The present invention provides sensors based on micromachined ultrasonic transducer technology. The sensors preferably include a plurality of sensor elements, but may include only one sensor element. Arrays of sensors are also provided. Sensor elements include a functionalized membrane supported over a substrate by a support frame. The functionalized membrane, support frame and substrate together form a vacuum gap. The sensor element is connected to an electrical circuit, which is configured to operate the sensor element at or near an open circuit resonance condition. The mechanical resonance frequency of the functionalized membrane is responsive to binding of an agent to the membrane. Thus, the sensor element also includes a detector, where the detector provides a sensor output responsive to the mechanical resonance frequency of the sensor element.