Poly(aniline) doped with 5-formyl-2-furansulfonic acid: A humidity memory

We present a new concept for a humidity sensor based on doped poly(aniline) that shows irreversible changes of its sheet resistance and capacitance upon exposure to humid conditions, i.e. a humidity memory. Therein humidity triggers an irreversible dedoping process, which results in a phase separation and crystallization of the hydrated form of its protonating agent, as suggested by combining FT-IR and UV–Vis-NIR spectroscopies, optical microscopy, dc-conductivity, and frequency dependent capacitance measurements. We also introduce a simple concept to activate the sensor and to monitor it through non-contact capacitance measurements. We anticipate that the proposed concept could be implemented for functional packaging to qualitatively detect whether their contents have been exposed to humidity during their transportation and storage history.