An optochemical sensor element suitable for sensing an analyte comprises a polymeric material in which at least a portion of the polymer material is solvent crazed to provide a multiplicity of pores of controlled nanometer size, and an indicator dye impregnated into the pores, in which optochemical sensor element the indicator dye is a long-decay photoluminescent dye selected from the group consisting of: phosphorescent platinum(II)—and palladium (II) complexes of porphyrin dyes such as octaethylporphine, coproporphyrin, octaethylporphine-ketone, benzoporphine, tetra (pentafluorophenyl) porphine, chlorin e6; fluorescent complexes of ruthenium (II), osmium(II), iridium(III) and europium (III); or derivatives or close analogs of these dyes. The sensor element in the presence of the analyte alters a photoluminescence parameter thus allowing sensing and/or quantification of the analyte.