A surface acoustic wave resonant sensor for measuring a sample comprising a single port surface acoustic wave (SAW) resonator comprising an interdigital transducer and at least one reflective grating. The sensor is provided with a region for receiving the sample, said region being in communication with the at least one reflective grating and the IDT is separated acoustically and electrically from the region for receiving a sample such that the IDT is not mass sensitive to the sample. The sensor is especially suitable for bio sensing applications.