Rare Earth Element Biosensing in Chemically Complex Waters: Interference Mechanisms, Operating Windows, and Mitigation for a Lanmodulin Enzymatic Switch

Rare earth elements (REE) are increasingly mobilised from mine residues and other industrial sources, creating a need for rapid screening of chemically complex waters where conventional ICP-based analysis is impractical on site. Here, we evaluate a lanthanide-gated enzymatic switch biosensor, for matrix-aware monitoring of mine-influenced waters. We quantified REE-dependent activation and systematic interference from co-occurring metals and anions typical of mine waters, and defined operating windows that preserve lanthanide responsiveness. REE activation was readily detectable at ~100 nM, whereas inhibition emerged in the low-micromolar range and became pronounced near ~10 µM for key interferents, corresponding to a practical tolerance of roughly 1:100 (REE:inhibiting element). Inhibition was strongest for Al III , Fe III , Ga III , Zn II , and Sn II . Combined machine learning, molecular dynamics, and density functional theory analyses indicate distinct inhibition modes, including coordination of Fe III , Zn II , and Sn II at the LanM EF4 hand loop and favourable binding of [Al(OH) 4 ] - /[Ga(OH) 4 ] - near the TEM-1 substrate-recognition region. Consistent with these predictions, citrate reduced inhibition from hydrolysing metals, especially Al III and Sn II . Together, these findings define matrix-dependent operating envelopes, mechanistic design rules, and practical mitigation strategies for deploying REE biosensors in chemically complex waters.