Archive for February, 2008

For the first time, researchers at North Carolina State University have demonstrated that microscopic “two-faced” spheres whose halves are physically or chemically different – so-called Janus particles – will move like stealthy submarines when an alternating electrical field is applied to liquid surrounding the particles.A paper describing the research, published in the Feb. 8, 2008, edition of Physical Review Letters, advances knowledge about how potential “smart” materials – think of tiny engines or sensors – can move around and respond to changes in their environment. Janus particles could be used as microscopic mixers, molecular “shuttles,” self-propelling microsensors or means of targeted drug delivery.

North Carolina State University research shows microscopic “two-faced” particles moving perpendicular to the direction of an electrical field, not in the direction of the field, as would be expected.

The researchers – Dr. Orlin Velev, associate professor of chemical and biomolecular engineering at NC State and lead author of the paper; Sumit Gangwal, an NC State graduate student; Dr. Olivier Cayre, a post-doctoral researcher in Velev’s lab; and Dr. Martin Bazant from Massachusetts Institute of Technology – created tiny two-faced gold and plastic particles and applied low frequency alternating current to the water containing the particles. The electric field was of voltage and frequency similar to the ones you’d get if you plugged a device into a socket in your home or office.

Velev says the micrometer-sized particles convert the electrical field into liquid motion around them and then unexpectedly propel themselves perpendicular to the direction of the powered electrodes – not in the direction of the electrical field, as would be expected. The particles always travel in the same orientation: with the plastic “face” as the front of the mini-submarine and the metallic “face” in the rear, Velev added.

The phenomenon – called “induced-charge electrophoresis,” which had been predicted in a theoretical model by the MIT collaborator – had not been demonstrated previously.

The term “Janus particle” comes from the name of a Roman god with two faces. Velev says that these materials have the potential to perform a variety of applications.

“You can imagine other types of Janus particles comprising a ’smart gel’ that responds to a change in its environment and then releases drugs, for example,” Velev says. Fabricating these responsive materials on the microscale and nanoscale is an exciting and rapidly developing area of science, he adds.

“We are able to create tiny Janus particles of the same size and shape and are beginning to learn how to give them functionality,” Velev said. “The next step is to create more complex particles that are able to perform more specialized functions in addition to propelling themselves around.”

The research is funded by the National Science Foundation and a Camile and Henry Dreyfus Teacher-Scholar grant.

“Induced Charge Electrophoresis of Metallodielectric Particles”
Authors: Sumit Gangwal, Olivier J. Cayre and Dr. Orlin D. Velev, NC State University; Dr. Martin Z. Bazant, Massachusetts Institute of Technology
Published: Feb. 4, 2008, in Physical Review Letters

Indiana University Bloomington chemists have designed an organic molecule that binds negatively charged ions, a feat they hope will lead to the development of a whole new molecular toolbox for biologists, chemists and medical researchers who want to remove chlorine, fluorine and other negatively charged ions from their solutions.”What we’ve done is create an efficient synthesis that gives us access to a whole new family of binding agents,” said Amar Flood, who reports the discovery with postdoctoral scholar Yongjun Li in Angewandte Chemie this week. “The synthesis is extremely modular, as well, so we imagine these molecules can be easily modified to bind a wide variety of negative ions with great specificity.”

Chelating agents are small molecules that grab atoms (or, sometimes, even smaller molecules) out of a solution and hold onto them. Chelators play a valuable role in both nature and laboratory settings. For example, the human protein calmodulin not only grabs positively charged calcium ions out of the solution surrounding it, it also influences cell processes according to how many calcium ions it has grabbed. In labs, EDTA (ethylenediaminetetraacetic acid) is frequently used to remove calcium and magnesium ions so that chemical reactions go faster or more efficiently.

Many organic molecules exist that can bind positively charged ions, or cations, and this has much to do with the fact that it is easy to synthesize organic molecules with negatively charged parts. It is those negatively charged parts that interact with positive ions, or cations, grabbing them out of solution and holding onto them so the cations cannot react or interfere with other processes.

Attempts at manufacturing organic binding agents with positively charged parts is not hard, but designing them in such a way that they don’t attract the attention of solvent molecules has been a major challenge for chemists.

Flood and Li’s solution was to create a donut-shaped organic molecule whose center would serve as the binding spot. A halide ion might fit snugly inside the hole, but the arrangement of atoms surrounding the hole would exclude any solutions.

This complex organic molecule is capable of binding a negatively charged ion (chloride). The versatile, easy-to-make molecule may represent a new family of binding agents for use in biology and medicine

Flood also wanted the synthesis of such a molecule to be cheap, easy and flexible, so he looked to the “click chemistry” devised by Scripps Research Institute chemist K. Barry Sharpless. Click chemistry is an efficient method of joining molecules together to form larger molecules. Flood’s particular application of click chemistry results in an eight-member macrocycle with a 3.7 angstrom hole in the middle. The more-or-less symmetrical molecule that Flood and Li built contains four triazole rings sporting three nitrogen atoms each. It is presumed the nitrogens withdraw electron density from the carbon and hydrogen atoms closest to the molecule’s hole, thereby creating an alluring binding spot for fluorine or chlorine ions. This binding is made all the more orderly because the macrocycle is preorganized to host its anionic guest.

“This thing is so easy to make,” Flood said. “The triazole moiety has got more character to it than meets the eye. It’s not just a byproduct of the click chemistry. We see lots of potential in it.”

The other four members of Flood and Li’s eight-member ring are entirely substitutable. Modifying these may give the chelator different binding affinities for a given anion.

The work Flood and Li describe in their Angewandte Chemie paper was funded by Indiana University.