Recent developments and sustainability in monitoring chlorine residuals for water quality control: a critical review†

Clean and safe water is a vital resource for human life. To ensure that consumable water is bacteria-free, water treatment, including the widely used chlorination process, is performed. Free chlorine resulting from the chlorination process in consumable water is a dangerous analyte and it is one of the vital parameters in water quality monitoring. Global guidelines state that free chlorine in consumable water should be controlled at 0.2–5.0 mg L; deviations from this concentration range could cause consumers to suffer from dire health effects. To control the concentration within the said range, various methods for free chlorine monitoring have been developed in recent years, categorized into conventional, optical and electrochemical methods. However, limitations such as high cost and complexity of analysis prevent these conventional methods from meeting the “Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free and Deliverable to end users” criteria for diagnostic tests set by the World Health Organization. Paper-based methods are therefore introduced to replace the conventional methods in the hope of meeting the criteria. However, the paper-based methods are still confined to the lab scale and are highly dependent on chemicals for the detection of free chlorine. Therefore, the capabilities of carbon quantum dots are introduced as a suitable indicator for free chlorine measurement. Using carbon quantum dots as an indicator is recommended for the future development of sustainable portable paper-based sensors due to their excellent absorption and fluorescent properties; in addition, carbon quantum dots can be synthesized from natural resources.