Article pubs.acs.org/cm
Thermal Energy Dissipation by SiO2‑Coated PlasmonicSuperparamagnetic Nanoparticles in Alternating Magnetic Fields Georgios A. Sotiriou,†,# Michelle A. Visbal-Onufrak,‡,○ Alexandra Teleki,†,¶ Eduardo J. Juan,‡ Ann M. Hirt,§ Sotiris E. Pratsinis,*,† and Carlos Rinaldi*,∥,⊥ †
Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland Department of Electrical & Computer Engineering, University of Puerto Rico, Mayagüez, Puerto Rico § Institute of Geophysics, Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland ∥ Department of Chemical Engineering, University of Puerto Rico, Mayagüez, Puerto Rico ⊥ J. Crayton Pruitt Family Department of Biomedical Engineering and Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States ‡
ABSTRACT: Multifunctional nanoparticles show great potential in the biomedical field and may help the diagnosis and therapy of diseases. Superparamagnetic nanoparticles are especially attractive because of their ability to dissipate thermal energy in an alternating magnetic field. Furthermore, plasmonic nanoparticles can be effectively used in non- or minimally invasive therapy of tumors exploiting their plasmonic photothermal effect. Here, hybrid plasmonicmagnetic Ag/Fe2O3 nanoparticles are made by flame aerosol technology. These nanoparticles can be in situ encapsulated with an amorphous nanothin SiO2 film to facilitate their dispersion and block any toxicity from Ag/Fe2O3. Detailed physicochemical characterization, including electron microscopy, electron dispersive X-ray spectroscopy, and X-ray diffraction, is performed. Furthermore, their magnetic properties are characterized in detail by monitoring their hysteresis, first-order-reversal-curves, and isothermal remanent magnetization. Finally, the effect of SiO2 and Agcontent on the specific absorption rate (SAR) of the hybrid Ag/Fe2O3 nanoparticles is investigated. The obtained understanding will help the rational design and engineering of multifunctional hybrid nanoprobes targeting specific biomedical applications. KEYWORDS: specific absorption rate, magnetic fluid hyperthermia, theranostics, core−shell nanoparticles
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INTRODUCTION Magnetic nanoparticles (MNPs) are being widely studied as tools for cancer therapeutics due to their ability to transform the energy of an alternating magnetic field to thermal energy in what is often called magnetic fluid hyperthermia.1,2 MNPs dissipate thermal energy by magnetic relaxation through the Brownian and Néel mechanisms.3 In Brownian relaxation, the magnetic moment of the MNPs is locked to their crystal axis and aligns with the applied field by physical particle rotation. In Néel relaxation, the magnetic moment rotates within the crystal by the applied magnetic field. In both cases the relaxation process transforms a portion of the energy of the alternating magnetic field into heat that is dissipated to the surrounding. Therefore, upon the MNP accumulation by passive or active targeting4 at tumor sites and the application of an alternating magnetic field, the cancer cells may be destroyed by this rapid local temperature increase.5 The rate at which magnetic nanoparticles dissipate power is characterized by calorimetric measurements of the initial temperature increase and is commonly expressed as the specific absorption rate (SAR). The SAR is defined as the thermal dissipation per unit mass of magnetic fluid under an alternating magnetic field:6 © 2013 American Chemical Society
SAR =
∑ cimi dT mMNP dt
≈ t=0
csolventmsolvent dT dt mMNP
t=0
(1)
where the sum is over all components in the particle suspension, ci is the specific heat capacity of species i, mi is the mass of species i, mMNP is the mass of nanoparticles in the sample, (dT/dt)|t=0 is the initial rate of temperature rise, csolvent is the specific heat capacity of the solvent, and msolvent is the mass of the solvent. The expression on the far right is accurate for cases where the particles are in dilute suspension in the medium. The SAR of magnetic nanostructures is related to the parameters of the applied alternating magnetic field. This heat dissipation is also dependent upon particle size and magnetic domain type, as its hydrodynamic volume and magnetic core diameter are linked to the effective Brownian and Néel magnetic relaxation mechanisms.7 Further parameters include particle concentration and size distribution, as well as surface coating. The latter affects particle−particle interactions and the colloidal stability of nanoparticles.8 Received: August 28, 2013 Revised: October 21, 2013 Published: October 23, 2013 4603
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tube. The HMDSO vapor was supplied by bubbling nitrogen through approximately 350 mL of liquid HMDSO in a 500 mL glass flask. The SiO2 amount was kept constant in the product particles and was calculated assuming saturated conditions19 (bubbler temperature 10 °C and nitrogen flow rate 0.5 L/min) corresponding to 23 wt % for pure Fe2O3 core particles20 (SiO2 wt% = mSiO2/(mSiO2 + mFe2O3)). Silicacoated pure Fe2O3 particles were made under identical conditions in the absence, however, of the Ag precursor. Particle Characterization. High-resolution transmission electron microscopy (HRTEM) was performed with a CM30ST microscope (FEI; LaB6 cathode, operated at 300 kV, point resolution ≈ 2 Å) and scanning transmission electron microscopy (STEM) on a Tecnai F30 (FEI; field emission gun, operated at 300 kV). The STEM images were recorded with a high-angle annular dark field (HAADF) detector revealing the Ag particles with bright Z contrast. The electron beam could be set to selected areas to determine material composition by energy dispersive X-ray spectroscopy (EDXS; detector (EDAX) attached to the Tecnai F30 microscope). Product particles were dispersed in ethanol and deposited onto a perforated carbon foil supported on a copper grid. X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 Advance spectrometer (Cu Kα, 40 kV, 40 mA). The crystallite size of silver and iron oxide was determined using the TOPAS 3 software and fitting only the main diffraction peaks. From the XRD patterns it is possible to calculate an average crystal size using Rietveld analysis.21 Magnetic measurements were made on a Princeton Measurements Corporation vibrating sample magnetometer (VSM). Magnetic hysteresis and first-order-reversal-curves (FORC) measurements are detailed elsewhere.22 FORC measurements were made using a 1.0 T saturation field and an averaging time of 100 ms with 121 FORCs. The measurement started by applying the positive saturation field, pausing 1 s, and ramping down to some reversal field, Ha. The magnetization was then measured as the field, Hb, increases from Ha back to saturation. The magnetization at any applied Hb along the reversal curve with a reversal field of Ha is defined as M(Ha,Hb) where Hb ≥ Ha. Acquisition of an isothermal remanent magnetization (IRM) was used to identify the ferromagnetic minerals in a material because the remanent saturation magnetization and field needed to reach magnetic saturation is dependent primarily on mineral composition. Samples were first magnetized in a 1 T field along one direction and then incrementally magnetized in an increasing DC field in the opposite direction. The DC field was applied for 1 s and the remanent magnetization was measured at each step 1 s after removal of the field. The field strength needed to cancel the original field is known as the remanence coercive force (Hcr). SAR Measurements. Instrumentation. To characterize ferrofluids by their SAR, a commercial induction heater (RDO Induction, Model HFI 3-135/400, Washington, NJ, USA) was used. The RDO induction heating coil, a 4-turn copper tubing coil (length = 2.97 cm, diameter = 3.17 cm) operates at a frequency of 233 kHz and magnetic field intensities of up to 51.56 kA/m. A Durachill HD chiller (Polyscience, Co, Niles, USA) was used to circulate cold water inside the coil and remove the heat from resistive losses in the coil. Fluoroptic immersion probes (Lumasense Technologies, Oakland, NJ, USA) were utilized for performing temperature measurements of the samples subjected to an AC magnetic field, for subsequent determination of their SAR values. Sample Preparation. The volumetric power dissipation depends on the out-of-phase magnetic susceptibility of the ferrofluid χ″, the frequency f, and the square of the magnetic field intensity of the applied AC field, H according to23
When magnetic nanoparticles are combined with metallic nanoparticles (e.g., gold or silver), hybrid magnetic-plasmonic nanostructures can be made.9 Plasmons are collective oscillations of the conduction band electrons upon their interaction with light.10 Such hybrid structures exhibit a number of advantages, especially in biomedical applications. For example, they can be detected by multiple imaging techniques (e.g., magnetic resonance11 and photoacoustic imaging12) exploiting both the magnetic and plasmonic components. Furthermore, they can also serve as multifunctional non- or minimially invasive therapeutic agents using the magnetic hyperthermia and plasmonic photothermal effects that involve the tissue heating by magnetic fields13 and light,14 respectively. Typically, a surface coating is applied on such bionanoparticles. A coating can protect the core particles from degradation, facilitate dispersion, and inhibit interactions (e.g., toxicity) with biological systems. Another important requirement of the surface coating is the possibility to easily modify its surface,15 i.e., to attach specific (bio)molecules16 for active targeting of certain biomoieties.17 Recently, hybrid Janus-like plasmonic-magnetic Ag/Fe2O3 nanostructures were made by scalable flame aerosol technology, hermetically encapsulated with a nanothin amorphous SiO2 film.18 This SiO2 shell hindered flocculation and allowed the easy dispersion of such nanoparticles in aqueous and biological buffer (PBS) solutions without any additional functionalization step. The hermetic coating also inhibited the release of toxic Ag+ ions from the nanosilver surface minimizing their toxicity against HeLa cells. Furthermore, these particles could be successfully biofunctionalized with the hDC-sign antibody that selectively binds on the corresponding cell-membrane receptor. The nanoparticle performance as biomarkers was explored by selectively binding them with live Raji and HeLa cells enabling their detection under dark-field microscopy. Here, the therapeutic potential of these hybrid nanoparticles by magnetic fluid hyperthermia is explored. Special emphasis is given on the SAR of these particles and the effect of Ag-content, as well as the presence of a SiO2 coating on the hybrid Ag/Fe2O3 nanoparticles. The detailed physicochemical and magnetic characterization of such hybrid plasmonic-magnetic nanostructures gives useful insight on the magnetic properties that influence their SAR performance.
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EXPERIMENTAL SECTION
Hybrid SiO2-Coated Ag/Fe2O3 Nanoparticle Synthesis. Silicacoated Ag/Fe2O3 particles were made in one-step with an enclosed flame aerosol reactor, described in detail elsewhere.18 In brief, the composite core Ag/Fe2O3 nanoparticles were made by flame spray pyrolysis of precursor solutions containing iron(III) acetylacetonate (Sigma Aldrich, purity ≥97%) and silver acetate (Sigma Aldrich, purity ≥99%) dissolved in 2-ethylhexanoic acid and acetonitrile (both Sigma Aldrich, purity ≥97%, volume ratio 1:1, stirring 100 °C for 30 min). The precursor solutions were fed at 5 mL/min to the flame spray pyrolysis (FSP) reactor and dispersed by 5 L/min O2 forming a spray. The spray of the precursor solution (pressure drop = 1.5 bar at the nozzle tip) was ignited by a ring-shaped, premixed methane/oxygen flame (1.5/3.2 L/min) and sheathed by 40 L/min O2 (all gases Pan Gas, purity >99%). The Fe precursor concentration was kept constant at 0.5 M, while corresponding amounts of silver acetate were added to reach the nominal Ag wt%, which was defined as x = mAg/(mAg + mFe2O3), and called as Ag-content x. The freshly formed composite Ag/Fe2O3 particles were coated in-flight by swirling injection of hexamethyldisiloxane (HMDSO, Sigma Aldrich, purity ≥ 99%) vapor along with 15 L/min nitrogen (PanGas, purity >99.9%) at room temperature through a metallic ring with 16 equidistant openings. The ring was placed on top of a 20 cm long quartz glass tube. The reactor was terminated by a 30 cm quartz glass
P = μ0 πχ ″fH2
(2)
The samples in powdered form were suspended in 2.0 mL of deionized water at a concentration of 1.0% w/w of Fe2O3 (nominal value). Samples were sonicated for 60 min or until a homogeneous suspension was obtained. The SARs were determined from the rate of temperature rise upon application of an alternating magnetic field to samples that have reached thermal equilibrium using eq 1, where Cwater = 4.18 J/g·°C, C1% agar = 4.96 J/g·°C (measured using a TA Instruments 4604
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Figure 1. HAADF-STEM (Z contrast) (a,b) and HR-TEM (e,f) images of uncoated (a,e) and SiO2-coated (b,f) 10Ag/Fe2O3 nanoparticles with the corresponding EDX spectra (c,d). Primary particle size distributions of Fe2O3 (g) and Ag (h) for the SiO2-coated 35Ag/Fe2O3 nanoparticles. The average diameter dp, total particles counted N, and the geometric standard deviation σg are also shown. The solid line is a log-normal fit. 1.0% w/w was diluted to 0.5% w/w by adding 800 μL of deionized water into a glass vial, for a total of 1.2 mL sample volume. For fixing the samples using agar, 800 μL from the sample suspended in water at 1.0% w/w was diluted to 0.5% w/w by adding 800 μL of 2.0% agar solution into a glass vial, for a total of 1.2 mL sample volume.
Q2000 Differential Scanning Calorimeter), and mNP = 0.005 g corresponding to 0.5% w/w in 1 g of sample. Two separate sample batches at 0.5% w/w were prepared for SAR characterization, suspended in deionized water or agar. To prepare the samples suspended in water, 800 μL from the sample in water at 4605
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Figure 2. XRD patterns of the uncoated (a) and SiO2-coated (b) xAg/Fe2O3 nanoparticles for varying Ag-content x = 0−50 wt %. The peak positions of Ag (square), γ-Fe2O3 (circle), and α-Fe2O3 (triangle) are also shown. The estimated average crystal sizes for γ-Fe2O3 (circles) and Ag (triangles) are shown for the uncoated (filled symbols) and SiO2-coated (open symbols) samples. Experimental Procedure. The sample vial was fixed in place inside the heating coil using a PMMA sample holder to provide thermal insulation. After the sample temperature reached equilibrium with room temperature (23 °C ± 2 °C) a magnetic field with intensity of H = 10 kA/m was applied for a minimum of 100 s. The temperature data of the sample was recorded throughout the experiment utilizing an acquisition rate of 1 s. The initial slope of temperature as a function of time was obtained by performing linear regression of the initial portion of the temperature profile, corresponding to the first 10 to 40 s immediately after applying the magnetic field. The sample was cooled back down to room temperature, and the experiment was repeated using increasing magnetic field intensity values of 20, 30, 40, and 50 kA/m. This procedure was repeated for all samples suspended in water and fixed in agar.
The higher-proton number Ag nanoparticles appear brighter in these STEM images and are attached to the Fe2O3 nanoparticles (diffuse gray). Even though their SiO2 coating in Figure 1b cannot be easily detected, the core particle morphology is not altered by its presence. The elemental composition of both uncoated and SiO2-coated Ag/Fe2O3 is confirmed by energy dispersive X-ray (EDX) spectroscopy (Figure 1c,d). For the uncoated sample (Figure 1c), Ag and Fe are present and additionally Si is detected at 2 keV in the SiO2-coated sample (Figure 1d). The Cu and C originate from the employed carboncoated copper foil. It should be noted that flame synthesis results in high-purity products without any contamination,24 which is verified here by the EDX data and is essential for biomedical applications.25 Figure 1e shows a high-resolution TEM image of the uncoated hybrid nanoparticles in which no coating is present. However, the corresponding image of the SiO2-coated Ag/Fe2O3 nanoparticles (Figure 1f) shows them (Ag is the darker part here) fully
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RESULTS AND DISCUSSION Morphology. Hybrid plasmonic-magnetic nanoparticles were made with a varying nominal Ag-content x (x = 0 to 50 wt %: xAg/Fe2O3).18 Figure 1 shows STEM images of uncoated (a) and SiO2-coated (b) 10Ag/Fe2O3 nanoparticles. 4606
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encapsulated by a nanothin, hermetic amorphous SiO2 layer of approximately 2 nm thickness.26 Furthermore, the Fe2O3 crystal lattices are also visible verifying the core crystallinity. Figure 1 also shows the primary particle size distributions of Fe2O3 (g) and Ag (h) for the SiO2-coated 35Ag/Fe2O3 nanoparticles. The solid line is a log-normal fit of the data. Both the Fe2O3 and Ag nanoparticles exhibit a unimodal particle size distribution20,27 with geometric standard deviations σg = 1.40 and 1.52, respectively. Figure 2 shows the XRD patterns of uncoated (a) and SiO2coated (b) xAg/Fe2O3 for varying Ag-content x, which is the mass fraction of Ag in the uncoated Ag/Fe2O3 particles. The patterns of both uncoated and SiO2-coated particles are similar, indicating that the presence of SiO2 does not influence the crystallinity of the core Ag26 and Fe2O320 particles because the SiO2 precursor
Table 1. Crystal Phase Composition in Mass Fractions of the Different Crystal Structures in the Uncoated and SiO2-Coated Samples for a Varying Ag-Content x, As Determined by Rietveld Analysis from the XRD Patterns Rietveld analysis
uncoated
SiO2-coated
nominal Ag-content x in xAg/Fe2O3
γ-Fe2O3 (wt %)
α-Fe2O3 (wt %)
Ag (wt %)
0 10 35 50 0 10 35 50
82 79 58 28 83 77 54 27
18 14 15 20 17 15 14 22
0 7 27 52 0 8 32 51
Figure 3. Hysteresis magnetization for uncoated (green lines) and SiO2-coated (red lines) xAg/Fe2O3 nanoparticles with x = 10 (a) and 35 wt % (b). Magnification at low magnetic fields for each graph are shown (c,d) along with their coercivities Hc. 4607
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was injected after the core particle formation. For pure Fe2O3 (x = 0 wt %), mostly the characteristic peaks for γ-Fe2O3 (circles) appear,28 with some α-Fe2O3 (hematite, triangle).20 Hematite exhibits lower magnetization than maghemite,29 and therefore, maghemite is desired for biomedical applications. For increasing Ag-content x, the characteristic peak corresponding to Ag metal (squares) emerges26 and dominates the pattern for x = 50 wt %. Furthermore, the main peak attributed to the α-Fe2O3 is more pronounced for increasing x, indicating a larger α-Fe2O3 content. This could be attributed to the higher combustion enthalpy of the employed precursor solutions for the largest Ag-content30 and thus higher temperatures within the enclosed flame aerosol reactor.20 Figure 2c shows the average crystal sizes of γ-Fe2O3 and Ag as a function of Ag-content x. The crystal size of the γ-Fe2O3 remains practically constant at 15 nm for Ag-content up to x = 35 wt %. In contrast, the Ag crystal size increases monotonically from 10 to 20 nm for a higher Ag-content x because of the higher Ag precursor concentration during synthesis.31 The crystal phase composition can also be estimated by Rietveld analysis21 and is shown in Table 1 for both uncoated and SiO2-coated samples for various Ag-contents x. First, it can be seen that the measured Ag-content is consistent with the nominal one. Furthermore, the total α-Fe2O3 content varies from 15 to 20 wt % independent of Ag-content. For the highest Ag-content (x = 50 wt %), however, the undesired α-Fe2O3 fraction is 40− 45 wt % of the total Fe2O3, indicating the low superparamagnetic performance of these nanoparticles. As a result, the magnetic analysis and performance was focused on the xAg/Fe2O3 nanoparticles for x = 10 and 35 wt %. Magnetic Properties. Magnetic hysteresis measurements for both uncoated and SiO2-coated Ag/Fe2O3 nanoparticles exhibit a wasp-waisted hysteresis loop that results from a combination of magnetically blocked and superparamagnetic particles (Figure 3) for both x = 10 (a) and 35 wt % (b), typical for flame-made Fe2O3-containing nanoparticles at these conditions.18 Even though the average γ-Fe2O3 crystal size is well below the superparamagnetic limit (
