For a typical experiment, five quartz FUV range spectrophotometer cuvettes (R-3010-T, Spectrocell Inc, USA) were equipped with a magnetic stirring bar and used for reactions and UV-vis absorbance measurements. In one of the cuvettes, a DBCO control was prepared: 600 µL of 0.35 µmol/mL DBCO-PEG 4 -alcohol solution in PBS was diluted with 2400 µL of PBS. The NPs control was prepared in a second cuvette: 600 µL of 1 mg/mL nanoparticles solution (N N P = 4.14 • 10 15 ) in PBS was diluted with 2400 µL of PBS. In three sample cuvettes: 600 µL of 0.35 µmol/mL DBCO-PEG 4 -alcohol solution in PBS, 600 µL of 1 mg/mL nanoparticles solution in PBS, and 1800 µL of PBS were mixed. There is at least 30x excess of DBCO w.r.t. number of particles in each sample. This means that for NPs with 2, 5 or 10 azide groups per particle, there is at least a 15, 6, or 3x excess of DBCO, respectively.