Design of Localized Surface Plasmon Resonance-based Fiber Probe for the Detection of Energetic Materials

Abstract

In recent decades, optical sensors have become prominent for their exceptional detection capabilities and diverse applications. Integrating optical sensors with plasmonic, using light characteristics such as absorbance, transmittance, or reflectance, offers a promising approach for developing high-throughput sensors. Traditional spectroscopic techniques often lack sensitivity, but evanescent wave phenomena in optical waveguides provide a compelling alternative. The strength of the evanescent field is affected by factors like the extinction coefficient of an analyte, concentration, interaction area, light wavelength, and refractive indices of the medium and corresponding waveguide. Combining the evanescent wave absorption with the plasmonic resonance behavior of noble metal nanoparticles advances the development of cost-effective, user-friendly, highly efficient, and sensitive devices for field applications. Understanding nanoparticles' mechanisms is crucial, as their localized surface plasmon resonance (LSPR) gets affected by the particle size, shape, and interaction with the surrounding medium. These parameters influence the sensors' resonance frequency and sensitivity, which makes them suitable for various sensing applications. Fiber optics-based evanescent wave absorption spectroscopy (FOEWS) has been explored in the visible and near-infrared spectral regions with various fiber probe designs, like U-bent, tapered, coiled, and straight fibers, enhancing the evanescent wave interactions with absorbing media. Yet, bent geometry is often preferred over straight decladded and tapered fiber sensors due to its higher sensitivity and deeper penetration depth. Bending the fiber reduces the coupling of cladding modes into the core, which minimizes noise and enhances the signal-tonoise ratio. Furthermore, bent fiber sensors are more compact and flexible, making them suitable for applications in limited-space environments. The research focuses on enhancing the parameters of a multi-mode optical fiber by modifying its geometry through a bending process. This method improves penetration depth, mode coupling, reliability, durability, and mechanical stability while offering more precise control than tapering. An optical fiber of a core diameter 200 μm was used to achieve efficient bend radii. The bare fiber probe was experimentally evaluated using two different sucrose solutions with respective refractive indices to assess its sensing capabilities and efficiency. The proposed geometry is functionalized with noble metal nanoparticles, primarily gold, due to their ease of functionalization with various molecules. This enhances sensor specificity and iii enables targeted drug delivery systems. These noble gold nanoparticles (GNPs) are biocompatible, low in toxicity, and suitable for different biomedical applications, including photothermal therapy and imaging. They are stable, easy to synthesize in various shapes and sizes and maintain their properties under different conditions, ensuring reliability and versatility. However, fabricating nano-structured periodic arrays of gold by conventional methods is challenging and costly. An alternative method involving the random immobilization of colloidal nanoparticles on probe surfaces via silanization is highlighted in the research work. This method is economically viable but introduces variability in particle distribution, affecting inter-particle interactions and sensor sensitivity. Systematic studies on the random distribution of nanoparticles are essential to develop guidelines for plasmonic-based optical sensors suitable for large-scale commercial and research applications. Using evanescent wave absorption methods based on fiber optic and attenuated total reflection techniques, different theoretical frameworks were monitored to understand the LSPR spectrum. The immobilization of GNPs onto the exposed fiber core demonstrated a linear absorbance increase at 530 nm, indicating more nanoparticles adhering to the silanized fiber core and providing insights into the binding mechanism. The proposed plasmonic probe effectively facilitates refractive index (RI) measurements of various chemical compounds and monitors thermal changes due to temperature variations. The methodology for developing the plasmonic probe begins with modeling its behaviour using COMSOL Multiphysics to simulate a gold monolayer's surface plasmon resonance (SPR) response. This simulation accounts for the interaction between the gold layer and a chemical compound coating, specifically Potassium Nitrate, to understand the sensing behaviour and coupling phenomenon at the metal-dielectric interface. The sensor demonstrates high sensitivity through shifts in the angle of incidence induced by three different refractive indices of KNO3, varying from 1.33 to 1.56. By observing these shifts in the SPR response based on reflectivity, the simulation provides critical insights into how variations in refractive index within the same compound affect the probe's performance. Once the model's reliability is observed, the functionalized probe undergoes detailed absorption spectroscopy to analyze analyte interactions with the GNP layer. The probe is exposed to five different concentrations of sucrose solution, and changes in absorbance at 530 nm are measured. The observed linear increase in absorbance demonstrates the probe’s high sensitivity, effectively detecting refractive index variations with an approximate change of iv 0.056 RIU. This makes the probe suitable for applications requiring precise monitoring of solution concentrations. The subsequent findings highlight the potential of LSPR-based optical fiber sensors in numerous fields, including bio-chemical sensing and environmental monitoring, where accurate and sensitive detection of refractive index changes is crucial. The research further explores enhancing fiber optic chemical sensing by using porphyrin molecules instead of gold nanoparticles (GNPs). Porphyrin molecules, abundant in nature and known for their unique optical properties, offer a low-cost solution for developing plasmonic sensors, essential for photonic devices. A monolayer of these molecules was applied to the exposed core portion of a U-bent fiber to analyze the specific behavioural patterns induced by various nitrates (potassium nitrate, calcium nitrate, cadmium nitrate, nitric oxide) and chlorides (potassium chloride, calcium chloride, cadmium chloride, hydrochloric acid). The specificity of these compounds was effectively determined using unsupervised learning algorithms like principal component analysis (PCA), which helped differentiate them into two groups based on their acidic characteristics and optical patterns. This optimized fiber probe was also used to develop highly sensitive nanoparticle fiber probes to detect thermal changes within conductive materials. These probes leverage the high surface area-to-volume ratio of nanoparticles, enhancing their responsiveness to minute thermal fluctuations. Their ability to detect exothermic reactions and provide real-time monitoring makes them invaluable for safety and security applications. Time dynamics analysis offers insights into material behaviour, which is crucial for identifying hazardous conditions of explosive reactions. The rapid response and detailed time dynamics analysis of the nanoparticle fiber probe are essential for defence, security, and military applications, enabling quick detection and analysis of explosive materials to enhance operational safety and effectiveness. This study aids the industrial and research-oriented community in optimizing plasmonic interactions for maximum sensitivity and exploiting fiber optic probes to monitor local heating dynamics from metal nanoparticles interacting with light. It covers systematic development, detailed characterization, and practical applications, highlighting the sensors' enhanced sensitivity due to localized surface plasmon resonance (LSPR). These capabilities make the sensors effective for identifying energetic compounds and tracking exothermic reactions in various real-world scenarios.