Bigger can be better, and that may be true for quantum dots, when they are used for labeling cells. New research suggests that cells may try to digest smaller quantum dots in a process known as autophagy.Cells have lysosomes, membrane-bound organelles that contain digestive enzymes. In autophagy, lysosomes consume the cell’s other organelles. This can serve as a defense mechanism against foreign objects such as bacteria or, in this case, quantum dots.Oleksandr Seleverstov and colleagues at the University of Leipzig in Germany and at Emory University in Atlanta studied cellular responses to the quantum dots. They labeled human mesenchymal stem cell cultures with protein-conjugated 605- and 525-nm quantum dots. The red quantum dots were approximately two times larger than the green ones, but both consisted of the same chemical components. Thus, the researchers excluded factors other than size that might affect the cells. For fluorescence observations, they employed a Carl Zeiss microscope.During the 52-day observation period, the red quantum dots retained their fluorescence. However, the green ones lost their fluorescence after two to seven days, depending on their concentration when added to the cells. The scientists detected several sites of autophagy, and the quantum dots appeared in the same regions. Autophagy can sometimes kill cells and, indeed, 2 to 5 percent of cells that received the 525-nm quantum dots exhibited the signs of autophagy activation and were dead after 48 to 96 hours. The investigators reported these findings in the Nov. 8 online publication of Nano Letters.Whereas small quantum dots may induce autophagy, making them of little use for labeling, the researchers noted that these quantum dots would be ideal for studying autophagy, a little-understood process. Furthermore, the small dots could be used to kill cancer cells in a targeted manner, by causing them to die from autophagy.