Author: C. Dendrinou-Samara (samkat@chem.auth.gr) Affiliation: Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. In now days, the ability to carry out diagnosis and therapy simultaneously (termed as theranostics) is gaining popularity thanks to unique features provided by nanosystems for both diagnosis and therapy. Superparamagnetic spinel ferrites MFe2O4 (M=Mn, Fe, Co, Ni, etc) are currently considered among the most successful inorganic nanoparticles for medical applications, including contrast enhancement in magnetic resonance imaging (MRI), magnetically guided drug delivery and hyperthermia cancer therapy. Bioapplications require magnetic nanoparticles (MNPs) with well-defined composition, narrow size distribution, and high saturation magnetization values for enhanced interaction with an externally appied nagnetic field. This enhancement is very important for the successful and low dosage performance of MNPs in medicine. We have undertaken a study where we focus on synthetic parameters to control the size, composition and magnetization of ferrite nanoparticles MFe2O4 where M=Mn, Co, Ni, in an atempt to enforce their performance in bio applications. Specifically, the NPs designed as fluorescence agents, magnetic fluid hyperthermia heat mediators and negative contrast agents for MRI. For the prepatation the solvothermal method has een selected as a simple and eco-friendly route providing products that exhibit high crystallinity, even for sizes under 10nm. Since hydrophilicity is esential for in vitro and in vivo applications, phase transfer becomes necessary and the hydrophobic NPs converted to hydrophilic by (i) an additional coating layer with the positively charged molecule cetyltrimethyl ammonium bromide; (ii) ligand exchange through 2, 3-dimeraptosuccinic acid and (iii) encapsulation into the hydrophobic cores of the micellar structures of copolymers that also prepared by us. Moreover, shifting form the synthesis of individual MNPs to their assembly into secondary structures, nanoclusters of the same and/or different building blocks were prepared while by combining magnetic and non-magnetic features of NPs, heterostructures of different ratios such as NiFe2O4@Cu2O have been synthesized with antifungal and magnetomechanical properties.