Continuous Monitoring of Free Chlorine Level and pH Using an Array of Carbon Nanotube Chemiresistors

Free chlorine (FC) is the most commonly used disinfectant in water treatment plants, both as a primary disinfectant for pathogen removal and as a secondary disinfectant to suppress pathogen growth in the distribution system. The "disinfectant power" is dependent on the concentration of FC as well as the pH of the water. Continuous monitoring of FC level and pH is crucial to ensure safe drinking water; however, currently used methods involve either frequent calibration or reagents. Here, an array of single-walled carbon nanotube (SWCNT) chemiresistors is demonstrated for the continuous monitoring of FC at different pHs. The SWCNT chemiresistors were noncovalently functionalized with cobalt phthalocyanine and anthraquinone. The array has been shown to differentiate FC concentrations ranging from 0.03 to 2.1 mg/L within a pH range of 6.5 to 9.5. In addition, sensor design has been improved from our previous devices to incorporate components to facilitate mass fabrication. This design was tested over a wide range (0.015-10 mg/L) of FC. The limit of detection (LOD) of the sensor was calculated to be 0.001 mg/L. An electronic reset function is incorporated into the sensors to be able to continuously monitor the concentration of FC. The durability of the sensors is demonstrated with repeated measurements in simulated tap water. Overall, this study presents electrical sensor-based continuous monitoring of FC.