[PhysicsWeb, Belle Dumé]Scientists from the Argonne National Laboratory in the US have developed a new type of magnetic sensor that can detect biomolecules. The device relies on measuring the Brownian relaxation of magnetic nanoparticles when they are bound to target biomolecules. The technique could have applications in medicine and the detection of bacteria and viruses in the environment (S-H Chung et al. 2004 Appl. Phys. Lett. 85 2971).
Seok-Hwan Chung and co-workers measure the change in the magnetic susceptibility of the nanoparticles in an alternating magnetic field. The susceptibility depends on the length of time it takes for the magnetic spins of the nanoparticles to "relax" to their original alignment after the magnetic field is removed.
There are two types of magnetic relaxation: in Brownian relaxation the particles rotate in solution due to their thermal energy, while in Néel relaxation the internal dipole moments of the particles rotate. Néel relaxation generally occurs for particles smaller than about 10 nanometres across, while Brownian relaxation dominates for larger particles. Sensing techniques that measure Néel relaxation times already exist, but they are not able to distinguish between different targets with similar properties.
Brownian relaxation shows up as a peak when magnetic susceptibility is plotted as a function of frequency. According to theory, this peak should move to lower frequencies if the radius of the nanoparticles is increased by, for example, binding target molecules to them.
Seok-Hwan Chung and co-workers measure the change in the magnetic susceptibility of the nanoparticles in an alternating magnetic field. The susceptibility depends on the length of time it takes for the magnetic spins of the nanoparticles to "relax" to their original alignment after the magnetic field is removed.
There are two types of magnetic relaxation: in Brownian relaxation the particles rotate in solution due to their thermal energy, while in Néel relaxation the internal dipole moments of the particles rotate. Néel relaxation generally occurs for particles smaller than about 10 nanometres across, while Brownian relaxation dominates for larger particles. Sensing techniques that measure Néel relaxation times already exist, but they are not able to distinguish between different targets with similar properties.
Brownian relaxation shows up as a peak when magnetic susceptibility is plotted as a function of frequency. According to theory, this peak should move to lower frequencies if the radius of the nanoparticles is increased by, for example, binding target molecules to them.
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