Adsorption-based sensors

Calibration measurements for adsorption based sensor systems. 

The self-assembly of an imprinted layer on the surface of a transducer was realized through the adsorption of the template on gold, Si02, or ln02 surfaces followed by treatment with an alkylthiol or organosilane (Hirsch et al. 2003). The first example of this type of sensor was reported in 1987 by Tabushi and coworkers (1987). Octadecylchlorosilane was chemisorbed in the presence of n-hexadecane onto tin dioxide or silicon dioxide for electrochemical detection of phylloquinone, menaqui-none, topopherol, cholesterol, and adamantane. Another MlP-based sensor was... 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

Fig. 9 Four waveguide-based sensors. Left Extrinsic sensors a in direct analyte measurements or c in indirect measurements by means of indicators or other reagents immobilized in membranes. Right Intrinsic sensors for measurements of changes in the light guided though the waveguide by b the adsorption of the analyte into its surface or by d the interaction of the analyte with a recognition phase, which is in direct contact with the waveguide...

Figure 34 shows the results for alcohol (methanol, ethanol, 1-propanol and 1-butanol), ketone (acetone and diacetyl), terpene (pinene and linalool), aldehyde (n-nonyl aldehyde) and ester (acetic acid n-amyl ester and n-butyric acid ethyl ester) of various concentrations. Because of the linear characteristics of the CTL-based sensor, the plots are located in a similar region for a certain type of gas of various concentrations where the Henry-type adsorption isotherm holds. Thus, we can identify these gases with various concentrations by simple data-processing. 

With lack of specificity and low sensitivity established as two major drawbacks of uncoated surfaces, it is clear that an important key to the performance of adsorption-based AW chemical sensors is the adsorbent coating material. All... 

Table 5.5 Examples of Adsorption>Based Acoustic Wave Sensors...

Immunologically based sensors show great potential, but there are a number of problems that may limit their performance. For example, the nonspecific adsorption of proteins and other large molecules can adversely affect the a rent sensitivity and selectivity. Strategies for minimizing this effect include the use of a reference crystal coated with a protein that does not specifically interact with the antigen or compound of interest [27], and deactivation of nonspecific adsorption sites. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

The demonstrated method is readily applicable to other systems. The ability both to enhance QD PL/responses and to suppress analyte adsorption induced wetting effects through utilization of porous structures is essential for these fluorescence-based sensors. Refined design of the platform through a creation of ordered periodic dielectric structures of desired dimensions, together with optical amplifications by surface plasmon resonance of noble metal nanoparticles, may further contribute to the sensor development. Overall this would offer benefits for employing QDs in a broader range of device applications. 

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