Addition of 25 mM phosphate to LUVs without transporter did not give rise to an emission enhancement (Figure , black curve), confirming the efficient encapsulation of the probe. In the presence of compound 1 (4 mol%, post-inserted from DMSO), a clear increase in the emission intensity was observed over time (Figure , blue curve), signature of a H2PO4 -/Cl -antiport transport mechanism. The observed dependence of the transport rate on the concentration of 1 (0.5-4 mol%, Figure ) further validates this assay, confirming that 1 transports phosphate. With control compounds 2 and 3 no phosphate transport activity was observed (Figure ), despite the activity of 3 as a Cl -transporter, supporting that phosphate is more difficult to transport than chloride.