Quartz crystal microbalance frequency

Fig. 2 Quartz crystal microbalance frequency shifts for cycles of alternate myoglobin/ds-DNA and cytochrome P450cam/ds-DNA adsorption on gold resonators coated with mixed mono-layers of mercaptoproionic acid/mercaptopropanol as first layer and PDDA as second layer. DNA was from salmon testes (ST) and calf thymus (CT). Average values are shown for five replicates of [Mb/ST ds-DNA] (0) and four replicates of [cyt P450cam/ST ds-DNA] ( ) films. (From Ref. [15] with permission. Copyright American Chemical Society.)...

To mimic the PG electrode surface for QCM measurements of layers adsorbed on the gold-quartz resonators, we first chemisorb a mixed monolayer of mercaptopropionic acid/mercaptopropanol. This layer is represented by the first point in Fig. 2, labeled MPA. The second layer is PDDA. Quartz crystal microbalance frequency decreasing in a roughly linear fashion and at regular intervals for the multiple adsorption steps demonstrates repeatable adsorption for the two DNA/en-zyme films. Relative precision of layer formation on multiple resonators within 15% can be achieved. Film thicknesses and component weights in Table 1 were obtained by analyzing the QCM data with Eqs. 1 and 2. 

Thickness of the quartz crystal Electrochemical quartz crystal microbalance Frequency... 

Figure 2. Quartz crystal microbalance frequency shift and its ratio versus the ellipsometrically determined film thickeness growth of as a function of successive dip cycles.

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

The first application of the quartz crystal microbalance in electrochemistry came with the work of Bruckenstein and Shay (1985) who proved that the Sauerbrey equation could still be applied to a quartz wafer one side of which was covered with electrolyte. Although they were able to establish that an electrolyte layer several hundred angstroms thick moved essentially with the quartz surface, they also showed that the thickness of this layer remained constant with potential so any change in frequency could be attributed to surface film formation. The authors showed that it was possible to take simultaneous measurements of the in situ frequency change accompanying electrolysis at a working electrode (comprising one of the electrical contacts to the crystal) as a function of the applied potential or current. They coined the acronym EQCM (electrochemical quartz crystal microbalance) for the technique. 

LB films of cadmium octadecanoate and other amphiphiles were transferred on a quartz-crystal microbalance (QCM, 9 MHz, AT-cut) as a substrate with a vertical dipping method. Frequencies of the QCM substrate were followed with time in air, after the QCM was raised from the interface. From the time courses of these frequency changes at each dipping cycle, the transfer amount of dry LB films (W ), the incorporated amount of water (W2), and its evaporation speed (v) could be obtained in nanogram level. 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

An application of an electrochemical quartz crystal microbalance (EQCM) in the study of the A11/HCIO4 system shows that even at a potential about 0.5 V more negative than the onset of AuO formation (the so-called preoxide region), the resonant frequency of the Au-covered quartz crystal decreases as that of the surface mass increases. A comparison of a voltammogram with the potential dependence of the micro-balance frequency for an Au electrode is shown in Figs. 6a and 6b. 

A technique for such measurements is the electrochemical quartz crystal microbalance (EQCM figure 14) [71]. Here, the working electrode is part of a quartz crystal oscillator that is mounted on the wall of the electrochemical cell and exposed to the electrolyte. The resonance frequency / of the quartz crystal is proportional to mass changes Am A/ Am. With base frequencies around 10 MHz, the determination of Am in the ng range is possible. 

Slip is not always a purely dissipative process, and some energy can be stored at the solid-liquid interface. In the case that storage and dissipation at the interface are independent processes, a two-parameter slip model can be used. This can occur for a surface oscillating in the shear direction. Such a situation involves bulk-mode acoustic wave devices operating in liquid, which is where our interest in hydrodynamic couphng effects stems from. This type of sensor, an example of which is the transverse-shear mode acoustic wave device, the oft-quoted quartz crystal microbalance (QCM), measures changes in acoustic properties, such as resonant frequency and dissipation, in response to perturbations at the surface-liquid interface of the device. 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Fig. 2 A Oscillator frequency changes with graft copolymerization of styrene (ST) on the dithiocarbamated copolymer-coated quartz crystal microbalance (QCM) in methano-lic solution with UV irradiation (light intensity 5mWcm ). Concentration of ST a 0, h 0.26, c 0.5, d 0.76 mol dm . B Relationship of oscillator frequency change calculated from A with concentration of ST...

The quartz crystal microbalance (QCM) is a piezoelectric device consisting of a thin (e.g.) quartz wafer sandwiched between two electrodes. A potential applied across the electrodes results in an oscillation of the quartz. The frequency of the oscillation, which can be measured accurately, is sensitive to mass loading. The relationship between frequency and mass loading is described by the Sauerbray equation ... 

A related, but less sensitive technology, uses the quartz crystal microbalance, in which the crystal has dimensions of millimeters. When analyte binds to a chemically sensitive layer on a gold coating on the surface of quartz, the oscillating frequency of the crystal is altered. See, for example, B. Zuo, S. Li, Z. Guo,... 

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