Rapid and reagent-saving immunoassay using innovative stirring actions of magnetic beads in microreactors in the sequential injection mode

We developed new ELISA techniques in sequential injection analysis (SIA) mode using microreactors with content of a few microliters. We immobilized antibodies on magnetic beads 1.0 μm in diameter, injected the beads into microreactors and applied rotating magnetic fields of several hundred gauss. Magnetic beads, suspended in liquid in density of approximately 109–1010 particles per millilitre, form a large number of thin rod clusters, whose length-wise axes are oriented in parallel with the magnetic field. We rotate the Nd magnets below the center of the microreactor by a tiny motor at about 2000–5000 rpm. These rotating clusters remarkably accelerate the binding rate of the antibodies with antigens in the liquid. The beads are trapped around the center of the rotating magnetic field even in the flowing liquid. This newly found phenomenon enables easy bead handling in microreactors. Modification of reactor walls with selected blocking reagents was essential, because protein-coated beads often stick to the wall surface and cannot move freely. Washing steps were also shortened.