Pencil-Drawn Chemiresistive Sensor for Free Chlorine in Water

Free chlorine concentration in drinking water is an important control parameter throughout the distribution network, since both too-low and too-high concentrations can lead to health hazards, with 0.5-2 ppm being a typically acceptable range. Although colorimetric and electrochemical methods exist for measuring free chlorine, they require either the addition of chemicals during manual sampling or maintenance intensive instrumentation, such as reference electrodes. We have previously demonstrated a simple, reliable, reagent-fee solid-state chemiresistive sensor for continuous monitoring of aqueous free chlorine concentrations based on a functionalized carbon nanotube film. Here, we show that the same sensing principle can be translated to lower cost materials-as simple as a line drawn between two contacts using a free IKEA pencil. While IKEA (HB grade, about 70% graphite) pencils work well enough for quantitative free chlorine sensors, the use of 9B grade pencils (90% + graphite) results in higher sensitivity. Bare pencil line sensors show a nonselective response to a wide range of aqueous species; pencil lines functionalized with a redox-active aniline oligomer (phenyl-capped aniline tetramer), however, are highly selective to oxidant species, namely free chlorine, in a demonstrated range of 0.06-60 ppm. The resulting sensors are cost-effective, durable, resettable, reusable, and resistant to fouling. In contrast to electrochemical sensors, they do not require the use of a reference electrode. They can be operated continuously online for drinking water quality monitoring.