Superparamagnetic relaxometry
Superparamagnetic relaxometry (SPMR) is a technique that uses very sensitive magnetic sensors and tiny magnetite nanoparticles to detect and locate tagged objects, like certain cells or disease markers.
The nanoparticles are usually about a few tens of nanometers in size. In their resting state they are essentially non-magnetic. When scientists apply a small external magnetic field (a few millitesla), the particles align with the field and behave like magnets with a noticeable magnetic moment. When the external field is removed, the particles relax back to their non-magnetic state, but they do so on a characteristic time that depends on their size and whether they are free in the solution or attached to a surface such as a cell.
The rate at which the particles lose their magnetization is very sensitive to particle size. For magnetite, particles around 25 nm are often used. If the particles are bound to a surface, they relax much more slowly (on the order of seconds) than if they are free (milliseconds). This difference helps researchers distinguish signals from bound particles from those that are unbound.
To read the decaying magnetic signal, researchers use very sensitive detectors such as SQUIDs (superconducting quantum interference devices) or advanced atomic magnetometers. In practice, a uniform magnetizing field is first created with coils (often Helmholtz coils) to saturate the particle moments. The field is then turned off quickly, and the sensors measure the decay of the magnetic signal over a short period, usually a few seconds, to capture the relaxation time.
From the measured decays, scientists map where the magnetic sources are and how strong their moments are. This is done by solving an inverse problem: matching the observed magnetic field to possible arrangements of nanoparticle sources. More measurements (from multiple sensor positions or times) give more accurate localization.
A major use of SPMR is detecting diseases like cancer. Nanoparticles can be coated with antibodies that target specific biological markers. After these functionalized particles bind to target cells or tissues, SPMR can locate them inside a sample, such as cell cultures, blood, or animal models.
Nanoparticles are attached to antibodies using chemical methods that link the antibodies to the particle surface. After binding, researchers confirm attachment using imaging and staining techniques, ensuring the particles are where they should be.
In short, SPMR combines smart magnetic particles with very sensitive detectors to find and map the location of targeted biological markers by watching how the particle magnetization fades after a brief magnetizing pulse.
This page was last edited on 2 February 2026, at 08:16 (CET).