The impact of Activation on the Performance of Optical Oxygen Sensing with the Luminescent Metal–Organic Framework MOF-76(Eu)

Abstract

The influence of activation as key parameter for oxygen sensing by luminescent metal–organic frameworks has been investigated and quantified for the archetype MOF-76(Eu). Activation at different conditions (regarding temperature and solvent-exchange for distinct vacuum pressure and heating time), shows an influence on the overall quenching, response time and cyclability due to different pore accessibility and surface area and therefore on the overall performance of the sensor. The optical sensing process is based on luminescence quenching, analyzed from high vacuum (10−7 bar) to ambient pressure by dosing oxygen from 0.01 bar to 1 bar. Strong influence of the different activation parameters is observed, as MOF-76(Eu) activated at 50 °C shows limited quenching of the luminescence intensity within 30 min, while methanol-exchange and subsequent activation at 250 °C leads to a quenching rate of 98.6 %. In addition, the sensor response occurs more than 1000 times faster within 0.2 s. These results correlate well with physisorption data, which reveal a significant change in porosity and surface area according to the degree of activation. For a better understanding of the involved processes, adsorption isotherms were recorded, surface areas determined and correlated to the photophysical parameters, including Stern-Volmer kinetics and cycling experiments for the differently activated MOF sensors.