Precise control over humidity and moisture levels is pertinent for various industrial processes dealing with, e.g. construction, chemical engineering and fabrication of pharmaceuticals and semiconductor devices. In order to improve the product yields and ensure safety and high quality, accurate and reliable humidity sensing is in great demand. The dominant humidity-sensing technologies in the market are based on permittivity changes in a polymer matrix induced by water adsorption from air, resulting in changes in capacitance or conductivity. These approaches are well established, but they are not suitable for all environments, such as those with large electromagnetic interference, high-voltage electricity, or explosive atmospheres. For several environments, remote humidity sensing would be the preferred, if not the only, option, with the additional requirement of simplicity and cost-effectiveness.
The OPTOSENSE project was set out to address this very challenge, aiming at developing a remote, all-optical detection scheme for measuring relative humidity and temperature, utilizing thin polymer films containing photoswitchable azobenzene compounds. Azobenzenes are widely used as light-responsive molecules in creating functional materials for applications ranging from non-linear optics to biotechnology. The light-sensitivity arises from the fully reversible light driven trans-cis isomerization upon which the molecules exhibit large spectral and geometrical changes. The cis-isomers of azobenzenes are metastable and the reverse isomerization can be induced by light, but it occurs also thermally. The thermal isomerization rate can be continuously tuned by proper molecular design, but according to our research discoveries during the ERC Starting Grant Project PHOTOTUNE (project number 679646), in a class of azobenzenes known as hydroxyazobenzenes, it can be strongly affected by environmental factors such as changes in humidity.
OPTOSENSE developed this basic research finding of dependence of thermal isomerization rate on environmental humidity to create a highly sensitive, fiber-optics-based humidity measurement device. The main result of the project is a standalone device that automatically measures the isomerization rate and temperature, calculates relative humidity and displays the current temperature and humidity reading. Tested over a 6-week continuous running, the device reports relative humidity that matches that of a commercial humidity sensor with one percentage point accuracy. In addition, we set up a molecular library of hydroxyazobenzenes with different pendant groups to optimize the device in terms of stability, response time, and operation at a specific wavelength of light.
With a proven concept, the technology is moving forward to preparation of commercialization with identified need in construction industry where simple while accurate measurement of high humidity is required. The operation principle of optical measurement of isomerization rate enables several benefits such as high accuracy, no need for sensor specific calibration, small size and contact-free reading of the sensor. Still, with all these features the sensor structure is kept simple allowing cost-effective manufacturing. Such a combination of features and affordability is not commercially available for humidity sensing. Patents for the measurement device and method are pending in Europe, USA, Canada and China.
Two distinct device schemes will be pushed forward: one where the active material is attached to the optical fiber head and another where the active material is placed into a sealed chamber. The former scheme is directed towards construction industry and other applications where humidity of air in a relatively large and accessible space is monitored. In the latter method, the sub-mm sensing thin film is placed during sealing into a vessel inside which humidity needs to be monitored. Readout is done through a transparent section in the vessel wall using a fiber-optic probe. This approach opens new possibilities for quality control in pharmaceuticals and electronics and for scientific research.
