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—so called
‘click chemistry’. 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.
Quantum dots made water-soluble by a coating can, in turn, be combined with polymers and be coupled to other quantum dots. |
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.
Synthesis of functionalized amphiphilic polymers for coating quantum dots