Researchers have developed a number of potassium ion (K⁺) probes to detect fluctuating K⁺ concentrations during a variety of biological processes. However, such probes are not sensitive enough to detect physiological fluctuations in living animals and it is not easy to monitor deep tissues with short-wavelength excitations that are in use so far. In a new report, Jianan Liu and a team of researchers in neuroscience, chemistry, and molecular engineering in China, describe a highly sensitive and selective nanosensor for near infrared (NIR) K⁺ ion imaging in living cells and animals. The team constructed the nanosensor by encapsulating upconversion nanoparticles (UCNPs) and a commercial potassium ion indicator in the hollow cavity of mesoporous silica nanoparticles and coated them with a K⁺ selective filter membrane. The membrane adsorbed K⁺ from the medium and filtered away any interfering cations. In its mechanism of action, UCNPs converted NIR to ultraviolet (UV) light to excite the potassium ion indicator and detect fluctuating potassium ion concentrations in cultured cells and in animal models of disease including mice and zebrafish larvae. The results are now published on Science Advances.

The most abundant intracellular cation potassium (K⁺) is extremely crucial in a variety of biological processes including neural transmission, heartbeat, muscle contraction and kidney function. Variations in the intracellular or extracellular K⁺ concentration (referred herein as [K⁺]) suggest abnormal physiological functions including heart dysfunction, cancer, and diabetes. As a result, researchers are keen to develop effective strategies to monitor the dynamics of [K⁺] fluctuations, specifically with direct optical imaging.

Most existing probes are not sensitive to K⁺ detection under physiological conditions and cannot differentiate fluctuations between [K⁺] and the accompanying sodium ion ([Na⁺]) during transmembrane transport in the Na⁺/K⁺ pumps. While fluorescence lifetime imaging can distinguish K⁺ and Na⁺ in water solution, the method requires specialized instruments. Most K⁺ sensors are also activated with short wavelength light including ultraviolet (UV) or visible light—leading to significant scattering and limited penetration depth when examining living tissues. In contrast, the proposed near-infrared (NIR) imaging technique will offer unique advantages during deep tissue imaging as a plausible alternative.

Designing the K⁺ nanosensor and characterizing its structureTo engineer the nanosensor, Liu et al. encapsulated upconversion nanoparticles (UCNPs) and a commercial K⁺ indicator—potassium-binding benzofuran isophthalate (PBFI) into the core of mesoporous silica nanoparticles (MSNs). The UCNPs were able to convert NIR light into UV light and excite the acceptor of the K⁺ indicator through luminescence resonance energy transfer. They shielded the outer surface of silica nanoparticles with a thin layer of K⁺ selective filter membrane with micropores created from carbonyl oxygen for specificity. The setup favored the free transfer of K⁺ through the membrane pore, while preventing other biologically relevant cations from diffusing through. The technique allowed them to detect slight fluctuations in [K⁺] in the solution. The team used transmission electron microscopy (TEM) to observe the well-controlled structure and appearance of the nanoparticles during each step of nanosensor construction. Dynamic light scattering confirmed the presence of a filter membrane on the surface of the shielded nanosensor.

Performance of the nanosensor in water and during [K⁺] fluctuations in cells.The team tested the enhanced sensitivity of the shielded nanosensor in a physiological range (0 to 150 mM) and showed a 12-fold increase in fluorescence intensity compared to unshielded nanosensors. The K⁺ probes had to display high selectivity against Na⁺, which Liu et al. verified using the shielded nanosensor by rapidly detecting consistent fluorescence sensitivity to fluctuating [K⁺], while remaining unaffected by increasing [Na⁺].

Since living cells rely on the sodium-potassium adenosine triphosphatase (Na⁺/K⁺ pump) to maintain a steep [K⁺] gradient across their plasma membrane, the process is partially responsible for the cell’s energy expenditure. Defects in cellular energy metabolism can lead to a loss of the [K⁺] gradient, while giving rise to extracellular [K⁺] known as [K⁺]_0, which the scientists monitored to obtain a valuable indicator of cell viability and growth. Thereafter, they increased the specificity of the nanosensor to detect cell death or proliferation rates by grafting polyethylene glycol (PEG) on the surface of nanosensors in a culture medium containing the human embryonic kidney 293 cell line. They then optimized the protocol by anchoring large numbers of nanosensors onto cell membranes using streptavidin-conjugated nanosensors to biotin-modified cells. The results highlighted improved sensitivity of shielded nanosensors to continuously monitor the K⁺ efflux.

Spreading waves in the mouse intact brainThe team then applied the shielded nanosensor to investigate cortical spreading depression (CSD) in the mouse brain as a wave-like propagation of neural activity. The process typically involves a slow propagation release of K⁺ in the cortical surface and could be triggered in the mouse brain via potassium chloride (KCl) incubation. The scientists simultaneously monitored the local field potential and optical signal through the surgical cranial window and observed a wave of increasing [K⁺]₀ propagate gradually across the cortex after stimulation. Liu et al. did not observe a wave in mice injected with unshielded nanosensors, indicating the importance of the outer filter for improved sensitivity of the nanosensor. The recorded wave velocity did not vary significantly from the values obtained using blood oxygen-level dependent magnetic resonance imaging (MRI) in patients with migraine aura.

To extend applications of the nanosensor, Liu et al. monitored neuronal calcium levels and extracellular potassium concentrations using zebrafish larvae. While a large increase in the extracellular potassium concentration can cause intense neuronal activation to cause CSD and epilepsy, no direct evidence exists to show changes in extracellular potassium during the disease. The team therefore engineered a disease model using zebrafish larvae to increase extracellular potassium concentrations and observed disease characteristic neuronal activation in specific brain regions.

In this way, Jianan Liu and colleagues engineered a potassium ion nanosensor with extremely high sensitivity and selectivity. The external coating of a selective filter membrane enhanced the selectivity, sensitivity, and kinetics of the device for rapid and quantitative [K⁺] detection in living cells and intact brains. The shielded nanosensor will have broad applications in brain research to improve the understanding of abnormal [K⁺]-related diseases. The method alongside optical fiber-based endoscope and photometry will allow real-time potassium imaging in freely moving animals.

A research team led by North Carolina State University has engineered a new catalyst that can more efficiently convert ethane into ethylene, which is used in a variety of manufacturing processes. The discovery could be used in a conversion process to drastically reduce ethylene production costs and cut related carbon dioxide emissions by up to 87%.

“Our lab previously proposed a technique for converting ethane into ethylene, and this new redox catalyst makes that technique more energy efficient and less expensive while reducing greenhouse gas emissions,” says Yunfei Gao, a postdoctoral scholar at NC State and lead author of a paper on the work. “Ethylene is an important feedstock for the plastics industry, among other uses, so this work could have a significant economic and environmental impact.”

“Ethane is a byproduct of shale gas production, and the improved efficiency of our new catalyst makes it feasible for energy extraction operations in remote locations to make better use of that ethane,” says Fanxing Li, corresponding author of the paper and an associate professor and University Faculty Scholar in NC State’s Department of Chemical Engineering.

“It is estimated that more than 200 million barrels of ethane are rejected each year in the lower 48 states due to the difficulty of transporting it from remote locations,” Li says. “With our catalyst and conversion technique, we think it would be cost effective to convert that ethane into ethylene. The ethylene could then be converted into liquid fuel, which is much easier to transport.

“The problem with current conversion techniques is that you can’t scale them down to a size that makes sense for remote energy extraction sites—but our system would work well in those locations.”

The new redox catalyst is a molten carbonate promoted mixed metal oxide, and the conversion process takes place at between 650 and 700 degrees Celsius with integrated ethane conversion and air separation. Current conversion techniques require temperatures higher than 800 degrees C.

“We estimate that the new redox catalyst and technique cut energy requirements by 60-87%,” Li says.

“Our technique would require an initial investment in the installation of new, modular chemical reactors, but the jump in efficiency and ability to convert stranded ethane would be significant,” Li says.

The paper, “A Molten Carbonate Shell Modified Perovskite Redox Catalyst for Anaerobic Oxidative Dehydrogenation of Ethane,” will be published April 24 in the journal Science Advances.