Sol-gel materials for electro-optical and optically active humidity-sensitive devices

One of the main research activities of the Sol-Gel Group@ICMM (GSG-ICMM) is devoted to smart photonic materials derived from the incorporation of optically active substances in glasses. Those smart materials allow the control of their optical properties by applying external stimuli (e.g., electric field). That is the case of gel-glass dispersed liquid crystals (GDLC), where a sol-gel derived matrix with large porosity and embedded with liquid crystal (LC), forms a thin film between transparent conductive substrates needed to apply the electric field. The dielectric anisotropy of the LC permits the reorientation of the encapsulated LC by means of electric fields, enabling the control of its light transmission properties, from opaque (scattering) to transparent states. A different route to obtain LC dispersion devices was designed, taking advantage of biological structures created by bacterial activity. In this case, biofilms of the bacterium Pseudomonas putida mt-2 were grown on transparent conductive substrates from microbiological culture media. The chemical composition and the tridimensional porous structure of the biofilm enabled its use as a biological template for a LC dispersion device, where the LC is embedded in the biofilm porosity. The electrical control of the light transmission properties of these devices was carried out by applying electric fields. This new and non-chemical strategy expanded the possibilities for creating smart photonic materials by using bacterial cell factories. In the same topic of materials for the smart control of light transmission, a new concept was stablished to obtain an optical thin film material that exhibits reversible humidity-responsive transmittance. This material consists on a porous structure, with embedded hygroscopic and deliquescent compounds, having light scattering properties that depend on the humidity of the air in contact with the material. With dry air the material displays an opaque state due to light scattering. When exposed to humid air, the material scavenges water from the air, filling up its porosity with water and becoming transparent to the incident light. Upon exposure to dry air, water is released from the thin film material, recovering its previous light scattering state. The developed thin films can change their transparency when exposed to air with different relative humidity (RH), adjusting the light throughput. Therefore, this material concept can be used to design new optical devices, having the advantage that they do not require LC, transparent conductive glass substrates or complex layer-by-layer architectures for operation as in conventional smart windows.