Electrochemical characterization of core@shell ...

J Nanopart Res (2013) 15:1813 DOI 10.1007/s11051-013-1813-0

RESEARCH PAPER

Electrochemical characterization of core@shell CoFe2O4/Au composite Francesco Carla` • Giulio Campo • Claudio Sangregorio • Andrea Caneschi • Ce´sar de Juliań Fernańdez • Lourdes I. Cabrera

Received: 21 January 2013 / Accepted: 19 June 2013 Ó Springer Science+Business Media Dordrecht 2013

Abstract In this paper, we address the synthesis and characterization of the core@shell composite magneto-plasmonic cobalt ferrite–gold (Co-ferrite/Au) nanosystem. The synthesis Co-ferrite/Au nanocomposite is not obvious, hence it was of interest to generate it in a simple straightforward method. Co-ferrite/Au nanocomposite was generated by synthesizing first by thermal decomposition Co-ferrite nanoparticles (NPs). On a second step, ionic gold (Au3?) was reduced at the surface of Co-ferrite NPs by ultrasound, to obtain the metallic Au shell. The characterization of the nanomaterial was achieved by

Electronic supplementary material The online version of this article (doi:10.1007/s11051-013-1813-0) contains supplementary material, which is available to authorized users. F. Carla` Dipartimento di Chimica, ‘‘Ugo Schiff’’, Universita` degli Studi di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, FI, Italy G. Campo C. Sangregorio A. Caneschi C. de Juliań Fernańdez L. I. Cabrera (&) Laboratorio di Magnetismo Molecolare, INSTM, Dipartimento di Chimica, Universita` degli Studi di Firenze, Polo Scientifico, Via della Lastruccia 3, 50019 Sesto Fiorentino, FI, Italy e-mail: [email protected] C. Sangregorio C. de Juliań Fernańdez C.N.R.—Istituto di Scienze e Tecnologie Molecolari (I.S.T.M.), Via C. Golgi 19, 20133 Milan, Italy

microscopy, spectroscopy, and performing magnetic measurements. However, what is attractive about our work is the use of electrochemical techniques as analytical tools. The key technique was cyclic voltammetry, which provided information about the nature and structure of the nanocomposite, allowing us to confirm the core@shell structure. Keywords Electrochemical techniques Hybrid material Magnetic material Magnetite Methylene blue Magnetic nanoparticles Fluorescent nanoparticles Abbreviations NP Nanoparticle kHz Kilohertz DMSA Dimercaptosuccinic acid DMSO Dimethyl sulfoxide ICP-EOS Inductively coupled plasma optical emission spectrometry TEM Transmission electron microscopy FT-IR Fourier transformed infrared XRD X-ray diffraction UV/vis Ultraviolet–visible SQUID Superconducting quantum interference device kOe Kilo-oersted H Magnetic field strength GPE Graphite paste electrode CV Cyclic voltammetry SCE Saturated calomel electrode

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SPR k CVs emu erg Hc Mr Ms Mr/Ms r vs

J Nanopart Res (2013) 15:1813

Surface plasmon resonance Wavelength Cyclic voltammograms Electromagnetic units Erg Coercivity Remanent magnetization Saturation magnetization Squareness ratio Mass density Against

Introduction In the last years, core@shell nanoclusters have become of great interest because they combine multiple properties not obtainable in individual materials that compose them (Levin et al. 2009). Their physical and chemical properties are strongly dependent on the structure of core, shell, and interface. This structure dependence allows the possibility to modify their properties by controlling their chemical composition and size of core and shell (Fan et al. 2010). Within these materials, those that exhibit magnetoplasmonic properties are very attractive. These materials have a magnetic core and a noble metal shell (Pellegrino et al. 2006). Even when core@shell nanocomposites retain the optical properties (surface plasmon resonance excitations) and magnetic properties from each of their component parts, these characteristics are modified by the core@shell structure (Levin et al. 2009). These alterations also depend on geometry and surrounding medium of the nanoparticle (Moores and Goettmann 2006). These magneto-plasmonic core@shell composites exhibit high potential and are promising materials for medical applications, such as biosensing, magnetic resonance imaging (MRI), and photothermal therapy, since they can be used for optical label with magnetic handle for sample collection (Goon et al. 2009; Brullot et al. 2012). Not only that, magneto-plasmonic composites also find utility in spintronics and spin injectors with higher efficiencies compared with existing injectors, or as magnetic field modulators, where the photoluminescence is controlled via applied magnetic fields (Kim et al. 2005).

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For these reasons, research has focused on the generation and study of magneto-plasmonic core@shell nanoparticles (NPs). For this matter, gold (Au) and cobalt ferrite (CoFe2O4) are two excellent options, which demonstrate unique properties at nanoscale. In fact, Au shows attractive optical properties, biological compatibility, and catalytic activity (Lin et al. 2001; Chen et al. 2003; Hyeon et al. 2007; Pita et al. 2008; Pradhan et al. 2008; Goon et al. 2009; Di-Guglielmo et al. 2010), while CoFe2O4 reveals many fascinating phenomena, such as charge ordering, metal–insulator transition known as the Verwey transition, etc. Hence, it is of interest to probe the effects resulting from the surface modification and constraints imposed by a core–shell arrangement on the nanoscale (Shubayev et al. 2009; Di-Guglielmo et al. 2010). Even when it is known that CoFe2O4 is toxic in biological systems due to the risk of Co2? dissolving into it, the core–shell composite CoFe2O4@Au is not. The Au shell besides supplying biocompatibility, it will protect the CoFe2O4 from oxidizing or dissolving, since Au is resistant to oxidation in an aqueous media or biological serum, as it has been stated by several research groups (Ullate et al. 2011; Gallo et al. 2010). For their biocompatibility, Au and CoFe2O4 together with other forms of iron oxide NPs have been studied for optical and magnetic biomedical applications (Liang et al. 2012). Several methods have been developed to obtain Aucoated cobalt ferrite (Co-ferrite). Most of the methods developed are based on the procedures followed for the synthesis of Fe3O4@Au NPs. The generation of homogeneous Fe3O4@Au composite has been obtained by Zhong et al. technique (Wang et al. 2005), where Fe3O4 NPs are used as seeds to grow over them the Au shell. However, the generation of CoFe2O4@Au is more complicated than for Fe3O4@Au, reason why the technique has been modified to synthesize them. Coating of Co-ferrite NPs with Au can be achieved in organic media (Pita et al. 2008; Goon et al. 2009) or aqueous media (Okitsu et al. 2005; Seino et al. 2006; Pradhan et al. 2008; Goon et al. 2009), where after the synthesis of CoFe2O4, Au precursor is reduced on the surface of CoFe2O4 NPs to generate the metallic Au shell (Pradhan et al. 2008). There are few reports where ultrasound is used as a tool for the synthesis of such materials (Vijayakumar et al. 2000; Okitsu et al. 2001, 2005; Seino et al. 2006; Pradhan et al. 2008; Quaresma et al. 2008; Teo et al. 2009).


Sonochemical synthesis of various types of NPs and nanostructured materials can be achieved due to the reaction routes induced by acoustic cavitation in solution, which provides extreme conditions of transient high temperatures and high pressures within the collapsing bubbles, shock wave generation, and radical formation (Okitsu et al. 2001, 2005). Characterization techniques that allow the identification and characterization of these nanomaterials include electron microscopy and scanning probe microscopies, X-ray and neutron diffraction, X-ray scattering, and various spectroscopies techniques (Joshi et al. 2008; Rao and Biswas 2009). However, in some cases it not possible to confirm by any of these techniques if the structure of a developed nanosystem was achieved. In such cases, electroanalytical techniques can be very useful. Electroanalytical methods can detect trace concentrations of an electroactive species in solution and supply useful information concerning its physical and chemical properties. The advantages of using electroanalytical techniques when compared to other analytical methods are low cost, selectivity, reliability, and broad applicability. For data acquisition, current and potential are the main electrochemical parameters for automation and control (Kounaves 1997). One of the most popular electroanalytical techniques is cyclic voltammetry (CV). There are few articles dealing with the electrochemical behavior of CoFe2O4 oxides (Laouini et al. 2011), using carbon paste electrodes. Most of the studies reported have been performed in alkaline medium (Barrado et al. 1999), such as KOH or phosphate buffer solutions (pH 7) to determine their electrochemical properties such as oxidation potentials, diffusion coefficients, electron transfer rates, etc. However, most analyses are focused in their feature potential application in lithium ion battery anodes (Ding et al. 2011; Crain et al. 2013), catalysts (Heli et al. 2012), sensors (Berchmans et al. 2011; Liu et al. 2011; Yang et al. 2011; Krishna et al. 2012; Shang et al. 2012) among others. Electrochemical characterizations performed in acidic media have been reported using CV. The voltammograms obtained represent the ‘‘electrochemical spectra’’ of solid or dissolved substances that can be used to characterize a material (Lorenzo et al. 1997). Vega-Rios was able to study core–shell

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composites by CV in acidic media to determine the nature and structure of NPs composed by polyaniline and polystyrene (Vega-Rios et al. 2013). These electrochemical techniques have also been used in the characterization and determination of lead ferrites, Fe2O3 and Fe3O4, in HClO4 and HCl media, providing different voltammogram shapes and potentials for species of the same chemical nature, but different structure (Lorenzo et al. 1997; Barrado et al. 1999). In this paper, the sonochemical reduction of Au(III) in an aqueous media to produce Au NPs was performed in the presence of a small amount of oleic acid and CoFe2O4 NPs dispersed in the same reaction solution, to generate core@shell NPs. Nevertheless, the characterization of the composite by conventional techniques was not very enlightening. Hence, CV technique has been used to confirm the core@shell structure of the synthesized nanomaterial.

Experimental Synthesis of Co-ferrite/Au composite Cobalt ferrite nanoparticle synthesis Co-ferrite NPs are prepared from the corresponding organometallic precursor, Fe(III) oleate and Co(II) oleate by thermal decomposition (Jana et al. 2004; Park et al. 2004; Han et al. 2007; Park et al. 2007; Kwon and Hyeon 2008; Roca et al. 2009). Co(II) oleate and Fe(III) oleate were generated by reacting the corresponding metal chlorides (FeCl36H2O and CoCl26H2O) with sodium oleate (Park et al. 2004; Bao et al. 2009). For the synthesis of Co ferrite, the metallic precursors were mixed in a ratio of 0.57 mmol of Fe(III) oleate for 0.29 mmol of Co(II) oleate in 25 mL of octyl ether. 2.55 mmol of oleic acid was added to the reaction mixture and slowly heated under nitrogen atmosphere, to 300 °C. The reaction mixture was aged at this temperature for 3 h. The final dark product was cooled to room temperature by retrieving the heating source. Co ferrite was washed with a mixture of toluene/heptane (1:1), coagulated with 2-propanol/ethanol (1:3), and centrifuged. The procedure was repeated numerous times, until the final solution was clear. The washed reaction product was then re-dispersed in hexane.

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Nanoparticles’ surface modification

Characterization techniques

To impart water stability, NPs were surface-functionalized with dimercaptosuccinic acid (DMSA) (Gupta and Gupta 2005; Shubayev et al. 2009). For DMSA functionalization, Co-ferrite NPs were coagulated from the hexane suspension by adding ethanol and centrifuging. A mixture of 25 mL of toluene and a solution of 90 mg of DMSA in 5 mL of dimethyl sulfoxide (DMSO) was added to 50 mg of Co-ferrite NPs. The mixture was sonicated for 5 min and mechanically stirred during 24 h at room temperature. DMSA functionalized Co-ferrite NPs were washed several times with ethanol, and redispersed in water. The aqueous suspension was basified to a pH value of 10, to achieve a better dispersion of the Co-ferrite NPs. The pH was then brought back to pH 7. The colloidal suspension was dialyzed for 3 days. The final concentration of the aqueous suspension was of 1 mg mL-1 of Co-ferrite NPs.

Instrumental analysis

Cobalt ferrite nanoparticles’ gold coating The coating of CoFe2O4–DMSA NPs with Au was achieved by a sonochemical process. An aqueous solution of 0.1 mM of HAuCl4 was prepared. For each 50 mL of HAuCl4 solution, 100 lL of oleic acid and 100 lL of 1 mg mL-1 Co ferrite suspension were added. The reaction mixture was deoxygenated by bubbling nitrogen gas during 1 h. The suspension was then placed under sonication in a sonication bath (Elmasonic S30 H, operated at 37 kHz) for 90 min, controlling the temperature not to be higher than 25 °C. The reaction product (Co ferrite/Au) was removed magnetically, and washed several times with ethanol. Gold nanoparticles Au NPs are generated to compare their optical properties with Co-ferrite/Au composite. Au NPs were achieved by sonosynthesis, in a similar way as the coating of Co ferrite was carried out. An aqueous solution of 0.1 mM of HAuCl4 was prepared. For each 50 mL of HAuCl4 solution, 100 lL of oleic acid was added. The solution was deoxygenated by bubbling nitrogen gas during 1 h, before placed under sonication in a sonication bath (Elmasonic S30 H, operated at 37 kHz) for 90 min, controlling the temperature not to be higher than 25 °C. Au NPs were removed by centrifugation, and washed several times with ethanol.

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The inductively coupled plasma optical emission spectrometry (ICP-OES) was performed with a PERKIN ELMER OPTIMA 2000 DV instrument. Transmission electron microscopy (TEM) analysis was performed on a PHILIPS/FEI CM12 Cryo-gatan UHRST 3500 instrument operated at an accelerating voltage of 120 kV. Samples for TEM and analysis were prepared by placing a drop of diluted NP solutions on carbon-coated copper grids, allowing the solvent to dry before the analysis was carried out. Particles morphology was studied, and particle size was determined from the measurement of 200 particles. The Fourier transform infrared (FTIR) spectra were performed in a FTIR spectrometer Perkin Elmer Spectrum BX sweeping the energy region between 4,000 and 500 cm-1. The measurement resolution is of 2 cm-1. X-ray diffraction (XRD) data were collected using a Bruker D8 Advance diffractometer ˚ ) radiation. The diffractowith Cu Ka (k = 1.5406 A grams were registered between 10° and 90° (2h values) with steps of 0.02° (2h) and a rate of 2 s step-1. The ultraviolet–visible (UV/Vis) spectra were performed in a Jasco V-670 spectrometer, recording the spectral region between 300 and 800 nm. The magnetic properties of the dried NPs were measured superconducting quantum interference device (SQUID) magnetometer used for magnetic characterization of the synthesized material was a MPMS XL, Quantum Design. Hysteresis loops were recorded at temperatures of 3 and 300 K after applying a saturation field of 50 kOe. Saturation magnetization values were obtained by extrapolation of the magnetization values to 1/H = 0 in the high field region. Electrochemical characterization set-up The graphite paste electrodes (GPE) with and without ferrite NPs were prepared according to the procedure described in (Barrado et al. 2001). Graphite powder (0.1 mg, Fisher, [99 % purity) and few drops of mineral oil (Aldrich, [99 %) were mixed together using an agate mortar to obtain a dry pasty mixture. To the GPE with ferrite NPs, about 5–10 mg of NPs (previously purified, washed and dried) were added to the graphite mixture and thoroughly homogenized.


Electrochemical characterization was performed by CV. The analysis was performed in an AMEL 551 potentiostat controlled by a LabView interface, using as a working electrode a GPE. A standard Metrohm glass cell was used for electrochemical measurement. CV was recorded using an AMEL 551 potentiostat controlled by a LabView interface. The counter electrode was a Pt wire, and the reference electrode was a Saturated Calomelan Electrode (SCE, Metrohm). It is worth to notice that all potentials are quoted with respect to SCE. The working electrode was designed and built at the workshop of the Chemistry Department. The graphite paste is placed in a TeflonÒ tube with a 3-mm diameter hole. A stainless steel screw can be used as a piston to push the graphite paste out of the TeflonÒ tube allowing the renewal of the surface layer (for detailed description see Supplementary Information). For the experiments with a bulk Au electrode, a polycrystalline Au disk of 3 mm in diameter (Metrohm, Autolab) was used as working electrode. The Au electrode was polished with alumina 0.05 lM particle size (Buehler Micropolish II), rinsed in double distilled water and cleaned in water bath ultrasonic cleaner for 15 min before each series of measurements. A 1 M HClO4 (65 % reagent grade, Fluka) solution and a 1 M NaCl (Merck, analytical reagent grade)/1 M HClO4 were used as electrolytes. All the solutions were freshly prepared and deaerated by bubbling nitrogen for 10 min before the beginning of each series of measurements.

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peak at 1,140 cm-1 is attributed to the C–O bond. The typical bands of spinel metal-oxide for iron was observed at 624 cm-1 (Sangmanee and Maensiri 2009; Naseri et al. 2010). The absence of Co3O4 (or CoOOH) and CoO was confirmed, since the weak shoulder at ca. 710 cm-1 and a peak at 450 cm-1, generally associated with the these oxides were not observed (Tirosh et al. 2006). However, oleic acid does not allow ferrite NPs to be stable in aqueous media. Since the sonochemical coating of Au is performed in an aqueous media, CoFe2O4 NP must be stable in water. Therefore, following the ligand exchange procedure (Gupta and Gupta 2005), oleic acid was displaced by DMSA. This molecule possesses a carboxylic group that can displace oleic acid from the particles’ surface. Furthermore, it has thiol groups in its structure that besides granting the hydrophilic property, also allow a bonding to metallic Au during Co ferrite coating. The presence of DMSA was confirmed by FTIR analysis (Fig. 1b). The characteristic FTIR band at 3,410 cm-1 is attributed to the stretching vibration of –OH, which may be from water present in the sample. The signals at 2,928 and 2,856 cm-1 correspond to the vibrations of the symmetric and asymmetric C–H2 stretching modes of the remaining oleic acid after DMSA coating. The spectrum showed bands at 1,650 and 1,535 cm-1 that were assigned to the vibration of –COOH, an indication of remaining free carboxylic acid groups of DMSA molecules that are present on the particle surface. The bands present at 1,476 and

Results and discussion Co-ferrite NPs were prepared by thermal decomposition of the corresponding metal oleates, which act effectively as a growth source of monodisperse NPs (Park et al. 2004). At the end of the reaction, it was confirmed by FTIR analysis (Fig. 1a) that the nanoparticle’s surface is covered by a layer of hydrophobic oleic acid, which prevents aggregation of the particle’s cores in organic solvents, such as hexane. The sharp bands at 2,924 and 2,855 cm-1 were attributed to the asymmetric and symmetric C–H vibrations of the methylene groups, respectively. The bands at 1,460 and 1,380 cm-1 were ascribed to the asymmetric and symmetric COO– stretches (Wang et al. 2010). The

Fig. 1 FTIR spectra of the metal oleate precursors (a) Coferrite NPs covered with oleic acid; and (b) Co-ferrite NPs modified with DMSA

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1,388 cm-1 are due to the COO– bond. Furthermore, at 1,048 and 1,004 cm-1 bands that correspond to the vibration of the bond S–CH are also observed. Moreover the absorption band at 590 cm-1 for stretching Fe–O bonds of the Co ferrite is observed. Once the CoFe2O4 NPs surface was functionalized with DMSA, the coating with Au could be performed applying ultrasound. An aqueous suspension was prepared, which contained Co ferrite–DMSA NPs, AuCl4- ions and oleic acid. The suspension showed a pale yellow color after bubbling nitrogen for 1 h. The color of the reaction suspension started to change from an initial pale yellow to reddish purple with increasing time of ultrasonic irradiation, as it has been reported in the literature (Okitsu et al. 2005). The sonochemical reduction of Au3? ions has been studied extensively by several groups (Okitsu et al. 2005; Pradhan et al. 2008). It has been reported that the sonochemical reduction of Au3? in the presence of an organic additive such as oleic acid, starts via radical reactions (Okitsu et al. 2005), generating Au NPs. Hence, when Au3? reduction is carried out in the presence of Co-ferrite NPs modified with DMSA, it is expected to anchor Au NPs to the surface of Co ferrite thanks to the thiol groups in DMSA, allowing the formation of the composite. The FTIR spectrum for Au-coated Co-ferrite NPs shows the presence of oleic acid at the final product. This is expected, since it was oleic acid the one used as the organic additive for the formation of reducing radicals in the sonic synthesis. In order to confirm the nature of the final nanosystem, the synthesized material was characterized using several techniques. ICP-OES analysis reveals the ratio between the elements present in the material. For Coferrite NP, the Co2?:Fe3? ratio was very close to 1:1.18, being richer in cobalt. For the generated core– shell nanosystem, Co-ferrite/Au NPs have a Co:Fe ratio of 1:2.1, being richer now in iron when compared to bare CoFe2O4 (Co:Fe 1:1.8). This change in the cobalt content could be due to a preferential dissolution of the Co2? among other iron ions of the NPs during the ligand exchange procedure in the Au coating step. The ratio of Co:Fe was always consistent and reproducible in each nanosystem. However, the amount of Au was not reproducible. The quantity of Au in different aliquots of the system gave different amounts of Au, varying from 26 to 6 mg L-1. These

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data are very striking, since it implies that the Au shell at the Co-ferrite NPs varies. This allows us to speculate about the possible nanostructural morphology that has been achieved (Fig. 2): (a) there is no control over the thickness of the Au shell deposited over the Co-ferrite surface; (b) Au is not present as a shell, but as smaller NPs anchored to the Co-ferrite surface, and to each other; (c) Au does not surround Co-ferrite NPs, but are present as individual Au NPs mixed with Co ferrite. Co-ferrite NPs and Co-ferrite/Au NPs were characterized by X-ray diffraction to compare the results. The diffractograms of Co ferrite samples showed diffraction peaks corresponding to the spinel structure (Fig. 3a). For the product Co-ferrite/Au particles of the sonochemical synthesis, characterized by X-ray diffraction (Fig. 3b), it was observed that the diffraction peaks registered correspond to the reflections of FCC Au (JCPDS No. 04-0784). The diffraction features appearing at 2h = 38.20°, 44.41°, and 64.54°, respectively, correspond to the (111), (200), and (200) planes of the standard cubic phase of Au (Khosroshahi and Nourbakhsh 2010). By comparing both diffractograms, the presence of Au in the composite was confirmed, whereas the presence of Co ferrite was not. These measurements cannot explain if Au was coating Co-ferrite NPs, or if it was present as individual Au NPs mixed with Co ferrite and in an excess, masking the diffraction peaks of Co-ferrite NPs. To have a better image of the nature of the composite, TEM analysis was performed. As previously mentioned, TEM analysis of bare Co-ferrite particles showed a circular or hexagonal shape with an average size distribution of ca. 15.6 ± 1.5 nm (Fig. 4a). The final nanosystem was also compared morphologically with pure Au NPs obtained by sonosynthesis. These Au NPs showed a circular shape, with an average size distribution of 4.5 ± 2.0 nm (Fig. 4b). Analogously, the TEM analysis of Co-ferrite NPs dispersed in AuCl4- solution sonicated, without the presence of oleic acid did not cause any observable changes. On the other hand, the presence of oleic acid during sonication resulted in the appearance of particles of morphology and size diverse from that of Co ferrite. TEM micrographies of the sonochemical reaction product with oleic acid also show big spherical shaped particles, which are surrounded or coated by smaller particles. It can be inferred that the bigger particles are Co ferrite, which


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Fig. 2 Schematic representation of the formation and possible structures of Co-ferrite/Au nanocomposite, where (a) has a core@shell structure, (b) has a gold NPs anchored to CoFe2O4, and (c) gold and CoFe2O4 are mixed

Fig. 3 X-ray diffractograms for Co-ferrite/Au NPs and Co-ferrite NPs

are assumed to be covered by metallic Au particles, forming a nanocomposite material (Fig. 4c). Observation of the particles in Fig. 4c suggests a high degree of agglomeration between Co-ferrites/Au particles. It cannot be excluded that the aggregation occurs while drying the sample drop on the TEM grid (Wilcoxon et al. 2000; Pita et al. 2008; Pradhan et al. 2008).

In appearance, it is not clear if Co ferrite is surrounded by a layer of Au and over it are Au NPs, or if it is only surrounded by smaller Au NPs. The EDXS was carried out in the sample zone represented in the inset of Fig. 4c. Elemental distribution is depicted in Fig. 5, indicating that Fe, Co, Au, and O are present. The signals assigned to Cu in the EDAX-image, are not from the sample, but from the

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Fig. 4 TEM micrographies of a cobalt ferrite NPs b gold NPs, and c cobalt ferrite NPs after the sonication process to coat them with gold. The inset in (c) shows in detail the coating of cobalt ferrite by apparently smaller NPs

sample grid. By the data collected, it can be inferred that in the generated material Au is present, as is Co ferrite. Several studies have proved that UV/Vis characterization of Au-coated Co-ferrite NPs can serve to examine the core@shell morphology. It has been shown that when Co-ferrite NPs have a Au coating, the surface plasmon resonance (SPR) peak is red shifted when compared with only Au NPs (Goon et al. 2009; Robinson et al. 2010).

Fig. 5 EDXS image of a composite NP. It is confirmed the presence of Fe, Co, Au, and O in the material

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In Fig. 6, the UV/Vis spectrum of ca. 4.5 nm diameter Au NPs (with a FCC crystal structure, data not shown here), Co-ferrite NPs and core@shell NPs, all in aqueous suspension are presented. Au NPs showed a strong absorption band due to SPR at 550 nm (Das et al. 2011). Optical scans of aqueous suspension of Co ferrite particles produced featureless spectra (Robinson et al. 2010), while Co-ferrite/Au NPs spectrum showed a SPR peak developed at 570 nm (Fig. 6), which was red-shifted and broader


compared to the narrow peak at 530 nm observed for Au NPs (Fig. 6), probably because of lack of a uniform Au shell around the cores may also, which can also affect SPR peak position (Lyon et al. 2004). The peak attributed to AuCl4- ions that appears around 320 nm (Iwamoto et al. 2003), was not observed. It has been established that the resonance position peak of CoFe2O4/Au depends on the resonant modes, associated with the various orientations of the particle axes relative to the electric field of light (Toderas et al. 2008), and hence its shape. Furthermore, the SPR of Au at CoFe2O4/Au is also influenced by the interaction of Au with the magnetic Co ferrite (Xu et al. 2007; Jain et al. 2009). Hence, the red-shift observed is most likely due to interaction of the Au with Co-ferrite core (Pradhan et al. 2008; Goon et al. 2009; Robinson et al. 2010). However, this does not suggest the existence of Au shell on the surface of CoFe2O4 NPs (Robinson et al. 2010). So far, all the techniques suggest that Co-ferrite NPs are coated with Au. However, these are not true proof that core@shell nanocomposites were achieved. In order to confirm that the Co-ferrite particles were coated by Au, an electrochemical analytical technique, CV was carried out. Electrochemical techniques can be generally used to get information about surface reactivity and surface reactions taking place at the solid–liquid interface. In the case of CoFe2O4/Au core@shell NPs, CV can be used as complementary technique to elementary and

Fig. 6 UV–visible spectra of (a) Au NPs with kmax = 550 nm, (b) Co ferrite with gold NPs with kmax = 570 nm, and (c) Coferrite NPs with no maximum being observed

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microscopy analysis in the determination of the chemical composition of the NPs surface and in the understanding of elements distribution in the core@shell structure. Given the differences in the electrochemical reactivity between the elements in the CoFe2O4/Au system (Au is a ‘‘noble metal’’ while cobalt and iron are not), a qualitative analysis of the NPs composition is a fairly easy task. Gold electrochemistry is a widely investigated and well known topic. In acid media, a Au polycrystalline electrode is stable in a wide potential range. Surface oxidations start at very positive potentials with the initial formation of a surface oxide monolayer (AuO) (Woods 1988; Tremiliosi-Filho et al. 2005). Under these conditions, cyclic voltammograms (CVs) of Au present a sharp and well-defined cathodic peak which is a clear evidence of the presence of Au. Iron, which is a less noble metal than Au, can be oxidized and dissolved in solution as Fe2? ion even at negative potentials (Beverskog and Puigdomenech 1996). Therefore, the anodic and cathodic peaks due to the Fe2?/Fe3? redox couple can be used to determine its presence at the electrode’s interface. As iron, cobalt can exist in solution in the Co2? and Co3? forms but the Co3? oxidation state can be reached only at a more positive potential than iron. The possibility of exploiting CV analysis to determine the presence of iron and Au at the electrode interface has been investigated in a model system using a Au electrode in an iron solution. Figure 7 shows the CVs of a Au bulk polycrystalline electrode performed in a 1.0 M HClO4 solution (a) and in a 1.0 mM Fe(NO3)3, 1.0 M HClO4 solution (b) at a scan rate of 50 mV s-1. The anodic peak (BO = 1.37 V vs. SCE) is assigned to Au oxide formation. The sharp cathodic peak (BR = 0.92 V vs. SCE) is assigned to the reduction of Au oxide monolayer (AuO) (Woods 1988; TremiliosiFilho et al. 2005). The broad peaks identified as AO and AR can be associated respectively with the Fe2? oxidation (AO = 0.53 V vs. SCE) and the Fe3? reduction (AR = 0.44 V vs. SCE) (Lide 2003). Under the same experimental conditions, CV can be used for the detection of iron and Au at the NP interface. CVs of NPs have been achieved by means of a GPE as a working electrode. The use of GPE for the electrochemical analysis of Co-ferrite system has been already demonstrated effective in the past (Barrado et al. 2001).

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Experiments were conducted for bare CoFe2O4 NPs as well as for CoFe2O4/Au NPs. Bare CoFe2O4 NPs were used as a reference for the Fe2?/Fe3? couple, which is similar to the system under investigation. CV of the bare CoFe2O4 NPs performed using a GPE (Fig. 8) showed that the voltammetric behavior was as expected (Barrado et al. 2001) and in agreement with the data reported in Fig. 7b for the model system. The anodic (AO) and cathodic (AR) peaks at 0.56 V versus SCE and 0.48 V versus SCE, respectively, in Fig. 8, can be assigned to the Fe2?/Fe3? couple. In case of CoFe2O4/Au core@shell NPs, the Au shell should protect the CoFe2O4 core avoiding its dissolution. Hence, if there is a Au shell covering completely (without defects) the NP core, the only voltammetric features present on the voltammograms should be those related with Au surface reactions. As a matter of fact in CVs of CoFe2O4/Au NPs measured in GPE (Fig. 8) only voltammetric features relative to Au oxide (AuO) reduction (BR = 0.88 V vs. SCE) are visible, stating that the Au coating protects effectively the inner part of the particle.

Fig. 7 CVs relative to the a characterization of the gold electrode performed in 1 M HClO4, as the electrolyte. CVs relative to the b Fe3?/Fe2? electrochemical reactions on gold polycrystalline electrode in 1 M HClO4. AO and AR peaks correspond, respectively, to the Fe2? oxidation and Fe3? reduction. BR peak corresponds to the gold oxide reduction. Scan rate was 50 mV s-1

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The differences between the two scans for potentials more positive than 1.0 V versus SCE (oxygen evolution region) can be explained considering that CoFe2O4 act as a catalyst for the oxygen evolution reaction (Singh et al. 1999). Nevertheless, a further proof of the core@shell composition can be obtained by electrochemistry. Indeed a controlled oxidation and dissolution of the NP can be used to analyze the composition of the outer and inner part of the NPs. As showed in previous experiments, Au oxidation in acidic media leads to the formation of a solid oxide. Besides, its dissolution can be achieved oxidizing Au in the presence of chloride ions, when the AuCl4complex is formed (Gaur and Schmid 1970; HerreraGallego et al. 1975). Oxidation and reduction potentials of the species under analysis are dependent on the electrolyte composition, consequently in the presence of chloride, the anodic and cathodic peaks of the iron and Au species are expected to be shifted to more negative potential due to the formation of complexes with chloride (more data about electrochemical behavior of the iron–Au-chloride system can be found in the supplementary information). CVs (first and second scan) of CoFe2O4/Au NPs in 1.0 M HClO4/1.0 M NaCl solution are reported Fig. 9. Scans were carried out at a scan rate of 50 mV s-1. In the second scan, both anodic (AO = 0.50 V vs. SCE) and cathodic (AR = 0.30 V vs. SCE) peak related with the presence of the Fe2?/Fe3? couple can be identified while in the first scan only the cathodic peak (AR) is visible (although overlapped with Au species AuCl4- reduction peak). The fact that the iron can be detected only after Au oxidation and dissolution supports the idea that Au is acting as a barrier between the CoFe2O4 core and the solution, confirming the hypothesis about of core@shell structure for CoFe2O4/Au system. It is worth to note that the Au oxidation peak (which is visible on the GPE due to the finite amount of Au on the surface, contrary to Au bulk electrode behavior) is shifted to more positive potential in the first scan. This can be explained considering that in the first scan Au is organized in NPs while after its dissolution Au is randomly deposited on the graphite electrode surface, meaning that Au is more stable in the NP form.


Page 11 of 16

Fig. 8 CVs of GPEs modified with CoFe2O4 and CoFe2O4/Au NPs in 1 M HClO4 solution. Scan rate was 50 mV s-1. Anodic and cathodic peak relative to the Fe2?/Fe3? couple (AO, AR) are detected only with the CoFe2O4–GPE electrode. In the case of core@shell NPs, the gold shell protects the iron/cobalt core from dissolution, hence the visible features is the cathodic peak associated to the gold oxide reduction (indicated in figure as BR)

Fig. 9 CVs of CoFe2O4/Au NPs GPE performed in 1.0 M HClO4/1.0 M NaCl solution. Scan rate was 50 mV s-1. First (dashed line) and second (continuous line) scan. AO and AR peaks correspond, respectively, to the Fe2? oxidation and Fe3?

reduction. Bo1 and Bo2 peaks can be related with the gold oxidation and dissolution processes, while BR peak corresponds to the AuCl4- reduction

It is easy to assume from all the analytical techniques that the Co-ferrite/Au composite consists of a shell of Au protecting the core of Co ferrite. It is still uncertain what the precise morphology of these core@shell NPs is. Judging by TEM, it is possible that for the Au shell to

be surrounded by smaller Au NPs (Fig. 10). This morphology could also explain why ICP-OES values for Au changed from one sample to another. After full prove was obtained that Co-ferrite NPs were coated with Au, magnetic properties of these NPs

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and of the Co-ferrite NPs were investigated. Figure 11 shows the T = 3 and 300 K hysteresis loops for both samples while Table 1 summarizes the main magnetic properties. At low temperature, T = 3 K (Fig. 11a), it was observed that both samples exhibit well open hysteresis loops characteristics of these ferrimagnetic nanomaterials. Ms at 50 kOe is 77 emu g-1 for Co ferrite, and 41 emu g-1 for Co ferrite/Au. These values are smaller than Co ferrite bulk value, 88 emu g-1. In the case of the hybrid samples, this value is expected due to Au’s mass contribution. In the case of bare ferrite NPs, it is observed that even at 50 kOe, hysteresis loops are not closed, and magnetic saturation was not reached. Hence, since the real Ms value was not reached during the measurement, it is reasonable for the calculated magnetization at 50 kOe to be smaller than that for bulk CoFe2O4. The more evident fingerprint of the Co-ferrite NPs is the large coercive field (Hc) that both loops present. CoFe2O4 sample had a Hc ca. 7.4 kOe, while CoFe2O4/Au NPs


had a Hc ca. 8.9 kOe. In fact, it has been widely evidenced that when Co2? ions replace Fe2? at the octahedral site in the spinel structure (Xie et al. 2006; Fan et al. 2010), there is a rise in the magnetocrystalline anisotropy (Vestal et al. 2004; Chakrabarti et al. 2005; Ahmed et al. 2010). Considering the simple model of Stoner Wolfarth (Tannous and Gieraltowski 2008), Hc is proportional to the effective magnetic anisotropy, Kef, and inversely proportional to the magnetization saturation. In such case, Kef / A 4pMs q Hc , where q is the mass density and A is a constant that depends on the symmetry of magnetic anisotropy (Antoshina et al. 2003). Taking as example A = 1 (normally [2), we obtain Kef is 4.7 9 107 erg cm-3 that is larger than that of bulk CoFe2O4 (2 9 106 erg cm-3) in agreement with many other studies. Often this increase of the anisotropy in NPs is associated to surface or spin glass effects (Peddis et al. 2010, 2011). However, in the case of the Co-ferrite NPs other explanations is possible, several studies have found Mr/Ms values between 0.6

Fig. 10 Schematic representation of the core@shell CoFe2O4/Au structure synthesised by the sonochemical technique

Fig. 11 Magnetization curves for ferrite NPs of 12 nm mean size at RT (a) and 5 K (b)

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Table 1 Magnetic properties of ferrites nanoparticles of 12 nm mean size Ferrite

300 K Hc (Oe)

3K -1

Ms (emu g )

-1

Mr (emu g )

Mr/Ms

Hc (Oe)

Ms (emu g-1)

Mr (emu g-1)

Mr/Ms

CoFe2O4

420

62

9

0.15

7,400

77

42

0.60

CoFe2O4/Au

518

32

7

0.22

8,900

41

29

0.67

and 0.8 (Ngo et al. 2001; Maaz et al. 2006; CedenõMattei et al. 2010; Fantechi et al. 2012), where Mr/Ms around these values (0.7–0.8) are fingerprint of cubic magnetocrystalline anisotropy (Usov and Peschany 1995) while the 0.5 value is characteristic of uniaxial anisotropies. This symmetry of the anisotropy is often observed in ferrite NPs when surface effects are domain on the cubic magnetocrystalline anisotropy. In our samples, Mr/Ms at T = 3 K for bare Co-ferrite NPs is 0.6, a value that increases to almost 0.7 in Co-ferrite/ Au NPs. Both values indicate that the effective magnetic anisotropy, in bare and caped Co-ferrite NPs, is an interplay between the surface (uniaxial) anisotropy and the core (cubic) anisotropy surface of the bulk Co ferrite (Slonczewski 1958). Analyzing the coercive field for both samples, it was observed that for bare CoFe2O4 NPs, its value 7.2 kOe increases to 8.9 kOe in the capped ones. Several reasons can be provided to explain such increase, like the change of the magnetic surface of core@shell system when capped with Au (Maaz et al. 2006; Fan et al. 2010; Gyergyek et al. 2010) as previously was discussed; or the modifications of the reversal process due to the change of the interparticle interactions by increase of the interparticle distance because of the capping. However, we will point out a more basic difference that has straightforward influence, and this is the composition of Co ferrite; bare Co-ferrite NPs are richer in cobalt Fe/Co = 1.8, in contrast with the almost stoichiometric Fe/Co = 2.1 for Co ferrite/Au. In particular, magnetic anisotropy depends on the Fe/Co (Tachiki 1960; Fantechi et al. 2012), where for a maximum value of magnetic anisotropy is for Fe/Co near three, i.e., for ferrites richer in Fe (Kishimoto et al. 1979; Calero-DdelC 2007), what explains the largest coercive field observed in the composite. Figure 11b shows that both samples exhibit open hysteresis loops at 300 K indicating that NPs are not superparamagnetic at this temperature as it is common

in most ferrite NPs. Superparamagnetism is a thermal driven demagnetization process where magnetic NPs exhibit no hysteresis and thus have zero magnetic coercivities and no remanence magnetization. The temperature above which NPs are fully superparamagnetic (blocking temperature) is proportional to the effective anisotropy and the particle volume. In Coferrite NPs, due to the high magnetic anisotropy, complete thermal demagnetization is not reached but this effect decreases Hc and Mr/Ms with respect to the values at 3 K. Regarding the differences between bare and capped NPs it was observed at low temperatures that Mr/Ms and Hc are higher in the capped ones, reaching a value of Hc = 420 Oe for Co ferrite and 520 Oe for Au capped NPs. These results confirm the improvement of the hard magnetic properties of Coferrite NPs when these are capped with Au.

Conclusion The core@shell nanosystem CoFe2O4/Au was achieved by sonosynthesis procedure. Even if several characterization techniques like XRD, TEM, and UV– Vis spectroscopy were used a coherent description of the growth of the core@shell morphology was not reached. The characterization of the material was performed, and the nature of the material was confirmed by CV using GPE. This last analytical technique offered great advantages in contrast with the other techniques used. It consisted of low cost simple equipment, which runs at room temperature. The system used to perform the analysis was very simple, and the measurements were obtained using nontoxic substances. The magnetic properties of CoFe2O4/Au NPs are defined by the presence of Au. The reduction in the magnetization density value and its high anisotropy are due to its composition. In fact, the composite exhibits a higher magnetic anisotropy than bare Co

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ferrite. Actually, NPs are blocked at room temperature with average coercive field. Furthermore, these hybrid NPs exhibit also optical features due to the plasmonic response of Au, which was confirmed by a weak absorption band at around 550 nm in the UV/Vis spectrum. Moreover, as confirmed by voltammetric experiments, a Au shell assures chemical stability of Co ferrite, which makes it suitable for a variety of biomedical applications, such as biosensors, and in other bioanalytical uses. Acknowledgments The authors acknowledge funding support from the European commission U (NMP3-SL-2008-214107Nanomagma) and Fondazione Cariplo through the Project No. 2010–0612 ‘‘Chemical synthesis and characterization of magnetoplasmonic nano-heterostructures’’.

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