Fiber-Optic Thorium Ion Sensor Based on the Breathing Effect of Metal-Organic Frameworks

This paper introduces a fiber-optic thorium ion (Th⁴⁺) sensor based on the breathing effect of a UiO-AZO thin film. The sensor integrates a Mach-Zehnder interferometer (MZI) with a surface plasmon resonance (SPR) sensing element for simultaneous measurements. Under visible (Vis) light, the MZI sensor exhibits a sensitivity to Th⁴⁺ of 0.315 nm/nM. Under ultraviolet (UV) light conditions, the sensitivity is 1.68 nm/nM, and the limit of detection (LOD) as low as 0.8 nM. SPR is excited by Au-TiO2 and senses temperature changes through PDMS, with a sensitivity reaching -1.23 nm/°C. The reversible adsorption and desorption of Th⁴⁺ are controlled by switching between Vis and UV light, which enables in-situ calibration.