Acoustic Love-wave sensor for K+ concentration in H,O solutions

In this Paper we present an acoustic wave delay line sensor for the detection of K+ ion concentrations in water solutions. Horizontally polarized shear acoustic modes propagating in quartz plate (Love Modes) are used in order to minimize acoustic energy loss in the liquid. The device consists of a quartz plate with one of the free surfaces covered with a thin membrane that selectively sensitive to K+ (1). Love waves are generated and detected by interdigital transducers located on the opposite surface of the plate. The advantage of this arrangement is that electroacoustic transducers are separated from the liquid environment under test. The operation of the device is based on the changes produced by the K ions captured by the membrane on the phase velocity of the acoustic waves. A differential structure is used with only one line covered by the sensitive membrane in order to compensate common mode error sources. The different sensing mechanisms of the device have been theoretically investigated by calculating the phase velocity shifts produced by changes in the physical properties of the membrane such as elastic constants, mass density, electrical conductivity. Measurements of the sensor response vs different K+ concentrations and for different acoustic modes are reported and compared with theoretical results.