Researchers from Jiangnan University (Wuxi, China), the University of Michigan (Ann Arbor, MI), and Rice University (Houston, TX) have demonstrated that slightly twisted pairs of gold nanorods have a circular polarizing effect on light that is ten times stronger than the best liquid-crystal-based circular polarizers.
A pair of gold nanorods, bound to each other by strands of DNA or by oppositely charged ions, becomes slightly twisted. The small dihedral angle between two adjacent nanorods breaks the centrosymetric nature of the two identical cylinders, inducing chiral properties. This twist creates geometric chirality of the nanorod pair and introduces a spin in light waves passing through it that is so strong, it can be observed with a single pair of nanorods. The scientists demonstrated that the twist of nanorod pairs in a clockwise direction leads to levo-rotating enantiomers, while the counter-clockwise direction results in light rotation in the opposite direction. For a given nanorod length and aspect ratio, the chiral optical activity is the greatest for nanorod pairs with equal length. The gold nanorods for the experiment were prepared using a standard seeded-growth method, with assembly triggered by the hybridization of DNA oligomers in a standard polymerase chain reaction (PCR) process. The scientists are working on applications of nanorod pairs for detection of cancer markers and environmental threats. Contact Nick Kotov at [email protected].
