Polymer/Metal−Organic Framework Composite Sensors for Gas Detection
Authors: Hong Tao , Culp Jeffrey , Kim Ki-Joong , Ohodnicki Paul R 
Published: February 2020

Methane (CH4), the primary composition of the natural gas, is a colorless and odorless gas with a melting point of -184 ºC and a boiling point of -164 ºC. It is considered as a greenhouse gas with a global warming potential far greater than CO2 per molecule, which could significantly contribute to undesired global warming through even moderate amounts of leakage from natural gas recovery and transportation, and can become dangerously explosive for concentrations above the lower explosive limit. Given the fact that the U.S. has nearly 3 million miles of natural gas pipelines, for both safety and environmental concerns, there is an interest in monitoring pipelines for integrity and leak detection. Unfortunately, current sensing technologies do not meet all the practical requirements for widespread deployment and accurate measurement of low level methane leaks or in-pipe gas composition monitoring. Thus, the development of advanced gas sensing devices which are of sufficiently low cost for ubiquitous deployment both internal and external to pipelines, and which can be interrogated remotely over long distances would be highly beneficial.
In order to meet the practical requirements of advanced sensor technology for natural gas infrastructure applications, our proposed research seeks to develop a series of polymer/ metal−organic frameworks (MOF) composite thin coating films. The concept targets to exploit the excellent gas adsorption capability and CH4/N2 and CO2/N2 selectivity of highly engineered MOF materials, while a polymer binder is used to keep the well-dispersed mixture composites on the optical fiber surface. The list of possible candidates includes a wide range of polymer/MOF combinations and we herein report the well-dispersed Fe(Pyz)Ni(CN)4 MOF particles blended with the dilute Poly(styrene-butadiene-styrene) (SBS) solution. The suspension is dip-coated onto the optical fiber surface to form a thin layer of film (in nm scale). Unlike pure MOF-based thin coating films which typically rely on time-consuming and complex deposition process, e.g. layer-by-layer technique, this rapid and facile deposition approach offers us the opportunity to efficiently cast materials on long-length (in kilometer scale) distributed platform with a wide range of different candidate MOFs and with high film uniformity and quality. The gas sensing response of the coated fiber is evaluated at room temperature with the transmittance (%T) of the sensor monitored in various gases. When gas molecules are adsorbed by the sensing layer, the refractive index is modified and is recorded by the spectrometer to be correlated with the analyte of interest (Figure 1). A monotonic increase of gas sensing performance is observed by the increase of MOF content in the composites (Figure 1 inset), and is attributed to the enhanced gas adsorption capacity of the MOF matrix. Moreover, a variety of characterizations are performed to investigate the physical/chemical/morphological properties of the polymer/MOF composites and unravel the fundamental understanding of the sensing mechanism.
Figure 1. Gas sensing data of a pure SBS polymer and SBS/MOF coated optical fiber. Sensing CH4 and CO2 by monitoring the change in light transmission at 459 nm for SBS/MOF composite and 1944 nm for pure SBS. The inset figure shows the pure CH4 and CO2 sensing response (T%) as a function of MOF wt%.

Figure 1