Four rhesus monkeys injected with tiny luminescent crystals called quantum dots showed no signs of ill effects over a one-year period, according to a study just published in Nature Nanotechnology. 

Previously, researchers had found conflicting results when they looked at the safety of quantum dots—the crystals had toxic side-effects in cell cultures but not in rodents and other small animals.

Quantum dots are inorganic nanoscale crystals that emit different colored light depending on their size. They glow brighter and for longer periods of time than many other fluorescent molecules. Researchers are currently exploring a variety of uses for quantum dots, from the light-emitting diodes of electronic displays, to the light-absorbing layer of solar cells, and as medical tracers of brain tumor cells.

Although the non-human primates in the study did not exhibit any signs of illness in response to the quantum dots, a heavy metal component of the crystals (cadmium) did build up in the primates’ liver, spleen and kidneys after 90 days. This concerning build up requires further investigation and may mean that the best use of quantum dots in people will be single-dose applications such as an injection of the luminescent dye to identify a tumor for surgical removal, said the researchers in a release.