The quantification via UV-Vis yielded identical results to the fluorescence quenching method, see Figure . Both 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), do not involve any purification, and are independent of the magnetic properties of the nanoparticles. An important requirement is colloidal stability throughout the duration of the quantification measurements. Aggregation would result in scattering that would impact both fluorescence and UV-Vis spectroscopy. The methods could be potentially applied to other aqueous buffers because the click reaction is largely insensitive to pH. One limitation of the fluorescence quenching method is the need for electron-accepting ligands, but it can be an interesting alternative for NPs with high absorbance around 308 nm. To demonstrate the broad utility of our azide quantification strategy based on UV-Vis, we prepared hafnium oxide nanoparticles with a different ligand composition. We chose a (sulfobetaine-azide) phosphonic acid block copolymer as the capping ligand (Figure ), which has been shown to stabilize several different NPs (e.g, iron oxide) in physiological media while reducing protein corona formation. We first stabilized hafnia nanoparticles in water using (6-{2-[2-(2-methoxy-ethoxy)-ethoxy]-ethoxy}-hexyl)phosphonic acid and subsequently exchanged the phosphonate ligands for the polyphosphonate (Figure ). The nanoparticles were purified by size exclusion chromatography (SEC). The final NPs were characterized by DLS, TGA and NMR to confirm, respectively, the colloidal stability, the inorganic fraction, and the success of the ligand exchange, see Figure . P NMR spectroscopy is particularly useful for detecting bound polyphosphonate, in addition to a small amount of residual phosphonate (Figure ). The UV-Vis quantification of azides was carried out as described above, and the corresponding absorbance spectra and kinetic curves are plotted in Figure .The raw spectra can be found in Figure . Interestingly, the reactions were completed much later than for catechol-stabilized particles, which can be attributed to the lower concentration of particles used for this experiment and also to the sterical hindrance and accessibility of azide groups surrounded by polymer chains. The change of ∆A after 12 h of the reaction was used to calculate the number of azide groups per NP: 4.23 ± 0.21 N 3 /NP. This is in agreement with previously published estimates for the number of accessible azide groups in NPs with sim-ilar polymer ligands determined using fluorophore labels. Therefore, this UV-tracking method is a promising alternative for determining the amount of derivatizable azide groups without the need for labelling with dyes and the associated purification steps.