Carbon Nanotube-Based\nChemiresistive Sensor Array\nfor Dissolved Gases

Abstract

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