IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Magnetic Interfaces and Nanostructures Monday Sessions
       Session MI+EL-MoM

Paper MI+EL-MoM7
Performance of the BARC Magnetoresistive Sensor*

Monday, October 29, 2001, 11:40 am, Room 110

Session: Magnetic Devices
Presenter: J.C. Rife, Naval Research Laboratory
Authors: J.C. Rife, Naval Research Laboratory
R.J. Colton, Naval Research Laboratory
M. Miller, Naval Research Laboratory
M.A. Piani, Nova Research, Inc.
C.R. Tamanaha, Geo-Centers, Inc.
P.E. Sheehan, Naval Research Laboratory
L.J. Whitman, Naval Research Laboratory
Correspondent: Click to Email
The Bead ARray Counter (BARC) is a microfabricated chip for quantitatively detecting and identifying biological molecules using giant magnetoresistive (GMR) sensors.@footnote 1@ The assay is based on highly selective biomolecular binding to the surfaces of numerous individually addressable GMR sensors, followed by labeling of captured molecules with magnetic beads. An externally applied AC magnetic field magnetizes the beads and a lock-in amplifier detects changes of 10@super -@@super 7@ in resistance of the GMR sensors, limited by Johnson and 1/f noise. Overall sensitivity is a convolution of chemical and magnetic/electronic sensitivities. Our current sensors can determine target concentrations from 10 fM to 1 nM with loading of one hundred to more than a thousand beads. In principle, each sensor could detect one bead/one captured molecule. Present electronic sensitivity is restricted, in part, by the properties of the commercial 2.8 µm diameter composite polymer/ferrimagnetic beads that result in signal levels a factor of ten below the electronic noise floor. We find 10 to 100x improved signal with solid, soft ferromagnetic beads of the same size that yield the theoretical susceptibility of solid magnetic spheres, but chemical functionalization of the surfaces is not yet resolved. We have measured bead signals versus magnetizing field to have an approximately square-law dependence determined by the magnetoresistance response curve. We have also measured the bead signal versus position across the 2 µm wide GMR sensor and generated a simple model of the local resistivity change. Finally, We have developed an overall model for the GMR sensor response that agrees in large part with the measurements. The model should enable sensor and magnetic physical design to be optimized for maximal chemical and electronic sensitivity. *Supported by the Defense Advanced Research Projects Agency. @FootnoteText@ @footnote 1@ Edelstein et al., Biosensors & Bioelectronics 14, 805 (2000).