CoFe 2 O 4 magnetic nanoparticles (MNPs) were synthesized in a new synthetic system from metal acetylacetonates in the presence of octadecylamine (ODA) in autoclaves with subcritical water. By altering the water extent and ratio of the precursors, four samples were isolated. High crystallinity MNPs of various sizes (26, 15, 18, 9 nm) and saturation magnetization (71, 87, 93, 64 emu g -1 ) were formed. The organic coating was characterized by FTIR spectroscopy, 1 H NMR and 13 C NMR spectroscopy, and thermogravimetric analysis (TGA), and the free amine groups were detected by a ninhy-
[a]3645 drin assay. Under the present conditions, the CoFe 2 O 4 MNPs were covered by a bilayer consisting of N-octadecylamide and ODA. The MNPs were converted into fluorescence probes through the formation of a sulfonamide bond between ODA and sulforhodamine B (SRB). NMR relaxometric measurements were performed for MNPs modified with SRB, cetyltrimethylammonium bromide, and by 2,3-dimercaptosuccinic acid. The bimodal magnetic fluorescent MNPs showed relaxivity values of up to r 2 = 219.5 mM -1 s -1 , which renders them promising as dual agents.