Monitoring disinfectants is essential to maintain water free from pathogens. Permanganate (MnO4-, applied as the potassium salt) is a commonly used disinfectant in drinking water and wastewater treatment plants. To ensure the proper level of permanganate during and after the water treatment process, it is necessary to monitor the concentration of MnO4-. Current methods for MnO4- detection either lack the suitability for low concentrated samples (direct colorimetry) or require reagents and sample preparation. To tackle these issues, we introduced a reagent-free chemiresistive sensing platform capable of continuous measurement of low concentrated MnO4-. Chemiresistive sensors consist of two parallel contacts connected by a layer of carbon nanotubes (CNTs). Change in concentration of the analyte results in resistance change in sensors and was recorded as current change by applying a constant voltage. We have functionalized the nanocarbon substrate with five different redox-active molecules where 4 of them are pH-responsive and one is not responsive to the pH range used for the analysis. The sensors were shown to operate over a range of 5 µg/L to 3 mg/L of MnO4-. These redox-active molecules create active sites on the substrate, and when in contact with the analyte, dopant molecules attached on the surface react with analytes and give responses. These responses varied in magnitude depending on pH and the redox-active molecule. The sensor responses at different pHs were then analyzed using principal component analysis (PCA) to identify the pH of the permanganate solution. PCA analysis resulted in a clear separation of different pH solutions. We have therefore demonstrated a nanocarbon-based sensing array that is capable of simultaneously and continuously measuring low concentrations of MnO4- and the pH of the aqueous analyte.

Chemiresistive carbon nanotube sensors for aqueous permanganate

ACS Fall 2021

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