Optical humidity sensor using methylene blue immobilized on a hydrophilic polymer

An optical humidity sensor based on a membrane of hydrophilic polymer has been developed. It is a solid state sensor where the components of the membrane are hydrophilic polymer hydroxypropyl methylcellulose (HPMC), methylene blue to indicate humidity changes and a non-ionic surfactant. The mechanism of the sensor is based on the methachromasy related to the dimerization of methylene blue observed upon exposure to humidity. Measurement were carried out in the range of 0-99.5% relative humidity with a detection limit of 0.034%RH, showing a linear response, with good reproducibility (S.D. 0.27 at 1.78%RH, n = 5) and stability, fast response and recovery times (60 and 90 s, respectively) and reversibility up to 50% at room temperature (291 K). This sensor displayed sufficient sensitivity that it could be used to detect low RH in environmental applications. Possible interferents were studied such as O₂, CO₂, CH₄ and H₂ gases, which did not show any interference whereas SO₂ and H₂S cause serious interferences on the humidity sensor.