Gold-plated nanotubes are effective non-toxic contrast agents for photothermal and photoacoustic imaging

Techniques used to image tumours and infections improve when the carbon nanotube ’contrast agents’ are gold plated, researchers in the US have discovered. The gold plating reduces the nanotubes’ potential toxicity while boosting their effect as contrast agents, thereby allowing far fewer of them to be used for the same effect.

Doctors use both photoacoustic and photothermal imaging to examine diseased tissue. The techniques involve shining a laser onto the tissue and measuring either the emitted heat - that is, the infrared radiation - or ultrasound. By adding contrast agents, such as pigmented biomolecules, the tissue responds better to the laser and the images become clearer.

Recently, scientists have found that carbon nanotubes might outperform biomolecules as contrast agents. However, they suffer from poor near-infrared absorption, and questions abound about their toxicity.

Now, a team led by Jin-Woo Kim at the University of Arkansas, US, together with a group led by Vladimir Zharov at the University of Arkansas for Medical Sciences, US, has found that gold plating solves both these issues. Gold is chemically inert, so it rules-out potential toxicity, and it has a high near-infrared absorption. Moreover, gold-plated nanotubes are smaller than gold nanorods (also used as contrast agents), which means they might prove more effective at penetrating certain biological barriers, and the plating process leaves an empty core that could be filled with a payload of drugs.


Source: © Nature Nanotechnology

Gold-plated nanotube formation

Kim, Zharov and colleagues apply the gold plating by submerging carbon nanotubes and gold chloride in water. At room temperature, the nanotubes reduce the gold chloride, leaving a thin layer of gold on their surface. ’All this happens in water without any other chemicals, so we may say our process is an environmentally "green" one,’ Kim and Zharov note.

In tests, the researchers showed that the gold-plated nanotubes displayed near-infrared absorption at least two orders of magnitude greater than their non-plated counterparts. ’Simply speaking, two orders [of magnitude] higher concentrations of carbon nanotubes would be required to have the same photothermal responsiveness as gold-plated nanotubes,’ they said. 

Jon Cartwright