LEUVEN, Belgium – A new nanoscale
light-manipulation method that optically
detects single molecules could be used in
a variety of photochemistry applications
and help advance technologies for visualizing single molecules and multiple-mole-cule interactions.
Progress in optically detecting single
molecules has been hindered by their weak
optical response. Currently, researchers use
metal nanostructures to focus light into
tiny zones called “hot spots,” which excite
electrons on the surface, causing them to
oscillate coherently. When shone on a molecule, and with the help of these oscillating
electrons, the focused light can increase a
molecule’s optical signal to 100 billion
times its normal strength, a level detectable
by optical microscopes.
The current method, however, has two
limitations: The first is that hot spots can
become too hot; the second is that they are
very small. This means that the heat from
hot spots can melt the nanostructure, destroying its ability to channel light effectively. And hot spots produce only a very
small cross section in which interaction
with molecules can take place. For a single molecule to become detectable, it
must find the hot spot.
To overcome these drawbacks, Dr.
Ventsislav Valev and colleagues at
Katholieke Universiteit Leuven sought to
nanoengineer larger spots. The international team began by shining circularly
Shining circularly polarized light on ring-shaped nanostructures increases the opportunity for interaction with
molecules. Courtesy of Katholieke Universiteit Leuven.
polarized light on nanostructures and
found that this could increase the useful
area. When they shone light on square-ring-shaped gold nanostructures, the scientists observed that the entire surface of the
nanostructures was successfully activated.
“Essentially, light is constituted of elec-
tric and magnetic fields moving through
space,” Valev said. “While with linearly
polarized light the fields move in a linear,
forward direction, with circularly polar-
ized light, they rotate in a spiral-like
The circularly polarized light imparts a
sense of rotation on the electron density in
ring-shaped gold nanostructures, thus trap-
ping the light in the rings and forming
“loops of light.” The loops cause excited
electrons to oscillate coherently on the full
surface of the square-ringed nanostruc-
tures, rather than in a few concentrated hot
spots. This increases the opportunity for
interaction with molecules.
Antennas capture, upconvert weak IR light
Inspiration from nature: (left) a natural photosynthesis system with light-harvesting (LH) molecules and a reactive center (RC); (right) a schematic representation of the nanocrystal that realizes the upconversion (UC) with
the attached antennas in green. Courtesy of University of Groningen.
GRONINGEN, Netherlands – A new technique that uses special molecules as light
antennas to harvest the energy from weak
infrared light and amplify the process
3300 times could lead to improved medical imaging methods.
Materials scientists and chemists from
the University of Groningen and from the
FOM Foundation harvested infrared light
– which has too little energy to release
electrons in solar cells – more efficiently
by modifying an organic dye that acts as
light antennas to transmit the energy to the
nanoparticles to which they are attached.
These particles subsequently convert
two weak captured photons into a
single strong, energy-rich photon in a