A Model for Detection of Immobilized Superparamagnetic Nanosphere Assay Labels using Giant Magnetoresistive Sensors

Commercially available superparamagnetic nanospheres are commonly used in a wide range of biological applications, particularly in magnetically assisted separations. A new and potentially significant technology involves the use of these particles as labels in magnetoresistive assay applications. In these assays, magnetic bead labels are used like fluorescent labels except that the beads are excited and detected with magnetic fields rather than with photons. A major advantage of this technique is that the means for excitation and detection are easily integrable on a silicon circuit. A preliminary study of this technique demonstrated its basic feasibility, and projected a sensitivity of better than 10-12 Molar [Baselt et al., Biosensors and Bioelectronics, 13, 731 (1998)]. The present paper examines the theoretical signal to noise ratio of this type of assay for the special case of a single magnetic bead being detected by a single giant magnetoresistive (GMR) detector. Assuming experimentally observed and reasonable parameters for the magnetic label and the sensitivity of the GMR detector, the signal to M. Tondra, et.al