Nonspecific adsorbents

The inner layer (closest to the electrode), known as the inner Helmholtz plane (IHP), contains solvent molecules and specifically adsorbed ions (which are not hilly solvated). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholtz plane (OHP), reflects the imaginary plane passing through the center of solvated ions at then closest approach to the surface. The solvated ions are nonspecifically adsorbed and are attracted to the surface by long-range coulombic forces. Both Helmholtz layers represent the compact layer. Such a compact layer of charges is strongly held by the electrode and can survive even when the electrode is pulled out of the solution. The Helmholtz model does not take into account the thermal motion of ions, which loosens them from the compact layer. 

Sandwich-type sensors are applicable for measuring large antigens that are capable of binding two different antibodies. Such sensors utilize an antibody that binds the analyte-antigen, which then binds an enzyme-labeled second antibody. After removal of the nonspecifically adsorbed label, the probe is placed into the substrate-containing solution, and the extent of the enzymatic reaction is monitored... 

Guidelli and co-workers336-338 measured the potential of zero charge by chronocoulometry. They found that the pzc was independent of the electrolyte concentration in both NaC104 and Na2S04. However, Ea=0 in the presence of sulfates was ca. 40 mV more negative. These authors have explained this apparent discrepancy in terms of the perturbation of the solvent structure at the interface by the ions at the electrode surface, which are, however, nonspecifically adsorbed. 

Further STM and SXS smdies [Wu et al., 1998] concerning this phenomenon indicated that the presence of specifically and nonspecifically adsorbing anions as well as organic molecules (e.g., pyridine, bipyridine, and uracil) may also lift the reconstructed surface by exhibiting a structural transition, and it has been extensively studied and reviewed in [Kolb, 1996]. 

Exchangeable ions (EXC), sometimes including ions nonspecifically adsorbed and specifically absorbed on the surface of various soil components, such as carbonate, organic matter, Fe, Mn, Si, and Al oxides, and clay minerals. This part is controlled by adsorption-desorption processes. 

Thus, cell adhesion is determined by nonspecifically adsorbed serum proteins on the surface. Therefore, it is important to consider the characteristics of adsorbed proteins including the amount, composition, and conformation or orientation. 

The electrified interface is generally referred to as the electric double layer (EDL). This name originates from the simple parallel plate capacitor model of the interface attributed to Helmholtz.1,9 In this model, the charge on the surface of the electrode is balanced by a plane of charge (in the form of nonspecifically adsorbed ions) equal in magnitude, but opposite in sign, in the solution. These ions have only a coulombic interaction with the electrode surface, and the plane they form is called the outer Helmholtz plane (OHP). Helmholtz s model assumes a linear variation of potential from the electrode to the OHP. The bulk solution begins immediately beyond the OHP and is constant in potential (see Fig. 1). 

The results of this TIR/FRAP study are that 80% of the nonspecific binding of fluorescein-labeled insulin to the external face of red cell membranes is reversible within <100s, and the mean residency time of the reversibly adsorbed insulin ranges from 0.4 s to 20 s. Surface diffusion of nonspecifically adsorbed insulin (as investigated by an TIR intersecting beam interference fringe pattern see Figure 7.5) was immeasurably small it is insufficient to carry a typical insulin more than 0.3 /xm before desorption. 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

Nonspecific adsorbents. No functional groups or exchange ions on the surface. Adsorbents of this type are carbon black, boron nitride and polymeric saturated hydrocarbons (e.g., polyethylene). ... 

Next comes a layer of nonspecifically adsorbed counterions with their hydration shell. Still, the permittivity is significantly reduced because the water molecules are not free to rotate. This layer specifies the outer Helmholtz plane. Finally there is the diffuse layer. A detailed discussion of the structure of the electric double layer at a metal surface is included in Ref. [65],... 

When reactants or intermediates are adsorbed, the rate of the reaction may no longer be related to the concentration by a simple law. This situation is best understood where a reactant is nonspecifically adsorbed in the outer -> Helmholtz plane. The effect of such adsorption on the electrode kinetics is usually termed the -> Frumkin effect. Physical and chemical adsorption on the electrode surface is usually described by means of an -> adsorption isotherm and kinetic equations compatible with various isotherms such as the - Langmuir, -> Temkin, -> Frumkin isotherms are known. 

In immunoaffinity chromatography adsorption phase is performed in physiological conditions followed by a wash with high ionic strength buffers to eliminate nonspecifically adsorbed proteins. Elution of human IgG is obtained by a deforming buffer such as a 0.2 M glycine-HCl, pH 2.7. 

Figure 1. Schematic picture of the metal/solution interphase in the case of nonspecific (a) and specific (b) anionic adsorption, x = 0, x = P and x = d are die electrode surface plane, the plane of closest approach for the specifically adsorbed anions, and that for the nonspecifically adsorbed ions. Curve 1 represents the potential-distance profile. In (b), curve 1 results from the combination of curve 2, expressing die contribution from the charge density as of the specifically adsorbed anions, and curve 3, expressing die contribution from die charge density Om on the metal. The potential difference, ft1 — d> across die inner layer is the same in (a) and (b). (Reprinted from Ref.7 with permission from the Am. Chem. Soc.)...

Here, rSi 0 and r, ads are the surface excesses of nonspecifically adsorbed S molecules and of those chemisorbed with pet, while y is the interfacial tension. Let us assume for simplicity that water is in such a large excess with respect to the other components of the solution phase as to allow us to set dfuw = 0 at constant temperature T and pressure P. Noting that in the pure metal phase dpM is also equal to zero at constant T and P, and replacing lSads from Eq. (3) into Eq. (4), after rearrangement we get ... 

The -vk contribution to / in Eq. (26) from the desorbing water molecules can be roughly estimated by assuming that the potential varies linearly with the distance x in the compact layer enclosed between the electrode surface plane x = 0 and the outer Helmholtz plane x = d. In the presence of a strong excess of a nonspecifically adsorbed supporting electrolyte or upon correction for the potential difference across the diffuse layer, the electric potential in the bulk... 

The primary requirement for a solid phase material is either that it be capable of nonspecifically adsorbing useful amounts of the desired reagent (usually a protein) or that it be possible to chemically link the solid phase to the reagent. Since the solid phase in an immunoassay is usually used only once and discarded, it is also desirable that the material be relatively inexpensive and obtainable in quantity. Ready availability of large, uniform lots of solid phase material (e.g., plastic) is important for test reproducibility. Uniformity in the total surface area on which the immune reactions take place is important for precision thus the shape of the solid phase is important. Solid phases that have been used for RIA or ELISA include the following ones ... 

Kiselev reviewed and made fundamental studies of adsorbents in gas chromatography. He attempted to eliminate the heterogeneity of the surfaces of adsorbent solids to improve their selectivity. He classified adsorbents in terms of their specificity. Graphitized carbon black is a nonspecific adsorbent and silica and zeolite are specific adsorbents. The adsorptive properties and the tailing of peaks can be greatly modified by the addition of a small amount of liquid to a large-area solid. The maximum temperature limit is then that of the liquid. 

Thus, the determination of Qi and Ci values on the basis of measurements carried out at ordinary temperatures in these cases becomes difii-cult. In such cases, however, one can approximately estimate the additional contribution of the energy of specific interaction arising from TT-bonds or the functional groups at a somewhat higher a—e.g., when values of a correspond to 1 molecule per cavity. AQspecific is determined by subtracting the heat of adsorption of the reference molecule (incapable to specific interaction) from the heat of adsorption of the specifically adsorbed molecule 10, 28S0). Both molecules must have similar geometry, close values of polarizability and of heat of adsorption on nonspecific adsorbent (6). 

Berezin, G.I. Kiselev, A.V. Adsorbate-adsorbate association on a homogenous surface of a nonspecific adsorbate. 

Enzyme-Linked Immunosorbent Assay. LlSAis a heterogeneous EIA technique that is widely used in clinical analyses. In this type of assay, one of the reaction components is nonspecifically adsorbed or covalently bound to the surface of a solid phase, such as that of a microtiter well, a magnetic particle, or a plastic bead. This attachment facilitates separation of bound- and free-labeled reactants. In the most common approach to using the ELISA technique, an aliquot of sample or calibrator containing the antigen to be quantitated is added to and allowed to bind with a solid phase antibody. After washing, enzyme-labeled antibody is... 

Fig. 8.5. Reverse capture assays allow an improvement of detectability by a decrease in background staining. Two oligomers (labeled and polyadenylated) are hybridized with the target (I, II) and captured with paramagnetic beads (PMB) covered with oligo-dT hairs (III). Nonspecifically adsorbed oligomers are usually tighter bound than hybrids to dT-hairs, allowing a selective desorption of specific hybrids (IV). These can be recaptured (V), followed by the determination of the amount of label bound.

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