The Eu(III) F -Transport Assay Having confirmed that 1a-1c predominantly perform as highly active H + /Cl -co-transporters, it was envisaged that the tetralactams would also be capable of efficient electroneutral F -co-transport. However, the ability of the series to perform electroneutral F -co-transport could not be analysed using the standard suite of assays because the F -cationophore coupled transport assay does not work in the presence of monensin. Previously, Butler synthesised a luminescent Eu(III) probe capable of detecting F -in H2O. The probe is capable of F -detection between 2-260 μM with no noticeable interference from a range of competing anions. Using the same probe, an assay capable of direct F -/Cl -transport detection was developed by Butler, Valkenier, and co-workers. The synthetic vesicles were made the same way as the regular HPTS assay and contained an internal solution of NaCl (225 mM), HEPES (0.5 mM), and the Eu(III) probe (50 μM) (Figure ). The vesicles were then suspended in an external solution of NaCl (225 mM) and HEPES (0.5 mM), with both solutions being buffered to pH 7.0. A NaF base pulse with a final concentration of 3 mM was added to the system, and when no leakage was detected, the transporters were added at various concentrations as DMSO solutions (Figure ). For further information on the fitted experimental data and associated spectra see Table and Figure -S177 in the ESI.