Lanthanide Binding Peptide Surfactants at Air-Aqueous Interfaces for Interfacial Separation of Rare Earth Elements

We are developing an all-aqueous, bio-inspired approach using peptides as surfactants that selectively bind rare earth element (REE) cations and adsorb at the air/water interface to enable green capture and separation. REEs are essential components in modern electronic devices and clean energy technologies which must be separated from feedstocks of aqueous mixtures. Their selective capture is particularly challenging owing to their similarity in size and charge. Lanthanide binding tags (LBTs) are amphiphilic peptide sequences based on the binding loop in the evolutionarily conserved EF-hand metal binding motif. We study LBTs optimized for coordination to Tb$^{3+}$ using a suite of experimental methods including luminescence spectroscopy, surface tensiometry, x-ray reflectivity and x-ray fluorescence near total reflection, and find that these LBTs capture Tb$^{3+}$ in bulk and adsorb at the interface. Molecular dynamics show that the binding pocket remains intact upon adsorption. We find that, if the net negative charge on the peptide results in a negatively charged complex, excess cations are recruited to the interface by non-selective Coulombic interactions that compromise selective REE capture. If, however, the net negative charge on the binding loop is -3, resulting in a neutral complex, a 1:1 surface ratio of cation to peptide is achieved. We demonstrate selective interfacial extraction from an equimolar mixture of Tb$^{3+}$ and La$^{3+}$, validating an LBT-mediated interfacial separation of REEs.