![]() ![]() Typical superparamagnetic limits are on the order of one to several dozen nanometers. ![]() When the size of the nanoparticles is below a certain threshold, the superparamagnetic limit, the thermal energy exceeds the magnetic crystalline anisotropy energy of the particles. These beads consist of magnetic nanoparticles that are encapsulated in a polymer shell. In recent years, however, superparamagnetic beads have attracted interest as alternative labels in immuno assays. In sandwich immuno assays, only target antigens are immobilized on the surface, which are functionalized with a second antibody.Ĭommon labels employed in immuno assays are enzymes (ELISA), radioactive isotopes (RIA), or fluorophores. In a washing step, unbound labels are removed so that only labels bound to antigen molecules can be measured by a suitable detection scheme. Labeled antibodies that specifically bind the target antigen are then introduced into the system, binding to the antigens on the surface. ![]() In direct immunoassays, proteins in the sample are immobilized on the chip surface. These, so-called, Micro Total Analysis Systems (µTAS) often employ detectable labels that are connected to an antibody probe. Recent advances in lab-on-a-chip technologies have made it possible to carry out complete immuno assay procedures on the area of one chip of several cm 2. Immuno assays are an established method in medical analysis to determine the concentration of a macromolecular analyte (antigen) in solution. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. ![]()
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