There is thus clearly a need for precise azide quantification methods that can be applied to various NPs. In this study, we developed two methods that require no purification, use a low amount of functionalized NPs, and are independent of the inorganic core of the NPs. As a model system, we take sub-5 nm hafnium oxide NPs (HfO 2 NPs), which have been recently explored as radiosensitizers for radiotherapy and as contrasts agents for X-ray computed tomography. We capped the HfO 2 surface with catechols, which were previously shown to provide the strongest binding and therefore the best colloidal stability for industrially and biomedically relevant metal oxide NPs such as hafnium oxide, 37 titanium oxide, and iron oxide NPs. We varied the amount of azide-carrying catechols in the ligand shell from 0 to 10 per NP to test the possibility of quantification of low amounts of functional groups necessary for precise bioconjugation. The first method leverages the fluorescence quenching effect of the nitrocatechol ligands on covalently bound dye molecules by photoinduced charge transfer (PCT). The second method is more general and is based on the UV-tracking of the DBCO triple bond absorbance upon click-reaction. Both methods gave highly comparable results and were able to accurately determine the number of azide groups on the surface of the NPs. The second method was successfully generalized to NPs with polyphosphonate ligands.