Fentanyl, an opioid painkiller, is the leading cause of accidental opioid overdose as many recreational drugs are unknowingly laced with it. Our solution: Quantifen! Quantifen is a non-invasive wearable biosensor adapted for fentanyl detection in sweat. Taking the form of a temporary tattoo consisting of conductive ink layers and cellular-functionalized iontophoretic hydrogels, biological detection is converted into electrical output. This output is transferred as data to our app, warning the user of fentanyl consumption via mobile alerts or by contacting emergency services. The biosensor consists of a genetic circuit which produces glucose oxidase (GOx) in response to fentanyl binding to receptor protein FEN21. GOx undergoes electrochemical reactions, creating current which is carried via screen-printed conductive inks to a printed circuit board (PCB) in the wearable device; it then communicates with the user’s smartphone. The biosensor can be adapted for detection of other drugs or small molecules due to its modularity.