Coating breakthrough holds promise for gold nanorod applications

A team of US researchers has fine-tuned a technique for coating gold nanorods with silica shells that they claim could pave the way for a wide variety of biomedical applications.

Gold nanorods have a lot of potential applications because they have a surface plasmon resonance – meaning they can absorb and scatter light. By controlling the dimensions of the nanorods – specifically their aspect ratio – its possible to control the wavelength of light they absorb.

“This characteristic makes gold nanorods attractive for use in catalysis, security materials and a range of biomedical applications, such as diagnostics, imaging, and cancer therapy,” said Joe Tracy, a materials science and engineering researcher at NC State Univeristy who is senior author of a recent paper on the improved technique.

Gold nanorods are efficient for photothermal heating, the process of converting absorbed light into heat. If too much light is shined on gold nanorods, however, they can lose their rod shape and change into spheres, losing their desirable optical properties.

One way to help gold nanorods retain their shape during photothermal heating is to coat them with silica shells, which confine the nanorods to their original shape but allow light to pass through. For different applications, it is important to be able to control the shell thicknesses. With thin shells, the change in size of the nanorods is minimal, and the gold nanorods can still pack into dense assemblies. Thicker shells, however, can act as buffers, preventing nanorods from bunching closely together and shielding them from their environment.

Silica shells also provide a surface that can be functionalised using well-understood chemical techniques. For example, the shells could be functionalised to fluoresce in the presence of specific proteins or to target tumors.

“The silica shells offer multiple benefits – and our modified approach to coating gold nanorods with silica shells has two distinct advantages,” Tracy said in a statement. “First, we have demonstrated that our technique can be carried out on a large scale – up to 190 milligrams. Second, we offer improved control over shell thickness. We can consistently create uniform shells as thin as 2nm.”