Sensitive and selective Cd2+ sensing with a hydrogel-functionalized Fabry-Pérot fiber sensor

Achieving highly sensitive detection of ultra-low concentration cadmium ions (Cd²⁺) in water remains a critical challenge. This work proposes a novel fiber-optic Cd²⁺ sensor by integrating a Cd²⁺-responsive hydrogel with a refractive-index-sensitive Fabry-Pérot (F-P) interferometer fabricated via large-offset fusion splicing. The hydrogel, synthesized from allylthiourea/acrylic acid/acrylamide monomers, exhibits exceptional specificity and adsorption toward Cd²⁺ while forming a robust bond with the F-P fiber interferometer. The sensor demonstrates ultrahigh sensitivity of 8.58 × 10⁷ nm/(mol/L) and a detection limit of 9.435 × 10⁻¹⁰ mol/L (0.1057 ppb)—two orders of magnitude below the World Health Organization (WHO) drinking water standard. The sensor also shows excellent selectivity (least 4.17 times higher response to Cd²⁺ than Cd²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Pb²⁺, Ca²⁺, Mg²⁺ and soybean whey protein), stability (0.027 nm spectral drift deviation), and consistency (2.393 % sensitivity deviation across sensors). Thus, the proposed sensor provides a reliable solution for in situ monitoring of trace Cd²⁺ in environmental waters, showing significant potential for environmental monitoring applications.