Passive adsorption

One of the simplest methods of attaching biomolecules to hydrophobic polymeric particles is the use of passive adsorption. Some of the earliest examples related to the use of particles in immunoassays include the use of non-covalently adsorbed antibody or antigen onto latex microspheres. Protein adsorption onto hydrophobic particles takes place through strong interactions 

Polystyrene polystyrene/PMMA copolymer PMMA polystyrene/polyacrylic acid copolymer 

The following protocol for passive adsorption is based on methods reported for use with hydrophobic polymeric particles, such as polystyrene latex beads or copolymers of the same. Other polymer particle types also may be used in this process, provided they have the necessary hydrophobic character to promote adsorption. For particular proteins, conditions may need to be optimized to take into consideration maximal protein stability and activity after adsorption. Some proteins may undergo extensive denaturation after immobilization onto hydrophobic surfaces therefore, covalent methods of coupling onto more hydrophilic particle surfaces may be a better choice for maintaining native protein structure and long-term stability. 

Dilute the particles to a mass concentration of 10 mg/ml (1 percent) in coating buffer at a pH near the pi of the protein being adsorbed. If particle aggregation becomes a problem, the initial particle concentration may be reduced to 5 mg/ml. Some typical coating buffer suggestions include (a) 10 mM sodium phosphate, 0.15 M NaCl, pH 7.4 (PBS) (b) 50 mM sodium borate, pH 8.5 (c) 50 mM sodium acetate, pH 3.6-5.6 (d) 25 mM MES [2-(N-morpholino)ethane sulfonic acid], pH 6.1 or (e) 50 mM sodium bicarbonate, 

Resuspend the particles to a final concentration of 1 percent in coating buffer containing a preservative. Avoid changing the composition of the storage buffer from that used to coat the protein, as any pH or compositional changes often result in elution of some of the adsorbed protein. 

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FIGURE 2.23 Schematic diagram showing the routes of possible removal of drug from the receptor compartment. Upon diffusion into the compartment, the drug may be removed by passive adsorption en route. This will cause a constant decrease in the steady-state concentration of the drag at the site of the receptor until the adsorption process is saturated. 

Preparation of microtiter plates. A constant amount of the coating antigen is bound to the surface of polystyrene microtiter plate wells by passive adsorption. After a predetermined incubation time, the plate is washed to remove unbound coating antigen. 

Figure 2 Immobilized antigen ELISA format. Antigen is immobilized to a solid phase by passive adsorption. Following removal of unbound antigen, analyte (free H) and antigen (H-protein) compete for a fixed number of primary antibody (Y) binding sites. Unbound materials are removed (dotted line). Secondary antibody-enzyme conjugate (Y-E) is added to bind to primary antibody followed by another wash step. Substrate (A) for the enzyme is added to detect the bound enzyme. The amount of colored product ( ) detected is inversely proportional to the amount of analyte present...

Bioaccumulation An enrichment of a particular chemical caused by either passive adsorption from seawater or active uptake followed by retention in living tissues or hard parts as a result of nonexcretion. 

QDs have been shown to be quite stable to metabohc degradation (with the exception of problems associated with heavy metal leachants) [ 177,180]. QDs can be conjugated to the linker [181] (e.g., avidin, protein A or protein G, or a secondary antibody) by covalent binding, passive adsorption, multivalent chelation or by electrostatic interactions [182,183]. 

Research on the various biochemical systems affected by tumor promoters, in particular TPA, has been recently reviewed by Diamond et al. (54) and Werner and coworkers (55) have reviewed the early effects of phorbol esters on the membranes of cultured cells. The latter group reports that TPA causes permeability changes in 3T3 cell membranes and experimental evidence is cited that phorbol esters interact specifically with a membrane-specific macromolecule rather than passive adsorption by the membrane lipid matrix. One of the earliest observed effects of TPA is a significant modification in the transport of potassium, sodium and phosphate. Lee and Weinstein (56) have found that the addition of phorbol esters immediately stimulated the uptake of 2-deoxyglucose in... 

Within the model of a semiconductor particle with two surfaces of different ( active and passive ) adsorption layer types, one may quantitatively describe the initial part of kinetic curves. Denote the total surface area of the colloidal particle as 2, and the surface area of the colloidal particle blocked by PAA as 2PAA. Let us call the colloidal particle surface area free of PAA as the working surface and denote it as 2W = 2 - 2PAA. 

The most popular form of this technique is solid-phase heterogeneous ELISA, and the inherent use of passive adsorption facilitates flexibility in assay design. ELISA is generally classified as direct, indirect, sandwich (double-antibody) or competitive. Principles of each of these formats are briefly discussed below. 

The isolation and concentration of petroleum products can be performed in several ways. The most efficient method is passive adsorption. In this method, the sample along with a tube filled with Tenax TA adsorbent is placed in a thermostated (60-70 °C) tightly closed container, such as a glass jar, for over 10 h. Under these conditions, a balance between compounds present in the headspace of the sample and the sample adsorbed on the polymer adsorbent is established. Adsorbed compounds are subjected to thermodesorbtion then, the desorbed compounds together with the carrier gas are injected onto a GC column, where they are separated and then identified. This approach has enabled easy detection and identification of trace amounts of petroleum products. Headspace analysis with passive adsorption on Tenax TA is normally used for separation and concentration of analytes. Gas chromatography coupled with an autothermal desorber and a mass spectrometer (ATD-GC-MS) is the best technique for separation of multicomponent mixtures... 

Borusiewicz, R., Zadora, G., Zigba-Palus, J. Application of head-space analysis with passive adsorption for forensic purposes in the automated thermal desorption-gas chromatography-mass spectrometry system. Chromatographia 60, 133-142 (2004)... 

Usually, immobilization of antibodies can be performed either directly by passive adsorption, covalent coupling, or indirectly by using a solid surface pre-coated with protein A or G [25] or a secondary immunoglobulin (Ab2, anti-Abi) [26] for a favourable orientation of the antibody on the solid surfece with the binding sites towards the sample solution. This orientated immobilization of antibodies is important for the assay performance, taking into account the special characteristics of solid-phase immunoassays (see Section 9.3.4.4). 

Selective Catalytic Oxidation (SCO) Passive Adsorption Hydrodesulfurization (HDS)... 

The selectivity of immunosensors for steroid analytes is achieved with the use of appropriately selected monoclonal antibodies. The carbon working electrode provides a suitable surface for passive adsorption of proteins, and can therefore be tailored with an appropriate antibody, so that it will act as an immunoactive surface upon which an immunoaffinity assay can be performed an electrochemical signal can then be generated by monitoring the production of an electroactive species at the underlying electrode surface. We and other workers have found that to retain maximum monoclonal antibody activity, it is desirable to use a primary antibody (rabbit IgG), which serves both to capture (e.g., from a culture medium) and to orientate the mAb. Hence this approach... 

Sponholtz, D.K. (1995) Immunoassays in Microplate Wells Optimization of Binding by Passive Adsorption. Technical Bulletin, IVD Technology, pp. 12 15. Available at http // www.devicelink.com/ivdt/95/03.html. 

For instance, what model should be assumed for the atomic structure of a surface The simplest picture of a surface is a planar terrace, a static array of passive adsorption sites. For a crystal, such terraces are slices through the bulk stacking sequence, a cleavage of the bulk. Is the planar terrace a reasonable model Some unrelaxed bulk terminations are polar and thus inherently unstable (see below). But for non-polar surfaces, planarity is often favoured in order to minimise the surface energy (Section 3). Thus on the timescales of diffraction techniques, such as LEED and XAFS, many surfaces are indeed observed to be... 

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