A Compact and High-Sensitivity Pb²⁺ Fiber Optic Sensor Based on Hydrogel-Coated Interferometer

Lead pollution, especially in water, threatens human health. Therefore, it is crucial to quickly and accurately determine the content of lead ions (Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup>) in drinking water. In this paper, a highly sensitive Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> optical fiber sensor based on a hydrogel-coated Mach-Zehnder interferometer (MZI) is presented. The hydrogel, crosslinked by hydroxyethyl methacrylate (HEMA), acrylic acid, and acrylamide, exhibits excellent specificity, and its porous structure enhances adsorption, ensuring high detection sensitivity and accuracy. Additionally, the MZI with a refractive index sensitivity of −11235.4 nm/RIU is fabricated by fusing three sections of NCF with a lateral offset structure. Then, the hydrogel is coated onto the misalignment area to complete the sensor. The chemical reaction between HEMA and Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> alters the refractive index of the hydrogel, allowing the MZI to detect Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup>. The sensor has a compact structure with a sensing length of only 0.6 mm. Experimental results show that the proposed Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> sensor has a high detection sensitivity of 4.933×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> nm/(mol/L) and a limit of detection (LOD) of 2.007×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> mol/L (0.4199 ppb). Through comparison, the response of Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> is 3.3 times higher than that of other heavy metal ions, indicating that the sensor has good selectivity. In addition, the sensor has excellent repeatability and can be used as a disposable detection product, ensuring the accuracy of the detection results. The sensor demonstrates significant potential applications for Pb<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> detection.