Substances, and where they are found on surfaces: this question is more and more of interest in an ever-increasing number of applications. The new analytical method developed by researchers at the ETH Zurich is rousing great interest in this arena. As reported by ETH professor Renato Zenobi’s research group in the Journal of Physical Chemistry(1), they have succeeded in localizing and precisely determining the chemical nature of individual molecules on a surface.„I am convinced that this work is a milestone” Professor Zenobi said. “Through our analysis, we have broken through to a new dimension and can now identify substances on a scale of just ten nanometres.”

Chemical fingerprint

In order to detect single molecules, scientists previously relied on the fluorescence method. This method, however, does not permit an absolutely reliable identification of the substances found. The method newly developed by the ETH researchers, on the other hand, is based on Raman spectroscopy, which supplies a true fingerprint of the molecule. In this procedure, the sample to be investigated is irradiated with laser light. The majority of the light is immediately scattered, but a part is absorbed by the molecules and then re-emitted as clearly defined Raman radiation. Zenobi explained the principle further, saying „This radiation is like a finger print because all substances have identifying characteristics.”

Amplification factor of several millions

The principle of this method of analysis has actually been known for some time, but a limitation has been that individual molecules emit a signal that is too weak to be measured. The ETH researchers, however, have now succeeded in enormously amplifying the signal with a special experimental set-up. „For some time now, we have known that Raman radiation is more intense when the sample is placed on a silver or gold substrate” Zenobi said. “In addition, earlier research showed that a comparable effect is obtained if a silver or gold tip is scanned over the sample.”

By combining the two principles, Zenobi has now succeeded in developing 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 force 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.

When passed under a metal tip the sample emits a characteristic signal of its composition, which gives off a specific frequency (vertical axis). The strength of the signal changes in the course of time (horizontal axis). Scientists consider this ‘blinking’ to be a clear indication that only single molecules are being measured. large

Threefold confirmation

By performing measurements on two different substances, the researchers were able to show that in principle, all compounds may be determined by this method. They are also sure that they can actually detect individual molecules by this procedure. For example, if the sample substance on the gold surface is more dilute, signals of the same intensity as before are measured but at fewer locations where molecules are still present. The precision of the procedure is also demonstrated by the fact that the signals measured fluctuate when observed over a period of a few seconds. The researchers consider this to arise from molecular motion. If the Raman signals measured were to originate from a cluster of molecules, one would not expect such fluctuation. Finally, a third indication that adds confidence to this interpretation is the observation that for some molecules the Raman signal disappears suddenly and does not return. According to the researchers, this occurs when the molecules are decomposed by the laser light.

The researchers envisage numerous applications for their new method. In principle, it is now possible to determine with high precision on thin samples of a material which substances are present and where. Such measurements could supply useful information in biology, environmental analysis, and also in the preparation of new materials.