Potential application of MOFs for optical oxygen sensing requires their immobilization into/onto solid supports to obtain a sensing element with acceptable mechanical stability. A notable exception is the application of MOF (micro)crystals in form of aqueous dispersion. For all the experiments, PCN-224 was chosen as the model system but results are expected to be applicable to the metalated congeners due to their high structural similarity. A straightforward way to prepare sensing materials is to disperse crystals in a polymeric matrix. To do so, the free crystals (Fig. ) were first dispersed in a polymer solution in an organic solvent that was evaporated after film casting. Hyflon AD 60 was chosen for its high oxygen permeability and chemical inertness. Unfortunately, a drastic loss of sensitivity was observed after immobilization (Fig. ). This may be due to clogging of the pores by flexible polymer chains or entrapped solvent (perfluorodecaline) that hinders oxygen from entering the MOF freely. Notably, perfluorodecaline is much less volatile that water (b.p. 142 °C) and is much larger so it may be very difficult to remove it once it gets entrapped in the pores of the MOF. Moreover, removal of residual solvent from the perfluorinated polymer itself was reported to be challenging. As an alternative approach, the growth of the MOF on different substrates was investigated. PCN-224 was successfully grown on heat-stabilized poly(acrylonitrile) (PAN) nanofibers (Fig. ). PAN nanofibers were prepared via an electrospinning technique and a cross-linking step at high temperature (280 °C) that renders the fibres insoluble in DMF. This property enables the MOF grow from a solution of the precursors in DMF. SEM revealed MOF crystals grown not only on the surface of the fibres but also around the fibres (Fig. ) and PXRD confirmed the structure of PCN-224 grown on the fibrous support (Fig. ). The oxygen sensitivity of the obtained material is very similar to that of the individual crystals (Fig. ). This is not the case for several other composite materials. MOF crystals grown on different Si-based materials such as a glass fibre filter (Fig. , Fig. ) and aluminium-supported silica-gel (Fig. , Fig. ) exhibited some decrease of oxygen sensitivity (Fig. ). The decrease in sensitivity may be due to blocking of some part of the crystals by the virtually oxygen-impermeable glass/silica-gel backbone. PCN-224 was also grown on a flat polyamide (PA) filter (Fig. , Fig. ) and a similar decrease in the sensitivity was observed (Fig. ). For the flat silica-gel and PA supports the individual crystals are comparable in size and the oxygen sensitivity is also very similar. Finally, the sensing properties of PCN-224 grown on PAN nanofibers were investigated in more detail. The response of the material is very fast and fully reversible (Fig. ) and is generally similar to that of the free crystals. It should be noted that since the compact PCN-224 crystals are grown around the individual PAN fibres, their position in the sensor material is fixed and there is no possibility of movement of individual crystals anymore. This allows for a very fast exchange of the gas atmosphere so that the measured response times are closer to the true values (in particular when compared to the measurement performed with the free crystals presented in Fig. ). The full response (t100) in both directions was accomplished within 1s (Fig. ), which again appears to reflect the time needed for the gas exchange. The true response times in terms of more commonly used t95 (time needed for 95% of the signal change) are expected to be well below 1 s. During 19 consecutive calibration cycles, almost no drift (less than 2% over a continuous illumination of more than one hour and 19 consecutive cycles) was found and no change in sensing behaviour was observed. Therefore, immobilization by growing PCN-224-related materials on PAN nanofibers can be considered highly promising for preparation of solid state (sensing) materials.