The elemental analysis was performed using an Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) instrument (PERKIN ELMER mod. OPTIMA 2100 DV). Typically, 25 µL of a MnxFe3-xO4 MNPs suspension were digested in 1 mL of aqua regia in a volumetric flask at 60 °C overnight. Afterward, the flask was filled up with deionized water.