Resonance frequency change

LOD for 1,2-xylene at the SAW chemosensor was as low as 0.1 pL L 1 at the appreciable resonant frequency change of 6 Hz. In contrast, LOD for 1,2-xylene at the QCM chemosensor, coated with a similar film, was rather high and equal to 4 pL L 1 at the resonant frequency change of merely 0.7 Hz. Apparently, 40-fold lower LOD for the SAW chemosensor indicates its prospective application for mass-sensitive gas sensing. 

Fig. 4 Resonant frequency changes with time due to repetitive FIA melamine injections, for the MIP-QCM chemosensor. Melamine concentration is indicated with number at each curve. Inset shows FIA calibration plots for (1) melamine and its interfering compounds, such as (2) ammeline, (3) cyanuric acid, and (4) cyromazine. Volume of the injected sample solution was 100 pL. The flow rate of the 1 mM FIC1 carrier solution was 35 pL min-1. The MIP film was prepared by electropolymerization of 0.3 mM bis(2,2 -bithienyl)-benzo-[18-crown-6]methane functional monomer and 0.3 mM 3,3 -bis[2,2 -bis(2,2 -bithiophene-5-yl)]thianaphthene cross-linking monomer, in the presence of 0.1 mM melamine, in the trihexyl(tetradecyl)phosphonium tris(pentafluor-oethy 1)-trifluorophosphate ionic liquid ACN (1 1 v/v) solution, which was 0.9 mM in trifluoroacetic acid (pH = 3.0). The melamine template was extracted from the MIP film with 0.01 M NaOH before the determinations (adapted from [134])...

A biomimetic MIP recognition material was devised for determination of a paracetamol (also known as acetominphen) pain reliever using a QCM transducer [109]. To this end, MIP was prepared with two different functional monomers, namely VPD and MAA. An EGDMA cross-linker and AIBN initiator were also present in the solution. The thermo-radical polymerization was performed at 60 °C. The resulting MIP powder was dispersed in a PVC matrix and then drop coated onto a quartz resonator. A linear decrease of the resonant frequency change with the increase of the paracetamol concentration ranged from 50 nM to 10 mM at LOD of 50 nM paracetamol. The chemosensor was successfully applied for the paracetamol determination in real samples, such as human serum and urine. 

Detectability of analytes of small molecules and low molecular mass can be increased by preconcentration of the analyte in the test solution. Then, a reasonable amount of the analyte should bind to the receptor sites of MIP showing a pronounced decrease in the measured resonant frequency change. 

Application of MIP chemosensors imprinted with two different templates can also improve detectability. For instance, polycyclic aromatic hydrocarbons (PAHs) have been determined using polymers molecularly imprinted with two different PAH templates [155]. Compared with MIPs of a single-template imprint, the detection signal of the resonant frequency change was enhanced by a factor of five and LOD for pyrene was as low as 60 pg L 1. This two-template largely improved detectability can be attributed to easier accessibility of the recognition sites through different diffusion pathways. 

A quartz crystal sensor chip was bonded with a microfluidic glass chip for acoustic wave detection (see Figure 7.46). The sensor was operated in the thickness-shear mode (TSM). This has allowed rat heart muscle cell contraction to be studied based on the measurement of the resonant frequency changes [133]. 

The EQCM comprises a quartz crystal oscillator, in which one of the Au exciting electrodes is also exposed to the solution and acts as the working electrode in a conventional (here, three electrode) cell. Provided any surface film is rigidly coupled to the underlying electrode changes in inertial mass (Am) of the electrode result in crystal resonant frequency changes (A/) that are described by the Sauerbrey equation [11] ... 

Severin, E. J. Lewis, N. S., Relationships among resonant frequency changes on a coated quartz crystal microbalance, thickness changes, and resistance responses of polymer-carbon black composite chemiresistors, Analytical Chemistry 2000, 72, 2008-2015. 

The method can only be used for product gases that are condensed quantitatively under reaction conditions these include water, carbon dioxide, and ammonia. Reactions can be studied only at low pressures and significant investigations of dehydrations using this technique have been published (23-25). This technique can alternatively use a quartz crystal with the reactant sample deposited on or otherwise attached to it and the mass loss monitored through resonance frequency changes as described earlier. 

Oscillator circuits are a cost-efficient alternative to impedance analysis and ring-down [12,13]. Naturally, most sensors rim on oscillator circuits. Some advanced circuits provide a measure of the dissipation (such as the peak resistance, Ri, see Sect. 6) in addition to the frequency. Most oscillators operate on one harmonic only. Oscillators can be more stable than ring-down and impedance analysis because the latter two techniques periodically turn the crystal on and off in one way or another, whereas oscillators just run quietly on one fixed frequency. If the signal-to-noise ratio is the primary concern, no technique can beat oscillators. There is one pitfall with the use of oscillators worth mentioning the theory below pertains to the series resonance frequency (simply called resonance frequency). The output frequency of an oscillator circuit, on the other hand, usually is not the series resonance frequency (Fig. 2). For instance, phase-locked-loop oscillators keep the phase constant. Many oscillators run at the zero-phase frequency (B = 0, Fig. 2). Importantly, the difference between the zero-phase frequency and the series resonance frequency changes if the bandwidth or the parallel capacitance change (Sect. 6). The... 

For a quartz thermometer, the resonant frequency of a quartz crystal resonator is strongly related to the temperature variation. With high resolution, the temperature change can be directly determined from the frequency change of a quartz crystal thermometer. A quartz thermometer developed for use between -80 and 250°C [85] has a resolution of 0.1 mK. If used at the same temperature, a comparable precision can be achieved. However, with temperature cycling, hysteresis can reduce its repeatability. An accuracy of 0.05°C can be achieved with calibration. Nevertheless the temperature resolution for the quartz resonator is found to be less accurate at lower temperatures Over the temperature range from 4.2 to 400 K, the temperature resolution with the resonant frequency change for a YS cut quartz crystal thermometer drops from 1 kHz/K at 300 K to 80 Hz/K at 4.2 K [86]. 

Causes of Resonant Frequency Changes in a Quartz Sensor... 

QCM can be described as a thickness-shear mode resonator, since weight change is measured on the base of the resonance frequency change. The acoustic wave propagates in a direction perpendicular to the crystal surface. The quartz crystal plate has to be cut to a specific orientation with respect to the ciystal axis to attain this acoustic propagation properties. AT-cut crystals are typically used for piezoelectric crystal resonators[7]. The use of quartz crystal microbalances as chemical sensors has its origins in the work of Sauerbrey[8] and King [9] who... 

The resonance frequency change (Af) according to the thin film formation on the surface of the quartz resonator is,... 

When a mass is added on the surface and the mass is negligibly small, the series resonance frequency change (Af) due to the mass added on the surface is given by,... 

Figure 6. Resonance frequency change of a bare QCM in CO2 with respect to pressure. Symbols are measured values and lines are derived from calculation[19].

Figure 7 shows the results of solubilization experiments in a static system. Each line indicates the resonance frequency change over time under different conditions. There are two types of lines - solid lines and lines with symbols. Solid lines are the results attained in the liquid state of CO2 (10 MPa, 20 C), while lines with symbols are in the supercritical state (10 MPa, 45 C). Since the resonance frequency of a quartz crystal is directly affected by the properties of the ambient fluid, reference measurements using a bare quartz crystal resonator were performed before the experiment. Noisy signals were observed when CO2 was purged into the system. The results of the reference measurements indicate that about 70-80 seconds are required until the stabilization of the system. The data recorded during this stabilization period in ignored. 

QCM measurements directly give in-situ kinetic information of reactions inside the cell. Figure 8 shows the tight correlation of dissolution with the square root of time. The resonance frequency change was converted to that of surface mass using eq.(3). This linear dependency indicates a strong possibility of diffusion-controlled dissolution. The rate-limiting step, in this case, seems to be diffusion of Cu(acac)2 to the ambient fluid. 

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