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 ~100nM, whereas inhibition emerged in the low-micromolar range and became pronounced near ~10µM for key interferents, corresponding to a practical tolerance of roughly 1:100 (REE:inhibiting element). Inhibition was strongest for AlIII, FeIII, GaIII, ZnII, and SnII. Combined machine learning, molecular dynamics, and density functional theory analyses indicate distinct inhibition modes, including coordination of FeIII, ZnII, and SnII 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 AlIII and SnII. Together, these findings define matrix-dependent operating envelopes, mechanistic design rules, and practical mitigation strategies for deploying REE biosensors in chemically complex waters.