It does not depend on the chemical nature of the stabilizing ligand, as we demonstrated for both catechol and polyphosphonate ligands. In the future, both methods could be partially automated using a plate reader, which would significantly speed up the quantification process. Both developed methods are capable of detecting low amounts of surface azides (as low as 0.8 nmol for fluorescence quenching and 13.8 nmol for UV-Vis). Compared with other optical methods like dye labeling, our methods do not require any purification. Furthermore, these methods are orthogonal to the magnetic properties of NPs and could be used for the quantification of surface azides on, e.g., iron oxide NPs. We believe that this quantitative analysis of surface functional groups will play an important role in precise bioconjugation, which is crucial for biomedical applications of NPs.