New Imaging Technique in Surface Chemistry

As being a surface chemist, I keep on tracking the signs of progress made so far in the Surface Chemistry field. Recently, I found good news about the imaging tool in surface chemistry. Here is the story:

Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) Laboratory for Fundamental BioPhotonics (LBP) have developed a microscope that can track, in real time, 3D spatial changes in the molecular structure and chemistry of confined systems, such as curved surfaces and pores to understand the geological, catalytic, biological and chemical processes which are driven by surface chemical heterogeneities, electrostatic fields and flow. They predict that this may enable the further development of new materials and microtechnology.

“An optical imaging tool to visualize surface chemistry in real time has been developed. This system basically images the interfacial chemistry in the microscopically confined geometry of a simple glass micro-capillary. The glass is covered with hydroxyl (-OH) groups that can lose a proton, a much-studied chemical reaction that is important in geology, chemistry, and technology. A 100-micron long capillary displayed a remarkable spread in surface OH bond dissociation constant of a factor of a billion.”

The developed microscope was used to image the surface chemical structure of the inside of a glass microcapillary. Surface potential maps were designed from the millisecond images, and the chemical reaction constant of each 188nm-wide pixel was evaluated. Amazingly, this very simple system which is used in many devices displayed a stunning spread in surface heterogeneity. The researchers' findings have been published in Science. It is believed that this method will be a plus point in understanding fundamental (electro)chemical, geological and catalytic processes and for building new devices.

Second-harmonic imaging

Imaging of surface potential and chemical process at the surface.
Image: taken from Google (28th July, 2017)

Sylvie Roke, director of the Julia Jacobi Chair of Photomedicine at EPFL, has developed a unique set of optical tools to study water and aqueous interfaces on the nanoscale. She uses second-harmonic and sum-frequency generation, which are optical processes in which two photons of a certain color are converted into a new color. "The second-harmonic process involves 1000 nm femtosecond photons i.e., 0.00000000000001-second bursts of light -- being converted into 500 nm photons, and this occurs only at interfaces," says Roke. "It is therefore ideal for interfacial microscopy. Unfortunately, the process is very inefficient. But by using a number of optical tricks, such as wide field imaging and light shaping, we were able to enhance both the imaging throughput and the resolution, bringing the time to record an image down from minutes to 250 milliseconds."

Surprising surface chemistry

The researchers then imaged the deprotonation reaction of the inner silica capillary/water interface in real time. Silica is one of the most abundant minerals on earth, and its interaction with water shapes our climate and environment. Although many researchers have characterized the properties of the silica/water interface, there is no consensus on its chemical reactivity. Roke continues: "Our data shows why there is a remarkable spread in surface reactivity, even on a very small portion of a capillary. Our data will help in the development of theoretical models that are more effective at capturing this surprising complexity. In addition, our imaging method can be used for a wide variety of processes, such as for analyzing the real-time functioning of a fuel cell, or for seeing which structural facet of a mineral is most chemically active. We could also gain more insight into nanochannels and both artificial and natural pores.

References
 1. Carlos Macias-Romero, Igor Nahalka, Halil I. Okur, Sylvie Roke. Optical imaging of surface chemistry and dynamics in confinement. Science, July 2017 DOI: 10.1126/science.aal4346
2. Science Daily, July 28, 2017 Issue (www.sciencedialy.com)