The present work describes approaches to the development of optical humidity sensors using silica xerogel as a matrix for humidity-sensing compounds. Humidity sensing finds applications in many areas, ranging from weather forecasting to the storage of hazardous materials. The majority of commercial humidity sensors rely on costly and often environmentally-unfriendly readout electronics that additionally require a stable power source to ensure continuous operation. Some simple humidity sensors, however, use visible changes in optical properties with respect to humidity, such as cobalt(II)chloride-containing desiccants that turn pink in response to the presence of water vapor. Optical humidity sensors are desirable for applications in which cost and operational downtime must be minimized. Furthermore, optical humidity sensors utilize chemical changes to make measurements, which allows them to achieve significantly faster response times than conventional sensors such as capacitive humidity sensors. The sensing compounds are often embedded in chemically-inert and highly-porous supports such as blotting paper in order to minimize cost and response time. The porosity, surface area and chemical stability of silica sol-gel materials render them attractive as supports for this application. In this work, silica gels were synthesized by the catalytic hydrolysis and condensation of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). Small quantities of selected humidity-sensitive dye solutions were added to the gel recipes to produce corresponding dye-doped silica gels. Wet gels were converted to xerogels by rate-controlled ambient-pressure drying. Dye-doped xerogels were characterized by UV/Vis absorption and luminescence spectroscopy at preset humidities. For example, xerogels doped with the humidity-sensitive dyes methylene blue and salicylideneaniline were found to respond to changes in humidity by a change in absorption and emission peak ratio, respectively. Alkyl functionalization of TMOS-based gels using 25% (v/v) methyltrimethoxysilane (MTMS) in the precursor recipe to increase hydrophobicity and alter the bulk water adsorption kinetics of the gels was also investigated.

Development of silica-xerogel-based optical humidity sensors

ACS Spring 2022

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