Fast‐Responding O₂ Gas Sensor Based on Luminescent Europium Metal‐Organic Frameworks (MOF‐76)

<jats:title>Abstract</jats:title><jats:p>The increasing demand for advanced sensing technologies drives the development of chemical sensors using innovative materials. In gas sensing, optical sensors are often used to detect gases such as CO, NO<jats:italic><jats:sub>x</jats:sub></jats:italic>, and O<jats:sub>2</jats:sub>. Oxygen sensors typically incorporate dyes into oxygen‐permeable matrices like polymers, silica, or zeolites. Alternatively, semiconductor surface chemistry can enable O<jats:sub>2</jats:sub> detection. However, these approaches are often limited by slow response and recovery times and low selectivity, restricting their practical applications. The metal‐organic framework MOF‐76(Eu) and its yttrium‐modified variant, MOF‐76(Eu/Y) are reported to exhibit highly reversible and fast optical responses to varying O<jats:sub>2</jats:sub> concentrations. Time‐resolved emission measurements are performed over short (seconds) and long (hours) timescales using N<jats:sub>2</jats:sub> and synthetic air mixtures. Cross‐sensitivity to humidity is analyzed. Multichannel scaling photon‐counting experiments confirm quenching at the linker level, as the emission lifetime remains nearly constant. Yttrium significantly improves stability and performance at room temperature. Structural and optical changes induced by yttrium are investigated. Additionally, MIL‐78(Eu), another Eu‐BTC‐based MOF with a different coordination environment, is synthesized. Unlike MOF‐76(Eu), MIL‐78(Eu) exhibits distinct optical properties but lacks a reversible response to O<jats:sub>2</jats:sub>. These results highlight the potential of MOF‐76‐based materials for high‐performance O<jats:sub>2</jats:sub> sensing.</jats:p>