Experimental Methods Anhydrous solvents were collected from an Inert Corp PureSolv MD7 solven purification system. Deionised water was collected from a Merck Millipore Milli-Q™ reference ultrapure water purification system. The nuclear magnetic resonance (NMR) spectra were recorded at 300 K using a Bruker AVIII 500, Bruker Avance DPX 400, or a Bruker Avance 600 spectrometer. 1 H and 13 C NMR spectra were recorded at the indicated frequency, and calibrated to the residual DMSO-d6 solvent peak (δ = 2.50 ppm and δ = 39.52 ppm, respectively). The resonances are reported as chemical shifts in ppm, multiplicity, coupling constant in Hz, and by their relative integral. Melting points were recorded manually using a Standford Research Systems Optimelt machine, and data were reported as a range ( o C) . Low resolution mass spectrometry was performed with a Bruker amaZon SL mass spectrometer using electrospray ionisation. High resolution mass spectra were recorded on a Bruker Apex II Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer with a 7.0 T magnet, fitted with an off-axis analytic electrospray source with quadrupole mass analyser, and are reported as m/z (relative intensity). To prepare the vesicles, a chloroform solution of POPC (37.5 mM, 4 mL) was transferred to a pre-weighed round-bottom flask, and the solvent was removed using a rotary evaporator. The pressure was lowered slowly to ensure the formation of a smooth lipid film. Subsequently, the film was dried in vacuo for 4-24 h, and the mass of lipid was recorded. The lipids were rehydrated with 4 mL of the respective internal solution (this number should correspond to the volume of POPC solution used initially) and vortexed until all lipids were removed from the sides of the flask and were suspended in solution. The lipids were subjected to 9 cycles of freeze-thaw by freezing using a dry ice/acetone bath and thawing in lukewarm water. Following this, the vesicles were left to rest at room temperature for 30 min. The lipids were extruded through a 200 nm polycarbonate membrane 25 times to form monodisperse vesicles. Only 1 mL of solution was extruded at a time before being collected. Finally, any residual unencapsulated salt from the internal solution was either removed via dialysis for 12 h in the desired external solution or through size-exclusion using a B19 column packed with hydrated G-25 Sephadex®, which had been pre-saturated with the respective external solution. The lipid suspensions were diluted with the external solution to afford a stock solution (10 mL) of a known concentration. The ISE assayd were conducted on the Fisherbrand™ Accumet™ Chloride Combination Electrode, and the HPTS transport fluorescence data was recorded on an Agilent Cary Eclipse Fluorescence Spectrophotometer.