Abstract
Sensing of metal contaminants at ultralow concentrations in aqueous environments is vital in today's overpopulated world, with an extremely stringent limit (<5 ppb) for Cd(2+) ions in drinking water. Here, we utilize sonochemically exfoliated molybdenum disulfide (MoS(2)) nanosheets functionalized with l-cysteine (Cys) as highly sensitive and selective two-dimensional (2D) materials for solid-state chemiresistors. We specifically targeted Cd(2+) ions due to their high toxicity at low concentrations. MoS(2)-Cys nanosheets are fabricated using an ad hoc, low-complexity, one-pot synthesis method. Porous MoS(2)-Cys thin films with a high surface area are assembled from these nanosheets. Two-terminal chemiresistors incorporating MoS(2)-Cys films are demonstrated to be preferentially sensitive to Cd(2+) ions at neutral pH, irrespective of other metal ions present in water flowing through the device. A 5 ppb concentration of the Cd(2+) ions in the water stream increases the device resistivity by 20 times. Our devices operate at broad (1-500 ppb) range and fast (∼1 s) response times. Cd(2+) is selectively detected because of preferential, size-driven adsorption at the interstitials between l-cysteine functional groups, combined with pH-controlled charge transfer that removes electronic gap states from MoS(2). MoS(2)-Cys-based chemiresistors can be deployed in-line to detect metal ions without any need for additional offline measurements.