Multichannel quartz crystal microbalance

Arrays of quartz crystal resonators are fabricated on a single quartz wafer as a multichannel quartz crystal microbalance (MQCM). Three types of four-channel array of 10-MHz resonators are prepared and tested. Mechanical oscillation of each channel is entrapped within the channel almost completely, so that the interference between the channels via the quartz crystal plate is almost negligible. A mass change on each channel is quantitatively evaluated on the basis of Sauerbrey's law. Thus, each channel of a MQCM device can be used as an independent QCM. Influence from a longitudinal wave generated from another channel is also negligible compared to the influence from the wave from the monitored channel itself. The simultaneous oscillation of channels is also possible. The potential applicability of MQCM to the two-dimensional mapping of mass changes is demonstrated.     

Calixarene monolayers as quartz crystal microbalance sensing elements in aqueous solution

We have examined p-tert-butylcalix[4]arenetetrathiolate (BCAT) monolayers for their potential use as molecular recognition elements for in situ aqueous chemical sensors. Spectroscopic and wetting studies of BCAT monolayers on Au{111} reveal that the calixarene molecules exist in monolayers, preferentially oriented with their phenyl rings parallel to the surface normal axis. Using quartz crystal microbalance (QCM) sensors with gold-coated electrodes, the chemical specificity of monolayers and thin films to a variety of aromatic and aliphatic analytes in aqueous solution was examined. The response of BCAT sensors was compared to the responses of p-tert-butylcalix[4]arene (BCA)- and decanethiolate (DT)-coated QCM electrodes. BCAT is very selective for alkylbenzenes, much more so than either its spray-coated thin-film analogue, BCA, or the highly ordered DT monolayer. From these measurements, the factors behind molecular differentiation in each film are explored. Drawing upon these findings, the roles of cavitation and film order in molecular recognition for calixarene films are discussed.     

Proximity effect in quartz crystal microbalance

When an object approaches a vibrating quartz crystal microbalance (QCM) the resonant frequency changes. This "proximity effect" was seen at the distance of 10 mm in air and became more pronounced as the distance decreased. This effect depends on the quality factor (Q-factor) value of a QCM, conductivity of the object, and electrical connection of the object to QCM electrodes. A special setup was constructed to test the impact of the proximity effect on a QCM. Damping fluid was placed on one side of QCM, to change the Q-factor. A conducting metal disk was brought close to the other side of the QCM exposed to air. By varying the distance between the QCM and an object (metal disk), a shift in frequency was observed. This proximity effect was largest (>200 Hz for 10 MHz QCM) when the Q-factor was low and a conducting metal disk (e.g., Cu) was electrically shorted to the proximal (nearest) QCM electrode. The finite element modeling showed that the proximity effect was likely due to interaction of the object with the fringing electromagnetic field of the QCM. A simple modified Butterworth Van-Dyke model was used to describe this effect. It must be recognized that this effect may lead to large experimental artifacts in a variety of analytical QCM applications where the Q-factor changes. Therefore, in order to avoid artifacts, QCM and similar mass acoustic devices should not operate in the low Q-factor (<1000) regime.     

Isolated electrodeless high-frequency quartz crystal microbalance for immunosensors

This paper presents a contactless technique to measure shear bulk wave resonance frequencies of an isolated quartz crystal in a flow cell. The line antenna placed outside the cell generates and detects the resonance frequencies in a wireless-electrodeless manner. It is revealed that this mechanism relies on the quasistatic electric field. A 0.3-mm-thick AT-cut quartz was used, and its overtone resonance frequencies up to 80 MHz were measured in liquids. Exact vibrational analysis was carried out for a triple-layered resonator system consisting of the adsorbed material layer, the electrode film, and the quartz plate. It predicts higher frequency sensitivity to the adsorbed material at higher modes when the electrode layer is removed. The 13th overtone (72-MHz resonance frequency) was used to detect human immunoglobulin G with concentrations between 0.1 and 20 microg/mL captured by protein A immobilized on one side of the crystal. The real-time measurement of the frequency response yielded the equilibrium constant KA=5.21 x 10(7) M(-1).     

Pyrene-modified quartz crystal microbalance for the detection of polynitroaromatic compounds

The synthesis of a dithiol-functionalized pyrene derivative is reported, together with studies of interactions between this receptor (and other related pyrenes) and nitroaromatic compounds (NACs), in both solution and in the solid state. Spectroscopic analysis in solution and X-ray crystallographic analysis of cocrystals of pyrene and NACs in the solid state indicate that supramolecular interactions lead to the formation of defined π-π stacked complexes. The dithiol-functionalized pyrene derivative can be used to modify the surface of a gold quartz crystal microbalance (QCM) to create a unique π-electron rich surface, which is able to interact with electron poor aromatic compounds. For example, exposure of the modified QCM surface to the nitroaromatic compound 2,4-dinitrotoluene (DNT) in solution results in a reduction in the resonant frequency of the QCM as a result of supramolecular interactions between the electron-rich pyrenyl surface layer and the electron-poor DNT molecules. These results suggest the potential use of such modified QCM surfaces for the detection of explosive NACs.     

Organic vapor sensing with ionic liquids entrapped in alumina nanopores on quartz crystal resonators

We report on a concept for vapor sensing with the quartz crystal microbalance where the vapor phase is absorbed into small droplets of an ionic liquid. The liquid is contained in the pores of a nanoporous alumina layer, created on the front electrode of the quartz crystal by anodization. Ionic liquids are attractive for vapor sensing because--being liquids--they equilibrate very fast, while at the same time having negligible vapor pressure. Containing the ionic liquids inside cylindrical cavities of a solid matrix removes all problems related to the liquid's softness as well as the possibility of dewetting and flow. The absence of viscoelastic effects is evidenced by the fact that the bandwidth of the resonance remains unchanged during the uptake of solvent vapor. The Henry constants for a number of solvents have been measured.     

Photo-click immobilization on quartz crystal microbalance sensors for selective carbohydrate-protein interaction analyses

A photoclick method based on azide photoligation and Cu-catalyzed azide-alkyne cycloaddition has been evaluated for the immobilization of carbohydrates to polymeric materials. The biomolecular recognition properties of the materials have been investigated with regard to applicable polymeric substrates and selectivity of protein binding. The method was used to functionalize a range of polymeric surfaces (polystyrene, polyacrylamide, poly(ethylene glycol), poly(2-ethyl-2-oxazoline), and polypropene) with various carbohydrate structures (based on α-D-mannose, β-D-galactose, and N-acetyl-β-D-glucosamine). The functionalized surfaces were evaluated in real-time studies of protein-carbohydrate interactions using a quartz crystal microbalance flow-through system with a series of different carbohydrate-binding proteins (lectins). The method proved to be robust and versatile, resulting in a range of efficient sensors showing high and predictable protein selectivities.     

Surface chemical activation of quartz crystal microbalance gold electrodes — analysis by frequency changes, contact angle measurements and grazing angle FTIR

A hydroxyalkanethiol, HS(CH₂)₁₂OH was deposited onto the gold electrodes of quartz crystal microbalances and gold-coated quartz microscope slides in order to aid in subsequent immobilization of antibodies. FTIR and contact angle measurements confirm the presence of thiol on the fresh gold substrates. In contrast, the change in frequency of the quartz crystal during thiol adsorption indicates an apparent decrease in mass rather than the expected increase due to thiol coverage of the surface. This anomalous behavior appears to be partly due to the removal of organic contaminants from the “aged” gold surface by solvent action. In addition, there may be some dissolution of gold by the thiol solution. QCM frequency changes and FTIR have also been used to monitor subsequent carbodiimide activation of the thiol, as well as antibody binding to the activated surface. The thiolated gold surface was used to fabricate an ordered layer of IgG molecules for probing various immuno-reactions. This was demonstrated by the binding of anti-IgG to IgG in the ratio of ca. 1:1.     

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.     

Conventional detection of 2,4-dinitrophenol using quartz crystal microbalance

We present conventional detection of 2,4-dinitrophenol (DNP) for using the competitive reaction between DNP and DNP-conjugated albumin onto DNP antibody immobilized quartz crystal microbalance (QCM). This QCM method allows detection of DNP concentration in the range of 0.01 to 100 ng/ml; linear correlation obtains DNP concentration from 1 to 100 ng/ml.     

Piezoelectric quartz crystal microbalance sensor for trace aqueous cyanide ion determination

Using selective reaction chemistry, our present research has developed an online, real-time sensor capable of monitoring toxic cyanide at both drinking water standard and environmental regulatory concentrations. Through the use of a flow cell, aqueous samples containing cyanide are reacted with a gold electrode of a piezoelectric crystal to indirectly sense cyanide concentration by the dissolution of metallic gold. The quartz crystal is an AT-cut wafer sandwiched between two neoprene O-rings within the liquid flow cell. The presence of cyanide in solution results in the selective formation of a soluble dicyano-gold complex according to the Elsner reaction: 4Au + 8CN- + 2H2O + O2 <=> 4Au(CN)2- + 4OH-. The resulting loss of gold from the electrode is detected by the piezoelectric crystal as a resonant frequency change. Since free cyanide is a weak acid (pKa = 9.3), available protons compete for cyanide ligands. Therefore, increased sample pH provides higher sensitivity. The detection limits at pH 12 are 16.1 and 2.7 ppb for analysis times of 10 min and 1 h, respectively. The incorporation of the flow cell improves both analyte sensitivity and instrument precision, with an average signal intensity drift of only 5% over a 2-h analysis. The calibrations show excellent linearity over a variety of cyanide concentrations ranging from low ppb to hundreds of ppm. This detection method offers the advantage of selectively detecting cyanides posing a biohazard while avoiding detection of stable metal cyanides. This aspect of the system is based on competitive exchange of available metals and gold with cyanide ligands. Stable metal cyanide complexes possess a higher formation constant than cyanoaurate. This detection system has been configured into a flow injection analysis array for simple adaptation to automation. Anions commonly found in natural waters have been examined for interference effects. Additionally, the sensor is free from interference by aqueous cyanide analogues including thiocyanate. The developed detection system provides rapid cyanide determinations with little sample preparation or instrument supervision.     

氧化锌纳米线尿酸氧化酶固定性能

本文利用石英晶体微天平研究了氧化锌纳米线对尿酸氧化酶的固定性能和作为尿酸传感器的应用.本文在PBS溶液中对尿酸氧化酶进行固定,并且利用固定的传感器作为尿酸传感器,能检测浓度范围从5.0 × 10-6到8×10-5 molL-1的尿酸含量,所有结果都说明氧化锌纳米线是一种很好的生物传感器的材料.     

Effects of mass layer dimension on a finite quartz crystal microbalance

In this paper, a gradient crystal plasticity model in a polycrystalline grain structure is investigated. Hereby, the focus is on the influence of the grain boundary conditions. A new type of grain boundary conditions is introduced, the so-called micro-flexible boundary condition. In particular, it is compared to existing grain boundary conditions of plastic slip. Numerical results are given for the stress?Cstrain response as well as for the plastic slip field in the grain structure.     

Combination of the quartz crystal microbalance and the electrochemical technique for the characterization of self-assembled monolayers of octadecylmercaptan on gold surfaces

Quartz crystal microbalance (QCM) technique has been applied to the direct mass measurement of a self-assembled monolayer of octadecylmercaptan (OM) on a gold surface deposited onto a quartz-crystal. Repeated rinsing of the modified electrodes with used solvent was conducted to get self-assembled monolayers of OM. From the resonance frequency change, 2.5 nm molecular occupied area of OM on gold was obtained. The advantage of the combination of QCM and electrochemical methods was emphasized for the characterization of self-assembled monolayers.     

Immunosensor for detection of inhibitory neurotransmitter gamma-aminobutyric acid using quartz crystal microbalance

A piezoelectric immunosensor for sensing the low molecular weight neurotransmitter gamma-aminobutyric acid (GABA), one of two major inhibitory neurotransmitters in the central nervous system, is described. The sensing interface consists of a dextran layer covalently attached to a self-assembled monolayer of thiolamine compound on the surface of gold electrodes of the crystals. The dextran layer is further modified with GABA molecules to act as the biosensing layer. The affinity binding of monoclonal anti-GABA antibody on the modified piezoelectric crystals is studied in real time without any additional labels. The equilibrium association constant, K(eq) for binding between anti-GABA antibody and GABA molecules is 14.5 microg x mL (-1). The detection limit for anti-GABA is approximately 10 nM. The sensitivity of the sensor at a concentration corresponding to half-maximal response is 13.6 ng/mL x Hz. The functionalized sensor substrate is subsequently used for competitive determination of different concentrations of free GABA (range of 5 microM-50 mM) in PBS-BSA buffer. The detection limit of the immunosensor for sensing GABA with maximum sensitivity is approximately 42 microM.     

SAW synchronous multimode resonator with gold electrodes on quartz

A surface acoustic wave synchronous multimode resonator with gold electrodes on ST-cut quartz was designed, fabricated, and measured. At a frequency of about 194.3 MHz, an insertion loss of about 14 dB, and loaded and unloaded quality factors of 11,500 and 14,500, respectively, were obtained.     

Method for measuring the losses and loading of a quartz crystal microbalance

A novel, experimentally simple, and highly sensitive method for measuring the loading of a quartz crystal resonator was developed. The method is based on the use of double-sideband suppressed-carrier modulated high-frequency signal, which is swept through the resonance range of the resonator. Induced current in the resonator is passed through a capacitor, and the voltage over the capacitor is demodulated on an analog multiplier. The phase and amplitude information is carried to the frequency-doubled modulation signal and measured on a conventional low-frequency two-phase lock-in amplifier. A complex dimensionless loading parameter is obtained from the experimental data by nonlinear model fitting. The real and complex parts of this loading parameter have a simple relationship with other parameters commonly used for characterizing the resonator loading. The performance of the method was demonstrated by measuring a series of different glycerol-water mixtures ranging from 0 up to 100% glycerol. The results were close to the shear acoustic impedance of these mixtures measured and calculated from their viscosities and densities.     

Quartz crystal microbalance sensor for organic vapor detection based on molecularly imprinted polymers

Molecularly imprinted polymers on quartz crystal microbalances (QCM) are examined for their ability to detect vapors of small organic molecules with greater sensitivity and selectivity than the traditional amorphous polymer coatings. Hydroquinone and phenol serve as noncovalently bound templates that generate shape-selective cavities in a poly(acrylic) or poly(methacrylic) polymer matrix. The imprinted polymers are immobilized on the piezoelectric crystal surface via a precoated poly(isobutylene) layer. The behavior of the imprinted polymer films is characterized by the dynamic and steady-state response of the QCM frequency to pulses of organic vapors in dry air. The apparent partition coefficients are determined for imprinted and nonimprinted polymers prepared by two synthetic methods and for varying mole ratios of template to monomer. The hydroquinone-imprinted polymers and, to a lesser extent, the phenol-imprinted polymers exhibit greater sensitivity and higher selectivity than the nonimprinted polymers toward organic vapors that are structurally related to the templates. These results indicate that molecularly imprinted polymers are promising for the development of selective piezoelectric sensors for organic vapor detection.     

Measurement of polystyrene photodegradation rate using a quartz crystal microbalance

Polystyrene is a very popular polymer utilised in the manufacture of various consumer products. This polymer is very cheap; however, after its usage, the slowness of its photodegradation leads to environmental pollution. In this report, the author presents a technique to systematically measure the rate of photodegradation of a thin polystyrene film. The said film was made to coat a quartz crystal microbalance (QCM) sensor. In order to detect polymer degradation and the reduction in the molecular weight, the resonance frequency of the sensor was monitored for 24 h. Results revealed that QCM sensor irradiation with ultraviolet light with a wavelength of 365 nm and optical power of 1.5 mW caused a quite significant change in the polymer structure.     

涂敷ZnO纳米线的石英晶体微天平的湿度敏感特性

采用石英晶体微天平传感器电极上旋涂纳米氧化锌薄膜,构造湿度传感敏感元件,实现对湿度检测.介绍了检测系统的组成,纳米氧化锌的制备.通过DAQ数据采集卡和Labview软件实现数据的实时采集.实验结果表明旋涂纳米氧化锌薄膜的传感器频率随测试的相对湿度变化明显,用纳米氧化锌作为敏感元件检测湿度具有很高的灵敏度和重复性.