Development of silica-xerogel-based optical humidity sensors

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 humidity sensors, however, take advantage of visible changes in the optical properties of safe and ecologically-safe compounds with respect to water vapor content in order to measure ambient humidity. 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 indicator solutions were added to the gel recipes to produce corresponding indicator-doped silica gels. Wet gels were converted to xerogels by rate-controlled ambient-pressure drying. Indicator-doped xerogels were characterized by UV/Vis absorption spectroscopy at preset humidities. For example, xerogels doped with the humidity-sensitive indicators methylene blue and cobalt(II) chloride were found to respond to changes in humidity by a visually-observable change in absorption and emission peak ratio, respectively.