Detecting ammonia in ambient air with high sensitivity and ultra-fast responsivity is crucial given its implications on human health. The response of such sensors should also be reversible to use them for continuous monitoring. Herein, we report a reversible ammonia (NH3) sensor based on 2D ZnO nanoflakes at 250 °C. The sensor exhibited a maximum response of 80% and sub-15 s response and recovery times upon exposure of 0.6–3 ppm NH3. Further, we formulated and corrected the baseline drift with a simple and straightforward baseline manipulation method. The excellent response of the sensor indicates the feasibility of using it in diverse applications where high sensitivity and rapid response are crucial. © 2020