Development of Solid-State Chemiresistive Devices for Simultaneous Detection of Nitrate, Nitrite and Ammonium Ions in Aqueous Solutions
Authors: Darestani-Farahani Maryam , Selvaganapathy Ponnambalam Ravi , Kruse Peter 
Published: July 2022

The presence of various ions is an important factor in evaluating water quality. Nitrate, nitrite and ammonium in water can cause health issues and pose environmental concerns1,2. Some sources of the nitrogen species are from natural water and some of them arise from industrial and agricultural activities. Currently commercially available sensors for measuring nitrogen compounds in water are based on colorimetric techniques and potentiometric methods incorporating ion selective electrodes. Progress still needs to be made towards fabricating devices with less instrumentation costs, less complexity, less maintenance and without need for any reagents to facilitate measurement of species1. Chemiresistive sensors are solid-state devices which can be simply fabricated from two contacts and a conductive sensing material affixed on a suitable substrate. They operate by detecting modulations in resistance of the conducting film due to surface charge transfer as a result of interactions with the analyte(s)3.

Here, we demonstrate chemiresistive devices capable of quantifying aqueous nitrate, nitrite and ammonium ions selectively. Due to challenges in the aqueous phase such as ionic strength effect, probability of side reactions, non-specific bonding on the surface, low interaction energy between analyte and surface, chemiresistive technology has not been developed extensively in water quality sensors4,5. In this study, we have overcome the issues in water by coating the chemiresistive devices with selective membranes. If the fabricated sensors are used as an array, the total nitrogen concentration in water can be measured online which is a significant advance since nitrate, nitrite and ammonium may interconvert and a single nitrate, nitrite or ammonium sensor by itself cannot give the total amount of nitrogen in a sample. For device fabrication, p-doped carbon nanotubes were selected as a sensitive conductive layer which were modified with selective membranes to improve the sensing performance. Nitrite sensors worked over a dynamic range of 67 ppb to 67 ppm with a 27.6% response at 67 ppm. Nitrate showed 13.2% response from 2.2 ppm to 220 ppm. Ammonium devices operated over a dynamic range of 10 ppb to 100 ppm with a 23.6% response at 100 ppm. The proposed response mechanism involves both an electrostatic gating effect and surface charge transfer. Compared with paper-based colorimetric sensors, the proposed devices perform better with a lower detection limit and the ability to perform continuous online measurements. Moreover, the chemiresistive responses of the devices were compared with their potentiometric responses and found to be equally sensitive but more selective. Unlike ion-selective electrodes, the resulting devices do not require the use of reference electrodes and are therefore potentially more robust for use in continuous water analyzers and in resource-poor settings. The chemiresistive devices showed low interferences and good reversibility. They were also tested in river water samples and showed satisfactory results. The fabricated devices are an advanced proof of concept and have the potential to replace the current technology.

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