QCM Operation in Liquids: An Explanation of Measured Variations in Frequency and Q Factor with Liquid Conductivity

Recently, several reports have shown that when one side of a quartz crystal microbalance (QCM) is exposed to a liquid, the parallel (but not the series) resonant frequency is influenced by the conductivity and dielectric constant of the liquid. The effect is still controversial and constitutes a serious complication in many applications of the QCM in liquid environments. One suggestion has been that acoustically induced surface charges couple to charged species in the conducting liquid. To explore this effect, we have measured the parallel and the series mode resonance frequencies, and the corresponding Q factors, for a QCM with one side facing a liquid. These four quantities have all been measured versus liquid conductivity, using a recently developed experimental setup. It allows the simultaneous measurement of the resonant frequency and the Q factor of an oscillating quartz crystal, intermittently disconnected from the driving circuit. Based on these results, a simple model together with an equivalent circuit for a quartz crystal exposed to a liquid is presented. The analysis shows that it is not necessary to infer the existence of surface charges (or other microscopic phenomena such as electrical double layers) to account for the influence of the liquid's electrical properties on the resonant frequency. Our results show that the contacting conductive liquid, in effect, enlarges the electrode area on the liquid side and thereby changes the parallel resonant frequency. By proper design of the QCM measurement, perturbing effects due to the liquid's electrical properties can be circumvented.     

Nanoscale superconductor-magnetic metal structures: Critical current enhancement and proximity effect

We focus on some aspects of proximity effect in coupled superconductor-magnetic metal artificial structures and discuss mechanisms that may enhance superconductivity of such systems. Superconductor-ferromagnet tunnel junctions are considered where an inversion/enhancement of the Josephson current is due to magnetic proximity effect. Special attention has been paid to another way of enhancing superconductivity of a proximity coupled superconductor-magnetic metal system—the field-induced enhancement. This mechanism can be realized if the effective exchange field between the conducting electrons and localized spins of magnetic ions in a magnetic spacer can counteract the effect of an external magnetic field, i.e., an enhancement due to the so-called Jaccarino-Peter compensation effect.     

Nanoscale Superconductor-Magnetic Metal Structures: Critical Current Enhancement and Proximity Effect

No Heading We focus on some aspects of proximity effect in coupled superconductor-magnetic metal artificial structures and discuss mechanisms that may enhance superconductivity of such systems. Superconductor-ferromagnet tunnel junctions are considered where an inversion/enhancement of the Josephson current is due to magnetic proximity effect. Special attention has been paid to another way of enhancing superconductivity of a proximity coupled superconductor-magnetic metal system - the field-induced enhancement. This mechanism can be realized if the effective exchange field between the conducting electrons and localized spins of magnetic ions in a magnetic spacer can counteract the effect of an external magnetic field, i.e., an enhancement due to the so-called Jaccarino-Peter compensation effect.PACS numbers: 74.78.Fk, 74.50.+r, 75.70.Cn     

Stability analysis of an acoustically levitated disk

In this paper, a model is developed for the stability analysis of an acoustically levitated disk on the basis of analyzing eddy acoustic streaming and acoustic viscous stress. In the model, the effect of the acoustic streaming outside the boundary layer that is on the surface of the levitated disk is properly taken into account. Also, the calculation of sound field and acoustic viscous stress is limited to the range that has a dominant effect on the stability. By this method, we obtain a quite accurate solution of the stability coefficient. For the small horizontal shift of a large levitated disk, the model is verified by the good agreement between the experimental and theoretical results. By means of this model and relevant experiments, some factors that affect the stability of the levitated disk are investigated, and useful guidelines for design and application are obtained. It is found that the range from the edge to the outermost nodal circle of the disk-shaped vibrator has a large effect on the stability of the levitated disk. To stabilize the levitated disk by acoustic viscous force, the distance between the edge and the outermost nodal circle of the vibrator must be larger than a critical value, which is determined by the driving frequency and the sound velocity of the fluid between the levitated disk and the vibrator. When this condition is satisfied, increasing the distance between the edge and the outermost nodal circle leads to a decrease in the stability. It is also found that the property of the fluid between the levitated disk and the vibrator has a large effect on the stability. It is easier to stabilize the levitated disk in steam than in air, but more difficult to do so in carbon dioxide and hydrogen. In addition, theoretical results show that increasing the weight per unit area of the levitated object increases the stability for a given vibrator velocity. The distribution of the acoustic viscous stress and the dependence of the stability coefficient and the holding force on the horizontal shift of the levitated disk, which are obtained by this study, also are useful to a better understanding of the stability of the levitated disk.     

Experimental Determination of the Q-factors of Microcantilevers Coated With Thin Metal Films

Abstract:  Microelectromechanical systems (MEMS) are typically fabricated out of materials that are mechanically sound at the microscale, but can be relatively poor electrical conductors. For this reason, areas of MEMS can be coated with various thin metal films to provide electrical pathways. These films, however, may drastically alter mechanical properties of the device. In this paper we investigate how metallization of microcantilevers affects the quality factors, (Q). Using two sets of silicon microcantilevers that are coated with aluminium films from 5 nm to 30 nm thick, on one side and two sides, respectively, the Q-factors are experimentally determined using the ring-down method. The ring-down method entails mechanically exciting the microcantilevers at their fundamental resonance frequency, abruptly stopping the excitation, and then measuring the decay of oscillation amplitude as a function of time. From this ring-down curve, the Q-factor of each microcantilever can be determined. Results show that the greater the thickness of the aluminium film, the lower the Q-factor will be. We also show a significant temperature dependency of the Q-factor of aluminium coated microcantilevers.     

Planar HTS structures for high-power applications in communication systems

The use of rotational symmetric modes of superconductive disk (ring) resonators in order to increase the power handling capability of planar filters has been proposed. Several YBCO disk resonators on LaAlO₃ and sapphire substrates have been measured as one- and two ports. The unloaded Q-factor of the resonators has been measured to be greater than 10⁵ up to the oscillating power of 50 kW. Experimental results obtained show that a disk resonator can be used as a building block for low insertion loss, sharp skirt high-power planar filters at low GHz frequency range and temperatures of around 60 K.     

Noncontact measurement of vibration using airborne ultrasound

A noncontact ultrasonic method for measuring the surface normal vibration of objects was studied. The instrument consists of a pair of 420 kHz ultrasonic air transducers. One is used to emit ultrasounds toward the moving surface, and the other receives the ultrasound reflected from the object under test. Two effects induce a phase modulation on the received signal. The first effect results from the variation of the round trip time interval tau required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Doppler effect directly proportional to the surface displacement. The second effect results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound field emitted in the air by the vibrating surface. This latter phenomenon, which is a volume effect, is proportional to the velocity of the vibrating surface and increases with the distance between the transducers and the surface under test. The relative contribution of the Doppler and parametric effects are evaluated, and both have to be taken into account for ultrasonic interferometry in air.     

QCM Operation in Liquids: Constant Sensitivity during Formation of Extended Protein Multilayers by Affinity

The quartz crystal microbalance (QCM) is a well-established tool in mass-sensitive detection. Due to recent improvements in experimental procedures, QCMs are finding increasing attention for applications in liquids. One important application is bioaffinity measurements for analytical or research purposes. The effect of the formation of solid films at a QCM surface, especially in gases or vacuum, is well understood. However, the situation is more complex in bioaffinity applications due to the comparably high viscosity of the liquid and the softness of the biological overlayer. Typically frequency responses found for protein layers exceed the values expected from simple models. The use of a hydrogel extending several hundred nanometers from the transducer surface as interacting matrix is common in bioaffinity applications and further increases complexity. Pure mass-related effects as well as viscosity-mediated effects may contribute to the overall frequency response observed experimentally. To improve our understanding of the effects during the formation of extended biological overlayers we have investigated systematically the formation of protein multilayers with a QCM in situ. The attenuation of the QCM oscillation by the liquid leads to a broadening of the resonance frequency. We have overcome this limitation by frequency-dependent admittance analysis and by curve fitting of the resulting admittance. A time resolution of 5 s and a noise of 0.2 Hz has been achieved with 6-MHz AT-cut quartz crystals operating in liquids. Protein multilayers were formed by successive incubations with a biotin-albumin conjugate and streptavidin. Frequency responses for dry protein layers in air were in agreement with mass changes estimated from the Sauerbrey equation. However, in water, the corresponding frequency decrease was increased by a factor of 4, thereby indicating that significant amounts of water are embedded in the hydrated protein layer. Unexpectedly a constant frequency decrease per layer was found during the successive formation of up to 20 protein layers (～400 nm). Neither noise nor drift increased with the number of protein layers. These results indicate that, despite the high hydration of the protein layers, viscosity-induced effects play a negligible role and that the frequency decrease reflects primarily mass changes at the surface.     

Band-Pass Filters for 1.8 GHz Frequency Range Using Double-Sided YBCO/Au Films on CeO2-Buffered Sapphire

Microstrip band-pass filters for the frequency range of 1.8 GHz were manufactured using double-sided YBa₂Cu₃O_7– (YBCO) films on CeO₂-buffered 2-in. sapphire wafers. Their S-parameters were measured. To protect YBCO films against degradation in air, they were coated with gold. Microwave surface resistance R_s(T) was measured by the copper end-plate and microstrip resonator techniques. The Au coating was proven to suppress Q-factor of resonators and to drastically increase the insertion losses of filters. A significant peak of R_s(d_Au) for 77 K at d_Au 20 nm was experimentally observed. The peak is much higher and occurs at much lower d_Au values than what is predicted by a simple electrodynamic calculation for the bilayer film. We suggest that this is due to a surface roughness, which was observed by atomic force microscopy. A dynamic proximity effect with an account of Andreev reflection may be also important to understand the obtained results.     

Design and evaluation of an antiparallel coupled resonator for chemical sensor applications

In this paper, a new type of quartz crystal resonator in which the electrodes are located on one side has been developed for chemical sensing. The resonator has two electrodes for exciting thickness shear mode (TSM) vibrations on one side of the crystal and a conductive layer on the other side. These electrodes are capacitively coupled with the electric fields in opposite directions, forming an antiparallel coupled resonator (ACR). The resonant characteristics of the ACR were evaluated as a function of gap width between the two electrodes used to excite the TSM. The conductance value was observed to increase with decreasing gap width. We also discovered that the gap should be parallel with the crystallographic x-axis to obtain the highest sensitivity. The frequency response to a viscous loading was almost same as that of a standard quartz crystal microbalance (QCM). The ACR sensor is an attractive alternative to a QCM chemical sensor because it can be easily integrated into packaging and film coatings.     

Influence of Biodopants on PEDOT Biomaterial Polymers: Using QCM‐D to Characterize Polymer Interactions with Proteins and Living Cells

Organic conducting polymers (OCPs) are currently the subject of intense research in the area of biomaterials and bioelectronics. Of the OCPs, poly(3,4‐ethylenedioxythiophene) (PEDOT) has attracted significant interest, however there has been little work on investigating the incorporation of biological compounds as the dopant species in the polymer which are aimed at enhancing the biocompatibility and biofunctionality of the material. Here, we incorporate the biological dopants dextran sulphate, chondroitin sulphate, and alginate, into PEDOT polymers and investigate their influence on a suite of physicochemical and electrochemical properties. We employ QCM‐D to study the mass of adsorption and the viscoelastic properties of the important extracellular matrix proteins fibronectin and collagen. Furthermore, we use QCM‐D to study the adhesion of PC12 neural cells to the PEDOT‐biodopant polymers with and without an adsorbed protein conditioning layer. QCM‐D was found to be an excellent tool with which to study conducting polymer–biological interactions, with this report the first time that QCM‐D has been used to study cell interactions with conducting polymer biomaterials.     

环形凹槽结构借助金属圆盘和金属锥尖实现增强聚束效应

基于传统“牛眼”结构对光波的聚束原理,介绍一种新颖的亚波长光聚束组合结构,在入射端的一定距离添加金属圆盘和出射端添加金属锥尖,实现聚柬能力的增强,FDTD仿真结果表明,基于金属圆盘和金属薄膜之间形成强烈共振的波导腔,使得远场光束中心能量增强 利用金属锥尖对出射光激发的表面等离激元进行局域和收集,抑制了远场光束的边锋,进一步增强了中心光束的能量,角分布变窄。该结构聚束作用和锥尖顶端局域场增强作用可广泛地应用于亚波长光学元器件设计、生物医疗探测等领域。     

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。     

Exploring a pseudo-regression model of transnational cooperation in science

This paper reports the results of an empirical study on the impact of three proximity measures: geographical distance, thematic distance and socio-economic distance among the set of 45 scientifically most advanced countries on their cooperation network. In network data, individuals (viz. countries) are linked to one another and the relationships are nested and embedded in groups, with the result that statistical assumptions of independence underlying ordinary least squares regression are systematically violated. Hence, we have used a non-parametric regression procedure, Quadratic Assignment Procedure (QAP), for regressing the matrix of transnational cooperation on the matrices of three proximity measures: geographic proximity, thematic proximity and socio-economic proximity. The results indicate that all the three proximity measures have the expected negative effect on transnational cooperation. Geographic proximity has greater impact than the other proximity measures. This revised version was published online in August 2006 with corrections to the Cover Date.     

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%.     

Phasor-Difference Method for Measuring the Amplitude and the Phase of Small-Amplitude Vibrations with TV Holography

A new, to the author's knowledge, method for time-averaged TV holography measurements of small-amplitude vibrations is presented. In time-averaged TV holography with sinusoidal phase modulation of the reference arm of the interferometer, two phasors describe the object vibration and the modulation of the reference arm. By inversion of the squared zero-order Bessel function of the first kind, it is possible to measure the distance between these two phasors. The distances from an object-vibration phasor to a number of known reference phasors are measured to determine the amplitude and the phase of the object vibration. The method is demonstrated by the measurement of a vibration mode of a circular metal disk. The results are compared with theoretical data and with data obtained by a commonly used method in phase-modulated TV holography.     

A Resonant Cavity Approach for the Nondestructive Determination of Complex Permittivity at Microwave Frequencies

A nondestructive microwave cavity approach for measuring complex permittivities of materials in sheet form is described. The resonant cavity is a section of a rectangular waveguide terminated by a thin rigid and large flange containing a small rectangular iris opening. The iris is placed in firm contact with one side of the dielectric sample while the other side is backed with a highly conducting plate. Variations of the cavity resonant frequency and Q-factor caused by the dielectric can be related to its complex permittivity through the consideration of equivalent admittance of this open-ended dielectricloaded aperture at resonance. Experimental determination of aperture admittance of a loaded iris is made and the results compared with theoretical calculations. The validity of this technique is confirmed by evaluating the resonant cavity characteristics by loading it with dielectrics of known permittivities and comparing the results with theoretical results. The permittivity of a lossy dielectric slab is measured and the value obtained by this method is compared with those found by other techniques. In all these cases the agreement between theory and measurements is satisfactory. Utility of this technique in evaluating the local inhomogeneities of permittivity of sheets is demonstrated. Measurement errors and limitations of this technique are pointed out.     

Recognition of dengue virus protein using epitope-mediated molecularly imprinted film

Molecularly imprinted film was fabricated in the presence of a pentadecapeptide onto a quartz crystal microbalance (QCM) chip. This 15-mer peptide has been known as the linear epitope of the dengue virus NS1 protein. Imprinting resulted in an increased polymer affinity toward the corresponding templates but also to the virus protein. Direct detection of the dengue virus protein was achieved quantitatively. The QCM chip response to the NS1 protein was obtained using epitope-mediated imprinting demonstrating a comparable frequency shift in chips immobilized with monoclonal antibodies. The binding effect was further enhanced and confirmed using a monoclonal antibody to form a sandwich with the MIP-NS1 protein complex on the chip. No pretreatment was required.     

Q-factor measurement of quasi-optical dielectric resonators under conditions of the whispering gallery mode degeneration removal

An approach that allows one to determine the unloaded quality factor of a quasi-optical dielectric resonator (QDR) under conditions of the resonance line splitting corresponding to twofold degenerative whispering gallery (WG) mode has been proposed. The resonator is represented by an equivalent network as two oscillatory circuits with a certain coupling between them. The approach allows one to determine the Q-factor of a single oscillatory circuit using three measured parameters, namely: 1) ratio between specific amplitudes of the resonator amplitude-frequency response (or its second derivative); 2) resonant frequency; and 3) frequency splitting of the response (or its respective second derivative). The proposed technique is illustrated by the measurements in the millimeter-wave range for sapphire quasi-optical resonators with conducting and high-T/sub c/ superconducting (HTS) film endplates.     

Quartz crystal microbalance detection of glutathione-protected nanoclusters using antibody recognition

A quartz crystal microbalance (QCM) immunosensor was developed for the quantitative detection of glutathione-protected nanoclusters. Advantages intrinsic to QCM were employed to make it an attractive alternative to other immunosensing techniques. We have addressed challenges in the area of QCM mass sensing through experimental correlation between damping resistance and frequency change for a reliable mass measurement. Electrode functionalization was optimized with the use of protein A to immobilize and present polyclonal IgG for antigen binding. This method was developed for the detection of glutathione (antigen)-protected clusters of nanometer size with high surface area and thiolate valency. Quantitation of glutathione-nanocluster binding to immobilized polyclonal antibody provides equilibrium constants (K(a) = (3.6 +/- 0.2) x 10(5) M(-1)) and kinetic rate constants (k(f) = (5.4 +/- 0.7) x 10(1) M(-1) s(-1) and k(r) = (1.5 +/- 0.4) x10(-4) s(-1)) comparable to literature reports. These observations further imply that immunoreactive nanoparticles have potential in medical diagnostics and materials assembly.