Hydrogen sulfide (H2S), known as an odor of rotten eggs, is widely spread across industries including iron smelters, landfills, food processing plants, and breweries. The extreme toxicity of H2S in the environment makes H2S of significance for safety and human health. Various Nitro-functionalized metal-organic frameworks (MOFs) have been designed and synthesized to realize H2S detection based on the “turn-on” effect, where the -NO2 groups are reduced to -NH2 groups and the fluorescence enhancement is observed after being exposed to H2S treatment. Most researchers ascribed such a “turn-on” effect to “local transformation” of MOF in aqueous suspension (i.e., solid-state MOFs are dispersed in liquid evenly) using NaHS or Na2S as the H2S source. However, no direct evidence was given to the existence of -NH2 groups attached to the MOF surface. In addition, peak broadening phenomenon in XRD patterns was observed in previously reported Zr-based, Ce-based, and Al-based MOFs, which indicated all these MOFs experienced partial degradation and crystal consuming during H2S treatment. No researchers noticed such peak broadening phenomenon and it is still unclear whether these -NH2 groups attached to the surface of MOFs or the free ones released in the solution that really contribute to the fluorescence enhancement. To explore the real fluorophores in MOF itself or in the solution, Al-MIL-53-NO2 was utilized as a model MOF in this study. Specifically, the supernatant and the undissolved particles separated from Al-MIL-53-NO2 suspension after being exposed to H2S were analyzed systematically. The results indicated that it is the free BDC-NH2 (2-aminobenzene-1, 4-dicarboxylic acid) in the solution rather than the Al-MIL-53-NH2 that gives rise to the “turn-on” effect. During the H2S detection, the crude Al-MIL-53-NO2 was partially degraded by OH- attack in the solution and then the shedding BDC-NO2 (2-nitrobenzene-1,4-dicarboxylic acid) was reduced to BDC-NH2. In addition, with the existence of fluorophores BDC-NH2 dissolved in the solution, the supernatant can be used for quantitative H2S detection as well. The result of this work unravels the origin of the “turn-on” effect of Al-MIL-53-NO2 in H2S sensing and provides new insights into the ways of fluorophore location in probe-consuming sensing application.

Did we really use the intrinsic fluorescent properties of nitro-factualized metal-organic frameworks (MOFs) to realize H2S detection in an aqueous system?

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.