Nano Lett. doi:10.1021/nl200384p (2011)

Superparamagnetic nanoparticles have been used in affinity assays to detect biomolecules. Magnetic nanoparticles that have captured target molecules in a fluid sample form clusters that can be monitored through changes either in the optical transmittance of the sample or in the transverse relaxation time of the clusters in nuclear magnetic resonance. However, these measurements cannot resolve few-particle clusters against a large background of single nanoparticles and therefore limit the sensitivity and speed of the assay. Menno Prins and colleagues at Philips Research and Eindhoven University of Technology have now developed a technique to selectively actuate, characterize and detect clusters of magnetic nanoparticles for highly sensitive and rapid detection of biomolecules.

The Dutch team focused a laser beam on the centre of a cuvette containing magnetic nanoparticles and target biomolecules. They used four electromagnets to drive nanoparticles that have captured target molecules to form chains through interparticle bonding. When the magnetic field is removed to allow unbound nanoparticles to disperse, optical scattering of two-particle clusters is measured and the intensity of the scattered light is correlated to the number, size distribution and magnetic properties of the clusters. Sensitive and selective detection of two-particle clusters is possible because their magnetic shape anisotropy enables frequency controlled rotation and their optical anisotropy generates scattered light. The technique is able to detect biomolecules down to 400 fm in buffer and 5 pm in human plasma in less than three minutes.

This method, which integrates magnetic actuation with optical detection, is potentially useful for various studies in biochemistry and colloidal interactions.