Magnetic Alignment for Plasmonic Control of Gold Nanorods Coated with Iron Oxide Nanoparticles

Gold nanorods are gold nanoparticles with a rod-shaped morphology. They have a unique optical property known as surface plasmon resonance (SPR), which is due to the collective oscillation of electrons on the surface of the gold nanorod. This SPR property can be tuned by adjusting the aspect ratio (length to width ratio) of the nanorod, which allows for control of the absorption and scattering properties of gold nanorods in the visible and near-infrared regions of the electromagnetic spectrum. The tunable optical properties of gold nanorods make them useful for a wide range of applications, including biomedical imaging, cancer diagnosis and treatment, biosensors, catalysis, and environmental monitoring. They can be functionalized with different molecules to enhance their properties and target specific applications. For example, gold nanorods can be used as contrast agents for biomedical imaging techniques such as photoacoustic imaging and optical coherence tomography. They have high absorption and scattering properties in the near-infrared region, which allows them to penetrate deep into tissue. Moreover, gold nanorods can be used as catalysts for various chemical reactions. They have a high surface area and can be functionalized with different groups to enhance their catalytic properties. They also can be used for environmental monitoring of pollutants such as heavy metals and pesticides when functionalized with recognition molecules to selectively detect and quantify these pollutants.

Numerous techniques for synthesizing, aligning and assembling of gold nanorods have been developed. Gold nanorods are typically synthesized using a seed-mediated growth method in which gold ions are reduced in the presence of a surfactant to form small gold nanocrystals or “seeds”. These seeds are then used as nucleation sites for the growth of the gold nanorods. Whereas some of the available alignment techniques can endow a high degree of ordering, dynamic alignment of gold nanorods dispersed in liquids using applied electric and magnetic fields is more appealing. This can be attributed to two main reasons: (1) remarkable speed and reversibility allow for quick switching, and (2) yields gold nanorods with strong polarization-dependent extinction.

However, aligning using electric fields requires small electrode separation and high field strength, which is relatively difficult to achieve. Alternately, aligning with magnetic fields has emerged as a promising approach because it can be performed over large sample volumes and does not require direct contact of a solution with electrodes. Unfortunately, there are limited studies on gold nanorods alignment using magnetic fields due to the challenges in achieving adequate magnetic anisotropy. Thus, synthetic approaches that impair the optical properties have been employed for the magnetic alignment of gold nanorods.

Herein, Professor Mehedi Rizvi, Dr. William Crumpler, and Professor Joseph Tracy from North Carolina State University in collaboration with Dr. Ruosong Wang, Dr. Jonas Schubert, Professor Christian Rossner, Professor Amy Oldenburg and Professor Andreas Fery from Leibniz-Institut für Polymerforschung Dresden investigated the magnetic alignment and plasmonic control of gold nanorods coated with superparamagnetic iron oxide nanoparticles. In their approach, the magnetic gold nanorods were obtained by depositing cationic polyethyleneimine functionalized iron oxide nanoparticles on the surface of anionic gold nanorods coated with bovine serum albumin under optimized conditions. Their work is currently published in the peer-reviewed journal, Advanced Materials.

The research team showed that large magnetic Gold nanorods aligned well without degrading the optical properties, while the small ones aligned partially. The resulting magnetic Gold nanorods exhibited distinct optical properties similar to that of plasmonic Gold nanorods , with the ability to track dynamic fields of low frequencies at least 17 Hz in water. The magnetic anisotropy induced by the magnetic dipolar interactions between the adjacent iron oxide nanoparticles caused the parallel alignment of the magnetic Gold nanorods with the applied magnetic field, enabling noncontact magnetic manipulation of the longitudinal and transverse surface plasmon resonances.

The study of the sizes of GNR cores and iron oxide satellite nanoparticles revealed that both were necessary to obtain significant magnetic field alignment even in low magnetic fields. A comprehensive framework was introduced to analyze the alignment of magnetic Gold nanorods  from the perspectives of their structural characterization and optical properties that can yield sufficient energy to overcome the Brownian motion, an inherent problem that impedes the magnetic alignment between the magnetic Gold nanorods  with the magnetic field. The framework was based on multiple techniques. Notably, this approach can be extended to magnetic alignment of all types of anisotropic nanostructures with magnetic satellite nanoparticles and

In summary, the study brought to light a new lower limit to determine the minimal size of nanostructures coated with magnetic satellite nanoparticles that can successfully undergo magnetic alignment by aligning magnetic Gold nanorods with low aspect ratios. Magnetic Gold nanorods exhibited multiple functionalities, which is beneficial for numerous applications like multimodal imaging and therapy. In a statement to Advances in Engineering, Professor Joseph Tracy, the corresponding author explained their new approach would aid the design and analysis of magnetic alignment of a wide range of anisotropic nanostructures.