Proton-Conductive 3D Ln^III Metal-Organic Frameworks for Formic Acid Impedance Sensing

Metal-organic frameworks (MOFs) as new classes of proton-conducting materials have been highlighted in recent years. Nevertheless, the exploration of proton-conducting MOFs as formic acid sensors is extremely lacking. Herein, we prepared two highly stable 3D isostructural lanthanide(III) MOFs, {(M(μ₃ -HPhIDC)(μ₂ -C₂ O₄ )_0.5 (H₂ O))⋅2 H₂ O}_n (M=Tb (ZZU-1); Eu (ZZU-2)) (H₃ PhIDC=2-phenyl-1H-imidazole-4,5-dicarboxylic acid), in which the coordinated and uncoordinated water molecules and uncoordinated imidazole N atoms play decisive roles for the high-performance proton conduction and recognition ability for formic acid. Both ZZU-1 and ZZU-2 show temperature- and humidity-dependent proton-conducting characteristics with high 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 HCOOH in aqueous solution. 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 can 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.