Wan Y. Shih

Effect of length, width, and mode on the mass detection sensitivity of piezoelectric unimorph cantilevers

We have investigated both experimentally and theoretically the resonance frequency change of a piezoelectric unimorph cantilever due to the mass loaded at the tip of the cantilever. The piezoelectric cantilever was composed of a lead zirconate titanate (PZT) layer and a stainless steel layer. The dependence of the resonance frequency shift, Δf, with respect to a loaded mass, Δm, on the cantilever length, L, width, w, and the resonance modes was examined. For Δm much smaller than the effective mass of the cantilever, we showed that Δf/Δm increased with an increasing eigen value, νn2, and decreasing length, L, and decreasing width, w, as Δfn/Δm=−(νn2/4π)(1/L3w)(1/0.23612ρ)Ẽ/ρ, where Ẽ and ρ are the effective Young’s modulus and effective density of the unimorph cantilever that depends on the thickness fraction, Young’s modulus and density of each of the individual layers. Thus, given the same Ẽ and ρ by maintaining the same layer thickness fractions of the individual layers, Δf/Δm is increased by a fact...

Simultaneous liquid viscosity and density determination with piezoelectric unimorph cantilevers

We have examined both experimentally and theoretically a piezoelectric unimorph cantilever as a liquid viscosity-and-density sensor. The fabricated piezoelectric unimorph consisted of a PbO⋅ZrO2⋅TiO2 (PZT) layer on a thin stainless-steel plate. In addition to a driving electrode, a sensing electrode was placed on top of the PZT layer, permitting the direct measurement of the resonance frequency. The cantilever was tested using water–glycerol solutions of different compositions. In all three of the tested modes, the resonance frequency decreased while the width of the resonance peak increased with increasing glycerol content. To account for the liquid effect, we consider the cantilever as a sphere of radius R oscillating in a liquid. By including the high and low frequency terms in the induced mass and the damping coefficient of the liquid, we show that for a given liquid density and viscosity the oscillating-sphere model predicts a resonance frequency and peak width that closely agree with experiment. Fur...

In situ cell detection using piezoelectric lead zirconate titanate-stainless steel cantilevers

We have investigated piezoelectric lead zirconate titanate (PZT)-stainless steel cantilevers as real-time in-water cell detectors using yeast cells as a model system. Earlier studies have shown that mass changes of a cantilever can be detected by monitoring the resonance frequency shift. In this study, two PZT-stainless steel cantilevers with different sensitivities were used to detect the presence of yeast cells in a suspension. The stainless steel cantilever tip was coated with poly-L-lysine that attracted yeast cells from the suspension, and immobilized them on the cantilever surface. After immersing the poly-L-lysine coated tip in a yeast suspension, the flexural resonance frequency of the cantilever was monitored with time. The flexural resonance frequency decreased with time in agreement with the optical micrographs that showed increasing amount of adsorbed yeast cells with time. The resonance frequency shifts are further shown to be consistent with both the mass of immobilized cells on the poly-L-l...

Non-heavy-metal ZnS quantum dots with bright blue photoluminescence by a one-step aqueous synthesis

We have examined the aqueous synthesis of non-heavy-metal ZnS quantum dots (QDs) using 3-mercaptopropionic acid (MPA) as the capping molecule at various pH and MPA:Zn:S ratios. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) indicated that the aqueous ZnS QDs were 3–5 nm in size with a zinc blende structure. We showed that, at pH 12 with a MPA:Zn:S = 8:4:1 ratio, the ZnS QDs with optimal blue emission could be obtained in a one-step, room-temperature aqueous process that exhibited a quantum yield of 31%, higher than that of the commercial CdSe/ZnS core–shell QDs. The present ZnS QDs could pass through a 50 kD filter. This indicated that they were smaller than 5 nm in size, consistent with those estimated from the UV–vis absorption edge and the TEM image. At a lower pH (e.g. pH = 8), the room-temperature synthesized ZnS QDs exhibited no photoluminescence. Although further hydrothermal annealing at 100 °C could improve the photoluminescence of the ZnS QDs, the resultant emission was not as bright as that obtained at pH 12 at room temperature. The blue emission of aqueous ZnS QDs was likely the result of trap-state emissions involving the defect states of the QDs. The present ZnS QDs were bright, small and contained non-heavy-metal elements, thus offering the potential for in vivo bioimaging.

Flow Energy Harvesting Using Piezoelectric Cantilevers With Cylindrical Extension

We present a new piezoelectric flow energy harvester (PFEH) based on a piezoelectric cantilever (PEC) with a cylindrical extension. The flow-induced vibration of the cylindrical extension causes the PEC to vibrate at the natural frequency of the PFEH. The PFEH provides a low-cost, compact, and scalable power source for small electronics by harvesting energy from ambient flows such as wind and water streams. Prototypes were tested in both laminar and turbulent air flows, demonstrating the feasibility of the design. Turbulence excitation was found to be the dominant driving mechanism of the PFEH with additional vortex shedding excitation contribution in the lock-in region.

Self-exciting, self-sensing PbZr0.53Ti0.47O3∕SiO2 piezoelectric microcantilevers with femtogram/Hertz sensitivity

Piezoelectric microcantilever sensors (PEMSs) consisting of a piezoelectric layer bonded to a nonpiezoelectric layer offer the advantages of electrical self-actuation and self-detection. Here we report PEMSs 60–300μm in length fabricated from 1.5-μm-thick sol-gel PbZr0.53Ti0.47O3 (PZT) films with a 2μm grain size, a dielectric constant of 1600, and a saturation polarization of 55±5μC∕cm2. The PEMSs exhibited up to four resonance peaks with quality factors Q ranging from 120 to 320. In humidity sensing tests, a PEMS with a 60×25μm PZT∕SiO2 section and a 24×20μm SiO2 extension exhibited 1×10−15g∕Hz mass sensitivity, two orders of magnitude better than the sensitivity of the current PZT PEMS.

Soft-materials elastic and shear moduli measurement using piezoelectric cantilevers

We have developed a soft-material elastic modulus and shear modulus sensor using piezoelectric cantilevers. A piezoelectric cantilever is consisted of a highly piezoelectric layer, e.g., lead–zirconate–titanate bonded to a nonpiezoelectric layer, e.g., stainless steel. Applying an electric field in the thickness direction causes a piezoelectric cantilever to bend, generating an axial displacement or force. When a piezoelectric cantilever is in contact with an object, this electric field-generated axial displacement is reduced due to the resistance by the object. With a proper design of the piezoelectric cantilever’s geometry, its axial displacements with and without contacting the object could be accurately measured. From these measurements the elastic modulus of the object can be deduced. In this study, we tailored the piezoelectric cantilevers for measuring the elastic and shear moduli of tissue-like soft materials with forces in the submilli Newton to milliNewton range. Elastic moduli and shear moduli ...

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 ...

80-MHz intravascular ultrasound transducer using PMN-PT free-standing film

[Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 (PMN-PT) free-standing film of comparable piezoelectric properties to bulk material with thickness of 30 μm has been fabricated using a modified precursor coating approach. At 1 kHz, the dielectric permittivity and loss were 4364 and 0.033, respectively. The remnant polarization and coercive field were 28 μC/ cm2 and 18.43 kV/cm. The electromechanical coupling coefficient kt was measured to be 0.55, which was close to that of bulk PMN-PT single-crystal material. Based on this film, high-frequency (82 MHz) miniature ultrasonic transducers were fabricated with 65% bandwidth and 23 dB insertion loss. Axial and lateral resolutions were determined to be as high as 35 and 176 μ m. In vitro intravascular imaging on healthy rabbit aorta was performed using the thin film transducers. In comparison with a 35-MHz IVUS transducer, the 80-MHz transducer showed superior resolution and contrast with satisfactory penetration depth. The imaging results suggest that PMN-PT free-standing thin film technology is a feasible and efficient way to fabricate very-high-frequency ultrasonic transducers.

Highly Sensitive Detection of HER2 Extracellular Domain in the Serum of Breast Cancer Patients by Piezoelectric Microcantilevers

Rapid and sensitive detection of serum tumor biomarkers are needed to monitor cancer patients for disease progression. Highly sensitive piezoelectric microcantilever sensors (PEMS) offer an attractive tool for biomarker detection; however, their utility in the complex environment encountered in serum has yet to be determined. As a proof of concept, we have functionalized PEMS with antibodies that specifically bind to HER2, a biomarker (antigen) that is commonly overexpressed in the blood of breast cancer patients. The function and sensitivity of these anti-HER2 PEMS biosensors was initially assessed using recombinant HER2 spiked into human serum. Their ability to detect native HER2 present in the serum of breast cancer patients was then determined. We have found that the anti-HER2 PEMS were able to accurately detect both recombinant and naturally occurring HER2 at clinically relevant levels (>2 ng/mL). This indicates that PEMS-based biosensors provide a potentially effective tool for biomarker detection.