Mercury is one of the most acute toxic heavy metals at trace levels and its detection at parts per billion (ppb) scale in a low-cost, simple way remains challenging. Here, we report a novel "Turn-On" fluorescence sensor based on aptamer-templated ZnO quantum dots (QDs), which was further developed into an electronic detection device and utilized for the rapid detection of mercury ions (Hg 2+). Binding of Hg 2+ with the aptamer leads to the formation of a duplex, T-Hg 2+-T, and this acts as a template for the formation of ZnO QDs. With an increase in the concentration of Hg 2+ , we observed an increase in duplex formation, leading to enhancement in the fluorescence. The limit of detection of the device is 0.1 ppb (0.5 nM), and experimental analysis has a linear range of detection between 0.1 and 10,000 ppb. Electron microscopy studies ascertained the crystalline nature of the aptamer-templated ZnO QDs in the presence of Hg 2+. Finally, the sensing was implemented as a simple working electronic detection device prototype. A photodiode coupled with an LED source was connected to an Arduino Nanoboard for the development of the device. To the best of our knowledge, this is the first study to report an enhancement in the fluorescence of the aptamer-templated ZnO QDs with an increase in the concentration of Hg 2+. Moreover, this is the first report of an analyte addition during the synthesis of QDs, leading to fluorescence-based sensing of the added analyte. Thus, a novel fluorescence sensor based on aptamer-templated ZnO QDs has been demonstrated with high sensitivity and selectivity, resulting in a simple electronic detection device.