Detection of single micron-sized magnetic bead and magnetic nanoparticles using spin valve sensors for biological applications

TLDR

The sensors polarize a magnetic microbead with a dc field and modulate its magnetization with an orthogonal ac field, producing a magnetoresistance reduction that is detected via a lock‑in voltage measurement. The spin‑valve sensors detected a single 2.8 µm superparamagnetic bead, yielding 1.2 mV rms (5.2 mΩ) from a 3 µm sensor and 3.8 mV rms (11.9 mΩ) from a 2.5 µm sensor; simulations confirm the results and indicate the sensors can detect 1–10 11‑nm Co nanoparticles, making them suitable for DNA fragment detection.

Abstract

We have fabricated a series of highly sensitive spin valve sensors on a micron scale that successfully detected the presence of a single superparamagnetic bead (Dynabeads M-280, 2.8 μm in diameter), and thus showed suitability for identifying biomolecules labeled by such magnetic beads. By polarizing the magnetic microbead on a spin valve sensor with a dc magnetic field and modulating its magnetization with an orthogonal ac magnetic field, we observed a magnetoresistance (MR) signal reduction caused by the magnetic dipole field from the bead that partially cancelled the applied fields to the spin valve. A lock-in technique was used to measure a voltage signal due to the MR reduction. A signal of 1.2 mV rms or 5.2 mΩ of resistance reduction was obtained from a 3 μm wide sensor and a signal of 3.8 mV rms or 11.9 mΩ from a 2.5 μm wide sensor. Micromagnetic simulations were also performed for the spin valve sensors with a single bead and gave results consistent with experiments. Further experiments and simulations suggested that these sensors or their variations can detect 1–10 Co nanoparticles with a diameter of ∼11 nm, and are suitable for DNA fragment detection.