Many industrial processes rely on chemical reactions occurring along surfaces, such as the Haber-Bosch process that produces ammonia on the surface of iron for fertilizer and other industrial products. It may seem surprising, then, that there are presently no good ways to observe these reactions on the molecular level in real-time because of their speed and scale (they occur on a scale smaller than the width of a human hair).
Earlier this year, however, researchers at the École Polytechnique Fédérale de Lausanne, a research institution in Lausanne, Switzerland, constructed and tested of a new type of microscope that can do just that—look at surface chemistry on the micro-scale in real-time. Using their microscope, the researchers measured, in a matter of milliseconds, the variation in chemical properties of a glass surface over micrometers. Their results were published in August 2017 in Science.
The first of its kind, this microscope is known as a “wide-field, structured illumination, second harmonic microscope,” which, as its name suggests, utilizes a phenomenon called second harmonic generation (SHG). SHG is a process that allows two photons—particles of light—to combine under certain conditions, resulting in a new photon with exactly twice the frequency. This is similar to how two water droplets, when colliding with suitable orientations and speeds, can combine to form a droplet with precisely twice the volume.
For SHG, the new photon forms only when two original photons interact with a non-centrosymmetric material—a material without a central point about which the molecules can be reflected. For example, a pizza in five slices is non-centrosymmetric, while a pizza in six slices is centrosymmetric. The net number of photons generated by SHG depends on the orientation of this non-centrosymmetric material.
This property is what allows the researchers to use SHG in their microscope. First, to reveal underlying chemical structures of a glass-water interface, the researchers wetted the surface, causing water molecules to selectively cluster on areas to which they were more attracted, akin to how shaking a tray of sand with a strip of glue collects sand along that strip. Next, the researchers shined light onto the whole surface, showering it with photons. Because water molecules are non-centrosymmetric, the amount of SHG depends on the orientation of the water molecules, which in turn depends on the surface’s underlying chemical structure. By measuring the output of SHG photons coming from different positions on the curved glass surface, the researchers were able to reconstruct the chemical properties of the different parts of the surface in 3D.
Since this microscope captures the entire surface all at once, it captures images more than 5000 times faster than current second harmonic microscopes. Most importantly, this increase in speed does not sacrifice image quality. Sylvie Roke, the principle investigator of the project, was herself pleasantly surprised about the sensitivity of the microscope. “What I find amazing is that there are so few oriented water molecules—just one nanometer of oriented water molecules—and I can see them so brightly,” Roke said.
The advantages of this new microscope hardly end there. Sapun H. Parekh of the Max Planck Institute for Polymer Research in Mainz, Germany, added that this microscope offers the additional benefit of being operational even without precise control of the environment—unlike most other methods to study surface chemistry that require ultrahigh vacuums to operate. One straightforward application of this microscope could be to improve the industrial manufacturing of surfaces, such as solar cells. By examining the surface chemistry of manufactured surfaces in real-time, we can get immediate feedback on the manufacturing processes, making its optimization easier and more efficient.
Roke foresees even more exciting applications for the new microscope, such as imaging biological processes like neurons firing. “Neurons communicate by electrostatic signals that travel across the membrane,” Roke said. “What if I could use this method to [image the] membrane of the neuron while it is signaling to other neurons, without modifying the neuron?” This would be an impressive step forward to understanding how neurons function, since current technologies only allow us to observe neurons by modifying them.
While there are still some nuances in second harmonic microscopy that remain to be explicitly addressed, the researchers have certainly accomplished a proof of concept with the new instrument. As Parekh puts it, “What they showed is not the end application—it’s just the beginning.”