Fundamental study on latex reagents for agglutination tests

Influence of particle size on the binding activity of proteins adsorbed onto gold nanoparticles

We used optical extinction spectroscopy to study the structure of proteins adsorbed onto gold nanoparticles of sizes 5-60 nm and their resulting biological binding activity. For these studies, proteins differing in size and shape, with well-characterized and specific interactions-rabbit immunoglobulin G (IgG), goat anti-rabbit IgG (anti-IgG), Staphylococcal protein A, streptavidin, and biotin-were used as model systems. Protein interaction with gold nanoparticles was probed by optical extinction measurements of localized surface plasmon resonance (LSPR) of the gold nanoparticles. Binding of the ligands in solution to protein molecules already immobilized on the surface of gold causes a small but detectable shift in the LSPR peak of the gold nanoparticles. This shift can be used to probe the binding activity of the adsorbed protein. Within the context of Mie theory calculations, the thickness of the adsorbed protein layer as well as its apparent refractive index is shown to depend on the size of the gold nanoparticle. The results suggest that proteins can adopt different orientations that depend on the size of the gold nanospheres. These different orientations, in turn, can result in different levels of biological activity. For example, we find that IgG adsorbed on spheres with diameter ≥20 nm does not bind to protein A. This study illustrates the principle that the size of nanoparticles can strongly influence the binding activity of adsorbed proteins. In addition to the importance of this in cases of direct exposure of proteins to nanoparticles, the results have implications for proteins adsorbed to materials with nanometer scale surface roughness.

A semi-quantitative dipstick assay for microcystin

An immunochromatographic lateral flow dipstick assay for the fast detection of microcystin-LR was developed. Colloid gold particles with diameters of 40 nm were used as red-colored antibody labels for the visual detection of the antigen. The new dipstick sensor is capable of detecting down to 5 microg x l(-1) (ppb; total inversion of the color signal) or 1 ppb (observation of color grading) of microcystin-LR. The course of the labeling reaction was observed via spectrometric wave shifts caused by the change of particle size during the binding of antibodies. Different stabilizing reagents showed that especially bovine serum albumin (BSA) and casein increase the assays sensitivity and the conjugate stability. Performance of the dipsticks was quantified by pattern processing of capture zone CCD images. Storage stability of dipsticks and conjugate suspensions over 115 days under different conditions were monitored. The ready-to-use dipsticks were successfully tested with microcystin-LR-spiked samples of outdoor drinking- and salt water and applied to the tissue of microcystin-fed mussels.

Electrochemiluminescence of Ru(II) complexes immobilized on a magnetic microbead surface: distribution of magnetic microbeads on the electrode surface and effect of azide ion

The electrochemiluminescence (ECL) of magnetic microbeads modified with tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) was studied in the presence of tri-n-propylamine (TPA) to develop highly sensitive ECL detection system, where the employed microbead has a diameter of 4.5 microm. The ECL signal of the [Ru(bpy)3]2+ derivative-modified magnetic microbeads was found to be affected by the geometrical distribution of the magnetic microbeads on the electrode surface. The ECL peak intensity increased with increasing the number of the beads on the electrode surfaces up to 1.6 x 10(6) beads cm(-2), although above 1.6 x 10(6) beads cm(-2), it decreased. The ECL decrease arises from the physical prevention of the ECL from reaching the photomultiplier tube by the excessive beads. The observed peak ECL signal of the [Ru(bpy)3]2+ derivative-modified magnetic microbeads in the presence of NaN3, which serves as a preservative substance, mainly appeared at a potential of +0.90 V vs Ag/AgCl where [Ru(bpy)3]2+ is hardly oxidized, whereas the ECL signal in the absence of NaN3 appeared at a potential of +1.15 V. The presence of NaN3 on the electrode surface retards formation of an oxide layer on the electrode surfaces and promotes TPA oxidation. The ECL response at +0.90 V was mainly attributed to ECL reaction of excited-state [Ru(bpy)3]2+* formed by oxidation of [Ru(bpy)3]+ with TPA radical cation, where the [Ru(bpy)3]+ was generated by reduction of [Ru(bpy)3]2+ with TPA radical.

A review of factors affecting the performances of latex agglutination tests

The present review describes the different strategies followed to improve the performance of latex agglutination tests. The analysis is mainly focused on the diverse parameters that affect the final colloidal stability of the immunoprotein-latex system. These parameters include: the surface properties of polymer carriers; the different kind of antibodies usually employed; the use of BSA as stabilizer; the co-adsorption of various macromolecules (BSA, surfactants and lipids) and antibodies; recent approaches to colloidal stability at high ionic strengths due to hydration forces; and the covalent coupling of antibodies on functionalized latex particles. Special emphasis is given to the relation between electrophoretic mobility and the colloidal stability of the sensitized particles and how this knowledge can be utilized for a better understanding of the immunoagglutination kinetic.

Colloidal stability of IgG- and IgY-coated latex microspheres

The stabilization of antibody-latex complexes at high salt concentration is an event that cannot be explained by the widespread DLVO theory. Adsorption of antibodies on polystyrene latex usually leads to a loss in colloidal stability. However, after the expected particle aggregation induced by an increase in ionic strength, an 'anomalous' restabilization occurs when the electrolyte concentration increases even more. This non-DLVO behaviour can be explained taking into account the hydration forces, which become significant in hydrophilic surfaces. This restabilization has already been observed in different protein latex complexes. In the present work, a study on the stability patterns of polystyrene particles covered independently by mammalian and chicken antibodies has been performed. This study reveals that avian antibodies present a more hydrophobic surface than that of mammalian antibodies. In addition, it has been possible to obtain some information about the molecular orientation of the adsorbed antibodies from the stability experiments. This information has been corroborated by an immunoreactivity study.

Forces acting on particle-enhanced immunoassays

The aim of the present work is to study the role of the different forces involved in the agglutination of immuno gamma-globulin (IgG) covered latex particles due to antigen-antibody reaction. An experimental investigation on the adsorption of IgG molecules on three latexes with different surface charge densities is described. Photon correlation spectroscopy was used to determine the hydrodynamic layer thickness of the IgG molecules adsorbed on the latexes. In order to get an insight into the forces acting between two antibody-covered particles approaching each other, the colloidal stability and immunoreactivity of these biocomplexes were studied. They can be stabilized by electrostatic or hydration forces. The immunological agglutination of IgG-immobilized latex particles due to the addition of the antigen was quantified through scattered light intensity measurements. The immunoresponse increases with ionic strength of the medium until a maximum value is achieved. Above this maximum, the immunoreactivity decreases.

Particle enhanced immunoaggregation of F(ab')2 molecules

The immunological agglutination reactions of physically absorbed F(ab')2 molecules onto anionic and cationic latex particles have been investigated by means of optical absorbance measurements. These measurements have been conducted under different conditions to determine the most influential factors. Surface F(ab')2 and BSA densities, particle concentration in the reaction medium and polyethylene glycol concentration are some of these factors. Sensitized cationic and anionic latexes differ considerably with respect to their colloidal stability and reactivity. As a general rule, the sensitized cationic latex has a relatively higher colloidal stability and hence, it provides reagents with a better optical response. Less than 0.025 microgram/ml of C-reactive protein has been detected using this particle enhanced optical immunoassay.

The adsorption of F(ab')2 on positively and negatively charged polystyrene beads

An experimental study on the adsorption of F(ab')2 molecules onto positively and negatively charged polystyrene beads is described. Adsorption isotherms at low ionic strength and different pH were performed. In all cases the adsorption isotherms showed well-defined plateau. The positively charged polystyrene beads showed a higher adsorption of F(ab')2 molecules over a range of pH. This latex sample reached a maximum adsorption at pH 7, whereas the negatively charged sample showed a maximum at pH 5. The isoelectric points of the F(ab')2-PS complexes were around 7.5 and 4.5 respectively. The differences in the colloidal stability of the sensitized beads are not explained by the electrophoretic mobility values. The colloidal stability of the F(ab')2-PS complexes is not only determined by the electrical charge of the complexes but also by certain properties of the protein molecules, which can be closely related to the conformational changes undergone by the F(ab')2 molecules in the adsorption process on polymer surfaces.

Preparation of peptide-carrying microspheres with bioactivity on platelets

A tetrapeptide RGDS (Arg-Gly-Asp-Ser), which is one of the active sites of cell adhesive proteins, was synthesized according to the solution method and covalently coupled onto monodisperse microspheres. The biospecific activity of the RGDS-carrying microspheres was investigated by measuring the stimulatory effect of the microspheres on platelets. Large and tightly packed platelet aggregation accompanied with drastic ATP release was induced by the RGDS-carrying microspheres. It was confirmed that the specific platelet aggregation was attributed to the activity of the immobilized RGDS by comparison with those induced by other microspheres.