Researchers have identified a new chemistry approach that could remove
micropollutants from the environment.
Micropollutants are biological or chemical contaminants that make their way
into ground and surface waters in trace quantities.
Using a pioneering imaging technique, Cornell University researchers
obtained a high-resolution snapshot of how ligands, molecules that bind to
other molecules or metals, interact with the surface of nanoparticles. In
doing so, they made an unexpected breakthrough discovery. They determined
that by varying the concentration of an individual ligand they could control
the shape of the particle it attached too.
This approach could result in an array of daily applications, including
developing chemical sensors that are sensitive at a very low level to a
specific chemical in the environment.
"Professor Peng Chen's work allows for deep insights into molecular
adsorption processes, which is important to understand for designing
molecular sensors, catalysts, and schemes to clean up micro-pollutants in
the environment," said Dr. James Parker, program manager, U.S. Army Combat
Capabilities Development Command, known as DEVCOM, Army Research Laboratory.
"This research is also important for designing and engineering
stimuli-responsive materials with specialized function that could not be
found in regular, bulk materials."
The research, published in Nature Communications, studied interactions of
ligands and gained new understanding of the strength, or affinity of ligand
adsorption as well as how multiple ligands cooperate, or don't, with each
other.
"When the molecule adsorbs on the surface of a nanoscale material, it also
actually protects the surface and makes it more stable," said Dr. Peng Chen,
the Peter J.W. Debye Professor of Chemistry in the College of Arts and
Sciences at Cornell University, who led the research. "This can be utilized
to control how nanoscale particles grow and become their eventual shape. And
we found we can do this with just one ligand. You don't do any other trick.
You just decrease the concentration or increase the concentration, and you
can change the shape."
Understanding how ligands interact with the surface of nanoparticles has
been a challenge to study. Adsorbed ligands are difficult to identify
because there are other molecules in the mix, and nanoparticle surfaces are
uneven and multifaceted, which means they require incredibly high spatial
resolution to be scrutinized.
A nanoparticle's size and surface structures, or facets, are intrinsically
tied to the particle's potential applications. The larger the particle, the
more atoms fit inside it, while smaller particles have less available space
internally but a greater surface volume ratio for atoms to sit atop, where
they can be utilized for processes such as catalysis and adsorption. The
different types of structures the atoms and molecules form on these surface
facets are directly correlated with the particle's shape.
Scientists have used several imaging methods to survey these particles, but
until now, they haven't been able to obtain nanometer resolution to really
explore the nooks and crannies of the multiple surface facets and quantify
the affinity, or strength, of a ligand's adsorption. The research team was
able to do just that by employing a method of their own devising called
COMPetition Enabled Imaging Technique with Super-Resolution or COMPEITS.
The process works by introducing a molecule that reacts with the particle
surface and generates a fluorescent reaction. A nonfluorescent molecule is
then sent to bind to the surface, where its reaction competes with the
fluorescent signal. The resulting decrease in fluorescence, essentially
creating a negative image, can then be measured and mapped with super high
resolution.
Using COMPEITS on a gold nanoparticle, the team was able to quantify the
strength of ligand adsorption, and they discovered ligand behavior can be
very diverse. Ligands, it turns out, are fair-weather friends of a sort, at
some sites they cooperate to help each other adsorb, but at other sites they
can impair each other's efforts. The researchers also discovered that
sometimes this positive and negative cooperativity exists at the same site.
In addition, the researchers learned that the surface density of adsorbed
ligands can determine which facet is dominant. This crossover inspired the
team to vary the concentrations of individual ligands as a way to tune the
shape of the particle itself.
"For us, this has opened more possibilities," Chen said. "For example, one
way to remove micropollutants, such as pesticides, from the environment is
to adsorb micro-portions on the surface of some adsorbent particle. After it
is adsorbed on the surface of the particle, if the particle is a catalyst,
it can catalyze the destruction of the micropollutants."
Reference:
Rong Ye, Ming Zhao, Xianwen Mao, Zhaohong Wang, Diego A. Garzón, Heting Pu,
Zhiheng Zhao, Peng Chen. Nanoscale cooperative adsorption for materials
control. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021-24590-y