How to align gold nanorods without losing their powers

"We've demonstrated that we can bring the nanorods into alignment and that the process does not adversely affect the optical properties of the gold nanorods," says Joe Tracy. (Credit: cobalt123/Flickr)

A new technique uses magnetic fields to align gold nanorods while preserving their underlying optical properties, report researchers.

“Gold nanorods are of interest because they can absorb and scatter specific wavelengths of light, making them attractive for use in applications such as biomedical imaging, sensors, and other technologies,” says Joe Tracy, corresponding author of a paper on the work and a professor of materials science and engineering at North Carolina State University.

It is possible to tune the wavelengths of light absorbed and scattered by engineering the dimensions of the gold nanorods. Magnetically controlling their orientation makes it possible to further control and modulate which wavelengths the nanorods respond to.

“In other words, if you can control the alignment of gold nanorods, you have greater control over their optical properties,” Tracy says. “And using magnetic fields to control that alignment means that you can control the alignment without actually touching the nanorods.”

In their technique, the researchers synthesize separate solutions of gold nanorods and iron oxide nanoparticles. Mixing the solutions drives assembly of the iron oxide nanoparticles onto the surface of the gold nanorods. The resulting “coated” nanorods can then be controlled using a low-strength magnetic field.

“We’ve characterized both what is happening during this process and how well it works,” Tracy says. “We’ve demonstrated that we can bring the nanorods into alignment and that the process does not adversely affect the optical properties of the gold nanorods.”

“In addition, to the best of our knowledge, these nanorods have the smallest aspect ratio of any elongated nanoparticle that has been ‘decorated’ with iron oxide nanoparticles and aligned using magnetic fields,” says Mehedi Rizvi, first author of the paper and a PhD student at NC State.

“In order for this technique to work, we’ve had to optimize many aspects of the system, including the dimensions of the gold nanorods, the size of the iron oxide nanoparticles, and the relative concentrations of both nanorods and nanoparticles in solution,” Rizvi says.

“We are currently in the process of exploring potential applications in imaging based on the multifunctional properties of magnetic-overcoated gold nanorods,” Tracy says.

The paper appears in the journal Advanced Materials. Coauthors are from NC State; the Institute for Physical Chemistry and Polymer Physics; Technische Universität Dresden; and the University of North Carolina, Chapel Hill.

Support came from the National Science Foundation, the Alexander von Humboldt Foundation, Deutsche Forschungsgemeinschaft , Fonds der Chemischen Industrie, and the China Scholarship Council.

Source: NC State