Kinetics of bioconjugate nanoparticle label binding in a sandwich-type immunoassay

Analytical methods of antibody surface coverage and orientation on bio-functionalized magnetic beads: application to immunocapture of TNF-α

The use of magnetic beads bio-functionalized by antibodies (Ab) is constantly increasing with a wide range of biomedical applications. However, despite an urgent need for current methods to monitor Ab’s grafting process and orientation, existing methods are still either cumbersome and/or limited. In this work, we propose a new simple and rapid analytical approach to evaluate antibody orientation and density on magnetic beads. This approach relies on the cleavage by IdeS, a highly specific protease for human immunoglobulin G (hIgG), of immobilized antibodies. The F(ab)₂ and Fc fragments could be then accurately quantified by size exclusion chromatography (SEC)-coupled to fluorescent detection (FLD), and the ratio of these fragments was used to give insight on the IgG orientation at the bead surface. Four different commercially available magnetic beads, bearing carboxyl groups, tosyl groups, streptavidin, or protein G on their surface have been used in this study. Results obtained showed that this approach ensures reliable information on hIgG orientation and bead surface coverage. Protein G magnetic beads demonstrated an optimal orientation of antibodies for antigen capture (75% of accessible F(ab)₂ fragment) compared to tosylactivated, carboxylated, and streptavidin ones. Capture efficiency of the different functionalized beads towards human TNF-α immunocapture, a biomarker of inflammation, has been also compared. Protein G beads provided a more efficient capture compared to other beads. In the future, this approach could be applied to any type of surface and beads to assess hIgG coverage and orientation after any type of immobilization.

Super-sensitive time-resolved fluoroimmunoassay for thyroid-stimulating hormone utilizing europium(III) nanoparticle labels achieved by protein corona stabilization, short binding time, and serum preprocessing

Thyrotropin or thyroid-stimulating hormone (TSH) is used as a marker for thyroid function. More precise and more sensitive immunoassays are needed to facilitate continuous monitoring of thyroid dysfunctions and to assess the efficacy of the selected therapy and dosage of medication. Moreover, most thyroid diseases are autoimmune diseases making TSH assays very prone to immunoassay interferences due to autoantibodies in the sample matrix. We have developed a super-sensitive TSH immunoassay utilizing nanoparticle labels with a detection limit of 60 nU L-1 in preprocessed serum samples by reducing nonspecific binding. The developed preprocessing step by affinity purification removed interfering compounds and improved the recovery of spiked TSH from serum. The sensitivity enhancement was achieved by stabilization of the protein corona of the nanoparticle bioconjugates and a spot-coated configuration of the active solid-phase that reduced sedimentation of the nanoparticle bioconjugates and their contact time with antibody-coated solid phase, thus making use of the higher association rate of specific binding due to high avidity nanoparticle bioconjugates. Graphical Abstract We were able to decrease the lowest limit of detection and increase sensitivity of TSH immunoassay using Eu(III)-nanoparticles. The improvement was achieved by decreasing binding time of nanoparticle bioconjugates by small capture area and fast circular rotation. Also, we applied a step to stabilize protein corona of the nanoparticles and a serum-preprocessing step with a structurally related antibody.