Immobilization techniques layered

Not all of the ions in the diffuse layer are necessarily mobile. Sometimes the distinction is made between the location of the tme interface, an intermediate interface called the Stem layer (5) where there are immobilized diffuse layer ions, and a surface of shear where the bulk fluid begins to move freely. The potential at the surface of shear is called the zeta potential. The only methods available to measure the zeta potential involve moving the surface relative to the bulk. Because the zeta potential is defined as the potential at the surface where the bulk fluid may move under shear, this is by definition the potential that is measured by these techniques (3). 

For an assessment of different immobilization techniques, CLEAs of HbHNL, PaHNL, and MeHNL were compared with aqua gels of these enzymes [68]. When the CLEAs were used in the presence of a buffer layer that is under conditions comparable to the aqua gels (see also Section 2.3), aU of them were stable [68, 80] and PaHNL can be recycled up to 10 times without loss of activity [81]. The real advantage of the immobiUzation as CLEAs only becomes obvious when they are employed in pure organic solvents. Here MeHNL displays remarkable stability... 

In the extrinsic technique, light from a suitable source travels along the FO to the distal end where an immobilized sensing layer is located. Reflected, scattered or emitted light returns from the sample by a second fiber or by bifurcation of the original fiber. The emitted light is interpreted at the detector and is a measure of the concentration of the analyte of interest. The simplest FO biosensor uses absorbance measurements to determine any changes in... 

Ultrafiltration of an enzyme solution through a UF membrane does not always result in gel layer formation. Unless a gel layer is formed, this immobilization technique cannot be used for flow systems lacking effective enzyme immobilization. In any event, soluble enzyme membrane reactors can be useful in order to perform kinetic analysis at high enzyme concentrations. Once steady state is attained, the theoretical model permits calculation of reaction rates in both regions. Once the layer is formed it behaves like a secondary membrane,34 capable of separating compounds of different molecular weight in the mixture as well as catalyzing a chemical reaction. 

Several immobilization techniques have been developed to overcome the instability problems of SLMs, such as by the gelation or microencapsulation of the liquid phase, covering the surface layers (contained liquid membranes). An overview on stabilization techniques for SLMs developed over the last 10 years can be found in Ref. [126]. 

The adsorption method at controlled potential or without potential application called "wet adsorption" [16, 17] is the easiest way to immobilize DNA (or probes) onto carbon transducers [2, 18, 19]. There is no need of special reagents, expensive labeled nucleic acids, or long experimental steps in adsorption-based immobilization technique. Hovewer, random immobilization of DNA were obtained with this technique and nucleic acids bound weakly to the surface as parallel layers. Additionally, it is possible to aglomerate DNA onto the surface and when the electrode is rinsed stringently, noncovalently bound DNA can be removed from the transducer surface. 

Not only the components of the sensing layer are important but also the strategies of integration between them and with the primary transducer are of paramount importance. Passive adsorption is one of the simplest and most frequently used immobilization techniques, based mainly on weak noncovalent bindings. However, random protein orientation can lead to the obstruction of functional binding sites and can result in the loss of affinity or activity. Thus, covalent attachment of a protein layer on a chemically functionalized surface produces a more stable layer with correct orientation. This influences sensitivity and specificity of the immunoassay as density of immobilized protein can be better controlled and nonspecific adsorption may be decreased [155]. On the other hand, covalent procedures are usually longer and more tedious and are less justified when disposable surfaces can be used. 

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