Silicon quantum dots promising for clinical deep-tissue imaging

Silicon nanocrystals called quantum dots caused no health problems in monkeys three months after large doses were injected, marking a step forward in bringing them into clinics as biomedical imaging agents. A new study at the University at Buffalo (UB) in New York has found that silicon quantum dots may be a safe tool for diagnostic imaging in humans. They absorb and emit near-infrared light, which makes them ideal for seeing deeper into tissue than traditional fluorescence-based techniques.

Co-lead author Folarin Erogbogbo, PhD, a UB research assistant professor who has since accepted a new position as an assistant professor of biomedical engineering at San Jose State University, notes that silicon quantum dots don't contain materials like cadmium that are found in other quantum dots, and are generally considered to be nontoxic.

The researchers tested the silicon quantum dots in rhesus macaques and mice, injecting each animal with 200 mg of the particles per kilogram of the animal's weight. Blood tests taken for three months afterward showed no signs of toxicity in either the mice or monkeys, and all of the animals appeared healthy over the course of the study. The subjects ate, drank, groomed, explored, and urinated normally.

The silicon quantum dots did, however, gather and stay in the livers and spleens of the mice, resulting in side effects including inflammation and spotty death of liver cells. But this did not happen with the rhesus macaques: The monkeys' organs appeared normal, without the damage seen in the mice.

This discrepancy raises the question of how useful toxicity studies on mice can be in determining a nanocrystal's potential effect on humans, says co-author Paras Prasad, PhD, SUNY Distinguished Professor in chemistry, physics, electrical engineering, and medicine at UB, and executive director of UB's Institute for Lasers, Photonics, and Biophotonics.

Quantum dots and other nanoparticles—because of their tiny size—can access parts of the body where larger particles cannot. Due to this and other factors, the differences in anatomic scale between mice and primates may matter more in nanomedicine than in other pharmaceutical fields, Prasad says. "Even at high doses, we didn't see any adverse side effects at all in monkeys despite the problems in mice," he says.

The fact that the silicon did not biodegrade in the mice was very surprising, says co-author Mark Swihart, PhD, a UB professor of chemical and biological engineer and co-director of UB's New York State Center of Excellence in Materials Informatics. "Generally, people assume that silicon quantum dots will biodegrade," he says. "We didn't see that happen, and we think this might be due to the fact that we capped the surface with organic, FDA-approved molecules to keep the quantum dots from degrading too fast."

The study was a collaboration between the University at Buffalo, Chinese PLA General Hospital in China, San Jose State University, Nanyang Technological University in Singapore, and Korea University in South Korea. It is part of a larger body of research that many of the team members have been conducting to investigate the effect of various nanoparticles in animal models.

Full details of the work appear in the journal ACS Nano; for more information, please visit pubs.acs.org/doi/abs/10.1021/nn4029234.