Wei-Heng Shih

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.

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.

Induced voltage of piezoelectric unimorph cantilevers of different nonpiezoelectric/piezoelectric length ratios

Piezoelectric cantilevers are widely used in sensing and energy harvesting devices. For both applications, a higher induced voltage from a given mechanical excitation is desirable to increase the sensitivity or energy conversion efficiency of the devices. In this study, we examined the effect of the length ratio of the nonpiezoelectric layer to the piezoelectric layer on the induced voltage of the piezoelectric unimorph cantilever due to a concentrated force applied at the cantilever tip. The cantilever was made of lead zirconate titanate (PZT)–stainless steel (SS) unimorph. The length of the PZT layer was fixed while that of the SS layer was varied. The induced voltage per unit tip displacement was obtained by measuring the induced voltage in the PZT layer and dividing it by the corresponding tip displacement of the cantilever and the induced voltage per unit force was obtained by dividing the induced voltage per unit tip displacement by the effective spring constant of the cantilever. The results showed that the induced voltage per unit force increased with an increasing SS/PZT length ratio, indicating that under constant force conditions, the optimal induced voltage occurs when the SS layer is longer than the PZT layer. In contrast, the induced voltage per unit tip displacement exhibited a maximum when the SS/PZT length ratio is unity, indicating that under constant tip displacement conditions, the optimal induced voltage occurs when the PZT layer and the SS layer have the same length. A theoretical analysis based on the Euler–Bernoulli beam theory was carried out to correlate the induced voltage of the cantilever to the tip displacement and force. The experimental results were consistent with the prediction of the theoretical analysis.

Label-free, all-electrical, in situ human epidermal growth receptor 2 detection

Using 3-mercaptopropyltrimethoxysilane (MPS)-coated (PbMg 1/3 Nb 2/3 O3)0.63-(PbTiO3)0.37 (PMN-PT)/tin and lead zirconate titanate/glass piezoelectric microcantilever sensors (PEMSs) with single-chain variable fragment (scFv) immobilized on the MPS surface, we have demonstrated real-time, label-free detection of human epidermal growth factor receptor 2 (Her2) in a background of 1 mg/ml bovine serum albumin. Coupled with a scFv with a KD of 3.4 x 10(-8)M, the MPS-insulated PMN-PT/tin PEMS 560 microm long and 720 microm wide exhibited a Her2 concentration sensitivity of 5 ng/ml in a background of 1 mg/ml BSA.

Mechanism of flexural resonance frequency shift of a piezoelectric microcantilever sensor during humidity detection

We have examined the flexural resonance frequency shift of a piezoelectric microcantilever sensor (PEMS) during humidity detection and have shown that the flexural resonance frequency shift of the PEMS during detection was a result of Youngs modulus change of its piezoelectric layer. Because of the piezoelectric layers Youngs modulus change, the PEMS flexural resonance frequency shift was more than 300 times larger than could be accounted for by mass loading.

Label-free flow-enhanced specific detection of Bacillus anthracis using a piezoelectric microcantilever sensor

Differentiation between species of similar biological structure is of critical importance in biosensing applications. Here, we report specific detection of Bacillus anthracis (BA) spores from that of close relatives, such as B. thuringiensis (BT), B. cereus (BC), and B. subtilis (BS) by varying the flow speed of the sampling liquid over the surface of a piezoelectric microcantilever sensor (PEMS). Spore binding to the anti-BA spore IgG coated PEMS surface is determined by monitoring the resonance frequency change in the sensors impedance vs. frequency spectrum. Flow increases the resonance frequency shift at lower flow rates until the impingement force from the flow overcomes the binding strength of the antigen and decreases the resonance frequency shift at higher flow rates. We showed that the change from increasing to decreasing resonance frequency shift occurred at a lower fluid flow speed for BT, BC, and BS spores than for BA spores. This trend reduces the cross reactivity ratio of BC, BS, and BT to the anti-BA spore IgG immobilized PEMS from around 0.4 at low flow velocities to less than 0.05 at 3.8 mm s(-1). This cross reactivity ratio of 0.05 was essentially negligible considering the experimental uncertainty. The use of the same flow that is used for detection to further distinguish the specific binding (BA to anti-BA spore antibody) from nonspecific binding (BT, BC, and BS to anti-BA spore antibody) is unique and has great potential in the detection of general biological species.

3-mercaptopropyltrimethoxysilane as insulating coating and surface for protein immobilization for piezoelectric microcantilever sensors

We have examined coating (PbMg(13)Nb(23)O(3))(0.63)-(PbTiO(3))(0.37) (PMN-PT)/tin and lead zirconate titanate (PZT)/glass piezoelectric microcantilever sensor (PEMS) with 3-mercaptopropyl-trimethoxysilane (MPS) by a simple solution method to electrically insulate the PEMS for in-water applications. In contrast to earlier methytrimethoxysilane insulation coating, the MPS coating also facilitated receptor immobilization on the sensor surface via bonding of its sulhydryl group to a bifunctional linker, sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate. We showed that a MPS coating of 21 nm in thickness is sufficient to electrically insulate and provide immobilization surface to the PEMS for in-liquid electrical self-excitation and self-sensing. The in-phosphate buffered saline solution resonance spectra were stable with Q values ranging from 41 to 55. The mass detection sensitivities were determined to be 5x10(-11) and 8x10(-12) gHz for the MPS-insulated PZT-glass and PMN-PT/tin PEMSs, respectively.