 This review discusses the recent developments in the preparation, microstructure, and magnetic properties of iron oxide nanoparticles, ONPs, as well as their potential applications in biology. To make them more suitable for practical use, ONPs need to combine properties like high magnetic saturation, stability, biocompatibility, and interactive functions at the surface. Surface modification can be achieved through organic materials or inorganic materials such as polymers, biomolecules, silica, metals, etc. The review also considers the current directions for the synthesis, surface functionalization, and bio-application of IONPs, as well as future trends and prospects. This article was authored by Wei Wu, Zhao Wei Wu, Taikyung Yu, and others.