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Principle of SFG spectroscopy

Powerfull nondestructive technique for in‑situstudies of surfaces and interfaces

和频(SFG)光谱

Ekspla is proud of the employment of a customized version of the EKSPLA SFG spectrometer.

Despite the huge progress humanity has made over several centuries, some diseases still do not have a remedy and lots of things are still unknown about them. Neurodegenerative diseases like Alzheimer’s and Parkinson’s are two of them. Understanding on how these diseases are evolving could help to research new methods and aids in fighting them.

Researchers found that more than 30 various diseases, neurodegenerative diseases like Alzheimer’s and Parkinson’s among them, are associated with amyloids. Amyloids are defined as β-sheet-rich structures consisting of vast amounts of self-assembled proteins. The aggregation process is very complex; it depends on a variety of parameters such as the sequence of the protein and its concentration, the acidity of the solution, salt concentration, and even on the interaction with biological surfaces, e.g., a cell membrane. Studies of that process are the object of various research projects. A group of scientists from the Department of Organic Chemistry at the Center for Physical Sciences and Technology (FTMC, Vilnius, Lithuania) are among them. Employing innovative laser spectroscopy methods like vibrational sum-frequency generation, they found that aggregates with a parallel and antiparallel β-sheet structure together with smaller unordered aggregates and denaturated proteins are adsorbed to both lipid/water and air/water interfaces.

Langmuir, the highly reputed ACS (American Chemical Society) journal, in it’s May issue, published an interesting study called “Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces” which was featured on the journal’s cover.

Source: S. Strazdaitė, E. Navakauskas, J. Kirschner, T. Sneideris, and G. Niaura. Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces. Langmuir 2020, 36, 17, 4766–4775.

The cover art illustrates the technique employed in the article – a surface-specific technique, vibrational sum-frequency generation spectroscopy. Ekspla is proud that a customized version of the EKSPLA SFG spectrometer was employed in this study.

WHAT IS SFG SPECTROSCOPY?

Sum Frequency Generation Vibrational Spectroscopy is a powerful and versatile method for in-situ investigation of surfaces and interfaces. SFG is a second order nonlinear process, which is allowed only in media without inversion. In a SFG-VS experiment a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). The SFG signal is generated in the visible spectral range. Detectors used to detect the visible spectrum are PMT or CCD symmetry.

At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SF signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions.

WHAT ARE THE MAIN COMPONENTS OF AN SFG SPECTROMETER SYSTEM?

A sum-frequency generation (SFG) spectrometer is based on a picosecond pump laser and optical parametric generator (OPG) with a difference frequency generation (DFG) extension. The system uses a solid state mode-locked Nd:YAG laser featuring high pulse duration and energy stability. Fundamental laser radiation splits into several channels in the multichannel beam delivery unit. Two of these beams pump OPG and DFG. A small part of the laser output beam, usually with doubled frequency (532 nm), directs to the VIS channel of the SFG spectrometer. The DFG output beam pumps the spectrometer IR channel.

A detection system, installed in the “Spectroscopy module”, consists of a monochromator with high stray light rejection and a gated PMT based SF signal detector. The feature of such a design is the ability to perform measurements in room lighting. A second parallel detection channel is available as an option.

Schematic layout of SFG Classic spectrometer.

HOW IS THE SYSTEM CONTROLLED?

A PC with a dedicated software application controls all system components. The program contains many useful instruments for automatic SFG spectra recording, dynamics monitoring, X-Y sample mapping, azimuthal scanning and system parameter monitoring.

OTHER APPLICATIONS

SFG spectroscopy is a versatile tool, employed in various applications among different disciplines such as life sciences, chemistry and material research. A list of the latest publications can be found here.

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