IEEE SENSORS JOURNAL, VOL. 13, NO. 6, JUNE 2013 2341

Iron Oxide-Gold Core-Shell Nanoparticles asMultimodal Imaging Contrast Agent

Luca Menichetti, Leonardo Manzoni, Luigi Paduano, A. Flori, Claudia Kusmic, Daniele De Marchi,Sergio Casciaro, Francesco Conversano, Massimo Lombardi, Vincenzo Positano, and Daniela Arosio

Abstract— Nanoparticles (NPs) are emerging as a potentialmedical tool for novel diagnostic, drug delivery, and therapeuticapproaches. Among them, a spherical NP with a core-shellstructure is a way to combine multiple functionalities on thenanoscale. In this paper, we describe the preparation characteri-zation and applications of core-shell iron oxide-gold nanoparticles(Fe3O4@Au NPs). A comprehensive set of experiments, includingtransmission electron microscopy, dynamic light scattering, smallangle neutron scattering, and ultraviolet visible spectroscopyis applied to characterize their chemical, physical, and opticalproperties. We also study their applicability as contrast agentsfor magnetic resonance imaging (MRI): the measurement oflongitudinal and transverse relaxation times of Fe3O4@Au NPs invitro and in vivo allowed the assessment of longitudinal (R1) andtransverse (R2) relaxivities at 1.5 and 3 T. Finally, a procedurefor functionalizing NPs with integrin targeting cyclic Arginine-Glycine-Aspartate peptidomimetic is reported, leading to thedevelopment of nanoscale probes for αvβ3 integrin, particularlyattractive in terms of resolution and 3-D imaging capabilities.The resulting multifunctional nanoprobes offer suitable blood-circulation time and contrast for microimaging as well as forgradient-echo MRI, and could enable new imaging magnetoplas-monic applications.

Index Terms— Core-shell nanoparticles, gradient-echo,magnetic resonance imaging (MRI), integrin, magneticnanoparticles, nanoprobe, RGD functionalized nanoparticles.

Manuscript received December 14, 2012; accepted March 6, 2013. Dateof publication March 20, 2013; date of current version April 26, 2013. Thiswork was supported in part by the National Research Council and a grantfrom the Fondazione Monte dei Paschi di Siena, Italy. This is an expandedpaper from the IEEE SENSORS 2011 Conference. The associate editorcoordinating the review of this paper and approving it for publication wasDr. Chang-Soo Kim.

L. Menichetti is with the Institute of Clinical Physiology, CNR, PisaI-56124, Italy, and also with the CNR–Regione Toscana FondazioneG. Monasterio, Pisa 56124, Italy (e-mail: luca.menichetti@ifc.cnr.it).

L. Manzoni and D. Arosio are with Institute of Sciences and moleculartechnology, CNR, Milan 20133, Italy (e-mail: leonardo.manzoni@istm.cnr.it;daniela.arosio@istm.cnr.it).

L. Paduano is with the Chemistry Department, University of Naples“Federico II,” Naples 80138, Italy (e-mail: luigi.paduano@unina.it).

A. Flori is with the Istituto di Scienze della Vita, Scuola SuperioreSant’Anna, Pisa 56127, Italy (e-mail: alessandra.flori@sssup.it).

C. Kusmic is with the Institute of Clinical Physiology, CNR, Pisa I-56124,Italy (e-mail: kusmic@ifc.cnr.it).

D. De Marchi, M. Lombardi, and V. Positano are with the CNR–RegioneToscana Fondazione G. Monasterio, Pisa 56124, Italy (e-mail: demada@ftgm.it; lomass@ftgm.it; vincenzo.positano@ftgm.it).

S. Casciaro and F. Conversano are with the Institute of Clinical Physiology,CNR, Lecce 73100, Italy (e-mail: casciaro@ifc.cnr.it; conversano@ifc.cnr.it).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2253215

I. INTRODUCTION

AMONG the number of inorganic nanoparticles (NPs)potentially able to display both diagnostic and

therapeutic properties, iron oxide/gold core-shell NPs(Fe3O4@Au NPs) have recently attracted increasing attentiondue to their hybrid nature. Fe3O4@Au NPs possess bothmagnetic and optical properties and can also be considereda smart platform for three-dimensional multipresentation(i.e., multivalency) due to their globular shape, tunable size,and simple surface chemistry [1]–[4].

Thus, Fe3O4@Au nanoparticles, which consist of an Auplasmonic coating and an iron oxide core, could providea promising platform for the development of multimodalimaging tools [1], [5]–[13].

They combine two features in a single NP displaying bothmagnetic and plasmonic properties. Iron oxide forms focalclusters of magnetic inhomogeneity, leading to a dramaticreduction in the protons’ transverse relaxation time that can beassessed by spin-echo (T2) techniques or gradient-echo (T2*)techniques. Hence, they act as a potent negative magneticresonance imaging (MRI) contrast agent, well-detectable alsoat very low concentration. Moreover, their plasmonic proper-ties make them suitable for dark field spectroscopy, SurfaceEnhanced Raman Spectroscopy [4] and for photoacousticdetection [12]–[14].

In biomedical applications, in particular for in vivo imaging,nanoparticles should possess good colloidal stability and lowtoxicity in a biological environment. Therefore, modifyingthe surface of nanoparticles is essential to endow them withhydrophilic properties. Gold coating of magnetic NPs repre-sent one of several established solutions for meeting theserequirements, both because gold is considered rather biocom-patible and at the same time it furnish an easily functionaliz-able surface [3], [13]–[16]. In this study, we explored the useof polyethileneglycol (PEG) as a coating in order to obtainbiocompatible, water soluble nanoparticles and we also pre-pared cyclic-RGD (cRGD) peptidomimetic [17] functionalizenanoparticles targeted to integrin αvβ3. Integrins has becomeincreasingly important in assessment of a number of patho-logical conditions, such as tumors, inflammation, degenerativediseases and remodelling [17]–[20]. Of the integrin receptors,αvβ3 have been identified as a promising target for diagnostic,therapeutic and potentially in combined (“theranostic”) inter-ventions.

1530-437X/$31.00 © 2013 IEEE

2342 IEEE SENSORS JOURNAL, VOL. 13, NO. 6, JUNE 2013

The aim of this study is to characterize functionalized,water soluble Fe3O4@Au nanoparticles with transmissionelectron microscopy (TEM), dynamic light scattering (DLS),small angle neutron scattering (SANS), and ultraviolet visiblespectroscopy (UV–Vis). Moreover, we tested their applicabilityas contrast agents in MRI in both phantom and in vivoexperimental studies.

II. MATERIALS AND METHODS

A. Synthesis of Fe3O4@Au NPs and PEG Coating

Fe3O4@Au NPs (DA409) were prepared following theexperimental procedure reported by L. Wang et al. [9]. In orderto obtain stable water-soluble NPs, the native coating wasexchanged with PEG-SH (MW 5000, Sigma-Aldrich). Theexchange reaction was performed in chloroform overnight.Subsequently the solvent was evaporated and the PEG-coatednanoparticles were re-dissolved in milliQ water and finallypurified by centrifugal filtration (10000 MWCO, Vivaspinfilters) to give (Fe3O4@Au)-PEG (DA411).

B. Functionalization of NPs With cRGD Sequences

The PEG-coated NPs were dissolved in water and theRGD ligand, functionalized with a short polyethyleneglycoliclinker ending with a thiol group obtained after reductionof the disulfide bridge [21], was added. The particles wereleft to react overnight and then were purified. Finally, theRGD functionalized NPs (Fe3O4@Au)-RGD (DA417) werelyophilized.

C. Nanoparticle Characterization

(Fe3O4@Au)-PEG DA411 and (Fe3O4@Au)-RGD DA417NPs were characterized by transmission electron microscopy(TEM), dynamic light scattering (DLS), small angle neu-tron scattering (SANS) measurements and UV/Visible spec-troscopy.

For TEM characterization, a single drop (10 μl) of aqueoussolution of gold-covered iron oxide NPs (ca. 0.1 mg/ml inmilliQ water) was placed on a Formvar/carbon-coated coppergrid (Electron Microscopy Science) and left to dry in airfor several hours. The sample grids were examined usingan EFTEM LEO 912AB microscope (Zeiss), operating at80 kV. The average diameter of the Fe3O4@Au was obtainedfrom evaluation of several TEM micrographs by means ofan algorithm for automatic image analysis developed by ourgroup.

For dynamic light-scattering investigations, samples DA411and DA417 were dissolved in a 0.1 M NaH2PO4/Na2HPO4buffer at pH 7.4 while hydrophilic nanoparticles DA409 weredissolved in chloroform (concentration, c = 0.413 g/ml andc = 0.446 g/ml respectively and c = 0.194 g/ml for dilutedsamples). Dissolved samples were positioned in the cell 30 minbefore data collection in order to stabilize the temperature byusing a thermostat bath (25.00 ± 0.05 °C). Data were collectedat θ = 90°, with a setup composed by a Photocor compactgoniometer, a SMD 6000 Laser Quantum 50 mW light source

operating at 5325 Å and a PMT and a correlator obtained fromCorrelator.com.

SANS investigations were performed using a SANS2Dspectrometer located at an ISIS pulsed neutron source. Thebeam used employs neutrons at wavelengths λ ranging from2 to 14 Å, which are detected by time-of-flight analysis andrecorded with a (96.5 cm2) 3He-CF4 detector, with 5-mm res-olution. Sample-detector distance can be varied between 2 mand 12 m. Such a setup allowed collecting data of scatteringvector modulus q = 4π

/λ sin

(θ/

2)

ranged between 0.006 to0.284 Å−1, where θ is the scattering angle. The samples stud-ied were contained in 1-mm path length, Hellma quartz cells at25 °C. The experimental raw data were converted to absoluteintensity following a previously reported procedure [22].

UV/Vis spectra were measured with a Thermo Scien-tific, Evolution 600 UV-Visible spectrophotometer. Sampleswere dissolved in milliQ water at a concentration between0.1–0.35 mg/ml.

D. MR Phantom Imaging

For the MRI studies a stock solution containing2.54 ± 0.01 mg of (Fe3O4@Au)-PEG DA411 NPs wasprepared with a final concentration of 1 mg/ml. Samplesat different DA411 NP concentrations, in the range 0.005-1mg/ml, were prepared by dilution of the mother solution forrelaxation time measurements (sample volume = 1.5 ml). MRIexperiments were performed using a 1.5 T GE Signa CVi anda 3 T GE Signa HDx scanner with a commercially availablebirdcage coil. Longitudinal (T1) and transverse (T2) relaxationtimes were assessed respectively with standard inversion-recovery and multi-echo spin-echo pulse sequences.

E. MR in Vivo Imaging

MRI acquisitions were performed using a 1.5 T GE SignaExcite scanner with a knee phased array coil; T∗

2 relax-ation time was assessed by a multi-echo gradient-echo pulsesequence [23]. Adult male Wistar rats (300-400 g) were anes-thetized by intraperitoneal injection of Zoletil®100 + xylazine(50 and 3 mg/kg, respectively) and their femoral vein exposedfor the injection of 300 μl of isotonic (Fe3O4@Au)-PEGDA411solution at the concentration of 1 mg/ml. T∗

2 imageswere acquired until 5 min after the injection and 25 min afterthe injection start; the acquisition time was about 60 s.

All the experimental protocols conformed to the “GuidingPrinciples for Research Involving Animals and HumanBeings” approved by the Council of the AmericanPhysiological Society.

III. RESULTS AND DISCUSSION

The Fe3O4@Au NPs DA409 (Fig. 1) were preparedfollowing the experimental procedure as previouslyreported [9]. To obtain stable water soluble nanoparticles,the native coating was exchanged with PEG-SH (MW 5000).The cRGD was introduced in a further exchange step afterreduction of the disulfide bridge between the terminal thiolgroup of short polyethyleneglycolic chains (Fig. 1). The

MENICHETTI et al.: IRON OXIDE-GOLD CORE-SHELL NPs 2343

O leic A cid S -PEG (5000)N aBH 4

SO

OC O N H cR G D

SO

OC O N H cR G D

SO

OC O N H cR G D

7

7S-PE G (5000 )+

7DA 411 DA 417DA 409

cRGD =N

OH N

Asp

Arg

O

G ly

Fig. 1. Experimental methodology for preparation of Fe3O4@Au nanoparticles.

Fig. 2. (a) TEM micrograph and size distribution of (Fe3O4@Au)-PEG NPs DA411 (scale bar 50 nm), TEM-average diameter: 6.53 ± 0.99 nm. (b) TEMmicrograph and size distribution of (Fe3O4@Au)-RGD NPs DA417 (scale bar 50 nm), TEM-average diameter: 6.11 ± 0.96 nm. (c) Radius distributionfunctions obtained at 90° by means of DLS measurements for DA411 (Mean hydrodynamic radii 12.4 ± 0.2 nm). (d) Radius distribution functions obtained at90° by means of DLS measurements for DA417 (Mean hydrodynamic radii 22.3 ± 0.9), (e) and (f) Scattering cross sections obtained from SANS measurementsfor DA411 and DA417, respectively, along with the fitting curves obtained as described. In addition, to allow better visualization, data are multiplied for ascale factor, as indicated.

cRGD functionalized NPs presented a PEG5000/cRGD ratioof about 55:45, calculated on the basis of weight changeduring the exchange reaction. (Fe3O4@Au)-PEG DA411 and(Fe3O4@Au)-cRGD NPs DA417 were finally characterizedby TEM, DLS, SANS, and UV/Visible spectroscopy. TEMaverage diameters were calculated from evaluation of severalTEM micrographs by means of an algorithm for automaticimage analysis developed by our group on this purpose.Results are reported in Table I. By comparing the meandiameter of native uncoated Fe3O4 NPs (5.23 ± 1.1 nm) withthat of gold -coated NPs (6.53 ± 0.99 nm) it results that thethickness of the gold shell is about 0.65 nm. TEM analysisshows that PEGilated NPs maintain a uniform dispersion and

TABLE I

CHARACTERIZATION OF SYNTHESIZED NANOPARTICLES

Sample TEM diameter (nm) DLS RH (nm) λmax (nm)

DA409 6.53 ± 0.99 5.7 ± 0.2 522

DA411 6.12 ± 0.88 12.4 ± 0.2 516

DA417 6.11 ± 0.96 22.3 ± 0.9 509

RH = hydrodynamic radius

average diameters are almost unaltered after coating exchange(Fig. 2a) and cRGD introduction (Fig. 2B).

Analysis of DLS data reveals bimodal distribution functionsof the hydrodynamic radius in both freshly prepared samples

2344 IEEE SENSORS JOURNAL, VOL. 13, NO. 6, JUNE 2013

(a)

(b) (c) (d)

Fig. 3. (a) Phantom multiecho T2 images (odd echoes shown). (b) and (c) Typical signal decay curves for two different concentrations (range of concentration0.005–1 mg/ml Fe3O4@Au NPs). (d) Transverse relaxation rates (R2) versus concentration of Fe3O4@Au NPs obtained at 1.5 and 3 T.

(a) (b)

Fig. 4. (a) Longitudinal relaxation rates (R1) with respect to concentration of Fe3O4@Au NPs (range of concentration 0.005–1.0 mg/ml) obtained at1.5 and 3 T. (b) Longitudinal (R1) and transverse (R2) relaxation rates at 1.5 T of water protons in homogeneous aqueous dispersions of Fe3O4@Au NPs atdifferent concentrations (0.005–1.0 mg/ml).

of DA411 and DA417: i.e., two different populations arepresent in the systems, as clearly seen in Fig. 2c, and 2d.Hydrodynamic radii Rh of the smaller populations obtainedfrom DLS measurements, and reported in Table I, areset at values of 12.4 ± 0.2 nm and 22.3 ± 0.9 nm for(Fe3O4@Au)-PEG and (Fe3O4@Au)-cRGD nanoparticles,respectively. These values are satisfactory for synthesizedPEG-functionalized NPs. In contrast, the presence of thepopulations located at higher Rh values (∼ 80/200 nm) wasunexpected. To investigate the nature of these populations,both samples were subjected to sonication treatments, asindicated in Fig. 2c and 2c, and reanalyzed. Comparison of thetraces in each figure showed a decrease in the larger population

along with a substantial constancy of the smaller population.However, after a prolonged time (i.e., approximately 6 h)no further change in the distribution functions was observed.The presence of larger aggregates could be attributed tocoalescence phenomena among particles. With regard tothis aspect, DA411 NPs solution was seeped through a100-nm HPLC filter. On the filtered part, DLS investigationsshowed that the larger population was reduced as expected.Furthermore, after 24 h the situation was substantiallyunvaried, indicating the absence of an equilibrium betweenthe two populations, and supporting the hypothesis thatcoalescence phenomena occurred. The same conclusion canbe conjectured for the DA417 sample, even though no filtration

MENICHETTI et al.: IRON OXIDE-GOLD CORE-SHELL NPs 2345

Fig. 5. Overlay of gradient-echo T∗2 maps and anatomical MR images obtained after 5 and 25 min after administration of the contrast media. Liver is

indicated by arrows. T∗2 values in liver decreased from ∼30 to ∼8 ms between the two experiments.

TABLE II

SANS CHARACTERIZATION OF

SYNTHESIZED NANOPARTICLES

Sample DA411 DA417

R0/Å 48.5 ± 0.9 -

τ1/Å 6.5 ± 0.5 6.1 ± 0.9

τ2/Å 105 ± 18 146 ± 9

ρ0/Å−2 6.84 · 10−6 6.84 · 10−6

ρ1/Å−2 4.5 · 10−6 4.5 · 10−6

ρ2/Å−2 5.65 · 10−6 5.57 · 10−6

ρs/Å−2 - 6.34 · 10−6

was possible, because of the quite comparable value exhibitedby the large population with respect to the smaller one.

SANS measurements were performed on both filteredDA411 and extensively sonicated DA417 systems. The scat-tering cross-sections obtained are reported in Fig. 2 e/f: for abetter presentation of data, cross-sections were multiplied bya scaling factor, as indicated in the plots. According to TEMand DLS observations, data have been interpreted in terms ofa collection of spheres composed of an iron oxide core with aradius R0 and two different shells: the first one composed of alayer of gold, whose thickness is τ1, and the second one com-posed of the chains linked to the nanoparticle surface, whichis different between the two samples (thickness = τ2). Core,first shell, second shell and solvent scattering length densitieswere indicated with ρ0, ρ1, ρ2 and ρs , respectively. Scatteringcross-sections of a dilute system containing such objects wassusceptible to a theoretical expression [24]. By fitting thisfunction, and by evaluating scattering length densities, it was

possible to obtain information on R0, τ1 and τ2, reported inTable II.

Currently, it must be noted that for the DA417 sample,R0 value could be determined only by rough calculationsbecause of the missing Guinier’s regime; this was in agreementwith the structure as well as with DLS distribution that showeda larger RH value for DA417.

Analysis of the data reported in Table II indicated a substan-tial constancy of the inner gold shell. The most external shell,which contains the functionalized moieties, was larger for theDA417 sample, according to the expected structure of the NPs.

The UV/Vis spectra of the synthesized NPs revealed thepresence of surface plasmon band characteristic of goldNPs, thus confirming the Fe3O4@Au core-shell morphology.The λmax values, comprised between 516-509 nm, were inaccordance with the small size of the analyzed particles [25].

The T2-weighted images of (Fe3O4@Au)-PEG DA411 NPsat different concentrations recorded at 1.5 and 3 T in thephantom showed a significant signal attenuation with increas-ing concentration from 0.005 to 1 mg/ml (Fig. 3 a-c). Thecorresponding relaxation rates (R2 = 1/T2) exhibited a lineartrend with (Fe3O4@Au)-PEG NPs concentration as reportedin Fig. 3-d. A linear trend was also observed for longitudinalrelaxation rates (R1 = 1/T1), as shown in Fig. 4-a. Thelongitudinal (r1) and transverse (r2) relaxivities were alsoevaluated both at 1.5 and 3 T: we obtained a slightly decreasedvalue for r2 at 3 T respect to 1.5 T (estimated difference of11.4 %) as reported in Table III and we found a decreasedr1 at 3 T with respect to 1.5 T (Table III). The ratio betweentransverse and longitudinal relaxivities was also assessed at1.5 and 3 T: in consequence of the values produced for r1 andr2 we obtained an increased value for r2/r1 at 3 T with respectto 1.5 T as reported in Table III (Fig. 4-b). Moreover, the ratio

2346 IEEE SENSORS JOURNAL, VOL. 13, NO. 6, JUNE 2013

TABLE III

LONGITUDINAL (R1) AND TRANSVERSE (R2)

RELAXIVITIES AND RATIO R2/R1

R1 (ms−1 mL mg−1) R2 (ms−1 mL mg−1) R2/R1

1.5 T (3.62 ± 0.76)10−4 (2.1 ± 0.2)10−2 58.0 ± 13.4

3 T (2.88 ± 0.80)10−4 (1.86 ± 0.06)10−2 64.6 ± 18.1

r2/r1 resulted comparable with those of other negative contrastagents [26], [27].

In order to study in vivo the fate of (Fe3O4@Au)-PEGNPs a MRI dynamic acquisition was performed in healthyrats; an important decrease in the T∗

2 value in the liver wasreported in T∗

2 maps depicted in Fig. 5. The decrease in T∗2

was likely related to the existence of a higher concentration of(Fe3O4@Au)-PEG NPs in the liver tissue at 25 min. The T∗

2value, assessed 5 min and 25 min after the injection start, were30.3 ms and 8.9 ms respectively, with a negative enhancementof about 70%.

Although the thick Au-shell contributed to decreasingthe saturation magnetization of the magnetic component ofNP [28], thus reducing the relaxivities and increasing thedistance between the magnetic core and the solvent watermolecules, the accumulation of (Fe3O4@Au)-PEG NPs wastracked in vivo using a commercial scanner, demonstratingtheir suitability for MRI and suitable blood-circulation timefor in vivo applications.

IV. CONCLUSION

We reported the synthesis of hydrophilic (Fe3O4@Au)-PEG and (Fe3O4@Au)-cRGD NPs and their characteriza-tion with transmission electron microscopy (TEM), dynamiclight scattering (DLS), small angle neutron scattering (SANS)and ultraviolet visible spectroscopy (UV–Vis). TEM analysisshowed that NPs samples were characterized by small size (ca.6 nm in diameter) and uniform dispersion. DLS and SANSmeasurements confirmed the NP structure with a small innercore and a hydrophilic external shell composed of PEG chainsof greater hydrodynamic radius (Rh = 12–22 nm). UV–Visspectroscopy revealed the presence of the characteristic surfaceplasmon band, thus indirectly confirming the Fe3O4@Aumorphology.

The relaxivity data obtained in this study both at 1.5 and3T in the phantom confirmed the modulation of R1 and R2values by NP concentration. The demonstrated increase inR2 is expected to be associated with a even larger increasein R2*, as shown in in-vivo experiments with endogenousiron [29]. It was confirmed by in-vivo imaging experiments,where NPs act as negative MRI contrast agents. Moreover,the presence of cRGD peptidomimetic ligands on the surfaceof this Fe3O4@Au-system could allow new applications innanomedicine, based on their magnetic and optical propertiesthat render this nanostructure a potential targeting probe, withsuitable contrast and blood-circulation time for microimagingtechniques and gradient-echo MRI.

ACKNOWLEDGMENT

The authors thank Dr. Nadia Santo of CIMA (CentroInterdipartimentale di Microscopia Avanzata) of the Universityof Milan for TEM analysis. Author L. P. wishes to thank theRutherford Appleton Laboratory for provision of beam time.Author L. M. wishes to thank Dr. Giovanni D. Aquaro’s forhis expertise and advice in experimental MRI animal imaging,and Mr. Luca Serasini for image editing.

REFERENCES

[1] E. D. Smolensky, M. C. Neary, Y. Zhou, T. S. Berquo, and V. C. Pierre,“Fe3O4@organic@Au: Core-shell nanocomposites with high saturationmagnetisation as magnetoplasmonic MRI contrast agents,” Chem. Com-mun., vol. 47, no. 7, pp. 2149–2151, 2011.

[2] M. Kumagai, T. K. Sarma, H. Cabral, S. Kaida, M. Sekino, N. Herlam-bang, K. Osada, M. R. Kano, N. Nishiyama, and K. Kataoka, “Enhancedin vivo magnetic resonance imaging of tumors by PEGylated Iron-Oxide–Gold Core–Shell nanoparticles with prolonged blood circulationproperties,” Macromol. Rapid Commun., vol. 31, no. 17, pp. 1521–1528,Sep. 2010.

[3] J. Zhang, M. Post, T. Veres, Z. J. Jakubek, J. Guan, D. Wang,F. Normandin, Y. Deslandes, and B. Simard, “Laser-assisted synthesis ofsuperparamagnetic Fe@Au Core–Shell Nanoparticles,” J. Phys. Chem.B, vol. 110, no. 14, pp. 7122–7128, 2006.

[4] I. S. Lim, P. N. Njoki, H.-Y. Park, X. Wang, L. Wang, D. Mott, andC.-J. Zhong, “Gold and magnetic oxide/gold core/shell nanoparticles asbio-functional nanoprobes,” Nanotechnol., vol. 19, no. 30, p. 305102,Jul. 2008.

[5] Z. Xu, Y. Hou, and S. Sun, “Magnetic core/shell Fe3O4/Au andFe3O4/Au/Ag nanoparticles with tunable plasmonic properties,” J. Amer.Chem. Soc., vol. 129, no. 28, pp. 8698–8699, 2007.

[6] S. Kayal and R. V. Ramanujan, “Anti-cancer drug loaded iron-gold core-shell nanoparticles (Fe@Au) for magnetic drug targeting,” J. Nanosci.Nanotechnol., vol. 10, no. 9, pp. 1–3, Sep. 2010.

[7] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst,and R. N. Muller, “Magnetic iron oxide nanoparticles: Synthesis, stabi-lization, vectorization, physicochemical characterizations, and biologicalapplications,” Chem. Rev., vol. 108, no. 6, pp. 2064–2110, Jun. 2008.

[8] A. Llevot and D. Astruc, “Applications of vectorized gold nanoparticlesto the diagnosis and therapy of cancer,” Chem. Soc. Rev., vol. 41, no. 1,pp. 242–257, Jan. 2012.

[9] L. Wang, J. Luo, Q. Fan, M. Suzuki, I. S. Suzuki, M. H. Engelhard,N. Lin, N. Kim, J. Q. Wang, and C.-J. Zhong, “Monodispersed core-shell Fe3O4@Au nanoparticles,” J. Phys. Chem. B, vol. 109, no. 46,pp. 21593–21601, Oct. 2005.

[10] H. Kojima, Y. Mukai, M. Yoshikawa, K. Kamei, T. Yoshikawa,M. Morita, T. Inubushi, T. A. Yamamoto, Y. Yoshioka, N. Okada,S. Seino, and S. Nakagawa, “Simple PEG conjugation of SPIO via anAu-S bond improves its tumor,” Bioconjugate Chem., vol. 21, no. 6,pp. 1026–1031, Jun. 2010.

[11] D.-H. Kim, E. A. Rozhkova, T. Rajh, S. D. Bader, and V. Novosad,“Synthesis of hybrid gold/iron oxide nanoparticles in block copoly-mer micelles for imaging, drug delivery, and magnetic hyperthermia,”IEEE Trans. Magn., vol. 45, no. 10, pp. 4821–4824, Oct. 2009.

[12] M. A. Malvindi, A. Greco, F. Conversano, A. Figuerola, M. Corti,M. Bonora, A. Lascialfari, H. A. Doumari, M. Moscardini, R. Cingolani,G. Gigli, S. Casciaro, T. Pellegrino, and A. Ragusa, “Magnetic/silicananocomposites as dual-mode contrast agents for combined magneticresonance imaging and ultrasonography,” Adv. Funct. Mater., vol. 21,no. 13, pp. 2548–2555, Jul. 2011.

[13] F. Conversano, A. Greco, E. Casciaro, A. Ragusa, A. Lay-Ekuakille, andS. Casciaro, “Harmonic ultrasound imaging of nanosized contrast agentsfor multimodal molecular diagnoses,” IEEE Trans. Instrum. Meas.,vol. 61, no. 7, pp. 1848–1856, Jul. 2012.

[14] Y. Jin, C. Jia, S.-W. Huang, M. O’Donnell, and X. Gao, “Multifunctionalnanoparticles as coupled contrast agents,” Nature Commun., vol. 1,pp. 1–8, Jul. 2010.

[15] R. Arvizio, R. Bhattacharya, and P. Mukherjee, “Gold nanoparticles:Opportunities and challenges in nanomedicine,” Expert Opin DrugDeliv., vol. 7, no. 6, pp. 753–763, Jun. 2010.

[16] E. Boisselier and D. Astruc, “Gold nanoparticles in nanomedicine:Preparations, imaging, diagnostics, therapies and toxicity,” Chem. Soc.Rev., vol. 38, no. 6, pp. 1759–1782, Apr. 2009.

MENICHETTI et al.: IRON OXIDE-GOLD CORE-SHELL NPs 2347

[17] L. Manzoni, L. Belvisi, D. Arosio, M. Civera, M. Pilkington-Miksa,D. Potenza, A. Caprini, E. M. V. Araldi, E. Monferini, M. Mancino,F. Podestà, C. Scolastico, “Cyclic RGD-containing functionalized azabi-cycloalkane peptides as potent integrin antagonists for tumor targeting,”Chem. Med. Chem., vol. 4, no. 4, pp. 615–632, Apr. 2009.

[18] M. Millard, S. Odde, and N. Neamati, “Integrin targeted therapeutics,”Theranostics, vol. 1, pp. 154–188, Feb. 2011.

[19] J. S. Desgrosselier and D. A. Cheresh, “Integrins in cancer: Biologicalimplications and therapeutic opportunities,” Nat. Rev. Cancer, vol. 10,no. 1, pp. 9–22, Jan. 2010.

[20] G. C. Alghisi and C. Rüegg, “Vascular integrins in tumor angiogen-esis: Mediators and therapeutic targets,” Endothelium, vol. 13, no. 2,pp. 113–135, 2006.

[21] D. Arosio, L. Manzoni, E. M. Araldi, and C. Scolastico, “Cyclic RGDfunctionalized gold nanoparticles for tumor targeting,” BioconjugateChem., vol. 22, no. 4, pp. 664–672, Apr. 2011.

[22] G. D. Wignall and F. S. Bates, “Absolute calibration of small-angleneutron scattering data,” J. Appl. Crystallogr., vol. 20, no. 1, pp. 28–40,Feb. 1987.

[23] V. Positano, A. Pepe, M. F. Santarelli, M. Scattini, D. De Marchi,A. Ramazzotti, G. Forni, C. Borgna-Pignatti, M. E. Lai, M. Midiri,A. Maggio, M. Lombardi, and L. Landini, “Standardized T2* map ofnormal human heart in vivo to correct T2* segmental artifacts,” NMRBiomed., vol. 20, no. 6, pp. 578–590, Oct. 2007.

[24] A. Guinier and G. Fournet, Small-Angle Scattering of X-Rays. New York,NY, USA: Wiley, 1955.

[25] S. Link and M. A. El-Sayed, “Spectral properties and relaxation dynam-ics of surface plasmon electronic oscillations in gold and silver nanodotsand nanorods,” J. Phys. Chem. B, vol. 103, no. 40, pp. 8410–8426,Sep. 1999.

[26] L. Calucci, G. Ciofani, D. De Marchi, C. Forte, A. Menciassi,L. Menichetti, and V. Positano, “Boron nitride nanotubes as T2-weighted MRI contrast agents,” J. Phys. Chem. Lett., vol. 1, no. 17,pp. 2561–2565, Aug. 2010.

[27] L. Menichetti, D. De Marchi, L. Calucci, G. Ciofani, A. Menciassi, andC. Forte, “Boron nitride nanotubes for boron neutron capture therapyas contrast agents in magnetic resonance imaging at 3 T,” Appl. Radiat.Isotopes, vol. 69, no. 12, pp. 1725–1727, Dec. 2011.

[28] L. Wang, H.-J. Park, I. I. S. Lim, M. J. Schadt, D. Mott, J. Luo, X. Wang,and C.-J. Zhong, “Core@shell nanomaterials: Gold-coated magneticoxide nanoparticles,” J. Mater. Chem., vol. 18, no. 23, pp. 2629–2635,Feb. 2008.

[29] J. C. Wood, M. Otto-Duessel, M. Aguilar, H. Nick, M. D. Nelson,T. D. Coates, H. Pollack, and R. Moats, “Cardiac iron determinescardiac T2*, T2, and T1 in the gerbil model of iron cardiomyopathy,”Circulation, vol. 112, no. 4, pp. 535–543, Jul. 2005

Luca Menichetti, photograph and biography are not available at the time ofpublication.

Leonardo Manzoni, photograph and biography are not available at the timeof publication.

Luigi Paduano, photograph and biography are not available at the time ofpublication.

A. Flori, photograph and biography are not available at the time of publica-tion.

Claudia Kusmic, photograph and biography are not available at the time ofpublication.

Daniele De Marchi, photograph and biography are not available at the timeof publication.

Sergio Casciaro, photograph and biography are not available at the time ofpublication.

Francesco Conversano, photograph and biography are not available at thetime of publication.

Massimo Lombardi, photograph and biography are not available at the timeof publication.

Vincenzo Positano, photograph and biography are not available at the timeof publication.

Daniela Arosio, photograph and biography are not available at the time ofpublication.