Metal-organic framework functionalized polymer coating for fiber optical methane sensors

Abstract

Functional polymer coating integrated with optical fiber is an intriguing approach to develop low-cost point and distributed fiber sensors for large-scale applications. This paper presents the development of gas-sensitive coatings for both single-mode and multi-mode optical fibers through the application of polymer/nanocrystalline metal-organic frameworks (MOFs) composites as methane sensors for monitoring natural gas infrastructure. Silicone polymers based on PDMS with optimized optical and mechanical properties were developed as host materials for the well-known metal organic framework ZIF-8. Integration of ZIF-8 nanocrystals within the PDMS polymer modified the physical properties of the material and led to an enhancement of the CH4 solubility and permeability of the resulting film. The refractive indices, viscosities, and mechanical properties of the ZIF-8 functionalized polymers were optimized by adjusting the ZIF-8/polymer weight ratio and subsequently used as the fiber coating. Both multi-mode and single-mode optical fibers coated with the MOF-functionalized polymers showed scaled changes in transmitted power upon exposure to various concentration of CH4 in a N2 carrier gas. The variations in transmitted power through the fiber were the result of changes in evanescent wave interactions with the sensor coating due to shifts in the polymer cladding refractive index upon CH4 sorption. A methane detection limit of 1 % in nitrogen was achieved using both multimode fiber and D-shaped single mode fibers. Overall, our paper presents a low-cost approach to perform point and distributed sensing for CH4 through an innovative method of functional polymer material integration on optical fiber sensor platforms.