In a next step, response times and the reversibility were assessed. As shown in Fig. the PCN-224 crystals fully reversibly respond to the alteration between oxygen-free atmosphere and air. The same holds for the two metalated MOF crystals. The dynamic response times t95 are within several seconds ( 6 s going from anoxic conditions to air and  7 s in the reverse direction). Although such response times are comparable to most state-of-the-art oxygen sensors based on polymer-immobilized indicators, they appear to be much longer than expected from extreme oxygen permeability of the MOF. As will be discussed below these values are likely to be greatly overestimated due to the time needed for the gas atmosphere to be exchanged in the chamber with the crystals.