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

An acoustic delay-line sensor for the detection of K+ ion concentrations in water solutions is described. It uses shear horizontal (SH) acoustic plate modes propagating in plates 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 sorbs K ions. The membrane consists of a mixture of polyvinylchloride (PVC), dibutylsebacate, tetrahydrofuran and valinomycin, which is a well-known ligand for K+. SH 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, which are particularly severe for the specific case of devices operating in liquid environments. The time response on K+ absorption and desorption, as well as the response versus K+ concentration, are reported here.