1 Unveiling the Physicochemical Features of CoFe 2 O 4 NPs Synthesized via a Variant Hydrothermal Method: NMR Relaxometric Properties Violetta Georgiadou, a Vassilis Tangoulis, a Ioannis Arvanitidis, b Orestis Kalogirou, b Catherine Dendrinou-Samara* a a Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. E-mail: samkat@chem.auth.gr b Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece * Corresponding author, Email: samkat@chem.auth.gr, Tel: +30-2310-99-7876 http://users.auth.gr/samkat ABSTRACT A series of CoFe 2 O 4 nanoparticles were formed through a variant hydrothermal synthesis based in a self-assembly oil-water system in autoclaves at 200 °C in the presence of octadecylamine and the trivalent iron and cobalt acetylacetonates. The variation of the water content, the different valence of cobalt precursor (Co(II) and Co(III)) as well as Fe: Co precursor ratios (2:1 and 1:1) were studied. CoFe 2 O 4 nanoparticles with a size range 9-16 nm of high crystallinity and enhanced saturation magnetization (~89 emu g -1 ) have been isolated and characterized. Raman spectroscopy provided information concerning the lattice strain, while incorporation of Co 2+ at T d sites of the spinel indicated a different inversion degree (0.67-0.60) among the samples. EPR studies showed that EPR signal and spin relaxation process were size dependent and influenced by aggregation effects. CoFe 2 O 4 nanoparticles were converted to dual agents via a reaction between the free amine groups of the organic coating and the sulfonyl group of the fluorescent dye sulforhodamine B acid chloride (SRB) and NMR relaxometric properties were measured. The relatively high transverse relaxivity values, r 2 (232.0-130.3 mM -1 s -1 ) were attributed to nanocluster effects in aqueous suspensions in respect with the amount of SRB and encourage their potential application as versatile agents in theranostics.