Method for preparing superfine magnetic iron oxide nanoparticles by using liquid nitrogen as reaction medium

Abstract

The invention relates to a method for preparing superfine magnetic iron oxide nanoparticles by using liquid nitrogen as a reaction medium, which comprises the following steps: S1, dropping a mixed aqueous solution containing Fe < 3 + > and Fe < 2 + > as a precursor solution and ammonia water into the liquid nitrogen at the same time; s2, stirring while adding ammonia water, and when the mixed solution becomes black, adding oleic acid for particle coating to obtain nanoparticles; and S3, sequentially washing the nano-particles with deionized water and acetone, collecting, filling liquid nitrogen into the obtained nano-particles, and freeze-drying to obtain the superfine magnetic iron oxide nano-particles. The method is easy to operate, the agglomeration phenomenon is effectively prevented under the low-temperature condition of liquid nitrogen, and the prepared nanoparticles have better dispersity and magnetism; due to the use of liquid nitrogen in the reaction process and a subsequent freeze-drying method, the formation of superfine nano-particle powder with uniform size is effectively realized, and the obtained nano-particles have superparamagnetism; the method is environment-friendly and low in cost.

Description

Technical Field

[0001] The invention relates to a method for preparing ultrafine magnetic iron oxide nanoparticles by utilizing liquid nitrogen as a reaction medium, and belongs to the technical field of preparation of superparamagnetic nanoparticles. Background Art

[0002] Nanoparticles are extremely small particles, ranging in diameter from one nanometer to hundreds of nanometers. They have a wide range of applications in various fields, including cosmetic dyes, functional coatings, biological research tools, medical imaging and treatment, magnetic recording media, quantum dots, and uniform nanoscale semiconductors. Nanoparticles can exist as simple molecular aggregates or as multilayer structures constructed from different materials. For example, basic nanoparticles made of magnetite or hematite are highly versatile and can be widely used in magnetic fields, such as MRI contrast agents, cell separation tools, and data storage systems.

[0003] In addition to their compact size, superparamagnetic iron oxide nanoparticles (SPIONs) exhibit a narrow particle size distribution. Among various synthesis methods, hydrolysis of metal precursors in organic solvents is the most commonly used method, which effectively reduces particle aggregation. However, before in vivo injection, these particles need to be transferred from the organic solvent to an aqueous solution. This requires replacing the hydrophobic ligands on the nanoparticle surface with hydrophilic ligands, a complex operation that requires multiple reagents and is costly.

[0004] There are many methods for synthesizing superparamagnetic iron oxide nanoparticles (SPIONs), including liquid phase method, two-phase method, sol-gel method, gas phase / aerosol method, polyol method, hydrothermal method, sonolysis method, microwave radiation method and biological method. Among them, the liquid phase method is the most mature and simple method, which can efficiently synthesize magnetic nanoparticles with a large yield and has good surface modification potential. In addition, the water solubility and biocompatibility of these nanoparticles make them suitable for applications in biology and biomedicine. However, many existing iron oxide nanoparticle synthesis methods are costly and unsuitable for large-scale production because they require expensive chemical reagents and long reaction times. The reported methods usually use oven drying, which not only consumes a lot of energy but also may affect the uniform distribution of nanoparticles, resulting in low uniformity of the finished product.

[0005] One of the main challenges in the in vivo application of superparamagnetic iron oxide nanoparticles (SPIONs) is preventing them from prematurely aggregating and being consumed by natural biological processes. Jose Fernando Contadini et al. report a method for synthesizing functionalized magnetic iron oxide nanoparticles via coprecipitation. This method uses high-temperature (60-90°C) precipitation followed by pH adjustment by the addition of acid or base. Subsequently, the surface is treated with various surfactants at high temperatures and washed with alcohol. The resulting nanoparticles have an average particle size of 50 nanometers.

[0006] Another study, reported by Pattayil et al., synthesized curcumin-coated magnetic nanoparticles for biomedical applications. This method synthesized biocompatible and stable ultrasmall superparamagnetic iron oxide nanoparticles coated with curcumin or its derivatives via a coprecipitation method without the need for linkers or binders. This method also employed high-temperature synthesis, resulting in uneven particle size distribution (US 2016 / 0310620A1).

[0007] At present, there is an urgent need to develop a low-temperature method for the synthesis of superparamagnetic iron oxide nanoparticles, which can achieve a narrow particle size distribution close to the particle size of ultrafine powder; at the same time, the method should be environmentally friendly and cost-effective, especially in terms of energy consumption.

[0008] This paper proposes a novel, environmentally friendly, low-temperature synthesis method for synthesizing superparamagnetic iron oxide nanoparticles coated with oleic acid. The method is not only applicable to various coated nanoparticles but also has the potential to be expanded to the synthesis of multi-metal nanoparticles. Summary of the Invention

[0009] (1) Technical issues to be resolved

[0010] In order to solve the above problems in the prior art, the present invention provides a method for preparing ultrafine magnetic iron oxide nanoparticles using liquid nitrogen as a reaction medium.

[0011] (2) Technical solution

[0012] In order to achieve the above objectives, the main technical solutions adopted by the present invention include:

[0013] A method for preparing ultrafine magnetic iron oxide nanoparticles using liquid nitrogen as a reaction medium comprises the following steps:

[0014] S1, containing Fe 3+ and Fe 2+ The mixed aqueous solution is used as a precursor solution and is dropped into liquid nitrogen at the same time as ammonia water;

[0015] S2. Adding ammonia water while stirring the mixed solution. When the mixed solution turns black, adding oleic acid to coat the particles to obtain nanoparticles.

[0016] S3. The nanoparticles are washed with deionized water and acetone in sequence and then collected. The obtained nanoparticles are charged into liquid nitrogen for freeze-drying to obtain ultrafine magnetic iron oxide nanoparticles.

[0017] In the above method, preferably, in step S1, the Fe 3+ and Fe 2+ The molar ratio of Fe is 1.8~2.2:1, 3+ The final concentration in the precursor solution is 0.2-3 mol / L.

[0018] Furthermore, preferably, the Fe 3+ and Fe 2+ The molar ratio is 2:1.

[0019] As described above, preferably, in step S1, the Fe 3+ The source is at least one of FeCl3, FeCl3·6H2O, Fe2(SO4)3, Fe(NO3)3·9H2O, Fe 2+ The source is at least one of FeCl2, FeSO4, Fe(NO3)2, and FeSO4·7H2O.

[0020] In the above method, preferably, in step S1, the volume ratio of the precursor solution to liquid nitrogen is 4 to 5:1, and concentrated aqueous ammonia and the precursor solution are simultaneously added dropwise to the liquid nitrogen at the same rate of 0.1 to 0.2 mL / s. The aqueous ammonia is added in excess, with the volume being 20% ​​to 50% of the volume of the precursor solution. During this process, excessive liquid nitrogen can cause the liquid to freeze, affecting the reaction process. Therefore, the volume ratio of the precursor solution to liquid nitrogen is preferably 4 to 5:1.

[0021] In the above method, preferably, in step S1, the concentration of the ammonia water is 13.4 mol / L to 15.8 mol / L, and the rate of adding the ammonia water is 0.1 to 0.2 mL / s.

[0022] In the above method, preferably, in step S2, the stirring rate is 400-600 rpm.

[0023] In the above method, preferably, in step S2, the amount of oleic acid is Fe 2+ 0.03 to 0.4 times the molar amount of

[0024] In the above method, preferably, in step S2, stirring is continued for 15 to 60 minutes after adding oleic acid.

[0025] In a preferred embodiment, in step S3, the washing is stopped after the acetone supernatant becomes transparent and colorless.

[0026] In a preferred embodiment, in step S3, after washing, the washing solvent is removed, the nanoparticles are collected, and liquid nitrogen is added to the nanoparticles; then freeze-drying is performed, and the freeze-drying temperature is -20 to -40°C and the time is 4 to 18 hours.

[0027] Furthermore, the amount of liquid nitrogen used is sufficient to cover the nanoparticles. The functions of the liquid nitrogen introduced during this process include: (1) rapid freezing and structural protection: the extremely low temperature of liquid nitrogen (about -196°C) can instantly freeze the water in the sample to form tiny ice crystals. This helps to avoid the destruction or agglomeration of the nanoparticle structure caused by ice crystal growth. When the residual solvent (such as water, acetone) after washing is rapidly frozen, the low temperature of liquid nitrogen can inhibit the Brownian motion of the nanoparticles during the freezing process and reduce the particle collision and aggregation caused by slow freezing. (2) inert gas protection: the nitrogen released after the liquid nitrogen evaporates can replace the air around the sample, forming an inert protective layer to prevent Fe3O4 from being oxidized during the freezing or freeze-drying process (for example, generating byproducts such as Fe2O3). Stabilizing the chemical environment: the chemical inertness of nitrogen can prevent other gases (such as CO2, water vapor) from interfering with the functional groups on the surface of the nanoparticles (such as oleic acid ligands), thereby maintaining the dispersion and stability of the particles. (3) Rapid freezing with liquid nitrogen ensures a uniform temperature gradient within the sample, providing a uniform distribution of ice crystals for subsequent freeze-drying (sublimation drying), thereby improving drying efficiency. (4) Maintaining dispersion: In the frozen state, the van der Waals forces between particles are suppressed, which helps maintain the dispersion of the nanoparticles and prevents agglomeration after freeze-drying.

[0028] (3) Beneficial effects

[0029] The beneficial effects of the present invention are:

[0030] The method for preparing ultrafine iron oxide nanoparticles using liquid nitrogen, provided by the present invention, is a novel, low-cost, and environmentally friendly method. It employs a simple process to synthesize uniformly sized ultrafine iron oxide nanoparticles under low-temperature liquid nitrogen conditions. This low-temperature method effectively prevents agglomeration, resulting in nanoparticles with improved dispersibility (ultrafine powder) and magnetic properties. The combined use of liquid nitrogen during the reaction and subsequent freeze-drying effectively achieves the formation of a uniformly sized ultrafine nanoparticle powder, and the resulting nanoparticles exhibit superparamagnetism. Experiments have demonstrated that the resulting iron oxide nanoparticles exhibit spherical structures with smooth, uniform surfaces, and particle sizes ranging from 3 nm to 13 nm, with an average diameter of 6.3 nm, demonstrating ultrafine properties.

[0031] The method provided by the present invention for preparing ultrafine iron oxide nanoparticles using liquid nitrogen avoids the use of high temperatures and provides a rapid and energy-saving synthesis route compared to traditional methods. This technology requires minimal equipment and significantly reduces energy consumption, making it both environmentally friendly and low-cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of the reaction principle of the method of the present invention;

[0033] Figure 2 Schematic diagram of the process of the present invention;

[0034] Figure 3 The SEM analysis results of the nano-iron particles obtained in Example 2 were magnified 100,000 times;

[0035] Figure 4 The SEM analysis results of the nano-iron particles obtained in Example 2 were magnified 21,000 times;

[0036] Figure 5 The SEM analysis results of the nano-iron particles obtained in Example 2 were magnified 50,000 times;

[0037] Figure 6 The SEM analysis results of the nano-iron particles obtained in Example 2 were magnified 21,000 times;

[0038] Figure 7 TEM images of oleic acid-coated nano-iron Fe3O4 particles in Example 2 at different magnifications;

[0039] Figure 8 FTIR spectrum results of oleic acid-coated nano-iron Fe3O4 particles in Example 2;

[0040] Figure 9 This is the VSM image of the oleic acid-coated nano-iron Fe3O4 particles in Example 2. DETAILED DESCRIPTION

[0041] The inventive principle of the present invention is to convert Fe 3+ and Fe 2+ The two ions are preferably mixed in a molar ratio of 2:1 to form a precursor solution, which is also the key to the formation of Fe3O4; 3+ / Fe 2+ The mixed solution and ammonia water were added into liquid nitrogen and frozen and mixed. Figure 1As shown, frozen droplets are formed under the influence of the low temperature environment of liquid nitrogen. These droplets quickly settle due to the nucleation and growth process. Oleic acid is then added to coat the particles. Finally, freeze-drying in liquid nitrogen (i.e., lyophilization) is performed to prepare oleic acid-coated ultrafine iron oxide nanoparticles. The use of liquid nitrogen to provide freeze-thaw conditions avoids the use of high temperatures, providing a rapid and energy-efficient synthesis route compared to traditional methods. This technology requires minimal equipment and significantly reduces energy consumption, enabling the synthesis of nanoparticles within 5 to 10 minutes.

[0042] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below through specific implementation methods in conjunction with the accompanying drawings.

[0043] Example 1

[0044] To synthesize Fe3O4 nanoparticles, FeCl3·6H2O and FeSO4·7H2O were first dissolved in deionized water. 3+ and Fe 2+ The solutions were mixed in a molar ratio of 2:1 and stirred for 10 minutes. 3+ The final concentration in the mixed solution is 0.2-3 mol / L. Next, the concentrated ammonia solution and the iron ion mixed solution are transferred to pipettes respectively. In a 1L glass beaker, 100ml of liquid nitrogen is added, and then 500ml of the iron ion mixed solution and 150ml of concentrated ammonia water are slowly added dropwise. The drop rate is controlled at 0.1ml / s and stirred at 500rpm. When the solution turns black, 3ml of oleic acid is added. The final Fe3O4 nanoparticles are washed sequentially with deionized water and acetone and then collected. After removing the washing solvent, 100ml of liquid nitrogen is added to the product, and then freeze-dried overnight at -40°C.

[0045] Example 2

[0046] Weigh 130.5g of FeCl3·6H20 and 69.5g of FeSO4·7H2O, dissolve them in 200ml of deionized water, stir them evenly at 500rpm for 10 minutes, and then combine FeCl3·6H20 and FeSO4·7H2O. 3+ and Fe 2+ The mixed solution was stirred for 10 minutes to obtain a precursor solution. Figure 2As shown, 150 ml of concentrated ammonia with a mass concentration of 30% was then added dropwise at a rate of 0.1 ml / s, along with the precursor solution, into 100 ml of liquid nitrogen. When the precursor solution turned from light brown to black, the reaction was considered to be terminated. 3 ml of oleic acid was added for particle coating, and stirring was continued for half an hour until the solution exhibited a uniform and stable black color, indicating the completion of the coating process. The resulting Fe3O4 nanoparticles were washed with deionized water and then acetone until the acetone supernatant was transparent and colorless. The acetone was removed, and the resulting nano-iron particles were placed in a freeze dryer and 100 ml of liquid nitrogen was added. Finally, they were dried overnight (12 hours) at -40°C to obtain the finished oleic acid-coated Fe3O4.

[0047] The obtained products were tested by scanning electron microscopy (SEM), transmission electron microscopy, Fourier transform infrared spectroscopy (FTIR), and vibrating sample magnetometer (VSM). The scanning electron microscopy showed the morphology and size distribution of oleic acid-coated ultrafine iron oxide nanoparticles prepared by the freeze mixing method. The results are as follows: Figure 3 、 Figure 4 、 Figure 5 and Figure 6 As shown, Figure 3-Figure 6 The following are SEM images of the Fe3O4 particles obtained in Example 2 at different magnifications and scales. The nanoparticles have a spherical structure and a smooth surface, indicating that they grow uniformly. The particle size range is 3nm to 13nm, with an average diameter of 6.3nm, which verifies the ultrafine characteristics of the nanoparticles. The observed micro-aggregation phenomenon indicates an effective stabilization effect during the synthesis process. The uniformity of particle size and morphology is attributed to the precise control of precipitation conditions during the reaction process, such as low-temperature reaction conditions and the use of stabilizers. The nanoparticles with stable structure and uniform size indicate that they have undergone a rapid nucleation stage, followed by a controlled growth process. These structural features make this type of nanoparticles an ideal material for synthesizing magnetic liquids and biomedical applications, especially in targeted drug delivery and magnetic resonance imaging.

[0048] Transmission electron microscopy (TEM) images provide detailed information on the morphology, size, and crystal structure of the synthesized ultrafine magnetic iron oxide nanoparticles. Figure 7As shown in the figure, a, b, c, and d are TEM images at different magnifications. The oleic acid-modified Fe3O4 nanoparticles are spherical with a narrow size distribution (5-13 nm). The nanoparticles exhibit a predominantly spherical morphology, indicating isotropic growth during the synthesis process. The particles are well dispersed and agglomeration is not evident, indicating that the surfactant used effectively stabilizes the nanoparticles. The uniform morphology and good crystallinity ensure consistent magnetic properties, making these nanoparticles ideal for magnetic liquids and medical applications.

[0049] FTIR spectroscopy confirmed the presence of oleic acid on the surface of Fe3O4 nanoparticles. Figure 8 As shown. At 3350-3500cm -1 A characteristic broad band is observed in the range of 550-650 cm, which is attributed to water or active groups from oleic acid. The broadening of the absorption band in this region is believed to be due to the introduction of three OH groups in the oleic acid molecule. Another characteristic peak appears at 550-650 cm -1 range, indicating that Fe3O4 nanoparticles were synthesized. In addition, the 1660cm -1 and 1400.5cm -1 The significant absorption peaks at are related to the stretching vibrations of CO-Fe, COC and CH2, respectively. These peaks are attributed to the active groups of oleic acid, which activate the surface of Fe3O4 nanoparticles.

[0050] The VSM image of Fe3O4 particles obtained in Example 2 is as follows Figure 9 The VSM graph in the figure shows the magnetic behavior of the synthesized magnetic iron oxide nanoparticles, revealing their key magnetic properties. The VSM graph exhibits a typical symmetrical S-shaped hysteresis loop, indicating that these nanoparticles are superparamagnetic. The saturation magnetization (Ms) of the nanoparticles is 61.77 emu / g, and the remanent magnetization (Mr) and coercivity (Hc) are close to zero, indicating that they exhibit superparamagnetism at room temperature. The absence of remanent magnetization and coercivity further confirms that these nanoparticles are superparamagnetic, that is, they only exhibit magnetization when an external magnetic field is applied and no residual magnetization remains when the field is removed. The high Ms value reflects the strong magnetic response of the iron oxide nanoparticles, making them suitable for magnetic fluids and biomedical applications. The observed Ms value of 61.77 emu / g is higher than that of most previously reported high-quality magnetite nanoparticles, demonstrating the effectiveness of the synthesis process. The superparamagnetic behavior and high magnetization are consistent with the ultrafine size of the nanoparticles, ensuring their single-domain nature.

[0051] Comparative Example 1

[0052] Weigh 130.5g of FeCl3·6H20 and 69.5g of FeSO4·7H2O, dissolve them in 200ml of deionized water, stir them evenly at 500rpm for 10 minutes, and then combine FeCl3·6H20 and FeSO4·7H2O. 3+ and Fe 2+ The Fe3O4 nanoparticles were washed with deionized water and acetone, and the acetone supernatant was transparent and colorless. Acetone was removed by magnetic separation, and the nano iron particles obtained were dispersed in kerosene. The iron particles average particle size obtained was 30nm, and agglomeration was observed. It was confirmed that the particles generated were larger than the particles obtained in Example 2 without introducing liquid nitrogen.

[0053] The method of the present invention has the following advantages: (1) The method uses a simple one-step process to synthesize iron oxide nanoparticles coated with a surfactant (oleic acid as a surfactant) at a low temperature of -40°C. Compared with high-temperature synthesis methods, this low-temperature method effectively prevents agglomeration, and the resulting nanoparticles have better dispersibility (ultrafine powder) and improved magnetic properties.

[0054] (2) The combined use of liquid nitrogen during the reaction and the subsequent freeze-drying method effectively achieved the formation of ultrafine nanoparticle powders of uniform size.

[0055] (3) The obtained nanoparticles have superparamagnetism, and their magnetic value is 61.77emu / g.

[0056] (4) The method is both environmentally friendly and low-cost in terms of energy consumption.

[0057] (5) This method can be extended to the synthesis of bimetallic or trimetallic nanoparticles for various applications.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the above-disclosed embodiments into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above-disclosed embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention shall remain within the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine magnetic iron oxide nanoparticles using liquid nitrogen as a reaction medium, characterized in that: It includes the following steps: S1, containing Fe 3+ and Fe 2+ The mixed aqueous solution is used as a precursor solution and is dropped into liquid nitrogen at the same time as ammonia water; S2. Adding ammonia water while stirring the mixed solution. When the mixed solution turns black, adding oleic acid to coat the particles to obtain nanoparticles. S3. The nanoparticles are washed with deionized water and acetone in sequence and then collected. The obtained nanoparticles are charged into liquid nitrogen for freeze-drying to obtain ultrafine magnetic iron oxide nanoparticles.

2. The method according to claim 1, wherein In step S1, the Fe 3+ and Fe 2+ The molar ratio of Fe is 1.8~2.2:1, 3+ The final concentration in the precursor solution is 0.2-3 mol / L.

3. The method according to claim 1, wherein In step S1, in step S1, the Fe 3+ The source is at least one of FeCl3, FeCl3·6H2O, Fe2(SO4)3, Fe(NO3)3·9H2O, Fe 2+ The source is at least one of FeCl2, FeSO4, Fe(NO3)2, and FeSO4·7H2O.

4. The method according to claim 1, wherein In step S1 , the volume ratio of the precursor solution to liquid nitrogen is 4 to 5:1, and the volume of ammonia water added is 20% to 50% of the volume of the precursor solution.

5. The method according to claim 1, wherein In step S1 , the concentration of the ammonia water is 13.4 mol / L to 15.8 mol / L, and the ammonia water and the precursor solution are simultaneously added dropwise into the liquid nitrogen at the same rate of 0.1 to 0.2 mL / s.

6. The method according to claim 1, wherein In step S2, the stirring speed is 400-600 rpm.

7. The method according to claim 1, wherein In step S2, the amount of oleic acid is Fe 2+ 0.03 to 0.4 times the molar amount of 8. The method according to claim 1, wherein In step S2, oleic acid is added and stirring is continued for 15 to 60 minutes.

9. The method according to claim 1, wherein In step S3, the washing is stopped when the acetone supernatant becomes transparent and colorless.

10. The method according to claim 1, wherein In step S3, after washing, the washing solvent is removed, the nanoparticles are collected, and liquid nitrogen is added to the nanoparticles; then freeze-drying is performed, and the freeze-drying temperature is -20 to -40°C and the time is 4 to 18 hours.

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