Provided herein are methods and devices for rapidly and accurately heating fluid droplets surrounded by a gas-phase medium, such as air. Sub-nanoliter fluid droplets can be rapidly heated by nanoscale field effect transistors via microwave heating by an applied AC voltage to the FET. The heating is in a well-defined interior portion of the fluid droplet, with minimal heating of the outer portion of the fluid droplet, thereby minimizing evaporation. In this manner, rapid thermal cycling is possible, including independently and in parallel for a plurality of droplets. Accordingly, the methods and devices provided herein are used in point-of-care detection such as by PCR that is high speed, robust and of low cost.