Lab-in-a-fiber for ultrasensitive Hg2+ detection

Accurate detection of molecules at ultra-low concentrations is essential for applications such as environmental monitoring, early-stage diagnostics, and precision medicine. Among various sensing platforms, optical fiber and waveguide-based systems are particularly valuable due to their immunity to electromagnetic interference, absence of electrical hazards, flexibility, and low attenuation over long distances. Lab-on-a-fiber (LoF) integrates sensing elements directly onto optical fibers, transforming them into versatile platforms for diverse sensing applications. However, LoF faces limitations in sensitivity due to the reliance on surface-adsorbed analytes and weak interactions with evanescent waves. Lab-in-a-fiber (LiF) sensors improve sensitivity by modifying the fiber’s geometry to create microfluidic channels, but they are hindered by significant optical losses and structural complexity. To overcome these challenges, we propose a novel LiF sensor by coaxially grafting a hydrogel waveguide with optical fibers (LiF-hydrogel). This hybrid structure combines the advantages of conventional waveguides with the infiltrability properties of hydrogels, allowing analyte molecules to penetrate the waveguide and enhancing sensitivity. This design achieves an exceptionally low detection limit for the fluorescent spectrum of rhodamine B (Rh B), reaching approximately 2.0 × 10 −20 M, equivalent to the detection of only 4–6 individual Rh B molecules. Furthermore, by embedding Rh B-lactam-ethylenediamine within the hydrogel waveguide, the LiF-hydrogel can selectively detect mercury (Hg 2+ ) ions with a detection limit of approximately 1.0 × 10 −13 M in aqueous solutions, underscoring its remarkable sensitivity.