Unraveling the origin of the “Turn-On” effect of Al-MIL-53-NO2 during H2S detection

Abstract

Nitro-functionalized metal–organic frameworks (MOFs), such as Al-MIL-53-NO₂, have been widely used in quantitative hydrogen sulfide (H₂S) detection based on the “turn-on” effect, where fluorescence enhancements were observed upon contact with H₂S. This was believed to be caused by the fact that the electron-withdrawing –NO₂ groups in the initial non-luminescent MOFs were reduced to electron-donating –NH₂ groups in the sensing process. However, since most H₂S detection is conducted in a suspension system consisting of MOFs and solvents, it is still unclear whether these –NH₂ groups are on MOFs or in the liquid. Using Al-MIL-53-NO₂ as a model MOF, this work aims to answer this question. Specifically, the supernatant and undissolved particles separated from the Al-MIL-53-NO₂ suspensions after being exposed to H₂S were analyzed systematically. The results showed that it is the free BDC-NH₂ (2-aminobenzene-1,4-dicarboxylic acid) in the solution rather than the formation of Al-MIL-53-NH₂ that really caused the fluorescence enhancement. In particular, the formed BDC-NH₂ was reduced from the shedded BDC-NO₂ (2-nitrobenzene-1,4-dicarboxylic acid) during the decomposition of Al-MIL-53-NO₂, which was attacked by OH⁻ in the NaHS solution. We anticipate that this work will offer new ways of tracing fluorophores for MOF-based sensing applications in aqueous systems.