Single-walled Carbon Nanotube Chemiresistive Sensors for the Identification and Quantification of Disinfectants
Authors: Akbar Md Ali , Sharif Omar , Selvaganapathy Ponnambalam Ravi , Kruse Peter 
Published: November 2021

Disinfectants are essential to keep water safe as they kill pathogens by oxidizing the cell membrane. Free chlorine, potassium permanganate, monochloramine, hydrogen peroxide, chlorine dioxide, ozone, and hypobromous acid are the most commonly used disinfectants for the disinfection of water.1 Monitoring the concentration of the disinfectant is crucial as the effectiveness mainly depends on the amount of disinfectants present in water.2 Insufficient levels of disinfectant in drinking water could impose health risks as pathogens could regrow in the distribution system. Standard methods for measuring the disinfectant levels are colorimetric and titrimetric.3 For free chlorine, DPD colorimetric method is well established.1
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4 For potassium permanganate (KMnO4), colorimetric methods (persulfate and periodate) and spectroscopic methods are available.1
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5 Currently, the monitoring of disinfectants is done on samples collected at the treatment plant and in different locations throughout the distribution system. This reagent-based measurement of discrete samples requires reagents and spectroscopic readout devices and can suffer from interference. Therefore, these methods are not suitable for the continuous monitoring of the disinfectant.6 Furthermore, there is no known method that can differentiate between multiple disinfectants in real samples without any previous knowledge.
Two of the most important water quality parameters are pH and oxidation-reduction potential (ORP). Disinfectants undergo different reactions at different pH, resulting in different active species for different pH. ORP measurement can be used to monitor whether the disinfection was successful as oxidants’ reactivity in water depends on redox conditions.7 ORP of a solution is associated with the pH and the concentration of the disinfectant. At a fixed pH, the ORP of a disinfectant will depend on the concentration. Therefore, just by knowing the ORP, we cannot distinguish disinfectant unless the pH or concentration of the oxidant is known.8
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9

To sort out these parameters, we propose to introduce a chemiresistive sensing array capable of distinguishing and quantifying disinfectants. Due to simpler fabrication techniques, lower cost and the ability to sense various analytes through appropriate ligand, chemiresistors have great promise in water quality sensing.6
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10 Previously reported chemiresistive sensor for continuous measurement of free chlorine used carbon nanotube (CNT) substrate functionalized with a redox-active aniline oligomer named phenyl capped aniline tetramer (PCAT).11 Oxidation of PCAT attached to the surface of CNT by free chlorine leads to a change in the oxidation state of the oligoaniline; this change in oxidation state in the molecule changes the doping characteristics of CNT leading to a change in resistance of the CNT film. This resistance change was used to quantify the concentration of free chlorine.11

In this work, we have constructed an array of single-walled carbon nanotubes (SWCNT) sensors functionalized with five redox-active molecules. Structurally different from each other, these molecules will create different active sites. Upon interaction with analytes, each molecule will give different magnitudes of responses. Sensor responses are collected for free chlorine and KMnO4 over a range of concentrations for pH 6.5 and 7.5. In general, sensors give increasing responses to the increasing concentration and decreasing pH of the solution. Though the responses from the sensors have a similar trend to the change of the parameters their varied magnitudes make them suitable for an array. Sensor responses are then analyzed with principal component analysis (PCA). PCA analysis shows clear separation between the analytes and as well as to different pH and concentrations of the solutions (figure). We have therefore demonstrated an array of chemiresistive sensors that can distinguish and quantify disinfectants.

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Figure 1