Effect of polyelectrolyte multilayers on the response of a quartz crystal microbalance

The effect of a polyelectrolyte (PE) multilayers made by a layer-by-layer technique on the response of a quartz crystal microbalance (QCM) is studied by using novel mathematical methods based on the M?bius transformations and their matrix representations in the complex plane. In the first method, the basic properties of the M?bius transformation are used for obtaining the PE bilayer matrix from the QCM impedance measurements taken at four different numbers of layers. In the second method, nonlinear fitting with concomitant error estimation is used for obtaining the elements of the bilayer matrix. The methods are applied to a multilayer composed of 150 bilayers of poly(sodium 4-styrenesulfonate) and poly(diallyldimethylammonium) chloride on a quartz crystal resonator. The structure of the system is discussed, and the bulk acoustic impedance and areal mass density of the bilayer are calculated from the layer matrix.     

Photoderivatized polymer thin films at quartz crystal microbalance surfaces: sensors for carbohydrate-protein interactions

Photoderivatized polymer-coated gold surfaces have been developed following a perfluorophenylazide-based double ligation strategy. Gold-plated quartz crystal microbalance (QCM) crystals were initially covalently functionalized with a monolayer of poly(ethylene glycol) (PEG), using photo- or thermolytic nitrene formation and insertion. The polymer surfaces were subsequently used as substrates for photoinsertion of carbohydrate-derivatized photoprobes, yielding different recognition motifs for selective protein binding. The resulting robust and biocompatible sensor surfaces were applied to a flow-through QCM instrument for monitoring lectin-carbohydrate interactions in real time. The results clearly show the predicted lectin selectivity, demonstrating the applicability of the approach.     

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.     

Development of circuit for measuring both Q variation and resonant frequency shift of quartz crystal microbalance

Although frequency shift is often used for QCM (Quartz Crystal Microbalance) measurement, Q of a resonator also varies when viscous loading occurs. However, it is difficult to measure Q variation in real time in comparison with resonant frequency shift. Furthermore, oscillation frequency shift deviates from real resonant one in case of large viscous loading. Here, the authors have developed the circuit based upon motional admittance method for measuring both Q variation and real resonant frequency shift. It was applied to a quartz resonator gas sensor, and its Q variation was measured by this circuit. Moreover, it was compared with a conventional oscillation circuit and the frequency shift of the former was remarkably different from that of the latter in case of large Q variation. When a quartz resonator coated with PEG20M (PolyEthylene Glycol 20M) membrane was exposed to water vapor, its Q decreased from 38700 to 3650 and a frequency shift of the oscillation circuit was only about one sixth of this circuit.     

An extended Butterworth-Van Dyke model for quartz crystal microbalance applications in viscoelastic fluid media

An extended Butterworth-Van Dyke (EBVD) model with frequency-independent parameters for the characterization of a resonant compound formed by a quartz crystal in contact with a finite viscoelastic layer contacting a semi-infinite viscoelastic medium is extracted by analysis of the lumped element model. The formulation of the EBVD model is compared with the complete expression of the electrical admittance of the loaded quartz derived from the transmission line model (TLM). Relative deviations between them do not exceed 3% around 1% bandwidth near resonance. An extended Martin and Granstaff's model and an explicit expression for the frequency shift that supposes an extension of Kanazawa's model for viscoelastic media are obtained. An analysis of the errors associated with the extraction of shear parameters of the coating for different materials prove that, to obtain an error less than 5% in the shear parameters determination, the viscoelastic contribution, defined as the relative error in the thickness computed from the frequency shift by Sauerbrey equation, must be greater than a limit that depends on Q, which is defined as the ratio of the shear storage modulus (G') to shear loss modulus (G"). In the materials studied polymers in the transition range or in the rubbery state with Q=1 and 10, the viscoelastic contribution must be higher than 15% and 50%, respectively, for a 5% limit error in the shear parameters extraction. A criterion for a practical determination of the appropriate viscoelastic regimes is indicated     

Charge transfer property of self-assembled viologen derivative by electrochemical quartz crystal microbalance response

Viologen modified electrodes have been extensively investigated with quartz crystal microbalance (QCM), which has been known as a nano-gram order mass detector, because of their highly reversible electrochemical properties, especially the first reduction-oxidation cycle of V2+ <--> V*+. The purpose of this work was to study the charge transfer characteristics of self-assembled monolayer (SAM) by changing electrolyte solutions where the cations and anions are different. The redox peak currents were nearly equal charges during redox processes and showed an excellent linear interrelation between the scan rates and second redox peak currents. The charge transfer of self-assembled viologen monolayer was determined by the mass change during the cyclic voltammetry (CV). The total frequency change was about 17.8 Hz, 19.6 Hz, 9.5 Hz, and 8.4 Hz. From this data, we could know the transferred mass was about 19.0 ng, 20.9 ng, 10.2 ng, and 9.0 ng. Finally, the electrochemical quartz crystal microbalance (EQCM) has been employed to monitor the electrochemically induced adsorption of self-assembled monolayer.     

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.     

A contribution to solve the problem of coating properties extraction in quartz crystal microbalance applications

The problem of coating properties extraction in quartz crystal resonator (QCR) applications is one of the challenging tasks of QCR applications, not completely solved even in theoretical conditions. The present work demonstrates that the problem is determined only for a set of three parameters of the coating, which concentrate the four coating properties. The following parameters are chosen: the surface mass density m8 = p1h1, the loss tangent tan delta1 = G'1/G'1, and the magnitude of the characteristic impedance /Z1(c)/ = (p1/G1/)(1/2). An algorithm is proposed that permits, in ideal conditions, an unambiguous extraction of these parameters, starting only from the admittance spectrum of a three-layer compound resonator, obtained from the one-dimensional transmission-line model (TLM) around the resonance. The algorithm introduces three additional improvements in relation to other routines: the calculation time is drastically reduced, the problem of erroneous solutions related to relative minima in typical fitting routines is eliminated, and a systematic error analysis in the extraction of the coating properties in real practice can be carried out. The use of the proposed algorithm as a tool for studying the effects of different phenomena such as slipping or surface roughness is introduced.     

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.     

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.     

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.     

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.     

A hybrid humidity sensor using optical waveguides on a quartz crystal microbalance

In this study, slab and ridge optical waveguides (OWGs) made of fluorinated polyimides were deposited on a quartz crystal microbalance (QCM), and hybrid sensors using OWG spectroscopy and the QCM technique were prepared. Polyvinyl alcohol (PVA) film with CoCl₂ was deposited on the OWGs, and the characteristics of humidity sensing were investigated. A prism coupler was used to enter a He–Ne laser beam (λ = 632.8 nm) to the slab OWG. The output light intensity markedly changed due to chromism of the CoCl₂ as a result of humidity sorption, and this change was dependent on the incident angle of the laser beam to the slab OWG. During the measurement of output light, the QCM frequency was simultaneously monitored. The humidity dependence of the sensor with the slab OWG was also investigated in the range from 15 to 85%. For the sensor with the ridge OWG, white light was entered by butt-coupling, and the characteristics of humidity sensing were investigated by observing the output light spectrum and the QCM frequency.     

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.     

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.     

Investigating biomechanical noise in neuroblastoma cells using the quartz crystal microbalance

Quantifying cellular behaviour by motility and morphology changes is increasingly important in formulating an understanding of fundamental physiological phenomena and cellular mechanisms of disease. However, cells are complex biological units, which often respond to external environmental factors by manifesting subtle responses that may be difficult to interpret using conventional biophysical measurements. This paper describes the adaptation of the quartz crystal microbalance (QCM) to monitor neuroblastoma cells undergoing environmental stress wherein the frequency stability of the device can be correlated to changes in cellular state. By employing time domain analysis of the resulting frequency fluctuations, it is possible to study the variations in cellular motility and distinguish between different cell states induced by applied external heat stress. The changes in the frequency fluctuation data are correlated to phenotypical physical response recorded using optical microscopy under identical conditions of environmental stress. This technique, by probing the associated biomechanical noise, paves the way for its use in monitoring cell activity, and intrinsic motility and morphology changes, as well as the modulation resulting from the action of drugs, toxins and environmental stress.     

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.     

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.     

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.