"Plasmonics with a Twist"

news story image

Nature is abundant with structures exhibiting chirality or "handedness." Professor Oded Rabin captured these images of magnificent chiral cacti during a visit to the botanical gardens. His spectroscopy tools capture signals from molecules and nanostructures with similar shapes, but (much, much) smaller in size.

In the high-stakes world of drug production, one configuration of a molecule can have the intended beneficial effect on a patient's health, while another has no effect--or possibly a toxic one. 

For example, some drug molecules are "chiral," exhibiting "handedness." A "left" configuration of such a molecule might perform a therapeutic task differently from a "right" configuration, or not at all. Manufacturers go to great difficulty and expense in an attempt to create compounds containing only those configurations that are correct for the intended task.  One of their tools in this effort is spectroscopy, a set of techniques that allow researchers to determine the identity, ratio and concentration of molecules in a sample based on how they scatter or absorb light. 

Using metallic, highly conductive "plasmonic" nanoparticles, Oded Rabin, an assistant professor in the Clark School's Department of Materials Science and Engineering with a joint appointment in the Institute for Research in Electronics and Applied Physics, seeks to enhance spectroscopic methods used to evaluate molecular configurations. Rabin hopes to enable detection and identification of chiral molecules at much lower concentrations than presently possible and give drug manufacturers and medical researchers new ways to ensure the efficacy and safety of compounds and understand complex diseases. 

Based on his proposal to perform this research, Rabin has been awarded a five-year, $540,000 National Science Foundation Faculty Early Career Development (CAREER) Award. The award recognizes the nation's outstanding junior faculty members who effectively integrate research and education in their work.

Rabin and his group have designed novel plasmonic nanoparticles that resemble pinwheels oriented to the left and right. After these particles have been mixed with chiral molecules, the whole sample is exposed to a beam of circularly polarized light, which, like the chiral molecules, has "left" or "right" attributes.

"When light is circularly polarized, it means that its electric field is spinning around, clockwise or counterclockwise, as the beam travels," Rabin explains. "It's like throwing a football. The ball travels along a path, and spins as it goes."

When the circularly-polarized light illuminates the sample, it sets off sophisticated and selective interactions. Molecules with a left configuration will absorb the "screw-like" light differently than those with a right configuration, just like a left hand will fit more comfortably in a left-handed glove than a right-handed one. The nanoparticles make this selectivity more pronounced, allowing the spectrometer to detect and identify the target molecules at much lower concentrations.

While pharmaceutical companies would benefit most from Rabin's technique in the near term, it could also help researchers observe and study the progression of diseases such as bovine spongiform encephalopathy ("mad-cow disease"), in which proteins, chain-like molecules which need to fold in a precise manner in order to function, refold in an incorrect and ultimately deadly way.

Rabin is the third MSE faculty member in the past year to receive a CAREER Award, preceded by assistant professors John Cumings and Joonil Seog in 2011.

Visit the NSF CAREER Award web site »
Visit Professor Rabin's homepage »
Visit Professor Rabin's Materials and Interface NanoTechnology web site »

Published February 9, 2012