Multi-modal sensing of molecules

A team of physicists and engineers in the US has demonstrated a way of simultaneously making optical and electronic measurements on the same molecule. The researchers claim that the advance could lay the foundation for mass-produced single-molecule sensors.

Rice University scientists use tiny gaps between gold electrodes to simultaneously perform electronic and optical measurements on the same molecule. These scanning electron microscope images show electrodes and gaps on a silicon chip, and the colour insets show optical signals due to the chip (top) and a gap (bottom)

The study led by Douglas Natelson at Rice University in Houston, Texas, was performed on a device consisting of two electrodes separated by a molecule-sized gap. With an electric current the researchers measured conduction through a single molecule in the gap, and the light-focusing properties of the electrodes allowed the molecule to be identified by a unique optical fingerprint.

Mark Ratner, a chemist at Northwestern University in Illinois, describes the work by Natelson and his colleagues as a lovely study that clarifies the geometric fluctuation mechanism of conductance by direct measurement of the spectra.

“What’s been done here is to use vibrational spectroscopy on a moving electronic charge,” says Ratner. “That’s been done before using a different method called inelastic tunnelling, but this is the first time that it’s been done on a single molecule using Raman spectroscopy.” The approach may be a little controversial, but Ratner feels that the Rice researchers have made a very strong case for what they are doing.

“The primary application would be in chemical sensing,” says Natelson. “Our structures are a way of making enormously sensitive optical ‘hotspots’ routinely and in pre-defined locations.” He adds that the fabrication process is within the capabilities of standard manufacturing methods, and the nanostructures are ready made for highly sensitive chemical detection.

“In our previous paper we showed that within tens of seconds it is possible to detect the spectrum of adsorbed air pollution if there aren’t already molecules on the devices,” says Natelson, whose research group has patent pending on the nanostructures as Raman sensors.

Further reading: “Simultaneous Measurements of Electronic Conduction and Raman Response in Molecular Junctions”, Ward et al., Nano Lett. (2008).

Figure: Rice University scientists use tiny gaps between gold electrodes to simultaneously perform electronic and optical measurements on the same molecule. These scanning electron microscope images show electrodes and gaps on a silicon chip, and the colour insets show optical signals due to the chip (top) and a gap (bottom) (source: Douglas Natelson/Rice University).

Article first published in Nanomaterials News.