Advances in lanthanide probes for selective anion recognition and sensing in water.

Selective anion recognition in aqueous media is an important focus in supramolecular chemistry, driven by both the intrinsic challenges of binding anions in highly competitive environments and the significant potential for applications in sensing, imaging, and sequestration. The structural diversity of anions necessitates the synthesis of host molecules with structures tailored for selective binding, overcoming high hydration energies and eliciting sensitive optical signals. Emissive lanthanide complexes, particularly those of europium(iii) and terbium(iii), continue to offer distinct advantages for anion sensing in aqueous and biological environments. Their long luminescence lifetimes enable time gating to remove background autofluorescence, while their narrow emission bands allow for precise ratiometric analysis. Variations in ligand structure and geometry can fine-tune binding selectivity and photophysical behaviour, enabling rapid and reversible anion sensing. Over the last five years, new and augmented ligand designs have enabled enhanced selectivity by controlling steric demand at the metal centre, matching host-guest charge and shape and utilising secondary non-covalent interactions for recognition. This review highlights advances over the past five years in lanthanide host design, showing how these developments have improved mechanistic understanding of binding and enabled new anion sensors and imaging probes that function in aqueous and biological media.