Conventional antennas, widely used to transmit radio or TV signals, can be used at optical frequencies if they are shrunk to the nanoscale, which could have potential applications in nanophotonics. The nanoantennas can also be used to generate electronic surface waves known as "surface plamons". This is done by confining electromagnetic waves – typically at the interface between metallic nanostructures (usually made of gold) and a dielectric (usually air) – that have dimensions smaller than half the wavelength of incident light.

When the oscillation frequency of the created plasmons matches that of the incident electromagnetic waves a phenomenon known as "localized surface plasmon resonance" (LSPR) occurs, which concentrates the electromagnetic field into an even smaller space – around 100 nm3. Any object brought into this so-called locally confined field – or "nanofocus" – will affect the LSPR in such a way that it can then be detected using a technique called dark-field microscopy – a technique where only scattered light makes up an image.

Paul Alivisatos and colleagues have now used such a set-up to detect single particles and atoms. The researchers created a novel set-up whereby they carefully placed a single palladium nanoparticle in the focus of a nanoantenna made of gold. The interaction between the gold and the palladium nanoparticle creates an LSPR so that any particle that is brought near the vicinity changes the dielectric function of the palladium particle as it absorbs or releases it. "Light scattered by the system can be collected by a dark-field microscope and the change in the LSPR read out in real time," says LBNL researcher Laura Na Liu.

The antenna enhancement effect can be controlled by changing the distance between the palladium nanoparticle and the gold antenna. The shape of the nanoantenna is important too, so that antennas that form a pointed tip are especially good for plasmonic sensing.

The researchers say that the device could be used to detect flammable gases, like hydrogen, that might easily be ignited by electricity during measurements with conventional sensors. Detecting small amounts of hydrogen is becoming increasingly important for developing fuel cells, especially as the gas can explode or ignite at concentrations of as little as 4%. And replacing the palladium with other nanocatalysts, such as ruthenium, platinum or magnesium, means that it could be used to detect gases such as carbon dioxide and nitrous oxides.

Alivisatos says that the new device, and the way it is made, provides a general blueprint for amplifying plasmonic sensing signals using single particles that "should pave the road for optically observing chemical reactions and catalytic activities in nanoreactors." The device could also serve as a bridge between plasmonics and biochemistry, adds Liu, because it offers a unique tool for probing biochemical processes using light. The technique employed dispenses with the need to use fluorescent markers to label molecules for subsequent detection.

"This work very elegantly shows that nanoantennas can be used to pick up very small changes in a satellite nanoparticle which may be optimised for specific chemicals," says Otto Muskens from the University of Southampton in the UK, who was not involved in the work. "This is an important next step in plasmonic biosensing with many possible applications."

The findings were detailed in Nature Materials.