Frequencies mass sensors

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

These piezoelectric crystal oscillators are very accurate mass sensors because their resonant frequencies can be measured precisely with relatively simple electronic circuitry. For certain quartz crystals, the resonant frequency is inversely related to the crystal thickness. A crystal resonating at 5 megahertz is typically 300 micrometers thick. If material is coated or adsorbed on the crystal surface, the resonant frequency will change (decrease) in proportion to the amount of material added. The effect of adsorbed mass on the oscillator frequency varies according to the operational mode of the device. In any case, interpretation of mass via changes in frequency or amplitude assumes that the coated films are rigidly elastic and infinitesimally thin (that is, an extension of the crystal). 

The major advantages of mass sensors are the simplicity of their construction and operation, low weight, and small power requirements. In addition, their operating principle depends on a highly reliable phenomenon. The measurement of the frequency shift is one of the simplest and most accurate physical measurements. 

This is the correct name for most popular mass sensors, although they are better known as Quartz Crystal Microbalances (QCMs). A piezoelectric crystal vibrating in its resonance mode is a harmonic oscillator. For microgravimetric applications, it is necessary to develop quantitative relationships between the relative shift of the resonant frequency and the added mass. In the following derivation, the added mass is treated as added thickness of the oscillator, which makes the derivation more intuitively accessible. 

Quartz crystals have a characteristic oscillation frequency which varies according to their mass. Although crystal wafers have been used as mass sensors in vacuum and gas-phase experiments for many years, it is only recently that they have been employed in contact with liquids or solutions. Quartz crystal wafers can be used as electrodes by depositing a thin film of electrode material on the exposed surface, and interfacial mass changes can then be monitored. It is then known as the electrochemical QCM or EQCM. It is a direct, but non-selective, probe of mass transport. 

Another state-of-the-art detection system contains a surface acoustic wave (SAW) device, which is based on a piezoelectric crystal whose resonant frequency is sensitive to tiny changes in its mass—it can sense a change of 10-1° g/cm2. In one use of this device as a detector it was coated with a thin film of zeolite, a silicate mineral. Zeolite has intricate passages of a very uniform size. Thus it can act as a molecular sieve, allowing only molecules of a certain size to pass through onto the detector, where their accumulation changes the mass and therefore alters the detector frequency. This sensor has been used to detect amounts of methyl alcohol (CH3OH) as low as 10 9 g. 

The piezoelectric mass sensor presenis an excellent example of a transducer converting a property of the analyte, mass in this case, to a change in an electrical quantity, the resonant frequency of the quart/ crystal. This example also illusiralcs ihe dislinenon hciwecn a... 

The detection of chemical analytes can be based on changes in one or more of physical characteristics of thin film or layer in contact with the device surface. Some of the intrinsic film properties that can be utilized for detection include mass/area, elastic stiffness (modulus) viscoelasticity, viscosity electrical conductivity, and permittivity. In addition, changes in extrinsic variables such as temperature and pressure also produce a sensor response [31, 32]. SAW sensor works as a mass sensor, when analytes of interest comes in contact of polymer film, which is coated, on SAW device. Desired toxic gas is exposed to the device through the carrier gas such as Na and then gas molecules are desorbed [5, 33, 34]. Device has been tested for determining the characteristics such as (i) frequency shift with different concentration of vapor (ii) transient response for response and recovery times (iii) transient response for reproducibility of the sensor. Since arrangement has been made to compensate the... 

Another type of aerosol monitoring device relies on the behavior of a piezoelectric crystal, whose oscillation frequency changes with the mass of aerosol deposited on it. It is called piezoelectric mass sensor. After each sampling period, the concentration of the aerosol is displayed and the crystal is automatically cleaned and ready for the next measurement. Sampling efficiency is affected by both the mass and the size of the particles. Very low sensitivity is observed when the particle size is larger than 10 pm in diameter or larger masses of particles are collected. 

If the mass deposited on the electrodes is not rigid and measurements are performed in liquid phase, the TSM resonator does not behave like a simple mass sensor anymore, but provides valuable information about the viscous and viscoelastic properties of the deposited material. Many of the applications of the QCM, in which polymer films, biomolecules, or even whole cells are investigated, benefit from this additional information. In all these cases, the Sauerbrey equation does not apply strictly, and it might become impossible to extract corresponding mass binding quantities from merely recording changes in resonance frequency. 

Fig. 1.10 Principle of operation of mass sensors by frequency-shift measurement (Reprinted with permission from Fanget et al. (2011). Copyright 2001 Elsevier)...

Sorption and surface adsorption of chemical agents by a mat of polymer nanofibers can be detected gravimetrically using quartz crystal microbalance (QCM) detectors (Ding et al. 2004b, 2005c Kwoun et al. 2001). The mass sensor is essentially a circular quartz crystal with thin metal elecfiodes deposited on either side of it. The resonant frequency of the piezoelectric crystal in an AC field depends on the crystal characteristics as well as the mass of material deposited on its surface. The Sauerbrey equation predicts a linear relationship... 

S.-J. Kim, T. Ono, M. Esashi, Capacitive resonant mass sensor with frequency demodulation detection based on resonant circuit. Applied Physics Letters 88 (2006) 053116. 

Mass sensors measure the change in mass upon interaction with the analyte. There are two main types of mass sensors quartz crystal microbalance (QCM) and surface acoustic wave (SAW). QCM measures the mass per unit area by measuring the change in frequency of a quartz crystal resonator. SAW uses a piezoelectric sensor to convert an electric signal into a mechanical wave that is then reconverted into an electric signal. Changes in amplitude, phase, frequency, or time delay between the input and output electrical signals are used to measure the concentration of the analyte. 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

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). 

Direct Mass Measurement One type of densitometer measures the natural vibration frequency and relates the amplitude to changes in density. The density sensor is a U-shaped tube held stationaiy at its node points and allowed to vibrate at its natural frequency. At the curved end of the U is an electrochemical device that periodically strikes the tube. At the other end of the U, the fluid is continuously passed through the tube. Between strikes, the tube vibrates at its natural frequency. The frequency changes directly in proportion to changes in density. A pickup device at the cui ved end of the U measures the frequency and electronically determines the fluid density. This technique is usefiil because it is not affec ted by the optical properties of the fluid. However, particulate matter in the process fluid can affect the accuracy. 

The si/e of mass within the accelerometer determines the self-resonant frequency of the sensors. The smaller the mass, the higher the frequency Accelerometers are usually operated in a range below this self-resonant, freqncnc)/. 

The tapered-element oscillating microbalance (TEOM) sensor, as described by Patashnick and Rupprecht, consists of an oscillating tapered tube with a filter at its free end (Fig. 13.40). The mass of the filter increases due to the collected aerosol and produces a shift in the oscillation frequency of the tapered tube that is directly related to mass. 

Nonlinear optical organic materials such as porphyrins, dyes, and phthalocyanines provide optical limiting properties for photonic devices to control light frequency and intensity in a predictable manner. The optical limit of CNTs composites is saturated at CNTs exceeding 3.8wt% relative to the polymer mass (Chen et al., 2002). Polymer/ CNT composites could also be used to protect human eyes, for example, optical elements, optical sensors, and optical switching (Cao et al., 2002). 

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