Methyl red based metal–organic frameworks for the selective and tuneable sensing of ammonia gas

Abstract

This work examines a family of MOFs that are colourimetric sensors to acid and base vapours. With the strongest colour change associated with strong base vapours such as ammonia, this material can act as an ammonia sensor for applications in health and safety. A methyl red based linker is synthesized and incorporated into a UiO-66 based metal–organic framework (MOF). Using terephthalic acid and the methyl red based linker, four different methyl red MOF compositions were synthesized and characterized. The degree of incorporation ranged from 10–100%. The as-synthesized MOF was an red-orange colour with the colour depending on the amount of methyl red based linker incorporated. Exposure of the MOF to acid vapour produced an irreversible, with respect to time, darker red colour. Exposing the MOF to ammonia vapour produced a stark and reversible, with respect to time, colour change to yellow. No other base vapours were able to elicit this response making this MOF selective for ammonia vapour. The sensitivity of the MOF to ammonia is proportional to the degree of methyl red linker incorporation. We further incorporate the MOFs into a mixed matrix membrane using polyvinylidene fluoride as the polymer host. As with the parent MOFs, the membranes demonstrated a strong colourimetric response to ammonia. These materials have potential applications in future shipping vessels that will utilize ammonia as a fuel source.