At the Institute of
Bioengineering and Nanotechnology (IBN), A*STAR, in Singapore, Jun Yang
and Jackie Ying* have developed a new protocol for the synthesis of tiny metal
and semiconductor crystals that are a few billionths of a meter in size. The
efficiency and structural control provided by this method could revolutionize
the production of nanocrystals and their hybrids, which have diverse applications
in medicine, electronics and energy.
 | | Fig. 1: Transmission electron micrograph images of nanoscale composites of noble metal and semiconductor crystals produced using a new synthesis protocol. Top left, silver-sulfide/silver (Ag2S–Ag); top right, cadmium-sulfide/silver (CdS–Ag); bottom left, cadmium-sulfide/gold (CdS–Au); bottom right, lead-sulfide/gold (PbS–Au).
© 2009 Institute of Bioengineering and Nanotechnology, A*STAR
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Typically, synthesis of a metal nanocrystal begins when a reducing agent is
mixed into a solution of metal ions. The reducing agent donates electrons to
the ions, causing metal atoms to crystallize out of the solution. If left unchecked,
the metal crystals agglomerate into larger and larger complexes. Consequently,
scientists use a capping agent—normally a thin monolayer of long organic
molecules—to cover the nanocrystals, arrest growth and preserve their
size.
One drawback with this procedure is that metal ions tend to remain in an aqueous
solution, which does not mix well with the organic liquid required to dissolve
the capping agents. To remedy this, chemists add a phase transfer agent, which
is a molecule that can bind to the water-dissolved metal ions and move them
into an organic liquid. Currently available phase transfer agents, however,
are only active with specifically charged systems, limiting their use.
Yang and Ying discovered that a molecule called dodecylamine (DDA) in ethanol
solution could act as a highly general phase transfer agent for nanocrystal
synthesis. Using the DDA agent, they transferred a wide variety of metal ions—silver,
gold, iridium, palladium and platinum, to name a few—to an organic medium
with greater than 95% efficiency, and readily turned the ions into uniform,
stable nanocrystals.
The DDA-based protocol could also be used to create an assortment of nanoscale
composite materials. For example, the researchers produced new nanocrystal alloys
using two types of noble metal ions in the starting mixture. In a ‘core–shell’
process, they grew another material around an existing nanocrystal to yield
novel metal–metal and metal–semiconductor hybrids. Ying says that
these nanocomposites are of great interest because they allow specific functionalities
to be tailored into the materials.
Several applications, such as chemical catalysis and biocompatible fluorescent
imaging, were possible with the synthesized nanocrystals. Furthermore, the DDA-based
protocol may be used for critical environmental purposes such as extraction
of heavy metals from contaminated soil or water. Such applications, says Ying,
are natural extensions of nanocrystal research that demonstrate their promising
possibilities.
The A*STAR affiliated authors in this highlight are from the Institute of Bioengineering
and Nanotechnology
* Yang, J. & Ying, J. Y. A general phase-transfer protocol for metal ions
and its application in nanocrystal synthesis. Nature Materials 8, 683–689
(2009).
Posted November 24th, 2009
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