Using tiny amounts of silver to find even tinier amounts of arsenic

Ensuring water is safe from arsenic is important if we are to avoid poisoning millions of people, and the task has been made easier after a French team found a new way to detect it at even very low concentrations.

Arsenic occurs naturally in the earth’s crust, but can get into drinking water and soils via groundwater. The World Health Organization recommends that arsenic (As) levels in drinking water do not exceed 10 micrograms per litre: a microgram is a millionth of a gram.

It’s difficult to detect  at this low, which makes arsenic monitoring expensive and time-consuming. Worldwide, it’s likely that 140 million people have drinking water with arsenic levels above the recommended amounts.

What’s more, there are two commonly occurring types of arsenic – As(III) and As(V) – which are hard to differentiate. As(III) is more toxic than As(V), so being unable to tell which is which makes it harder to judge the danger of arsenic contamination.

The researchers, who have published their findings in the Journal of Applied Physics, have developed sensors that can spot arsenic at concentrations of one microgram per litre, and discern the two forms.

The sensors are made by submerging glass plates in a liquid that contains dissolved silver, which provokes a chemical reaction that deposits tiny amounts of silver onto the glass.

Photo of silver film on glass plate next to microscope image of silver surface
Left: A sample of a silver nanostructured film, deposited by the electroless technique on a glass plate. Right: A zoom in on the silver film at 1.5 micrometre scale, by atomic force microscopy. Credit: Dominique Vouagner

The silver forms nanometre-sized structures that hold arsenic-containing molecules.

Then, the arsenic can be spotted with a technique called surface-enhanced Raman spectroscopy (SERS): hitting the surface with a laser, and spotting arsenic through the unique way it scatters light.

SERS can also be tuned to spot other contaminants.

“By using SERS, we can detect and speciate pollutants even at the lowest concentration,” says study co-author Dominique Vouagner.

Read more: Toxic groundwater threatens millions

Their method for making the sensors – called an ‘electroless’ method – also rendered them much more sensitive than the conventional way of making SERS sensors.

Even better: it’s fairly easy and safe to do.

“Our technique for developing this SERS substrate makes it simple to manufacture because the electroless films can be easily deposited on various substrates,” says Vouagner.

“Plus, the starting compounds have low environmental toxicity, which is a benefit for detection measurements in natural as well as potable water.”

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