Chemists fabricate single-molecule magnets for MR

February 15, 2007

A class of materials known as single-molecule magnets might boost the contrast of MR scans and their specificity. One such nanomagnet, dubbed Fe8, is especially promising, but investigators will have to find some way to make it last longer on the shelf.

A class of materials known as single-molecule magnets might boost the contrast of MR scans and their specificity. One such nanomagnet, dubbed Fe8, is especially promising, but investigators will have to find some way to make it last longer on the shelf.

Researchers from Florida State University in Tallahassee, the University of Colorado, and the National Institute of Standards and Technology have synthesized and tested Fe8 for its potential as an MRI contrast agent. Composed of eight iron atoms, Fe8 produces a strong magnetic field, according to Naresh Dalal, Dirac professor of chemistry and biochemistry at FSU.

"This is obviously important in generating a very clear image with an MRI device," Dalal said. "Fe8 is nontoxic and water-soluble, making it safe for injection into the body."

Single-molecule magnets are essentially nanomagnets, or nanomagnetic particles, demonstrating a magnetic spin yet exhibiting no appreciable magnetic interaction with other molecules. Early research into single-molecule magnets focused on their potential use in magnetic memory, as their uniform size, solubility, and lack of interaction make them ideal candidates as storage materials.

The magnetic coating on the surface of a CD is about 1 micron thick. But if a CD could be coated instead with a film of single-molecule magnets, it would be only about a nanometer thick.

The researchers allude to the potential of Fe8 in magnetic storage in their paper, "Efficacy of the single-molecule magnet Fe8 for magnetic resonance imaging contrast agent over a broad range of concentration," published in the current issue of Polyhedron. They also note its potential in the area of quantum computing, which could lead to much faster computers than the current generation.

The focus of the paper, however, is on the use of Fe8 as an MR contrast agent - and for good reason.

The researchers believe their work resolves a debate over the potential of single-molecule magnets as MRI contrast agents. They demonstrated that iron-containing magnets just 2 nanometers wide, dissolved in water, provide reasonable contrast in nonclinical MR images obtained using scanners at The Children's Hospital in Denver. The caveat is that the concentration of the nanomagnet must be under a certain threshold.

Previous studies by other research groups had reached conflicting conclusions on the utility of molecular nanomagnets for MRI, but they failed to account for concentration. Making novel magnetic measurements, the current researchers were able to monitor the molecules' decomposition and magnetic properties at varying concentrations.

An MRI agent fashioned from nanomagnets would differ markedly from current contrast media, which are one of two types: magnetic ions or particles that alter local magnetic fields. Magnetic ions change the properties of hydrogen in tissue, offering the advantage of consistent identical design but providing low contrast. Particles composed of thousands of atoms or crystals that alter local magnetic fields produce variations in the contrast seen in MR images, but they exhibit irregular designs and magnetic properties that are difficult to control.

The consistency and high contrast of nanomolecular magnets provide advantages over current media, as does their propensity for modification, according to the NIST researchers. These agents might be developed to "turn on" their magnetic properties only when they bind to a target molecule or cell, providing unprecedented specificity.

As one of the strongest such magnets ever created, Fe8, packaged as a contrast agent, could go a long way toward boosting the resolution of an MR image. There is, however, a big obstacle standing in the way.

The shelf life of Fe8 could be a problem. Like other single-molecule magnets, Fe8 is not very stable. It breaks down in water within a few hours. This would not a problem once injected into the body, and it could even be an advantage. But when it comes to shelf life, this particular property of Fe8 limits its utility as a contrast agent.

"We're now looking at ways to increase the stability so that these magnets can be stored for long periods of time and transported easily," Dalal said.