Nanoprobe reveals new details of cell death

01 Feb 2022

Nanjing University sensor monitors changes in refractive index without markers.

Developments in the design of nanoscale probes and sensors promise to reveal new details of the inner workings of cells, without the need for potentially harmful added fluorescent markers.

A project at China's Nanjing University has now designed a compact nanosensor designed to monitor the process of cell death, or apoptosis, occurring as part of normal growth and decay cycles.

Published in Advanced Photonics, the device could provide a new understanding of internal variations during the apoptosis process. It also indicates how future non-fluorescent fiber devices may help to investigate cellular events.

The project envisaged an optical device adhered to the tip of a tapered fiber, allowing the nanodevice to be freely moved, flexibly directed, and able to safely penetrate the plasma membrane and enter biological cells.

"The scale of all-fiber functional modules is on the order of tens or hundreds of micrometers, which is not applicable compared to the sizes of most common biological cells," noted the project in its paper. "Fortunately, one-dimensional nanowires compensate for this deficiency."

The project built a complete functional module by integrating 660-nanometer zinc oxide (ZnO) nanogratings into a single 800-nanometer diameter nanowire. This design offered low toxicity and high biological compatibility, while serving as a waveguiding matrix with strong light confinement in the aqueous environment of a cell.

"With optical nanogratings etched on the ZnO nanowire, the integrated fiber sensor is functionalized with the capability of simultaneously sending and receiving optical signals, being sensitive to changes in the surrounding environment," commented the team.

Novel studies combining photonics with biochemistry

One property of interest for the device's initial trials was refractive index (RI) of the cell nucleus. A gradual increase in RI can occur as apoptosis proceeds, illustrating an increase in molecular density and a decrease in cell volume.

In trials using human breast cancer cells forced into apoptosis with hydrogen peroxide, the nanoprobe was combined with a micromanipulator, microscope and CCD camera to orient the ZnO nanowire into the cell. Low-power 655-nanometer light traveled through the fiber-optic taper and into the cell, and reflected light transmitted back through the taper ws used to calculate an RI value.

The results showed a decrease in the nuclear volume and an increase in the nuclear RI during cell apoptosis, with the final mean RI being 0.0040 higher than that of normal living cells.

This is the first study into tracking and sensing apoptosis with nanogratings in living cells, according to the project team, which believes that a nanoprobe approach is only limited by the size of the cell rather than by the type of cell being examined.

"Quantitative probing and analysis of the RI naturally present in single living cells during apoptosis may offer insight into the life activities within cells," noted the project. "Our strategy creates a precedent that allows a passive fiber nanodevice for in situ dynamic intracellular monitoring, and may provide new ideas for a number of novel studies combining photonics with biochemistry."