A Highly Proton-Conductive 3D Ionic Cadmium–Organic Framework for Ammonia and Amines Impedance Sensing

Lately, the progressive
study of metal–organic frameworks
(MOFs) for the detection of ammonia and amines has made infusive achievements.
Nevertheless, the investigation of proton-conductive MOFs used to
detect the low concentrations of ammonia and amine gases at different
relative humidities (RHs) at room temperature is relatively restricted.
Herein, by solvothermal reaction of Cd­(NO3)2 with 2-methyl-1H-imidazole-4,5-dicarboxylic acid
(H3MIDC), a three-dimensional ionic MOF {Na­[Cd­(MIDC)]}n (1) bearing ordered one-dimensional
channels was successfully synthesized. Our research indicates that
the uncoordination carboxylate sites are beneficial to proton transfer
and the recognition of ammonia and amine compounds. The optimized
proton conductivity of 1 reaches a high value of 1.04
× 10–3 S·cm–1 (100 °C,
98% RH). The room temperature sensing properties of ammonia and amine
gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily,
the detection limits of MOF 1 toward ammonia, methylamine,
dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5,
1, and 4 ppm, respectively, which is
one of the best room-temperature sensors for ammonia among previous
sensors based on proton-conductive MOFs. The proton conducting and
sensing mechanisms were highlighted as well.