Mass detection sensitivity of piezoelectric cantilevers with a nonpiezoelectric extension

A piezoelectric cantilever (PEC) is a transverse transducer consisting of a piezoelectric layer, e.g., lead zirconate titanate (PZT), bonded to a nonpiezoelectric layer, e.g., stainless steel, which has a uniform cross section throughout the length. A PEC with a thin nonpiezoelectric extension, on the other hand, has two distinctive sections each with its own thickness and transverse mass-density and elastic-modulus profiles. A piezoelectric cantilever with a nonpiezoelectric extension has been increasingly used as an in situ biosensor that has the advantage of dipping only the nonpiezoelectric extension part in an aqueous solution without electrically insulating the piezoelectric section. In this study, we examined both experimentally and theoretically the effect of the thin nonpiezoelectric extension, in particular, its length ratio to the piezoelectric part on the vibration wave form and mass detection sensitivity of a PEC. We showed that the nonpiezoelectric extension caused substantial distortion to the vibration wave form. Due to the wave form distortion, the mass detection sensitivity of a PEC with a nonpiezoelectric extension can be higher than that of a PEC of the same length without a nonpiezoelectric extension. Using PECs consisting of a 0.25 mm thick PZT layer and a 0.07 mm thick stainless steel of various lengths, l₁, and a 0.07 mm thick nonpiezoelectric stainless steel extension of various lengths, l₂, we showed that for any l₁, the maximum mass detection sensitivities of first, second, and third modes occurred at l₂/l₁=0.6, 0.28, and 0.16 and the maximum values were, respectively, 1.42, 3.9, and 6.7 times the mass detection sensitivity of a PEC of the same l₁ without a - nonpiezoelectric extension.