Title: Facile synthesis of superparamagnetic iron oxide nanoparticles with tunable size: from individual nanoparticles to nanoclusters
Abstract: Micro & Nano LettersVolume 12, Issue 10 p. 749-753 ArticleFree Access Facile synthesis of superparamagnetic iron oxide nanoparticles with tunable size: from individual nanoparticles to nanoclusters Xuan Mi, Xuan Mi School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorMeikui Tong, Meikui Tong School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorJia Cai, Jia Cai Changzheng Hospital, Secondary Military Medical University, Shanghai, 200003 People's Republic of ChinaSearch for more papers by this authorHui Su, Hui Su State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240 People's Republic of ChinaSearch for more papers by this authorShiyuan Liu, Shiyuan Liu Changzheng Hospital, Secondary Military Medical University, Shanghai, 200003 People's Republic of ChinaSearch for more papers by this authorYongjie Ma, Yongjie Ma School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorXunbin Wei, Xunbin Wei School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorChunfu Zhang, Corresponding Author Chunfu Zhang [email protected] School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of China Department of Nuclear Medicine, Rui Jin Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, 200025 People's Republic of ChinaSearch for more papers by this author Xuan Mi, Xuan Mi School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorMeikui Tong, Meikui Tong School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorJia Cai, Jia Cai Changzheng Hospital, Secondary Military Medical University, Shanghai, 200003 People's Republic of ChinaSearch for more papers by this authorHui Su, Hui Su State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200240 People's Republic of ChinaSearch for more papers by this authorShiyuan Liu, Shiyuan Liu Changzheng Hospital, Secondary Military Medical University, Shanghai, 200003 People's Republic of ChinaSearch for more papers by this authorYongjie Ma, Yongjie Ma School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorXunbin Wei, Xunbin Wei School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of ChinaSearch for more papers by this authorChunfu Zhang, Corresponding Author Chunfu Zhang [email protected] School of Biomedical Engineering, Shanghai Jiao Tong University Affiliated Sixth People's Hospital, Shanghai Jiao Tong University, Shanghai, 200030 People's Republic of China Department of Nuclear Medicine, Rui Jin Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, 200025 People's Republic of ChinaSearch for more papers by this author First published: 01 October 2017 https://doi.org/10.1049/mnl.2017.0192AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Superparamagnetic iron oxide nanoparticles (SPIONs) have a wide variety of biomedical applications. Synthesis of SPIONs with well-defined, tunable size in a facile way is highly desirable. In this work, a one-pot polyol method was developed for preparation of SPIONs with tunable size and high quality in the presence of poly (acrylic acid) (PAA). By adjusting the amount of PAA used, the isolated nanoparticles and nanoparticle clusters were able to be synthesised. As-synthesised nanoparticles and nanoclusters were highly stable in physiological conditions. The size of the nanoparticles was 7 ± 1 nm and that for the nanoclusters was 20 ± 2 and 32 ± 3 nm, respectively. The nanoparticles demonstrated good magnetic resonance imaging T1 and T2 dual contrast effects (r1 = 12.2 mM−1 s−1 and r2 = 47.5 mM−1 s−1), while the nanoclusters exhibited a superior T2 contrast effect with the transverse relaxivities of 198 mM−1 s−1 (20 ± 2 nm) and 410 mM−1 s−1 (32 ± 3 nm), respectively. More importantly, both the nanoparticles and nanoclusters were highly tolerable by cells. 1 Introduction Superparamagnetic iron oxide nanoparticles (SPIONs) have a wide variety of applications in the biomedical field such as cell separation, contrast-enhanced magnetic resonance imaging (MRI) and drug delivery [[1]-[3]], which put a high demand for synthesis of SPIONs with well-defined size and high quality. SPIONs are conventionally synthesised by coprecipitation of ferric and ferrous ions under the alkaline conditions, which are convenient and easily scaled up [[4]]. However, SPIONs synthesised by this method have a broad size distribution, poor crystallinity and low saturation magnetisation [[5], [6]]. Moreover, to prevent aggregation of the particles, surface modification is often necessary. Monodispersed SPIONs with high crystallinity and tunable sizes have been synthesised by thermal decomposition of iron precursors (e.g. ferric acetylacetonate [Fe(acac)3] and ferric oleate) in high-boiling organic solvents in the presence of surfactants [[7], [8]]. However, SPIONs produced by this way are poorly dispersed in aqueous solution due to their hydrophobic surface coating, and further hydrophilic surface modification is necessary for biomedical applications. Thermal decomposition of iron precursors in polar solvents such as ethylene glycol (EG), di(EG) (DEG) and triethylene glycol (TREG) has been explored to synthesise hydrophilic SPIONs with well-defined size [[9]-[12]]. However, the colloidal stability of the nanoparticles in physiological conditions is poor, which makes them far from optimal for biomedical applications [[10], [11]]. To address these issues, recently, we have developed a microwave-heating procedure for production of SPIONs in ultra-large scale using ferric chloride as an iron precursor and DEG as a solvent in the presence of poly (acrylic acid) (PAA) [[13]]. As-prepared SPIONs (designated as PAA@USPIOs) were monodispersed, with size around 4.5 nm and highly stable in physiological conditions. Moreover, the PAA@USPIOs demonstrated superior MRI T1 contrast effect both in vitro and in vivo. Compared to SPIONs, SPIONs clusters are more sensitive for T2-weighted MRI [[14], [15]], which is beneficial for early detection of diseases [[6], [16], [17]]. Thus, in recent years, different strategies have been explored for the fabrication of SPIONs clusters [[18]]. For example, amphiphilic, polyelectrolyte block copolymers and liposomes have been frequently used to assemble SPIONs, forming hydrophilic magnetic clusters [[19]]. However, these strategies were often complex and it was difficult to control the size of the clusters. Therefore, a facile method for preparation of SPIONs clusters with well-defined size is highly desirable. In this Letter, a novel one-pot method using Fe(acac)3 as iron precursor, TREG as solvent and PAA as coating material was explored to prepare SPIONs. By simply varying the amount of PAA used, the sizes of SPIONs could be tuned from isolated nanoparticles to nanoparticle clusters. Nanoparticles exhibited both T1 and T2 MRI contrast effects, while the nanoclusters demonstrated a superior T2 contrast effect. 2 Materials and methods Materials: PAA (PAA, molecular weight (MW) 5000) and Fe(acac)3 were bought from across (NJ, USA). PAA was dried in vacuum at 90°C for 12 h before use. Other chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without further purification. Synthesis of PAA-coated SPIONs (PAA@SPIONs): PAA@SPIONs were prepared by thermal decomposition of Fe(acac)3 in TREG solvent in the presence of PAA. Briefly, Fe(acac)3 (1 mmol) and PAA (1, 3 or 6 mmol) were mixed into 30 ml of TREG. After sonicated for 1 min, the mixture was quickly heated to 185°C using an oil bath under the protection of nitrogen gas, and then held at this temperature for 30 min. Subsequently, the mixture was heated to 280°C and kept at this temperature for another 30 min, producing a black, homogeneous, colloidal suspension. The suspension was cooled down to the ambient temperature, and the product was precipitated with ethyl acetate. The precipitation was collected by a magnet, washed with ethanol three times and then dispersed in deionised water. 2.1 Characterisations of PAA@SPIONs Transmission electron microscopy (TEM): The morphology and size of the particles were examined using a Japan Electron Optics Laboratory 2100F TEM operating at an acceleration voltage of 200 kV. Samples were prepared by dropping the particle suspensions on the carbon-coated copper grid and dried at the ambient temperature for 12 h. The particle size distributions were analysed using Image J analysis software by measuring the diameters of no less than 100 particles. X-ray diffraction (XRD): The crystal structure and phase composition of the particles were determined by XRD. XRD measurement was performed using a D8 Advanced Avinci X-ray polycrystalline diffractometer (Bruker, Germany) at 40 kV with Cu/Kα radiation (λ = 0. 1542 nm). Fourier transform infrared (FTIR): To verify PAA surface coating, FTIR analysis was performed using a Nicolet 6700 FTIR spectrometer (Thermo Fisher, USA). For this purpose, all samples were ground and mixed with potassium bromide and then pressed to form pellets. Spectra were recorded in the wave number interval between 4000 and 400 cm−1. The background spectrum was subtracted from the sample spectrum. Each spectrum was acquired three times, and an average of the three measurements was taken and analysed. Thermal gravimetric analysis (TGA): To determine the amount of PAA surface coating, TGA of the particles was performed. For this purpose, the particles were lyophilised and TGA was performed using a TG209F1 TG analyser (NETZSCH Group, Germany). The samples were heated from the ambient temperature to 800°C with an increase step of 10°C per minute under the protection of nitrogen gas. Dynamic light scattering: The hydrodynamic sizes and zeta potentials of the particles were measured using a Malvern Zetasizer NanoZSP instrument (Malvern, Britain). To this end, the particles were dispersed in phosphate buffer saline (PBS, pH 7.4) and the measurements were performed in triplicate. Magnetic properties: Magnetic properties of the particles were evaluated using a PPMS-9T (EC-II) physical property measurement system (Quantum Design, America). The magnetisation (M, emu/g) of the samples was measured as a function of the magnetic field (H, Oe) at 300 K. Nuclear magnetic resonance (NMR) relaxometry: NMR relaxometry was measured at 37°C by an NMR spectrometer (Minispec, mq60, Brucker, Germany) with ac magnetic field frequency of 60 MHz (1.41 T). A Carr–Purcell–Meiboom–Gill spin echo sequence and an inversion recovery (IR) pulse sequence were used to determine T2 and T1 relaxation times, respectively. The iron concentrations were determined using an atomic absorption spectrophotometer (AAS, Z-2000, Hitachi, Japan). The T1, 2 relaxivities (r1, 2) were deduced by fitting inverse relaxation times (1/T1, 2) as a function of the iron concentrations. MRI of the particle suspensions: Both the nanoparticles and nanoclusters were suspended in water in plastic vials at different concentrations (25, 50, 100 and 200 μM) and placed in a water container. MRI was performed on a MAGNETOM Trio 3T MRI scanner (Siemens, Germany) using a clinical head coil with T1-weighted (repetition time (TR) = 500 ms, echo time (TE) = 30 ms, field of view (FOV) = 20 × 20 mm, matrix = 256 × 256 and slice thickness = 7 mm) and T2-weighted (TR = 3000 ms, TE = 102 ms, FOV = 20 × 20 mm, matrix = 256 × 256, and slice thickness = 7 mm) spin echo sequences. Cytotoxicity of the particles: Cytotoxicity of the particles was evaluated by CCK-8 assay (CCK-8; Dojindo Laboratories Co., Ltd., Kumamoto, Japan) using a human non-small-cell lung carcinoma cell line (H1299) according to the manufacturer's procedure [[17], [20]]. For this purpose, the cells (5 × 103) were seeded in each well of 96-well plate and incubated with the particles at the concentrations of 20, 40, 80, 160 and 320 μg Fe/ml for different periods of time (10, 24 and 48 h). After incubation, the cell culture media were removed and the cells were washed with PBS (0.1 M, pH = 7.4) three times. Subsequently, fresh cell culture medium (90 μl) was added into each well, followed by adding 10 μl of CCK-8 solution. Then, the cells were further incubated for 1 h. After incubation, the culture media containing unreacted CCK-8 were carefully removed. The absorbance (optical density) was measured by a microplate reader at a wavelength of 450 nm. The cell viability was expressed as the percentage of absorbance of the cells incubated with the particles to that of the cells maintained in a normal culture medium. 3 Results and discussion In this Letter, a facile, one-pot method has been developed to synthesise SPIONs with tunable sizes, in which Fe(acac)3 was used as the iron precursor, TREG as the solvent and PAA as the surface coating material. By varying the amount of PAA used, isolated nanoparticles and nanoparticle clusters with well-defined sizes were synthesised. Fig. 1 showed the representative TEM images and size distributions of the as-synthesised particles. Both nanoparticles and nanoclusters were roughly spherical and uniform in size. When 1 mmol of PAA was used, nanoparticles with the size of 7 ± 1 nm were prepared (Fig. 1a). Increasing the amount of PAA to 3 or 6 mmol, nanoclusters with size of 20 ± 2 nm (Fig. 1b) or 32 ± 3 nm (Fig. 1c) were obtained. TREG acted not only as the polar solvent with a high-boiling point, but also as the reductive agent [[21], [22]]. During polyol process, ferric ions were partially reduced to ferrous ions by TREG and hydrolysed along with the resulting ferrous ions. The hydrates dehydrated simultaneously at high temperature (280°C), forming IO nanocrystal nuclei [[23]]. The size tuning effect of PAA could be interpreted as the result of the interactions of PAA molecules and the nanocrystal nuclei [[24]]. At the lower PAA concentration, the nanocrystal nucleuses were mainly coated by TREG (MW 150) solvent through coordination of oxygen atoms in hydroxyl groups and iron ions on the particle surface. With the increase of PAA concentration in the solvent, PAA would dominantly coat on the particles and the chance of a PAA molecule (MW 5000) binding with multi-nanocrystal nuclei enhanced. PAA@SPIONs clusters were thus formed by assembly and subsequent growth of these newly born nanoparticles. The hydrodynamic diameters of the nanoparticles and clusters in PBS (pH 7.4) were 26 ± 3, 37 ± 2 and 76 ± 4 nm. Fig. 1Open in figure viewerPowerPoint TEM images of the isolated nanoparticles and nanoparticle clusters (left) and the corresponding size distributions (right) a TEM size of the nanoparticles was 7 ± 1 nm b, c TEM sizes for the nanoclusters were 20 ± 2 and 32 ± 3 nm XRD revealed that both the nanoparticles and nanoclusters were highly crystalline and XRD patterns showed peaks at 30.11, 35.41, 43.11, 53.41, 57.11 and 62.61° (Fig. 2a), which can be indexed to (220), (311), (400), (422), (511) and (440) lattice planes of the spinel structure (JCPDS Card No. 01-082-1533) known for magnetite crystal [[25]]. No other peaks were detected, indicating that the as-synthesised particles are pure phase Fe3O4 [[26]]. Close inspection of TEM images of the nanoclusters revealed that the clusters were composed of small primary grains. Calculating with Debye–Scherrer formula from peak (311), the average sizes of the crystallite were determined to be 7.6, 8.3 and 10.2 nm, respectively, for 7 ± 1 nm nanoparticles and 20 ± 2 and 32 ± 3 nm clusters. Fig. 2Open in figure viewerPowerPoint Identification of crystal structure and surface coating of the particles a XRD patterns of the nanoparticles b FTIR of the nanoclusters PAA surface coating was evaluated with FTIR. As shown in Fig. 2b, the peak around 3400 cm−1 was attributed to the absorbed water [[27]] and that at 1725 cm−1 was characteristic of C = O stretching mode of carboxylic groups. The peak at 1430 cm−1 could be assigned to the asymmetric C–O stretching mode of carboxylate groups covalently bonded with ferric or ferrous ions on the surface of the particles [[28]], which indicated that PAA coating was achieved by carboxylic groups coordinating with iron atoms [[13]]. The band around 580 cm−1 was the vibration of Fe–O bond [[25]]. In addition, the absorption band at 2365 cm−1 on the spectra referred to the vibration of the adsorbed carbon dioxide in the sample [[29], [30]]. To determine the mass fraction of PAA coated on the particles, TGA was performed (Fig. 3). The weight loss occurred in two stages. The first slight weight loss from ∼40 to 150°C corresponded to the removal of the physically adsorbed water molecules [[31]]. The weight loss between 150 and 800°C was ascribed to the removal of the surface-coated PAA. PAA coating accounted for 42.3% of the total weight of the nanoparticles (7 ± 1 nm), while that for 20 ± 2 and 32 ± 3 nm nanoclusters were 33.0 and 21.8%, respectively. The TGA results were summarised in Table 1, which indicated that more PAA was coated on the smaller particles. Zeta potential of 7 ± 1 nm nanoparticles was −35.9 mV and that for 20 ± 2 and 32 ± 3 nm nanoclusters were −25.6 and −23.5 mV, respectively. Fig. 3Open in figure viewerPowerPoint TGA curves of the nanoparticles and nanoclusters. PAA coating accounts for 42.3% (7 ± 1 nm), 33.0% (20 ± 2 nm) and 21.8% (32 ± 2 nm) of the total weight of the nanoparticles and nanoclusters, respectively Table 1. TGA analyses of the nanoparticles and nanoclusters Samples, nm Weight loss (30–150°C), % Weight loss (150–800°C), % Residue (750°C), % 7 6.4 42.3 50.9 20 4.1 33.0 62.9 32 3.1 21. 8 75.1 Superparamagnetism is critical for biomedical applications of IONs. To examine if the particles are superparamagnetic, susceptibility of the particles as a function of applied magnetic field was evaluated. As shown in Fig. 4, the saturation magnetisation of the nanoparticles was 56.2 emu/g and that for the nanoclusters were 55.6 (20 ± 2 nm) and 57.4 emu/g (32 ± 3 nm), respectively. No pronounced remanence or coercivity (Fig. 4 right column) was observed, which indicated that both nanoparticles and nanoclusters were superparamagnetic at 300 K [[32]]. Fig. 4Open in figure viewerPowerPoint Magnetisation curves a Nanoparticles measured at 300 K b, c Nanoclusters measured at 300 K Right column: magnification of the curves at low magnetic field. No pronounced remanence observed indicates that both nanoparticles and nanoclusters are superparamagnetic at 300 K We next investigated the efficiency of the particles for MRI by determining these relaxivities at 1.4 T. Both the longitudinal (r1) and transverse (r2) relaxivities were closely correlated to the particle sizes (Figs. 5a and b). The r2 relaxivity of the nanoparticles was 47.5 mM−1 s−1, and that for 20 and 32 nm nanoclusters were 198 and 410 mM−1 s−1, respectively. On the contrary, the r1 relaxivity decreased significantly as the particle size increased. The nanoparticles exhibited an r1 value of 12.15 mM−1 s−1, whereas that for 20 and 32 nm nanoclusters were only 3.9 and 1.0 mM−1 s−1, respectively. These results were consistent with previous observations that clustering would dramatically enhance the T2 effect, while diminish the T1 effect of SPIONs [[33], [34]]. Nanoclusters of 32 nm had an r2 value of 410 mM−1 s−1, which was among the highest reported for IO nanomaterials [[15], [18]]. This ultra-high r2 value is attributed to the increased number of constituent nanocrystals and the enhanced interactions of their magnetic moments [[15], [18]]. Fig. 5Open in figure viewerPowerPoint MRI property of the particles a, b T1 and T2 relaxation rates of the nanoparticles and nanoclusters as a function of iron concentration c T1- and T2-weighted MRI images of aqueous suspensions of the nanoparticles and the nanoclusters at different iron concentrations To evaluate the potentials of these particles as MRI contrast agent, MRI of the particle water suspensions was conducted. Both T1- and T2-weighted images revealed a strong dependence of signal intensity on the particle concentration (in iron) (Fig 5c). For the nanoparticles, MRI signal intensity in T1-weighted MR images increased with an increase in iron concentration, whereas that in T2-weighted MR images attenuated. In contrast, for the nanoclusters, the signal intensity in both T1- and T2-weighted MR images decreased with an increase in iron concentration. Moreover, the hypointensity in T2-weighted MR images was more pronounced than that in T1-weighted images, and the nanoclusters with 32 nm size had a greater decrease in signal intensity. These observations indicated that the nanoparticles are potentials for T1- and T2-weighted dual contrast MRI [[35], [36]], whereas the nanoclusters could only be used for T2-weighted MRI [[37]]. Cytotoxicity was evaluated by CCK-8 assay. As shown in Fig. 6, no cytoxic effects were noted for both the nanoparticles and the nanoclusters at the conditions investigated. Even treated with these materials at the high concentration (320 μg/ml) for a long period of time (48 h), the cell death was marginal. Consistent with previous reports [[13], [38]], our results indicated that both the nanoparticles and nanoclusters were biocompatible and had no obvious toxicity to the cells. Fig. 6Open in figure viewerPowerPoint In vitro cytotoxicity of the nanoparticles and nanoclusters. Relative viabilities of H1299 cells were evaluated after being incubated with a Various doses of nanoparticles b 20 ± 2 nm nanoclusters at different concentrations for different periods of time c 32 ± 3 nm nanoclusters at different concentrations for different periods of time 4 Conclusion In summary, we have developed a novel one-pot approach for preparation of monodispersed SPIONs with tunable sizes, ranging from isolated nanoparticles to nanoparticle clusters. 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Waters E.A. et al.: 'Ultrasmall, water-soluble magnetite nanoparticles with high relaxivity for magnetic resonance imaging', J. Phys. Chem. C, 2009, 113, pp. 20855– 20860 (doi: 10.1021/jp907216g) Volume12, Issue10October 2017Pages 749-753 FiguresReferencesRelatedInformation
Publication Year: 2017
Publication Date: 2017-06-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 6
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