Bioinspired polydopamine (PDA) nanocoating poses one important measure to precisely customize functionalized surfaces, while its dynamic growth characteristics are still not fully understood. Herein, we develop a high-sensitivity and rapid-response fiber-optic modal interferometer (FMI) to operando probe the growth dynamics of PDA nanocoating at the solid–liquid interface. The proposed FMI was treated with HF corrosion to enhance the surface evanescent field, thereby improving the surface refractive index sensitivity. Thus, the FMI successfully monitored the adhesion behavior of the polydopamine coating on the surface of the optical fiber and the subsequent adsorption of biomolecules. Through further analysis of surface evanescent wave field, a model was established to correlate FMI’s interference spectra with PDA nanocoating’s thickness. The accuracy of these results was validated through multiple ex situ characterization and spectroscopic techniques. Finally, the fiber-optic modal interferometer successfully revealed growth kinetic characteristics of the PDA coating induced by CuSO4/H2O2 under diverse operational conditions. This work may advance the evolution of surface-sensitive measurement technologies and surface phenolic chemistry.
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.
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