In the absence of NPs, the fluorescence intensity increases linearly with the dye concentration (green data points in Figure ). In case NPs have only NDA-(EG) 2 -OCH 3 ligands (Figure ), the dependence is also linear and differs only slightly from the dye alone. For the samples where NDA-(EG) 2 -N 3 ligands are present, two linear regions are observed. The first linear region has a lower slope than the second linear region. Furthermore, the latter has a slope comparable to that of the dye alone (see also Table ). We conclude that the fluorescence of AF488 is quenched until the dye:NP ratio is equal to the valency of the NPs, i.e., the number of azide groups on the NP surface. Above that concentration, there is additional free dye, and its fluorescence is not quenched by PCT. Hence, PL quenching by nitrodopamine is only observed when the dye is covalently bound to nitrodopamine-based ligands. This was confirmed by control experiments in which AF488 was added to either free NDA-(EG) 2 -OCH 3 , free NDA-(EG) 2 -N 3 (see Figure ). Fluorescence is quenched by NDA-(EG) 2 -N 3 (until the stoichiometry reaches 1:1), but not by NDA-(EG) 2 -OCH 3 .