Thin-film sensors diffusion

Gas sensors usually incorporate a conventional ion-selective electrode surrounded by a thin film of an intermediate electrolyte solution and enclosed by a gas-permeable membrane. An internal reference electrode is usually included, so that the sensor represents a complete electrochemical cell. The gas (of interest) in the sample solution diffuses through the membrane and comes to equilibrium with the internal electrolyte solution. In the internal compartment, between the membrane and the ion-selective electrode, the gas undergoes a chemical reaction, consuming or forming an ion to be detected by the ion-selective electrode. (Protonation equilibria in conjunction with a pH electrode are most common.) Since the local activity of this ion is proportional to the amount of gas dissolved in the sample, the electrode response is directly related to the concentration of the gas in the sample. The response is usually linear over a range of typically four orders of magnitude the upper limit is determined by the concentration of the inner electrolyte solution. The permeable membrane is the key to the electrode s gas selectivity. Two types of polymeric material, microporous and homogeneous, are used to form the... 

A different analyser method based upon the same effect (deviating thermal conductivities of gaseous components) is shown in Fig. 6.127. The gas to be analysed diffuses into the measuring cell. Here, a thermal conductivity sensor made of three superimposed silicon chips shows a balanced (zero) output of two thin film resistors fitted on a membrane on the chip in the middle of this stack. One of these thin film resistors is exposed to the gas to be measured. Due to its thermal conductivity, the pair of thin film resistors show an unbalanced signal output. 

Thin films of many polymeric materials exhibit good adhesive i operties and are easily applied to most substrates. In addition, relatively rapid diffusion and a high capacity for organic solutes make amorphous rubbery polymers attractive as sensor coatings. An example of this rapid and sensitive detection is shown in Figure 5.17, the response of a polyisobutylene-coated SAW device to trichlraoeth-... 

Surface acoustic waves (SAW), which are sensitive to surface changes, are especially sensitive to mass loading and theoretically orders of magnitude more sensitive than bulk acoustic waves [43]. Adsorption of gas onto the device surface causes a perturbation in the propagation velocity of the surface acoustic wave, this effect can be used to observe very small changes in mass density of 10 g/cm (the film has to be deposited on a piezoelectric substrate). SAW device can be useful as sensors for vapour or solution species and as monitors for thin film properties such as diffusivity. They can be used for example as a mass sensor or microbalance to determine the adsorption isotherms of small thin film samples (only 0.2 cm of sample are required in the cell) [42]. 

Diffusion of materials out of a PSi sensor was monitored by Koh and coworkers in a proof-of-concept device for monitored drug delivery applications [29], Poly-methyl methacrylate (PMMA) containing 0.25 mg/ml caffeine was first cast onto a freestanding PSi thin film. Exposure of this composite material to a pH 7 buffer caused a time-dependent decrease in the intensity of reflection at a fixed wavelength, or alternatively a time-dependent blue shift of the peak, corresponding to diffusion of the small molecule out of the PMMA-PSi composite. These data correlated well with appearance of caffeine in the buffer solution, as measured by UV-Vis spectrophotometry. 

Thin films of nanostructured metals and semiconductors (e.g., Pt, Sn, CdTe) can be prepared by electrodeposition of the metal ions doped into the Hi LLC phase [40,47,48]. Similar to the precipitation of CdS, these films can retain the symmetry of the LLC template during the deposition. These materials allow one to combine well-defined porous nanostructures, high specific surface areas, electrical connectivity, fast electrolyte diffusion, and good mechanical and electrochemical stability. With this approach, hexago-nally structured semiconductor films of uniform thickness can be prepared. Nanostructured thin films of this type are proposed to have relevance in catalysis, batteries, fuel cells, capacitors, and sensors. 

Fig. 7.3.1 shows the principle of the piezoresistive sensor. Diffused resistors (gages) are formed on the thin-walled section called the diaphragm. An applied pressure is detected via the piezoresistive effect, which is the change in electrical resistance when a stress is applied to the diaphragm. The sensitivity is determined by the material, diameter, and thickness of the diaphragm. The thin-film piezoresistive sensor offers low sensitivity because the piezoresistive coefficient of thin-film silicon is less than one-third of that of single-crystal silicon. 

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is ... 

Shea s current work involves the synthesis of imprinted polymer films and studies of the transport properties and mechanisms of these materials. There are several reports of transport properties in the current literature but the conclusions are in some cases contradictory [17]. The general consensus seems to be that imprinted polymers provide a facilitated diffusion path for the template molecule that increases the flux of this compound compared with other similar compounds. Mechanistically, however, it is by no means clear why this should be so. Possible explanations include a hopping mechanism transporting molecules from site to site, or a simple explanation based on the imprinted sites increasing the concentration of template in the microenvironment of the polymer film and hence increasing the flux. Work on deposition of thin films of imprinted polymer is being applied to sensor research. 

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