Top figure shows hydrogen (red) absorbed on a palladium nanoparticle, resulting in weak light scattering and barely detectable spectral changes. Bottom figure shows gold antenna enhancing light scattering and producing an easy to detect spectral shift. Image: Alivisatos group |
Coveted technical capabilities such as the observation of
single catalytic processes in nanoreactors, or the optical detection of low
concentrations of biochemical agents and gases are an important step closer to
fruition. Researchers with the U.S. Department of Energy (DOE)’s Lawrence
Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers
at the Univ. of Stuttgart
in Germany,
report the first experimental demonstration of antenna-enhanced gas sensing at
the single particle level. By placing a palladium nanoparticle on the focusing
tip of a gold nanoantenna, they were able to clearly detect changes in the
palladium’s optical properties upon exposure to hydrogen.
“We have demonstrated resonant antenna-enhanced
single-particle hydrogen sensing in the visible region and presented a
fabrication approach to the positioning of a single palladium nanoparticle in
the nanofocus of a gold nanoantenna,” says Paul Alivisatos, Berkeley Lab’s
director and the leader of this research. “Our concept provides a general
blueprint for amplifying plasmonic sensing signals at the single-particle level
and should pave the road for the optical observation of chemical reactions and
catalytic activities in nanoreactors, and for local biosensing.”
A paper on this work titled “Nanoantenna-enhanced gas
sensing in a single tailored nanofocus” can be found in Nature Materials.
One of the hottest new fields in technology is plasmonics—the
confinement of electromagnetic waves in dimensions smaller than half-the-wavelength
of the incident photons in free space. Typically this is done at the interface
between metallic nanostructures, usually gold, and a dielectric, usually air.
The confinement of the electromagnetic waves in these metallic nanostructures
generates electronic surface waves called “plasmons.” A matching of the
oscillation frequency between plasmons and the incident electromagnetic waves
gives rise to a phenomenon known as localized surface plasmon resonance (LSPR),
which can concentrate the electromagnetic field into a volume less than a few
hundred cubic nanometers. Any object brought into this locally confined field—referred
to as the nanofocus—will influence the LSPR in a manner that can be detected
via dark-field microscopy.
“Nanofocusing has immediate implications for plasmonic
sensing,” says Laura Na Liu, lead author of the Nature Materials author, who was at the time the work was done a
member of Alivisatos’ research group but is now with Rice Univ. “Metallic
nanostructures with sharp corners and edges that form a pointed tip are
especially favorable for plasmonic sensing because the field strengths of the
electromagnetic waves are so strongly enhanced over such an extremely small
sensing volume.”
Plasmonic sensing is especially promising for the detection
of flammable gases such as hydrogen, where the use of sensors that require
electrical measurements pose safety issues because of the potential threat from
sparking. Hydrogen, for example, can ignite or explode in concentrations of
only 4%. Palladium was seen as a prime candidate for the plasmonic sensing of
hydrogen because it readily and rapidly absorbs hydrogen that alters its
electrical and dielectric properties. However, the LSPRs of palladium
nanoparticles yield broad spectral profiles that make detecting changes
extremely difficult.
Scanning electron microscopy image showing a palladium nanoparticle with a gold antenna to enhance plasmonic sensing. Image: Alivisatos group |
“In our resonant antenna-enhanced scheme, we use double
electron-beam lithography in combination with a double lift-off procedure to
precisely position a single palladium nanoparticle in the nanofocus of a gold
nanoantenna,” Liu says. “The strongly enhanced gold-particle plasmon
near-fields can sense the change in the dielectric function of the proximal
palladium nanoparticle as it absorbs or releases hydrogen. Light scattered by
the system is collected by a dark-field microscope with attached spectrometer
and the LSPR change is read out in real time.”
Alivisatos, Liu, and their co-authors found that the antenna
enhancement effect could be controlled by changing the distance between the
palladium nanoparticle and the gold antenna, and by changing the shape of the
antenna.
“By amplifying sensing signals at the single-particle level,
we eliminate the statistical and average characteristics inherent to ensemble
measurements,” Liu says. “Moreover, our antenna-enhanced plasmonic sensing
technique comprises a noninvasive scheme that is biocompatible and can be used
in aqueous environments, making it applicable to a variety of physical and
biochemical materials.”
For example, by replacing the palladium nanoparticle with
other nanocatalysts, such as ruthenium, platinum, or magnesium, Liu says their
antenna-enhanced plasmonic sensing scheme can be used to monitor the presence
of numerous other important gases in addition to hydrogen, including carbon
dioxide and the nitrous oxides. This technique also offers a promising
plasmonic sensing alternative to the fluorescent detection of catalysis, which
depends upon the challenging task of finding appropriate fluorophores.
Antenna-enhanced plasmonic sensing also holds potential for the observation of
single chemical or biological events.
“We believe our antenna-enhanced sensing technique can serve
as a bridge between plasmonics and biochemistry,” Liu says. “Plasmonic sensing
offers a unique tool for optically probing biochemical processes that are
optically inactive in nature. In addition, since plasmonic nanostructures made
from gold or silver do not bleach or blink, they allow for continuous
observation, an essential capability for in-situ monitoring of
biochemical behavior.”
