According to a new report in the journal Science, Dartmouth researchers have developed a self-powered pump that uses natural light and chemistry to target and remove specific water pollutants.
As water enters the pump, a wavelength of light activates a synthetic molecular receptor designed to bond to negatively charged ions, or anions, a class of pollutants linked to metabolic disruptions in plants and animals. A second wavelength deactivates the receptors as water exits the pump and causes them to release the contaminants, trapping them in a non-reactive substrate until they can be safely discarded.
The study’s senior author and chair of the Department of Chemistry at Dartmouth, Professor Ivan Aprahamian, said, “This is a proof of concept that you can use a synthetic receptor to convert light energy into the chemical potential for removing a contaminant from a waste source.”
The pump is currently calibrated to well-known pollutants such as chloride and bromide. Professor Aprahamian said the researchers are working to expand its use to target other anion-rich pollutants, such as radioactive waste and the phosphates and nitrates in agricultural runoff that cause massive dead zones.
“Ideally, multiple receptors can be in the same solution, and they can be activated with different wavelengths of light,” Professor Aprahamian said. “You can also target and collect each of these anions separately.”
The synthetic receptor’s unusual ability to trap and discharge negatively charged molecules allowed the researchers to control the flow of chloride ions from a low-concentration solution on one end of a U-shaped tube to a high-concentration solution on the other. Over a 12-hour period, the study reports, they moved eight per cent of chloride ions against the concentration gradient across a membrane embedded with the synthetic receptors.
The researchers focused on chloride for two reasons. During winter, stormwater laden with road salt raises chloride levels in waterways, harming plants and animals. Secondly, the transport of chloride ions also plays a key role in healthy cell functioning. The disease cystic fibrosis is caused by cells being unable to pump out excess chloride. The trapped ions cause dehydration in cells, leading to a buildup of thick mucus in the lungs, among other organs.
In absolute terms, the chloride ions were driven almost 1.4 inches – the width of the membrane separating both ends of the tube. Relative to the receptor’s tiny size, they covered an impressive distance, fuelled by light alone. “It’s the equivalent of kicking a soccer ball the length of 65,000 football fields,” Professor Aprahamian said.
Professor Aprahamian’s lab has long focused on synthetic compounds known as hydrazones, which switch on and off when exposed to light. During the COVID pandemic, PhD student Baihao Shao proposed enhancing the hydrazone receptor to collect and release target anions when switched on and off.
Professor Aprahamian tried to dissuade him. “I told him that while it is a great idea, I do not think it will be competitive with the other impressive photoswitchable receptors in the literature,” he said. “Luckily, Baihao ignored me, and he went ahead and designed the receptor.”
Professor Aprahamian says the receptor can be controlled by a renewable energy source – light – and is also relatively easy to make and modify. Researchers created the receptors by stitching them together using ‘click chemistry,’ a Nobel Prize-winning technique chemist Barry Sharpless helped invent years after graduating from Dartmouth.
In another Nobel connection, the study demonstrates the potential of molecular machines eight years after three chemists received the 2016 Nobel Prize in Chemistry for their work developing synthetic versions. Molecular machines are naturally abundant, powered by ATP in animal cells and the sun in plant cells. In humans, tiny molecular machines carry out much of the work within cells, from replicating DNA to ferrying materials across the cell membrane.
For decades, scientists have tried to replicate these miniaturised workhorses outside of the body, with dreams of applying them to tasks like environmental cleanup, drug delivery, and the diagnosis and treatment of disease. However, artificial molecular machines have proven easier to design on paper than to implement in real life.
“We want to mimic such biological processes, using sunlight as the energy source to create autonomous and self-sustaining filtration systems,” Professor Aprahamian said.
Image: Ivan Aprahamian, Dartmouth College
