The light emitted by quantum dots is both more intense and longer lasting than that produced by the fluorescent markers commonly used in medical and biological applications. Yet these nano-scale light sources still suffer from one major drawback: they do not dissolve in water. Researchers at the University of Twente's MESA+ Institute for Nanotechnology and at the A*STAR agency in Singapore have found a way to remedy this. They have developed a coating which allows quantum dots to be used inside the human body, even inside living cells. The researchers published details of their coating recipe in the October issue of Nature Protocols.
The new coating enables quantum dots, which are semiconductor nanocrystals, to literally cast light on biological processes. These dots are "nuggets," consisting of several hundred to several thousand atoms, that emit visible light when they are exposed to invisible UV radiation, for example. They range from a few nanometres to several tens of nanometres in size. The coating's benefits are not limited to improved solubility in water alone. Other molecules can "lock on" to its surface. This could make coated quantum dots sensitive to certain substances, for example, or allow them to bind to specific types of cells, such as tumour cells.
Scientists studying biological processes often use fluorescent tags that bind to biomolecules. This makes it relatively easy to track such molecules, even inside living cells. Quantum dots are a better option. They emit long-lasting, bright light, the colour of which depends on the size of the quantum dots used. For a number of reasons, including their toxicity, they were previously unsuitable for use in living organisms.
The researchers therefore developed an amphiphilic coating, i.e. one with both hydrophobic and hydrophilic properties. The "water hating" side of the polymer material attaches to the surface of the quantum dot. Its exposed hydrophilic side then makes the quantum dot/coating combination soluble in water. The coating builds up on the surface of the quantum dot through a process of self-assembly. The coating polymer has the added benefit that other molecules can be bound to it. Another important plus is that it does not adversely affect the quantum dot's light-emitting properties.
The study is a collaborative venture between the University of Twente's MESA+ Institute for Nanotechnology and the A*STAR agency's Institute of Materials Research and Engineering, in Singapore. It is headed by Professor Julius Vancso, Professor of Materials Science and Technology of Polymers at the University of Twente, who is also a visiting scientist at the Singapore institute.
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