Laser-engraved ammonia sensor integrating a natural deep eutectic solvent

Abstract

Ammonia is a naturally-occurring gas, present throughout the atmosphere at sub-ppb concentrations. As result of various processes, significantly higher concentrations can be found in the areas surrounding industrial and agricultural operations. In such cases, the presence of NH3 in air is not only associated with a very pungent odor but can also affect the environment, animals, plants, and humans. Aiming to provide a simple platform to monitor NH3 in air, a paper-derived gas sensor, integrating a natural deep eutectic solvent (NADES) was developed. The herein described paper-derived ammonia gas sensor was fabricated using direct laser scribing, to form a carbon-based interdigitated array. Characterization and optimization (using chemometrics) were performed to develop a material with the lowest possible resistivity. A unique feature of the proposed sensor is that it integrates a natural deep eutectic solvent formed with lactic acid:glucose:water (LGH). This NADES conveys selectivity toward ammonia (over methanol, ethanol, 1-propanol, and water) and greatly enhances the sensitivity of the sensor (over the plain carbon array). The resistance of the sensor was dependent on the NH3 concentration in the 0.11–40.6% range and rendered an estimated limit of detection of 0.33%. Although the sensitivity of the sensor is lower than that of others based on metallic oxides, the herein described approach represents a low-cost, portable, and simple way to address the need of emerging markets such as the NH3-based fuel industry or waste treatment where most of the current sensors would be saturated.