Carbon Nanotube-Based Chemiresistive Sensor Array for Dissolved Gases

Dissolved gases such as oxygen (DO) and ammonia (dNH₃) are among the most consequential parameters for the assessment of water quality. Since the concentrations of DO and dNH₃ are interdependent through the nitrogen cycle, simultaneous monitoring can be useful in many applications. For example, in wastewater treatment, aeration baths are used to adjust the rate of removal of ammonia by the bioactive sludge. Here, we have developed a sensing array which can monitor dissolved molecular oxygen (DO) and dissolved un-ionized ammonia (dNH₃) continuously and simultaneously. This was achieved by functionalizing two sensors made from single-walled carbon nanotube (SWCNT) films with two different molecules: phenyl-capped aniline tetramer (PCAT) and iron phthalocyanine (II) (FePc). It was found that the FePc-doped SWCNT (FePc@SWCNT) sensors demonstrated good sensitivity and selectivity to DO, compared to dNH₃. Conversely, we found that the PCAT-doped SWCNT (PCAT@SWCNT) sensors demonstrate greater sensitivity to ammonia. Investigating the effect of different PCAT salts as a dopant, we describe the following series of sensor responses to ammonia: chloride < crotonate < fumarate. Additionally, we coated our sensors with thin PDMS membranes, which are selectively permeable to gases, over ionic species. Finally, using principal component analysis (PCA) and partial least-squares discriminant analysis, it was possible to discriminate between responses to DO and dNH₃, with 100% accuracy. As a result, here, we have developed a compelling proof-of-concept for the use of a single sensing substrate, doped with molecules with distinct mechanisms of interaction with two different analytes, to simultaneously monitor concentrations of two dissolved gases, in this example, DO and dNH₃.