Mass sensors

The optimum domain of applicability of mass sensors can be evaluated by considering some general aspects of behavior of these devices, from the point of view of the basic species-sensor interaction. 

Clearly, a mass-related signal will be obtained only if the species-sensor interaction results in a net change of mass of the chemically selective layer attached to the device. Thus, an equilibrium binding will yield a measurable signal. On the 

Different types of mass sensors discussed in this chapter are summarized in Fig. 4.1 (Grate et al., 1993). It shows the schematics, principal modes of oscillations, and the common acronyms. Their basic properties and sensitivities are summarized in Tables 4.1 and 4.2, respectively. These mass sensors are based on piezoelectric oscillators. More recent additions to mass sensors are microfabricated cantilevers, which do not rely for their operation on the piezoelectric effect. 

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. 

Device Type Wave Type Particle Displacement Relative to Wave Propagation Direction Transverse Component Displacement Relative to the Sensing Surface Media Plate Thickness Factors Determining Frequency3 Typical Frequency (MHz) Example  

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As for miniaturisation, examples are UV/VIS spectrometers using a linearly variable filter rather than a diffraction grating as the wavelength separation device (matchbox size), and double-focusing mass sensors (postage stamp size). 

Figure 1.11 — Average number of papers on (bio)chemical sensors published annually, based on data from Janata s biannual review. E electrochemical sensors ISEs ion-selective electrodes P potentiometric sensors A amperometric sensors C conductimetric sensors O optical sensors M mass sensors T thermal sensors. (Adapted from [23] with permission of the American Chemical Society).

Figure 2.9 — General types of continuous-flow electric, thermal and mass sensors. S sample. SMZ sensitive microzone. E electrode. PC piezoelectric crystal. T thermistor. For details, see text. (Reproduced from [1] with permission of the Royal Society of Chemistry).

Spectrophotometry, 42 Absorbance, 42 Infrared, 44 Luminescence, 45 Raman, 48 Fiber Optics, 50 Refractive Index, 52 Piezoelectric Mass Sensors, 53 New Chemistry, 54 Immunochemistry, 54 Polymers and New Materials, 56 Recognition Chemistry, 57 Chromatography and Electrophoresis, 61 Flow Injection Analysis and Continuous Flow Analysis, 63 Robotics, 65 Chemometrics, 68 Communications, 70... 

PIEZOELECTRIC MASS SENSORS (ALDER AND MCCALLUM, 1983 CAREY AND KOWALSKI, 1986 ... 

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 use of piezoelectric mass sensors for solution phase measurements is still under development. Whereas considerable success has been achieved with these devices for detecting the mass of electrodeposited analytes in... 

Although QCM has originally been devised as a mass sensor for operation in vacuum or gas, it also appeared to be suitable for measurements of the mass and visco-elastic changes at a solid-liquid interface. That was possible due to elaboration of the dedicated oscillator circuitry [115]. The Sauerbrey Equation (2) was derived for resonator oscillations in vacuum. However, it also holds for solution measurements provided that (1) the deposited film is rigid and (2) it is evenly... 

This form of selectivity applies to sensors that operate in the steady-state regime. The prime examples are thermal and amperometric sensors. It is somewhat limited for potentiometric sensors and it is least suitable for mass sensors. The minimum necessary kinetic background information can be found in Appendix B. 

The geometry shown here corresponds to a semi-infinite planar diffusion. Other geometries (e.g., radial geometries) typical for microsensors can be used. The enzyme-containing layer is usually a hydrogel, whose optimum thickness depends on the enzymatic reaction, on the operating pH, and on the activity of the enzyme (i.e., on the Km). Enzymes can be used with nearly any transduction principle, that is, thermal, electrochemical, or optical sensors. They are not, however, generally suitable for mass sensors, for several reasons. The most fundamental one is the fact... 

Antibodies against small molecules (haptens) can be readily prepared. It has been suggested that Ab against, for example, pesticide gas molecules can be immobilized on mass sensors and used for sensing of such gaseous compounds. 

Moreover, impedance analysis provides a powerful approach to the study of interfaces. Finally, mass sensors have high sensitivity, and can be used with a variety of selective layers for sensing of a very broad range of compounds. 

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. 

Table 4.3 Selected values of acoustic impedances of materials used as electrodes in mass sensors...

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