New sensor detects valuable rare earth terbium from non-traditional sources

A new luminescent sensor can detect terbium, a valuable rare earth element, from complex environmental samples such as acid mine waste. The sensor, developed by researchers at Penn State, takes advantage of a protein that binds very specifically to rare earth elements and could be exploited to help develop a home supply of these metals, which are used in technologies such as smart phones, electric car batteries. , and energy efficient lighting. An article describing the sensor appeared on August 25 in the Journal of the American Chemical Society.

Terbium, one of the rarest rare earth elements, produces the green color of cell phone screens and is also used in high efficiency lighting and semiconductor devices. However, there are a variety of chemical, environmental and political challenges to obtaining terbium and other rare earth elements from the environment. The development of new sources of these metals also requires robust detection methods, which poses another challenge. For example, the gold standard for detecting rare earth elements in a sample – a type of mass spectrometry called ICP-MS – is expensive and not portable. Portable methods, however, are not as sensitive and do not work well in complex environmental samples, where acidic conditions and other metals can interfere with detection.

“There is currently no national supply chain of rare earth elements like terbium, but they are actually quite abundant in non-traditional sources in the United States, including coal by-products,” acid mine drainage and e-waste, “said Joseph Cotruvo, Jr., assistant professor and Louis Martarano career development chemistry professor at Penn State, member of the Penn State Center for Critical Minerals and lead author of the study. “In this study, we developed a luminescence-based sensor that can be used to detect and even quantify low concentrations of terbium in complex acidic samples.”

The new sensor is based on lanmodulin, a protein that researchers previously discovered that binds almost a billion times better to rare earth elements than to other metals. The selectivity of the protein to bind to rare earth elements is ideal for a sensor, as it is more likely to bind to rare earths instead of other metals that are common in environmental samples.

To optimize lanmodulin as a terbium scavenger in particular, the researchers modified the protein by adding the amino acid tryptophan to the protein.

“Tryptophan is what’s called a ‘sensitizer’ for terbium, which means that the light absorbed by tryptophan can be transmitted to terbium, which terbium then emits at a different wavelength,” Cotruvo said. . “The green color of this show is actually one of the main reasons terbium is used in technologies like smart phone screens. For our purposes, when the compound tryptophan-lanmodulin binds to terbium, we can observe the light emitted, or luminescence, to measure the concentration of terbium in the sample. “

Researchers have developed many variations of the tryptophan-lanmodulin sensor, optimizing the location of tryptophan so that it does not interfere with the ability of lanmodulin to bind to rare earth elements. These variants provided important information on the main characteristics of the protein that allow it to bind to rare earths with such high selectivity. Then they tested the most promising variant to determine the lowest concentration of terbium the sensor could detect under idealized conditions, without any other metal that might interfere. Even under very acidic conditions, such as those found in acid mine drainage, the sensor could detect environmentally relevant terbium levels. “One of the challenges of extracting rare earth elements is that you have to extract them from rock,” Cotruvo said. “With acid mine drainage, nature has done it for us before, but looking for rare earths is like finding a needle in a haystack. We have existing infrastructure to treat acid mine drainage sites in active and inactive mines to mitigate their environmental impact. If we can identify the sites containing the most valuable rare earth elements using sensors, we can better focus mining efforts to turn waste streams into sources of income. “

Next, the researchers tested the sensor in real samples from an acidic mine drainage treatment facility in Pennsylvania, an acidic sample with many other metals present and very low levels of terbium – 3 parts per billion. The sensor determined a concentration of terbium in the sample that was comparable to what they detected with the “gold standard” method, suggesting that the new sensor is a viable means of detecting low concentrations of terbium in environmental samples. complex.

“We plan to further optimize the sensor so that it is even more sensitive and can be used more easily,” Cotruvo said. “We also hope to target other specific rare earth elements with this approach.”