Preparation of extremely small and uniform sized, iron oxide-based paramagnetic or pseudo-paramagnetic nanoparticles and MRI T1 contrast agents using the same

A method for preparation of iron oxide nanoparticles, comprising:

• reacting an iron complex having iron as a central atom and a carboxylate group having 10 to 22 carbon atoms that is bonded to the central atom in a ligand form;
• a C10 to C22 fatty acid;
• and a C10 to C22 aliphatic alcohol or C10 to C22 aliphatic amine to prepare iron oxide nanoparticles having an average size of 3.5 nm or less and being paramagnetic or pseudo-paramagnetic at a temperature of 20K or higher, wherein the iron oxide nanoparticle preparation is performed by raising the temperature from room temperature to 200 to 250° C. with a temperature elevation rate of 10° C./m or more, and conducting reaction at 200 to 250° C. for 5 to 60 minutes.