"Although other research groups have used quantum dots to either target or image cells, we believe this is the first time in vivo targeting and imaging has been achieved simultaneously," said Xiaohu Gao of Emory University and Georgia Institute of Technology.

Gao and colleagues used cadmium selenide-zinc sulphide (CdSe-ZnS) core-shell quantum dots with a diameter of 5 nm. They coated the nanoparticles with a protective layer of an ABC triblock copolymer and then a layer of poly (ethylene glycol). The triblock polymer protects the quantum dots from enzymes and other biomolecules and also acts to prevent leakage of toxic cadmium and selenium ions. Finally, they attached the quantum dots to a monoclonal antibody that targets prostate-specific membrane antigen on the prostate tumour cell surfaces.

When the team injected the quantum dots into the circulatory system of mice with solid prostate tumours the dots accumulated selectively at the site of the tumour. The scientists detected the dots by fluorescence imaging. They also tried "passive" targeting of the tumour by injecting quantum dots that weren't conjugated to antibodies. Although the quantum dots accumulated at the tumour sites as they leaked from the porous angiogenic blood vessels that grow in tumours, the process was slower and less efficient than for the quantum dots conjugated to antibodies.

"This is a new class of quantum dot conjugates designed specifically for complex in vivo applications," said Shuming Nie of Emory University and Georgia Institute of Technology. "They are stable over a broad range of pH and salt conditions and maintain their stability even after treatment with hydrochloric acid."

The researchers say they believe this is an important step in the quest to eventually use nanotechnology to target, image and treat cancer, cardiovascular plaques and neurodegenerative disease in humans.

"The larger surface area provided by quantum dots should allow the conjugation of multiple agents, and we envision the development of diagnostic and therapeutic dual-modality quantum dots," said Nie.

The scientists reported their work in Nature Biotechnology.