Dual-MZI sensitivity-enhanced U-shaped microfiber Vernier probe for inflammatory biomarker SAA detection

A sensitivity-enhanced dual Mach–Zehnder interferometer (MZI) fiber-optic biosensing platform for highly sensitive immunoassay detection of the inflammatory biomarker serum amyloid A (SAA) is presented in this study. By integrating a U-shaped microfiber biosensing probe with a double-tapered single-mode fiber amplifier, optical Vernier signal amplification was achieved through dual-MZI superposition. Here, the double-tapered single-mode fiber serves as the reference MZI, while the U-shaped microfiber functions as the sensing MZI. By precisely matching the free spectral ranges of the two MZIs, the optical Vernier effect was generated, significantly enhancing sensor sensitivity. Within the refractive index range of 1.3345–1.3414, the dual-MZI sensor demonstrated a sensitivity of 6752.05 nm/RIU, representing a 4.46-fold improvement over conventional single-MZI configurations. To enable high-sensitivity detection of SAA across varying concentrations, the fiber-optic probe surface was functionalized with specific molecular recognition elements. The proposed platform achieved a limit of detection as low as 2.4 ng/ml for SAA, with a rapid response time of 10 min. This work highlights the potential of dual-MZI Vernier-enhanced fiber-optic probes for biomolecular sensing and provides a promising strategy for point-of-care SAA detection.