength for low concentration detection of neurochemicals. This comprehensive study involved detection of neurotransmitters on both silver and gold nanoparticles at 3 different excitation wavelengths to determine ideal conditions for detection.SERS can also be combined with spatially offset Raman spectroscopy (SORS), termed SESORS, to detect low concentration analytes in subsurface layers of diffusely scattering media. SORS takes advantage of the spatial component of light in which the photons that travel deeper into the material traverse laterally before being scattered out at the surface. By collecting the Raman scattered light at a point that is spatially offset from the incident illumination point, signal can be obtained from the deep layers. SESORS was used to detect neurotransmitters in a brain tissue mimic through an animal skull. Initial experiments involved optimization of the volume and order of the addition of the SERS probe as well as determining the optimal offset needed for the skull. These experiments were followed by finding limits of detection of various neurotransmitters through the skull including melatonin, serotonin, dopamine, epinephrine, and norepinephrine, as well as determining the feasibility of this technique for an in vivo detection technique using a mouse model.

Portions of this document were previously published in Analytical Chemistry and ACS Chemical Neuroscience.