Researchers mimic bacteria to produce magnetic nanoparticles

Interdisciplinary Ames Laboratory team uses bioinspired approach

When it comes to designing something, it’s hard to find a better source of inspiration than Mother Nature. Using that principle, a diverse, interdisciplinary group of researchers at the U.S. Department of Energy’s Ames Laboratory is mimicking bacteria to synthesize magnetic nanoparticles that could be used for drug targeting and delivery, in magnetic inks and high-density memory devices, or as magnetic seals in motors.

Commercial room-temperature synthesis of ferromagnetic nanoparticles is difficult because the particles form rapidly, resulting in agglomerated clusters of particles with less than ideal crystalline and magnetic properties. Size also matters. As particles get smaller, their magnetic properties, particularly with regard to temperature, also diminish.

However, several strains of bacteria produce magnetite (Fe3O4) – fine, uniform nanoparticles that have desirable magnetic properties. These magnetotactic bacteria use a protein to form crystalline particles about 50 nanometers in size. These crystals are bound by membranes to form chains of particles which the bacteria use like a compass needle to orient themselves with the Earth’s magnetic field.

To see if researchers could learn from the bacteria, Surya Mallapragada, Ames Laboratory Materials Chemistry and Biomolecular Materials program director pulled together a team that included microbiologists, biochemists, material chemists, chemical engineers, materials scientists and physicists from Ames Laboratory and Iowa State University.

As a starting point, former ISU microbiologist Dennis Bazylinski, now at the University of Nevada-Las Vegas, isolated several strains of magnetotactic bacteria for use in the study.

Based on earlier work by a Japanese research team, Ames Laboratory biochemist Marit Nilsen-Hamilton looked at several proteins known to bind iron, including Mms6 found in magnetotactic bacteria, which she cloned from the bacteria. “This protein is associated with the membranes that surround the magnetite crystals,” Nilsen-Hamilton said, “and each bacterium appears to make particles with their own unique crystal structure.”

Ames Lab chemist Tanya Prozorov tried synthesizing crystals, using the proteins with various concentrations of reagents in an aqueous solution, but the particles formed quickly, were small and lacked specific crystal morphology. At the suggestion of Ames Lab senior physicist and crystal growth expert Paul Canfield, the team used polymer gels developed by Mallapragada and Balaji Narasimhan, who are both Ames Lab scientists as well as ISU chemical engineers, to slow down the reaction and help control formation of the nanocrystals and minimize aggregation.

“It’s simple chemistry,” Prozorov said, “and you can judge the reaction by the color, watching it go from yellow to green to black as the crystals form. Once the crystals precipitate out, we use a magnet to concentrate the particles at the bottom of the flask, then separate them out to study them further.”