The infinite delicacy of the gaze
Exactly? Swiss researchers want to know more about it, down to the last molecule. Ever since nanoengineers have been building structures from individual atoms and molecular biologists have been following the chemistry of nature down to the smallest detail, there has been a demand for technologies that make the world of tiny things visible.
A chemical reaction is like a criminal case: It is usually known which substances are involved and what comes out later. However, only hypotheses are usually possible about the exact course of events, which appear as apparently clear models in the textbooks. Not enough if chemists want to achieve the precision of organisms.
Up until now, there has been a big gap in the methods of exploring the world on a small scale. The scanning tunneling microscope provides an atomically accurate image of chemical structures, the surface of which it scans the sea floor in a similar way to an echo sounder. The only result, however, is an extremely precise topography – peaks and valleys of molecular surfaces. Structures can be measured in this way, but in no way determined. Or as Renato Zenobi from ETH Zurich puts it: "You can use it to determine the abdominal circumference of bacteria. But we want to know: is that a pore protein over there? Is this a lipid over here?"
Another tool on the molecular dissection table is laser tweezers and Raman spectroscopy. In this way, individual microparticles such as red blood cells can be fixed in the beam of a laser. Researchers from Bonn have even used it to move individual cesium atoms. And the Raman spectrum of the illuminated and decelerated particle also provides information about its chemical structure. This is due to the Raman effect: when light hits molecules, some of the rays transfer energy to the molecular bonds – physicists call this "inelastic scattering". Since the energy determines the frequency and thus the color of the light, the emitted light changes its color spectrum.
This color change is like a fingerprint of the molecule: the shift in the spectrum allows conclusions to be drawn about the type of molecule bonds – and thus about the molecule as a whole. If you can measure them - because only one in ten billion light particles is emitted differently. The majority is scattered undistorted, "elastic". The Raman effect is therefore not measurable with small molecules.
This is where Zenobi and his team came in: They increased the Raman effect. It increases when the illuminated molecules lie on a gold surface. And it increases even more when an extremely fine silver tip is brought up to within a few atom diameters of the sample, which creates a kind of feedback. So why not combine both methods to make the Raman spectrum of single molecules measurable?
The Zurich team covered a gold plate with extremely thin layers of sample molecules: a dye and a simple organic compound. They brought the sharp silver tip very close to the surface, switched on the laser - and caught the Raman spectrum of a tiny section with a side length of ten nanometers. In tens of millions of amplifications. Molecule specific.
Ten times ten nanometers: There are only a handful of sample molecules on this surface. This is important because it enabled the researchers to prove that they really did capture the Raman radiation from individual molecules. On the one hand, the intensity and frequency of the radiation fluctuated – caused by the movement of the individual molecules photographed in the image section. If an unexpected number were in focus, the spectrum would not fluctuate - it is easier for someone to be missing unnoticed in the class photo than in the individual portrait. In addition, the Raman effect sometimes suddenly disappeared: the laser light destroyed the irradiated molecules. The death of a molecule only means the abrupt end of the Raman effect if a few of them are really on the scene.
The amplified Raman spectrum can actually be used to distinguish and localize individual molecules. And in real time. Zenobi sees just one of the many possible applications for microbiology: soon it will be possible to use this method to observe bacteria, experienced master builders of nanostructures, building their colonies – molecule by molecule. Their technology of clinging to surfaces as a biofilm in a wafer-thin layer of molecular slime has aroused great interest among researchers. You know the chemical compounds that are used – but now it might be possible to follow "live" which molecule is being maneuvered where and when.
