Photoactivated acidochromic elastomeric films for on demand acidic vapor sensing

Abstract

The acid vapor sensing properties of photochromic elastomeric composite films based on spiropyran molecules embedded in poly(dimethylsiloxane) matrices are investigated. The conversion of the spiropyran molecules to their zwitterionic merocyanine isomers upon UV irradiation is essential for the acidochromic response of the composite system, in order to detect vapors of hydrochloric, trifluoroacetic, formic and acetic acids by means of the protonation of the phenolate ions. The formation of the protonated merocyanine species is accompanied by a change in the absorption spectrum and a clear color transition of the film. Differences in the protonation rate are observed for different acids due to the relative acidity of the vapors, and for different concentrations of merocyanine. The acidochromic detection can be switched on and off several times by UV irradiation and subsequent thermal regeneration of the non-polar spiropyran form with an excellent resistance to photodegradation. The facile method of preparation combined with the fast and reversible optical response activated on demand upon photoexcitation renders this material suitable for the development of portable sensing devices for environmental and industrial applications.