Soft X-rays observe carbon-based structures in living cells

Wednesday, 12 June, 2024

Soft X-rays observe carbon-based structures in living cells

Japanese researchers have used a powerful laser, called a soft X-ray free electron laser (SXFEL), to emit ultrafast pulses of illumination at the speed of femtoseconds, or quadrillionths of a second. This enabled them to capture images of carbon-based structures in living cells — which have not been seen before with other instruments — before the soft X-ray radiation damaged them.

While hard X-rays are used when you go through airport security or need to have a broken limb examined, soft X-rays are more typically restricted to research — from studying biology and chemistry to minerals and meteorites. Soft X-rays are able to provide chemical information about samples and detailed images at the subcellular level, but their use has been limited due to the very specialised equipment required, as well as the damage they cause to living cells.

A research team led by the University of Tokyo (UTokyo) has now constructed a new soft X-ray microscope through which they could view live mammalian cells, taking images of carbon structures within these cells. Carbon is one of the main elements for life, so this provides a new window into a vital part of ourselves.

The microscope has two key components: a soft X-ray free electron laser and highly precise Wolter mirrors, a type of mirror widely used in X-ray telescopes for observing space. The mirrors were made using technology created by lead author Satoru Egawa, an assistant professor at UTokyo.

“A soft X-ray free electron laser provided pulse illumination at the speed of tens of femtoseconds (with one femtosecond being one-millionth of one-billionth of a second),” Egawa said. “The ultrashort duration of the radiation pulses enabled us to take an image before the structure of the living cell was altered by radiation damage.

“We used Wolter mirrors for illumination and imaging. These mirrors provide a wide field of view, can withstand irradiation from the powerful lasers and exhibit no colour distortion, making them ideal for observing samples at various wavelengths.”

Although soft X-ray free electron lasers have previously been used to study smaller viruses and bacteria, mammalian cells were too big to be studied this way. However, by using Wolter mirrors, the team could achieve a wider field of view and use a thicker sample holder which could hold larger cells. The resulting images, published in the journal Optica, showed details about carbon content in the cells that had not been seen through other methods, such as electron microscopy and fluorescence microscopy.

“It was surprising for us to find a carbon pathway between the nucleolus (a structure in the cell’s nucleus, involved in cell function and survival) and the nuclear membrane (which envelops the nucleus), which had not been observed with visible light microscopes,” Egawa said.

Brighter soft X-ray free electron lasers are also available, which would enable even clearer images with less grainy ‘noise’. By adding brighter lasers and more precise Wolter mirrors, the team hopes to upgrade the microscope so that it can observe more biochemical elements. With this, it could also help to illuminate some of the vital reactions and interactions which take place within living cells.

Image caption: Soft X-ray pulses generated from SXFEL undulators, concentrated by Kirkpatrick-Baez mirrors (not shown) and a condenser Wolter mirror, are directed towards a sample under observation. The transmitted light reflects off the objective Wolter mirror, forming an image on a CCD detector. An aperture downstream of the objective mirror eliminates unwanted scattered light. The cells and the culture medium are enclosed in a vacuum-sealed liquid cell holder. Cells are located with visible light transmission imaging using the Wolter mirrors before soft X-ray imaging. Image© 2024 Egawa et al/Optica.

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