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Researchers at the University of Tokyo have built a microscope that can detect a signal over an intensity range 14 times wider than conventional microscopes. Moreover, the observations are made label-free, that is, without the use of additional dyes.

This means the method is gentle on cells and adequate for long-term observations, holding potential for testing and quality control applications in the pharmaceutical and biotechnology industries. The findings are published in the journal Nature Communications.

Microscopes have played a pivotal role in the development of science since the 16th century. However, progress has required not only more sensitive and accurate equipment and analysis, but also more specialized ones. Therefore, modern, cutting-edge techniques have had to straddle trade-offs.

Quantitative phase microscopy (QPM) leverages forward-scattered light and can detect structures at the microscale (in this study, over 100 nanometers), but not smaller. Consequently, this technique has been primarily used to take static pictures of relatively complex cell structures. Interferometric scattering (iSCAT) microscopy, on the other hand, exploits back-scattered light and can detect structures as small as single proteins.

As such, it can be used to "track" single particles, allowing insight into dynamical changes within the cell, but it cannot provide the comprehensive view that QPM can.

"I would like to understand dynamic processes inside living cells using noninvasive methods," says Kohki Horie, one of the first authors.

Thus, the research team, including Horie, Keiichiro Toda, Takuma Nakamura, and Takuro Ideguchi, set out to investigate whether measuring both directions of light simultaneously could overcome the trade-off and reveal a wide range of sizes and motions from the same image.

To test the idea and confirm their newly built microscope was working as hoped, the researchers set out to observe what happened during cell death. They recorded one image encoding information from both forward and backward-traveling light.

"Our biggest challenge," Toda, another first author, explains, "was cleanly separating two kinds of signals from a single image while keeping noise low and avoiding mixing between them."

As a result, they were able to quantify not only the motion of cell structures (micro) but also of tiny particles (nano). Additionally, by comparing the forward and back-scattered light, they could also estimate each particle's size and refractive index, a property describing how much light bends or scatters when passing through particles.

"We plan to study even smaller particles," Toda says, already thinking about future research, "such as exosomes and viruses, and to estimate their size and refractive index in different samples. We also want to reveal how living cells move toward death by controlling their state and double-checking our results with other techniques."