Tiny ruler is golden ticket to see diseases in action

MAKE room in your pencil case: there's a new gold standard for measuring things on the tiniest scale. It could lead to better sensors for finding disease biomarkers.

Ryan Hill and colleagues at Duke University in Durham, North Carolina, have made an ultra-precise tool called a plasmon ruler. Shining light on metallic nanomaterial creates surface ripples of electrons, called plasmons. When two structures are very near each other, their plasmons become coupled, and they share a specific frequency. Slipping something between them, such as a target molecule, causes a shift in frequency, which changes the wavelengths of light reflected by the ruler. This corresponds to a measurement.

Previous plasmon rulers, made by carving very narrow channels in gold to create nanorods, were limited to nanoscale distances. The latest version creates a cavity between gold film and gold nanoparticles. The particles are tethered to the film by tiny chains of molecules with precise lengths. The chains are spaced so that the molecules being measured can slide between them. This ruler can measure things down to 1 angstrom - roughly the width of a single hydrogen atom (ACS Nano, DOI: 10.1021/nn3035809).

Optical tools and scanning microscopes can also measure angstrom lengths. But because the plasmon ruler makes measurements with its physical structure, it is better able to track real-time changes in distance, such as the structural morphing of proteins as they interact with viruses or bacteria, says Hill.

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