Magnetic immunoassay

Magnetic immunoassay (MIA) is a novel type of diagnostic immunoassay using magnetic beads as labels in lieu of conventional enzymes (ELISA), radioisotopes (RIA) or fluorescent moieties (fluorescent immunoassays). This assay involves the specific binding of an antibody to its antigen, where a magnetic label is conjugated to one element of the pair. The presence of magnetic beads is then detected by a magnetic reader (magnetometer) which measures the magnetic field change induced by the beads. The signal measured by the magnetometer is proportional to the analyte (virus, toxin, bacteria, cardiac marker,etc.) quantity in the initial sample.

Magnetic labels

Magnetic beads are made of nanometric-sized iron oxide particles encapsulated or glued together with polymers. These magnetic beads range from 35nm up to 4.5μm. The component magnetic nanoparticles range from 5 to 50nm and exhibit a unique quality referred to as superparamagnetism in the presence of an externally applied magnetic field. First discovered by Frenchman Louis Néel, Nobel Physics Prize winner in 1970, this superparamagnetic quality has already been used for medical application in Magnetic Resonance Imaging (MRI) and in biological separations, but not yet for labeling in commercial diagnostic applications. Magnetic labels exhibit several features very well adapted for such applications:

• they are not affected by reagent chemistry or photo-bleaching and are therefore stable over time,
• the magnetic background in a biomolecular sample is usually insignificant,
• sample turbidity or staining have no impact on magnetic properties,
• magnetic beads can be manipulated remotely by magnetism.

Magnetometers

A simple instrument can detect the presence and measure the total magnetic signal of a sample, however the challenge of developing an effective MIA is to separate naturally occurring magnetic background (noise) from the weak magnetically labeled target (signal). Various approaches and devices have been employed to achieve a meaningful signal-to-noise ratio (SNR) for bio-sensing applications: giant magneto-resistive sensors and spin valves, piezo-resistive cantilevers, inductive sensors, superconducting quantum interference devices, anisotropic magneto-resistive rings, and miniature Hall sensors. But improving SNR often requires a complex instrument to provide repeated scanning and extrapolation through data processing, or precise alignment of target and sensor of miniature and matching size. Beyond this requirement, MIA that exploits the non-linear magnetic properties of magnetic labels can effectively use the intrinsic ability of a magnetic field to pass through plastic, water, nitrocellulose, and other materials, thus allowing for true volumetric measurements in various immunoassay formats. Unlike conventional methods that measure the susceptibility of superparamagnetic materials, a MIA based on non-linear magnetization eliminates the impact of linear dia- or paramagnetic materials such as sample matrix, consumable plastics and/or nitrocellulose. Although the intrinsic magnetism of these materials is very weak, with typical susceptibility values of –10⁻⁵ (dia) or +10⁻³ (para), when one is investigating very small quantities of superparamagnetic materials, such as nanograms per test, the background signal generated by ancillary materials cannot be ignored. In MIA based on non-linear magnetic properties of magnetic labels the beads are exposed to an alternating magnetic field at two frequencies, f1 and f2. In the presence of non-linear materials such as superparamagnetic labels, a signal can be recorded at combinatorial frequencies, for example, at f = f1 ± 2×f2. This signal is exactly proportional to the amount of magnetic material inside the reading coil.

This technology,makes magnetic immunoassay possible in a variety of formats such as:

• conventional lateral flow test by replacing gold labels with magnetic labels,
• vertical flow tests allowing for the interrogation of rare analytes (such as bacteria) in large-volume samples
• microfluidic applications and biochip

It was also described for in vivo applications and for multiparametric testing.

References

1. Design and performance of GMR sensors for the detection of magnetic microbeads in biosensors, Rife et al., Sensors and Actuators A: Physical, Volume 107, Issue 3, (2003), 209-18 [1]
2. Magnetic Immunoassays, P.I.Nikitin, P.M. Vetoshko, T.I Ksenevich, Sensor Letters, Vol. 5, 1-4, 2007 [2]
3. Quantitative real-time in vivo detection of magnetic nanoparticles by their nonlinear magnetization, M. Nikitin, M. Torno, H. Chen, A. Rosengart, P. Nikitin Journal of Applied Physics (2008) 103, 07A304 [3]