Multitasking miniatures: tailor-made particles are versatile.

A new class of tiny particles fashioned from metal and organic building blocks may lead to novel catalysts and sensors, say the chemists who synthesized the structures. They've created nanoscale and microscale spheres that can contain a diverse array of chemical ingredients.

Complexes called metal-organic coordination polymers form when dissolved metal ions and organic molecules combine into a chain with an alternating pattern. In some solvents, the bonds are strong enough to hold and the polymers can precipitate, but in other solutions, the polymers' bonds fall apart. Previously, chemists had made stable polymers only as macroscale solids.

Chad A. Mirkin and Moonhyun Oh of Northwestern University in Evanston, Ill., found that by slowly adding ether to metal ions and organic molecules in a solvent, they could interrupt the polymerization process. They were left with porous, spherical, polymer particles between 1 and 2 micrometers in diameter. The spheres, each a long chain wrapped up like a ball of yarn, remained stable in a variety of solvents and even when dry. The researchers report their results in the Dec. 1 Nature.

The work is "surprising because of the stability [of the particles] and the fact that they are forming spheres," says chemist Charles M. Drain of Hunter College of the City University of New York.

By introducing ether more quickly, Mirkin and Oh made smaller spheres, in the 100-to-200-nanometer range. Particles of all sizes reverted to their building blocks when the researchers added an excess of the original solvent.

Because the particles can be made from a wide range of building blocks, notes Mirkin, they can potentially fulfill many functions. For example, by using organic molecules containing two extra metal ions, Mirkin created particles that could have catalytic activity. The spheres' layered, porous structure provides a large surface area for catalytic reactions to take place.

Because the bonds linking the chain of metal and organic components are reversible, chemists could recover their expensive, carefully designed organic molecules after each use, says Mirkin.

Mirkin and Oh have also tweaked the optical properties of particles incorporating metal ion pairs. For instance, adding methanol to a suspension of particles containing zinc turned the mix yellow, while adding dimethyl sultbxide turned it red. This property, Mirkin suggests, might enable chemists to create sensors whose color indicates the presence of target molecules.

"You can tailor [one particle] in almost an infinite number of ways" says Mirkin. "It's a pathway that people can follow to generate new nano- and micromaterials," says chemist John A. Shelnutt of Sandia National Laboratories in Albuquerque.