## 2 Materials and Methods  
_Chemicals._ HAuCl\({}_{4}\)\(\times\) 3H\({}_{2}\)O (\(\geq\)99.5%) was purchased from Carl Roth. (NH\({}_{4}\))\({}_{2}\)Fe(SO\({}_{4}\))\({}_{2}\)\(\times\) 6H\({}_{2}\)O (\(\geq\)99%), FeCl\({}_{3}\)\(\times\) 6H\({}_{2}\)O (97%), 3-mercaptoethanolpropionic acid (\(\geq\)99%), NaBH\({}_{4}\) (\(\geq\)96%), ethanol (\(\geq\)95%), fetal calf serum (FCS), penicillin-streptomycin-solution, sodium pyruvate, phosphate buffered saline (PBS), non-essential amino acids (MEM), trypsin/EDTA, 3-amino-7-dimethylamino-2-methyl- phenazine hydrochloride (neutral red), crystal violet (98%), and 2',7'-dichlor fluorescein diacetate (DCFH-DA) (95%) were bought from Sigma-Aldrich (Munich, Germany, 2018). 1-methyl-3-(dodecylphosphonic acid) imidazolium bromide from Sikemia (Montpellier, France, 2018). DMEM, GlutaMAX Supplement, MitoSOX\({}^{\text{TM}}\) Red Mitochondrial Superoxide Indicator and 3-(p-hydroxyphenyl) fluorescein (HPF) from Thermo Fischer Scientific (Karlsruhe, Germany, 2018) and glacial acetic acid and DMSO (99.7%) from Merck. Millipore (Darmstadt, Germany, 2018) water was used in all experiments. _Synthesis of the Fe\({}_{3}\)O\({}_{4}\)@Au pNPs._ 3-Mercaptopropionic acid (MPA) stabilized Fe\({}_{3}\)O\({}_{4}\)@Au pNPs were obtained through alkaline co-precipitation of Fe\({}_{3}\)O\({}_{4}\) NPs in aqueous HAuCl\({}_{4}\) solution, and were subsequently stabilized with 3-mercaptoethanolpropionic acid (MPA). 21 mL HCl solution was prepared by adding 1 mL of 1 M HCl to 20 mL of water. This solution was heated up to 80 \({}^{\circ}\)C under stirring. 2 mL of aqueous HAuCl\({}_{4}\) solution (25 mM) were added at once. 3 mL of aqueous ammonium iron(II) sulfate hexahydrate solution (100 mM) and 0.05 mL of aqueous MPA (10 mM) solution were added. After a reaction time of 1 min 3 mL ammonia (30 wt.%) were added. The resulting black suspension was refluxed for 30 min. The resulting MPA-stabilized Fe\({}_{3}\)O\({}_{4}\)@Au pNPs were collected by magnetic decantation and washed thrice with 20 mL ultrapure water. _Synthesis of Fe\({}_{3}\)O\({}_{4}\) nanoparticles._ Fe\({}_{3}\)O\({}_{4}\) nanoparticles (Fe\({}_{3}\)O\({}_{4}\) NPs) were synthesized following [PERSON]'s procedure under argon [28]. _Surface-coating procedure._ For attaining biocompatibility, the initially MPA-stabilized Fe\({}_{3}\)O\({}_{4}\)@Au pNPs and pristine Fe\({}_{3}\)O\({}_{4}\) NPs were coated with 1-methyl-3-(dodecylphosphonic acid) imidazolium bromide (Imidazolium-PA). 15 mg of the Fe\({}_{3}\)O\({}_{4}\)@Au pNPs or Fe\({}_{3}\)O\({}_{4}\) NPs were dispersed in water by sonication. Afterwards 40 mM of a methanolic solution of Imidazolium-PA were subjoined, and the mixture was sonicated for 30 min. The Fe\({}_{3}\)O\({}_{4}\)@Au pNPs and Fe\({}_{3}\)O\({}_{4}\) NPs were magnetically isolated and washed thrice. For the Fe\({}_{3}\)O\({}_{4}\) NPs the whole procedure was carried out under argon. _Synthesis of gold nanocluster._ MPA-stabilized gold nanoclusters (AuNCs) were prepared by dissolving HAuCl\({}_{4}\) (35.7 umol) in methanol (7 mL), followed by the addition of MPA (142.8 umol). The mixture was cooled in an ice bath for 30 min. NaBH\({}_{4}\) dissolved in water (0.2 M, 357 umol, 0 \({}^{\circ}\)C), was then rapidly injected into the AuCl\({}_{4}\)-MPA-mixture under vigorous stirring. The reaction mixture changed its color to dark red-brown under bubbling with nitrogen. After 1 hour of reaction in the ice bath, the mixture was centrifuged (8500 rpm, 10 min). The collected precipitate was subsequently washed with methanol and after the removal of the solvent the precipitate was dried in vacuum and the product was obtained as a black powder. _Characterization of the nanoparticles._ UV-Vis absorption and Fourier-Transform Infrared (FTIR) spectra were measured using a Perkin Elmer Lambda 2 (Perkin Elmer, Rodgau, Germany, 1987) and a Shimadzu IR Prestige-21 (Shimadzu Europa GmbH, Duisburg, Germany, 2002)) device, respectively.