The development of nanotechnology has opened up a new field for the research of various materials. Nowadays, a variety of nanomaterials have been fully applied in the field of autocorrelation, and the value of nanomaterials has been fully affirmed. Therefore, the development of new nanomaterials has become a necessity of modern society, with great development prospects. Nanometer Fe3O4 has the characteristics of small particle size, high specific surface area and magnetic sensitivity. It has been widely applied in the fields of biological separation [1-2], targeting [3-5], tumor magnetic hyperthermia [6-7] and immune detection [8-9]. Therefore, the preparation of Fe3O4 magnetic nanoparticles and the research and summary of some characteristics of Fe3O4 magnetic nanoparticles have guiding significance for the development of some related fields in China, and also provide convenience for some participating researchers. The preparation and modification of nano Fe3O4 particles are briefly described in this paper.
Preparation of 1 nanometer Fe3O4 particle
1.1 solid phase synthesis method
1.1.1 ball grinding method 
It can be divided into ordinary ball grinding method and high-energy ball grinding method. Ordinary ball grinding method refers to crushing Fe3O4 coarse particles with a particle size of tens of microns into fine particles in a ball mill through the impact between steel balls or between steel balls and the inner wall of the grinding tank. High energy ball milling method is to use high energy ball mill for mechanical alloying of raw materials to synthesize nanometer spinel ferrite. The grain size of ball milling products is not uniform and impurities are easily introduced.
1.1.2 pyrolysis method
Iron organic matter decomposition method is to decompose iron precursors (such as Fe(CO)5, Fe(CuP)3, etc.) at high temperature to produce iron atoms, and then generate iron nanoparticles from the iron atoms, and control the oxidation of iron nanoparticles to obtain iron oxide. The nanoparticles obtained by this method have high crystallinity, controllable particle size and narrow distribution . Using the decomposition method of high-temperature organic precursors, du yunghua et al.  prepared Fe3O4 nanoparticles with good monodispersity using Fe(acac)3 as the precursors, with an average particle size of (6.4±0.9)nm, and obtained orderly monolayer and multilayer structures arranged in a regular manner through self-assembly of these particles.
1.2 liquid phase synthesis
Precipitation method includes co-precipitation method, oxidation precipitation method, reduction precipitation method, alternating current precipitation method and complex decomposition method, etc. . Due to its high yield and low cost, the coprecipitation method has been widely used in the preparation, but the preparation process is complicated and the products prepared are easy to aggregate and oxidize. Zhang xin et al.  used the co-precipitation method to prepare Fe3O4 with particle size within 20 nm under the condition that the volume ratio of Fe3+ and Fe2+ solutions was 1:1 and the reaction temperature was 30 ℃. Wu jun-hua et al.  studied the effect of reaction temperature on the particle size of nano-fe3o4. The particle size obtained at 20 ℃ and 80 ℃ was at least 2 ~ 4 nm. The most difficult problem of coprecipitation is how to disperse Fe3O4 nanoparticles and keep them from agglomerating. For this reason, many scholars have improved the co-precipitation method by adding surfactant to cover the particle surface after the formation of Fe3O4 particles, so as to reduce agglomeration. Cheng haibin et al.  used an improved co-precipitation method to produce nano-fe3o4 composite particles with sodium dodecyl benzene sulfonate (SDBS) as the surfactant, which could be dispersed stably in a wider pH range (1-9). The nano-fe3o4 particles generated by the co-precipitation method are extremely unstable, and their stability is inversely proportional to the pH value. The deposition occurs immediately when standing in the strongly alkaline medium. With the decrease of pH value, the stability is improved, but precipitation will be precipitated after several minutes of standing.
1.2.2 hydrothermal synthesis
The method USES the characteristic that the solubility of some hydroxides in water is higher than that of the corresponding oxides in water under high temperature and high pressure. Liu Yi etc.  with FeSO4 · 7 H2O, (NH4) 2 Fe (SO4) 2 · 6 H2O and NaOH, NH3 · H2O as raw materials, with KC1O4 and – KNO3 as oxidant, the hydrothermal synthesis method in 110 ℃ 14 h reaction, successful synthesis of the cubic phase of the six-party flake Fe3O4 nanocrystals, and Fe3O4 single-crystal nanorods, with Fe3O4 single-crystal nanorods was 60 nm in diameter, length is between 0175 ~ 1175 um, the saturation magnetization of 39154 emu/g. Chen D et al.  used N2 as the environmental gas, and Fe (OMOE) 2 was reflued in MOE for 4 h, and then a certain amount of mixed solution of MOE and H2O was added under magnetic agitation, and the suspension obtained was reacted in hydrothermal kettle to obtain Fe3O4 nanoparticles with different particle sizes. Using ferrous chloride (FeC12·4H2O), ferric chloride (FeC13·6H2O), Na2SO3, H2O2, NaOH and hydrazine hydrate as raw materials, zhou xiaoli et al.  successfully prepared magnetic Fe3O4 nanoparticles by hydrothermal synthesis. The effects of different surfactants and oxidants on the synthesis process were also studied. The advantage of this method is that the oxide can be generated directly, which avoids the step of conversion to oxide through calcination in the general liquid phase synthesis method and greatly reduces the hard agglomeration.
1.2.3 microemulsion method 
Water-in-oil (W/O) or water-in-oil (O/W) microemulsions are formed by surfactant, oil phase, water phase and cosolvent.the reaction is confined to the micro-reactor of microemulsion droplet. Thus, nanometer powders with narrow particle size distribution, regular morphology and good dispersibility were obtained. At the same time, the nucleation and growth can be controlled by controlling the volume of water in microemulsion droplets and the concentration of various reactants, so as to obtain monodisperse nanoparticles of various sizes. The nano-magnetic Fe3O4 prepared by microemulsion method has uniform particle size, small particle size, good dispersion and mostly spherical. However, this method consumes a large amount of emulsifier and has a low yield, so it is expensive and not suitable for mass production.
1.2.4 hydrolysis method 
The hydrolysis method can be divided into two types: Massart method and titration hydrolysis method. And what we say commonly hydrolysis means titration hydrolysis. The hydrolysis method has a low requirement for equipment, and the reaction can be carried out under relatively mild conditions. The raw materials used are cheap inorganic salts, the process is simple, the reaction product is of high purity, and the particle dispersion is relatively good. To some extent, the problem of reunion has been solved. However, during the preparation of this method, many factors affecting the particle size and magnetic properties of the powder (such as reactant concentration, reaction temperature, precipitant concentration and adding speed, stirring condition and pH value, etc.) should be considered, so that the technological parameters of the experiment must be strictly controlled. Qiu Xing screen  by hydrolysis titration and Massart synthesis method, respectively, the preparation of the Fe3O4 nanoparticles at about 8 nm in diameter, transmission electron microscope found that the Fe3O4 nanoparticles prepared by hydrolyzing titration are mainly spherical, particle size is uniform, while the Massart get Fe3O4 nanoparticles were prepared from the ball to the cube of a variety of forms, and broad particle size distribution. Zhongbing Huang et al.  used hydrolysis method to prepare Fe3O4 nanoparticles with particle size of about 10 nm and narrow particle size distribution.
1.2.5 sol-gel method
The homogeneous sol of metal oxide or metal hydroxide was prepared by the hydrolysis and polymerization of metal alcohol brine, and then condensed into transparent gel. LouMinYi etc.  after ultrasonic dispersion of nano-sized Fe3O4 magnetic particles in thick poly SiO2 sol, with acetone, deionized water, ammonia mixed to form water-in-oil emulsion, finally after solvent replacement, washing and heat treatment, the preparation of the particle size distribution are mainly distributed in about 20 nm, spherical monodisperse magnetic microspheres, and showed good superparamagnetism and magnetic responsiveness. This study is characterized by the use of two-step catalysis of acid and base, which utilizes the respective characteristics of the two catalysis methods and has complementary advantages. At the same time, sol-gel method and emulsion pelletizing technology are combined to solve the problem of poor sphericity of SiO2 magnetic microspheres. This method has been widely used for preparing single-component or multi-component molecular-grade mixtures with high purity, uniform particle size and high chemical activity at low temperature. However, the metal alcohols of the precursors are highly toxic, sensitive to trace moisture and expensive.
1.2.6 biological template synthesis
Because DNA binding proteins, small heat shock proteins, listeria bacteria, ferritin and other internal hole structures, they can be used as templates to synthesize magnetic nano-fe3o4 particles. For example, ferritin holes have an inner diameter of 8nm and an outer diameter of 12nm, DNA binding protein holes have an inner diameter of 6nm and an outer diameter of 9nm. The inner diameter of the tobacco Mosaic virus hole was 8nm and the outer diameter was 12nm. Using these cavitation structures, the researchers were able to synthesize nano-fe3o4 particles.
1.2.7 microwave hydrothermal method
Microwave hydrothermal method was proposed by Roy  of the university of Pennsylvania in 1992. The preparation of fine powder by microwave hydrothermal method USES the microwave field as the heat source, and the reaction medium reacts in the special pressure-resistant reaction kettle that can pass through the microwave field. Under the action of microwave radiation, the polar molecule receives the energy of microwave radiation, and the dipole rotates at a high speed of hundreds of millions of times per second to produce the thermal effect. Because microwave hydrothermal method has the characteristics of fast heating speed, sensitive reaction and uniform heating system, it can quickly prepare nanoparticles with narrow particle size distribution and uniform morphology. Therefore, microwave hydrothermal method has great potential research value in the preparation of ultrafine powder. The researcher hai yanbing et al.  could prepare Fe3O4 nanoparticles with an average particle size of 30 nm by using microwave oven for 8 s, with a yield of up to 90% and good dispersion.
1.2.8 oxidation method 
Oxidation method is the most commonly used method for the preparation of ultrafine Fe3O4. It is to mix iron salts and lye of a certain concentration and precipitate them to form Fe(OH)2. After air stirring at constant temperature, part of Fe2+ in Fe(OH)2 is oxidized to Fe3+ to directly obtain Fe3O4 powder.