Design and Analysis of Evanescent-Wave Based D-Shaped Fiber-Optic Photonic Sensing Device for Pathogen Detection

Abstract

Optical sensors have emerged as transformative tools in sensing technology, offering good detection sensitivity and adaptability across diverse applications. The integration of plasmonic elements with optical waveguides has unlocked new frontiers in high-throughput optical sensing, leveraging properties such as absorbance, transmittance, and reflectance. While conventional spectroscopic techniques often face challenges like limited applicable sensitivity and impracticality for field applications, evanescent wave-based optical sensors provide a compelling alternative. The efficacy of evanescent wave absorption is governed by factors including the analyte's extinction coefficient, concentration, interaction area, wavelength of light, and the refractive indices of the medium and waveguide. By coupling evanescent wave absorption with the localized surface plasmon resonance (LSPR) effect of noble metal nanoparticles, researchers are developing cost-effective, user-friendly, and highly sensitive sensors tailored for field deployment. This study focuses on fiber optics-based evanescent wave sensors (FOS), emphasizing D-shaped optical fibers. This geometry enhances interaction between the evanescent field and analytes, enabling deeper field penetration, minimizing noise, and optimizing the signal-to-noise ratio. Gold nanoparticles (AuNPs), chosen for their biocompatibility, low toxicity, and ease of functionalization, play a pivotal role in advancing sensor performance. By leveraging these nanoparticles' size- and shape-dependent optical properties, this research work demonstrates the development of a robust LSPR sensing platform for agricultural applications. The proposed sensor integrates a D-shaped fiber optic geometry functionalized with AuNPs using a cost-effective silanization-based nanoparticle immobilization process. This method achieves uniform nanoparticle distribution, ensuring enhanced plasmonic interactions and reproducible sensor performance. A systematic investigation of nanoparticle distribution and binding dynamics confirmed an optimized LSPR response at 540 nm, demonstrating high sensitivity to refractive index changes. For bare probes, the refractive index sensitivity was evaluated using sucrose solutions, yielding a linear correlation between sensor area and sensitivity. The functionalization with amine groups and subsequent AuNP immobilization further improved detection sensitivity, achieving a limit of detection (LOD) of 0.118 ± 0.047 × 10⁻³ RIU with a sensitivity of 1.025 ΔA/Δn. The sensor's biosensing capabilities were validated using immunoglobulin antibody (HIgG) detection, achieving an LOD of 0.6 μg/mL through antigen-antibody conjugation. Further, extended its application to plant pathogen detection, the sensor was adapted for Begomovirus DNA sensing by immobilizing complementary DNA (cDNA) probes on the AuNP-functionalized surface. ii The dynamic LSPR absorption response confirmed successful hybridization with single-stranded viral DNA (ssDNA) extracted from Tomato Leaf Curl New Delhi Virus-infected leaves. This sensor achieved an LOD of 85 ± 5 ng/μL, demonstrating comparable sensitivity and specificity with other standard detection such as polymerase chain reaction and enzyme-linked immunosorbent assay. This research work demonstrated the versatility and potential of D-shaped fiber optic LSPR sensors for real-time, on-site detection of plant pathogens. By optimizing probe geometry and functionalization techniques, the developed sensor achieves remarkable sensitivity and selectivity, making it a valuable tool for agricultural disease management. The conceptualized optical sensing platform, based on digital electronic logic states, further enhances the sensor's utility for versatile diagnostic systems. Beyond agriculture, this technology shows promise for broader applications, including environmental monitoring and healthcare diagnostics. The findings presented herein offer a robust proof-of-concept for advanced optical sensing technologies, paving the way for scalable and cost-effective solutions to address challenges in agriculture. By redesigning D-shaped fiber arrays and optimizing plasmonic interactions, this research established a benchmark for the development of next-generation optical biosensors for plant pathogen detection. The demonstrated ability to detect various genera of begomo-viruses and the potential for multi-target sensing highlight its profound impact on improving crop yield and disease control strategies, ensuring a significant contribution to the global agricultural production landscape.