New sensor for detecting rare earths in acid mine waste
Device can be used to improve the supply of these metals to the United States
Byron King (B_K)May 9, 2025
Penn State University researchers have developed a luminescence sensor that can detect terbium, one of the rarest rare earth elements, in complex environmental samples such as acid mine waste. In a peer-reviewed article published in the Journal of the American Chemical Society, the scientists explain that the sensor takes advantage of a protein that binds very specifically to rare earth elements. This means that the device could be used to improve the US supply of these metals. According to the researchers, while there is currently no domestic supply chain for rare earths like terbium, they are quite abundant in nontraditional sources like coal by-products, acid mine sewage and electronic scrap. However, robust detection methods are required to unlock these REE sources, and the gold standard has been to use a type of mass spectrometry called ICP-MS, which is expensive and not portable. Portable methods, on the other hand, are not as sensitive and are not suitable for complex environmental samples in which acidic conditions and other metals can interfere with the detection. The protein Lanmodulin was developed into a sensor that can detect terbium in complex environments such as acid mine drains. The sensor pictured here emits green light when bound to terbium. (Image by Emily Featherstone, courtesy Penn State University). In contrast to these methods, Penn State University's development relies on lanmodulin, a protein that can bind to rare earths almost a billion times better than other metals. The selectivity of the protein in binding rare earths is ideal for a sensor because it is most likely to bind to rare earths rather than other metals that are commonly found in environmental samples. In order to optimize Lanmodulin specifically as a sensor for terbium, the researchers modified the protein by adding the amino acid tryptophan to it. "Tryptophan is what is known as a 'sensitizer' for terbium, which means that light absorbed by tryptophan can be passed on to the terbium, which then emits at a different wavelength," said Joseph Cotruvo Jr. "The green color of this emission is one of the main reasons why terbium is used in technologies such as smartphone displays. For our purposes, when the tryptophan-lanmodulin compound binds to terbium, we can observe the light or luminescence emitted to determine the concentration of terbium in the sample. " The researchers developed numerous variants of the tryptophan lanmodulin sensor, optimizing the position of the tryptophan so that it does not interfere with the ability of the lanmodulin to bind to rare earths. These variants provided important insights into the key features of the protein that enable it to bind rare earths with such high selectivity. Then they tested the most promising variant in order to determine the lowest concentration of terbium that the sensor can detect under idealized conditions - without interfering other metals. Even under strongly acidic conditions, the sensor was able to detect environmentally relevant amounts of terbium. "One of the challenges in extracting rare earths is that you have to get them out of the rock," says Cotruvo. "With acid mine drainage, nature has already done it for us, but finding the rare earths is like finding a needle in a haystack If we can use sensors to identify the locations with the most valuable rare earths, we can better focus mining efforts to turn waste streams into sources of income ”. Next, the researchers tested the sensor on actual samples from an acid mine sewage treatment plant in Pennsylvania - an acidic sample containing many other metals and very low levels of terbium - 3 parts per billion. The sensor detected a concentration of terbium in the sample that was comparable to the gold standard method, suggesting that the new sensor is a viable way of detecting low concentrations of terbium in complex environmental samples. "We plan to further optimize the sensor so that it is even more sensitive and easier to use," said Cotruvo. "We hope that with this approach we can also detect other specific rare earths.
