Detection of Magnetic Nanobeads by Self-Assembly of Superparamagnetic Microbeads for Biosensing

Detection of magnetically labeled biomolecules is a promising method for fast and inexpensive medical diagnosis. Recently, there has been an increasing interest in using magnetic labels with diameters of 20-150 nm-dimensions that are similar to actual biomolecules. However, detection of small numbers of nanometer-scale magnetic beads (labels) distributed over large surface areas by magnetoresistive devices is extremely challenging due to the intrinsic noise and design limitations of giant magnetoresistance (GMR) and Hall-type of biosensors. Here, we describe proof of principle experiments on exploiting magnetically induced self-assembly of superparamagnetic beads for detecting superparamagnetic nanobeads with diameters of less than 150 nm. We successfully detected low areal densities of ~ 130 nm diameter superparamagnetic ldquotarget beadsrdquo immobilized over millimeter-sized substrates, by optically monitoring the ldquocapturerdquo of easily visible, micrometer-sized superparamagnetic beads by the targets. Our procedure could readily be extended for the detection of magnetically labeled biomolecules.