On the validity of rapid optical sensing of dioxygen by means of sensitivity, stability, and reversibility for archetype MOFs post-synthetically modified with Eu3+

Abstract

Pressure dependent optical dioxygen sensing was studied by a comparison of six archetype metal–organic frameworks (MOFs: UiO-66(Zr), UiO-67(Zr), UiO-67(Zr)-bipy, MIL-68(In), MIL-100(In) and DUT-5(Al)) based on the quenching of the photoluminescence emission of the MOFs post-synthetically modified with Eu³⁺ in the pore systems. MOF-76(Eu) as an example of a luminescent LnMOF with Eu³⁺ ions as coordinative centers of the framework was included in the comparison. Pressure dependence was investigated starting with high vacuum (10⁻⁷ bar) to ambient pressure by quantitatively analyzing the response of the Eu³⁺ photoluminescence emission towards oxygen. The MOFs show fast response, leading to reversible “turn-off” and “turn-on” effects most prominent for Eu³⁺@MIL-68 directly observable by a luminescence emission quenching efficiency of 93.8% within 2 s. Intensity changes were already measured at oxygen pressures of 10⁻⁵ bar, indicating high sensitivity. Furthermore, the sensing process follows the Stern–Volmer relationship, although a certain pressure dependence also affects the kinetics especially in the low-pressure region, so that applying only Stern–Volmer is not valid over the whole pressure range, as the quenching rate increases with decreasing pressure. In addition, the process is reversible and robust, which was elaborated by multiple sorption/desorption studies including cycling over ten sensing cycles. Selectivity of the processes were evaluated for other atmospheric gases N₂, CO₂ indicating only minor quenching that still allows oxygen detection for a p_O2 of 10⁻⁵ bar. Altogether, this work presents analysis-based proof on the validity of broad and rapid sensitive on-the-fly optical oxygen sensing with lanthanide-loaded MOFs.