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.近年来，用于氨气与胺类检测的金属有机框架（MOFs）研究已取得斐然进展。然而，针对室温下不同相对湿度（RH）环境中低浓度氨气与胺类气体检测的质子传导型MOFs研究，仍相对有限。本文通过硝酸镉（Cd(NO3)2）与2-甲基-1H-咪唑-4,5-二羧酸（H3MIDC）的溶剂热反应，成功合成出一种拥有有序一维孔道的三维离子型MOF {Na[Cd(MIDC)]}n（标记为化合物1）。本研究表明，未配位的羧酸位点有利于质子传输以及氨气与胺类化合物的识别。该化合物的最优质子传导率可达1.04 × 10–3 S·cm–1（100 ℃，98% RH）。我们分别在68%、85%与98% RH的环境下，考察了该材料室温下对氨气及胺类气体的传感性能。令人满意的是，MOF 1对氨气、甲胺、二甲胺、三甲胺及乙胺的检测限分别为0.05、0.1、0.5、1与4 ppm，其是目前已报道的基于质子传导型MOFs的氨气室温传感器中性能最优的器件之一。本文同时对质子传导与传感的作用机制进行了深入阐释。