We hypothesized that functionalizing UCNPs with low-vibrational-energy Sn2S6 4-would increase their UCL by reducing non-radiative relaxation pathways. To quantitatively assess the effect of Sn2S6 4-ligands on the UCL, we measured the UCL from the same solution of ligandstripped UCNPs before and after titrating them with Sn2S6 4-ligands, with overnight equilibration used to ensure complete exchange. We compared Sn2S6 4--capped UCNPs with stripped UNCPs (rather than with oleate-capped ligands) as this ensures consistency in the UCNP concentration and type of solvent (NMF).