The inside of a living cell is a surprising patchwork of environments. For example, pH levels can vary dramatically in different organelles, despite them being just a couple hundred nanometers apart. pH changes within the cell can be a signal of disease or may help drive processes such as cell division and growth. Scientists have managed to identify differences in pH across cell types—and even between different organelles—by measuring pH in batches of isolated cells and organelles. But real-time pH monitoring inside live single cells has been elusive.
Now, a team at Pohang University of Science and Technology has developed a probe that can accomplish this task. The device consists of a nanosized filament that slips into and out of a cell’s organelles without harming the delicate membranes that hold the cell together (Nano Lett., 2022 DOI: 10.1021/acs.nanolett.2c02185).
“Monitoring pH dynamics in cellular compartments allows [us] to obtain comprehensive information on a single living cell,” say study authors Seung Soo Oh and Jung Ho Je in an email.
The key feature of the probe is a nanowire made from poly(vinylbenzyl azide), a polymer thin enough—about 200 nanometers in diameter—yet strong enough to pierce cleanly through a cell’s membrane without damaging it. The wire is coated with pH-sensitive fluorescein molecules—whose fluorescence intensity shifts depending on the pH of their environment—and then attached to the end of an optical fiber.
The researchers can send a laser through one end of the optical fiber down to the nanowire tip on the other end. There, the laser hits the fluorescein molecules and causes them to light up. The fluorescein’s luminescence travels back up the fiber to be read by a spectrometer that translates the light’s intensity to a pH level.
The team used the device to compare pH in the cell’s nucleus with that in the surrounding liquid, known as the cytosol, and found that the nucleus was significantly more acidic. Also, while monitoring the cell division process, the probe sensed that pH inside the nucleus rises and falls as a cell divides. The researchers write that this is the first time this pH shift, which happens independent of the rest of the larger cell, has been observed directly in mammalian cells.
“It’s a clever way to use this nanowire,” says Wenbin Lin, a chemist at the University of Chicago who has worked on intracellular pH sensing. “They can pinpoint exactly where they want to stick this fiber tool.”
The probe could detect how agents such as chemotherapy drugs or hormones affect pH levels, and therefore cells’ functioning, says Lin. The same probe design could also be adapted to measure many other intracellular parameters important to the cell’s physiology, such as sodium or magnesium flux, he adds.