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Researchers at ETH Zürich, Switzerland, have developed an analytical procedure for localising and precisely determining the chemical nature of individual molecules on a surface, and say identification on a scale of just 10 nanometres is possible by their method.
In order to detect single molecules, scientists now rely on the fluorescence method. But while the technique makes it possible to pinpoint the presence of an individual molecule it is not possible to identify them.
The method newly developed by the ETH researchers, led by Renato Zenobi, supplies a fingerprint of a molecule. The sample to be investigated is irradiated with laser light. Most of the light is immediately scattered, but a part is absorbed by the molecules and then re-emitted as clearly defined Raman radiation, which is analysed to identify the substances present on the surface of the sample.
While the principle of this method of analysis has been known for some time, individual molecules emit a signal that is too weak to be measured. The ETH researchers have succeeded in amplifying the signal. Raman radiation is more intense when the sample is placed on a silver or gold substrate. The researchers have obtained a comparable effect by scanning a silver or gold tip over the sample during the measurement.
By combining the two principles, Zenobi and his team have developed a high-resolution analytical method. The sample is deposited on a gold surface. During measurement, a silver tip of roughly the size used in a scanning tunnelling microscope is moved over the sample. Between the tip and the gold substrate, over an area of approximately 10 by 10 nanometres, a strong electrical field is generated that amplifies the Raman signal by a factor of 107.
The researchers have shown that, in principle, all compounds may be identified by this method, and say there numerous potential applications. In principle, it is now possible to determine with high precision , which substances are present and where. Such measurements could be useful in biology, environmental analysis, and the development of new materials.
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