Threshold Volatile Organic Compounds Sensor Based on Polymer Microtip ^†

Abstract

The development of optical fiber sensor technology has led to interest in the use of new elements for functionalizing the fiber end face. The modification of this part of the fiber can be conducted by forming a polymer microtip (PM). The technology of manufacturing PMs on single-mode and multi-mode fibers is widely described in the literature. Geometry and optical properties of PMs depend on technological process parameters, optical fiber type, and monomer mixture composition. PM can be considered in the context of sensor applications as a transducer that is sensitive to refractive index (RI) changes. RI sensors were an inspiration to explore the possibility of using PM to detect VOCs in air. Five selected volatile compounds were tested: trimethyl phosphate (TMP), 1,4-thioxane (THX), acetone, toluene, and aqueous ammonia. The principle of the sensor operation was based on the phenomenon of attraction and repulsion of the condensed VOCs. Attracted VOC molecules cover the PM surface and form a liquid film that changes the RI at the interface between the PM and the external material. The return loss level changes rapidly when the PM is introduced into the glass vial filled with the test compound. The proposed sensor acts as a threshold, showing only the presence of condensed VOC, but with no ability to determine its concentration and type of substance. Such a sensor can be used in warning systems to monitor leaks in volatile chemical transport lines, as well as to monitor accumulations of hazardous liquid substances that easily transit into the vapor phase. It acts as an early warning system in the event of the condensation process of the monitored dangerous chemical agent.