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  • Spinning Fibers Produce Aligned Nanorods
Aug 2011
CHAPEL HILL, N.C., Aug. 22, 2011 — A new way to align masses of gold nanorods could aid the development of potent biomedical imaging technologies.

Aligning gold nanorods is important because the way they respond to light depends on the direction in which the particles are pointed. To control the optical response of the nanorods, all of the nanorods should be readily aligned.

Researchers from North Carolina State University have developed a simple, scalable way to align gold nanorods using electrospun polymer “nano-/microfibers.” Electrospinning is a way of producing fibers, with a liquid polymer being discharged from a needle and then solidifying in the way that spiders form their web material. The researchers produced fibers from 40 nm to 3 μm in diameter — thus, nano-/microfibers.

Researchers have developed a way to align gold nanorods using electrospun polymer "nano-/microfibers." (Photo: North Carolina State University)

The investigators mixed the gold nanorods into the polymer solution, causing them to be incorporated directly into the polymer. The nanorods align when the fibers form. The force experienced by the liquid polymer as it is emitted from the electrospinning needle creates “streamlines” in the polymer solution.

“The nanorods are forced into alignment with these streamlines, like logs in a river that swing into alignment with the current,” said Joe Tracy, co-author of a recent paper published in Langmuir describing the study. “And as the polymer solidifies, the aligned nanorods are locked into place.”

“What makes this result truly exciting is that the alignment is multiscale, or simultaneously achieved at different length scales,” added co-author Rich Spontak. “The nanorods are aligned at nanoscale dimensions, whereas the fibers are aligned at larger length scales.”

This approach has been used in the past to align other kinds of nanorods, but this is the first time it has been done with gold nanorods. “To the best of our knowledge, this is also the first time nanorods of this size have been aligned in electrospun fibers,” Tracy said, referring to the fact that the study focused on relatively short nanorods.

Specifically, the researchers used nanorods with an aspect ratio of 3:1. That means that a nanorod measuring 10 nm wide would be 31 nm long. The nanorods in the study were approximately 49 nm long. The aspect ratio affects the way the nanorods interact with light and, therefore, their optical properties.

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aspect ratio
With respect to pictorial displays, the ratio of the width to the height. The television standard in the US is 4:3. High-definition or wide-screen television will have a ratio of 16:9.
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