The dissertation is devoted to the investigating the effects and mechanisms of enhancing the optical transitions of thymine molecules using SERS and SEIRA spectroscopy methods. It involves the utilization of diverse range of enhancement components including noble metal particles, photonic structures, carbon materials, and their combinations.
The active development of nanotechnologies in recent years has led to significant attention from the scientific community towards the properties of nanoparticles and carbon materials. One of the most actively researched directions remains the utilization of specially designed nanomaterials to enhance the spectral optical signal in analytical methods, particularly in Surface-Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Infrared Absorption (SEIRA). Metallic nanoparticles in these systems generate plasmonic oscillations, contributing to a substantial increase in the electromagnetic field on their surface. This enhanced field interacts with the molecules of the substance, amplifying the intensity of light scattering (in the case of SERS) or the absorption of infrared light (in the case of SEIRA). The acquisition of additional information about the interaction of molecules on the nanoscale, as well as the ability to analyze signals from very small amounts of substances, makes these methods highly powerful for studying physical, chemical, or biological systems at the molecular level.
The dissertation work presents research findings on the mechanisms of enhancing optical transitions of thymine molecules using various amplifying structures.
Firstly, the enhancing properties of gold nanoparticles in the form of nanostars and their composites with graphene nanoflakes as enhancing elements in Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy were experimentally investigated. Considering that the realization of these effects is based on two primary mechanisms - electromagnetic and chemical, a separate analysis was proposed to assess the impact of the size and shape of noble metal nanoparticles on the electromagnetic component of enhancement and the influence of the chemical mechanism by adding carbon nanoparticles to the composite. It has been demonstrated that nanoparticles with a red-shifted infrared plasmonic peak exhibit superior properties for the SEIRA effect. Thymine molecules adsorbed on gold nanostructures show an enhancement absorption coefficient up to 10 times, depending on the type of molecular group. Simultaneously, a significant dependence of the enhancement coefficient on the molecular group of thymine is observed when deposited on graphene complexes. This coefficient reaches 25 times for the N1-H group, indicating the orientation of the thymine molecule on graphene.
Secondly, the potential use of a composite with silver nanoparticles in combination with reduced graphene oxide as an active nanostructure for Surface-Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Infrared Absorption (SEIRA) experiments has been experimentally explored, particularly for the detection of thymine and adenine molecules. It has been shown that signals from samples prepared from solutions with a lower concentration of the test molecule exhibit significantly better enhancement, likely due to a moderate fraction of the analyte material in contact with nanosized silver and graphene inclusions. The maximum achieved enhancement coefficient for adenine is lower than for thymine, attributed to the better suitability of the used red excitation for inducing charge transfer processes from graphene to the LUMO level of the thymine molecule.
Thirdly, the enhancing effect of graphene flakes and photonic structures in Raman spectroscopy has been investigated. To analyze the enhancement mechanism, a dielectric resonant waveguide structure with volume modulation of the refractive index, such as a volume grating based on a polymeric nanocomposite on a glass substrate, was employed. The potential application of both periodic resonant waveguide structures and their combinations with graphene nanoflakes as enhancing substrates for the Surface-Enhanced Raman Spectroscopy (SERS) effect has been experimentally demonstrated. It has been experimentally confirmed that periodic waveguide structures can serve as enhancing surfaces, leading to a significant signal enhancement of adsorbed molecules (up to 102 times).
Keywords: SERS effect, SEIRA effect, infrared absorption enhancement factor, Raman spectroscopy enhancement factor, thymine, metal nanoparticles, carbon nanomaterials, graphene, reduced graphene oxide, resonant photonic structures.
