Detection of 8 nm Diameter Superparamagnetic Beads by Magnetically-Induced Manipulation of Micrometer-Sized Magnetic Beads: A Novel Protocol for Magnetically-Labeled Biosensing

Biosensing platforms based on functionalized superparamagnetic beads acting as “magnetic-labels” show tremendous potential for rapid and highly-sensitive point of care medical diagnosis. Recently, for greater quantification, there are increasing demands for the use of magnetic labels with diameters of 8–150 nm, which are comparable in size to actual biomolecules. However, detection of small numbers of sub-150 nm sized magnetic beads by magnetoresistive device-based platforms is extremely challenging due to the intrinsic noise of these electronic devices. Here, we describe an easy and economical detection method of low areal densities of 8-nm-diameter superparamagnetic “target beads” immobilized over millimeter-sized substrates by optically monitoring the magnetically induced “capture” of easily visible, micrometer sized superparamagnetic “columnar beads” by the targets in less than a few tens of seconds.