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PubMed2001

Photoionization of radiation-induced traps in quartz and alkali feldspars.

For the optimization of luminescence dating and dosimetry techniques on the basis of the optically stimulated luminescence, the stimulation spectra of quartz and alkali feldspars were measured in the spectral region of 250-1100 nm using optically stimulated afterglow. Optically stimulated luminescence in all studied spectral regions is induced by the same kind of deep traps, that produce thermoluminescence in the regions of palaeodosimetric peaks for both minerals. The mechanism for photoionization of deep traps was proposed as being due to delocalization of the excited state of the corresponding lattice defects. The excited state overlaps the zone states; i.e. is situated in the conduction band. Because of the high quantum yield of deep electron trap ionization in the UV spectral region, the present aim was to study the possibility of using UV-stimulation for palaeodose reconstruction.

Journal Article↗
PubMed2004

Quantitative analyses of Streptococcus mutans biofilms with quartz crystal microbalance, microjet impingement and confocal microscopy.

Microbial biofilm formation can be influenced by many physiological and genetic factors. The conventional microtiter plate assay provides useful but limited information about biofilm formation. With the fast expansion of the biofilm research field, there are urgent needs for more informative techniques to quantify the major parameters of a biofilm, such as adhesive strength and total biomass. It would be even more ideal if these measurements could be conducted in a real-time, non-invasive manner. In this study, we used quartz crystal microbalance (QCM) and microjet impingement (MJI) to measure total biomass and adhesive strength, respectively, of S. mutans biofilms formed under different sucrose concentrations. In conjunction with confocal laser scanning microscopy (CLSM) and the COMSTAT software, we show that sucrose concentration affects the biofilm strength, total biomass, and architecture in both qualitative and quantitative manners. Our data correlate well with previous observations about the effect of sucrose on the adherence of S. mutans to the tooth surface, and demonstrate that QCM is a useful tool for studying the kinetics of biofilm formation in real time and that MJI is a sensitive, easy-to-use device to measure the adhesive strength of a biofilm.

Journal Article↗
PubMed2000

Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators

The selection of sensitive coatings is a critical task in the design and implementation of chemical sensors using coated thickness shear mode quartz crystal resonators (QCRs) for detection in liquid environments. This design or selection is performed through a study of the sorption process in terms of the partition coefficients of the analytes in the coatings. The partition coefficient, which is controlled by the chemical and physical properties of the coating materials, determines the inherent selectivity and sensitivity toward analyte molecules. The selection of the coatings is logically determined by the interactions between coating and target analyte molecules, but can also be made through a systematic variation of the coating's properties. The determination of the partition coefficients is only accurate if all contributions to the total measured frequency shifts, deltafs, of the coated QCR can be established. While mass loading is often assumed to be the dominant factor used in determining partition coefficients, viscoelastic effects may also contribute to deltafs. Both the effect of viscoelastic properties and the effect of mass loading on the sensor responses are investigated by using a network analyzer and oscillator circuit and by characterizing the total mechanical impedance of the loaded sensor. Different types of coatings including rubbery and glassy polymers are investigated, and the targeted analytes include classes of polar compounds (methanol), nonpolar compounds (toluene, xylenes), and chlorinated hydrocarbons (trichloroethylene, tetrachloroethylene, etc). It is seen that changes in viscoelastic properties due to analyte sorption may be significant enough to place the sensor in the nongravimetric regime. However, for most applications involving the detection of relatively low concentrations of organic compounds and the use of acoustically thin films, changes in the complex shear modulus of the coatings contribute less than 5% of the total shift in the series resonant frequency, depending on the coating. In that case, the measured deltafs and, hence, the calculated approximate classification and selection of the coatings for operation in a complex solution of water/analyte molecules.

Journal Article↗
PubMed2001

Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking: a quartz crystal microbalance with dissipation monitoring, ellipsometry, and surface plasmon resonance study.

We have measured the time-resolved adsorption kinetics of the mussel adhesive protein (Mefp-1) on a nonpolar, methyl-terminated (thiolated) gold surface, using three independent techniques: quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance, and ellipsometry. The QCM-D and ellipsometry data shows that, after adsorption to saturation of Mefp-1, cross-linking of the protein layer using NaIO4 transforms it from an extended (approximately 20 nm), water-rich, and hydrogel-like state to a much thinner (approximately 5 nm), compact, and less water-rich state. Furthermore, we show how quantitative data about the thickness, shear elastic modulus, and shear viscosity of the protein film can be obtained with the QCM-D technique, even beyond the Sauerbrey regime, if frequency (f) and energy dissipation (D) measurements measured at multiple harmonics are combined with theoretical simulations using a Voight-based viscoelastic model. The modeling result was confirmed by substituting H2O for D2O. As expected, the D2O substitution does not influence the actual adsorption behavior, but resulted in expected differences in the estimated effective density and shear viscosity. These results provide new insight and understanding about the adsorption kinetics and crosslinking behavior of Mefp-1. They also demonstrate how the above three techniques complement each other for biomolecule adsorption studies.

Adsorption↗
PubMed2002

Performances and limits of a parallel oscillator for electrochemical quartz crystal microbalances.

This paper describes a driving circuit for an electrochemical quartz crystal microbalance (EQCM) adapted to a wide range of applications. The oscillator is a Miller-type parallel oscillator using an operational transconductance amplifier (OTA). A theoretical study of the oscillating circuit led to the analytical expression of the microbalance frequency as well as to an overestimation of the error on the mass measurement. The reliability of the EQCM was then experimentally verified through electrochemical copper deposition and dissolution. The limit of operation of the EQCM was also investigated, both analytically and experimentally. This work shows that parallel oscillators using few electronic components allow a very reliable EQCM to be obtained for mass measurements on metallic films, even if they are highly damped.

Journal Article↗
PubMed2002

Ionic liquids: a new class of sensing materials for detection of organic vapors based on the use of a quartz crystal microbalance.

A QCM device employing ionic liquids as the sensing materials for organic vapors has been developed and evaluated. The sensing mechanism is based on the fact that the viscosity of the ionic liquid membrane decreases rapidly due to solubilization of analytes in the ionic liquids. This change in viscosity, which varies with the chemical species of the vapors and the types of ionic liquids, results in a frequency shift of the corresponding quartz crystal. The QCM sensor demonstrated a rapid response (average response time of less than 2 s) to organic vapors with an excellent reversibility because of the fast diffusion of analytes in ionic liquids. Furthermore, the ionic liquids, with zero vapor pressure and stable chemical properties, ensure a long-term shelf life for the sensor.

Journal Article↗
PubMed2001

Molecular-imprinted, polymer-coated quartz crystal microbalances for the detection of terpenes.

A piezoelectric sensor coated with an artificial biomimetic recognition element has been developed for the determination of L-menthol in the liquid phase. A highly specific noncovalently imprinted polymer (MIP) was cast in situ on to the surface of a gold-coated quartz crystal microbalance (QCM) electrode as a thin permeable film. Selective rebinding of the target analyte was observed as a frequency shift quantified by piezoelectric microgravimetry with the QCM. The detectability of L-menthol was 200 ppb with a response range of 0-1.0 ppm. The response of the MIP-QCM to a range of monoterpenes was investigated with the sensor binding menthol in favor of analogous compounds. The sensor was able to distinguish between the D- and L-enantiomers of menthol owing to the enantioselectivity of the imprinted sites. To our knowledge, this is the first report describing enantiomeric resolution within an MIP utilizing a single monomer-functional moiety interaction. It is envisaged that this technique could be employed to determine the concentration of terpenes in the atmosphere.

Journal Article↗
PubMed2003

UV vapor generation for determination of selenium by heated quartz tube atomic absorption spectrometry.

A new vapor generation technique utilizing UV irradiation coupled with atomic absorption for the determination of selenium in aqueous solutions is described. In the presence of low molecular weight organic acid solutions, inorganic selenium(IV) is converted by UV irradiation to volatile selenium species, which are then rapidly transported to a heated quartz tube atomizer for detection by atomic absorption spectrometry. Optimum conditions for photochemical vapor generation and interferences from concomitant elements were investigated. Identification of the volatile products using cryotrapping GC/MS analysis revealed that inorganic selenium(IV) is converted to volatile selenium hydride, selenium carbonyl, dimethyl selenide, and diethyl selenide in the presence of formic, acetic, propionic, and malonic acids, respectively. In acetic acid solution, the efficiency of generation was estimated to be 50 +/- 10%. No interference from Ni(2+) and Co(2+) at concentrations of 500 and 100 mg L(-)(1), respectively, was evident. A detection limit of 2.5 microg L(-)(1) and a relative sensitivity of 1.2 microg L(-)(1) (1% absorption) with a precision of 1.2% (RSD, n = 11) at 50 microg L(-)(1) were obtained.

Journal Article↗
PubMed2003

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.

Journal Article↗
PubMed2004

A model for the quartz crystal microbalance frequency response to wetting characteristics of corrugated surfaces.

We consider the effect of surface roughness, and its unique wetting behavior, on the response of a quartz crystal microbalance (QCM) resonator operating in contact with a fluid. The rough surface is modeled as sinusoidally corrugated particular to the case of a fixed relationship between amplitude and periodicity, as would arise from polishing with monodisperse spherical particles. The penetration of fluid into the troughs of the corrugations and the resulting meniscus are determined as a competition between surface tension and compression of the trapped gas. Liquid contained below the corrugation peaks, but above the gas/liquid meniscus, is trapped and behaves as an ideal mass layer, contributing a frequency shift that adds to that arising from liquid entrainment. This model allows QCM responses on rough surfaces to be described as a function of liquid properties and contact angle. This permits responses on hydrophobic surfaces to be understood in terms of incomplete surface wetting.

Journal Article↗
PubMed2004

Quartz crystal microbalance and infrared reflection absorption spectroscopy characterization of bisphenol A absorption in the poly(acrylate) thin films.

The absorption process of bisphenol A (BPA) in a number of poly(acrylate) thin films, such as poly(2-methoxyethyl acrylate) (PMEA), poly(ethyl acrylate) (PEA), poly(n-butyl methacrylate) (PBMA), and poly(methyl methacrylate) (PMMA), has been investigated by quartz crystal microbalance (QCM) and infrared reflection absorption spectroscopy (IRRAS) measurements. Both QCM and IRRAS measurements show that the BPA molecules absorb in PMEA, PEA, and PBMA thin films but not in PMMA thin film. The differences in the BPA absorption behavior are mainly attributed to the difference in the glass transition temperature (T(g)) between these polymers. This absorption behavior also depends on the BPA concentration and polymer film thickness. Furthermore, IRRAS characterization demonstrates that the hydrogen bonding is formed between the hydroxyl group in BPA and the carbonyl group in the poly(acrylate) thin films. BPA molecule absorbed in these polymer thin films can be removed by ethanol rinse treatment. By optimizing experimental conditions for the QCM electrode modified by PMEA thin film, detection limitation of approximately 1 ppb for BPA can be realized by the in situ QCM measurement. This method is expected to be a sensitive in situ detection way for trace BPA in the environmental study.

Journal Article↗
PubMed2005

A strategy for single supersaturated droplet analysis: confocal Raman investigations on the complicated hygroscopic properties of individual MgSO4 droplets on the quartz substrate.

We report a new strategy for single supersaturated droplet analysis, i.e., the complicated hygroscopic properties of MgSO4 aerosols under supersaturated state were studied through the micro-Raman observation on an individual MgSO4 droplet deposited on a quartz substrate in a relative-humidity-controlled chamber. Upon reduction of the ambient relative humidity (RH), MgSO4 droplets with tiny volume lost water but did not effloresce. Thus, a detailed spectral evolution of the symmetric stretching vibration band (v1-SO4(2-)) from free ions (at approximately 983 cm(-1)) to monodentate (approximately 995 cm(-1)) and then to bidentate contact ion pairs (CIPs) or more complex chain-structural compositions (approximately 1021 cm(-1)) was observed with the high signal-to-noise (S/N) confocal Raman spectra of the droplet with a diameter of approximately 80 microns. Such a transition process could be well-described by the changes of relative intensity at 983, 995, and 1021 cm(-1). Four steps, i.e., concentrated step, monodentate CIPs step, bidentate CIPs step, and gel step, were roughly observed in the dehumidifying-humidifying cycle according to the intensity ratios of I995/I983 and I1021/I983. Even though the area ratio of the O-H stretching band of water molecules to the v1-SO4(2-) band seemed reversible in the dehumidifying and humidifying processes, the intensity ratios of I995/I983 and I1021/I983 showed a hysteresis in the decomposition of CIPs in the humidifying process with the RH < 40%. The O-H stretching envelope of the MgSO4 droplet was also observed to be sensitive to the structural changes of the hydrogen bonding of water molecules in the four steps. The intensity ratio of Raman scattering for the components with strong hydrogen bonds to those with weak ones, i.e., I3224/I3431, was used to understand the effects of CIPs on the water structures of the first hydration layer of Mg2+. Good consistency on the hysteresis in the humidifying process was also observed from the ratio of I3224/I3431 changing with RH.

Journal Article↗
PubMed2006

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.

Journal Article↗
PubMed2007

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.

Journal Article↗
PubMed1997

Mismatch-sensitive hybridization detection by peptide nucleic acids immobilized on a quartz crystal microbalance.

A quartz crystal microbalance DNA hybridization biosensor, based on thiol-derivatized peptide nucleic acid (PNA) probes, offers unusual in situ differentiation of single-base mismatches. A large excess of a single-base mismatch oligonucleotide has no effect on the frequency response of the target. Such remarkable distinction between perfect matches and mismatches is illustrated by the detection of a common mutation in the p53 gene. The greater specificity of the new mass-sensitive indicatorless hybridization device over those of analogous PNA-based carbon electrodes is attributed to the formation of a PNA monolayer and the use of a hydrophilic ethylene glycol linker. The improved specificity is coupled to very fast (3-5 min) hybridization in a low-ionic-strength medium.

Biosensing Techniques↗
PubMed2001

Design and characterization of asparagine- and lysine-containing alanine-based helical peptides that bind selectively to A.T base pairs of oligonucleotides immobilized on a 27 mhz quartz crystal microbalance.

We have systematically designed and synthesized six kinds of 16-17 mer alanine-based peptides containing four to six lysine (K) and one to four asparagine (N) residues to achieve the selective binding to A.T base pairs of DNA duplexes. The position and number of K and N residues were changed in the helical structure according to common features of the DNA-binding proteins, in which K and N residues are expected to interact electrostatically with phosphate groups and to interact with A.T base pairs by hydrogen bonding, respectively. The time courses of binding of these peptides to dA(30).dT(30) and dG(30).dC(30) duplexes immobilized on a 27 MHz quartz crystal microbalance (QCM) were studied in 10 mM phosphate buffer (pH 7.5) and 40 mM NaCl at 10 degrees C. The maximum binding amounts (Deltam(max)) on a nanogram scale and binding constants (K(a)) could be obtained from the frequency decrease (mass increase) of the oligonucleotide-immobilized QCM. The conformation changes of the peptides upon binding to DNAs were monitored by circular dichroism (CD) spectroscopy. The four properly arranged N residues in the six-cationic K peptide, K6N4(d), resulted in a 5-fold higher affinity for A.T base pairs (K(a) = 5.9 x 10(5) M(-1)) than for G.C base pairs (K(a) = 1.2 x 10(5) M(-1)), and alpha-helices were clearly promoted by the binding to A.T base pairs from CD spectral changes.

Adenine↗
PubMed2006

Nonspecific hydrophobic interactions of a repressor protein, PhaR, with poly[(R)-3-hydroxybutyrate] film studied with a quartz crystal microbalance.

The gene expression for phasins (PhaP), which are predominantly polyhydroxyalkanoates (PHAs) granule-associated proteins, is regulated by a repressor protein of PhaR through the dual binding abilities of PhaR to the target DNAs and the granules. In this study, the binding functions of PhaR to poly[(R)-3-hydroxybutyrate] (P(3HB)) were investigated quantitatively by using a quartz crystal microbalance (QCM) technique. Adsorption of PhaR onto a melt-crystallized film of P(3HB) (cr-P(3HB)) was detected as a negative frequency shift of the QCM. The time course of the frequency changes observed for PhaR adsorption was composed of a quick frequency decrease at an initial stage and a subsequent slower frequency decrease for several hours, indicating multilayered adsorption of PhaR molecules onto cr-P(3HB). The initial rapid adsorption, which corresponds to direct adsorption of PhaR molecules onto a bare surface of cr-P(3HB), was a diffusion-controlled process. Strong interactions between PhaR and cr-P(3HB) were also observed as apparently irreversible adsorption. The comparative QCM measurement of PhaR adsorption onto various types of polymers with different aliphatic chemical structures revealed that PhaR was adsorbed onto the surfaces of polymers, including cr-P(3HB), mainly by nonspecific hydrophobic interactions. These results illustrate the high affinity and low specificity for adsorption of PhaR to P(3HB).

Adsorption↗
PubMedDate not supplied

Real-time measurement of anchorage-dependent cell adhesion using a quartz crystal microbalance.

An in situ technique based on the mass-sensitive piezoelectric quartz crystal microbalance (QCM) was applied to continuously monitor the attachment and detachment of anchorage-dependent mammalian cells on a metal surface. Specifically, we have demonstrated that the attachment of Vero cells to metal surfaces on the piezoelectrically active area of the QCM results in decreases in the QCM resonant frequency that can be monitored conveniently in real time. Lysis and detachment of Vero cells caused by vesicular stomatitis virus (VSV) infection can also be monitored readily. Our results demonstrate that the QCM is a viable technique for monitoring anchorage-dependent cell attachment and detachment on surfaces (caused by stimulatory, inhibitory, or cytopathic effects).

Animals↗