SEIRA stands for surface-enhanced infrared absorption and is a spectroscopy technique that complements the more widely used surface-enhanced Raman spectroscopy (SERS). SEIRA directly probes molecular vibrations through absorbed light as opposed to scattered light for SERS. “While SERS is sensitive to single molecules, it is limited to molecules that scatter light from a large cross-sectional area,” explains team member Lisa Brown. “SEIRA, however, can be used to probe virtually any molecule that is chemically compatible with the antenna.”

Single-molecule SEIRA would thus be able to detect a greater variety of molecules, and combining the two techniques, SEIRA and SERS, would allow us to build up a comprehensive data set of the vibrational signatures of molecules when they are excited with infrared light, she adds.

A nanoantenna is a metallic nanostructure that collects and focuses light at optical to infrared wavelengths – as opposed to a conventional television or radio antenna that work at radio frequencies. Optical antennas can control light at the nanometre scale and they localize, enhance and redirect light at this scale. As such, they will be crucial for developing nanophotonics devices in the future.

Focusing light into a very confined volume

The new nanoantenna, made by Naomi Halas’ and Peter Nordlander’s teams at Rice University, consists of two gold nanorods with a 10-20 nm gap between them and large semi-circular “fans” on either side (see figure). “With this structure, we can focus infrared light by more than 105 into a very confined volume,” Brown tells nanotechweb.org. “This design is more efficient than two nanorods alone because each fan portion serves as a storehouse of charge carriers (electrons and holes) that can flow towards the gap.”

With more charge carriers, the antenna produces a stronger electric field at the gap interface and underneath the antenna, we place a gold mirror film that reflects both incoming light and the light scattered by the antenna itself, she adds. It is this approach that ultimately boosts the field intensity by another factor of six and by using the fan-shaped antenna together with the gold mirror we can detect as few as 10,000 molecules using conventional infrared spectroscopy techniques.

Fano resonances

“SEIRA magnifies the intensity of signals from infrared spectroscopy, which involves directly exciting molecular vibrational modes in the mid-IR range,” explains Brown. “We designed our fan antennas to absorb light at the same frequency as the molecules being studied and the interference between the two systems leads to ‘inverted’ features, called Fano resonances, in the resultant spectrum. Fano resonances are caused by only a small number of molecules because the near–field is enhanced solely in the gap region.”

The researchers say that they now plan to adjust the overall design of the nanoantennas to increase the bandwidth and/or the intensity of the near-field enhancement in their samples. They are also going to try and make the nanostructures from materials other than gold. “Since most metals behave in a similar way at IR frequencies, we should be able to fabricate the antennas from different metals and adapt these to the compound being analysed,” adds Brown.

The research is detailed in Nano Letters DOI: 10.1021/nl504455s.