The intersection point of the two linear curves in Figures , represents the valence of the NPs. It is the amount of dye that was conjugated to the NDA-(EG) 2 -N 3 on the nanoparticle surface. To avoid any bias in the determination of the intersection, we opted for an automatic fitting using a piecewise linear function (see Methods and Supporting Information for further details). As can be seen in Table , the intersection found between two linear parts is indeed close to the target value of N 3 /NP and corresponds to the number of azide groups per particle. To test the repeatability and reproducibility of this method, we performed the azide quantification three times on the same sample (twice by the same operator and once by another, see Figures and Table ). All three independent experiments yielded the same result (within the experimental error). The above method based on fluorescence quenching can be applied only to particles with electron-accepting ligands. In this current section, we demonstrate a more general method that exploits the UV-Vis absorption band of the DBCO triple bond (alkyne) with a maximum at 308 nm (ε = 12000 L mol -1 cm -1 ), which disappears during cycloaddition (Figure ). To calculate the concentration of azide groups in the sample, we calculate the concentration of reacted DBCO using the Beer-Lambert law via: