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

To identify the rare earth element Terbium in complex environments such as acid mine drainage, the protein lanmodulin was developed into a sensor. This sensor emits green light when it is bound to terbium. Credit: Emily Featherston at Penn State
A new luminescent sensor is capable of detecting terbium from complex environmental samples such as acid mine waste. Penn State researchers developed the sensor by using a protein that binds specifically to rare earth elements. This protein could be used to develop a domestic supply. These metals are essential for technologies like smart phones, electric cars, and energy efficient lighting. The Journal of the American Chemical Society published Aug. 25 a paper that describes the sensor.

Terbium is one of the most rare of the rare earth elements. It produces the color green in cell phones and can also be used in high efficiency lighting and solid state devices. There are many challenges that must be overcome to obtain terbium or other rare earth elements from our environment. Another challenge is the development of new sources for these metals. This requires robust detection methods. The ICP-MS mass spectrometry is the most reliable method to detect rare earth elements in samples. However, it is not portable and expensive. However, portable methods are less sensitive and can't be used in complex environmental samples where other metals and acidic conditions may interfere with detection.

Joseph Cotruvo Jr., assistant professor of Chemistry and Louis Martarano Career development Professor of Chemistry at Penn State and senior author of this study, stated that although there is no current domestic supply chain for rare earth elements such as terbium, they are abundant in non-traditional sources like coal byproducts and acid mine drainage. We developed a luminescence-based detector that can detect low levels of terbium even in complex acidic samples.

Researchers previously discovered that lanmodulin is almost a billion-fold more effective at binding rare earth elements to metals than other proteins. Because the protein is more likely to bind to rare elements than other metals, it makes a great sensor.

The researchers modified lanmodulin to make it a better sensor for terbium by adding tryptophan to its protein.

Researchers could use a new sensor to detect the rare earth element Terbium in complex environmental samples such as acid mine drainage, which is polluting a Pennsylvania stream. Credit: Rachel Brennan, Penn State

Cotruvo stated that tryptophan is a "sensitizer" for terbium. This means that tryptophan's light can pass to the terbium and the terbium emits at a different wavelength. This emission's green color is one of the main reasons that terbium is used for technologies such as smart phone displays. For our purposes, when the tryptophan-lanmodulin compound binds to terbium, we can observe the emitted light, or luminescence, to measure the concentration of terbium in the sample."

The researchers developed many variants of the tryptophan-lanmodulin sensor, optimizing the location of the tryptophan so that it does not interfere with lanmodulin's ability to bind to rare earth elements. These variants gave insight into the proteins key characteristics that allow it to selectively bind rare earths. They then tested the most promising variant to determine which variant could detect the lowest concentrations of terbium in ideal conditions with no metals interfering. The sensor can detect terbium levels that are environmentally sensitive even in highly acidic conditions such as acid mine drainage. Cotruvo said that the only problem with extracting rare earth elements from rock is that it can be difficult to do so. Cotruvo said that acid mine drainage is something nature already does for us. However, searching for rare earths can be like looking for a needle in the haystack. To reduce their environmental impact, we have the infrastructure in place to treat acid mine drainage areas at active and inactive mines. We can focus our efforts on extracting the rare earth elements that are most valuable using sensors to identify these sites and turn them into revenue streams.

The sensor was then tested in acid mine drainage treatment facilities in Pennsylvania. This acidic sample contained many metals and low levels of terbium3ppb. The sensor detected a concentration of Terbium in the sample comparable to what was found with the "gold-standard" method. This suggests that the new sensor can be used to detect low levels of Terbium in complex environmental samples.

Cotruvo stated that the sensor will be further optimized to make it more sensitive and more user-friendly. We also plan to target other rare earth elements using this approach.

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More information: Emily R. Featherston and colleagues, Probing Lanmodulin’s Lanthanide Recognition through Sensitized Lumenescence Yields an A Platform for Quantifying Terbium in Acid Mine Drainage. Journal of the American Chemical Society (2021). Journal Information: Journal of the American Chemical Society Emily R. Featherston and colleagues, Probing Lanmodulin’s Lanthanide Recognition through Sensitized Lightening Yields a Platform to Quantify Terbium In Acid Mine Drainage. (2021). DOI: 10.1021/jacs.1c06360