Deep-tissue optical imaging may soon be getting easier – or at least, the images may soon be getting sharper. That’s because an international team of scientists have developed photoluminescent nanoparticles that are able to shine through over three centimeters (1.2 inches) of biological tissue. If attached to anomalies deep beneath the skin, the nanoparticles could allow those anomalies to be seen more clearly from the outside.

The nanoparticles both absorb and emit near-infrared light. They employ a process known as near-infrared-to-near-infrared up-conversion (NIR-to-NIR). This means that they absorb pairs of photons and combine them into single, higher-energy photons that are subsequently emitted. The near-infrared region of the electromagnetic spectrum is the one at which biological tissue absorbs and scatters light the least – as a result, the concentrated light emitted by the nanoparticles has little in the way of background “noise” to compete with.

The particles themselves are composed of “a nanocrystalline core containing thulium, sodium, ytterbium and fluorine, all encased inside a square, calcium-fluoride shell.” Because calcium-fluoride is common in the bones and teeth, its inclusion in the shell allows the nanoparticles to be biocompatible within the body. Additionally, it has been found to increase their photoluminescence.

So far, the scientists have injected the nanoparticles into mice, and inserted a capsule of them three centimeters into a slab of pork. In both cases, they were able to obtain “vibrant, high-contrast images of the particles shining through tissue.” They now hope to develop methods of attaching the particles to biological targets.

The research project included scientists from institutions in the U.S., China, South Korea and Sweden. It was led by the University at Buffalo’s Prof. Paras N. Prasad, and Gang Han, an assistant professor at University of Massachusetts Medical School. A paper on the research was published last week in the journal ACS Nano.