Abstract
In this work, we present the design and fabrication of a novel localized surface plasmon resonance (LSPR) chemical sensor based on a D-shaped multimode optical fiber decorated with gold nanoparticles (AuNPs). This sensor offers a simple, low-cost, and efficient approach for detecting low concentrations of chemical analytes with high sensitivity. The D-shaped multimode optical fiber geometry was achieved through a combination of side polishing and chemical etching, resulting in a smooth and precisely controlled sensing region. Scanning electron microscopy (SEM) was used to verify the geometric precision and surface morphology of the polished area. AuNPs with an average diameter of 56 nm were immobilized onto the D-shaped multimode optical fiber sensing region using a laser-assisted plasmon-mediated method. This approach enhanced both the localized surface plasmon resonance (LSPR) response and the sensitivity of the fiber surface. The successful deposition of AuNPs was confirmed by SEM and energy-dispersive X-ray spectroscopy (EDX), which revealed a dense and uniform nanoparticle distribution and confirmed the elemental composition. The dense immobilization of AuNPs induced measurable transmission loss and wavelength shifts, serving as the operational principle of the refractive index-based LSPR sensor. The sensor's performance was evaluated using permethrin, a widely used pesticide from the pyrethroid family, as the target analyte. Experimental results showed that the resonance wavelength shifted proportionally with increasing permethrin concentration in the range of 0 ppm – 3 ppm, achieving a detection limit as low as 0.12 ppm. The demonstrated sensor provides real-time, label-free detection and shows strong potential for practical applications in environmental monitoring, food safety, and agricultural residue analysis. Its compact size, ease of fabrication, and high sensitivity make it a promising candidate for field-deployable chemical sensing platforms.