A sensor includes a substrate 1 and at least one resonator, which includes an acoustic reflector 2, a piezoelectric layer 5, a first and second electrode 3, 4 placed on the same side of the piezoelectric layer 5, and a sensing layer 6. The sensing layer 6 reacts with a chemical or biological agent by absorption, adsorption, desorption or chemical reaction. As a result the individual frequency of a resonator changes and conclusions about the agent can be drawn. Such a sensor is very sensitive to an agent being sensed, especially when used in liquids.