Highly sensitive gold nanostar based optical fiber sensor with tunable plasmonic resonance

In this work, we present a detailed numerical and experimental investigation of highly sensitive optical fiber sensors based on localized surface plasmon resonance (LSPR). These sensors are enhanced by the deposition of nanoparticles (NPs) and nanostars (NSs) onto uncladded silica multi-mode optical fiber. The unique optical properties of NSs - featuring a 40 nm gold core surrounded by silver branches of variable size and shape - allows for precise tuning of the LSPR effect. For comparison, we also explored spherical gold NPs with a 40 nm diameter to assess performance differences. Our findings, both numerical and experimental, demonstrate that the LSPR wavelength and sensitivity to surrounding refractive index can be finely tuned by adjusting the morphology of the NS branches. This is achieved by varying the silver nitrate content during their synthesis. Using the Finite Element Method-based design tool we performed simplified study cases, that led to experimental sensitivity of approximately 560 nm/RIU for an LSPR wavelength near 810 nm. As a practical demonstration, the sensor was successfully employed to detect Thiram, a common agricultural pesticide, with a wide dynamic range from 10 pM to 100 µM and an impressive low limit of detection of 0.3 pM. Moreover, we investigated the sensor selectivity, stability and response to environmental temperature changes. This study emphasizes the simplicity, cost-effectiveness, and tunable performance of NS-based optical fiber sensors. By manipulating nanostructure morphology, we can significantly enhance sensor performance, positioning this technology as a highly promising solution for environmental monitoring, biomedical diagnostics, and chemical detection.