Highly sensitive detection of trace lead ions concentration based on a functional film-enhanced optical microfiber sensor

An optical fiber sensor for detecting the concentration of trace lead ions is proposed using a polydopamine-maleic acid (PDA-MA) functional membrane coated on the surface of a Mach-Zehnder microfiber interferometer. The PDA-MA functional film is wrapped by a one-step immersion method for in-situ growth on the interferometer surface. The microfiber surface has an excellent evanescent field possessing a sensitivity to the ambient RI of 1243.455 nm/RIU and a low temperature cross-reactivity of −0.04 nm/°C. The absorption of lead ions by carboxyl groups in the fiber-coated functional film changes the refractive index of the sensor surface, which reflects a macroscopic wavelength shift in the interference spectrum. The sensor effectively detects lead ions within 10−14-10−6 mol/L concentrations and has a sensitivity of 1.85 × 108 nm/(mol/L) in the ultra-low concentration range of 10−14-10−10 mol/L with a detection limit of 0.1678 ppb. The proposed sensor is not only simple and low cost to fabricate, but also has good stability and specificity. The success of this sensor shows that the development of optical microfiber sensor with high stability and accuracy is feasible, which has progressive significance for the research of optical environmental pollution sensor.