Heavy Metal Ion Sensing Based on a Balloon-Shaped Optical Fiber Interferometric Structure

Abstract

Mercury ions (Hg2), among the most toxic heavy-metal species, pose serious threats to ecosystems and human health, making their detection in aqueous environments of considerable importance. In this work, a balloon-shaped optical fiber intermodal interferometer functionalized with a layer-by-layer assembled chitosan/polyacrylic acid (CS/PAA) multilayer film is proposed for Hg2+ detection. To the best of our knowledge, this is the first integration of a CS/PAA multilayer with a balloon-shaped fiber interferometer for Hg2+ sensing. The balloon-shaped bending structure enhances core-cladding mode coupling and the interaction of the evanescent field with the sensitive film, while the amino and carboxyl groups of CS/PAA provide binding sites for Hg2+. After optimization, the coated sensor achieved a refractive-index sensitivity of 690.46 nm/RIU, compared with 298.50 nm/RIU for the bare sensor. Hg2+ adsorption changes the refractive index of the film and is converted into a measurable interference-dip wavelength shift. For Hg2+ detection, a linear response was obtained over 100- $800~\mu $ M with a wavelength sensitivity of 0.02723 nm/ $\mu $ M and a low temperature cross-sensitivity of 0.013 nm/°C. The sensor also showed a markedly stronger response to Hg2+ than to Cu2+, K+, Na+, Ag+, and Fe3+ and could be regenerated for repeated measurements. The integration of selective ion recognition, interferometric wavelength interrogation, and reversible regeneration provides a simple and reusable fiber-optic platform for Hg2+ monitoring in aqueous environments.