Sensing mechanisms sensor

When mass loading is not the dominant sensing mechanism, sensor response may not be linear with concentration in the film. This departure from linearity has been observed with polymer films [61,64,86]. An investigation into the possible effect of this nonlinearity on the evaluation of D values fiom SAW frequency transients indicated that errors in D values (factor of two error) could be obtained if the nonlinearity of the response is large [86). However, using small steps in partial pressure, this nonlinearity in the response can be minimized, allowing the effective evaluation of difiiision coefficients based on AW frequency transients. 

Chen D, Lei S, Chen Y (2011) A single polyairiline nanofiber field effect transistor and its gas sensing mechanisms. Sensors 11(7) 6509-6516. doi 10.3390/s110706509... 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Table 8.1 Overall sensitivity S of a microring sensor is listed based on two sensing mechanisms and two transducing schemes...

Other sensors are mostly grouped towards the triethylamine. In the case of porphyrins 6-8, 13 the coordinated metal is no longer able to drive the selectivity pattern and the presence of the peripheral alkyl chains completely shadows the coordination interactions. This result can explain the failure to observe the coordination interaction in the sensing mechanism of the metal complexes of the closely related alkyl chains functionalized phthalocyanines reported in the past by Gopel and coworkers [22]. 

This effect is complete in the case of porphyrins 6-8,13, which behave in the same way, independent of metal present in the inner core of the macrocycle. In conclusion, the introduction of these alkyl chains from one side gives a higher porosity to the molecular film and, as a consequence, both a speed up of the response and an increase of the sensor response. On the other hand there is a significant decrease of the importance of the selective interactions in the sensing mechanism of the organic material and, as a consequence, a lowering of the selectivity of the sensor. 

It is convenient to discuss the elemental aspects of the sensing mechanism assuming a regular crystal lattice, that is, a monocrystal. Indeed, the fundamental studies of the reactions at oxide surfaces have been done on such well-defined surfaces. On the other hand, practical sensors are prepared by techniques that yield polycrystalline or amorphous layers. For the dimensions of the film at which the surface conductance begins to dominate the overall conductance, the morphology of film becomes the... 

R(200 ppm of alcohol vapor in air) are 50 and 140 for tin oxide (Sn0x) and palladium-gold/SnOx (PdAu/SnOx) sensors, respectively while the values of R(50 ppm of N0X in air)/R(air) are 60 and 5 for SnOx and PdAu/SnOx sensors, respectively. Possible sensing mechanisms based on XPS, Kelvin probe, Hall and electrical conductivity measurements are also discussed. 

In this paper, the application of microelectronic processing technology to the fabrication of SnOx and PdAu/SnOx microsensors on silicon wafers is described, sensor responses to various gases in air are presented, and the possible sensing mechanisms are briefly discussed. 

The sensing mechanisms of the tin oxide based sensors have been discussed in many publications (9,10,11). The most widely accepted model for tin oxide based sensors operated at temperatures <400°C is based on the modulation of the depletion layer width in the semiconductor (sensor) due to chemisorption as illustrated schematically in Figure 6. For C2H 0H and Sn0x (or PdAu/Sn0x) interaction, the possible reaction steps may be expressed by the following equations ... 

Sensing mechanism of the modified sensor. The sensing mechanism in this modified sensor should be essentially the same as that of the unmodified one. It is noteworthy that a stationary short circuit current was obtained in spite of such sensor construction that the counter electrode was covered with Epoxy resin. Since the sensing electrode is placed in the same situation as the unmodified sensor, this fact indicates that the cathodic reaction is allowed to take place stationarily at the counter electrode. The proton conductor membrane is as thin as 0.2 mm, so that the reactant 02 and the produced H20 will permeate the membrane as shown in Figure 11. A part of H2 will naturally also permeate through the membrane, but the transfered H2 will be consumed by the reaction with 02 electro-chemically or catalytically at the counter electrode. 

Figure 8. Sensing mechanism of the amperometric sensor. "Reproduced with permission from Ref. 13. Copyright 1984, The Chemical Society of Japan. "...

Performances for CO. This modified sensor was found to be also sensitive to small amounts of CO in air. Figure 12 shows response curves to CO in air at room temperature. Although the value of short circuit current is rather small as compared with that for H2 detection, the response is still rapid enough. It is noteworthy that the current value is also in direct proportion to CO concentration as shown in Figure 13. The sensing mechanism is considered to be almost the same as in the case of " -tion. The anodic... 

The possible sensing mechanism of this sensor was proposed. 

Figure 11. Sensing mechanism of the modified amperometric sensor.

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