Gold nanocages – bioinert nanostructures with hollow interiors and ultrathin porous walls – can be designed to absorb strongly and scatter light in the near-infrared (IR) region of the electromagnetic spectrum (from 700 to 900 nm). Light at these wavelengths can penetrate deeply into soft biological tissue and so is perfect for optical imaging based on photoacoustic and optical coherence tomography.

Younan Xia at Georgia Tech, together with Yongjian Liu from Washington University School of Medicine and Cathy Cutler at the University of Missouri, are now saying that gold nanocages containing radioactive 198Au that emit Cerenkov light could also be used as contrast agents in luminescence imaging while carrying drugs to tumours. Cerenkov radiation is the light emitted during the decay of a radionuclide and the gold nanocages luminesce in the visible and near-infrared regions by high-energy radioactive beta decay. More importantly, the nanocages appear to accumulate in mice tumour cells after injection.

Reduced dose and robust labelling

The good thing about Cerenkov radiation is that no excitation light source is used, which means that autofluorescence from surrounding healthy tissue is minimized. This increases the signal-to-noise ratio and reduces the amount of radionuclide, or dose, required for in vivo imaging.

The researchers detected the Cerenkov luminescence emitted by the 198Au in live mice using the IVIS Lumina II XR system.

“The radioactive 198Au nanocages emit Cerenkov light but the physicochemical and other properties of these nanostructures remain unchanged,” explained team member Yucai Wang. “Our approach is thus a new and robust way to label a nanoparticle. In conventional radioisotope labelling (where the isotope is attached to a nanostructure by chemical means), there is always the worry that the label will ‘come off’ during treatment. By incorporating 198Au atoms into the walls of the gold nanocages, such an issue does not exist anymore.”

The technique developed in this research could be used for diagnosing cancer and to guide nanoparticle delivery for treating tumours, he told nanotechweb.org.

The team says that it will now try adding targeting moieties, such as peptides, to the gold nanocages in an attempt to improve drug delivery to cancer cells for photothermal treatment, chemotherapy, or a combination of the two.

The current work is reported in Nano Letters.