“Off-On” typed square fiber-optic strontium ion sensor based on breathing effect of flexible metal-organic framework

Breathing effect of flexible Metal-Organic Framework (MOF) can introduce a new sensing mechanism in optical fiber device. In this paper, we present a novel “off-on” typed square fiber optic strontium ion (Sr²⁺) sensor based on Au/UiO-SP nanofilm, which exhibits both photoinduced ion sensitivity enhancement behavior and temperature crosstalk compensation capability. This sensor adopts a single-mode fiber (SMF)-square fiber (SF)-SMF structure to construct multimode interference (MMI). Meanwhile, surface plasmon resonance (SPR) was excited by the deposition of Au nanofilm on the surface of SF. Among them, MMI serves as the temperature compensation unit and SPR serves as the strontium ion sensing unit. The post-modification of spiropyran (SP) on UiO-66-NH₂ can achieve “breathing effect”, that is, the molecular configuration of compound UiO-SP changes (from “off” state to “on” state) when exposed to ultraviolet irradiation, thus improving the adsorption efficiency of strontium ions and enhancing the detection sensitivity of the SPR sensor. The results showed that the sensitivities of the SPR sensors using UiO-66-NH₂, UiO-SP-off and UiO-SP-on could reach 1.36 nm/nM, 0.99 nm/nM and 2.12 nm/nM, respectively. Experiments showed that the strontium ion detection limit of the SPR sensor based on UiO-SP-on reached 2 nM, and the temperature crosstalk was 0.366 nm/℃. The temperature sensitivity of MMI was 0.386 nm/℃, and it had almost no response to changes in Sr²⁺ concentration. Therefore, it can be used as the temperature calibration for SPR sensors. This optical fiber device integrates the light-controlled respiration effect of flexible MOF, providing a new idea for the detection of strontium ions.