Sweet! Sugar-Coated Probe Yields Better Acid Test

By Marcia Goodrich | Published

A new type of nontoxic fluorescent probe could make it much easier to detect low pH in living cells.

The human body is engaged in a constant tightrope walk to maintain the right pH, because when our cells’ acid-alkaline balance goes wrong, it can go wrong in a big way.

Abnormal pH—in particular, abnormally low, acid pH—is a marker for maladies ranging from cystic fibrosis, stroke and rheumatoid arthritis to cancer and Alzheimer’s. Now, Michigan Technological University chemist Haiying Liu has developed new tools that could make it much easier to detect low pH in living cells. The discovery is the focus of a new study published in ACS Sensors (DOI: abs/10.1021/acssensors.7b00137.)

Measuring Acidity

To measure pH, researchers and medical personnel use fluorescent dyes, called probes, that glow in acidic conditions when activated by fluorescent light. The probes are used for diagnostic imaging—to visualize blood vessels and the digestive tract, for example—and they can help surgeons remove diseased tissue, including tumors. However, these probes are not perfect.

Haiying Lius acid-sensitive luminescent probe glows progressively brighter in living cells as the pH decreases, both under conventional fluorescence (CF) and under single-photon upconversion luminescence (FUCL).

The problems stem from the high-energy light they need to fluoresce. Light at these wavelengths not only switches on the probe; it can also cause biological structures to shine like glow sticks. This autoluminescence makes it hard to tell the difference between the probe and the surrounding tissues. High-energy light can even damage cells and cause the probe’s initial shine to fade into darkness, a process called photobleaching.

To fix these issues, you would need a probe that works in low-energy, near-infrared light. So chemistry professor Liu and his team developed two of them.

These probes have a lot going for them. They aren’t subject to photobleaching and autoluminescence. Plus, since near-infrared light can penetrate deep into tissues, they could give scientists and physicians a better look inside the body.

Sweet and Fluorescent

The chemistry of the two probes was inspired by rhodamine, which has been used in biotech applications for decades. “But the problem with rhodamine is that it can damage cells,” Liu said. “We needed a probe that was compatible with living tissues.”

So, Liu’s team sweetened its next-generation probes with a simple sugar found in many fruits: mannose. “We introduced the sugar to make the probes water soluble and less toxic,” said Liu. “That helps them penetrate cell membranes and makes them much friendlier.”

Liu’s team designed its probes to emit light in two different ways. First, they fluoresce in the conventional way when exposed to near-infrared light. In addition, they glow in the near-infrared spectrum at even lower energies, thanks to a different type of chemical response called single-photon frequency up-conversion luminescence (FUCL).

Tests under both types of light have been promising, showing that both probes are highly sensitive to pH and very gentle to living cells, even at high concentrations. Though the probes are similar chemically, one is slightly more sensitive to pH in cell cultures.

Next, Liu will study how well they detect low pH in mice, in hopes that the probes can help advance medical science and even save lives.

An article on their work “Luminescent Probes for Sensitive Detection of pH Changes in Live Cells through Two Near-Infrared Luminescence Channels,” was published online June 27 in ACS Sensors.

The coauthors are Liu, research scientist Shuwei Zhang, faculty member Christo Christov and PhD students Jianheng Bi, Fei Xie and Mingxi Fang of Michigan Tech’s Department of Chemistry; Tzu-Ho Chen, Fen-Tair Luo and Hsien-Ming Lee of the Institute of Chemistry, Chemical Biology and Molecular Biophysics in Taiwan; Associate Professor Feng Zhao and PhD students Avik Ghosh and Zichen Qian of Michigan Tech’s Department of Biomedical Engineering; and Jon Ainsley of Northumbria University, U.K.

Last Modified 2:44 PM, August 7, 2017

Michigan Technological University is a public research university, home to more than 7,000 students from 60 countries around the world. Founded in 1885, the University offers more than 120 undergraduate and graduate degree programs in science and technology, engineering, forestry, business and economics, health professions, humanities, mathematics, and social sciences. Our beautiful campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway and is just a few miles from Lake Superior.