Quantification of Nanoparticle Compositions

The iron (Fe), gold (Au), and gadolinium (Gd) contents of the obtained nanoparticles were quantified using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Spectrometric (ICP-OES) measurements were performed using an Agilent Technologies 5800 VDV ICP-OES. Fe, Gd, and Au standard solutions were prepared in about 5–10% Aqua regia matrix (HNO₃:HCl, 1:3). A 200 µL sample of each NP solution prepared were digested following the protocol: (1) evaporation to dryness, (2) re-dissolved in 1 mL of concentrated Aqua Regia and evaporated to dryness, (3) addition of 100 µL of 30% H₂O₂ + 100 µL of Milli-Q water are then added and evaporated to dryness and finally retaken in 5 mL of about 5–10% Aqua Regia for ICP measurements. The quantitative determinations were performed using the most accurate wavelength: 342.246 nm for Gd, 259.940 nm for Fe, and 267.594 nm for Au. All the measurements were performed in triplicate.