Researchers Find Way To Align Gold Nanorods On A Large Scale

Researchers from North Carolina State University have developed a simple, scalable way to align gold nanorods, particles with optical properties that could be used for emerging biomedical imaging technologies.

Aligning gold nanorods is important because they respond to light differently, depending on the direction in which the nanorods are pointed. To control the optical response of the nanorods, researchers want to ensure that all of the nanorods are aligned.

The NC State researchers developed a way to align the 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. The researchers produced fibers as thin as 40 nanometers (nm) in diameter and as thick as three microns in diameter – thus, nano/microfibers.

The researchers 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,” says Dr. Joe Tracy, an assistant professor of materials science and engineering at NC State and co-author of a paper describing the study. “And as the polymer solidifies, the aligned nanorods are locked into place.”

“Electrospinning efforts at NC State are world-class and have yielded a wide range of novel and functional materials,” adds Dr. Rich Spontak, a professor of chemical and biomolecular engineering and materials science and engineering at NC State and paper co-author. “What makes this result truly exciting is that the alignment is multiscale, or simultaneously achieved at different length scales. 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 says, referring to the fact that the study focused on relatively short nanorods.

Specifically, the researchers used nanorods with an aspect ratio of 3.1. For example, that means that a nanorod measuring 10 nm wide would be 31 nm long. The nanorods in the study were approximately 49 nm long.

This aspect ratio is important, because it affects the way the nanorods interact with light – and, therefore, their optical properties.

The paper, “Long-Range Alignment of Gold Nanorods in Electrospun Polymer Nano/Microfibers,” was published online Aug. 11 in Langmuir. The paper was co-authored by Tracy; Spontak; NC State Ph.D. students Kristen Roskov and Wei-Chen Wu; NC State undergraduate Krystian Kozek; UNC-Chapel Hill Ph.D. student Raghav Chhetri; and Dr. Amy Oldenburg, an associate professor at UNC-Chapel Hill. The research was funded by the National Science Foundation and NC State.

NC State’s Department of Materials Science and Engineering and Department of Chemical and Biomolecular Engineering are part of the university’s College of Engineering.

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Note to Editors: The study abstract follows.

“Long-Range Alignment of Gold Nanorods in Electrospun Polymer Nano/Microfibers”

Authors: Kristen E. Roskov, Krystian A. Kozek, Wei-Chen Wu, Richard J. Spontak and Joseph B. Tracy, North Carolina State University; Raghav K. Chhetri and Amy L. Oldenburg, University of North Carolina, Chapel Hill

Published: online Aug. 11 in Langmuir

Abstract: In this study, a scalable fabrication technique for controlling and maintaining the nanoscale orientation of gold nanorods (GNRs) with long-range macroscale order has been achieved through electrospinning. The volume fraction of GNRs with an average aspect ratio of 3.1 is varied from 0.006 to 0.045 in aqueous poly(ethylene oxide) solutions to generate electrospun fibers possessing different GNR concentrations and measuring 40–3000 nm in diameter. The GNRs within these fibers exhibit excellent alignment with their longitudinal axis parallel to the fiber axis n. According to microscopy analysis, the average deviant angle between the GNR axis and n increases modestly from 3.8 to 13.3° as the fiber diameter increases. Complementary electron diffraction measurements confirm preferred orientation of the {100} GNR planes. Optical absorbance spectroscopy measurements reveal that the longitudinal surface plasmon resonance bands of the aligned GNRs depend on the polarization angle and that maximum extinction occurs when the polarization is parallel to n.