Fe₃O₄ nanomaterials have attracted tremendous attention in the field of magnetic resonance imaging (MRI) because of their low cost, good biocompatibility, favorable superparamagnetic properties. This article focuses on the controlled preparation methods of superparamagnetic Fe₃O₄ nanoparticles, and provides an in-depth discussion of the key factors and their influence rules for particle size, crystallinity and magnetic properties. The current status of available methodologies and mechanisms for the assembly and surface modification of Fe₃O₄ nanoparticles are highlighted to generate high performance and multifunction. We also systematically summarize the effect of particle size, morphology and surface properties on the magnetic and toxic properties of Fe₃O₄ nanoparticles. Finally, the future opportunities and challenges of Fe₃O₄ nanomaterials used as MRI contrast agents are addressed to our understanding.

Contents
1 Introduction
2 Controllable preparation of Fe₃O₄ nanoparticles
2.1 Thermal decomposition
2.2 Coprecipitation
2.3 Solvothermal
2.4 Sol-gel
2.5 Microemulsion
2.6 Sonochemical
3 Assembly of Fe₃O₄ nanoparticles
3.1 Zero-dimensional sphere
3.2 One-dimensional nanochain
3.3 Multidimensional assembly
4 Surface functionalization of Fe₃O₄ nanoparticles
4.1 Modified by SiO₂
4.2 Modified by polymer
4.3 Multifunctional modification
5 Toxicity
6 MRI applications
6.1 Cell imaging
6.2 Tissue/organ imaging
7 Conclusion