Non-Uniform SERS Substrates to Three-Dimensional SERS Platforms: Optimizing Raman Spectroscopy for Advanced Applications

Abstract

Raman spectroscopy is a valuable technique for analyzing materials and their properties. However, due to the small scattering cross-section of many analytes, only weak signals with low signal-to-noise ratios (SNRs) are obtained. Researchers have explored various methodologies to overcome these limitations and enhance the power of this technique. Plasmonics, particularly Surface-Enhanced Raman Spectroscopy (SERS), has garnered global attention as a means to overcome weak Raman intensity. However, SERS has limitations and constraints, such as complex sample preparation and high costs, that must be overcome for broader applicability. This dissertation demonstrates that gradient two-dimensional (2D) SERS substrates with multiple resonances improve the analysis of numerous samples. Such substrates enable fast screening of various molecules. With conventional 2D substrates, the signal from molecules is only enhanced when they are very close (a few nanometers) to the substrate surface. To overcome this limitation, innovative, stable, and three-dimensional (3D) SERS substrates based on spherical silica microspheres (SMPs) are developed as carriers for metal nanoparticles (NPs). These novel substrates offer the advantage of allowing analyses in all spatial directions, showing multiple resonances, being cost-effectively synthesized, and being used in various solutions. They address some of the limitations of conventional SERS substrates. Furthermore, this doctoral thesis deals with advanced data analysis in SERS and Raman spectroscopy, which can lead to difficulties interpreting the data. Advanced data analysis is applied to identify subtle spectral differences and improve data interpretation. However, optimizing and maintaining spectrometers' performance and calibration stability over time is essential for reliable data interpretation. For this purpose, a quality factor is introduced, which represents the fit between theoretically calculated and experimentally measured spectral resolution (SR). This is applied to a commercial spectrometer used in the dissertation. In summary, this work addresses central challenges and weaknesses in Raman spectroscopy, improving its applicability and addressing the associated challenges.