To develop a quantification method for azide functional groups, we first designed a model system of NMR-compatible metal oxide nanoparticles stabilized with PEGylated catechol-based ligands. Hafnium oxide nanoparticles were synthesized from hafnium tert-butoxide in benzyl alcohol, TEM images show ellipsoidal-shaped nanoparticles with a major axis of (3.8 ± 0.6) nm and the minor axis of (2.4 ± 0.3) nm, see Figure . The colloidal stability of the nanoparticles after ligand exchange in water was confirmed by DLS (Figure ), yielding a mean hydrodynamic diameter of 5.8 nm, which is in agreement with TEM considering the additional ligand shell. The colloidal stability of the nanoparticles is not influenced by the azide content (Figure ). Finally, the organic content of the nanoparticle was determined to be 24% by TGA (Figure ) and further used to calculate the number of nanoparticles for stoichiometric reactions (see Supporting Information for calculation). The organic content is similar for particles with different azide content (see Figure ). reference NMR spectra (Figure ). Because of the broadening and the low content of bound NDA-(EG) 2 -N 3 , it is impossible to distinguish it from bound NDA-(EG) 2 -OCH 3 . Therefore, all three samples appear identical by H NMR, see Figure . Clearly, the characteristic peak of the -CH 2 -N 3 protons (labeled as f') of NDA-(EG) 2 -N 3 is too broad and too low in intensity to allow quantification of the shell composition using the qNMR method, which is often used in surface ligand quantification. To avoid the effects associated with line broadening, ligands can be released ("stripped") from the surface of previously functionalized particles for routine NMR analysis. Strong acids such as trifluoroacetic acid or HBF 4 can be used to release bound ligands. The NPs lose their stabilizing ligands and precipitate, allowing them to be separated from the ligand solution. This method has been used primarily for a qualitative analysis of the surface. We attempted to quantify the surface ligands of our hafnia NPs by stripping them with HBF 4 (Figure and Table ). All weakly bound carboxylate ligands were removed from the surface but only 30% of catechol ligands were effectively removed. Although some NDA-(EG) 2 -N 3 was detected, its quantification becomes unreliable due to the incomplete stripping. FTIR spectroscopy qualitatively confirmed the presence of azide groups (Figure ) on the surface of the NPs. The characteristic band of the azide group at 2100 cm -1 is visible for nanoparticles with targeted NDA-(EG) 2 -N 3 content, as opposed to nanoparticles with NDA-(EG) 2 -OCH 3 ligands alone. However, the IR band is relatively weak due to the low concentration of azide groups on the surface of the NPs, highlighting the need for sensitive methods of quantification.