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Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance
Journal Pre-proof Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance Mohaned Hammad, Sebastian Hardt, Benedikt Mues, Soma Salamon, Joachim Landers, Ioana Slabu, Heiko Wende, Christof Schulz, Hartmut Wiggers PII:
S0925-8388(20)30177-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153814
Reference:
JALCOM 153814
To appear in:
Journal of Alloys and Compounds
Received Date: 10 September 2019 Revised Date:
17 December 2019
Accepted Date: 10 January 2020
Please cite this article as: M. Hammad, S. Hardt, B. Mues, S. Salamon, J. Landers, I. Slabu, H. Wende, C. Schulz, H. Wiggers, Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153814. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Title page
Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance Mohaned Hammada*, Sebastian Hardtb, Benedikt Muesc, Soma Salamond, Joachim Landersd, Ioana Slabuc, Heiko Wended,e, Christof Schulza,e, Hartmut Wiggersa,e*
a
IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids, University of Duisburg-Essen, Duisburg, Germany
b
HSWmaterials GmbH, Kevelaer, Germany
c
Institute of Applied Medical Engineering, Helmholtz Institute, Medical Faculty, RWTH Aachen University, Aachen, Germany
d e
Faculty of Physics, University of Duisburg-Essen, Duisburg, Germany CENIDE, Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Duisburg,
Germany
Corresponding authors: Dr. Mohaned Hammad* Prof. Dr. Hartmut Wiggers* Institute for Combustion and Gas Dynamics – Reactive Fluids University of Duisburg-Essen Carl-Benz-Str. 199 47057 Duisburg, Germany
[email protected] [email protected]
1
Abstract Magnetic nanoparticle-mediated hyperthermia has shown great potential in cancer therapy. However, upscaling of the synthesis of iron oxide nanoparticle with the required narrow size distribution remains challenging. This paper describes the reproducible and scalable synthesis of citric acid-functionalized iron oxide nanoparticles optimized for hyperthermia treatment. Iron oxide nanoparticles were synthesized by a spray flame method, which is eco-friendly and cost-effective. To the best of our knowledge, this is the first study reporting spray-flame synthesis of small iron oxide nanoparticles (approx. 7 nm) with narrow size distribution (polydispersity index ≪ 0.1). The citric acid-coated iron oxide nanoparticles revealed a hydrodynamic size of approx. 37 nm and a high magnetic saturation of 69 Am2/kg at room temperature. The magnetic hyperthermia study showed a significantly enhanced value of the intrinsic loss power (4.8 nHm2/kg), which is 1.5-fold higher than the best commercially available equivalents. The improved heating efficiency and small hydrodynamic size of citric acidcoated iron oxide nanoparticles demonstrate that the system could potentially be used as a nanoplatform for hyperthermia treatment.
Keywords: magnetic nanoparticles, spray-flame synthesis, heating efficiency, Mößbauer spectroscopy
2
1
Introduction
The cancer burden has risen to 18.1 million new cases and claims the lives of 9.6 million people in 2018 around the world [1]. Usually, a combination of traditional remedies such as chemotherapy, radiation, and surgery is used for cancer therapy [2]. The adverse side effects of conventional treatment especially of chemotherapy have proved the need to explore alternative cancer therapies. Magnetic hyperthermia is a promising therapeutic method based on inductive heating of injected magnetic nanoparticles (MNPs) inside a tumor tissue by application of an external alternating magnetic field (AMF) [3]. For a low injected dose of MNPs, reaching a therapeutic temperature of 43 °C is significant challenge [4]. Therefore, the specific absorption rate (SAR) of the MNPs must be high enough to increase the efficacy of cell death hyperthermia and to reduce the dosage and toxicity in hyperthermia treatment. The SAR depends strongly on particle shape, particle size, and magnetic properties of MNPs, such as saturation magnetization, as well as on the AMF frequency and amplitude [5]. Tuning the intrinsic and extrinsic characteristics (i.e., crystallinity, agglomeration, magnetism, colloidal stability, interactions with biological media) of MNPs through the control of their size enhances the heating efficiency of MNPs for hyperthermia [6]. Considerable interest has been devoted to the synthesis of MNPs based on chemical and physical approaches such as wet-chemical precipitation [7], thermal decomposition of iron acetylacetonate precursor solution [8], solvothermal processing of iron (III) chloride precursor [9,10], template-free hydrothermal route using ferrocene as a precursor [11], physical and lithographic processing for 3D magnetic nanostructure fabrication [12], and solid-state reaction [13]. However, the production rates via these methods are limited and scale-up may not be straightforward. A promising but little explored approach is the spray-flame method (also called flame spray pyrolysis) that is especially intriguing because of its scale-up potential. Here, an inorganic precursor is dissolved in a combustible liquid that is processed in a turbulent flame with a short residence time at high temperature. This technique gives superior yields at high production rates up to 4 kg/h of nanoparticles on a pilot-scale [14]. However, there are several challenges associated with the spray-flame method, such as the production of nanoparticles with well-controlled size, shape, and morphology [15]. Pratsinis et al. reported the influence of solvent parameters such as the boiling point on the formation of homogeneous nanoparticles during spray-flame synthesis [16]. Adding of a carboxylic acid (2-ethylhexanoic acid) to the precursor solution that reacts with the precursor, forming a more volatile carboxylate, resulted in homogeneous nanoparticles [16]. Pristine iron oxide (IO) nanoparticles are prone to oxidation in air and can easily agglomerate in a colloidal suspension, both resulting in lower magnetization values and dispersibility [17]. For biomedical applications, IO nanoparticles are coated with biocompatible organic and inorganic materials to prevent agglomeration and improve the colloidal stability [18]. Citric acid presents several advantages in this context, such as the electrosteric (i.e., combined electrostatic and steric) 3
stabilization, biocompatibility, and its easy conjugation to other targeting biomolecules [19]. Some reports have focused on the influence of the citric acid coating on the SAR [20]. On the one side, SAR values became lower for citric acid-coated iron oxide nanoparticles (IO_CA) compared to IO, but on the other side, the colloidal stability improved. Li et al. described the effect of synthesis conditions on the magnetic properties of IO_CA nanoparticles [21]. The adsorption of citric acid onto the MNP surface is enhanced by raising the temperature from 30 to 90 °C and therefore decreases the hydrodynamic size of IO_CA nanoparticles. The objective of this study is to fabricate IO_CA nanoparticles and investigate their heating efficiencies for hyperthermia application. IO nanoparticles with sizes around 7 nm were synthesized using a spray-flame method. Subsequent functionalization with citric acid yielded highly dispersable IO_CA nanoparticles that are stable in aqueous solvents. The influence of the vacuum heating on the magnetic properties of IO nanoparticles is studied. Finally, the impact of the citric acid coating on the SAR is also investigated compared to pristine IO nanoparticles.
2 2.1
Material and methods Materials
The precursor iron(III)-nitrate nonahydrate (≥98 %) was purchased from VWR chemical. Solvents used for spray-flame synthesis (2-ethylhexanoic acid (99 %), absolute ethanol, acetic acid anhydride (≥99 %)), and citric acid (≥99.5 %) and tetramethylammonium hydroxide (TMOAH) (25 wt.% in H2O) used for particle functionalization were purchased from Sigma Aldrich. All chemicals were used as received without further purification.
2.2
Synthesis of iron oxide nanoparticles
The synthesis of IO nanoparticles was carried out in a laboratory-made enclosed spray-flame system described elsewhere [22] with a production rate of 10–20 g/h. A precursor solution was prepared by dissolving 0.5 mol of iron(III)-nitrate in 1 l of a 12.35:6.65:1 mixture of 2-ethylhexanoic acid: ethanol: acetic acid anhydride. The solution is supplied to the spray-flame reactor via a capillary with an inner diameter of 0.5 mm and an outer diameter of 0.9 mm, which is placed in the center of a pilot-flamesupported burner nozzle. An annular gap with an outer diameter of 1.6 mm coaxially surrounds the capillary. The precursor solution is continuously injected with a feed rate of 2 ml/min into the reactor by a system of two alternating syringe pumps. A dispersion gas flow of 10 standard liter per minute (slm) oxygen through the annular gap atomizes the solution. The combustion of the resulting spray is stabilized by a surrounding premixed methane/oxygen (5/10 slm) pilot flame fed from a second annular gap with an inner diameter of 11 mm and an outer diameter of 12.3 mm. An additional coaxial sheath gas flow of 300 slm air fed through a surrounding sintered-bronze ring stabilizes the gas flow in the reaction chamber. A rotary vane pump on the clean-gas side of a filter system ensures the desired
4
pressure within the reaction chamber. A baghouse filter is used to separate the particles from the exhaust gases, which are afterwards harvested from the filter.
2.3
Synthesis of IO_CA nanoparticles
The pristine IO nanoparticle powder (10 g) was heated for 8 hours at 270 °C under vacuum to remove unburned combustion residuals, CO2, and water from the surface. The heated IO nanoparticles, henceforth referred to as IO_270, were dispersed in 100 ml of TMAOH and sonicated for 10 minutes. Excess of TMAOH was removed by centrifugation. To get uniform dispersions, the treated nanoparticles were dispersed in 500 ml of water and sonicated for 15 minutes while kept in an ice bath. The mixture was then heated to 80 °C under vigorous mechanical stirring. Citric acid (5 mg/ml in water) was added into the solution under vigorous mechanical stirring. After 15 minutes, the solution was cooled to room temperature. The prepared IO_CA were collected by centrifugation and washed with de-ionized water for three times. Finally, the samples were dried in an oven under vacuum (50 °C) for 12 hours.
2.4 Methods of Characterization 2.4.1
Structural characterization
A PANalytical X-ray diffractometer X'Pert with a Cu Kα radiation (λ = 1.5406 Å) was used to determine the phase composition of all samples. The mean crystallite size was calculated from the XRD pattern using Rietveld refinement using the program “X'Pert HighScore Plus”. The calculated pattern for the fit was simulated using the crystallographic information file (CIF) from Inorganic Crystal Structure Database (ICSD 98-015-8745 and 98-017-2906). The profile was fit using Pseudo Voigt 3 (FJC Asymmetry) and the background was fitted using a shifted Chebyshev polynomial. Raman spectra were recorded at room temperature on a Renishaw inVia Raman microscope using a HeNe laser beam with a wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VersaProbe II by Ulvac-Phi with monochromatic Al Kα light at an emission angle of 45°. All binding energies were referenced to the carbon 1s peak at 284.8 eV. The morphology and microstructure of the samples were studied by transmission electron microscopy (TEM, JEM‐2200FSJEOL). Nitrogen adsorption/desorption isotherm measurements were carried out at 77 K with a Quantachrome NOVA2200 analyzer. The hydrodynamic size and zeta-potential measurements of IO and IO_CA in aqueous solutions were investigated by dynamic light scattering on a Zetasizer (Malvern Instruments). The water and hydrocarbon content is determined by thermogravimetric analysis (TGA, Netzsch 449 F1 Jupiter) with a heating rate of 5 K/min under the flow of synthetic air (250 ml/min). FTIR spectra were recorded using a Bruker Vertex 80 spectrometer in the 400–4000 cm−1 region.
5
2.4.2
Magnetic characterization
Field-dependent magnetization curves were recorded with the vibrating sample magnetometer option of a Quantum Design PPMS DynaCool. The saturation magnetization was measured at room temperature and a maximum applied magnetic field of 7200 kA/m. SAR measurements were performed to assess the heating performances of the MNPs. Calorimetric particle heating measurements were carried out with a custom-built hyperthermia setup (Trumpf Hüttinger, Freiburg, Germany), consisting of a DC generator, an AC-resonant oscillator and a water-cooled copper coil. The field amplitude ܪ was varied between 7 and 53 kA/m, and the frequency was set to
݂ = 270 kHz. 1 ml ferrofluid samples were prepared in 4 ml glass vials, which were wrapped in 1 mm-thick styrofoam to reduce heat losses and placed in the 7-turn coil. Samples were exposed to the alternating magnetic field for ca. 25 minuts starting at an initial temperature of ܶ = 308.15 K
(35 °C). During exposure to the alternating field, the sample temperature was recorded using a fiberoptic thermometer Luxtron 812 (LumaSense Inc., Santa Barbara, CA, USA). The measured temperature–time data, ܶ()ݐ, were fitted with the Box-Lucas function to calculate SAR. ௧
ܶ(ܶ = )ݐ୰୧ୱୣ ൬1 − exp ቀ− ఛቁ൰ + ܶ
(1)
from which the SAR value was calculated according to [23]: SAR =
ୢ் | ఘ ୢ௧ ௧→
(2)
with the specific heat capacity of water, ܿ = 4.187 J (g K)–1, the MNP weight fraction, ߩ, and dܶ/ d → ݐ(|ݐ0) =
்౨౩ , ఛ
derived from the Box-Lucas fitting parameters.
Further, the intrinsic loss parameter (ILP) is determined: [24] ILP =
ୗୖ ୌమ
(3)
The applied magnetic field should be smaller than the magnetic field leading to the saturation magnetization. The ILP allows comparing the heating efficiency of different MNPs independent of AMF settings.
3 3.1
Results and Discussion Structural characterizations
Crystal structure and crystallite size were characterized from X-ray diffraction (XRD) measurements. Figure 1 shows the XRD pattern of IO and IO_270 nanopowders, respectively. The (220), (311), (400), (422), (511), and (440) diffraction peaks of IO nanoparticles match the Bragg reflections of the spinel ferrite structure. This structure can be indexed to either magnetite (Fe3O4) or maghemite (γFe2O3), and it cannot be ruled out that both phases are present in the powder since the signals indexed
6
(511) and (400) suggest the existence of both phases due to some asymmetry and broadening. On heating, all peaks became more intense and narrower. This change can be attributed to a slight increase in crystallite size. The XRD pattern of the IO_270 nanoparticles with additional low-intensity (111), (620), and (533) diffraction peaks are matched to ICSD card 98-015-8745. The lattice constant calculated from Rietveld refinement increases with the heating of IO nanoparticles from 0.834 to 0.839 nm. This suggests that a probably existing maghemite phase in the IO nanopowder has been converted to magnetite when heating at 270 °C under vacuum [25]. The mean crystallite sizes calculated from the XRD pattern using Rietveld refinement of IO and IO_270 nanoparticles are 6.8 and 7.7 nm, respectively.
Figure 1. XRD patterns of IO and IO_270 nanopowders.
To support the XRD results concerning the phase composition, Raman spectroscopy was applied to identify and distinguish between maghemite and magnetite (Figure 2). The broad peaks of the IO sample around 350, 500, and 700 cm–1 are considered to originate from the maghemite phase [26] while the shoulder at 660 cm–1 can be attributed to magnetite [27]. Consistent with XRD, the transformation of maghemite to magnetite phase, accomplished by heating under vacuum, is confirmed by the Raman measurements since the peak intensities at 350, 500, and 700 cm–1 significantly lower while an increase in peak intensity at 660 cm–1 (magnetite) is observed.
7
Figure 2. Raman spectra of IO and IO_270 nanoparticles.
Moreover, XPS measurements were employed to further characterize the elemental components of the samples (Figure 3). The XPS spectrum of the IO nanoparticles shows two characteristic peaks centered at 711 and 725 eV, which have been assigned to Fe2p3/2 and Fe2p1/2, respectively (Figure 3a). Besides, the explicit development of a satellite peak in the IO spectrum around 719 eV confirms the presence of only Fe3+ species in the maghemite lattice. For the IO_270 sample, the Fe2p3/2 and Fe2p1/2 are centered at 710.7 eV and 724.5 eV, respectively. The reduced intensity of the satellite peak around 719 eV along with the shifted peak position of 2p3/2 and 2p1/2 confirms the existence of both Fe3+ and Fe2+ species, thus indicating the magnetite lattice [28]. Moreover, the O1s spectra (Figure 3b) also show that heating under vacuum leads to a change in chemical composition due to a release of surface contaminants. The signals indicating C–O and Fe–O–C bonds vanish supporting the assumption of particle’s surface cleaning by heating in vacuum, which is also confirmed by FTIR measurements (see Figure S1, supplementary materials).
Figure 3. XPS patterns of the (a) Fe2p and (b) O1s core-level photoelectron spectra for IO and IO_270 powders.
8
The specific surface area (SSA) of the IO and IO_270 samples was measured by nitrogen adsorptiondesorption isotherms analyzed by the BET (Brunauer-Emmett-Teller) theory [29]. Assuming monodisperse, spherical, non-aggregated particles, particle sizes of 8 and 16 nm, respectively, were calculated from the measured BET surface areas of 145 and 71 m2/g and an average density of 4.92 g/cm3. Especially the value obtained for IO_270 is significantly larger compared to the crystallite size obtained from XRD. This difference can be explained with the heating-induced agglomeration or partial sintering of the iron oxide nanoparticles. Particle size and morphology of IO, IO_270, and also IO_CA samples were investigated by TEM (Figure 4). The sizes of at least 300 particles per sample were measured and plotted as a histogram while the histogram was fitted to a log-normal particle size distribution. The fit results are mentioned and plotted as a solid lines in Fig. 4. In all cases, primary particles with an approximately spherical shape and count median diameters (CMD) of about 7 to 8 nm were observed which are in good agreement with the crystallite sizes measured from XRD results. There is almost no difference in shape between the particles before and after heating, however, a slightly pronounced agglomeration is found after heating (Figure 4b). Surprisingly, the amount of agglomerates is significantly reduced after functionalization with citric acid (Figure 4c). The nanoparticles are well dispersed, and the TEM images reveal the existence of many single, individual nanoparticles. Obviously, the functionalization of IONPs almost completely reverses the particle agglomeration observed after heating. In addition to calculating CMD, we have also measured the polydispersity index (PDI) of samples that describes the average uniformity of nanoparticles and was calculated according to: ଶ
PDI = ቀ ౝ ቁ , ଶ σ
(4)
where σ is the geometric standard deviation of the particle diameter distribution and a is the average particle diameter of the nanoparticles. PDI values of our materials are less than 0.1, indicating nearly monodisperse nanoparticle distributions [30].
9
Figure 4. TEM images of (a) IO, (b) IO_270, and (c) IO_CA nanoparticles and respective histograms of particle sizes fitted to a log-normal particle size distribution.
Zeta potential measurements are used as a measure for the electrostatic stability of the dispersions and show that the Zeta potential of IO_270 increases from −15 mV before surface functionalization to −43 mV after citric acid functionalization (Figure S3 supplementary materials). The high Zeta potential of the IO_CA nanoparticles guaranties a good dispersibility and high stability of the nanoparticles in the aqueous medium.
3.2
Magnetic measurements
In order to study the magnetic properties of the different samples, magnetization curves of the IO, IO_270, and IO_CA samples were measured at room temperature with a vibrating sample magnetometer. Figure 5 shows the M(H) loops of all three samples. The saturation magnetization of IO and IO_270 nanoparticles was found to be ca. 32 and 62 Am2/kg, respectively. The M(H) loops are closed, not showing any remanent magnetization indicating that the samples are superparamagnetic. The change in magnetization between the two samples is surprisingly high, even considering the removal of 11 wt.% surface contamination of the pristine IO nanoparticles (Figure S2) [31]. A further potential origin for the lower magnetization of the as-prepared IO sample could be the presence of a minor antiferromagnetic α-Fe2O3 phase, which would not be clearly visible in the XRD diffractogram in case of amorphous material or very small regions. This possibility will be further discussed in terms
10
of Mößbauer spectroscopy below. Other forces contributing to the enhanced magnetization in IO_270 also include the slightly higher average core diameter, which would lead to less pronounced surface spin frustration [32] as well as an improvement of the crystalline and magnetic order due to particle annealing [33]. Nevertheless, frustrated surface spins will result in a reduction of high field magnetization, even in the IO_270 sample, as compared to the bulk value of magnetite (87–92 Am2/kg) [34]. After functionalization with citric acid, the saturation magnetization surprisingly increased to about 69 Am2/kg, which is still higher than that of the IO_270 sample. This can be explained by the CA functionalization preventing direct contact of the IO particles and thus exchange interaction of surface spins, thereby presumably reducing surface spin disorder and enhancing high field magnetization [35]. Furthermore, the surface functionalization could, to some extent, compensate the broken symmetry at the particle surface, representing the main cause of surface spin canting, and thereby increase the high field magnetization [36], overcompensating the slight decrease introduced by negligible amounts of non-magnetic CA (ca. 3 wt.%, see Figure S2). The samples have a high magnetic response and are thus suitable for hyperthermia treatment.
Figure 5. Hysteresis loops at room temperature for IO, IO_270, and IO_CA nanoparticles
To further investigate the degree of spin alignment relative to external fields, Mössbauer spectroscopy was employed at low temperatures (4.3 K) and high fields (5 T). Spectra were recorded in a liquid helium bath cryostat with a superconducting split-pair magnet, in order to achieve low temperatures (4.3 K) and high magnetic fields (5 T) applied parallel to the γ-ray propagation direction. Additional spectra at 80 K were obtained with a liquid nitrogen bath cryostat. A sample heated to 250°C (IO 250) was used as a reference for IO_270, in order to discern the effects of the heating on the particle properties. Figure 6 shows the comparison between the heated IO_250 and the untreated IO samples, with clear differences being visible. At 80 K, the particles experience slow Néel-type relaxation, leading to an asymmetric broadening of the absorption lines, due to which contributions of tetrahedral (A-site) and octahedral (B-site) lattice positions cannot be resolved. A lower relaxation frequency in 11
the heated particle sample can be deduced from the less pronounced spectral deformation, which can be explained by the slightly higher particle diameter and the corresponding higher anisotropy energy [37,38], potentially also favored by particle–particle interaction within particle clusters, indicated by the higher hydrodynamic diameter. When applying high magnetic fields, the alignment of magnetic moments in the particles results in a splitting of A- and B-site contributions, due to the antiferromagnetic coupling of the sub-lattices. For the untreated sample, even when exposed to an external magnetic field of 5 T, there is a significant overlap between the A- and B-site sub-spectra, indicating considerable spin frustration. This can be quantified by the so-called canting angle (angle between the magnetic field and spin direction), which can be determined from the A23 ratio, given by the intensity ratios between lines 2 and 3. Average canting angles of ca. 43° and 36° can be extracted for IO and IO_250, respectively, clearly indicating a reduction in spin frustration as a result of the heating procedure. Still, the moderate spin frustration is insufficient to account for the distinct superposition of sub-spectra observed in sample IO, wherefore a higher degree of structural and magnetic disorder is expected here, resulting not only in increased spin canting but also in distributions of the hyperfine magnetic fields, closely connected to the local magnetic moment, no longer present in the heated sample IO_250. As mentioned in the magnetometry evaluation, the possible presence of a small fraction of poorly ordered iron oxide (hematite or maghemite) phase was taken into account by adding a further subspectrum to the untreated IO sample, showing especially high spin canting and no shift in line position induced by the external magnetic field. Considering its strong contribution to lines 2 and 5, this subspectrum can be modeled with a maximum relative spectral intensity of ca. 10 %. In addition to annealing effects and the removal of carbon contamination, the transformation of this minor fraction to well-ordered maghemite could contribute further to the surprisingly high increase in magnetization that was observed between IO and IO_270 in Figure 5, as this subspectrum in between A- and B-site position is absent in the annealed nanoparticle powder.
12
Figure 6. Mössbauer spectra for IO and IO_250 recorded at 80 K (top) and 4.3 K in an applied magnetic field of 5 T (bottom). Black dots represent experimental data, red lines the overall theoretical fit functions, green and blue lines the A- and B-site sub-spectra, orange line the poorly ordered iron oxide, respectively.
Figure S4 shows the temperature versus time, measured at 270 kHz frequency, for IO and IO_CA samples. The SAR values were normalized to the iron content of the nanoparticles measured by chemical analysis and validated by TGA and are plotted as a function of the field amplitude (Figure 7). The SAR values of IO_CA nanoparticles are higher compared to the IO nanoparticles SAR values. Agglomerations have a demagnetizing effect resulting in decreased saturation magnetization and in a lower heating efficiency compared to the one of IO_CA nanoparticle, e.g., SAR values at 53 kA/m are increased by a factor of 6 for IO_CA nanoparticles. Such high SAR values were reached before for iron oxide nanoparticles with bigger sizes [39]. The high SAR of IO_CA samples, especially at low magnetic field strength is very important for hyperthermia since it allows to reduce the unwanted heating of healthy tissues induced by eddy current [40].
13
Figure 7. SAR functions of IO and IO_CA samples.
For the heating efficiency of magnetic nanoparticles, the Néel and Brownian relaxation processes play a leading role and are expressed by τ = τ exp
ా ்
(5) and τ =
ଷ (౯ౚ )య ా ்
(6), respectively,
where τ0 is a length of time of the material usually between 10–9 to 10–11 s, Keff is the anisotropy constant of the material, Vm is the magnetic volume, kB is the Boltzmann constant, T is the temperature, η is the viscosity of the fluid , and rhyd is the hydrodynamic radius of the particles. There is a significant enhancement of the SAR when the particles are functionalized with citric acid, confirming that both relaxation mechanisms are present in IO_CA [6], which cause a relaxation given by ଵ த
ଵ
=த + ొ
ଵ , தా
(7)
where τ depends on rhyd to the power of three and τ depends exponentially on Keff. To estimate the rhyd value for the nanoparticles dispersed in the water, respective dispersions were characterized by dynamic light scattering. As is evident from Figure 8, the average rhyd of IO_CA nanoparticles (37±3.1 nm) is significantly smaller than that of the pristine IO nanoparticles. Moreover, the pristine IO sample has a bimodal particle size distribution, indicating the presence of stable agglomerates in the micrometer regime while the measurements of IO_CA sample hardly show any bigger agglomerates but a monomodal particle-size distribution. Thus, the measurements support the TEM investigations showing that the surface functionalization leads to de-agglomeration and stabilization of small particles. Binding of citric acid to the nanoparticles’ surface is sufficient to overcome the shortdistance van der Waals forces leading to particle agglomeration and to provide an electrosteric stabilization of the particle dispersions. According to Brownian relaxation equation, the Brownian relaxation time for pristine IO nanoparticles is longer compared to IO_CA nanoparticles because of the large rhyd of the IO agglomerates; therefore faster Brown relaxation contribution to the heating process of IO_CA sample is expected which 14
enhances their heating efficiency. It is reported that the well dispersed magnetic nanoparticles improve the heating efficiency through Brownian and Neel’s relaxations [41]. On the other hand, the Mr/Ms value of the IO_CA sample is close to zero, indicating single domain nanoparticles. Since the IO_CA nanoparticles are superparamagnetic [42], they are heated faster under the external field and are easily magnetized and demagnetized with almost zero hysteresis loss. SAR depends also on the saturation magnetization, spin canting effects, shape anisotropy, and anisotropy constant [43] as well as bigger hysteresis losses due to formation of chains, which happens when the magnetic energy overcomes the thermal energy [44]. We have shown before that heat treatment improves the crystallinity of the iron oxide nanoparticles and reduces spin frustration, which also has an enhancing effect on the heating properties of the nanoparticles [45]. Moreover, since the interparticle interactions are field dependent, chain formation of particles could occur at such a high magnetic field enhancing the heating performance [46].
Figure 8. Hydrodynamic size distribution of IO and IO_CA nanoparticles
According to equation 3, the average ILP values of IO_CA sample is 4.8 nHm2/kg. The ILP value of IO_CA is quite large compared to the best commercial equivalents (Table 1). ILP values of different commercial ferrofluids ranging between 2 and 4 nHm2/kg are sufficient for cancer cell death in magnetic hyperthermia which makes our material the best candidate for magnetic hyperthermia [24].
15
Table 1: ILP values of the IO and IO-CA samples compared to commercial ferrofluids [24]. Particle type
Coating
Dhyd / nm
ILP / (nHm2/kg)
Nanomag-D-spio
Carboxyl
20
2.31
Fluidmag-D
Starch
50
2.67
Resovist
Carboxydextran
60
3.1
IO
without coating
80 2600
0.6
37
4.8
IO_CA
4
Citric acid
Conclusions
In this article, we have presented a robust and scalable method to produce citric acid-coated iron oxide with controllable particle sizes for magnetic hyperthermia. The process occurs in two steps, namely the spray-flame synthesis of the iron oxide nanoparticles, and then its functionalization with citric acid. The higher SAR values of IO_CA compared to the ones of IO nanoparticles at low field amplitudes are attributed to the monodispersity of the IO_CA particles and de-agglomeration of nanoparticles due to citric acid coating. The system showed an enhanced heat performance on exposure to AMF that could overcome the limitations of using IO in medical applications. We have shown how heat treatment and coating can be employed during synthesis to improve the saturation magnetization of IO nanoparticles, with the effects of reduced spin frustration and higher anisotropy energy after heat treatment being clearly demonstrated on the local scale via Mößbauer spectroscopy. Therefore, by utilizing these advantageous effects, it is possible to develop a system with higher abilities to enhance the efficacy of cancer treatment by increasing the local heat upon applying AMF.
Acknowledgement The authors thank M. Heidelmann from the Interdisciplinary Center for Analytics on the Nanoscale, ICAN, B. Endres, P. Fortugno, A. Tarasov, and S. Apazeller (IVG UDE) for the TEM, BET, Raman Spectroscopy, XPS, and TGA measurements. This material is based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No.10077536, 'Development of fusion-bioceramic material and high-integration diagnostic kit with superparamagnetic ceramic-nanoparticle and single domain antibodies for hazardous agents difficult to detect' and the German Research Foundation (DFG) within the priority program “Nanoparticle Synthesis in Spray Flames” SPP1980, project number 375857056.
References [1]
F. Bray, J. Ferlay, I. Soerjomataram, R.L. Siegel, L.A. Torre, A. Jemal, Global cancer statistics 2018: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries, CA. Cancer J. Clin. 68 (2018) 394–424. https://doi.org/10.3322/caac.21492.
16
[2]
M. Arruebo, N. Vilaboa, B. Sáez-Gutierrez, J. Lambea, A. Tres, M. Valladares, A. GonzálezFernández, Assessment of the evolution of cancer treatment therapies, Cancers (Basel). 3 (2011) 3279–3330. https://doi.org/10.3390/cancers3033279.
[3]
M.M. Cruz, L.P. Ferreira, A.F. Alves, S.G. Mendo, P. Ferreira, M. Godinho, M.D. Carvalho, Chapter 19 - Nanoparticles for magnetic hyperthermia, in: A. Ficai, A.M.B.T.-N. for C.T. Grumezescu (Eds.), Micro Nano Technol., Elsevier, 2017: pp. 485–511. https://doi.org/https://doi.org/10.1016/B978-0-323-46144-3.00019-2.
[4]
D. Chang, M. Lim, J.A.C.M. Goos, R. Qiao, Y.Y. Ng, F.M. Mansfeld, M. Jackson, T.P. Davis, M. Kavallaris, Biologically Targeted Magnetic Hyperthermia: Potential and Limitations, Front. Pharmacol. 9 (2018) 831. https://doi.org/10.3389/fphar.2018.00831.
[5]
W. Xie, Z. Guo, F. Gao, Q. Gao, D. Wang, B.-S. Liaw, Q. Cai, X. Sun, X. Wang, L. Zhao, Shape-, size- and structure-controlled synthesis and biocompatibility of iron oxide nanoparticles for magnetic theranostics, Theranostics. 8 (2018) 3284–3307. https://doi.org/10.7150/thno.25220.
[6]
F. Arteaga-Cardona, K. Rojas-Rojas, R. Costo, M.A. Mendez-Rojas, A. Hernando, P. de la Presa, Improving the magnetic heating by disaggregating nanoparticles, J. Alloys Compd. 663 (2016) 636–644. https://doi.org/https://doi.org/10.1016/j.jallcom.2015.10.285.
[7]
S.H. Chaki, T.J. Malek, M.D. Chaudhary, J.P. Tailor, M.P. Deshpande, Magnetite Fe3O4 nanoparticles synthesis by wet chemical reduction and their characterization, Adv. Nat. Sci. Nanosci. Nanotechnol. 6 (2015) 35009. https://doi.org/10.1088/2043-6262/6/3/035009.
[8]
M. Hammad, R. Hempelmann, Enhanced specific absorption rate of bi-magnetic nanoparticles for heating applications, Mater. Chem. Phys. 188 (2017) 30–38. https://doi.org/http://dx.doi.org/10.1016/j.matchemphys.2016.12.009.
[9]
S. Li, T. Zhang, R. Tang, H. Qiu, C. Wang, Z. Zhou, Solvothermal synthesis and characterization of monodisperse superparamagnetic iron oxide nanoparticles, J. Magn. Magn. Mater. 379 (2015) 226–231. https://doi.org/https://doi.org/10.1016/j.jmmm.2014.12.054.
[10]
A. V Anupama, V. Kumaran, B. Sahoo, Application of monodisperse Fe3O4 submicrospheres in magnetorheological fluids, J. Ind. Eng. Chem. 67 (2018) 347–357. https://doi.org/https://doi.org/10.1016/j.jiec.2018.07.006.
[11]
A. V Anupama, V.B. Khopkar, V. Kumaran, B. Sahoo, Magnetic field dependent steady-state shear response of Fe3O4 micro-octahedron based magnetorheological fluids, Phys. Chem. Chem. Phys. 20 (2018) 20247–20256. https://doi.org/10.1039/C8CP02335B.
[12]
G. Williams, M. Hunt, B. Boehm, A. May, M. Taverne, D. Ho, S. Giblin, D. Read, J. Rarity, R. Allenspach, S. Ladak, Two-photon lithography for 3D magnetic nanostructure fabrication, Nano Res. 11 (2018) 845–854. https://doi.org/10.1007/s12274-017-1694-0.
[13]
L.E. Bykova, V.G. Myagkov, I.A. Tambasov, O.A. Bayukov, V.S. Zhigalov, K.P. Polyakova, G.N. Bondarenko, I. V Nemtsev, V. V Polyakov, G.S. Patrin, D.A. Velikanov, Solid-state synthesis of the ZnO-Fe3O4 nanocomposite: Structural and magnetic properties, Phys. Solid State. 57 (2015) 386–390. https://doi.org/10.1134/S1063783415020079.
[14]
R. Koirala, S.E. Pratsinis, A. Baiker, Synthesis of catalytic materials in flames: opportunities and challenges, Chem. Soc. Rev. 45 (2016) 3053–3068. https://doi.org/10.1039/C5CS00011D.
[15]
W.Y. Teoh, R. Amal, L. Mädler, Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication, Nanoscale. 2 (2010) 1324–1347. https://doi.org/10.1039/C0NR00017E.
[16]
R. Strobel, S.E. Pratsinis, Effect of solvent composition on oxide morphology during flame spray pyrolysis of metal nitrates, Phys. Chem. Chem. Phys. 13 (2011) 9246–9252. https://doi.org/10.1039/C0CP01416H. 17
[17]
W. Wu, Q. He, C. Jiang, Magnetic iron oxide nanoparticles: synthesis and surface functionalization strategies, Nanoscale Res. Lett. 3 (2008) 397–415. https://doi.org/10.1007/s11671-008-9174-9.
[18]
B.I. Kharisov, H.V.R. Dias, O. V Kharissova, A. Vázquez, Y. Peña, I. Gómez, Solubilization, dispersion and stabilization of magnetic nanoparticles in water and non-aqueous solvents: recent trends, RSC Adv. 4 (2014) 45354–45381. https://doi.org/10.1039/C4RA06902A.
[19]
C. Blanco-Andujar, D. Ortega, P. Southern, Q.A. Pankhurst, N.T.K. Thanh, High performance multi-core iron oxide nanoparticles for magnetic hyperthermia: microwave synthesis, and the role of core-to-core interactions, Nanoscale. 7 (2015) 1768–1775. https://doi.org/10.1039/C4NR06239F.
[20]
M.E. de Sousa, M.B. Fernández van Raap, P.C. Rivas, P. Mendoza Zélis, P. Girardin, G.A. Pasquevich, J.L. Alessandrini, D. Muraca, F.H. Sánchez, Stability and Relaxation Mechanisms of Citric Acid Coated Magnetite Nanoparticles for Magnetic Hyperthermia, J. Phys. Chem. C. 117 (2013) 5436–5445. https://doi.org/10.1021/jp311556b.
[21]
L. Li, K.Y. Mak, C.W. Leung, K.Y. Chan, W.K. Chan, W. Zhong, P.W.T. Pong, Effect of synthesis conditions on the properties of citric-acid coated iron oxide nanoparticles, Microelectron. Eng. 110 (2013) 329–334. https://doi.org/10.1016/j.mee.2013.02.045.
[22]
S. Hardt, I. Wlokas, C. Schulz, H. Wiggers, Impact of Ambient Pressure on Titania Nanoparticle Formation During Spray-Flame Synthesis, J. Nanosci. Nanotechnol. 15 (2015) 9449–9456. https://doi.org/10.1166/jnn.2015.10607.
[23]
R.R. Wildeboer, P. Southern, Q.A. Pankhurst, On the reliable measurement of specific absorption rates and intrinsic loss parameters in magnetic hyperthermia materials, J. Phys. D. Appl. Phys. 47 (2014) 495003. https://doi.org/10.1088/0022-3727/47/49/495003.
[24]
M. Kallumadil, M. Tada, T. Nakagawa, M. Abe, P. Southern, Q.A. Pankhurst, Suitability of commercial colloids for magnetic hyperthermia, J. Magn. Magn. Mater. 321 (2009) 1509– 1513. https://doi.org/https://doi.org/10.1016/j.jmmm.2009.02.075.
[25]
A. V Anupama, W. Keune, B. Sahoo, Thermally induced phase transformation in multi-phase iron oxide nanoparticles on vacuum annealing, J. Magn. Magn. Mater. 439 (2017) 156–166. https://doi.org/https://doi.org/10.1016/j.jmmm.2017.04.094.
[26]
D.L.A. de Faria, S. Venâncio Silva, M.T. de Oliveira, Raman microspectroscopy of some iron oxides and oxyhydroxides, J. Raman Spectrosc. 28 (1997) 873–878. https://doi.org/10.1002/(SICI)1097-4555(199711)28:11<873::AID-JRS177>3.0.CO;2-B.
[27]
S.J. Oh, D.C. Cook, H.E. Townsend, Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel, Hyperfine Interact. 112 (1998) 59–66. https://doi.org/10.1023/A:1011076308501.
[28]
T. Fujii, F.M.F. de Groot, G.A. Sawatzky, F.C. Voogt, T. Hibma, K. Okada, In situ XPS analysis of various iron oxide films grown by ${\mathrm{NO}}_{2}$-assisted molecularbeam epitaxy, Phys. Rev. B. 59 (1999) 3195–3202. https://doi.org/10.1103/PhysRevB.59.3195.
[29]
S. Brunauer, P.H. Emmett, E. Teller, Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc. 60 (1938) 309–319. https://doi.org/10.1021/ja01269a023.
[30]
J.M. Hughes, P.M. Budd, A. Grieve, P. Dutta, K. Tiede, J. Lewis, Highly monodisperse, lanthanide-containing polystyrene nanoparticles as potential standard reference materials for environmental “nano” fate analysis, J. Appl. Polym. Sci. 132 (2015). https://doi.org/10.1002/app.42061.
[31]
J.A. Cuenca, K. Bugler, S. Taylor, D. Morgan, P. Williams, J. Bauer, A. Porch, Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements, J. Phys. Condens. Matter. 28 (2016) 106002. https://doi.org/10.1088/095318
8984/28/10/106002. [32]
A. Demortière, P. Panissod, B.P. Pichon, G. Pourroy, D. Guillon, B. Donnio, S. Bégin-Colin, Size-dependent properties of magnetic iron oxide nanocrystals, Nanoscale. 3 (2011) 225–232. https://doi.org/10.1039/C0NR00521E.
[33]
M.P. Morales, C.J. Serna, F. Bødker, S. Mørup, Spin canting due to structural disorder in maghemite, J. Phys. Condens. Matter. 9 (1997) 5461–5467. https://doi.org/10.1088/09538984/9/25/013.
[34]
N. Lee, T. Hyeon, Designed synthesis of uniformly sized iron oxide nanoparticles for efficient magnetic resonance imaging contrast agents, Chem. Soc. Rev. 41 (2012) 2575–2589. https://doi.org/10.1039/C1CS15248C.
[35]
Y. Yuan, D. Rende, C.L. Altan, S. Bucak, R. Ozisik, D.-A. Borca-Tasciuc, Effect of Surface Modification on Magnetization of Iron Oxide Nanoparticle Colloids, Langmuir. 28 (2012) 13051–13059. https://doi.org/10.1021/la3022479.
[36]
T.J. Daou, J.M. Grenèche, G. Pourroy, S. Buathong, A. Derory, C. Ulhaq-Bouillet, B. Donnio, D. Guillon, S. Begin-Colin, Coupling Agent Effect on Magnetic Properties of Functionalized Magnetite-Based Nanoparticles, Chem. Mater. 20 (2008) 5869–5875. https://doi.org/10.1021/cm801405n.
[37]
J. Landers, F. Stromberg, M. Darbandi, C. Schöppner, W. Keune, H. Wende, Correlation of superparamagnetic relaxation with magnetic dipole interaction in capped iron-oxide nanoparticles, J. Phys. Condens. Matter. 27 (2014) 26002. https://doi.org/10.1088/09538984/27/2/026002.
[38]
J. Fock, M.F. Hansen, C. Frandsen, S. Mørup, On the interpretation of Mössbauer spectra of magnetic nanoparticles, J. Magn. Magn. Mater. 445 (2018) 11–21. https://doi.org/https://doi.org/10.1016/j.jmmm.2017.08.070.
[39]
P. Guardia, A. Riedinger, S. Nitti, G. Pugliese, S. Marras, A. Genovese, M.E. Materia, C. Lefevre, L. Manna, T. Pellegrino, One pot synthesis of monodisperse water soluble iron oxide nanocrystals with high values of the specific absorption rate, J. Mater. Chem. B. 2 (2014) 4426–4434. https://doi.org/10.1039/C4TB00061G.
[40]
R. V Stigliano, F. Shubitidze, J.D. Petryk, L. Shoshiashvili, A.A. Petryk, P.J. Hoopes, Mitigation of eddy current heating during magnetic nanoparticle hyperthermia therapy, Int. J. Hyperth. 32 (2016) 735–748. https://doi.org/10.1080/02656736.2016.1195018.
[41]
N.D. Thorat, V.M. Khot, A.B. Salunkhe, A.I. Prasad, R.S. Ningthoujam, S.H. Pawar, Surface functionalized LSMO nanoparticles with improved colloidal stability for hyperthermia applications, J. Phys. D. Appl. Phys. 46 (2013) 105003. https://doi.org/10.1088/00223727/46/10/105003.
[42]
S.P. Gubin, Y.A. Koksharov, G.B. Khomutov, G.Y. Yurkov, Magnetic nanoparticles: preparation, structure and properties, Russ. Chem. Rev. 74 (2005) 489–520. https://doi.org/10.1070/rc2005v074n06abeh000897.
[43]
A.A. McGhie, C. Marquina, K. O’Grady, G. Vallejo-Fernandez, Measurement of the distribution of anisotropy constants in magnetic nanoparticles for hyperthermia applications, J. Phys. D. Appl. Phys. 50 (2017) 455003. https://doi.org/10.1088/1361-6463/aa88ed.
[44]
I. Morales, R. Costo, N. Mille, B.G. Da Silva, J. Carrey, A. Hernando, P. De la Presa, High Frequency Hysteresis Losses on γ-Fe2O3 and Fe3O4: Susceptibility as a Magnetic Stamp for Chain Formation, Nanomater. . 8 (2018). https://doi.org/10.3390/nano8120970.
[45]
D. Bonvin, D.T.L. Alexander, A. Millán, R. Piñol, B. Sanz, G.F. Goya, A. Martínez, J.A.M. Bastiaansen, M. Stuber, K.J. Schenk, H. Hofmann, M. Mionić Ebersold, Tuning Properties of Iron Oxide Nanoparticles in Aqueous Synthesis without Ligands to Improve MRI Relaxivity 19
and SAR, Nanomater. (Basel, Switzerland). 7 (2017) 225. https://doi.org/10.3390/nano7080225. [46]
E. Myrovali, N. Maniotis, A. Makridis, A. Terzopoulou, V. Ntomprougkidis, K. Simeonidis, D. Sakellari, O. Kalogirou, T. Samaras, R. Salikhov, M. Spasova, M. Farle, U. Wiedwald, M. Angelakeris, Arrangement at the nanoscale: Effect on magnetic particle hyperthermia, Sci. Rep. 6 (2016) 37934. https://doi.org/10.1038/srep37934.
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Highlight
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Iron oxide nanoparticles were prepared by a spray-flame technique.
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Two-fold increase in the magnetization values for heated iron oxide nanoparticles than pristine iron oxide nanoparticles.
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Improved heating efficiency of citric acid-coated iron oxide nanoparticles compared to commercially available iron oxide nanoparticles.
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Citric acid-coated iron oxide nanoparticles could be potential candidates for hyperthermia.
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Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance Manuscript title:
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S0925-8388(20)30177-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.153814
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JALCOM 153814
To appear in:
Journal of Alloys and Compounds
Received Date: 10 September 2019 Revised Date:
17 December 2019
Accepted Date: 10 January 2020
Please cite this article as: M. Hammad, S. Hardt, B. Mues, S. Salamon, J. Landers, I. Slabu, H. Wende, C. Schulz, H. Wiggers, Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153814. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Title page
Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance Mohaned Hammada*, Sebastian Hardtb, Benedikt Muesc, Soma Salamond, Joachim Landersd, Ioana Slabuc, Heiko Wended,e, Christof Schulza,e, Hartmut Wiggersa,e*
a
IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids, University of Duisburg-Essen, Duisburg, Germany
b
HSWmaterials GmbH, Kevelaer, Germany
c
Institute of Applied Medical Engineering, Helmholtz Institute, Medical Faculty, RWTH Aachen University, Aachen, Germany
d e
Faculty of Physics, University of Duisburg-Essen, Duisburg, Germany CENIDE, Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Duisburg,
Germany
Corresponding authors: Dr. Mohaned Hammad* Prof. Dr. Hartmut Wiggers* Institute for Combustion and Gas Dynamics – Reactive Fluids University of Duisburg-Essen Carl-Benz-Str. 199 47057 Duisburg, Germany
[email protected] [email protected]
1
Abstract Magnetic nanoparticle-mediated hyperthermia has shown great potential in cancer therapy. However, upscaling of the synthesis of iron oxide nanoparticle with the required narrow size distribution remains challenging. This paper describes the reproducible and scalable synthesis of citric acid-functionalized iron oxide nanoparticles optimized for hyperthermia treatment. Iron oxide nanoparticles were synthesized by a spray flame method, which is eco-friendly and cost-effective. To the best of our knowledge, this is the first study reporting spray-flame synthesis of small iron oxide nanoparticles (approx. 7 nm) with narrow size distribution (polydispersity index ≪ 0.1). The citric acid-coated iron oxide nanoparticles revealed a hydrodynamic size of approx. 37 nm and a high magnetic saturation of 69 Am2/kg at room temperature. The magnetic hyperthermia study showed a significantly enhanced value of the intrinsic loss power (4.8 nHm2/kg), which is 1.5-fold higher than the best commercially available equivalents. The improved heating efficiency and small hydrodynamic size of citric acidcoated iron oxide nanoparticles demonstrate that the system could potentially be used as a nanoplatform for hyperthermia treatment.
Keywords: magnetic nanoparticles, spray-flame synthesis, heating efficiency, Mößbauer spectroscopy
2
1
Introduction
The cancer burden has risen to 18.1 million new cases and claims the lives of 9.6 million people in 2018 around the world [1]. Usually, a combination of traditional remedies such as chemotherapy, radiation, and surgery is used for cancer therapy [2]. The adverse side effects of conventional treatment especially of chemotherapy have proved the need to explore alternative cancer therapies. Magnetic hyperthermia is a promising therapeutic method based on inductive heating of injected magnetic nanoparticles (MNPs) inside a tumor tissue by application of an external alternating magnetic field (AMF) [3]. For a low injected dose of MNPs, reaching a therapeutic temperature of 43 °C is significant challenge [4]. Therefore, the specific absorption rate (SAR) of the MNPs must be high enough to increase the efficacy of cell death hyperthermia and to reduce the dosage and toxicity in hyperthermia treatment. The SAR depends strongly on particle shape, particle size, and magnetic properties of MNPs, such as saturation magnetization, as well as on the AMF frequency and amplitude [5]. Tuning the intrinsic and extrinsic characteristics (i.e., crystallinity, agglomeration, magnetism, colloidal stability, interactions with biological media) of MNPs through the control of their size enhances the heating efficiency of MNPs for hyperthermia [6]. Considerable interest has been devoted to the synthesis of MNPs based on chemical and physical approaches such as wet-chemical precipitation [7], thermal decomposition of iron acetylacetonate precursor solution [8], solvothermal processing of iron (III) chloride precursor [9,10], template-free hydrothermal route using ferrocene as a precursor [11], physical and lithographic processing for 3D magnetic nanostructure fabrication [12], and solid-state reaction [13]. However, the production rates via these methods are limited and scale-up may not be straightforward. A promising but little explored approach is the spray-flame method (also called flame spray pyrolysis) that is especially intriguing because of its scale-up potential. Here, an inorganic precursor is dissolved in a combustible liquid that is processed in a turbulent flame with a short residence time at high temperature. This technique gives superior yields at high production rates up to 4 kg/h of nanoparticles on a pilot-scale [14]. However, there are several challenges associated with the spray-flame method, such as the production of nanoparticles with well-controlled size, shape, and morphology [15]. Pratsinis et al. reported the influence of solvent parameters such as the boiling point on the formation of homogeneous nanoparticles during spray-flame synthesis [16]. Adding of a carboxylic acid (2-ethylhexanoic acid) to the precursor solution that reacts with the precursor, forming a more volatile carboxylate, resulted in homogeneous nanoparticles [16]. Pristine iron oxide (IO) nanoparticles are prone to oxidation in air and can easily agglomerate in a colloidal suspension, both resulting in lower magnetization values and dispersibility [17]. For biomedical applications, IO nanoparticles are coated with biocompatible organic and inorganic materials to prevent agglomeration and improve the colloidal stability [18]. Citric acid presents several advantages in this context, such as the electrosteric (i.e., combined electrostatic and steric) 3
stabilization, biocompatibility, and its easy conjugation to other targeting biomolecules [19]. Some reports have focused on the influence of the citric acid coating on the SAR [20]. On the one side, SAR values became lower for citric acid-coated iron oxide nanoparticles (IO_CA) compared to IO, but on the other side, the colloidal stability improved. Li et al. described the effect of synthesis conditions on the magnetic properties of IO_CA nanoparticles [21]. The adsorption of citric acid onto the MNP surface is enhanced by raising the temperature from 30 to 90 °C and therefore decreases the hydrodynamic size of IO_CA nanoparticles. The objective of this study is to fabricate IO_CA nanoparticles and investigate their heating efficiencies for hyperthermia application. IO nanoparticles with sizes around 7 nm were synthesized using a spray-flame method. Subsequent functionalization with citric acid yielded highly dispersable IO_CA nanoparticles that are stable in aqueous solvents. The influence of the vacuum heating on the magnetic properties of IO nanoparticles is studied. Finally, the impact of the citric acid coating on the SAR is also investigated compared to pristine IO nanoparticles.
2 2.1
Material and methods Materials
The precursor iron(III)-nitrate nonahydrate (≥98 %) was purchased from VWR chemical. Solvents used for spray-flame synthesis (2-ethylhexanoic acid (99 %), absolute ethanol, acetic acid anhydride (≥99 %)), and citric acid (≥99.5 %) and tetramethylammonium hydroxide (TMOAH) (25 wt.% in H2O) used for particle functionalization were purchased from Sigma Aldrich. All chemicals were used as received without further purification.
2.2
Synthesis of iron oxide nanoparticles
The synthesis of IO nanoparticles was carried out in a laboratory-made enclosed spray-flame system described elsewhere [22] with a production rate of 10–20 g/h. A precursor solution was prepared by dissolving 0.5 mol of iron(III)-nitrate in 1 l of a 12.35:6.65:1 mixture of 2-ethylhexanoic acid: ethanol: acetic acid anhydride. The solution is supplied to the spray-flame reactor via a capillary with an inner diameter of 0.5 mm and an outer diameter of 0.9 mm, which is placed in the center of a pilot-flamesupported burner nozzle. An annular gap with an outer diameter of 1.6 mm coaxially surrounds the capillary. The precursor solution is continuously injected with a feed rate of 2 ml/min into the reactor by a system of two alternating syringe pumps. A dispersion gas flow of 10 standard liter per minute (slm) oxygen through the annular gap atomizes the solution. The combustion of the resulting spray is stabilized by a surrounding premixed methane/oxygen (5/10 slm) pilot flame fed from a second annular gap with an inner diameter of 11 mm and an outer diameter of 12.3 mm. An additional coaxial sheath gas flow of 300 slm air fed through a surrounding sintered-bronze ring stabilizes the gas flow in the reaction chamber. A rotary vane pump on the clean-gas side of a filter system ensures the desired
4
pressure within the reaction chamber. A baghouse filter is used to separate the particles from the exhaust gases, which are afterwards harvested from the filter.
2.3
Synthesis of IO_CA nanoparticles
The pristine IO nanoparticle powder (10 g) was heated for 8 hours at 270 °C under vacuum to remove unburned combustion residuals, CO2, and water from the surface. The heated IO nanoparticles, henceforth referred to as IO_270, were dispersed in 100 ml of TMAOH and sonicated for 10 minutes. Excess of TMAOH was removed by centrifugation. To get uniform dispersions, the treated nanoparticles were dispersed in 500 ml of water and sonicated for 15 minutes while kept in an ice bath. The mixture was then heated to 80 °C under vigorous mechanical stirring. Citric acid (5 mg/ml in water) was added into the solution under vigorous mechanical stirring. After 15 minutes, the solution was cooled to room temperature. The prepared IO_CA were collected by centrifugation and washed with de-ionized water for three times. Finally, the samples were dried in an oven under vacuum (50 °C) for 12 hours.
2.4 Methods of Characterization 2.4.1
Structural characterization
A PANalytical X-ray diffractometer X'Pert with a Cu Kα radiation (λ = 1.5406 Å) was used to determine the phase composition of all samples. The mean crystallite size was calculated from the XRD pattern using Rietveld refinement using the program “X'Pert HighScore Plus”. The calculated pattern for the fit was simulated using the crystallographic information file (CIF) from Inorganic Crystal Structure Database (ICSD 98-015-8745 and 98-017-2906). The profile was fit using Pseudo Voigt 3 (FJC Asymmetry) and the background was fitted using a shifted Chebyshev polynomial. Raman spectra were recorded at room temperature on a Renishaw inVia Raman microscope using a HeNe laser beam with a wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VersaProbe II by Ulvac-Phi with monochromatic Al Kα light at an emission angle of 45°. All binding energies were referenced to the carbon 1s peak at 284.8 eV. The morphology and microstructure of the samples were studied by transmission electron microscopy (TEM, JEM‐2200FSJEOL). Nitrogen adsorption/desorption isotherm measurements were carried out at 77 K with a Quantachrome NOVA2200 analyzer. The hydrodynamic size and zeta-potential measurements of IO and IO_CA in aqueous solutions were investigated by dynamic light scattering on a Zetasizer (Malvern Instruments). The water and hydrocarbon content is determined by thermogravimetric analysis (TGA, Netzsch 449 F1 Jupiter) with a heating rate of 5 K/min under the flow of synthetic air (250 ml/min). FTIR spectra were recorded using a Bruker Vertex 80 spectrometer in the 400–4000 cm−1 region.
5
2.4.2
Magnetic characterization
Field-dependent magnetization curves were recorded with the vibrating sample magnetometer option of a Quantum Design PPMS DynaCool. The saturation magnetization was measured at room temperature and a maximum applied magnetic field of 7200 kA/m. SAR measurements were performed to assess the heating performances of the MNPs. Calorimetric particle heating measurements were carried out with a custom-built hyperthermia setup (Trumpf Hüttinger, Freiburg, Germany), consisting of a DC generator, an AC-resonant oscillator and a water-cooled copper coil. The field amplitude ܪ was varied between 7 and 53 kA/m, and the frequency was set to
݂ = 270 kHz. 1 ml ferrofluid samples were prepared in 4 ml glass vials, which were wrapped in 1 mm-thick styrofoam to reduce heat losses and placed in the 7-turn coil. Samples were exposed to the alternating magnetic field for ca. 25 minuts starting at an initial temperature of ܶ = 308.15 K
(35 °C). During exposure to the alternating field, the sample temperature was recorded using a fiberoptic thermometer Luxtron 812 (LumaSense Inc., Santa Barbara, CA, USA). The measured temperature–time data, ܶ()ݐ, were fitted with the Box-Lucas function to calculate SAR. ௧
ܶ(ܶ = )ݐ୰୧ୱୣ ൬1 − exp ቀ− ఛቁ൰ + ܶ
(1)
from which the SAR value was calculated according to [23]: SAR =
ୢ் | ఘ ୢ௧ ௧→
(2)
with the specific heat capacity of water, ܿ = 4.187 J (g K)–1, the MNP weight fraction, ߩ, and dܶ/ d → ݐ(|ݐ0) =
்౨౩ , ఛ
derived from the Box-Lucas fitting parameters.
Further, the intrinsic loss parameter (ILP) is determined: [24] ILP =
ୗୖ ୌమ
(3)
The applied magnetic field should be smaller than the magnetic field leading to the saturation magnetization. The ILP allows comparing the heating efficiency of different MNPs independent of AMF settings.
3 3.1
Results and Discussion Structural characterizations
Crystal structure and crystallite size were characterized from X-ray diffraction (XRD) measurements. Figure 1 shows the XRD pattern of IO and IO_270 nanopowders, respectively. The (220), (311), (400), (422), (511), and (440) diffraction peaks of IO nanoparticles match the Bragg reflections of the spinel ferrite structure. This structure can be indexed to either magnetite (Fe3O4) or maghemite (γFe2O3), and it cannot be ruled out that both phases are present in the powder since the signals indexed
6
(511) and (400) suggest the existence of both phases due to some asymmetry and broadening. On heating, all peaks became more intense and narrower. This change can be attributed to a slight increase in crystallite size. The XRD pattern of the IO_270 nanoparticles with additional low-intensity (111), (620), and (533) diffraction peaks are matched to ICSD card 98-015-8745. The lattice constant calculated from Rietveld refinement increases with the heating of IO nanoparticles from 0.834 to 0.839 nm. This suggests that a probably existing maghemite phase in the IO nanopowder has been converted to magnetite when heating at 270 °C under vacuum [25]. The mean crystallite sizes calculated from the XRD pattern using Rietveld refinement of IO and IO_270 nanoparticles are 6.8 and 7.7 nm, respectively.
Figure 1. XRD patterns of IO and IO_270 nanopowders.
To support the XRD results concerning the phase composition, Raman spectroscopy was applied to identify and distinguish between maghemite and magnetite (Figure 2). The broad peaks of the IO sample around 350, 500, and 700 cm–1 are considered to originate from the maghemite phase [26] while the shoulder at 660 cm–1 can be attributed to magnetite [27]. Consistent with XRD, the transformation of maghemite to magnetite phase, accomplished by heating under vacuum, is confirmed by the Raman measurements since the peak intensities at 350, 500, and 700 cm–1 significantly lower while an increase in peak intensity at 660 cm–1 (magnetite) is observed.
7
Figure 2. Raman spectra of IO and IO_270 nanoparticles.
Moreover, XPS measurements were employed to further characterize the elemental components of the samples (Figure 3). The XPS spectrum of the IO nanoparticles shows two characteristic peaks centered at 711 and 725 eV, which have been assigned to Fe2p3/2 and Fe2p1/2, respectively (Figure 3a). Besides, the explicit development of a satellite peak in the IO spectrum around 719 eV confirms the presence of only Fe3+ species in the maghemite lattice. For the IO_270 sample, the Fe2p3/2 and Fe2p1/2 are centered at 710.7 eV and 724.5 eV, respectively. The reduced intensity of the satellite peak around 719 eV along with the shifted peak position of 2p3/2 and 2p1/2 confirms the existence of both Fe3+ and Fe2+ species, thus indicating the magnetite lattice [28]. Moreover, the O1s spectra (Figure 3b) also show that heating under vacuum leads to a change in chemical composition due to a release of surface contaminants. The signals indicating C–O and Fe–O–C bonds vanish supporting the assumption of particle’s surface cleaning by heating in vacuum, which is also confirmed by FTIR measurements (see Figure S1, supplementary materials).
Figure 3. XPS patterns of the (a) Fe2p and (b) O1s core-level photoelectron spectra for IO and IO_270 powders.
8
The specific surface area (SSA) of the IO and IO_270 samples was measured by nitrogen adsorptiondesorption isotherms analyzed by the BET (Brunauer-Emmett-Teller) theory [29]. Assuming monodisperse, spherical, non-aggregated particles, particle sizes of 8 and 16 nm, respectively, were calculated from the measured BET surface areas of 145 and 71 m2/g and an average density of 4.92 g/cm3. Especially the value obtained for IO_270 is significantly larger compared to the crystallite size obtained from XRD. This difference can be explained with the heating-induced agglomeration or partial sintering of the iron oxide nanoparticles. Particle size and morphology of IO, IO_270, and also IO_CA samples were investigated by TEM (Figure 4). The sizes of at least 300 particles per sample were measured and plotted as a histogram while the histogram was fitted to a log-normal particle size distribution. The fit results are mentioned and plotted as a solid lines in Fig. 4. In all cases, primary particles with an approximately spherical shape and count median diameters (CMD) of about 7 to 8 nm were observed which are in good agreement with the crystallite sizes measured from XRD results. There is almost no difference in shape between the particles before and after heating, however, a slightly pronounced agglomeration is found after heating (Figure 4b). Surprisingly, the amount of agglomerates is significantly reduced after functionalization with citric acid (Figure 4c). The nanoparticles are well dispersed, and the TEM images reveal the existence of many single, individual nanoparticles. Obviously, the functionalization of IONPs almost completely reverses the particle agglomeration observed after heating. In addition to calculating CMD, we have also measured the polydispersity index (PDI) of samples that describes the average uniformity of nanoparticles and was calculated according to: ଶ
PDI = ቀ ౝ ቁ , ଶ σ
(4)
where σ is the geometric standard deviation of the particle diameter distribution and a is the average particle diameter of the nanoparticles. PDI values of our materials are less than 0.1, indicating nearly monodisperse nanoparticle distributions [30].
9
Figure 4. TEM images of (a) IO, (b) IO_270, and (c) IO_CA nanoparticles and respective histograms of particle sizes fitted to a log-normal particle size distribution.
Zeta potential measurements are used as a measure for the electrostatic stability of the dispersions and show that the Zeta potential of IO_270 increases from −15 mV before surface functionalization to −43 mV after citric acid functionalization (Figure S3 supplementary materials). The high Zeta potential of the IO_CA nanoparticles guaranties a good dispersibility and high stability of the nanoparticles in the aqueous medium.
3.2
Magnetic measurements
In order to study the magnetic properties of the different samples, magnetization curves of the IO, IO_270, and IO_CA samples were measured at room temperature with a vibrating sample magnetometer. Figure 5 shows the M(H) loops of all three samples. The saturation magnetization of IO and IO_270 nanoparticles was found to be ca. 32 and 62 Am2/kg, respectively. The M(H) loops are closed, not showing any remanent magnetization indicating that the samples are superparamagnetic. The change in magnetization between the two samples is surprisingly high, even considering the removal of 11 wt.% surface contamination of the pristine IO nanoparticles (Figure S2) [31]. A further potential origin for the lower magnetization of the as-prepared IO sample could be the presence of a minor antiferromagnetic α-Fe2O3 phase, which would not be clearly visible in the XRD diffractogram in case of amorphous material or very small regions. This possibility will be further discussed in terms
10
of Mößbauer spectroscopy below. Other forces contributing to the enhanced magnetization in IO_270 also include the slightly higher average core diameter, which would lead to less pronounced surface spin frustration [32] as well as an improvement of the crystalline and magnetic order due to particle annealing [33]. Nevertheless, frustrated surface spins will result in a reduction of high field magnetization, even in the IO_270 sample, as compared to the bulk value of magnetite (87–92 Am2/kg) [34]. After functionalization with citric acid, the saturation magnetization surprisingly increased to about 69 Am2/kg, which is still higher than that of the IO_270 sample. This can be explained by the CA functionalization preventing direct contact of the IO particles and thus exchange interaction of surface spins, thereby presumably reducing surface spin disorder and enhancing high field magnetization [35]. Furthermore, the surface functionalization could, to some extent, compensate the broken symmetry at the particle surface, representing the main cause of surface spin canting, and thereby increase the high field magnetization [36], overcompensating the slight decrease introduced by negligible amounts of non-magnetic CA (ca. 3 wt.%, see Figure S2). The samples have a high magnetic response and are thus suitable for hyperthermia treatment.
Figure 5. Hysteresis loops at room temperature for IO, IO_270, and IO_CA nanoparticles
To further investigate the degree of spin alignment relative to external fields, Mössbauer spectroscopy was employed at low temperatures (4.3 K) and high fields (5 T). Spectra were recorded in a liquid helium bath cryostat with a superconducting split-pair magnet, in order to achieve low temperatures (4.3 K) and high magnetic fields (5 T) applied parallel to the γ-ray propagation direction. Additional spectra at 80 K were obtained with a liquid nitrogen bath cryostat. A sample heated to 250°C (IO 250) was used as a reference for IO_270, in order to discern the effects of the heating on the particle properties. Figure 6 shows the comparison between the heated IO_250 and the untreated IO samples, with clear differences being visible. At 80 K, the particles experience slow Néel-type relaxation, leading to an asymmetric broadening of the absorption lines, due to which contributions of tetrahedral (A-site) and octahedral (B-site) lattice positions cannot be resolved. A lower relaxation frequency in 11
the heated particle sample can be deduced from the less pronounced spectral deformation, which can be explained by the slightly higher particle diameter and the corresponding higher anisotropy energy [37,38], potentially also favored by particle–particle interaction within particle clusters, indicated by the higher hydrodynamic diameter. When applying high magnetic fields, the alignment of magnetic moments in the particles results in a splitting of A- and B-site contributions, due to the antiferromagnetic coupling of the sub-lattices. For the untreated sample, even when exposed to an external magnetic field of 5 T, there is a significant overlap between the A- and B-site sub-spectra, indicating considerable spin frustration. This can be quantified by the so-called canting angle (angle between the magnetic field and spin direction), which can be determined from the A23 ratio, given by the intensity ratios between lines 2 and 3. Average canting angles of ca. 43° and 36° can be extracted for IO and IO_250, respectively, clearly indicating a reduction in spin frustration as a result of the heating procedure. Still, the moderate spin frustration is insufficient to account for the distinct superposition of sub-spectra observed in sample IO, wherefore a higher degree of structural and magnetic disorder is expected here, resulting not only in increased spin canting but also in distributions of the hyperfine magnetic fields, closely connected to the local magnetic moment, no longer present in the heated sample IO_250. As mentioned in the magnetometry evaluation, the possible presence of a small fraction of poorly ordered iron oxide (hematite or maghemite) phase was taken into account by adding a further subspectrum to the untreated IO sample, showing especially high spin canting and no shift in line position induced by the external magnetic field. Considering its strong contribution to lines 2 and 5, this subspectrum can be modeled with a maximum relative spectral intensity of ca. 10 %. In addition to annealing effects and the removal of carbon contamination, the transformation of this minor fraction to well-ordered maghemite could contribute further to the surprisingly high increase in magnetization that was observed between IO and IO_270 in Figure 5, as this subspectrum in between A- and B-site position is absent in the annealed nanoparticle powder.
12
Figure 6. Mössbauer spectra for IO and IO_250 recorded at 80 K (top) and 4.3 K in an applied magnetic field of 5 T (bottom). Black dots represent experimental data, red lines the overall theoretical fit functions, green and blue lines the A- and B-site sub-spectra, orange line the poorly ordered iron oxide, respectively.
Figure S4 shows the temperature versus time, measured at 270 kHz frequency, for IO and IO_CA samples. The SAR values were normalized to the iron content of the nanoparticles measured by chemical analysis and validated by TGA and are plotted as a function of the field amplitude (Figure 7). The SAR values of IO_CA nanoparticles are higher compared to the IO nanoparticles SAR values. Agglomerations have a demagnetizing effect resulting in decreased saturation magnetization and in a lower heating efficiency compared to the one of IO_CA nanoparticle, e.g., SAR values at 53 kA/m are increased by a factor of 6 for IO_CA nanoparticles. Such high SAR values were reached before for iron oxide nanoparticles with bigger sizes [39]. The high SAR of IO_CA samples, especially at low magnetic field strength is very important for hyperthermia since it allows to reduce the unwanted heating of healthy tissues induced by eddy current [40].
13
Figure 7. SAR functions of IO and IO_CA samples.
For the heating efficiency of magnetic nanoparticles, the Néel and Brownian relaxation processes play a leading role and are expressed by τ = τ exp
ా ்
(5) and τ =
ଷ (౯ౚ )య ా ்
(6), respectively,
where τ0 is a length of time of the material usually between 10–9 to 10–11 s, Keff is the anisotropy constant of the material, Vm is the magnetic volume, kB is the Boltzmann constant, T is the temperature, η is the viscosity of the fluid , and rhyd is the hydrodynamic radius of the particles. There is a significant enhancement of the SAR when the particles are functionalized with citric acid, confirming that both relaxation mechanisms are present in IO_CA [6], which cause a relaxation given by ଵ த
ଵ
=த + ొ
ଵ , தా
(7)
where τ depends on rhyd to the power of three and τ depends exponentially on Keff. To estimate the rhyd value for the nanoparticles dispersed in the water, respective dispersions were characterized by dynamic light scattering. As is evident from Figure 8, the average rhyd of IO_CA nanoparticles (37±3.1 nm) is significantly smaller than that of the pristine IO nanoparticles. Moreover, the pristine IO sample has a bimodal particle size distribution, indicating the presence of stable agglomerates in the micrometer regime while the measurements of IO_CA sample hardly show any bigger agglomerates but a monomodal particle-size distribution. Thus, the measurements support the TEM investigations showing that the surface functionalization leads to de-agglomeration and stabilization of small particles. Binding of citric acid to the nanoparticles’ surface is sufficient to overcome the shortdistance van der Waals forces leading to particle agglomeration and to provide an electrosteric stabilization of the particle dispersions. According to Brownian relaxation equation, the Brownian relaxation time for pristine IO nanoparticles is longer compared to IO_CA nanoparticles because of the large rhyd of the IO agglomerates; therefore faster Brown relaxation contribution to the heating process of IO_CA sample is expected which 14
enhances their heating efficiency. It is reported that the well dispersed magnetic nanoparticles improve the heating efficiency through Brownian and Neel’s relaxations [41]. On the other hand, the Mr/Ms value of the IO_CA sample is close to zero, indicating single domain nanoparticles. Since the IO_CA nanoparticles are superparamagnetic [42], they are heated faster under the external field and are easily magnetized and demagnetized with almost zero hysteresis loss. SAR depends also on the saturation magnetization, spin canting effects, shape anisotropy, and anisotropy constant [43] as well as bigger hysteresis losses due to formation of chains, which happens when the magnetic energy overcomes the thermal energy [44]. We have shown before that heat treatment improves the crystallinity of the iron oxide nanoparticles and reduces spin frustration, which also has an enhancing effect on the heating properties of the nanoparticles [45]. Moreover, since the interparticle interactions are field dependent, chain formation of particles could occur at such a high magnetic field enhancing the heating performance [46].
Figure 8. Hydrodynamic size distribution of IO and IO_CA nanoparticles
According to equation 3, the average ILP values of IO_CA sample is 4.8 nHm2/kg. The ILP value of IO_CA is quite large compared to the best commercial equivalents (Table 1). ILP values of different commercial ferrofluids ranging between 2 and 4 nHm2/kg are sufficient for cancer cell death in magnetic hyperthermia which makes our material the best candidate for magnetic hyperthermia [24].
15
Table 1: ILP values of the IO and IO-CA samples compared to commercial ferrofluids [24]. Particle type
Coating
Dhyd / nm
ILP / (nHm2/kg)
Nanomag-D-spio
Carboxyl
20
2.31
Fluidmag-D
Starch
50
2.67
Resovist
Carboxydextran
60
3.1
IO
without coating
80 2600
0.6
37
4.8
IO_CA
4
Citric acid
Conclusions
In this article, we have presented a robust and scalable method to produce citric acid-coated iron oxide with controllable particle sizes for magnetic hyperthermia. The process occurs in two steps, namely the spray-flame synthesis of the iron oxide nanoparticles, and then its functionalization with citric acid. The higher SAR values of IO_CA compared to the ones of IO nanoparticles at low field amplitudes are attributed to the monodispersity of the IO_CA particles and de-agglomeration of nanoparticles due to citric acid coating. The system showed an enhanced heat performance on exposure to AMF that could overcome the limitations of using IO in medical applications. We have shown how heat treatment and coating can be employed during synthesis to improve the saturation magnetization of IO nanoparticles, with the effects of reduced spin frustration and higher anisotropy energy after heat treatment being clearly demonstrated on the local scale via Mößbauer spectroscopy. Therefore, by utilizing these advantageous effects, it is possible to develop a system with higher abilities to enhance the efficacy of cancer treatment by increasing the local heat upon applying AMF.
Acknowledgement The authors thank M. Heidelmann from the Interdisciplinary Center for Analytics on the Nanoscale, ICAN, B. Endres, P. Fortugno, A. Tarasov, and S. Apazeller (IVG UDE) for the TEM, BET, Raman Spectroscopy, XPS, and TGA measurements. This material is based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No.10077536, 'Development of fusion-bioceramic material and high-integration diagnostic kit with superparamagnetic ceramic-nanoparticle and single domain antibodies for hazardous agents difficult to detect' and the German Research Foundation (DFG) within the priority program “Nanoparticle Synthesis in Spray Flames” SPP1980, project number 375857056.
References [1]
F. Bray, J. Ferlay, I. Soerjomataram, R.L. Siegel, L.A. Torre, A. Jemal, Global cancer statistics 2018: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries, CA. Cancer J. Clin. 68 (2018) 394–424. https://doi.org/10.3322/caac.21492.
16
[2]
M. Arruebo, N. Vilaboa, B. Sáez-Gutierrez, J. Lambea, A. Tres, M. Valladares, A. GonzálezFernández, Assessment of the evolution of cancer treatment therapies, Cancers (Basel). 3 (2011) 3279–3330. https://doi.org/10.3390/cancers3033279.
[3]
M.M. Cruz, L.P. Ferreira, A.F. Alves, S.G. Mendo, P. Ferreira, M. Godinho, M.D. Carvalho, Chapter 19 - Nanoparticles for magnetic hyperthermia, in: A. Ficai, A.M.B.T.-N. for C.T. Grumezescu (Eds.), Micro Nano Technol., Elsevier, 2017: pp. 485–511. https://doi.org/https://doi.org/10.1016/B978-0-323-46144-3.00019-2.
[4]
D. Chang, M. Lim, J.A.C.M. Goos, R. Qiao, Y.Y. Ng, F.M. Mansfeld, M. Jackson, T.P. Davis, M. Kavallaris, Biologically Targeted Magnetic Hyperthermia: Potential and Limitations, Front. Pharmacol. 9 (2018) 831. https://doi.org/10.3389/fphar.2018.00831.
[5]
W. Xie, Z. Guo, F. Gao, Q. Gao, D. Wang, B.-S. Liaw, Q. Cai, X. Sun, X. Wang, L. Zhao, Shape-, size- and structure-controlled synthesis and biocompatibility of iron oxide nanoparticles for magnetic theranostics, Theranostics. 8 (2018) 3284–3307. https://doi.org/10.7150/thno.25220.
[6]
F. Arteaga-Cardona, K. Rojas-Rojas, R. Costo, M.A. Mendez-Rojas, A. Hernando, P. de la Presa, Improving the magnetic heating by disaggregating nanoparticles, J. Alloys Compd. 663 (2016) 636–644. https://doi.org/https://doi.org/10.1016/j.jallcom.2015.10.285.
[7]
S.H. Chaki, T.J. Malek, M.D. Chaudhary, J.P. Tailor, M.P. Deshpande, Magnetite Fe3O4 nanoparticles synthesis by wet chemical reduction and their characterization, Adv. Nat. Sci. Nanosci. Nanotechnol. 6 (2015) 35009. https://doi.org/10.1088/2043-6262/6/3/035009.
[8]
M. Hammad, R. Hempelmann, Enhanced specific absorption rate of bi-magnetic nanoparticles for heating applications, Mater. Chem. Phys. 188 (2017) 30–38. https://doi.org/http://dx.doi.org/10.1016/j.matchemphys.2016.12.009.
[9]
S. Li, T. Zhang, R. Tang, H. Qiu, C. Wang, Z. Zhou, Solvothermal synthesis and characterization of monodisperse superparamagnetic iron oxide nanoparticles, J. Magn. Magn. Mater. 379 (2015) 226–231. https://doi.org/https://doi.org/10.1016/j.jmmm.2014.12.054.
[10]
A. V Anupama, V. Kumaran, B. Sahoo, Application of monodisperse Fe3O4 submicrospheres in magnetorheological fluids, J. Ind. Eng. Chem. 67 (2018) 347–357. https://doi.org/https://doi.org/10.1016/j.jiec.2018.07.006.
[11]
A. V Anupama, V.B. Khopkar, V. Kumaran, B. Sahoo, Magnetic field dependent steady-state shear response of Fe3O4 micro-octahedron based magnetorheological fluids, Phys. Chem. Chem. Phys. 20 (2018) 20247–20256. https://doi.org/10.1039/C8CP02335B.
[12]
G. Williams, M. Hunt, B. Boehm, A. May, M. Taverne, D. Ho, S. Giblin, D. Read, J. Rarity, R. Allenspach, S. Ladak, Two-photon lithography for 3D magnetic nanostructure fabrication, Nano Res. 11 (2018) 845–854. https://doi.org/10.1007/s12274-017-1694-0.
[13]
L.E. Bykova, V.G. Myagkov, I.A. Tambasov, O.A. Bayukov, V.S. Zhigalov, K.P. Polyakova, G.N. Bondarenko, I. V Nemtsev, V. V Polyakov, G.S. Patrin, D.A. Velikanov, Solid-state synthesis of the ZnO-Fe3O4 nanocomposite: Structural and magnetic properties, Phys. Solid State. 57 (2015) 386–390. https://doi.org/10.1134/S1063783415020079.
[14]
R. Koirala, S.E. Pratsinis, A. Baiker, Synthesis of catalytic materials in flames: opportunities and challenges, Chem. Soc. Rev. 45 (2016) 3053–3068. https://doi.org/10.1039/C5CS00011D.
[15]
W.Y. Teoh, R. Amal, L. Mädler, Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication, Nanoscale. 2 (2010) 1324–1347. https://doi.org/10.1039/C0NR00017E.
[16]
R. Strobel, S.E. Pratsinis, Effect of solvent composition on oxide morphology during flame spray pyrolysis of metal nitrates, Phys. Chem. Chem. Phys. 13 (2011) 9246–9252. https://doi.org/10.1039/C0CP01416H. 17
[17]
W. Wu, Q. He, C. Jiang, Magnetic iron oxide nanoparticles: synthesis and surface functionalization strategies, Nanoscale Res. Lett. 3 (2008) 397–415. https://doi.org/10.1007/s11671-008-9174-9.
[18]
B.I. Kharisov, H.V.R. Dias, O. V Kharissova, A. Vázquez, Y. Peña, I. Gómez, Solubilization, dispersion and stabilization of magnetic nanoparticles in water and non-aqueous solvents: recent trends, RSC Adv. 4 (2014) 45354–45381. https://doi.org/10.1039/C4RA06902A.
[19]
C. Blanco-Andujar, D. Ortega, P. Southern, Q.A. Pankhurst, N.T.K. Thanh, High performance multi-core iron oxide nanoparticles for magnetic hyperthermia: microwave synthesis, and the role of core-to-core interactions, Nanoscale. 7 (2015) 1768–1775. https://doi.org/10.1039/C4NR06239F.
[20]
M.E. de Sousa, M.B. Fernández van Raap, P.C. Rivas, P. Mendoza Zélis, P. Girardin, G.A. Pasquevich, J.L. Alessandrini, D. Muraca, F.H. Sánchez, Stability and Relaxation Mechanisms of Citric Acid Coated Magnetite Nanoparticles for Magnetic Hyperthermia, J. Phys. Chem. C. 117 (2013) 5436–5445. https://doi.org/10.1021/jp311556b.
[21]
L. Li, K.Y. Mak, C.W. Leung, K.Y. Chan, W.K. Chan, W. Zhong, P.W.T. Pong, Effect of synthesis conditions on the properties of citric-acid coated iron oxide nanoparticles, Microelectron. Eng. 110 (2013) 329–334. https://doi.org/10.1016/j.mee.2013.02.045.
[22]
S. Hardt, I. Wlokas, C. Schulz, H. Wiggers, Impact of Ambient Pressure on Titania Nanoparticle Formation During Spray-Flame Synthesis, J. Nanosci. Nanotechnol. 15 (2015) 9449–9456. https://doi.org/10.1166/jnn.2015.10607.
[23]
R.R. Wildeboer, P. Southern, Q.A. Pankhurst, On the reliable measurement of specific absorption rates and intrinsic loss parameters in magnetic hyperthermia materials, J. Phys. D. Appl. Phys. 47 (2014) 495003. https://doi.org/10.1088/0022-3727/47/49/495003.
[24]
M. Kallumadil, M. Tada, T. Nakagawa, M. Abe, P. Southern, Q.A. Pankhurst, Suitability of commercial colloids for magnetic hyperthermia, J. Magn. Magn. Mater. 321 (2009) 1509– 1513. https://doi.org/https://doi.org/10.1016/j.jmmm.2009.02.075.
[25]
A. V Anupama, W. Keune, B. Sahoo, Thermally induced phase transformation in multi-phase iron oxide nanoparticles on vacuum annealing, J. Magn. Magn. Mater. 439 (2017) 156–166. https://doi.org/https://doi.org/10.1016/j.jmmm.2017.04.094.
[26]
D.L.A. de Faria, S. Venâncio Silva, M.T. de Oliveira, Raman microspectroscopy of some iron oxides and oxyhydroxides, J. Raman Spectrosc. 28 (1997) 873–878. https://doi.org/10.1002/(SICI)1097-4555(199711)28:11<873::AID-JRS177>3.0.CO;2-B.
[27]
S.J. Oh, D.C. Cook, H.E. Townsend, Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel, Hyperfine Interact. 112 (1998) 59–66. https://doi.org/10.1023/A:1011076308501.
[28]
T. Fujii, F.M.F. de Groot, G.A. Sawatzky, F.C. Voogt, T. Hibma, K. Okada, In situ XPS analysis of various iron oxide films grown by ${\mathrm{NO}}_{2}$-assisted molecularbeam epitaxy, Phys. Rev. B. 59 (1999) 3195–3202. https://doi.org/10.1103/PhysRevB.59.3195.
[29]
S. Brunauer, P.H. Emmett, E. Teller, Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc. 60 (1938) 309–319. https://doi.org/10.1021/ja01269a023.
[30]
J.M. Hughes, P.M. Budd, A. Grieve, P. Dutta, K. Tiede, J. Lewis, Highly monodisperse, lanthanide-containing polystyrene nanoparticles as potential standard reference materials for environmental “nano” fate analysis, J. Appl. Polym. Sci. 132 (2015). https://doi.org/10.1002/app.42061.
[31]
J.A. Cuenca, K. Bugler, S. Taylor, D. Morgan, P. Williams, J. Bauer, A. Porch, Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements, J. Phys. Condens. Matter. 28 (2016) 106002. https://doi.org/10.1088/095318
8984/28/10/106002. [32]
A. Demortière, P. Panissod, B.P. Pichon, G. Pourroy, D. Guillon, B. Donnio, S. Bégin-Colin, Size-dependent properties of magnetic iron oxide nanocrystals, Nanoscale. 3 (2011) 225–232. https://doi.org/10.1039/C0NR00521E.
[33]
M.P. Morales, C.J. Serna, F. Bødker, S. Mørup, Spin canting due to structural disorder in maghemite, J. Phys. Condens. Matter. 9 (1997) 5461–5467. https://doi.org/10.1088/09538984/9/25/013.
[34]
N. Lee, T. Hyeon, Designed synthesis of uniformly sized iron oxide nanoparticles for efficient magnetic resonance imaging contrast agents, Chem. Soc. Rev. 41 (2012) 2575–2589. https://doi.org/10.1039/C1CS15248C.
[35]
Y. Yuan, D. Rende, C.L. Altan, S. Bucak, R. Ozisik, D.-A. Borca-Tasciuc, Effect of Surface Modification on Magnetization of Iron Oxide Nanoparticle Colloids, Langmuir. 28 (2012) 13051–13059. https://doi.org/10.1021/la3022479.
[36]
T.J. Daou, J.M. Grenèche, G. Pourroy, S. Buathong, A. Derory, C. Ulhaq-Bouillet, B. Donnio, D. Guillon, S. Begin-Colin, Coupling Agent Effect on Magnetic Properties of Functionalized Magnetite-Based Nanoparticles, Chem. Mater. 20 (2008) 5869–5875. https://doi.org/10.1021/cm801405n.
[37]
J. Landers, F. Stromberg, M. Darbandi, C. Schöppner, W. Keune, H. Wende, Correlation of superparamagnetic relaxation with magnetic dipole interaction in capped iron-oxide nanoparticles, J. Phys. Condens. Matter. 27 (2014) 26002. https://doi.org/10.1088/09538984/27/2/026002.
[38]
J. Fock, M.F. Hansen, C. Frandsen, S. Mørup, On the interpretation of Mössbauer spectra of magnetic nanoparticles, J. Magn. Magn. Mater. 445 (2018) 11–21. https://doi.org/https://doi.org/10.1016/j.jmmm.2017.08.070.
[39]
P. Guardia, A. Riedinger, S. Nitti, G. Pugliese, S. Marras, A. Genovese, M.E. Materia, C. Lefevre, L. Manna, T. Pellegrino, One pot synthesis of monodisperse water soluble iron oxide nanocrystals with high values of the specific absorption rate, J. Mater. Chem. B. 2 (2014) 4426–4434. https://doi.org/10.1039/C4TB00061G.
[40]
R. V Stigliano, F. Shubitidze, J.D. Petryk, L. Shoshiashvili, A.A. Petryk, P.J. Hoopes, Mitigation of eddy current heating during magnetic nanoparticle hyperthermia therapy, Int. J. Hyperth. 32 (2016) 735–748. https://doi.org/10.1080/02656736.2016.1195018.
[41]
N.D. Thorat, V.M. Khot, A.B. Salunkhe, A.I. Prasad, R.S. Ningthoujam, S.H. Pawar, Surface functionalized LSMO nanoparticles with improved colloidal stability for hyperthermia applications, J. Phys. D. Appl. Phys. 46 (2013) 105003. https://doi.org/10.1088/00223727/46/10/105003.
[42]
S.P. Gubin, Y.A. Koksharov, G.B. Khomutov, G.Y. Yurkov, Magnetic nanoparticles: preparation, structure and properties, Russ. Chem. Rev. 74 (2005) 489–520. https://doi.org/10.1070/rc2005v074n06abeh000897.
[43]
A.A. McGhie, C. Marquina, K. O’Grady, G. Vallejo-Fernandez, Measurement of the distribution of anisotropy constants in magnetic nanoparticles for hyperthermia applications, J. Phys. D. Appl. Phys. 50 (2017) 455003. https://doi.org/10.1088/1361-6463/aa88ed.
[44]
I. Morales, R. Costo, N. Mille, B.G. Da Silva, J. Carrey, A. Hernando, P. De la Presa, High Frequency Hysteresis Losses on γ-Fe2O3 and Fe3O4: Susceptibility as a Magnetic Stamp for Chain Formation, Nanomater. . 8 (2018). https://doi.org/10.3390/nano8120970.
[45]
D. Bonvin, D.T.L. Alexander, A. Millán, R. Piñol, B. Sanz, G.F. Goya, A. Martínez, J.A.M. Bastiaansen, M. Stuber, K.J. Schenk, H. Hofmann, M. Mionić Ebersold, Tuning Properties of Iron Oxide Nanoparticles in Aqueous Synthesis without Ligands to Improve MRI Relaxivity 19
and SAR, Nanomater. (Basel, Switzerland). 7 (2017) 225. https://doi.org/10.3390/nano7080225. [46]
E. Myrovali, N. Maniotis, A. Makridis, A. Terzopoulou, V. Ntomprougkidis, K. Simeonidis, D. Sakellari, O. Kalogirou, T. Samaras, R. Salikhov, M. Spasova, M. Farle, U. Wiedwald, M. Angelakeris, Arrangement at the nanoscale: Effect on magnetic particle hyperthermia, Sci. Rep. 6 (2016) 37934. https://doi.org/10.1038/srep37934.
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Highlight
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Iron oxide nanoparticles were prepared by a spray-flame technique.
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Two-fold increase in the magnetization values for heated iron oxide nanoparticles than pristine iron oxide nanoparticles.
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Improved heating efficiency of citric acid-coated iron oxide nanoparticles compared to commercially available iron oxide nanoparticles.
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Citric acid-coated iron oxide nanoparticles could be potential candidates for hyperthermia.
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Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance Manuscript title:
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