Acoustic sensors for physical, chemical and biochemical applications

Since the late 1950s, it has been recognized that one can make sensitive high-resolution sensors by exploiting the effects of various measurands on propagating ultrasonic waves. The earliest and best-known acoustic-wave sensor is the quartz-crystal microbalance (QCM) employed as a mass sensor for monitoring film thickness in thin-film deposition systems and by researchers studying chemical interactions with thin films deposited on the crystals. H. Wohltjen first demonstrated that a rugged and sensitive gravimetric acoustic sensor could be made in the form of a delay-line oscillator employing an electronic amplifier and a surface acoustic wave (SAW) delay line operating at hundreds of MHz. Additional ultrasonic sensor modes and structures have been demonstrated: flexural modes in thin micromachined plates and narrow rods, thickness resonators made by thin-film deposition rather than mechanical thinning of single-crystal piezoelectrics, and transverse mode devices such as SAW devices having surface modifications or a single-crystal plate-mode device. A figure of merit for a gravimetric sensor is Sm = Dm/(Dvp/vp), where vp = phase velocity, and Dvp = change of phase velocity caused by the added mass per unit area, Dm. Reported values of Sm have ranged from 10 to 1200 cm2/g. The minimum detectable added mass per unit area depends both on Sm and on the noise level of the specific acoustoelectric oscillator used. Among the significant findings made as these devices have been studied are: the advantages of measuring changes of both the phase velocity and the attenuation of the ultrasonic wave, the role of surface roughness in coupling a transverse-mode (shear) resonator to a neighboring liquid, and the acoustical effects produced by polymeric films used on the devices to promote the absorption of organic vapors. Demonstrated applications of these devices include: measuring the density and viscosity of liquids, determining the rheological properties of polymer films, quantitative detection of the concentrations of volatile organic vapors with sensor arrays, detecting the outputs of a gas chromatographic column, and both selective and sensitive detection of biochemically active compounds by employing antibody-antigen, enzyme-substrate, and other receptor-protein pairs.