Control and detection of humidity is most crucial in process and packaging applications. At present, mainly sensors with an electronic read-out are in use. This contribution introduces an alternative, directly optically (visually) readable humidity detection method. The used advanced hybrid metal/polymer surfactant films allows precise detection of water and solvent vapor levels without requiring an electrical reactant or power supply. More specifically, the sensor film can be easily obtained from copper nanoparticles protected by a 2-3 nm carbon shell, a polymeric dispersant and a wetting agent. The films showed an exceptional sensitivity with optical shifts in the visible light range of up to 50 nm for a change of 1% in relative humidity. This coloration response of the nanocomposite film could be attributed to the formation of a water-surfactant interference film when brought in contact to air with a certain humidity level.

Figure: Deposition of C/Cu dispersions on a glass substrate results in metallic copper films (a). Exposure to a stream of ethanol vapor resulted in rapid and reversible, intense iridescent coloration (b). Exposure to saturated water vapor (c) at room temperature provoked an intense green coloration that gradually shifted back to pink (d), blue (e), and orange (f) under ambient humidity.

References:

Athanassiou, E. K., R. N. Grass, et al. (2006). "Large-scale production of carbon-coated copper nanoparticles for sensor applications." Nanotechnology 17(6): 1668-1673.

Luechinger, N. A., S. Loher, et al. (2007). "Highly sensitive optical detection of humidity on polymer/metal nanoparticle hybrid films." Langmuir 23(6): 3473-3477.

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