Gold-Nanoparticle-Based Fiber Optic Sensor for Sensing the Refractive Index of Environmental Solutions

Abstract

Here an optical fiber modified with gold nanoparticles was successfully fabricated to sense the refractive index of a chemical solution surrounding its surface, with the potential of utilizing this sensor as a biosensing device. Gold nanoparticles, with an average diameter ~20 nm, were synthesized via the citrate reduction method and used to functionalize the glass core of an optical fiber. The sensing principle, which is based on localized surface plasmon resonance, requires the metal gold nanoparticles to be exposed to both the incident and absorbing lights, with the potential detection capabilities determined from the observed light intensity measurement. The optical fiber was uncladded at its center to expose the glass core fiber, and gold nanoparticles were immobilized on the exposed surface using a silane coupling agent. The sensitivity of the gold-nanoparticle-modified optical fibers in measuring the refractive index changes of a solution was compared for unclad lengths ranging between 1 and 2 cm. The attenuation of light depended on both the refractive index of solution and the length of the unclad fiber. The results showed that the fiber sensitivity increased as the unclad length increased due to greater amount of gold on the surface. Antibody-A, which has a specific binding to antigen-A, was then covalently linked to the fiber surface via an amide bond to demonstrate the potential bio-sensing platform of this sensor. The antibody-A functionalized optical fiber was used to detect the red blood cell samples in groups A, B, and O, where it effectively detected both the specific and non-specific binding signals. This fiber optic biosensor therefore provides a low-cost and simple fabrication setup that has potential field applications. Moreover, this setup could potentially be applied to detect other types of whole cell samples.