ARE quantum dots safe to inject into humans? These fluorescent nanoparticles are being touted as a non-invasive way to make images of internal body tissues and diagnose disease. But a study has shown they could do more harm than good, by breaking down in the body and releasing cadmium, a toxic heavy metal.
Quantum dots consist of a core of fluorescent cadmium selenide or cadmium sulphide within an inert layer of zinc sulphide. They can be coated with a layer containing proteins or other molecules designed to latch onto specific biomolecules within the body. The hope is that when injected into the bloodstream they will target tumour cells, for instance, or tissue inflamed by rheumatoid arthritis.
Illuminated from outside the body by infrared light, the quantum dots would fluoresce at precise frequencies and produce an infrared image. This offers great potential for identifying the precise sites of disease without resorting to surgery.
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But quantum dots may be more toxic than anyone realised, according to Sangeeta Bhatia and her colleagues at the University of California, San Diego. When they exposed the particles to oxidation by ultraviolet light and hydrogen peroxide, they found that within hours the dots had broken down and released cadmium (Nano Letters, vol 4, p 11). Because the immune system also releases oxidising chemicals in its attacks on foreign bodies, Bhatia suspects the dots could cause cadmium poisoning in humans. “This is probably something the [research] community doesn’t want to hear,” says Mike Sailor, who is a colleague of Bhatia’s at UC San Diego.
Other scientists who have tested quantum dots in animals say more research is needed to find out if they really are toxic. Byron Ballou at Carnegie Mellon University in Pittsburgh, Pennsylvania, has studied the way dots behave after they are injected into the tail veins of mice. “We’ve seen no evidence of toxicity in these mice, but we’ve commissioned work to study this in more detail,” he says. “I’d be extremely cautious using the dots in humans.”