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Fluorescent nanoparticles known as quantum dots can improve imaging technologies and drug targeting for cancers. But they can be unstable in the body and are often made from highly toxic semiconductor materials such as cadmium. Scientists have now found a way to coat quantum dots with a polymer to make them more stable and safer for biomedical uses (Bioconjugate Chem. 2015, DOI: 10.1021/acs.bioconjchem.5b00462).
Quantum dots fluoresce in a narrow range of visible wavelengths based on their size and can be used to label single cells, making them more specific than the radioactive dyes often used for medical imaging. Quantum dots also require simpler detection equipment and don’t generate harmful radioactive waste, says Alexander M. Seifalian of University College London.
To make a biocompatible coating for these nanoparticles, Seifalian and his colleagues first made a block copolymer by combining polyhedral oligomeric silsesquioxane (POSS)—a cagelike silica molecule—with poly(carbonate-urea) urethane (PCU), which is used to make artificial heart valves, nose grafts, and other human implants. The POSS-PCU copolymer forms small, water-soluble micelles, which are bluish-white in solution and appear “like skim milk,” says Seifalian.
When mixed with 10-nm-diameter, cadmium-based particles that fluoresce at 650 nm, the polymer reorganizes and encases the quantum dots. The polymer coating increased the particle size by about 33 nm, but did not significantly change the red color. Although uncoated dots bleached within 20 minutes of exposure to ultraviolet light and air, coated particles were stable for up to two hours.
To test biocompatibility, the researchers added the coated quantum dots to cultured mouse and human cells. After 24 hours, 85% of cells survived compared with only 65% of cells exposed to uncoated quantum dots. Cultures exposed to uncoated dots showed signs of cell death— the loss of cell membranes and lower cell density—while those exposed to the polymer-shielded dots had intact membranes and continued to divide. In future work, antibodies or drugs could be attached to the polymer surface to deliver them to specific diseased tissues or cells, Seifalian adds.
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