Paper-based printed electrical gas sensors and their application in food freshness and human breath monitoring

This thesis introduces an entirely new class of printed electrical gas sensors that are produced at near 'zero cost'. This technology exploits the intrinsic hygroscopic properties of cellulose fibres within paper; although it feels and looks dry, paper contains substantial amount of moisture, absorbed from the environment, enabling the use of wet chemical methods for sensing without manually adding water to the substrate. The sensors exhibit high sensitivity to water-soluble gases with a fast and reversible response. The sensors show comparable or better performance (especially at high relative humidity) than most commercial ammonia sensors at a fraction of their price (< $0.02 per sensor for the cost of materials for laboratory prototypes). This thesis demonstrates that the sensors proposed can be integrated into food packaging to monitor freshness of protein-rich food. Tests on poultry and cod fish fillet show that the sensors detect an increase in decomposition gases (mainly trimethylamine and ammonia) which correlates to the findings of microbial control experiments. The sensors are implemented into near-field-communication tags to function as wireless, battery-less gas sensors that can be interrogated with smartphones. In a next step this thesis demonstrates functionalized versions of the paper-based gas sensors where the pH of the paper is changed by adding sulfuric acid or sodium hydroxide. In the presence of alkaline or acidic gases the neutralization effect in the sensors decouple fluctuations in relative humidity from gas concentration changes. An array of sensors with different pH can distinguish between alkaline and acidic gases. The functionalized sensors quantitatively detect ammonia in a respiratory simulator where relative humidity varies widely with inhalation and exhalation. In healthy human subjects the sensors detect ammonia when a subject eats salty liquorice (an ammonium chloride containing candy) while breathing normally. This can be used to detect increased ammonia levels in human breath which can indicate a variety of renal and hepatic diseases.