Chemical sol–gel-based sensors for evaluation of environmental humidity

Crystal violet and chlorophenol red pH indicators were independently encapsulated into sol–gel matrices to be used as environmental humidity and acidity optical sensors, respectively. Absorption spectra of the prepared sensors were recorded before and after exposure under different controlled artificial atmospheres. Results showed the possibility to estimate the environmental relative humidity with the crystal violet-doped sensor in the range between 20 and 90%, since a colour change occurred after approximately 10 min exposure. On the other hand, the presence of environmental pollutants with acid properties, mainly SO2, was also possible to be detected by means of an optical absorption change of the chlorophenol red-doped sensor. Its response time was about 1 min. Both sensors are found to be reversible and reusable without showing optical fatigue. Consequently, the sensors are adequate for an easy and fast monitoring in one step of relative humidity and environmental acidity of engraved historical, artistic and cultural heritage assets and can help to improve the preventive conservation of this valuable legacy.