Water plays an important role in the economic world; only 3% of available water is potable, and with increasing demand, the need for safe drinking water is rising. The National Academy of Engineering identified “providing access to clean water” as one of the top 10 grand challenges for engineering in the 21^st century. Heavy metal ions, e.g., lead, mercury, and arsenic, are widely present in water systems. These heavy metal ions are poisonous and may lead to serious damage to organs, tissues and bones, and nervous systems of humans. Existing water quality monitoring mostly occurs at the water supply intake or water treatment plant, instead of along water distribution lines and at the point of use, which is considered inadequate given potential changes in water quality and associated risks within water distribution systems. Current detection methods for aqueous heavy metal ions are often too expensive or unsuitable for in-situ and real-time detection (an unmet need). A field-effect transistor (FET) device-based sensor is developed to specifically detect Pb²⁺ ions in an aqueous environment. Reduced graphene oxide (rGO), as the semiconducting channel material, is utilized in the FET device through a self-assembly method. An L-glutathione reduced (GSH) is employed as the capture probe for the label-free detection. By monitoring the electrical characteristics of the FET device, the performance of the sensor is measured and investigated. Compared with conventional detection technologies, this sensor enables real-time detection with a response time of 1-2 seconds. A lower detection limit for Pb²⁺ ions as low as 10 nM is achieved, which is much lower than the maximum contaminant level (MCL) for Pb²⁺ ions in drinking water advised by the World Health Organization (WHO). Furthermore, the rGO FET sensor is able to distinguish Pb²⁺ from other metal ions. Without any sample pretreatment, the platform is user-friendly.