Highly sensitive and tunable fiber optic biosensors exploiting localized surface plasmon resonance of gold nanostars

The main objective of this thesis is the design and fabrication of fiber optic biosensors based on Localized Surface Plasmon Resonance (LSPR) using gold nanostructures. When a noble metallic nanostructure is illuminated by an electromagnetic wave, the free electrons within the nanostructure begin to oscillate collectively with the incident EM field. The oscillation domain extends beyond the nanostructure dimensions, leading to the accumulation of charge at its surface and generating a strong localized electric field known as surface plasmon resonance (SPR). In nanostructures smaller than the wavelength of the incident light, this effect becomes localized referred to as LSPR. As a result, a portion of the incident electromagnetic spectrum is absorbed by the nanostructure. For noble metals such as gold and silver, this resonance typically occurs in the visible range. The position and intensity of the LSPR absorption band depend strongly on the size, shape, and material composition of the nanostructure, as well as on the surrounding refractive index (SRI). The SRI sensitivity of LSPR is a crucial property for biosensing applications. When biological components are immobilized onto the sensing surface, they alter the local refractive index near the nanostructure. This change leads to a measurable shift in the LSPR resonance wavelength or intensity, providing a label-free means of detection. Due to their excellent SRI sensitivity and chemical compatibility, gold nanostructures are particularly suitable for bioconjugation with antibodies, enabling the development of highly specific and selective biosensors for a variety of biological and chemical targets. In parallel, optical fibers offer a powerful platform for sensing applications because of their flexibility, compactness, and immunity to electromagnetic interference. They operate with low-power light sources and can be easily integrated into different environments. These features, combined with the high signal-to-noise ratio of optical measurements, allow fiber optic sensors to achieve an ultra-low limit of detection (LOD) and high sensitivity. Therefore, the integration of gold nanostructures with optical fibers represents a promising approach for developing advanced LSPR-based fiber optic biosensors. In this context, the thesis presents a comprehensive study on the design, simulation, fabrication, and characterization of fiber optic LSPR sensors optimized for biological detection. The initial chapters provide the theoretical background of both SPR and LSPR, along with examples of their implementation in bulk and fiber-optic configurations. Following this, numerical simulations were conducted to study the effect of the SRI on the resonance band of spherical nanoparticles (NPs) and to evaluate their SRI sensitivity. Further investigations were performed to examine the influence of particle size, shape, and aggregation state on the plasmonic response. For optimization purposes, gold nanostars (NSs) were also simulated to explore how geometrical parameters affect the LSPR resonance band and sensitivity. The next part of the work focuses on the experimental design of the fiber optic transducer. An uncladded multimode optical fiber was used to achieve evanescent wave interaction between the guided light and the surrounding medium. NPs and NSs were synthesized and deposited onto the functionalized fiber surface. The LSPR bands of these transducers were characterized in reflection mode and compared with the UV-Vis spectra of nanoparticles in solution, as well as with SEM and TEM images to confirm morphology and size distribution. The SRI sensitivity of the sensor was then calculated and analyzed. As a preliminary step toward biosensing applications, Thiram pesticide, which is a chemical compound known to interact naturally with gold nanostructures, was selected as a model analyte. The corresponding sensitivity and LOD were evaluated to estimate the biosensing performance of the proposed transducer. Subsequently, the sensing platform was biofunctionalized with specific antibodies for cortisol and ochratoxin A (OTA) to achieve selective and specific biosensing capabilities. These experiments were conducted at the University of Aveiro during a research exchange period, which significantly broadened my knowledge and experimental skills in the field of fiber optic biosensing. Finally, an additional study was carried out to enhance the SRI and biosensing sensitivity of Tilted Fiber Bragg Grating (TFBG) sensors through the incorporation of gold nanostructures. The modified TFBG sensors were applied for the detection of glyphosate, demonstrating good selectivity, repeatability, and potential for environmental monitoring applications. Overall, this thesis provides both theoretical and experimental insights into the development of plasmonic fiber optic biosensors. By combining the optical advantages of fiber platforms with the plasmonic properties of gold nanostructures, the presented work contributes to advancing high-performance, label-free sensors for biological and chemical detection, with promising applications in environmental monitoring, medical diagnostics, and food safety.