Water-soluble hollow spherical magnetite (Fe3O4) nanocages (ca. 100 nm) with high saturation magnetization are prepared in a one-pot reaction by sol-gel method and subsequent annealing to synthesise the maghemite (γ-Fe2O3) nanocages with similar nanostructures. The nanocages have been investigated by powder X-ray diffraction (XRD), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), and superconducting quantum interference device (SQUID). The results indicated that glutamic acid played an important role in the formation of the cage-like nanostructures.
KeywordsMagnetite Maghemite Sol-gel growth Nanocages
Magnetic nanoparticles are of great interest to researchers for their wide range a board of applications, including magnetic fluid, data storage, catalyst and biotechnology, owing to their unique magnetic properties such as superparamagnetic, low Curie temperature, and high coercivity, high susceptibility. . Currently, magnetic nanoparticles are used in important biological applications, mainly including magnetic bioseparation and detection of biological entities such as cell, protein, nucleic acids, enzyme, bacterials, and virus. [2, 3]. To this end, magnetic iron oxide nanoparticles have become strong candidates, and the application of small iron oxide nanoparticles in in vitro diagnostics has been practiced for nearly half a century [4, 5]. In addition, magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3) are promising and popular candidates since biocompatibility has been obtained. Usually, different biological applications require different morphologies and size of magnetic nanoparticles. Moreover, magnetic colloid particles offer attractive possibilities in bioseparation or biodetection and they should be made at dimensions comparable to those of a virus (20–500 nm), a protein (5–50 nm), or a DNA (10–100 nm) [6–10].
The internal structure and the external morphology of iron oxide nanoparticles have a significant influence on their practical applications. Particularly, the polymorphic nature of iron oxides and phase-transition studies in the nanoscale regime have attracted much attention due to its widely applications. Therefore, it is important to develop facile methods to regular both their surface morphology and size. However, it is still a technical challenge to control the size, shape, dispersibility and stability of iron oxide nanoparticles. Several preparation methods have also been reported on the synthesis of high quality of iron oxide nanoparticles, including co-precipitation, thermal decomposition, microemulsion, hydrothermal synthesis, and sonochemical method. [11, 12]. In these methods, co-precipitation was often employed for obtaining water-soluble and biocompatible iron oxide nanoparticles, but this method presents low control of the particle shape, generates broad size distributions, and cannot avoid aggregation. Thermal decomposition and microemulsion are generally stabilized in an organic solvent by surfactants. The hydrothermal synthetic route often requires high temperature and pressure . Moreover, it is important to note that using these methods at is difficult to obtain >50 nm iron oxide nanoparticles in a one-pot reaction without extra coating or seed-mediate processes .
Ferrous sulfate (FeSO4·7H2O, AR) and potassium hydroxide (KOH, AR) were purchased from Tianjin Kermel Chemical Reagent CO., Ltd., potassium nitrate (KNO3, AR) was purchased from Beijing Hongxing Chemical Reagent CO., Ltd., ethanol (C2H5OH, 95%, AR) andl(+)-glutamic acid (C5H9NO4, BR) were purchased from Sinopharm Chemical Reagent CO., Ltd., and all were used as received. The MagneticSphere Technology magnetic separation stand (MSS), purchased from Promega (Z5333), was used to separate magnetic particles using washing and selecting steps.
Preparing Hollow Spherical Magnetite Nanocages
For the synthesis of hollow spherical magnetite nanocages, in a typical synthesis, solution A was prepared by dissolving 2.02 g KNO3and 0.28 g KOH in 50 mL double distilled water; solution B was prepared by dissolving 0.070 g FeSO4·7H2O in 50 mL double distilled water. Then, the two solutions were mixed together under magnetic stirring at a rate ofca. 400 rpm. Two minutes later, solution C [prepared by dissolving 0.18 g glutamic acid (Gla) in 25 mL double distilled water] were added dropwise into the mixed solution. The reaction temperature was raised incrementally to 90 °C and kept for 3 h under argon (Ar) atmosphere. Meanwhile, the brown solution was observed to change black. After the mixture was cooled to room temperature, the precipitate products were magnetically separated by MSS, washed with ethanol and water two times, respectively, and then redispersed in ethanol (sample 1, S1). The same preparing process without added any Gla was used to obtained the sample 3 (S3).
Preparing Hollow Spherical Maghemite Nanocages
Precipitate S1 was subjected to a series of isochronal annealing at 500 °C (sample 2, S2) for 2 h in oxygen atmosphere, and the heating rate was 5 °C/min.
XRD patterns of the samples were recorded on a D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA. For TEM observations, S1 and S3 (powder samples redissolved in ethanol) were dropped on copper grids and observed on a JEOL JEM-2010 (HT) transmission electron microscope at an acceleration voltage of 150 kV. For HRTEM observations, S1 and S3 (the annealed powders redissolved in ethanol) were dropped on copper grids and observed on a JEOL JEM-2010 (FEF) field-emission transmission electron microscope at an acceleration voltage of 200 kV. Magnetic measurements were performed using a Quantum Design MPMS XL-7 SQUID magnetometer. The powder sample was filled in a diamagnetic plastic capsule, and the packed sample was then put in a diamagnetic plastic straw and impacted into a minimal volume for magnetic measurements. Background magnetic measurements were checked for the packing material. Fourier transform infrared spectrum (FT-IR) measurement was carried out on a Nicolet 5700 FT-IR Spectrometer. Vacuum-dried S1 samples were mixed and compressed with KBr to obtain pellets for FT-IR analysis.
Results and Discussion
Furthermore, such excellent magnetic properties indicate that as-prepared nanocages have strong responsivity and can be separated easily from the solution with the help of an external magnetic force. Figure 6B shows photographs of the Fe3O4nanocages and γ-Fe2O3nanocages before and after magnetic separation by an external magnetic field. This figure also illustrates the facile, fast separation process of the nanocages during the experiments.
In summary, we have demonstrated a facile one-pot reaction approach in generating hollow iron oxide nanocages. In this method, Gla plays an important role in the formation of magnetite nanocages with hollow structure. The subsequent annealing will decrease the size of the central hole of hollow nanocages. The iron oxide nanocages prepared can be well dispersed in aqueous solution and show good stability. The magnetic property measurements of Fe3O4nanocages show superparamagnetism with very high saturation magnetization close to the value of bulk Fe3O4(92 emu g−1). The synthetic strategy developed in this study may also be extended to the preparation of other magnetic nanoparticles, which also opens up new potential avenues for the nanostructural controlling and promising applications in various fields of nanotechnology.
The author thanks the National Nature Science Foundation of China (No. 10775109), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20070486069), and the Young Chenguang Project of Wuhan City (No. 200850731371) for financial support. The author thanks Associate Prof. H. -Y. Zhang of Tsinghua University for assistance with the SQUID measurements.