Researchers explain why nanotubes prefer potassium to sodium

Researchers explain why nanotubes prefer potassium to sodium

Xiao Cheng Zeng

It’s a counterintuitive finding that has puzzled researchers: Why do bigger atoms sometimes fare better at passing through the smallest of channels?

UNL researchers have co-authored a new study that helps reveal the chemistry behind the mystery, offering answers that may inform efforts to purify water and transport pharmaceutical drugs.

The study modeled the behavior of water-carried potassium and sodium ions -- positively or negatively charged atoms -- in artificial channels similar to those that regulate the passage of ions throughout cells in the body.

Biochemists have largely explained the workings of natural ion channels, which help balance the highly charged cocktail of potassium, sodium and other electrolytes that keep the heart pumping and nerves firing.

As reported in the journal Proceedings of the National Academy of Sciences, UNL chemists Xiao Cheng Zeng and Joseph Francisco led a study of two synthetic counterparts that could be used to filter or detect nanoparticles.

The team found that the smaller sodium ions accumulate more stable, robust “hydration shells” – essentially coats of water molecules – than do the larger potassium ions. Whereas potassium ions take on a single, short-lived shell, sodium ions attract two shells of molecules that forge longer-lasting bonds.

This extra baggage means that sodium ions require more energy to move from large volumes of water to the gateway of an ion channel. Even if they reach it, Zeng said, their shells can slow or even impede their migration when the water molecules begin binding even more tightly in the nanoscopically small tunnels.

In contrast, the size of the larger potassium ions can actually work in their favor, Zeng said. Because these ions occupy more of the channel, they tend to shuck off some of their looser water molecules as they enter, he said.

Having worked for years in a football-crazed state, Zeng compared the potassium ion to an offensive lineman and the sodium to a running back. The former tends to receive attention from fewer, less intense followers, while the latter often gets swarmed by autograph-seeking fanatics, he said.

“So when the running back moves through the tunnel, he has to work through a lot of surrounding fans who won’t let go very easily,” said Zeng, an Ameritas University Professor of chemistry. “It’s harder for him to move, or maybe he doesn’t have as much strength to move the fans. On the other hand, the lineman is bigger; when he goes into the tunnel, some of the fans have to move aside to make room.”

Zeng and Francisco, the dean of UNL’s College of Arts and Sciences, examined this ion selectivity in numerous carbon nanotubes and a synthetic organic nanopore that was co-developed by Zeng in 2012. The carbon nanotubes boast a smooth interior but are difficult to fabricate in a uniform size; the organic nanopore features a jagged interior of dangling molecules but can be produced with extreme precision.

After modeling various scenarios through the Holland Computing Center, the authors pinpointed a carbon nanotube configuration that is about 20 times better than the organic nanopore at selecting potassium while blocking sodium. The team designated it “likely one of the best” artificial nanochannels for this purpose.

According to Zeng, these nanochannels could eventually become viable candidates for the desalination of saltwater, an ever-appealing option as the world’s population continues to rise and its sources of freshwater diminish. They might also assist the removal of contaminants in drinking water, he said.

The channels could further lead to more precise transportation of pharmaceutical drugs, Zeng said, particularly as researchers explore how to design channels that filter or select species beyond potassium and sodium.

“Filtering a greater variety of ions is certainly one goal for us,” said Zeng. “We’re collaborating with other institutions to functionalize the inner surfaces of these pores and study the effects of different surface designs on selective ion transport.”

Zeng and Francisco co-authored the study with Hui Li, a former UNL postdoctoral researcher now at the Chinese Academy of Sciences. The team received support from the National Science Foundation and the National Science Foundation of China.