Absorption spectra of NPs. Upper panel: UV-visible spectra of Ag NPs (30 nm; a, red colour) and Au NPs (10 nm; b, green colour) exposed to cell culture medium (DMEM + 10% FBS) at 37°C. Fainter lines: NPs as synthesized. Darker lines: after 48 h in culture medium at 37°C. Red shift of the absorbance peak is an indication of protein corona formation. Lower panel: absorption spectra of the NPs used in this work. NP concentration is 1 × 1012 NPs/ml for Au, Ag and AgO and 1 × 1014 NPs/ml for CeO2, CoO and Fe3O4. At these concentrations, all NPs absorb in the visible range, where most of the biological tests have their readouts. In this figure: 340 nm is the absorption wavelength of NADH, measured in the LDH assay; 405 nm is the absorption wavelength of the chromophore p-nitroaniline (pNA) measured in the LAL assay for endotoxin determination; 440 nm, 490 nm and 540 nm are the readout for WST-1, MTS and MTT respectively.