However, they are not new, but what has hampered their use has been the lack of particles of standard size, ideally of less than 20 nm. All that changed when Shouheng Sun, leading a group at IBM’s T.J. Watson Research Center, published a remarkable paper on how this problem could be overcome, and not surprisingly it’s now found its way into the Hot Ten at #6. (The work also involved researchers at the IBM Almaden Research Center at San Jose and Stanford University, California. Sun is now based in the Chemistry Department of Brown University, Providence, Rhode Island.)

Fe₃O₄ nanoparticles are hydrophobic, but paper #6 describes a method of making them hydrophilic by coating them with a layer of tetramethylammonium 11-aminoundecanoate. After treatment they can be removed from solution by using a magnet and then can be dispersed in water—and they still give a good magnetic signal, suitable for spin valve sensor detection.

Sun and his group have published a series of papers on iron oxide nanoparticles and their potential applications in materials such as nanocomposites and for biodetection. Their most recent work in the latter area is reported in Langmuir (see D.B. Robinson, et al., 21[7]: 3096-3103, 2005) and the Journal of Physical Chemistry B (see S.G. Grancharov, et al., 109(26):13030-5, 2005). Currently he has been testing nanoparticles as labels for highly sensitive DNA sequence detection.

Speaking to Science Watch, Sun says: "Our nanoparticles show far higher magnetic signals than any of the known bio-entities, so they really stand out in a non-magnetic background of biomolecules. By labelling a target molecule with such a particle we are able, with magnetic sensors, to fish successfully for a needle in an ocean of other molecules and impurities."

Sun’s nanoparticles are of the same dimension as a cell (10-100 nm), a virus (20-450 nm), a protein (5-50 nm), or a gene (2 nm wide and 10-100 nm long). In principle they can be attached to any of these entities and manipulated by an external magnetic field.

"These favorable attributes provide a controllable mean of magnetically tagging or addressing all biomolecules, leading to potentially highly efficient bio-separation, highly sensitive biodetection and MRI contrast enhancement, as well as site-specific drug delivery," he says. "They may even offer a way to kill cancer cells without harming the healthy cells and tissues, by pulsing them with an alternating magnetic field, thereby allowing the transfer of magnetic energy to their surroundings in the form of enough heat to kill the cell."