Most conventional optical imaging techniques are not suitable for imaging through thick living tissue because light is strongly scattered by biological matter. This leads to poor spatial resolution and features such as tumours are difficult to make out.

Photoacoustic and photothermal imaging overcomes this problem by measuring the ultrasound and heat produced by photons that are fired into a sample. The technique works because the sample heats up when it absorbs photons, causing it to expand. Pressure waves propagate out from the expanding structures and these can then be detected by an ultrasound transducer. The heat can also be detected by a photodetector.

Although it is efficient, the technique still requires contrast agents because diseases, such as tumours and cancer cells, do not show natural photoacoustic and photothermal contrast. Carbon nanotubes are promising candidates as contrast agents because they offer high resolution and allow deep tissue imaging. However, they do not absorb light very well in the near-infrared, a range important for biological imaging.

Gold layer around CNTs
Now, Jin-Woo Kim of the University of Arkansas and Vladimir Zharov of the University of Arkansas for Medical Sciences have overcome this problem by depositing a thin layer of gold around carbon nanotubes. The gold layer enhances absorption of near-infrared light.

"Our golden nanotubes absorb near-infrared radiation at least two orders of magnitude more effectively than traditional nanotubes," explained Kim. "Simply speaking, 100 times more ordinary carbon nanotubes are needed to have the same photoacoustic and photothermal response as golden nanotubes." Fewer golden tubes are thus required, and less is always better when injecting nanoparticles into the body, he added.

The hollow core may also be used to potentially carry therapeutic payloads, such as drugs.

"The unique properties of golden nanotubes make them an effective alternative to existing nanoparticles and fluorescent labels for non-invasive targeting of molecular structures in vivo," Zharov told nanotechweb.org. "They may be used for a variety of biomedical applications, including highly target-specific lymphatic diagnosis and therapy, as well as to treat tumours and infections."

The researchers will now continue exploring the optical properties of these nanoparticles and use double- and multi-walled carbon nanotubes as the core. They will also try adding different biomarkers to the tubes to target different types of cancer cell and investigate the in vivo toxicity of the materials in animals.

The work was reported in Nature Nanotechnology.