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Showcasing research from the Department of Chemistry, Department of Physics, and Department of Radiology at Case Western Reserve University in Cleveland, Ohio, USA.

High-performance iron oxide nanoparticles for magnetic particle imaging – guided hyperthermia (hMPI)

We demonstrated focused magnetic hyperthermia heating by adapting the fi eld gradient used in magnetic particle imaging (MPI) using optimized iron oxide nanoparticles. By saturating the iron oxide nanoparticles outside of a fi eld free region (FFR) with an external static fi eld, we can selectively heat a target region, highlighting the potential for high-performance focused hyperthermia therapy through an MPI-guided approach (hMPI).

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PAPERMichal Otyepka, Radek Zbořil et al. Fluorinated graphenes as advanced biosensors – eff ect of fl uorine coverage on electron transfer properties and adsorption of biomolecules

Volume 8 Number 24 28 June 2016 Pages 12083–12374

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See Mark A. Griswold, Anna Cristina S. Samia et al., Nanoscale, 2016, 8, 12162.

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PAPER

Cite this: Nanoscale, 2016, 8, 12162

Received 4th March 2016,Accepted 15th May 2016

DOI: 10.1039/c6nr01877g

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High-performance iron oxide nanoparticles formagnetic particle imaging – guided hyperthermia(hMPI)†

Lisa M. Bauer,‡a Shu F. Situ,‡b Mark A. Griswold*a,c and Anna Cristina S. Samia*b

Magnetic particle imaging (MPI) is an emerging imaging modality that allows the direct and quantitative

mapping of iron oxide nanoparticles. In MPI, the development of tailored iron oxide nanoparticle tracers is

paramount to achieving high sensitivity and good spatial resolution. To date, most MPI tracers being deve-

loped for potential clinical applications are based on spherical undoped magnetite nanoparticles. For the

first time, we report on the systematic investigation of the effects of changes in chemical composition

and shape anisotropy on the MPI performance of iron oxide nanoparticle tracers. We observed a 2-fold

enhancement in MPI signal through selective doping of magnetite nanoparticles with zinc. Moreover, we

demonstrated focused magnetic hyperthermia heating by adapting the field gradient used in MPI. By

saturating the iron oxide nanoparticles outside of a field free region (FFR) with an external static field, we

can selectively heat a target region in our test sample. By comparing zinc-doped magnetite cubic nano-

particles with undoped spherical nanoparticles, we could show a 5-fold improvement in the specific

absorption rate (SAR) in magnetic hyperthermia while providing good MPI signal, thereby demonstrating

the potential for high-performance focused hyperthermia therapy through an MPI-guided approach

(hMPI).

Introduction

Magnetic particle imaging (MPI) is an emerging imagingmodality that is capable of real-time, high-resolution imagingof iron oxide nanoparticle (IONP)-based tracers.1 MPI has theability to directly detect IONPs, with no background signaloriginating from surrounding tissues, resulting in superb con-trast.2,3 With nanomolar sensitivities, zero-depth attenuation,and the biocompatibility of IONP-based tracers, MPI hasbecome a popular alternative to other imaging modalities suchas magnetic resonance imaging (MRI), positron emission

tomography (PET), and single photon emission computedtomography (SPECT), which frequently utilize tracer materialswith higher toxicity profiles.2,3 Recently, MPI has been shownto be a useful technique for angiography, stem cell tracking,and cancer imaging.4,5

The MPI imaging process relies on the combination ofstrong, static gradient magnetic fields and time-varying,spatially homogeneous magnetic fields for spatial localizationand excitation, respectively.1,6,7 A 3-dimensional gradient fieldmagnetically saturates all tracer IONPs outside of a field-freeregion (FFR), where the magnetic field is zero. IONPs insidethe FFR are able to freely align with excitation fields and con-tribute to MPI signal generation, while those outside of theFFR are saturated and unable to align.6,7 In order to obtain animage, the time-varying excitation magnetic fields sweep theFFR throughout the imaging field of view (FOV). As the FFRpasses over a tracer, it induces a magnetization reversal thatcan be detected by a nearby receiver coil. Subsequently, thedetected signals can be processed by two MPI image re-construction techniques: the “system matrix” approach, or x-spaceMPI.1,8–12 In x-space MPI, the shape of the magnetization rever-sal is represented by the point-spread function (PSF).6,7,13 Sincethe FFR trajectory is known at all times, the x-space reconstruc-tion follows a simple gridding procedure that maps the rawsignal to instantaneous FFR position, and reconstruction is

†Electronic supplementary information (ESI) available: Detailed IONP syntheticmethods, description of magnetic particle relaxometer set-up, TEM of referenceIONP (Senior Scientific PrecisionMRX™ 25 nm oleic acid-coated nanoparticles),concentration dependent PSF of all IONP samples, PSF and SAR of Zn-Sph andZn-Cube mixture sample, upper right quadrant of field-dependent hysteresiscurve labelled with static field strengths, and the magnetic hyperthermia temp-erature profiles with and without the presence of external magnetic fields. SeeDOI: 10.1039/c6nr01877g‡Equal author contributions.

aDepartment of Physics, Case Western Reserve University, 10900 Euclid Avenue,

Cleveland, OH 44106, USA. E-mail: mark.griswold@case.edubDepartment of Chemistry, Case Western Reserve University, 10900 Euclid Avenue,

Cleveland, OH 44106, USA. E-mail: anna.samia@case.educDepartment of Radiology, Case Western Reserve University and University Hospitals

of Cleveland, 11100 Euclid Avenue, Cleveland, OH 44106, USA

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essentially a convolution of the PSF with the spatial distributionof tracers.6,7 X-space MPI is useful as a quantitative imagingtechnique due to its linear and shift invariant (LSI) nature.14

Image resolution in MPI is determined by the full-width athalf-maximum (FWHM) of the PSF. A specialized device,known as a magnetic particle relaxometer, is typically used toevaluate the performance of tracer materials in MPI bymeasuring the 1-dimensional PSF.15 In addition to predictingimage quality, the PSF provides information about thedynamic relaxation processes governing tracer behavior.13 Inorder to generate a signal, MPI tracers must undergo a magneti-zation reversal, which requires overcoming viscous, thermal,and inertial torques by using a combination of Brownian andNéel relaxations.16 The ideal PSF, generated by tracers withminimal relaxation, is symmetric, with high signal-to-noiseratio (SNR) and small FWHM, corresponding to high imageresolution. On the other hand, strong relaxation can cause areduction in SNR, a shift of the PSF peak in the direction ofFFR motion, and a broadening of the PSF, leading to poor MPIimage resolution.

Much work has been directed toward developing tailoredMPI tracers for improved sensitivity and resolution.2,16–19

While initial imaging studies used a commercial MRI contrastagent (Resovist, Bayer-Schering Pharma) as a “gold standard”,recent work has shown that using a tailored IONP tracer gene-rates images with higher resolution.20,21 Developing tracersinvolves balancing various properties, such as core size, sizedistribution, capping-ligands, chemical compositions, andmagnetic properties.22–25 To date, magnetite spherical NPswith a diameter between 20–25 nm are considered the besttracer material for MPI.21–24

It is interesting to note that the tracers used in MPI aresimilar to those developed for biomedical applications such asmagnetic hyperthermia. The heat generation mechanism inmagnetic hyperthermia relies on the same Brownian and Néelrelaxation principles for magnetization reversal, which alsodictate the signal generation in MPI. Recently, the idea ofcombined magnetic hyperthermia and MPI has beenproposed.26–28 Therefore, there is great interest in seeking highperformance tracer materials from IONP optimized for mag-netic hyperthermia and potentially designing one agentmaterial for combined applications. Iron oxide-based nano-particles have been used in previous magnetic hyperthermiatreatments for cancer therapy due to their biocompatibilityand tunable magnetic properties that give rise to good heatingefficiency.29–34 Moreover, incorporation of other metaldopants, such as zinc, manganese, and nickel, can inducehigher saturation magnetization and further enhance mag-netic hyperthermia properties.33 For our study on the effect ofa metal dopant on the MPI signal and hyperthermia perform-ance, we selected zinc due to its biocompatibility.35 In additionto chemical modification, introducing shape anisotropy hasalso been shown to be effective in improving heating efficiencydue to the reduced surface anisotropy of cubic nanoparticles,compared to spherical shapes, and the tendency of cubes toarrange in chain-like formations.36 The increased saturation

magnetization in cubic nanoparticles results in significantenhancement in heat generation and this result has beenheavily explored for magnetic hyperthermia therapy.18,32,36

Although numerous studies have been done in optimizing theperformance of IONP in MPI and magnetic hyperthermia sep-arately, the usage of IONP in combined MPI and magnetichyperthermia applications has not yet been explored.

In this study, the potential of utilizing tailored IONP inMPI-guided magnetic hyperthermia therapy was experi-mentally investigated for the first time. We used magnetic par-ticle relaxometry to study the effect of zinc-dopant and IONPshape on the MPI signal by comparing spherical and cubicundoped magnetite (Fe3O4) and zinc-doped magnetite(Zn0.4Fe2.6O4) forms. The relaxometry experiments are comple-mented by a study of the magnetic hyperthermia performance.In addition, the magnetic hyperthermia performance of theIONP samples was also evaluated under external static gradientfields, mimicking the magnetic environment in MPI to investi-gate application in MPI-guided hyperthermia (hMPI) therapy.

Experimental sectionMagnetic nanoparticle synthesis and characterization

The oleic acid-capped IONPs were synthesized using a modi-fied thermal decomposition method based on previouslyreported procedures.35–41 The size, composition, and shape ofthe IONPs were tuned by varying the type of metal precursors,solvents, capping ligands, and heating rates during NP syn-thesis. For this study, spherical and cubic IONPs with equalvolumes (3600–3700 nm3) were evaluated for their perform-ance in MPI through relaxometry measurements. Moreover,the MPI performance of the synthesized IONPs was comparedto a commercially available IONP (PrecisionMRX™, 25 nmoleic acid-coated IONP), which served as a reference samplefor the MPI studies. Elemental analyses using atomic absorp-tion spectroscopy (AAS) were performed to evaluate the Znand Fe concentrations in the IONP samples. The powder X-raydiffraction patterns of the synthesized IONPs were collectedusing a Rigaku MiniFlex powder X-ray diffractometer usingCu-Kα radiation (λ = 0.154 nm) within a 2θ range of 25 to 65°.The magnetic characterization was performed with a QuantumDesign MPMS5 Superconducting Quantum Interference Device(SQUID) magnetometer. The field-dependent magnetic pro-perties of the IONP samples were obtained at 300 K from −5 Tto 5 T.

Magnetic particle relaxometry

Magnetic particle relaxometry experiments were performedusing Case Western Reserve University’s custom x-space mag-netic particle relaxometer. A simplified schematic of the relaxo-meter and PSF reconstruction is shown in Scheme 1.

Background signals were acquired prior to testing eachsample, and were subtracted from the averaged sample signals(N = 10 averages for both background and sample signals). TheSNR was calculated as the peak PSF value divided by the noise

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level of the relaxometer, which was obtained by acquiring thesignal from a sample of deionized water. The full width at halfmaximum (FWHM), which indicates image resolution, wasevaluated by finding the width of the PSF (in mT) at one-halfof the peak PSF value. To obtain an estimate of the imageresolution, the FWHM is divided by the strength of the staticgradient field. A measure of the PSF symmetry is obtained byfinding the location of the peak PSF value; a symmetric PSFshould have a peak shift of 0 mT.

Magnetic hyperthermia measurements

The magnetic hyperthermia measurements were performedusing an MSI Automation bench mount magnetic inductionheating system. The IONP samples were exposed to an alternat-ing magnetic field excitation at a fixed frequency ( f ) of 380kHz and a magnetic field amplitude (H) of 16 kA m−1, equi-valent to 200 Oe and 20 mT (the field strength was chosen tomatch the amplitude of the magnetic particle relaxometry exci-tation). All samples were measured inside an insulated NMRglass tube with an internal diameter of 7.5 mm, at an iron con-centration of 1.0 M. To evaluate the temperature profiles of thesamples upon excitation with a continuous alternating mag-netic field, the change in temperature of the samples wasmonitored with a fiber optic temperature probe (Neoptix T1™)and was recorded every 5 s. The sample temperature was

recorded for 3 min prior to turning the magnetic field on toobtain a stable baseline for the calculation of the specificabsorption rate (SAR) values. The SAR was obtained from theinitial slope over the first 10 s of the heating curve using thefollowing equation:

SAR ¼ CVsm

dTdt

where dT/dt is the initial slope of the heating curve, C is thevolumetric specific heat capacity of the solvent, Vs is thevolume of the sample, and m is the mass of magnetic material(i.e. g Fe) in the sample. The reported average SAR values arecalculated from three repeat measurements.

Results and discussionStructural and magnetic properties of the synthesized IONPs

To examine the effect of chemical composition and shapeanisotropy on the MPI signal and heating efficiency of IONPs,undoped magnetite spherical NPs (Fe3O4,Sph), magnetitecubic NPs (Fe3O4,Cube), zinc-doped magnetite spherical NPs(Zn0.4Fe2.6O4,Zn-Sph), and zinc-doped cubic NPs (Zn0.4Fe2.6O4,Zn-Cube) with equivalent volumes (3600–3700 nm3) were syn-thesized through a thermal decomposition approach.35–41 Thetransmission electron microscope (TEM) images of the syn-thesized IONP samples reveal well-defined morphologies andmonodisperse size distributions (Fig. 1a and Table 1). Theobtained powder X-ray diffraction patterns (PXRD) showcharacteristic peaks indicative of a cubic inverse spinel magne-tite (Fe3O4) crystal structure (Fig. 1b). The field-dependentmagnetic behaviors of the IONP samples were measured at300 K from −5 T to 5 T. The saturation magnetization (Ms) ofthe Sph sample was determined to be 101.5 emu per g Fe(Fig. 1c). Incorporation of 13% zinc-dopant (Zn0.4Fe2.6O4) andintroducing shape anisotropy increases the Ms value to 107.3,125.7, 130.4 emu per g Fe for Cube, Zn-Sph, and Zn-Cubesamples, respectively (Fig. 1c and Table 1). The as-synthesizedIONPs were dispersed in toluene for the relaxometry and mag-

Scheme 1 Schematic diagram of the magnetic particle relaxometerused in evaluating the MPI performance of the synthesized iron oxidenanoparticles (a). Illustration of the signal generation process in mag-netic particle relaxometry (b).

Fig. 1 Transmission electron microscopy images of magnetite spherical NPs (Fe3O4,Sph), cubic NPs (Fe3O4,Cube), and the corresponding zinc-doped magnetite spherical NPs (Zn0.4Fe2.6O4,Zn-Sph), and cubic NPs (Zn0.4Fe2.6O4,Zn-Cube) of equivalent volumes. The scale bars represent 50 nm(a). The powder X-ray diffraction patterns, (b) and the field-dependent magnetization curves of the IONP samples measured at 300 K (c). The insetdisplays the zoom-in portion of the field-dependent magnetization curves showing superparamagnetic behavior.

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netic hyperthermia measurements. In addition, commerciallyavailable 25 nm oleic acid-coated IONPs purchased fromSenior Scientific were used as a reference sample (Ref) in allmeasurements (Fig. S1†).

Magnetic particle relaxometry measurements

An overview of the magnetic particle relaxometry measurementis depicted in Scheme 1. The sample under evaluation isexposed to a sinusoidal magnetic field (frequency 16.8 kHz,amplitude 20 mT), which mimics the FFR movement acrossthe sample during an MPI experiment. The time dependentmagnetization is inductively detected by a receiver coil andgridded to the known instantaneous magnetic field value togenerate the PSF (Fig. S2 and S3†). The relaxometer is highlysensitive, and is a powerful tool to predict MPI tracer perform-ance during imaging. An ideal IONP tracer with a sharp hyster-esis and high Ms can produce signals that can be translatedinto a PSF with tall, narrow, and symmetric shape, ultimatelyleading to MPI image with high SNR and image resolution.Relaxometry measurements were performed on Sph, Cube, Zn-Sph, and Zn-Cube NP samples at 1 mg Fe in 100 μL of toluene(Fig. 2 and 3). To methodically compare the PSF of all four syn-thesized IONP samples, the NPs were prepared to have similarvolumes and capping ligands (Table 1). The effect of zinc-dopant and shape anisotropy on the PSF symmetry and SNRwere systematically evaluated and the results on the MPI signalare presented in the following sections.

Zinc-dopant effect on the MPI signal of IONPs

Previous studies have shown the increase in the Ms of magne-tite NPs through incorporation of metal dopants, and theresulting enhanced performance in magnetic hyperthermiaand MRI applications.35–37,42 The magnetic properties of mag-netite NPs can be tuned by systematically adjusting theamount of zinc metal dopant. Based on previous studies invol-ving zinc-doped magnetite NPs, the atomic ratio of the zinc toiron metal in the magnetite NPs used in our study was tunedto 0.4 to 2.6 (13% zinc dopant) to achieve the maximum Ms

value.41 The magnetic relaxation of zinc-doped IONPs wascompared to undoped magnetite NPs in Fig. 2. The PSF of Zn-Sph displayed a symmetric shape with a low shift in peak(0.9 mT), a 2-fold increase in SNR, and a 10% smaller FWHMthan the undoped Sph sample (Fig. 2a, b and Table 1). Theenhancement in SNR is even more pronounced in the cubicIONPs with approximately a 3-fold increase in SNR for the Zn-Cube sample as compared to the undoped Cube NP sample(Fig. 2c and d). The observed SNR enhancement in the zinc-

doped Zn-Sph and Zn-Cube samples compared to theirundoped magnetite NP counterparts demonstrate the positiveeffect of zinc-doping on the IONP tracer performance in MPI.This result can be attributed to the increase in Ms valuesobserved for the zinc-doped magnetite NPs (Fig. 1c andTable 1) due to the selective replacement of the Fe2+ with thediamagnetic Zn2+ metal in the tetrahedral sites of the spinelferrite lattice, which results to a larger overall net magneticmoment in the ferrimagnetically ordered magnetite system.41

Moreover, the enhancement in MPI signal is optimal for theZn-Sph sample, which maintains a symmetric PSF with highSNR. The SNR of all the IONP samples investigated are concen-tration dependent with linearly increasing SNR as the Fe con-centration increases (Fig. 2b and d). This result supports theapplicability of the synthesized IONPs in quantitative MPImeasurements.14

Shape effect on the MPI signal of IONPs

Shape anisotropic cubic IONPs have been demonstrated tohave improved performance in magnetic hyperthermia, due to

Table 1 Summary of the IONP structural and magnetic properties as well as key results from relaxometry and magnetic hyperthermiameasurements

Sample Diameter (nm) Volume (nm3) Ms (emu per g Fe) SNR FWHM (mT) PSF peak shift (mT) SAR (W per g Fe)

Sph 19.2 ± 1.3 3720 101.5 27495 11.9 2.1 189.6Cube 15.5 ± 1.1 3723 107.3 6491 16.2 3.6 356.2Zn-Sph 19.1 ± 1.0 3610 125.7 44787 10.7 0.9 438.6Zn-Cube 15.4 ± 1.1 3652 130.4 17739 16.4 4.8 1019.2

Fig. 2 The normalized point spread functions (PSFs) (a) and the corres-ponding signal to noise ratio (SNR) vs. Fe concentration plots (b) for themagnetite spherical NPs (Fe3O4,Sph) and zinc-doped magnetite spheri-cal NPs (Zn0.4Fe2.6O4,Zn-Sph). The normalized PSFs (c) and the corres-ponding SNR vs. Fe concentration plots (d) for the magnetite cubic NPs(Fe3O4,Cube), and zinc-doped magnetite cubic NPs (Zn0.4Fe2.6O4,Zn-Cube).

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the reduced surface spin canting effect and tendency toarrange in chain-like formation.35–37,41,42 Spherical IONPs havehigher spin disorder at the surface compared to cubic IONPs,leading to a lower overall magnetic moment for the sphericalcompared to the cubic nanoparticles.34,42,43 In our study, Zn-Cube and Cube IONPs of similar volumes as their sphericalcounterparts were synthesized to investigate the effect of shapeon the MPI performance. As seen in Fig. 3, Sph IONPs out-performed Cube IONPs in SNR and a similar result wasobserved for Zn-Cube and Zn-Sph (with a 2.5-fold greater SNRobserved for the Zn-Sph sample). This result can be partlyexplained by the greater tendency of magnetic cubic IONPs tospontaneously form chain-like arrangements.35–37 Such chain-like formations found in cubic IONPs is beneficial for magnetichyperthermia heating as demonstrated by several researchgroups.35–37,41,42 However, the increase in anisotropy can resultin asymmetry of the observed PSF, which is evident in themeasured MPI signal from the Cube and Zn-Cube samples,with the highest peak value shifts observed at 3.6 and 4.8 mT,respectively (Table 1). The observed peak shift in the cubicIONPs is consistent with pervious work on large sphericalmagnetite NPs with strong hysteretic reversal.45

MPI and magnetic hyperthermia performance of thesynthesized IONPs

The relaxometry measurements of the Sph, Cube, Zn-Sph, andZn-Cube samples were also compared to a commercially avail-able reference IONP sample (Ref, PrecisionMRX™) with anaverage diameter of 25 nm that was purchased from SeniorScientific. The observed SNR of the Ref IONP is the lowest in

the IONPs tested in this study with approximately an 8-foldreduction in SNR compared to the synthesized IONP samplewith the best MPI performance (Zn-Sph). The asymmetricshape was also seen in the PSF of the Ref IONP with peakvalue shift at 4.8 mT, which is the highest peak value shiftfound in this study (Fig. 4a and S4†).

In addition to showing the significant improvement in MPIperformance through incorporation of zinc-dopant in Fe3O4

NPs, we also demonstrated the effect of changes in chemicalcomposition and shape anisotropy on heat generation in mag-netic hyperthermia. The SAR value of the as-synthesizedundoped Sph sample was estimated to be 189.6 W g−1, whichis consistent to SAR values reported for the spherical magne-tite NPs prepared using the same synthetic method andsimilar magnetic hyperthermia measurement conditions (180W g−1 by Gonzales-Weimuller et al. and 185 W g−1 by Lartigueet al.) (Fig. 4b and Table 1).46,47 The SAR value of the undopedcubic IONPs in this study was estimated to be 356.2 W g−1

which is consistent with reported SAR values.44 A 2-foldincrease in SAR value was evaluated for the Zn-Sph comparedto the Sph sample during magnetic hyperthermia measure-ments (Fig. 4b). Furthermore, the Zn-Cube exhibited a 5-foldhigher SAR value compared to the Sph sample (Fig. 4b). Thecalculated SAR values and the enhancement in heatingefficiency in the Zn-doped and cubic IONPs are consistentwith reported studies.44,47,48

Since Zn-Sph provides the best signal generation in MPIand Zn-Cube has the highest SAR value, it is difficult to decideon a single IONP sample for a combined MPI and magnetichyperthermia approach. One possible approach would be touse a mixture of particles. To this end, the PSF and SAR of amixture containing 50% Zn-Sph and 50% Zn-Cube IONP weremeasured. As seen in Fig. S5,† the MPI performance and SARof the mixture lie in between that of the 100% Zn-Sph and100% Zn-Cube samples. Considering that the ultimate goal ofMPI-guided hyperthermia would be to achieve a therapeuticeffect, Zn-Cube IONP, with its superior hyperthermia perform-

Fig. 4 Point spread functions (PSFs) of undoped magnetite spherical(Fe3O4, Sph) and cubic NPs (Fe3O4, Cube), zinc-doped magnetite spheri-cal (Zn0.4Fe2.6O4,Zn-Sph) and cubic NPs (Zn0.4Fe2.6O4,Zn-Cube), andthe PSF of the reference sample (Ref) (a).The calculated specific absorp-tion rates (SARs) from the magnetic hyperthermia measurements forall the samples (b); the hyperthermia measurements were performed atH = 16 kA m−1 and f = 380 kHz, and the reported SAR values were aver-aged from three trials.

Fig. 3 The normalized point spread functions (PSFs) (a) and the corres-ponding SNR vs. Fe concentration plots (b) for the magnetite sphericalNPs (Fe3O4,Sph) and cubic NPs (Fe3O4,Cube). The normalized PSFs (c)and the corresponding SNR vs. Fe concentration plots (d) for the zinc-doped magnetite spherical NPs (Zn0.4Fe2.6O4,Zn-Sph), and cubic NPs(Zn0.4Fe2.6O4,Zn-Cube).

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ance, was used as a model tracer material to demonstratethe concept of MPI-guided magnetic hyperthermia therapy.Although the Zn-Sph IONP is the best performing MPI tracer,the MPI signal of the Zn-Cube sample is still higher than thesignal generation from the commercially available IONPsample.

To mimic the magnetic environment during an MPIimaging process, the hyperthermia experiments were carriedout under two external magnetic field application conditions:exposure to a (1) static magnetic field, and to a (2) gradientmagnetic field.

Controlling the magnetic hyperthermia properties of IONPswith the application of a static magnetic field

To investigate the effect of a saturating magnetic field duringhyperthermia measurements, a strong permanent magnet wasplaced next to the hyperthermia coil, and the distance betweenthe magnet and coil was varied to expose the IONP sample todifferent static magnetic field strengths, ranging from 2.6 mT(26 Oe) to 15.4 mT (154 Oe) (Fig. 5a). The impact of a staticmagnetic field on the magnetic hyperthermia heat generation

of the IONPs is seen in Fig. 5b. In the absence of an externalmagnetic field (B = 0 mT), the Zn-Cube has a high SAR value of1019.2 W per g Fe. As the magnitude of the external static mag-netic field is increased, the SAR decreases rapidly, and a 10-fold reduction in SAR is observed upon exposure of the IONPsample to 2.6 mT. As the static field is further increased to15.4 mT, the SAR continued to decrease to near-zero value.Similarly for the Sph IONP sample, there was an overall 10-foldreduction in SAR in the presence of a constant static field.This inverse relationship between the SAR value and fieldstrength of an external static field is a result of an effectivemagnetic saturation of the IONPs. Once saturated, the IONPsare unable to respond to the alternating magnetic field used inmagnetic hyperthermia, and consequently there is a signifi-cant decrease in the heat generation (Fig. S6†). Once saturated,the IONPs are unable to respond to the alternating magneticfield used in magnetic hyperthermia, and consequently thereis a significant decrease in the heat generation (Fig. S6†). Thereduction in heat generation can also be explained by correlat-ing the results to the field-dependent hysteresis curves. InFig. S7,† the strengths of the static magnetic field are labelledin the upper right quadrant of the hysteresis curves. With astatic magnetic field strength between 26 and 154 Oe, theIONP samples are being saturated to about 30–60% of theirsaturation magnetization, which leads to a decrease in SARvalues. It is important to note that the range of static fieldstrengths used in this experiment are similar to the static fieldstrengths used in MPI to saturate IONP tracers and achievespatial localization. This suggests that it should be possible toachieve suppression of the heat release in magnetic hyperther-mia and spatial localization in MPI with the same gradientfields.

Focused magnetic hyperthermia through the application of agradient magnetic field

In another set up, an external magnetic field gradient was uti-lized during magnetic hyperthermia measurements to moreclosely mimic the MPI environment. For this study, weimbedded Zn-Cube IONPs into a polyethylene matrix (seeESI†) to prepare nanocomposites (NCs) for the hyperthermiastudies. During the experiment, small strips of Zn-Cube NCs(1 mm × 3 mm) were placed inside a 3-D printed multi-welledsample holder (Fig. 6a), and were subjected to magnetichyperthermia treatment under a gradient magnetic field(0.47 T m−1) (Fig. 6a).

To visualize the heat generation of the Zn-Cube NCs, athermal camera (Seek Thermal Compact for Android) was usedto capture thermal images before and after 30 s of magnetichyperthermia excitation. In the absence of a static gradientfield, all of the NCs were able to efficiently generate heatregardless of location within the coil (Fig. 6b). Comparing thethermal images obtained during hyperthermia in the presenceof a static gradient field, it becomes evident that there is selec-tive heating of the NCs located in the central region of thesample holder, near the FFR (Fig. 6b). The thermal imageintensity from each experiment is presented in Fig. 6c. It is

Fig. 5 Schematic illustration of the magnetic hyperthermia measure-ment set-up under static field exposure. The table indicates the distanceof the magnet and the corresponding static field strength (a). Thespecific absorption rates (SARs) of magnetite spherical NPs (Fe3O4,Sph)and zinc-doped cubic NPs (Zn0.4Fe2.6O4,Zn-Cube) under an externalstatic field with varying field strengths (b). All the hyperthermia measure-ments were performed at H = 16 kA m−1 and f = 380 kHz, and thereported SAR values were averaged from three trials and normalized tothe SAR of Zn-Cube at zero field.

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evident that in the absence of gradient magnetic field, threedistinct peaks aligning with the location of three NC stripsplaced along the direction of the gradient field were seen, indi-cating a temperature increase in these three hot spots. In thepresence of an external gradient magnetic field, heat releasewas only observed in the central region of the sample holder.This result is a clear visual demonstration of focused magnetichyperthermia heating under similar field environment used inMPI. Furthermore, this study also demonstrates the potentialof Zn-Cube IONPs as high performance agents for MPI-guidedhyperthermia (hMPI).

Conclusions

In summary, we have demonstrated the first experimentalwork on systematically investigating the effect of zinc-dopingand shape anisotropy on the performance of IONP MPItracers. The zinc-doping of magnetite NPs resulted in a 2-foldenhancement in MPI signal, which could translate to improveSNR. In addition, the zinc-doped cubic IONPs were demon-strated to have outstanding heating efficiency in magnetichyperthermia, while providing higher SNR compared tocurrent commercially available MPI tracers, and comparablebehavior to synthesized undoped magnetite spheres. Further-more, we have investigated the influence of saturating mag-netic fields during magnetic hyperthermia measurements, inan effort to demonstrate an MPI-guided hyperthermia (hMPI)approach. Under the influence of a gradient magnetic fieldenvironment that is similarly present during MPI image acqui-sition, the zinc-doped cubic IONPs were demonstrated to besaturated outside of FFR leading to focused and selectiveheating within the FFR, thus making them an ideal agent forhMPI theranostic applications.

Acknowledgements

The authors thank Dr Michael Twieg for his assistance inobtaining thermal camera images. This work was supported inpart by NCI Training Programs in Cancer Pharmacology 5R25CA148052 (L. M. B.), NIH 1R24MH106053-01 (M. A. G.), and anNSF-CAREER Grant (DMR-1253358) from the Solid State andMaterials Chemistry Program (A. C. S. S.).

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Fig. 6 Illustration of customized 3-D printed multi-welled sample holder with corresponding measured field strengths, and a schematic illustrationof the magnetic hyperthermia measurement set-up under gradient magnetic field (a). Thermal images of the nanocomposites with zinc-dopedcubic IONPs (Zn0.4Fe2.6O4,Zn-Cube) during magnetic hyperthermia measurement with and without gradient magnetic field application (b). Thermalimage intensity profiles of the center lines along the gradient field direction acquired 30 s after magnetic hyperthermia measurements with andwithout the application of a gradient magnetic field (c). All the hyperthermia measurements were performed at H = 16 kA m−1 and f = 380 kHz.

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