Redox Active Molecule Induced Metal-Organic Framework Thin Film on Optical Fiber Towards Chemical Sensing of Carbon Dioxide

Abstract

Sensing of carbon dioxide (CO2) has proven to be a challenging area of research over the few last decades. Although nondispersive infrared absorption sensors are widely used as gas sensors, these are unsuitable for mobile sensing applications due to the necessity of a bulky and expensive optical system. In the case of chemi-resistive sensors based on metal oxides, elevated temperatures are required to promote reaction of surface-bound chemical species and they exhibit cross-sensitivity and significant baseline drift over the life of the sensors. Therefore, it is necessary to explore new types of sensing materials and systems that provide a sensitive, selective, and stable response for CO2 gas as well as sensing at atmospheric conditions. Waveguide-based optical fiber sensors have unique characteristics and flexible designs which make them well-suited to remote monitoring applications due to their inherent long- range transmission of light. Integrating functional materials such as metal-organic frameworks (MOFs) into optical fibers enable the measurement of optical property changes by adsorbing guest molecules and has led to the potential approach for the fabrication of chemical sensors due to their exceptional sorption capacity and high degree of reversibility even at near-ambient temperatures. Recently, efforts have also demonstrated the ability to infiltrate guest molecules into the pores of MOFs to induce emergent properties (guest-host interaction) which are important advantages for MOFs for use in optoelectronic devices such as chemical sensors. Examples of such properties include enhanced electronic conductivities compatible with chemi-resistive sensing platforms as well as increased optical absorption relevant for optical waveguide-based sensors. In the present work, MOF thin films were coated on an uncladded optical fiber through a simple solution method at room temperature, which is critical for manufacturability of optical fiber-based sensor devices. Then, the redox-active molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), was introduced into MOF pores, achieving enhanced electronic conductivity and optical absorption as well as selective closing of MOF pores and enhanced stability in water. It is observed that the activation of pristine MOF at different temperatures greatly affects the coordination of TCNQ guest molecule into pristine MOF pores. Sensing responses of TCNQ infiltrated MOF deposited on optical fibers showed a linear relationship between partial pressure of CO2 gas and the light transmission, indicative of a guest molecule-induced optical response modification. Furthermore, the sensors show high sensitivity to CO2 with rapid response time and excellent reversibility. A method for fabricating guest molecule induced MOF films on optical fibers and the capabilities of these sensor to reversibly detect a wide range of concentrations of CO2 gas will be presented.