Compiled by Photonics Spectra staff
The first reported experimental demonstration of antenna-enhanced gas sensing at the single-particle level holds great promise for plasmonic sensing.
By placing a palladium nanoparticle on the focusing tip of a gold nanoantenna, researchers with Lawrence Berkeley National Laboratory, in collaboration with researchers at the University of Stuttgart in Germany, clearly detected changes in the palladium’s optical properties when exposed to hydrogen. Their findings appeared online May 15, 2011, in Nature Materials (doi: 10.1038/nmat3029).
Plasmonics – the confinement of electromagnetic waves in dimensions smaller than half the wavelength of the incident photons in free space – has emerged as a popular field in technology. Typically, the process is done at the interface between metallic nanostructures (usually gold) and a dielectric (usually air). The confinement of electromagnetic waves within the metallic nanostructure generates electronic surface waves, or plasmons. When a matching of the oscillation frequency between plasmons and the incident electromagnetic waves occurs, a phenomenon known as localized surface plasmon resonance (LSPR) happens. This can concentrate the electromagnetic field into a volume less than a few hundred cubic nanometers. Any object brought into this locally confined field (nanofocus) will influence the LSPR in a way that can be detected via dark-field microscopy.
Plasmonic sensing holds promise for detection of flammable gases such as hydrogen, where using sensors that require electrical measurements poses safety issues because of the potential threat from sparking. Palladium was seen as a prime candidate for plasmonic sensing of hydrogen because it readily and rapidly absorbs hydrogen, which alters its electrical and dielectric properties. However, the LSPR of palladium nanoparticles yields broad spectral profiles that make detecting changes extremely difficult.
The scientists used double electron-beam lithography in combination with a double liftoff procedure to precisely position a single palladium nanoparticle in the nanofocus of a gold nanoantenna. The enhanced gold-particle plasmon near fields can sense changes in the dielectric function of the proximal palladium nanoparticle as it absorbs or releases hydrogen, the researchers said.
They found that the antenna enhancement effect could be controlled by changing the distance between the palladium nanoparticles and the gold antenna and also by changing the shape of the antenna. The scheme could also be used to monitor the presence of other important gases such as carbon dioxide and nitrous oxides if the palladium nanoparticles were replaced with other nanocatalysts.
The technique also offers a promising plasmonic sensing alternative to fluorescence detection of catalysis, which depends upon the challenging task of finding appropriate fluorophores. Antenna-enhanced plasmonic sensing could also assist in the observation of single-chemical or biological events.