Equilibrium sensors

The changes of activity coefficients over short distances and/or short time periods are almost always close to zero and can be neglected. The changes of activity coefficients are also negligible in the mass transport sensors (e.g., amperometric) there, too, concentration may be used in place of activity. Generally speaking, only the equilibrium sensors with partitioning or adsorption mechanisms depend on activities rather than on concentrations. 

For irreversible (non-equilibrium) sensors that utilize activated reaction ter-action mechanisms, sensitivity usually increases as a result of increased reaction rates at higher temperatures. This type of response behavior is illustrated in Ing-ure 5.5 for the reaction of a Pt-olefin-complex with ethyl acrylate [92d]. In contrast to the steady decrease in sensitivity for the reversible sensor, die SAW response rate (in Hz/min) increases with temperature. Thus, depending on the type of coating-analyte interactions being utilized, the sensor sensitivity can be improved by selecting an operating temperature consistent with the predominant response mechanism. 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

Fig. 4 Equilibrium surface plasmon resonance sensor response to staphylococcal entero-toxin B (SEB) in a solution of BSA in phosphate buffered saline (BSA-PBS). Reference-compensated equilibrium sensor response to different concentrations of SEB in BSA-PBS solution for direct and sandwich detection modes (a-SEB concentration 3 ixg/mL in BSA-PBS) [38]...

Figure 4. Equilibrium sensor response is directly proportional to the analyte concentration in the sample.

By using different membranes, it is possible to obtain potentiometric sensors for gases such as sulfur dioxide or nitrogen dioxide. Such sensors employ similar (acid-base) or other equilibrium processes. These devices, along with their equilibrium processes and internal electrodes, are summarized in Table 6-2. Membrane coverage... 

Luo K, Shi Z, Varesi J, Majumdar A (1997) Sensor nanofabrication, performance, and conduction mechanisms in scanning thermal microscopy. J Vac Sci Technol B 15 349-360 Majumdar A (1999) Scanning thermal microscopy. Annu Rev Mater Sci 29 505-585 Manghk RM, Wasekar VM, Zhang J (2001) Dynamic and equilibrium surface tension of aqueous surfactant and polymeric solutions. Exp Thermal Fluid Sd 25 55-64... 

Of all existing methods to monitor electrical properties while using semiconductor sensors, only two [5] have become widely implemented both in experimental practice and in industrial conditions. These are kinetic method, i.e. measurement of various electrical parameters under kinetic conditions, and stationary (equilibrium) method based on the measurement of steady-state parameters (conductivity, work function. Hall s electromotive force, etc.). 

Besides the experimental data mentioned above, the kinetic dependencies of oxide adsorption of various metals are also of great interest. These dependencies have been evaluated on the basis of the variation of sensitive element (film of zinc oxide) conductivity using tiie sensor method. The deduced dependencies and their experimental verification proved that for small occupation of the film surface by metal atoms the Boltzman statistics can be used to perform calculations concerning conductivity electrons of semiconductors, disregarding the surface charge effect as well as the effect of aggregation of adsorbed atoms in theoretical description of adsorption and ionization of adsorbed metal atoms. Considering the equilibrium vapour method, the study [32] shows that... 

It should be noted however that the method related to the measurements of equilibrium or stationary oxygen concentrations by means of sensors is not always applicable and convenient even in this particular case. Sometimes too much time is required to conduct these measure-... 

Gas-sensing electrodes. A gas-sensing electrode consists of a combination electrode that is normally used to detect a gas in its solution by immersion. The sensor contains the inner sensing element, usually a glass electrode or another ISE, and around this a layer of a 0.1 Af electrolyte, surrounded by a gas-permeable membrane. On immersion of the sensor this membrane contacts the solution of the gas which diffuses through it until an overall equilibrium is established, i.e., the partial pressure of the gas attains an equilibrium between sample solution and membrane and between membrane and sensor electrolyte. For a better understanding of the interaction between this electrolyte and the... 

Component to be determined Diffusing agent Equilibrium in sensor electrolyte ISE... 

Thermodynamic equilibrium in the sample has to be ensured. That requires that the sample is isothermal, otherwise the heat flux cannot be attributed to the single temperature indicated at the sensor. Furtheron, the sample has to be in reaction equilibrium, so there should be no subcooling of the sample. 

Their distinguishing feature is the high speed of response. This response develops on the scale of fluorescence lifetime of photophysical or photochemical events that provide the response and can be as short as 10 x-10 10 s. Because of that, the fluorescence reporting is never time-limiting, so that this limit comes from other factors, such as the rate of target - sensor mutual diffusion and the establishment of dynamic equilibrium between bound and unbound target. 

Kinetics evaluation software generates the values of ka (rates of complex formation) and kd (rates of complex dissociation) by fitting the data to interaction models. In a sensorgram, if binding occurs as sample passes over a prepared sensor surface, the response increases and is registered upon equilibrium, a constant signal is reached. The signal decreases when the sample is replaced with buffer, since the bound molecules dissociate. 

Mills A., Chang Q., McMurray N., Equilibrium studies on colorimetric plastic film sensors for carbon dioxide, Anal. Chem. 1992 64, 1383. 

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