The determination of iron concentration was performed following a previously reported protocol and to adjust iron concentration before glucose functionalization. In brief, 5 μl of MNPs was diluted in 45 μl of solvent (hexane or water) and digested with 100 μl of aqua regia solution (3:1 HCl:HNO3) at 60 °C for 15 min. Then, the samples were diluted up to 300 µl with miliQ water. At this point, 50 μl (in triplicate) was used for the iron quantification by mixing the digested samples with 60 µl of 0.25 M 1,2-dihydroxybenzene-3,5-disulfonic acid (Tiron). This molecule forms a coloured complex with iron, and it can be measured by spectrophotometry at λ = 480 nm using a microplate spectrophotometer (BioTek Synergy H1 UV/VIS, Agilent Technologies, Santa Clara, CA, USA) and compared with a standard calibration curve obtained with solutions of known iron concentrations (0-1000 μg Fe/mL).