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Three-Dimensional Imaging of Surface Chemistry and Dynamics in Confinement

Researchers at EPFL have created an optical imaging tool to envisage surface chemistry in real time. They imaged the interfacial chemistry in the microscopically confined geometry of a basic glass micro-capillary.

The glass is covered with hydroxyl (-OH) groups that can lose a proton – a well-researched chemical reaction that is crucial in chemistry, geology and technology. A 100 micron long capillary exhibited an extraordinary spread in surface OH bond dissociation constant of a factor of a billion. The research has been reported in Science.

© Alain Herzog / 2017 EPFL

Geological, biological, catalytic and chemical processes are compelled by surface chemical heterogeneities, electrostatic fields and flow. To comprehend these processes and to enable the additional development of new materials and microtechnology, Researchers at EPFL’s Laboratory for Fundamental BioPhotonics (LBP) have engineered a microscope that can monitor, in real time, 3D spatial changes in the molecular structure and chemistry of confined systems, such as pores and curved surfaces. The microscope was used to image the surface chemical structure of the inner side of a glass microcapillary.

Surface potential maps were created from the millisecond images, and the chemical reaction constant of each 188 nm wide pixel was established. Remarkably, this very simple system – which is used in a number of devices – exhibited an extraordinary spread in surface heterogeneity. Their technique will be an advantage for understanding fundamental (electro)chemical, catalytic and geological processes and for developing new devices.

Second-harmonic imaging

Sylvie Roke, Director of the Julia Jacobi Chair of Photomedicine at EPFL, has created a unique set of optical tools to examine water and aqueous interfaces on the nanoscale. She uses second-harmonic and sum-frequency generation, which are optical methods in which two photons of a specific color are converted into a new color.

The second-harmonic process involves 1000 nm femtosecond photons – that is, 0.00000000000001-second bursts of light – being converted into 500 nm photons, and this occurs only at interfaces. 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.

Sylvie Roke, Director of the Julia Jacobi Chair of Photomedicine at EPFL

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 plentiful minerals on earth, and its interaction with water shapes earth’s climate and environment. Although a number of Researchers have categorized the properties of the silica/water interface, there is no agreement on its chemical reactivity.

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.

Sylvie Roke, Director of the Julia Jacobi Chair of Photomedicine at EPFL

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