Magnetic iron oxide nanoparticles (MNPs) have been researched extensively due to their promising applications for biomedicine such as hyperthermia[
Journal of Inorganic Materials, Volume. 34, Issue 8, 899(2019)
Zn, Mn doped Fe3O4 magnetic nanoparticles have broad application prospects in biomedicine for their excellent magnetic properties. Therein, the most remarkable property of magnetic nanoparticles is size-dependent biomagnetic applications, and size variation also affect their magnetic characteristics. Therefore, based on the specific requirements of size for various biological applications, it is critical to regulate their size. In this study, we synthesized 5-20 nm Zn, Mn doped Fe3O4 magnetic nanoparticles by changing reflux time duration, varying metal precursor and adding oil phase reducing agent (1,2-hexadecanediol). It is found that addition of 1,2-hexadecanediol is beneficial to the formation of smaller nanoparticles, while metal chloride and longer reflux time are helpful to prepare larger particles. Additionally, there exists a positive correlation between particle size and saturation magnetization.
Magnetic iron oxide nanoparticles (MNPs) have been researched extensively due to their promising applications for biomedicine such as hyperthermia[
For decades, a variety of preparation methods of MNPs achieved in hydrolytic phase have been developed, including hydrothermal reaction[
Size-dependent biomagnetic applications are the most remarkable properties of MNPs[
1 Experimental
Material Oleic acid (OAc) and Oleylamin (OAm) were purchased from Sigma-Aldrich. Other chemicals were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd.
Preparation of 5 nm ZnMn-Fe3O4 nanoparticles Oleylamine (3 mmol), oleic acid (3 mmol), Fe(acac)3 (1 mmol), Zn(acac)2 (0.2 mmol) , Mn(acac)2 (0.3 mmol) and 1,2-hexadecanediol (5 mmol) were placed in a 50 mL three-neck round-bottom flask in the presence of 15 mL benzyl ether. The mixture was heated to 200 ℃ for 30 min and then to 300 ℃ for 1 h under argon atmosphere. After cooling to room temperature, excess ethanol was added to the solution, then a black powder precipitated and was collected by centrifugation. The magnetic nanoparticles were redispersed in cyclohexane.
Preparation of 10 nm ZnMn-Fe3O4 nanoparticles Oleylamine (3 mmol), oleic acid (3 mmol), Fe(acac)3 (1 mmol), Zn(acac)2 (0.2 mmol) and Mn(acac)2 (0.3 mmol) were placed in a 50 mL three-neck round-bottom flask in the presence of 15 mL benzyl ether. The mixture was heated to 200 ℃ for 30 min and then to 300 ℃ for 1 h under argon atmosphere. After cooling to room temperature, excess ethanol was added to the solution, then a black powder precipitated and was collected by centrifugation. The magnetic nanoparticles were redispersed in cyclohexane.
Preparation of 15 nm ZnMn-Fe3O4 nanoparticles Oleylamine (3 mmol), oleic acid (3 mmol), Fe(acac)3 (1 mmol), ZnCl2 (0.2 mmol) and MnCl2 (0.3 mmol) were placed in a 50 mL three-neck round-bottom flask in the presence of 15 mL benzyl ether. The mixture was heated to 200 ℃ for 30 min and then to 300 ℃ for 1 h under the atmosphere of argon. After cooling to room temperature, excess ethanol was added to the solution. The black precipitate was collected by centrifugation and washed with ethanol prior to redispersion in cyclohexane for further use. Similarly, by prolonging reaction time to 1.5 or 2 h, 20 nm ZnMn-Fe3O4 nanoparticles can be prepared.
Characterization Transmission electron microscopy (TEM) was performed on JEM-2100F instruments. The XRD studies were conducted via a Rigaku D/MAX-2250V diffractometer with a Cu Kα radiation source (40 kV, 120 Ma). Magnetic measurements were performed on a Vibrating Sample Magnetometer (VSM). FT-IR spectra were recorded by an IRPRESTIGE-21 spectrometer (Shimadzu) using KBr pellets. Size distribution were measured on Nano-Zetasizer (Malvern Instruments Ltd.).
2 Results and discussion
Reductive effect As shown in Fig. 1(a), monodispersed ZnMn-Fe3O4 nanoparticles about 5 nm were fabricated by decomposition of Fe(acac)2, Mn(acac)2 and Zn(acac)2 in the presence of oleylamine, oleic acid and 1,2-hexadecanediol. Under the same experimental conditions, if 1,2-hexadecanediol was not added, the size of particles increased from 5 nm to 10 nm (Fig. 1(b)), which were further proved by the alteration of size distribution (Fig. 1(c, d)). Organic-phase thermal decomposition in fact is a process that activates metal atoms to nucleate and grow[
Figure .TEM images of ZnMn-Fe3O4 nanoparticles and histograms of their size distributions obtained by Fe(acac)3, Mn(acac)2 and Zn(acac)2 with (a, c) and without (b, d) adding 1,2-hexadecanediol
Metal precursor effect Metal precursors play an important role in size regulation of magnetic nanoparticles. The size of nanoparticles synthesized by metal chloride as precursor is about 15 nm with narrow size distribution (Fig. 2(a, b)). The high resolution TEM (Fig. 2(c)) and selected area electron diffraction (SAED) pattern (Fig. 2(d)) suggest that the particles have high-quality crystallinity. In contrast, using acetylacetone as metal precursor, the size of obtained nanoparticles is about 10 nm (Fig. 1(b)). According to chemical bond theory, metal chloride is more stable than metal acetylacetone, thus it requires higher decomposition temperature. Therefore, the addition of metal chloride may lead to slower nucleation rate, more metal precursor could deposit surrounding the nuclei formed in mixture and resulting in larger nanoparticles.
Figure .TEM image (a), histograms of their size distributions (b), high-resolution TEM image (c) and selected area electron diffraction (SAED) pattern (d) of 15 nm-sized ZnMn-Fe3O4 nanoparticles prepared from Fe(acac)3, MnCl2 and ZnCl2
Reflux time effect It is also important to study the effect of reaction time on the size of nanoparticles. In this work, we found that larger particles could be obtained by longer reflux time. As shown in Fig. 3(a), when reflux time was postponed from 1 h to 1.5 h under the same reaction condition, the particles size increased from 15 nm (Fig. 2(a)) to 20 nm (Fig. 3(a)), which were further proved by the alteration of size distribution (Fig. 3(c)). However, further prolongation of the reflux time to 2 h resulted in no distinct increase in the size of nanoparticles (Fig. 3(b, d)), which indicates that the nanocrystals will stop growing when the reaction reaches homeostasis. In the meantime, it seems to produce new nanocrystal with longer reflux time (Fig. 3(b)).
Figure .TEM images of 20 nm-sized ZnMn-Fe3O4 nanoparticles synthesized from Fe(acac)3, MnCl2 and ZnCl2 with different reflux time durations of 1.5 h (a) and 2 h (b), and the corresponding histograms of size distributions of 1.5 h (c) and 2 h (d)
The XRD pattern (Fig. 4(a)) shows a single-phase spinel structure of 5-20 nm ZnMn-Fe3O4 nanoparticles and the relative intensity and position of all diffraction peaks/rings accord well with previously reported Zn, Mn doped Fe3O4 powder[
Figure .XRD patterns (a), FT-IR spectra (b) and energy dispersive X-ray spectroscopy (EDS) data (c) of 5 nm, 10 nm, 15 nm, and 20 nm-sized ZnMn-Fe3O4 nanoparticles
Size effect on magnetic characteristics Magnetic characteristics of MNPs with different size were measured by VSM at 300 K, and the results are shown in Fig. 5(a). The hysteresis curve of MNPs in 5-15 nm exhibits superparamagnetism without remnant magnetization. However, 20 nm particles are ferromagnetic with a coercivity of 150 Oe (1 Oe≈79.6 A/m). Saturation magnetization (Ms) value of MNPs gradually increased as the size of MNPs increased from 5 to 20 nm, and reached the maximum of 77.7 emu/g (emu/g=4π×10-7 Wb∙m/kg). As Fig. 5(b) shown, Ms and d-1 (the reciprocal of the nanoparticles size) display an approximate linear relationship, which is consistent with previous reports[
Figure .Magnetic hysteresis curve (a) and d-1-dependent Ms curve (b) of 5-20 nm-sized ZnMn-Fe3O4 nanoparticles
3 Conclusion
We have successfully synthesized 5-20 nm ZnMn-Fe3O4 by changing metal precursors, varying reflux time and adding 1,2-hexadecanediol. These ferrite nanoparticles with different particle sizes could adapt to the needs of extensive biomagnetic applications. Smaller sized nanoparticles could be obtained by introducing extra reducing power 1,2-hexadecanediol, and we can prepare larger particles by using more stable metal precursor and extending reflux time. These seem to indicate that powerful reducing agent and metal precursor with low decomposition temperature will yield smaller particles. Meanwhile, the magnetic characteristics of ZnMn-Fe3O4 are closely related to particle size. Understanding how the different parameters impact on particle size is extremely critical to develop a more predictive and controllable way to synthesize the MNPs.
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Tian-Sheng CHENG, Jiong PAN, Ying-Ying XU, Qun-Qun BAO, Ping HU, Jian-Lin SHI.
Category: RESEARCH LETTERS
Received: Jan. 6, 2019
Accepted: --
Published Online: Sep. 26, 2021
The Author Email: HU Ping (huping@mail.sic.ac.cn), SHI Jian-Lin (jlshi@mail.sic.ac.cn)