Ping Kao

Application of Micromachined

This paper presents the design, fabrication, and characterization of thermal infrared (IR) imaging arrays operating at room temperature which are based on Y-cut-quartz bulk acoustic wave resonators. A novel method of tracking the resonance frequency based upon the measurement of impedance is presented. High-frequency (240-MHz) micromachined resonators from Y-cut-quartz crystal cuts were fabricated using heterogeneous integration techniques on a silicon wafer. A temperature sensitivity of 22.16 kHz/°C was experimentally measured. IR measurements on the resonator pixel resulted in a noise equivalent power of 3.90 nW/Hz1/2, a detectivity D* of 1 × 105 cm · Hz1/2/W, and a noise equivalent temperature difference of 4 mK in the 8- to 14-μm wavelength range. The thermal frequency response of the resonator was determined to be faster than 33 Hz, demonstrating its applicability in video-rate uncooled IR imaging. This work represents the first comprehensive thermal characterization of micromachined F-cut-quartz resonators and their IR sensing response.

Y

A new class of nonlithographically prepared surface enhanced Raman spectroscopy (SERS) substrates based on metalized, nanostructured poly(p-xylylene) films has been developed and optimized for surface plasmon response with a view to applications of SERS detection of microbial pathogens, specifically, bacteria and viruses. The main emphasis has been on achieving high spot to spot, sample to sample reproducibility of the SERS signals while maintaining useful enhancement factors. The use of these surfaces, metalized with either Ag or Au, provides a noninvasive and nondestructive method for spectral fingerprint analyses of both bacteria and viruses. Examples are given for the detection of bacteria (E. coli and B. cereus) and viruses (respiratory syncytial virus and Coxsackievirus). Our method is able to distinguish Gram positive from Gram negative bacterial strains as well as enveloped and nonenveloped viruses. The results demonstrate the development of a new class of SERS substrates which can provide rapid, selective identification of infectious agents without amplification of cultures.

-Cut-Quartz Bulk Acoustic Wave Resonator for Infrared Sensing

This paper reviews the fabrication and performance of micromachined quartz resonator arrays. Using inductively coupled plasma etching techniques, we have successfully fabricated micromachined quartz resonator arrays with fundamental frequencies in the range of 25?85 MHz in an array format. These resonators have been experimentally evaluated for their performance in viscous (liquid) and viscoelastic (a biomolecular film in liquid) loading conditions. The paper discusses the ultimate sensitivity to mass and other properties of the adsorbates/contacting materials onto high-frequency quartz resonator surfaces. Measuring the frequency and Q-factor changes at the fundamental and third overtone of a 66 MHz resonator upon adsorption of immunoglobulin G (IgG) protein film on a hexadecanethiol functionalized surface, we were able to deduce: (i) the film thickness = 18 nm, (ii) density = 1040 kg m?3, (iii) elastic modulus = 6.7 MPa and (iv) viscosity = 5.5 mPa s. Furthermore, from the adsorption isotherm for the IgG film, two different Langmuir equilibrium constants (K) were deduced. In the low-concentration region K = 2.13 ? 108 M?1 and in the high-concentration region K = 6.53 ? 106 M?1 were obtained. The thickness and density values obtained for IgG are consistent with the bilayer model predicted from interfacial packing of spherical protein molecules as a function of the molecular weight, and K values are consistent with earlier reported values for adsorption of IgG films. This is the first reporting of the elastic modulus and viscosity of IgG films in phosphate buffer solution.

Bio-organism sensing via surface enhanced Raman spectroscopy on controlled metal/polymer nanostructured substrates

The maximum capacity of a hydrophobic adsorbent is interpreted in terms of square or hexagonal (cubic and face-centered-cubic, FCC) interfacial packing models of adsorbed blood proteins in a way that accommodates experimental measurements by the solution-depletion method and quartz-crystal-microbalance (QCM) for the human proteins serum albumin (HSA, 66 kDa), immunoglobulin G (IgG, 160 kDa), fibrinogen (Fib, 341 kDa), and immunoglobulin M (IgM, 1000 kDa). A simple analysis shows that adsorbent capacity is capped by a fixed mass/volume (e.g. mg/mL) surface-region (interphase) concentration and not molar concentration. Nearly analytical agreement between the packing models and experiment suggests that, at surface saturation, above-mentioned proteins assemble within the interphase in a manner that approximates a well-ordered array. HSA saturates a hydrophobic adsorbent with the equivalent of a single square or hexagonally-packed layer of hydrated molecules whereas the larger proteins occupy two-or-more layers, depending on the specific protein under consideration and analytical method used to measure adsorbate mass (solution depletion or QCM). Square or hexagonal (cubic and FCC) packing models cannot be clearly distinguished by comparison to experimental data. QCM measurement of adsorbent capacity is shown to be significantly different than that measured by solution depletion for similar hydrophobic adsorbents. The underlying reason is traced to the fact that QCM measures contribution of both core protein, water of hydration, and interphase water whereas solution depletion measures only the contribution of core protein. It is further shown that thickness of the interphase directly measured by QCM systematically exceeds that inferred from solution-depletion measurements, presumably because the static model used to interpret solution depletion does not accurately capture the complexities of the viscoelastic interfacial environment probed by QCM.

Fabrication and performance characteristics of high-frequency micromachined bulk acoustic wave quartz resonator arrays

We have designed and fabricated 25-microm-thick quartz resonators operating at a fundamental resonance frequency of approximately 62 MHz. The results show a substantial increase in the mass sensitivity compared to single monolithic commercial resonators operating at lower frequencies in the approximately 5-10-MHz range. The overall performance of the micromachined resonators is demonstrated for the example of human serum albumin protein adsorption from aqueous buffer solutions onto gold electrodes functionalized with self-assembled monolayers. The results show a saturation adsorption frequency change of 6.8 kHz as opposed to 40 Hz for a commercial approximately 5-MHz sensor under identical loading conditions. From the analysis of the adsorption isotherm, the equilibrium adsorption constant of the adsorption of the protein layer was found to be K = 8.03 x 10(6) M(-1), which is in agreement with the values reported in the literature. The high sensitivity of the miniaturized QCM devices can be a significant advantage in both vapor and solution adsorption analyses.

Volumetric interpretation of protein adsorption: Interfacial packing of protein adsorbed to hydrophobic surfaces from surface-saturating solution concentrations

In this paper, we use micromachined, high-frequency, quartz bulk acoustic wave resonators to systematically study the physical and viscoelastic properties of spontaneously adsorbed globular protein films with molecular weights (MWs) spanning three decades on hydrophobic surfaces. Specifically, changes in the frequency and the Q -factor of the micromachined resonator array were studied as a function of concentration for five proteins, namely human serum albumin (HSA), immunoglobulin G (IgG), human fibrinogen (Fib), alpha-2-macroglobulin (AMG), and immunoglobulin M (IgM) at the fundamental and third resonance modes. The results obtained were interpreted using equivalent electrical impedance models for the multilayer stack on the quartz crystal microbalances surface. Discrete changes in the protein adsorption and the viscoelastic behavior with solution concentration were observed for all the five protein films. The spherical core-shell protein model is used to provide a simple explanation of the results. The work presents the first systematic and quantitative evaluation of the density, thickness, viscosity, and elastic modulus of adsorbed globular protein films and demonstrates the advantages of using micromachined high-frequency bulk acoustic wave resonators for obtaining these types of data.

Human serum albumin adsorption study on 62-MHz miniaturized quartz gravimetric sensors.

Summary Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain length n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn molecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the gold–thiolate anchor and the nitrile tail groups located at the SAM–ambient interface. The packing density in these SAMs was found to be close to that of alkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination, with an average tilt angle of ~33–36° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation) was found to depend on the molecular length, being close to 45° for the films comprising the short OPE chains and ~53.5° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic marker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs.

Study of Adsorption of Globular Proteins on Hydrophobic Surfaces

This paper presents the design, fabrication, and characterization of temperature sensitive quartz resonators fabricated using heterogeneous integration methods for realizing high-density, thermal conductance fluctuation limited thermal sensors for infrared imaging and biochemical sensing applications. An integrated quartz sensor array using CMOS-compatible micromachining techniques has been designed and fabricated. 241 MHz micromachined resonators from F-cut quartz crystal cuts were fabricated with a temperature sensitivity of 22.162 kHz/°C. Infrared measurements on the resonator pixel resulted in a noise equivalent power (NEP) of 3.90 nW/Hz1/2, detectivity D* of 9.17 ×107 cmHz1/2/W, and noise equivalent temperature difference (NETD) in the 8–12 µm wavelength region of 4 mK and a response time of < 30 Hz. In a unique new application a remotely coupled thermal sensor configuration was used to monitor biochemical reactions in real time.

X-ray spectroscopy characterization of self-assembled monolayers of nitrile-substituted oligo(phenylene ethynylene)s with variable chain length

In this paper we use micromachined, high-frequency, quartz bulk acoustic wave resonators to systematically study the physical and viscoelastic properties of spontaneously adsorbed globular protein films with molecular weights (MW) spanning two orders of magnitude. Specifically, changes in the frequency and the Q-factor of the micromachined resonator array were studied as a function of concentration for three proteins, namely Human Serum albumin (HSA), Immunoglobulin G (IgG) and Human Fibrinogen (Fib) at the fundamental and third resonance modes. The results obtained were interpreted using equivalent electrical impedance models for the multilayer stack on the QCM surface. Discrete changes in the protein adsorption rate constant and the viscoelastic behavior was observed for all the three protein films. The spherical core-shell protein model is used to provide a simple explanation of the results. The work presented is a systematic and quantitative evaluation of the density, thickness, viscosity, and elastic modulus of the globular protein films, which was possible, due the use of the micromachined high frequency bulk acoustic wave resonators.

Micromachined quartz resonator-based high performance thermal sensors

This paper reports on the fabrication and application of Y-cut temperature-sensitive quartz resonators for infrared detection applications. The high thermal sensitivity of the Y-cut quartz crystal (90 ppm/K) and the high quality factor (≫10,000) of the piezoelectric bulk acoustic wave resonators fabricated from such crystals enable the application of these devices as extremely sensitive uncooled thermal infrared (IR) detectors. We report the fabrication and performance of inverted mesa devices with 1 mm diameter and 18 µm thickness with 6.8 kHz/K of temperature sensitivity at 90 MHz.