Chemicals and Materials 5,10,15,20-Tetrakis-(4-carboxyphenyl)-21,23H-porphyrin (H2TCPP) was purchased from Porphyrin-Laboratories, (Porphyrin-Laboratories GmbH, Gleschendorf, Germany, www.porphyrinlaboratories.com). Zirconyl chloride octahydrate (ZrOCl2 8H2O), N,N-dimethylformamide (DMF), acetic acid, potassium chloride and magnesium chloride were obtained from Roth (www.carlroth.com). Magnesium nitrate was acquired from Merck (www.merckmillipore.com). Poly(acrylonitrile) (PAN, Mw = 150000 g/mol) was from obtained from Sigma Aldrich (www.sigmaaldrich.com). Hyflon® AD 60 was obtained from Solvay (www.solvay.com) and perfluorodecalin was from Fluorochem (www.fluorochem.co.uk). The polyamide (PA) substrate was a Biodyne Plus®Transfer membrane with 0.45 µm pore size from Pall (Pall Corporation, Port Washington, USA, www.pall.com). Glass micro-fiber filters (Whatman,  4.7 cm) were purchased from Sigma Aldrich (www.sigmaaldrich.com). Silica-gel coated TLC-plates and acetone were from VWR (www.vwr.com). Polyethylene terephthalate (PET) foils (125 μm) (MELINEX 506) were supplied by Pütz (Pütz GmbH Co. Folien KG, Taunusstein, Germany; www.puetz-folien.com). Pt(II)TCPP was prepared similar to a previously described procedure and Pd(II)TCPP was prepared following a previously described microwave-assisted synthesis. Poly(trimethylsilylpropyne) (PTMSP) was synthesized according to literature. Synthesis of PCN-224. The synthesis of PCN-224 relies on a published recipe. In a typical experiment H2TCPP (6.2 mg, 7.8 µmol) and ZrOCl2 x 8H2O (32.2 mg, 0.1 mmol) were dispersed in a mixture of DMF (12.5 mL) and acetic acid (6.2 mL). The dispersion was sonicated for 20 min in a 50 mL round bottom flask and kept at 65 °C for 72 h. After cooling to ambient temperature, the PCN-224 crystals were collected by centrifugation and washed three times with DMF (10 mL) and three times with acetone (10 mL). For removal of residual DMF, the solid was dispersed in acetone (3 mL) and volatiles were allowed to evaporate upon heating the dispersion placed on a watch glass on a hotplate operated at 65 °C. This operation was repeated 4 times. Finally, the material was dried under vacuum at 65 °C until constant weight was reached. Synthesis of Pt(II)PCN-224 and Pd(II)PCN-224. Pt(II)PCN-224 and Pd(II)PCN-224 were prepared analogously to PCN-224 using Pt(II)TCPP (7.7 mg, 7.8 µmol) or Pd(II)TCPP (7 mg, 7.8 µmol) as the starting materials. At the final stage, the crystals were activated for 72 h in a glass vacuum oven (B-585 from Büchi, www.buchi.com) at 0.7 mbar and 120 °C. Immobilization of PCN-224. In order to embed PCN-224 into a Hyflon AD 60 layer, a "cocktail" containing 1 % (w/w) of MOF powder and 10 % (w/w) Hyflon AD 60 in perflurodecaline was prepared. This composition was knife-coated on PET support (thickness of the wet layer 25 µm) and dried in vacuum overnight. Immobilization experiments on glass fibres, electrospun PAN nanofibers (see ESI for more details on preparation of electrospun nanofibers), silica-gel TLC substrate and PA filter sheets were done by directly growing the PCN-224 MOF on the substrate. The flat support materials were cut in squares of approximately 10x10mm and were immersed in the solution containing MOF precursors and the synthesis was carried out as described above. After the synthesis, residual reactants were removed by soaking the materials in DMF (10 mL) and acetone (10 mL). Each solvent was exchanged three times. The materials were dried in vacuum overnight. Instruments and Methods. Powder X-ray diffraction (PXRD) measurements were performed at the XRD1 beamline at the Elettra Synchrotron in Trieste. This beamline has an operating wavelength of 1.4 Å with a beam size of 200 x 200 µm. The data was collected on a stationary Dectris Pilatus 2M detector which was mounted 400 mm away from the sample. The sample itself consisted of the MOF PCN-224 powder which was filled in a glass capillary (1.5 mm diameter and 0.02 mm wall thickness). All data were transformed to reciprocal space for analysis. All data conversion, treatment and analysis steps were performed with GIDVis. The morphology of the MOF powder samples was studied by a standard scanning electron microscope Jeol JSM-6490 LV (www.jeol.de). For preparing the samples the MOF powder was drop casted onto a Si wafer and carbon coated. H NMR spectra were recorded on a 300 MHz Avance III spectrometer from Bruker (www.bruker.com). Absorption spectra were measured on a Varian Cary 50 UV-VIS spectrophotometer from Agilent Technologies (www.agilent.com). Luminescence spectra were recorded on a Fluorolog-3 luminescence spectrometer from Horiba (www.Horiba.com) equipped with a NIR sensitive photomultiplier R2658 from Hamamatsu (www.hamamatsu.com). An optical fiber setup was used to guide the excitation light to a 10 mL Schlenk flask or a 3 mL screw-cap glass vial containing the MOF sample and to guide back the emission to the monochromator and photodetector. A custom-made 3D printed adapter (ESI, Fig. ) was used for fixation of optical fibers. Luminescence decay times were acquired on the same spectrometer equipped with the DeltaHub module. The set-up was similar to the one used in the intensity measurements but a 453 nm laser diode "NanoLED" from Horiba (www.horiba.com) was used for excitation of PCN-224 and a 405 nm "SpectraLED" from Horiba (www.horiba.com) for excitation of Pt(II)PCN-224 and Pd(II)PCN-224. DAS-6 Analysis software was used for data analysis. The composition of the calibration gas was adjusted with help of mass-flow controllers from Voegtlin (www.voegtlin.com. Aesch, Switzerland) with the total flow rate of 200 mL min -1 . For calibration of PCN-224 nitrogen (99.999% purity) and compressed air were used; for Pt(II)PCN-224 nitrogen of 99.9999% purity was mixed with the test gas (0.2% O2 in nitrogen) and for Pd(II)PCN-224 nitrogen of 99.99999% purity) was mixed with a test gas containing 20 ppm oxygen in nitrogen. All test gases were acquired from Linde gas (www.linde-gas.de). The temperature was 25 °C in all the experiments. In order to investigate the effect of humidity on the oxygen response of PCN-224, the gas compositions were bubbled through saturated aqueous solutions of MgCl2 (33 % r.h.), Mg(NO3)2 (53 % r.h.) and KCl (84 % r.h.) before entering the measurement compartment with MOF.