Fabrication and characterization of photo-responsive metal–organic framework membrane for gas sensing using planar optical waveguide sensor

Abstract

A facile, novel fabrication approach using UV light irradiation was proposed to fabricate a photo-responsive metal–organic framework (PR-MOF-1, [Zn2-(bdc)2-(dpNDI)]n, where bdc = benzene-1,4-dicarboxylic acid; dpNDI = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxy diimide) membrane. The morphology of the PR-MOF-1 framework evolved from a honeycomb porous structure to a densified ladder-layered structure after 60 min of UV-light illumination. The as-grown film was optically transparent and exhibited a greater sensing response to ethylenediamine (EDA) gas in the presence of interfering substances such as ammonia and dimethylamine as well as benzene, toluene, xylene, and styrene gases, as measured using an asymmetric planar optical waveguide gas sensor. When the EDA gas molecule was adsorbed on the surface of the membrane, charge transfer between them preferably occurred, leading to a change in the membrane surface conformation. As an ideal sensing material for EDA gas detection, the PR-MOF-1 membrane showed a relatively high surface sensitivity (11,000 times cm-1) after 60 min of growth, and it could quickly (in less than 2 s) detect 1 ppb of EDA gas with a significant response (S/N = 3.45). During the static gas adsorption process, the EDA gas adsorption kinetics fit well with pseudo-second-order (PSO) model, and the adsorption capacity (qe) on a unit surface showed a high value of 33.91 μg cm−2 at 283 K. The high selectivity and sensitivity of the PR-MOF-1 membrane for EDA gas indicate the effectiveness of the light irradiation method for alteration of the metal– organic framework membrane structure and control of the gas sensing properties.