Functionalized Reflective Structure Fiber-Optic Interferometric Sensor for Trace Detection of Lead Ions

Lead ions (Pb²⁺) are among the most prevalent toxic heavy-metal pollutants in daily human life, particularly in children and pregnant women. Although atomic absorption spectroscopy is the most commonly used method owing to its accuracy and reliability, it requires complex sample preparation and expensive equipment. Therefore, efficient detection of Pb²⁺ is currently the focus of optical sensing research. In this study, we develop a reflective fiber-optic interferometric sensor to detect trace levels of lead ions. The sensor is composed of a single-mode fiber, no-core fiber (NCF), and thin-core fiber (TCF). When light from the broadband light source is transmitted to the sensor via ports 1 and 2 of the fiber optic circulator, the light diverges and propagates forward in the NCF. Owing to the fiber-core mismatch of different optical fibers, the beams can excite the core and cladding modes in the TCF. When the beams are reflected back into the NCF, the core and cladding modes can effectively interfere in the NCF due to their optical path differences. Subsequently, the light signal is recorded by an optical spectrum analyzer through port 3 of the circulator. The TCF’s cladding is partially etched and coated with a functionalized hydrogel-sensing film made of 2-hydroxyethyl methacrylate (2-HEMA) as the recognition monomer. The oxygen atoms in the 2-HEMA are specifically matched with Pb²⁺ to form “-O-Pb-O-” cross-linked structures. Therefore, the absorption of Pb²⁺ by the hydrogel can change the effective refractive index of a new cladding of the TCF, formed by the sensing film and the TCF’s original cladding, thereby the Pb²⁺ concentration is detected by the change of the optical signal. Owing to the trace levels of the detected Pb²⁺ in aqueous solutions (in the ppt range), we employ an equation system to eliminate temperature interference and ensure accurate detection results under environmental temperature fluctuations. Additionally, for the same sensing length, the concentration sensitivity of fiber-optic sensors with reflective structures is twice that of the transmission structures, and the reflective structure is convenient for real-time remote detection. The experimental results show that the optimal sensitivity of the sensor is 1.926 × 10⁹ nm∙mol⁻¹ ∙L, and its detection limit can reach 2.0 × 10⁻¹¹ mol∙L⁻¹ (4.14 ppt, 1 ng∙L⁻¹ = 1 ppt), which is far lower than the standard (10 ppb, 1 μg∙L⁻¹ = 1 ppb) set by the World Health Organization. Moreover, the sensor exhibits good stability, specificity, and a wide detection range. Consequently, the designed reflective fiber optic sensor can provide broad prospects for environmental and human health monitoring.