Hank Hogan, email@example.com
CÓRDOBA, Argentina, and CAMBRIDGE, UK – Researchers intent on building plasmonic sensors to a set of specifications now have a new tool
to help in that quest, according to a group of investigators. The team has demonstrated
that electron tomography and electrodynamic simulations can create an accurate optical
model of highly irregular gold nanoparticles, the heart of many plasmonic sensors.
The method could prove a powerful aid in designing devices to
detect the faintest of substances, said researcher Eduardo A. Coronado. “The
ultimate goal is to have a very precise optical characterization of real nanoparticles
to address their capabilities to enhance the spectroscopic signal of nearby molecules,
which is useful in ultrasensitive detection.”
Coronado, a chemical sciences professor at the National University
of Córdoba, headed the part of the group in Argentina. Other members were from
the University of Cambridge, where an electron microscopy research team was led
by Paul A. Midgley.
By taking 2-D electron microscope images (left) at different angles and combining them, researchers
create a 3-D electron tomogram that reveals the real shape of a gold nanoparticle
(right). This provides accurate information for simulation of its optical properties
by the discrete dipole approximation (DDA) method without any assumption about its
shape or morphology. Courtesy of Eduardo A. Coronado, National University of Córdoba.
Gold and silver nanoparticles enhance faint Raman spectroscopy
and fluorescence microscopy signals. In the case of the first, this can be by many
orders of magnitude, sometimes a hundred trillionfold. The effect is not nearly
as great for fluorescence but is still significant. These are just some examples
of the ability of nanoparticles to potentially control, manipulate and amplify light,
making them of great interest to researchers and possibly of considerable commercial
Achieving the right response from nanoparticles requires understanding
the relationship between their structure and optical properties, both in the far-
and near-field regions. The latter is particularly important when it comes to determining
the precise enhancement of a signal and what is needed to invoke it.
There are some analytical methods that yield an exact answer to
the question about the relationship of a nanoparticle’s size, shape and composition
to its optical behavior. But, Coronado noted, they apply only to nanoparticles of
a regular shape – such as a sphere – that are tens of nanometers in
diameter. They don’t work for irregular shapes and smaller nanoparticles,
two categories that could be the majority in a given sample.
The solution demonstrated by the researchers in a Nano Letters
paper published earlier this year involves the combination of two technologies.
The first is electron tomography, a form of electron microscopy. In it, two-dimensional
images sensitive to nanoparticle composition are captured at different tilt angles.
From this data, the investigators reconstructed a highly accurate
three-dimensional picture of the gold nanoparticles; e.g., they achieved nanometer
resolution of the features of triangular particles measuring 100 nm or more on a
The second technology is electrodynamic simulations. The investigators
modeled the irregularly shaped gold nanoparticles, approximating them as an array
of discrete dipoles for the purpose of predicting their optical response.
Coronado noted that this modeling approach has been shown to be
accurate to within 5 or 10 percent when used on nanoparticles that are simple spheroids.
Applying it, however, requires accurate 3-D information about the object being modeled.
Being able to predict a nanoparticle’s optical characteristics
could prove useful. Not only could the right shape, size and composition be designed
for a given situation, but the right illumination and working distance also could
One application might be to mount a nanoparticle on the probe
tip of an atomic force microscope. That would then be used to detect the sequence
of bases in a strand of DNA molecule by molecule.
Such a capability could be a boon, Coronado said. “This
kind of technology would, for example, speed up considerably the process of DNA
sequencing, which now takes a lot of time and effort using PCR [polymerase chain
reaction] chemical procedures.”