Electro-synthesized Conjugated Salen Polymer-Glassy Carbon as Hydrochromic Reflective Filter for Humidity Detection: Introduction of Humidity Optical Sensor

A novel humidity optical sensor was fabricated based on adsorption of water vapor via the hydrochromic reflective filtration behavior of conjugated Salen polymer, immobilized on the surface of a glassy carbon (GC) electrode by cyclic voltammetry (CV). This process was attributed to the hydrophilic behavior of the electro-synthesized conjugated Salen-based polymer when contacting with water vapors especially during coordination with K+ through the electro-synthesis process. Optical image of the hydrochromic reflective filter was considered as appropriate detection system for relative humidity (RH) sensing purpose. The mechanism of the change in the color intensity was evaluated via optical filtration of a white laser diode as polychromatic optical source via filtration by the conjugated polymer during its reflection from a smooth surface such as GC. The GC electrode therefore acted as both i) the working electrode during the electrochemical synthesis by CV, and ii) light reflector (mirror) during the light radiation. In this study the electrochemical and optical properties of the GC was compared to a graphite or metal electrode. Parameters such as linearity, rise/fall time, sensitivity, and selectivity of each the fabricated humidity optical sensor were evaluated. Also, dependency of humidity sensor to temperature was investigated. The intensity of the blue component was linearly increased over a wide range of RH between 5 - 80%. The fabricated optical sensor had improved detection limit (0.17% RH), standard saturated limit (>80% RH), accepted relative standard deviation (RSD= 3.6%, n=3), short rise time (~9.5 s compared to the commercial RH sensor), good linearity (R=0.9971) and maximum~7.5 % RH hysteresis. Effects of interferences such as H2, CO, CO2, NOx, He, and volatile organic compounds (VOCs) were also considered. Results showed no noticeable interfering effects. This sensor was applicable for selective and reliable detection of %RH in different environmental samples without having any significant interferences.