Researchers led by Joseph Tracy at North Carolina State University have demonstrated a method for coating silica-covered gold nanorods with magnetite nanoparticles by changing the polarity of the solvent. Tracy’s work, published recently in Chemistry of Materials, is an efficient approach to creating uniform nanoparticle coatings and could be used for novel applications in sensing, imaging, and therapy. 

When purifying nanoparticles, scientists commonly manipulate the solvent environment or attached ligands. For example, adding a polar solvent to a suspension of nanoparticles in a nonpolar solution can cause the particles to fall out of solution and homoaggregate, or stick to each other, says Brian Chapman, a recent graduate from the Tracy group and first author of the article. Exchanging the ligands on the surface of the nanoparticles can also be used for direct assembly of nanoparticles, where careful choice of the ligand can give control over the structure of the final nanoparticle assembly. Ligand exchange, however, adds an additional synthetic step. 

In this work, Chapman and his colleagues added ethanol, a polar solvent, to a solution of silica-coated gold nanorods and magnetite nanoparticles. They found that, after adding just enough ethanol, the magnetite particles favored heteroaggregation by attaching to the silica layer covering the nanorods over homoaggregation. Adding too much ethanol, however, favored homoaggregation of the magnetite particles. “There’s a very fine balance between the energy barriers for homo- and heteroaggregation,” says Ning Liu of the University of Limerick, who was not affiliated with this study. 

Chapman and Tracy’s work stands out for the quality of the coating, speed at which it could be achieved, and magnetic response of the final product. The magnetite particles coated the nanorods uniformly and remained attached even after the composite particles were transferred to an aqueous solution. Heteroaggregation also occurred quickly—under five minutes. By placing a magnet outside of the container, Chapman and Tracy found that the particles could be controlled using an external magnetic field. These are significant properties for using the magnetite-coated silica-gold nanorods in real-world applications, according to Liu. 

As a next step, Tracy wants to further investigate the fundamental interactions between the magnetite nanoparticles and the solvent, silica-coating, and gold nanorods. “We’re trying to better understand the chemistry and how general it is,” says Tracy, by exploring different combinations of nanoparticles, solvents, and coatings. By investigating a wide set of parameters, potentially with the help of simulations, he and his group hope to develop a predictive model that materials researchers could use to create a whole suite of composite nanoparticles. 

Both the magnetite-coated silica-gold nanorods and future combinations of nanoparticles formed through heteroaggregation could be used in a variety of applications. The composite nanoparticles demonstrated by Chapman and Tracy could be used for simultaneous sensing or heating using both the plasmon resonance of the gold nanorods and magnetic response of the magnetite nanoparticles. Plasmon heating is currently being explored as a therapy to destroy cancer cells. Coating the gold nanorods with semiconductor quantum dots instead of magnetite nanoparticles might allow control over the optical properties of the composite nanorods. It is also possible to change the composition of the core material and the properties of the composite nanoparticles. In the future, it could be “a mix and match,” Tracy says, “between the particles you have and the properties you want.” 

Read the abstract in Chemistry of Materials.