A Janus nanoparticle. Image: Deborah Brewington/Vanderbilt University
New drug delivery
systems, solar cells, industrial catalysts, and video displays are among the
potential applications of special particles that possess two chemically
distinct sides. These particles are named after the two-faced Roman god Janus
and their twin chemical faces allow them to form novel structures and new
scientists have reduced the size of Janus particles down to a few nanometers in
diameter, their efforts have been hampered because they haven’t had a way to
accurately map the surfaces of the particles that they produce. This uncertainty
has made it difficult to evaluate the effectiveness of these particles for
various applications and to improve the methods researchers are using to
Now, a team of Vanderbilt University chemists has overcome this
obstacle by developing the first method that can rapidly and accurately map the
chemical properties of the smallest of these Janus nanoparticles.
The fact that Janus
particles have two chemically distinct faces makes them potentially more
valuable than chemically uniform particles. For example, one face can hold onto
drug molecules while the other is coated with linker molecules that bind to the
target cells. This advantage is greater when the different surfaces are cleanly
separated into hemispheres than when the two types of surfaces are intermixed.
nanoparticles (with sizes above 10 nm), researchers can use existing methods,
such as scanning electron microscopy, to map their surface composition. This
has helped researchers improve their manufacturing methods so they can produce
cleanly segregated Janus particles. However, conventional methods do not work
at sizes below 10 nm.
chemists—Associate Professor David Cliffel, Assistant Professor John McLean,
graduate student Kellen Harkness, and Lecturer Andrzej Balinski—took advantage
of the capabilities of a state-of-the-art instrument called an ion
mobility–mass spectrometer (IM-MS) that can simultaneously identify thousands
of individual particles.
The team coated the
surfaces of gold nanoparticles ranging in size from two to four nanometers with
two different chemical compounds. Then they broke the nanoparticles down into
clusters of four gold atoms and ran these fragments through the IM-MS.
Molecules from the
two coatings were still attached to the clusters. So, by analyzing the
resulting pattern, the chemists showed that they could distinguish between
original nanoparticles where the two surface compounds were completely
separated, those where they were randomly mixed and those that had an
intermediate degree of separation.
“There is no other
way to analyze structure at this scale except X-ray crystallography,” says
Cliffel, “and X-ray crystallography is extremely difficult and can take months
to get a single structure.”
“IM-MS isn’t quite as precise as X-ray
crystallography but it is extremely practical,” adds McLean, who has helped
pioneer the new instrument’s development. “It can provide structural
information in a few seconds. Two years ago a commercial version became
available so people who want to use it no longer have to build one for