A lateral field excited liquid acoustic wave sensor

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.     

A lateral-field-excited LiTaO3 high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO₃. Prototype LFE LiTaO₃ sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.     

Multiscale modeling of impact on heterogeneous viscoelastic solids containing evolving microcracks

Multiscale computational techniques play a major role in solving problems related to viscoelastic composites due to the complexities inherent to these materials. In this paper, a numerical procedure for multiscale modeling of impact on heterogeneous viscoelastic solids containing evolving microcracks is proposed in which the (global scale) homogenized viscoelastic incremental constitutive equations have the same form as the local‐scale viscoelastic incremental constitutive equations, but the homogenized tangent constitutive tensor and the homogenized incremental history‐dependent stress tensor at the global scale depend on the amount of damage accumulated at the local scale. Furthermore, the developed technique allows the computation of the full anisotropic incremental constitutive tensor of viscoelastic solids containing evolving cracks (and other kinds of heterogeneities) by solving the micromechanical problem only once at each material point and each time step. The procedure is basically developed by relating the local‐scale displacement field to the global‐scale strain tensor and using first‐order homogenization techniques. The finite element formulation is developed and some example problems are presented in order to verify the approach and demonstrate the model capabilities. Copyright © 2009 John Wiley & Sons, Ltd.     

A novel technique to immobilize DNA on surface of a quartz crystal microbalance by plasma treatment and graft polymerization

A quartz crystal microbalance (QCM) is well known to provide mass-sensitive devices in nanogram levels, because of the resonance frequency changes upon the adsorption on the electrode. It offers the possibility of monitoring hybridization in real time and with high selectivity. In this study, a biosensor system was developed for the detection of Vibrio parahaemolyticus via its oligonucleotide probe immobilized on the gold electrodes' surface of QCM. However, because the surface of QCM was an inorganic substance, it was difficult to immobilize the oligonucleotide probe. In this study, the plasma surface modification of QCM through deposition of hexamethyldisilazane (HMDSZ) films as an interlayer was investigated. The interlayer provided good adhesion to the substrate and had a uniform structure. The result indicates that plasma deposition was a useful technique to immobilize the oligonucleotide probe on the gold electrodes' surface via glutaraldehyde (GA) coupling. To improve immobilization, post treatments by surface grafting of acrylamide (AAm) and polyethyleneimine (PEI) treatment onto the electrodes were also performed. The result demonstrates that the shift of resonance frequency of QCM was improved via subsequent graft polymerization of AAm and PEI treatment onto the electrodes. The QCM sensor after plasma deposition and surface modification could provide detection sensitivity up to 86 ng/ml and kept at 88% detecting sensitivity after 19 days of storage at 0 °C. After washing with 0.1 M NaOH solution and 7 times of repeated use in detecting, the regeneration rate of QCM could be up to 60%.     

Impact of oxygen and argon plasma exposure on the roughness of gold film surfaces

The impact of oxygen and argon plasma exposure on the roughness of gold film Quartz Crystal Microbalance (QCM) electrodes is reported here, employing low levels of gas uptake and scanning tunneling microscope measurements to probe the post-exposure surface morphology. For equal exposure times, argon plasma bombardment is observed to produce both greater material removal and greater change in surface roughness. A possible explanation for this is that the oxygen plasma produces a protective gold oxide layer, which may remove the contaminants from the surface without creating defects in the gold surface. The result is also consistent with prior reports of chemical cleaning of the surface by reactive oxygen ions. Pentane gas adsorption on the argon bombarded QCM surfaces was, moreover, observed to occur at pressures that are several orders of magnitude lower than that for an unbombarded surface.     

QCM露点传感器水粘性影响的抑制方法研究

为了解决基于主控温式的石英晶体微天平（Quartz Crystal Microbalance, QCM）露点测量系统中冷凝水粘弹特性影响露点识别准确性的问题,对QCM电极进行疏水处理,改善凝结特性,减小水粘性引起的频率耗散,实现液态水质量变化引起的谐振频率偏移测量。在QCM电极上制备静态水接触角为133° ± 2°的疏水层并对其进行表征,将疏水电极与未经处理的电极用于露点识别实验,并与精密露点仪获得的标准露点进行比对。实验证明,通过疏水处理电极凝结面的方法能够有效提升QCM露点传感器的露点识别精度,为主控温式露点传感器结构的优化设计提供理论和实验依据。     

Stress analysis in anisotropic functionally graded materials by the MLPG method

A meshless method based on the local Petrov–Galerkin approach is proposed for stress analysis in two-dimensional (2D), anisotropic and linear elastic/viscoelastic solids with continuously varying material properties. The correspondence principle is applied for non-homogeneous, anisotropic and linear viscoelastic solids where the relaxation moduli are separable in space and time. The inertial dynamic term in the governing equations is considered too. A unit step function is used as the test functions in the local weak-form. It leads to local boundary integral equations (LBIEs). The analyzed domain is divided into small subdomains with a circular shape. The moving least squares (MLS) method is adopted for approximating the physical quantities in the LBIEs. For time-dependent problems, the Laplace-transform technique is utilized. Several numerical examples are given to verify the accuracy and the efficiency of the proposed method.     

In vitro real-time characterization of cell attachment and spreading

A method based on the piezoelectric quartz crystal microbalance (QCM) technique for in vitro real-time characterization of cell attachment and spreading on surfaces has been developed. The method simultaneously measures the resonant frequency, f, and the dissipation energy, D, of the oscillating system. The QCM responses are sensitive to very small amounts (a few hundreds) of cells and highly specific to surface chemical properties. The first results from deposition of cells on two polystyrene surfaces of different wettability in serum-containing medium are reported. It has previously been shown that a decrease in f is related to the degree of cell spreading. In our data it appears that the extent or quality of cell attachment is reflected in an increase in D caused by adhering cells. The combined information from f and D measured by this technique might therefore be useful to probe cell–surface interactions for biomaterials. © 1998 Kluwer Academic Publishers     

The SPH method for simulating a viscoelastic drop impact and spreading on an inclined plate

In the present work, the phenomenon of an Oldroyd-B drop impact and spreading on an inclined rigid plate at low impact angles is simulated numerically using the smoothed particle hydrodynamics (SPH) method. In order to remove the unphysical phenomenon of fracture and particle clustering in fluid stretching which is the so-called tensile instability, an artificial stress term is employed which has been successfully proposed in simulations of elastic solids. Particularly, the effects of surface inclination and the different regimes of drop impact and spreading on an inclined surface are investigated. The numerical results show the capability of the proposed scheme in handing the unsteady viscoelastic free surface flows. All numerical results of using the SPH method are in agreement with the available data.     

Simultaneous monitoring of protein adsorption kinetics using a quartz crystal microbalance and field-effect transistor integrated device

We developed an integrated device comprising a quartz crystal microbalance (QCM) and a field-effect transistor (FET) with a single common gold electrode in a flow chamber. An alternating current inducing oscillations in the piezoelectric quartz of the QCM sensor is electrically independent of the circuit for the FET output so that the two sensors in different detection mechanisms simultaneously record binding kinetics from a single protein solution on the same electrode. A conjunction of adsorbed mass from QCM with electric nature of bound protein from FET provided deeper understanding on a complex process of nonspecific protein adsorption and subsequent conformational changes at a solid/liquid interface. Lower apparent k(on) values obtained by FET than those obtained by QCM on hydrophobic surfaces are interpreted as preferred binding of protein molecules facing uncharged domains to the electrode surface, whereas higher k(off) values by FET than those by QCM imply active macromolecular rearrangements on the surfaces mainly driven by hydrophobic association in an aqueous medium. The advanced features of the combined sensor including in situ, label-free, and real-time monitoring provide information on structural dynamics, beyond measurements of affinities and kinetics in biological binding reactions.     

The interfacial behaviour of single poly(N,N-dimethylacrylamide) chains as a function of pH

We have studied the pH-dependent conformational behaviour of poly(N,N-dimethylacrylamide) (PDMAc) at silicon and gold surfaces using single-molecule force spectroscopy and a quartz crystal microbalance (QCM). Despite the pH dependence, nuclear magnetic resonance and titration experiments demonstrate that PDMAc is not a weak polybase. The interaction between single chains and a silicon surface (with native oxide layer intact) in aqueous solution was investigated using force spectroscopy. Single-molecule force measurements were performed using thiol-functionalized PDMAc and gold-coated AFM cantilevers. The forces of interaction between PDMAc and the native oxide-coated silicon surface vary with the pH. The shape of the retraction curve for low pH solution includes a greater number of 'train' conformations, which suggests a stronger interaction with the surface relative to the surrounding media at low pH. The adsorption behaviour of PDMAc, from liquid onto silicon surfaces, was monitored using a QCM, which shows greater PDMAc adsorption onto silicon at low pH. The force spectroscopy and QCM investigations confirm that the PDMAc chain is more extended and stiffer in low pH solution. We attribute the pH-dependent behaviour to an increased number of hydrogen bonding sites on the silicon surface at low pH.     

Detection of point mutation and insertion mutations in DNA using a quartz crystal microbalance and MutS, a mismatch binding protein

MutS protein is a mismatch binding protein that recognizes mispaired and unpaired base(s) in DNA. In this study, we incorporate the MutS protein-based mutation recognition into quartz crystal microbalance (QCM) measurements for DNA single-base substitution mutation and 1-4 base(s) insertion (or deletion) mutation detection. The method involves the immobilization of single-stranded probe DNA on a QCM surface, the hybridization of target DNA to form homoduplex or heteroduplex DNA, and finally the application of MutS protein for the mutation recognition. By measuring the MutS binding signal, DNA containing a T:G mismatch or unpaired base(s) is(are) discriminated against perfectly matched DNA at target concentrations ranging from 1nM to 5 microM. Furthermore, the QCM damping behavior upon MutS-DNA complex formation is studied using a Network Analyzer. The measured motional resistance changes per coupled MutS unit mass (deltaR/deltaf) are found to be indicative of the viscoelastic or structural properties of the bound protein, corresponding to different binding mechanisms. In addition, the deltaR/deltaf values vary remarkably when the MutS protein binds at different distances away from the QCM surface. Thus, these values can be used as a "fingerprint" for MutS mismatch recognition and also used to quantitatively locate the mutation site.     

Determining early adhesion of cells on polysaccharides/PCL surfaces by a quartz crystal microbalance

The early adhesions of cells to various biopolymers are important to their growths and proliferations. Here, the adhesion of cells (e.g., fibroblasts) on the electrode of a quartz crystal microbalance (QCM) that was coated by PCL or PEG/PCL and further adsorbed by chitosan (CS) or CS/hyaluronic acid (HA) layers, was examined by cell-counting technique, QCM method and MTS assay under a serum-free condition for 3 h. The surfaces on electrodes of the QCM were confirmed to have been modified by measuring their contact angles, FT-IR spectra and the weights of biopolymers affected the frequency shifts of the QCM. Among tested surfaces on electrodes, the adhesion of fibroblasts on a HA/CS/PCL surface was the most (e.g., 3.08 × 10⁵ cells/cm²) while that on a PEG/PCL surface was the least (e.g., 0.7 × 10⁵ cells/cm²), as determined by cell-counting technique. The frequency shift and the mass of adhering fibroblasts on HA/CS/PCL electrodes were ?3,537 ± 770 Hz and 3.78 ± 0.22 μg (n = 3), respectively, that were significantly exceeded those on other electrodes (?393 ± 58 Hz and 0.32 ± 0.06 μg, n = 3, respectively, for PEG/PCL electrodes). These results were consistent with cell-counting technique. Although MTS assay yielded similar results, it was less sensitive than the two aforementioned methods. In conclusion, modified electrodes of a QCM provide a convenient and sensitive method for examining the early adhesion of cells (e.g., 3 h) to biopolymer surfaces.     

Functionalized cubic mesostructured silica films

Cubic Pm3n mesoporous silica films have been prepared on silicon wafers and quartz crystal microbalance (QCM) devices covered with gold electrodes by a spin-coating process from preformed silica/CTABr/ethanol/water assemblies under acidic conditions. Post-synthesis functionalization of mesoporous films with amino- and thiol-containing organosilanes is performed in order to modify the mesoporous surface for further confinement of nanoscale structures. The type of mesophase structure and the functionalization process was followed with surface sensitive techniques such as grazing incidence diffraction (GID), reflection-absorption FT-IR spectroscopy and gravimetric measurements by applying QCMs technique. Nitrogen sorption data using QCM devices were obtained for the calcined and functionalized mesoporous films.     

A small-strain model to simulate the curing of thermosets

This contribution presents a newly developed phenomenological model to describe the curing process of thermosets undergoing small strain deformations. The governing equations are derived from a number of physical and chemical presuppositions and details of the numerical implementation within the finite element method are given. The curing of thermosets is a very complex process involving a series of chemical reactions which result in the conversion of liquid low molecular weight monomer mixtures into highly cross-linked solid macromolecular structures. This phase transition from a viscous fluid to a viscoelastic solid can be modelled by a constitutive relation which is based on a temporal evolution of shear modulus and relaxation time. Some numerical examples demonstrate the capability of the model to correctly represent the evolution of elastic and inelastic material properties as well as the volume shrinkage taking place during the curing process.     

Postbuckling of viscoelastic micro/nanobeams embedded in visco-Pasternak foundations based on the modified couple stress theory

This paper investigates the postbuckling analysis of a viscoelastic microbeam embedded in a double layer viscoelastic foundation. This viscoelastic microbeam is modeled using the Kelvin–Voigt model and the modified couple stress theory. A material length scale parameter is utilized to describe the size-dependent behavior of the viscoelastic microbeam. The visco-Pasternak foundation used in this study contains a viscoelastic medium and a shear layer. This microbeam is subjected to an axial compressive load at the beam ends which can change as a function of time. According to the Euler–Bernoulli beam theory and von-Karman nonlinearity, the time-dependent equations of motion are derived by Hamilton’s principle. The nonlinear equations of motion are directly solved under the simply supported boundary condition. Both time-dependent deflection and viscoelastic buckling load are investigated. Finally, the influences of the material length scale parameter, parameters of the visco-Pasternak foundation and the material viscosity coefficient on the dynamic postbuckling response are studied.

     

一维分数导数粘弹性饱和多孔介质层的稳态响应

固相骨架的应力-应变关系利用分数导数粘弹性Kelvin 模型来描述,在流相和固相微观不可压以及小变形的假定下建立了分数导数粘弹性饱和多孔介质层一维稳态响应的数学模型和运动控制方程,求得了分数导数粘弹性饱和多孔介质层一维稳态响应的固相位移和液相位移。通过数值算例分析了分数导数的阶数对稳态响应的影响。研究结果表明:固相位移和液相位移随频率的增大逐渐趋于零,在低频时,分数导数的阶数越大固相位移和液相位移越大。     

Harmonic wave propagation in viscoelastic heterogeneous materials Part II: Effective complex moduli

The effective complex moduli of a heterogeneous periodic viscoelastic medium has been determined through its dispersion and damping curves. These curves give the values of complex wave numbers in terms of frequencies and wave direction. The solution of Christoffel's equations of an equivalent viscoelastic homogeneous medium for judiciously selected wave directions provides us simple relations between complex wave numbers belonging to the heterogeneous medium and its effective complex moduli. Based on this method two examples have been treated. The one, a viscoelastic homogeneous material, to show the accuracy of this method. The other, a fiber composite material, to illustrate the applicability of the method.     

Electropolymerized molecularly imprinted polymer films of a bis-terthiophene dendron: folic acid quartz crystal microbalance sensing

A folic acid sensor was prepared via an electropolymerized molecularly imprinted polymer (E-MIP) film of a bis-terthiophene dendron on a quartz crystal microbalance (QCM). The cyclic voltammetry (CV) electrodeposition of the imprinted polymer film was monitored by electrochemical QCM or E-QCM, enabling in situ monitoring and characterization of E-MIP film formation and the viscoelastic behavior of the film. A key component of the E-MIP process is the use of a bifunctional monomer design to precomplex with the template and function as a cross-linker. The complex was electropolymerized and cross-linked by CV to form a polythiophene matrix. Stable cavities were formed that specifically fit the size and shape of the folic acid template. The same substrate surface was used for folic acid sensing. The predicted geometry of the 1:2 folic acid/terthiophene complex was obtained through semiempirical AM1 quantum calculations. The analytical performance, expressed through the figures of merit, of the sensor in aqueous solutions of the analyte was investigated. A relatively good linearity, R(2) = 0.985, was obtained within the concentration range 0-100 μM folic acid. The detection limit was found to be equal to 15.4 μM (6.8 μg). The relative cross selectivity of the folic acid imprinted polymer against the three molecules follows this trend: pteroic acid (= 50%) > caffeine (= 41%) > theophylline (= 6%). The potential and limitations of the E-MIP method were also discussed.     

Nonlabeled quartz crystal microbalance biosensor for bacterial detection using carbohydrate and lectin recognitions

High percentages of harmful microbes or their secreting toxins bind to specific carbohydrate sequences on human cells at the recognition and attachment sites. A number of studies also show that lectins react with specific structures of bacteria and fungi. In this report, we take advantage of the fact that a high percentage of microorganisms have both carbohydrate and lectin binding pockets at their surface. We demonstrate here for the first time that a carbohydrate nonlabeled mass sensor in combination with lectin-bacterial O-antigen recognition can be used for detection of high molecular weight bacterial targets with remarkably high sensitivity and enhanced specificity. A functional mannose self-assembled monolayer in combination with lectin concanavalin A (Con A) was used as molecular recognition elements for the detection of Escherichia coli W1485 using a quartz crytsal microbalance (QCM) as a transducer. The multivalent binding of Con A to the E. coli surface O-antigen favors the strong adhesion of E. coli to the mannose-modified QCM surface by forming bridges between these two. As a result, the contact area between cell and QCM surface that increases leads to rigid and strong attachment. Therefore, it enhances the binding between E. coli and the mannose. Our results show a significant improvement of the sensitivity and specificity of the carbohydrate QCM biosensor with a experimental detection limit of a few hundred bacterial cells. The linear range is from 7.5 x 10(2) to 7.5 x 10(7) cells/mL, which is four decades wider than the mannose-alone QCM sensor. The change of damping resistances for E. coli adhesion experiments was no more than 1.4%, suggesting that the bacterial attachment was rigid, rather than a viscoelastic behavior. Little nonspecific binding was observed for Staphylococcus aureus and other proteins (fetal bovine serum, Erythrina cristagalli lectin). Our approach not only overcomes the challenges of applying QCM technology for bacterial detection but also increases the binding of bacteria to their carbohydrate receptor through bacterial surface binding lectins that significantly enhanced specificity and sensitivity of QCM biosensors. Combining carbohydrate and lectin recognition events with an appropriate QCM transducer can yield sensor devices highly suitable for the fast, reversible, and straightforward on-line screening and detection of bacteria in food, water, and clinical and biodefense areas.