Liquid media sensors

Rijal,K., Mutharasan,R. Piezoelectric-excited millimeter-sized cantilever sensors detect density differences of a few micrograms/mL in liquid medium. Sensors and Actuators B Chemical2007, 121 (1), 237-244... 

The mechanical and electrical displacements for metalized and free surfaces at liquid-36 YX LiTa03 interface are shown in Figure 4.4a and 4.4b [22]. Most of the acoustic energy is conhned to within one wavelength from the surface of the substrate. When the surface is metalized and electrically shorted, the potential on the surface is zero. In this case, only the normalized displacement (U2) interacts with the liquid loading, and the phenomenon is called mechanical perturbation. If the surface is free and electrically open, then both U2 and normalized electric potential (d>) interact with the adjacent liquid medium (Figure 4.4b). Interactions of d> and the electrical properties of the liquid constitute the acoustoelectric interaction. The influence of both the mechanical and acoustoelectrical interactions on sensor response and material characterization is discussed in the subsequent sections. 

Another compelling feature of microcantilever sensors is that they can be operated in air, vacuum, or liquid (P). The danq>ing effect in a liquid medium, however, reduces the resonance response of a microcantilever. In most liquids, the observed resonance response is approximately an order of magnitude smaller than that in air. The bending response, however, remains unaffected by the presence of a liquid medium (10-14). Iberefore, the feasibility of operating a microcantilever in a solution with hi sensitivity makes the microcantilever an ideal choice for biosensors. In addition, highly selective biochemical interactions can be used to regulate molecular adsorption and tune microcantilever response. 

To date, the thermocapacitive flow sensor has only been demonstrated in the semiconductor field for measuring the gas flow rate. Many microfluidic chips and most BioMEMS chips work with a liquid medium. Possible future work includes applications of the thermocapacitive flow sensor in a liquid medium, where electrical insulation would be required. 

Notwithstanding the excellent analytical features inherent in molecular phosphorimetric measurements, their use has been impeded by the need for cumbersome cryogenic temperature techniques. The ability to stabilize the "triplet state" at room temperature by immobilization of the phosphor on a solid support [69,70] or in a liquid solution using an "ordered medium" [71] has opened new avenues for phosphorescence studies and analytical phosphorimetry. Room-temperature phosphorescence (RTF) has so far been used for the determination of trace amounts of many organic compounds of biochemical interest [69,72]. Retention of the phosphorescent species on a solid support housed in a flow-cell is an excellent way of "anchoring" it in order to avoid radiationless deactivation. A configuration such as that shown in Fig. 2.13.4 was used to implement a sensor based on this principle in order to determine aluminium in clinical samples (dialysis fluids and concen-... 

Bioelectrochemical sensors using ionic liquids as electrolyte medium... 

When it is important to control the water activity in a reactor, a water activity sensor is quite useful. The sensor should ideally measure the water activity in the liquid reaction medium. However, the sensors available are designed for gas phase measurements, and, provided there is effective enough equilibration between the liquid and gaseous phases, they can be used to control the water activity in the reactor. If the measured water activity is above the set point, drying is initiated, for example, by passing dry air through the reactor. On the other hand, if the water activity is too low, water can be added, either as liquid water or as humid air. Automatically controlled systems of this kind have been successfully used to monitor and control enzymatic reactions in organic media [13, 14]. 

In this chapter, a method is described to remove simultaneously cellulose and bleach fibre products in the same bath with an acidic medium. An account is also given of a sensor system for measuring and controlling the hydrogen peroxide concentration. For the first purpose, enzymes are used that catalyse the oxidation of the cellulose-structure. In this cellulose-removal process oxygen, dissolved in the process liquid, is consumed and hydrogen peroxide is formed (Equation3.7) ... 

Acoustical temperature sensors can theoretically measure temperature from the cryogenic range to plasma levels. Their accuracy can approach that of primary standards. Temperature measurements can be made not only in gases but also in liquids or solids, on the basis of the relationship between the sound velocity and temperature shown in Figure 3.163. The acoustic velocity can be detected by immersing a rod or wire into the fluid or by using the medium itself as an acoustic conductor. The sensor rod can measure the temperature at a point or, by means of a series of constrictions or indents, can profile or average the temperature within the medium. 

Figure 6.179 Cross-sectional view of an integrated leakage detection system for product pipelines (electrical pipe heating with thermal insulation and sensor cable). Medium organic/inorganic liquids...

An attractive feature of acoustic-wave-based chemical sensors is that they impose relatively few constraints on the materials that can be used as chemically selective coatings. In brief, the film must be uniform, adherent, thin, chemically and physically stable when in contact with its working medium (gas or liquid), and it must not electrically short-circuit the IDTs. 

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