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Two proton conductive 3D Ln(III) MOFs for formic acid impedance sensing.

Metal-organic frameworks (MOFs) as new types of proton conductive materials have been highlighted in recent years. However, the adoption of proton conductive MOFs as gas sensors for formic acid is extremely lacking. Herein, we prepared two highly stable 3D isostructural lanthanide(III) MOFs, {(M( μ 3 -HPhIDC)( μ 2 -C 2 O 4 ) 0.5 (H 2 O))·2H 2 O} n (M = Tb ( ZZU-1 ); Eu ( ZZU-2 )) (H 3 PhIDC = 2-phenyl-1H-imidazole-4,5-dicarboxylic acid), in which the coordinated and un-coordinated water molecules and uncoordinated imidazole N atoms all play crucial roles for the high-performance proton conduction and recognition materials for formic acid. Both ZZU-1 and ZZU-2 show temperature-dependent and humidity-dependent proton conducting properties with high proton conductivities of 8.95 × 10 -4 and 4.63 × 10 -4 S·cm -1 at 98% RH and 100 °C, respectively. Importantly, the impedance values of the two MOF-based sensors decrease upon exposure to formic acid vapor generated from formic aqueous solutions at 25 °C with good reproducibility. By comparing the changes of impedance values, we can indirectly determine the concentration of formic acid in water. The results showed that the lowest detectable concentrations of formic acid aqueous solutions are 1.2 × 10 -2 mol⋅L -1 by ZZU-1 and 2.0 × 10 -2 mol⋅L -1 by ZZU-2 . Furthermore, the two sensors enable to distinguish formic acid vapor from interfering vapors including MeOH, N-hexane, benzene, toluene, EtOH, acetone, acetic acid and butane. Our research provides a new platform of proton conductive MOFs-based sensors for detecting formic acid.