Frequency-Domain Approach To Determine Magnetic Address-Sensor Separation Distance Using the Harmonic Ratio Method

Abstract

In this work, we describe an approach to determine the distance separating a magnetic address from a scanning magnetoresistive sensor – a critical adjustable parameter for certain bioassay analyses where magnetic nanoparticles are used as labels. Our approach is leveraged from the harmonic ratio method (HRM), a method used in the hard drive industry to control the distance separating a magnetoresistive read head from its data platter with nanometer resolution. At the heart of the HRM is an amplitude comparison of a signal’s fundamental frequency to that of its harmonics. When the signal is derived from the magnetic field pattern of a periodic array of magnetic addresses, the harmonic ratio contains the information necessary to determine the separation between the address array and the read head. The elegance of the HRM is that there is no need of additional components to the detection platform to determine a separation distance; the streaming “bit signal” contains all the information needed. In this work we demonstrate that the tenets governing HRM used in the hard drive industry can be applied to the bioanalytical arena where submicron to 100 μm separations are required.

Colloidally Assembled Zinc Ferrite Magnetic Beads: Superparamagnetic Labels with High Magnetic Moments for MR Sensors

“…Detailed procedures for the fabrication of the assay substrate, MR sensor configuration, substrate readout, and signal analysis are described in the Supporting Information and elsewhere. 27,28,30 2.2.5. Magnetic Sandwich Immunoassay for the Detection of a Potential Pancreatic Cancer Marker.…”

“…Detailed procedures for the fabrication of the assay substrate, MR sensor configuration, substrate readout, and signal analysis are described in the Supporting Information and elsewhere. ,, …”

“…25−29 In previous work, we demonstrated that streptavidin (SA)coated μMBs (Dynabeads) bound to biotinylated gold addresses can be quantitatively measured using a scanning GMR readout method which mimics that used in hard disk drives. 27,28 In our approach, the separation distance between the assay substrate and the GMR sensor is on the order of micrometers (10−50 μm) 27,30 as opposed to tens or hundreds of nanometers found in other GMR sensors. [7][8][9]12 Because the magnetic dipole field of magnetic particles decreases rapidly with distance (i.e., a decrease proportional to 1/r 3 , where r is the separation between the particle and the sensor), 29 this architecture places a heavy emphasis on the use of MNP labels with a large m for low level detection.…”

“…Our interest in this area stems from the use of nanoparticles as labels in a diagnostic tool for the detection of disease markers that is based on giant magnetoresistance (GMR). − In previous work, we demonstrated that streptavidin (SA)-coated μMBs (Dynabeads) bound to biotinylated gold addresses can be quantitatively measured using a scanning GMR readout method which mimics that used in hard disk drives. , In our approach, the separation distance between the assay substrate and the GMR sensor is on the order of micrometers (10–50 μm) , as opposed to tens or hundreds of nanometers found in other GMR sensors. − , Because the magnetic dipole field of magnetic particles decreases rapidly with distance (i.e., a decrease proportional to 1/ r 3 , where r is the separation between the particle and the sensor), this architecture places a heavy emphasis on the use of MNP labels with a large m for low level detection. We have also found that the ballistic settling of μMBs can complicate the assay by inducing a larger level of nonspecifically adsorbed μMBs than those found with MNPs.…”