Magnetic iron oxide nanoparticles (NPs) are a type of functional material with wide applications in biotechnology and catalysis. This review discusses recent developments in the synthesis, structure, and magnetic properties of naked and surface-functionalized iron oxide NPs, as well as their applications. The particles must have high magnetic saturation, stability, biocompatibility, and surface interaction capabilities. The surface of iron oxide NPs can be modified with organic or inorganic materials such as polymers, biomolecules, silica, metals, etc. Challenges in synthesis and surface functionalization are considered, along with future trends and prospects. Iron oxide NPs have unique magnetic properties such as superparamagnetism, high coercivity, and low Curie temperature. They are used in magnetic fluids, data storage, catalysis, and bioapplications. Magnetic ferrofluids and data storage have led to the integration of magnetic NPs in commercial applications. Magnetic iron oxide NPs are also used in bioapplications such as magnetic bioseparation, detection of biological entities, clinic diagnosis and therapy, targeted drug delivery, and biological labels. Choosing materials for nanostructure materials and devices with adjustable physical and chemical properties is crucial. Magnetic iron oxide NPs are strong candidates, with in vitro diagnostic applications practiced for nearly half a century. The synthesis of iron oxide NPs has been extensively studied, with methods including co-precipitation, thermal decomposition, hydrothermal synthesis, microemulsion, and sonochemical synthesis. Co-precipitation is the most conventional method for producing Fe3O4 or γ-Fe2O3. Thermal decomposition involves the decomposition of iron precursors followed by oxidation to produce high-quality monodispersed NPs. Microemulsion allows for the synthesis of shape- and size-controlled NPs. Hydrothermal synthesis is used to produce NPs with controlled size and shape, and is effective for growing dislocation-free single crystal particles. Sonochemical synthesis uses ultrasound to generate novel materials with unusual properties. Surface functionalization of iron oxide NPs can be achieved using organic or inorganic materials. Organic materials such as small molecules, surfactants, and polymers can be used to improve biocompatibility and stability. Inorganic materials such as silica, metal, and metal oxides can be used to enhance antioxidation properties and expand the application scope. Surface functionalization strategies include core-shell, matrix, and shell-core-shell structures. Silica-coated NPs provide stability, avoid interparticle interactions, and prevent agglomeration. Polymers can be used to increase repulsive forces and balance magnetic and van der Waals attractive forces. Biological molecules such as proteins, antibodies, and biotin can be used to enhance biocompatibility and target specificity. Inorganic materials such as silica, metal, and metal oxides can be used to functionalize iron oxide NPs. Silica-coated NPs have several advantages, including stability in solution, prevention of interparticleMagnetic iron oxide nanoparticles (NPs) are a type of functional material with wide applications in biotechnology and catalysis. This review discusses recent developments in the synthesis, structure, and magnetic properties of naked and surface-functionalized iron oxide NPs, as well as their applications. The particles must have high magnetic saturation, stability, biocompatibility, and surface interaction capabilities. The surface of iron oxide NPs can be modified with organic or inorganic materials such as polymers, biomolecules, silica, metals, etc. Challenges in synthesis and surface functionalization are considered, along with future trends and prospects. Iron oxide NPs have unique magnetic properties such as superparamagnetism, high coercivity, and low Curie temperature. They are used in magnetic fluids, data storage, catalysis, and bioapplications. Magnetic ferrofluids and data storage have led to the integration of magnetic NPs in commercial applications. Magnetic iron oxide NPs are also used in bioapplications such as magnetic bioseparation, detection of biological entities, clinic diagnosis and therapy, targeted drug delivery, and biological labels. Choosing materials for nanostructure materials and devices with adjustable physical and chemical properties is crucial. Magnetic iron oxide NPs are strong candidates, with in vitro diagnostic applications practiced for nearly half a century. The synthesis of iron oxide NPs has been extensively studied, with methods including co-precipitation, thermal decomposition, hydrothermal synthesis, microemulsion, and sonochemical synthesis. Co-precipitation is the most conventional method for producing Fe3O4 or γ-Fe2O3. Thermal decomposition involves the decomposition of iron precursors followed by oxidation to produce high-quality monodispersed NPs. Microemulsion allows for the synthesis of shape- and size-controlled NPs. Hydrothermal synthesis is used to produce NPs with controlled size and shape, and is effective for growing dislocation-free single crystal particles. Sonochemical synthesis uses ultrasound to generate novel materials with unusual properties. Surface functionalization of iron oxide NPs can be achieved using organic or inorganic materials. Organic materials such as small molecules, surfactants, and polymers can be used to improve biocompatibility and stability. Inorganic materials such as silica, metal, and metal oxides can be used to enhance antioxidation properties and expand the application scope. Surface functionalization strategies include core-shell, matrix, and shell-core-shell structures. Silica-coated NPs provide stability, avoid interparticle interactions, and prevent agglomeration. Polymers can be used to increase repulsive forces and balance magnetic and van der Waals attractive forces. Biological molecules such as proteins, antibodies, and biotin can be used to enhance biocompatibility and target specificity. Inorganic materials such as silica, metal, and metal oxides can be used to functionalize iron oxide NPs. Silica-coated NPs have several advantages, including stability in solution, prevention of interparticle