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 assist ed separations. A new and potentially significant technology involves the u se 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 wi th photons. A major advantage of this technique is that the means for excit ation and detection are easily integrable on a silicon circuit. A prelimina ry study of this technique demonstrated its basic feasibility, and projecte d a sensitivity of better than 10(-12) molar [Baselt ct al., Biosensors Bio electronic 13, 731 (1998)]. In this article we examine the theoretical sign al to noise ratio of this type of assay for the special case of a single ma gnetic bead being detected by a single giant magnetoresistive (GMR) detecto r. Assuming experimentally observed and reasonable parameters for the magne tic label and the sensitivity of the GMR detector, the signal to noise rati o is calculated to be greater than 5000:1 for detection of a single 1 mu m diameter magnetic microsphere immobilized on the surface of a 1 mu m x 1 mu m GMR sensor. Based on this large signal to noise ratio, the detection for mat should be applicable to more complicated assays where linear quantifica tion is required or to assays requiring significantly smaller beads. Detect ion of microsphere labels approaching 10 nm may be possible upon further te chnological advances. (C) 2000 American Vacuum Society. [S0734-2101(00)1180 4-4].