a facile method for the synthesis of co-core au-shell nanohybrid

This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem., 2014, 38, 4107--4114 | 4107

Cite this: NewJ.Chem., 2014,

38, 4107

A facile method for the synthesis of Co-coreAu-shell nanohybrid†

Debasmita Sardar,ab S. K. Neogi,bc S. Bandyopadhyay,bc Biswarup Satpati,d

Ruchi Jain,e Chinnakonda S. Gopinathe and Tanushree Bala*ab

Heterostructured Co–Au core–shell nanoparticles have been synthesized by reducing AuCl4� ions on

cobalt nanoparticles after a minor but effective modification of cobalt surface by an amine. The core shell

morphology is emphatically confirmed by thorough investigation through UV-Vis spectroscopy, X-ray

photoelectron spectroscopy (XPS), and transmission electron microscopic analysis (TEM). The chemical

composition and topography were determined using STEM-HAADF analysis and EFTEM imaging. Fourier

transform infrared (FTIR) spectroscopy confirms the surface modification of Co nanoparticles and the

interactions involved between the ligands and the core and shell metals at various steps of the synthetic

process. The magnetic properties confirm the material to be superparamagnetic in nature.

Introduction

Current literature has been flooded with physical and chemicalrecipes for the synthesis of structures such as spherical nano-particles,1,2 nanowires,3,4 nanorods,5,6 nanoprisms,5,7 and nano-tubes,8 etc., due to the fundamental study of property-size basedrelationships along with diverse application potential. Among thevarious groups of nanomaterials, magnetic nanostructures9,10 haveattracted considerable attention due to their potential applicationin optics,9,11,12 electronics,13 catalysis,14 targeted drug deliverysystems,15 etc. The superparamagnetic16 properties of the magneticnanostructures are found to be imperative for biomedical applica-tions such as cell separation,17 biosensors,11 hyperthermia oftumours and MRI techniques.18 While their synthesis has beenpredominant, the trickiest part is to generate non-agglomeratedmagnetic nanoparticles, which are stable against aerial oxidation.In particular, magnetic metallic (Co, Ni, Fe)19 nanoparticles areextremely prone to oxidation with only a few studies reportingtheir synthesis. Enveloping the particles with noble metals hasserved as a route to overcome the quandary, and synthesizing these

nanoparticles in an organic medium20 with suitable ligands isanother common way-out. Though organic-medium based synth-esis can avoid aerial oxidation, here the particles have severelimitations towards their application for biological purposes.21 Thisis definitely a loophole in the synthesis of magnetic nanomaterialsin an organic medium. Moreover, the use of toxic solvents,20,22 hightemperature23 and sophisticated experimental setup make thesynthetic process further complicated. On the other hand, aqueousphase based techniques mainly involve reverse micelle-basedmethods where the yield of the product is relatively low. Veryrecently, Prasad and co-workers reported a facile water basedtechnique to generate monodispersed magnetic nanoparticles.24

The surfactants present on the surface of these nanoparticles canstabilize the particles against immediate aerial oxidation but theformation of oxide layers with time can not be overruled comple-tely.25 Both the stability and biocompatibility can be improved forthese particles by creating a bio-friendly interface on magneticnanoparticles by growing a gold shell. Gold-coated magnetic parti-cles have been widely reported. However, the use of strong reducingagents (e.g. NaBH4)26 in aqueous medium or carrying out thereaction in an organic medium has their own merits and demerits.Cheon and his group reported organic phase-based Au-shell for-mation under reflux conditions using TOP as a capping agent.Using an uncommon chemical-stabilizer, inert atmosphere27 ornon-green solvents28 still furnishes a lacuna in this regard.

Thus, in this paper, we report the conversion of water-basedcobalt nanoparticles to a stable core–shell system by creating agold shell via an amine mediated reduction method. Pristinecobalt nanoparticles have surfactants on the surface, which areprobably partially replaced by an amino acid, L-tryptophan(Trp). This in turn can act as the reducing agent yieldingCo-core Au-shell nanostructures. It is envisaged that core–shell

a Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata-700009,

India. E-mail: [email protected] CRNN, University of Calcutta, JD 2, Sector III, Salt Lake, Kolkata-700098, Indiac Department of Physics, University of Calcutta, 92 A.P.C. Road, Kolkata-700009,

Indiad Surface Physics and Material Science Division, Saha Institute of Nuclear Physics,

1/AF Bidhannagar, Kolkata-64, Indiae Catalysis Division and Center of Excellence on Surface Science, CSIR – National

Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411 008, India

† Electronic supplementary information (ESI) available: Zeta potential measure-ment data, TEM, STEM images of Co nanoparticles and detailed particle sizeanalysis for Co and Co-core Au-shell nanoparticles. See DOI: 10.1039/c4nj00733f

Received (in Montpellier, France)6th May 2014,Accepted 5th June 2014

DOI: 10.1039/c4nj00733f

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structures of a Co-core and Au-shell will not only render cobaltnanoparticles more chemically robust but also provide mani-pulative ability to modify the surface because Au is well-knownfor its ability to interact with a range of ligands besides makingthe structures bio-friendly. Ryan and co-workers have alreadyreported such an approach using semiconductor–gold hybridswhere Cyt C proteins molecules were immobilized on Au viasuitable ligands.29 Co and Co-core Au-shell nanomaterials werecharacterized by UV-visible spectroscopy, fluorescence spectro-scopy, high resolution transmission electron microscopy(TEM), Fourier transform infrared (FTIR) spectroscopy andX-ray photoelectron spectroscopy (XPS). The measurement ofmagnetic properties using VSM reveals the retention of super-paramagnetic character for the core–shell. The details of theinvestigation are presented below.

Experimental sectionMaterials

Cobalt chloride hexahydrate (CoCl2�6H2O), sodium dodecylsulfate (SDS) (C12H25NaO4S), oleic acid (cis-9-octadecenoic acid,C17H33COOH), L-tryptophan (Trp) (C11H12N2O2), sodium boro-hydride (NaBH4) and gold chloride (HAuCl4) were purchasedfrom Sigma-Aldrich and SRL and utilized as received.

Preparation of Co nanoparticles

In a typical experiment, 10 mL of an aqueous solution of1 � 10�2 M cobalt chloride (CoCl2) was taken with 10 mLaqueous solution of 1 � 10�1 M sodium dodecyl sulfate (SDS)and 1 mL of a methanolic solution of 1 � 10�2 M oleic acid.Because the total volume of the aqueous solution needs to bemaintained at 100 mL, the rest of the mixture was made up to100 mL with double distilled water. Then, 0.025 g of solidsodium borohydride (NaBH4) was added to initialize thereduction of cobalt ions. After the addition of the reducingagent, the reduction started and the solution turned grey-black almost immediately. To ensure complete reduction, thesolution was maintained at ambient conditions for 2 h. Then,the blackish solution was subjected to repetitive centrifuga-tion at 8000 rpm for 20 min, followed by the separation of thesupernatant and pellets. The pellets obtained were washedwith double distilled water and re-centrifuged with waterunder the abovementioned conditions. The pellets, after thesecond phase of centrifugation, were processed suitably forcharacterizations using UV-Vis spectroscopy, FTIR, TEM, ZETApotential measurement, XPS and VSM.

Preparation of Co-core Au-shell nanoparticles

After the formation of the Co nanoparticles, Trp was incorpo-rated to modify the surface. For this, an aqueous solution ofL-tryptophan was added to make the concentration of theoverall solution to 1 � 10�2 M. Various characterizations, e.g.FTIR, UV-Vis spectroscopy, ZETA potential measurement,fluorescence spectroscopy were carried out at this stage toensure the incorporation of Trp. Then, the Co nanoparticles

were allowed to react with a 1 � 10�3 M aqueous solution ofHAuCl4. The solution was then stirred overnight and centri-fuged at 8000 rpm for 20 min, followed by the separation ofsupernatant and pellets. The pellets were then collected andseparated for various characterizations, including magneticmeasurements. The different steps of the reaction are pre-sented in Scheme 1.

Characterization

UV-Vis spectroscopy. UV-Vis spectra were recorded by aPerkin-Elmer 25 Lambda UV-Vis spectrometer to monitor theoptical properties of the Co-core and Co-core Au-shell nano-particle solutions at a resolution of 1 nm.

Fluorescence spectroscopy. Fluorescence spectra wererecorded on a Perkin-Elmer LS55 fluorescence spectrometerfor the Trp modified Co nanoparticles and then for the Co-coreAu-shell nanoparticles.

Zeta potential. Zeta potential was evaluated by a BeckmanCOULTER DelsaTM NANO C particle analyzer to measure thezeta potential at the surface of the three solutions atdifferent steps.

Fourier transform infrared spectroscopy. Fourier transforminfrared (FTIR) spectroscopy was obtained on a Perkin-ElmerSpectrum 100 FT-IR spectrometer at a resolution of 4 cm�1 toevaluate bonding interactions at different steps after mixing thepowder samples with potassium bromide. The FTIR spectra ofpure SDS, Oleic acid and L-tryptophan were recorded forcomparison.

Transmission electron microscopy. Transmission electronmicroscopy (TEM) investigations were recorded using a FEI,Tecnai G2 F30, S-TWIN microscope operating at 300 kV. High-angle annular dark field scanning transmission electronmicroscopy (STEM-HAADF) was employed using the samemicroscope, which was equipped with a scanning unit and aHAADF detector from Fischione (model 3000). Compositionalanalysis was performed by the energy dispersive X-ray spectro-scopy (EDS, EDX Inc.) attachment on the Tecnai G2 F30.Energy-filtered TEM (EFTEM) measurements were carriedout using GIF Quantum SE (model 963). Samples for theseanalyses were prepared by simply drop casting the solution ona carbon coated copper grid and allowing the solutions to dry.

Scheme 1 Schematic representation of different steps involved in thesynthesis of CocoreAushell nanoparticles.

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X-ray photoelectron spectroscopy. X-ray photoelectron spec-tra (XPS) of the catalysts were recorded with a custom builtambient pressure photoelectron spectrometer (APPES) (Prevac,Poland) equipped with a VG Scienta’s R3000HP analyzer andMX650 monochromator.30 Monochromatic Al Ka X-rays weregenerated at 450 W and used for measuring the X-ray photo-electron spectrum (XPS) of the abovementioned samples. Basepressure in the analysis chamber was maintained in the rangeof 2 � 10�10 Torr. The energy resolution of the spectrometerwas set at 0.7 eV at a pass energy of 50 eV. Binding energy (BE)was calibrated with respect to Au 4f7/2 core level at 84.0 eV. Theerror in the reported BE values was within 0.1 eV. For peaksynthesis, a mixed Gaussian–Lorentzian function with a Shirleytype background subtraction was used. Samples were floodedwith low energy electrons for efficient charge neutralisation.

Vibrating sample magnetometer. Magnetic measurementsof the samples were performed by a superconducting quantuminterference device vibrating sample magnetometer (SQUID-VSM, Quantum Design). The magnetization measurements,dependent on both magnetic field and temperature havebeen performed. Field dependent magnetization (M–H) of thesample were measured at 300 K and 10 K. Temperaturedependent magnetization (M–T) were measured at 150 Oe,300 Oe and 600 Oe under zero field cool (ZFC) and field cool(FC) conditions.

Results and discussion

It is interesting to mention here that cobalt nanoparticlesprepared by the abovementioned technique were stable againstquick aerial oxidation probably because of the capping ofsurface ligands, oleic acid and SDS. In this scenario, a trans-metallation reaction could have been an effective approach forgenerating core–shell structures.31,32 This was a processwherein the shell materials could be deposited on the core bya galvanic exchange reaction between the two components.This method definitely had several advantages. Oleic acidcapped Co nanoparticles showed a negative value during thezeta potential measurement (ESI,† S1). This hindered theapproach of AuCl4

� to cobalt surface when a transmetallationreaction was aimed. Because of adverse surface charge onpristine cobalt nanoparticles, the surface was first modifiedwith an amino acid, L-tryptophan (Trp), by overnight stirringthe Co nanoparticle solution with Trp. The introduction of Trpchanged the potential to positive (ESI,† S1). Trp has beenknown not only for its ability to reduce AuCl4

� to Au0 but alsobecause of its capping efficiency for Au nanoparticles.33

Modified Co nanoparticles were washed several times to avoidthe presence of excess Trp in the medium, which might act asnucleation points for discrete Au nanoparticles. The reduction ofAuCl4

� to Au0 was indicated by UV-Vis spectroscopy (Fig. 1A). TheUV-Vis spectrum of the pristine oleic acid capped Co nanoparticles(Curve 1, Fig. 1A) was almost featureless with a monotonic increasein absorbance with a decrease in wavelength, and agreed wellwith those reported for Co nanoparticles.34–37 Trp modified Co

nanoparticles showed characteristic absorption of Trp at 279 nmin the UV region (Curve 2, Fig. 1A).38 This indicated the adsorptionof amino acid onto the Co surface. The development of a broadsurface plasmon resonance peak centred around 606 nm (Curve 3,Fig. 1A) after the addition of aqueous HAuCl4 to the Trp modifiedCo nanoparticles was attributed to metallic gold in the nanoscaleregime.34 Moreover, the photoluminescence (PL) spectra of theTrp-Co nanoparticles showed appreciable intensity at 374 nm(Curve 1, Fig. 1B) when excited at 279 nm, which was dampenedconsiderably (Curve 2, Fig. 1B) after the formation of Au shell. Theplausible reason could be efficient non-radiative decay after theformation of Au-shell, and this observation was in good agreementwith previous reports.39–42 In addition to this, pristine cobaltnanoparticles did not have any signature of N (not shown in thepaper) but for the core–shell structure showed N along with C, O,Co and Au in the EDX analysis, which clearly hinted towards thepresence of Trp on the surface of the Co nanoparticles (Fig. 2A).

The incorporation of the amino acid Trp on the surface ofthe Co nanoparticles was ensured from detailed FTIR (Fig. 2B)and XPS (Fig. 3) results. Thorough FTIR studies revealed thatthe carboxylic acid group of oleic acid was shifted to 1642 cm�1

compared to 1707 cm�1 for pure oleic acid due to interactionswith cobalt nanoparticles (Curve 1, Fig. 2B).43 The FTIR spectrachanged drastically after the interaction with Trp, which wasconfirmed by the presence of bands characteristic to Trp at1231, 1590, 1667, 2077, 2582 and 3408 cm�1 (Curve 2, Fig. 2B).The terminal and indole amine groups of Trp were found to berelatively free along with a clear shift in asymmetric stretchingof the –COO� group from 1614 cm�1 (pure Trp)44 to 1590 cm�1.This indicated a direct interaction of the carboxylic acid groupof Trp to the cobalt surface, which agreed well with literaturereporting a favorable interaction between cobalt/nickel nano-particles and carboxylic acid group containing ligands.45 Thesignatures of the indole amine group at 2582 and 3408 cm�1

were considerably dampened after the reduction of gold, whichvalidated that the reduction was initiated by the amine group ofthe indole-ring (Curve 3, Fig. 2B).33 The strong stretchingfrequency of the amine group at 1667 cm�1 was also found tobe transformed to a broad peak, while the –COO� stretching at1590 cm�1 was observed to be undisturbed. The proposition ofthe incorporation of Trp and the formation of Au-shell was

Fig. 1 (A) UV-Vis spectra of Co (Curve 1), Trp modified Co (Curve 2) andCocoreAushell nanoparticle (Curve 3) dispersed in an aqueous medium.(B) Fluorescence spectra of Trp modified Co (Curve 1) and CocoreAushell

nanoparticles (Curve 2) in an aqueous medium.

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further validated by XPS analysis of the samples (Fig. 3). Thecore level binding energy (BE) of the Co 2p3/2 obtained from

pristine Co nanoparticles could be fitted to a single peakcentered at 782.0 eV BE with a second component at 786.8 eV(Fig. 3A). The lower binding energy component (782.0 eV) wasdue to Co0 states, whereas the higher binding energy peaksmight arise due to Co(OH)2/CoO. The possibility of adsorbedCo2+(oleate) complexes on the surface of these nanoparticlescould not be ignored, which could also account for the shift tohigher BE. Similar results in XPS analysis were observed byZanchet et al.,46 and Couto et al.47 for colloidal Ni synthesizedvia completely different routes but using oleic acid as a cappingagent, and also by Gupta et al. for silver loaded by doxorubi-cin.48 This agreed well with FTIR observations. FTIR (Curve 1,Fig. 2B) indicated the presence of an –OH stretching frequencyin pristine cobalt along with Co–oleic acid interaction. Thefeeble S 2p spectra was fitted to two spin orbit doublets at168.8 and 170.7 eV BE, arising from S 2p3/2 and S 2p1/2 levels ofthe –SO4 group of SDS, respectively,49 and thus very minuteamounts of SDS were present on the surface (Fig. 3B). Thescenario changed considerably after the formation of theAu-shell on the Co nanoparticles. No signature of Co 2p wasobserved after core–shell formation. Basically, in XPS, electronsejected from a point buried deeper from the surface experi-ences higher number of inelastic collisions than the surfaceelectrons, and thereby loses energy to such an extent that theycontribute exclusively to the low kinetic energy background.The results indicated metallic Co to be present away from thesurface, which was now completely covered with metallic Au.On the other hand, peaks of Au 4f and N 1s agreed well withthe plausible mechanism of core–shell formation. The N 1sspectrum was deconvoluted into two components at bindingenergies of 397.8 and 399.9 eV (Fig. 3C). The lower bindingenergy peak corresponded to the first amine group (inset,Fig. 2B) and the higher one to the 2nd amine group, whichwas attached to the indole ring of Trp, which was adsorbed onthe surface of the Co–Au hybrid nanoparticles.50 The XPS of Au4f7/2 and 4f5/2 spin orbit core levels (Fig. 3D) of the Co-coreAu-shell nanoparticles were observed to be centered at 84.0 eV and87.7 eV BE, respectively, with an energy gap of 3.7 eV betweenthem. This undoubtedly corresponded to the Au0 state.51 Thisauthenticated our proposition of complete reduction of chloro-aurate ions in the presence of Trp.

Structural information was obtained using TEM. Pristine Conanoparticles are shown in ESI,† S2. The size distribution of thesame is presented in the ESI,† S3, as obtained from both TEMand DLS results. To investigate the chemical composition ofcore–shell structure, we performed STEM-HAADF analysis. Itprovided the Z-contrast image, where the intensity of scatteredelectrons is proportional to the square of the atomic number Z.Fig. 4A depicts the HAADF-STEM image of a Co/Au core–shellnanoparticle. EDX spectra from area 1 in Fig. 4A are plotted inFig. 4B. The spatial distributions of atomic contents across theCo/Au core–shell nanoparticles were obtained using an energydispersive X-ray spectroscopy (EDX) line profile. Fig. 4C showsthe EDX profiles for Co, and Au across line 2, as shown inFig. 4A. The high concentration of Co in the cores and thetendency for Au accretion to form a shell was clearly visible by a

Fig. 2 (A) EDX spectra analysis of the CocoreAushell nanoparticles. (B) FTIRspectra of the Co nanoparticles (Curve 1), Co after surface modificationwith Trp (Curve 2), and the same particles after the formation of the Aushell (Curve 3). (Inset B) The chemical structure of L-tryptophan.

Fig. 3 XPS of (A) Co 2p (B) S 2p from the Co nanoparticles and (C) N 1s, (D)Au 4f from the CocoreAushell nanoparticles.

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highly magnified bright field TEM image (Fig. 4D). For detaileddistribution of Co and Au in the core–shell nanoparticles, weperformed elemental mapping using EFTEM, as illustrated inFig. 4E–J. Energy filtered images were acquired using a contrastaperture of about 10 mrad to reduce aberrations (mostly chro-matic). Chemical maps from Co M (60 eV), and Au O (54 eV)edges were obtained using the jump-ratio method by acquiringtwo images (one post-edge and one pre-edge) to extract thebackground with an energy slit of 4 eV for Co and Au. Thecomposite image in Fig. 4J gave a clear confirmation of thecore–shell structure and elemental composition spatially sepa-rated. EDX data (Fig. 4C) obtained for these hybrid particlessuggested the presence of a thin Au-shell on the Co-core.All these images provided strong concurrent evidence for thecore–shell structure.

The temperature dependence of magnetization for theCocoreAushell nanoparticles is shown in Fig. 5, where the appliedmagnetic field was maintained at three different values of150 Oe, 300 Oe and 600 Oe. Temperature was varied betweenroom temperature to 300 K and 5 K. Curve 1 in Fig. 5A–Ccorrespond to magnetization against temperature (M–T) plotunder ZFC conditions, while Curve 2 is the measurementcarried out under FC conditions. As can be clearly seen, thecurves of temperature dependent ZFC and FC magnetizationswere typical of magnetic nanoparticles. Magnetic particlesbelow a certain size regime behaved as superparamagneticmaterials with characteristic features: (i) magnetization underZFC conditions measured using low magnetic fields displayeda maximum at a certain temperature, which is called theblocking temperature, TB, (ii) divergence in the magnetizationcurves measured under ZFC and FC conditions below theblocking temperature and (iii) magnetic hysteresis loop andremnant magnetization below TB, whereas hysteresis behaviourdisappeared above TB.

Core–shell particles showed a divergence between the mag-netization curves measured under ZFC and FC conditions up to187 K when the applied field was 150 Oe. Magnetization curves

with increasing field of 300 Oe diverged at relatively lowertemperature of 132 K (Fig. 5B), whereas these two curvesremained superimposed within the range of experimentaltemperature when the applied field was maintained at 600 Oe,(Fig. 5C). In fact, the bifurcation of magnetization curvesmeasured under ZFC and FC conditions should occur at rela-tively lower temperatures with increasing magnetic field, i.e.there would be a clear gradual lower temperature shift in TB

on increasing the applied field. This was also consistent withthe signature of the superparamagnetic nature of the sample.In general, for a superparamagnetic system at very low tempera-ture, spins were aligned and hence represented a frozen condi-tion. With little increase in temperature in presence of moderatemagnetic field (around hundreds of Oe) ordered spin orientationdevelops in this type of systems, resulting in an augmentation inmagnetization. Then, the system attained the highest magneti-zation at a particular temperature (blocking temperature, TB).Both thermal excitation and low applied magnetic field playedsignificant roles in orienting the spins or reaching the maximummagnetization. On further increasing the applied magnetic field(from 150 Oe to 300 Oe to 600 Oe), it became easier to achieve themaximum parallel spin orientation (maximum of magnetization),

Fig. 4 TEM images of the CocoreAushell nanoparticles (A, D–J), (B) EDXspectra obtained from the same region, as shown in the previous figure,showing the presence of both Co and Au, (C) chemical mapping of a singlenanoparticle highlighted in image (A).

Fig. 5 Temperature dependent magnetization of CocoreAushell nano-particles. The applied magnetic field was (A) 150 Oe. (B) 300 Oe. (C) 600 Oe.

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and hence a shift in TB at lower temperatures was observed. Thisnature is clearly demonstrated in Fig. 5. However, at low tempera-tures, (below 25 K) there was a sudden increase in magneticmoment, which might indicate the presence of free paramagneticspins in the system. Blocking temperature was actually related tothe size of the magnetic particles and the magnetocrystallineanisotropy constant (K) by the equation K = 25kBTB/V, where kB

and V were the Boltzmann constant and the volume of singleparticle, respectively, and TB was the blocking temperature. Bysubstituting the values for a B77.3 nm particle (average particlesize obtained from the particle size distribution calculation fromthe TEM images, ESI,† S4), we deduced K to be 26.7 � 102 ergcm�3, which was found to be much lower than bulk cobalt(45 � 105 erg cm�3).52 The difference between the values wasexpected because we calculated K using the average particle sizeand not the individual crystallite size, which actually controlled theindividual magnetic directions of the grains, and thus the blockingtemperature. It was worth noting that the magnetic core wasisolated in the diamagnetic gold shell but the magnetizationcurves under ZFC and FC conditions were devoid of a clearblocking temperature at a very low applied magnetic field due toslight polydispersity in particle size. In addition to this, severalother phenomenon related to the nanoscale regime such asstructural disorder, surface anisotropy, non-magnetic or weakmagnetic interfaces, lack of surface coordination for surfacemagnetic atoms and electron exchange between the capping agentand surface atoms are also known to influence magnetic proper-ties.53 One or several of these phenomena in tandem could alsohave resulted in the smaller values of the magnetic parametersobserved in our systems.

The field dependent magnetic behaviour data measured at10 K (below the blocking temperature, Fig. 6a) and at 300 K (wellabove the blocking temperature, Fig. 6b) are displayed in Fig. 6.It clearly indicated the strong hysteresis nature at low tempera-ture (10 K) and weak hysteresis nature at room temperature

(300 K), which indicated that the system possesses ferromagnetic(FM) ordering. In the high field region, M–H curves indicatedthat magnetic moments were unsaturated. This might bebecause of the coexistence of a paramagnetic (PM) phase alongwith the ferromagnetic (FM) phase.54 Typically, the featureobserved in the M–H curves was in accordance with thoseexpected for superparamagnetic nanoparticles. The M–H curvesof the sample at both 10 K and 300 K did not fit well with theBrillouin-function as should be the case of a purely PM or FMsystem. However, it fits quiet well with eqn (1) given below:55

M(H) = (2MSFM/p)tan�1[(H � Hci)/Hci tan{(pMR

FM)/2MSFM}] + wH

(1)

Respaud et al.55 described eqn (1) consisting of both FM andPM components, where MS

FM, MRFM, Hci and w were the saturation

magnetization, remanent magnetization, the intrinsic coercivity,and PM susceptibility, respectively. Subtracting the PM partsfrom experimental data, saturation of magnetization in the highfield region could be observed (as indicated by the red linesin Fig. 6). Saturation magnetic moment below the blockingtemperature (10 K) was significantly high (nearly 10 ten timesor I order of magnitude) in comparison to the saturationmagnetic moment above the blocking temperature (300 K). Thistype of significant or complete depletion of ferromagnetismabove the blocking temperature was a familiar feature for super-paramagnetic nanoparticles.56 The estimated ratio of FM to PMcontributions at 10 K was calculated to be 0.661 and the ratioremained same (within the experimental and calculation error)at 300 K, the exact calculated value was found to be 0.667. Boththe calculations were performed at H = 4000 Oe. It was indicativeof the regular tendency of magnetization and confirmed theabsence of carrier or the defect mediated FM nature of thesample.57 The constancy of the ratio of FM to PM moments at10 K and 300 K suggested that FM ordering originated from theCo–Au core shell structure and reflected the superparamagneticnature of Co nanoparticles.

Conclusion

An easy and effective method for the synthesis of cobalt-coregold-shell nanoparticles has been reported at ambient tempera-tures. Cobalt nanoparticles prepared were initially capped withsurfactants and hindered the approach of AuCl4

� for transme-tallation. Thus, the system behaved differently from its analo-gous Co-core Ag-shell hybrids where surfactants capped Conanoparticles could be used directly for transmetallation withAg+. A minor but extremely effective surface modification ofcobalt nanoparticles was successfully achieved to facilitateAu-shell formation: a comparative investigation is in progress.More importantly, the synthetic strategy mentioned here waspurely water based and did not involve costly and/or toxicchemicals at any stage of the process. This definitely suggestspotential applications including biological systems. The mag-netic characteristics of the hybrid nanoparticles were studied

Fig. 6 Magnetization against magnetic field (M–H) variation at (a) 10 Kand (b) 300 K. The solid blue line represents the theoretical fit of theexperimental M–H curves. The red data points represent saturationmagnetization after the subtraction of the paramagnetic contributionsfrom the experimental data points.

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extensively and were found to be in accordance with thoseexpected for nanoscale superparamagnetic particles. The blockingtemperature where the magnetic directions of individual grainsessentially remain invariant was determined to be B187 Kfor these particles at an applied field of 150 Oe, which shiftedto a relatively lower temperature with an increase in appliedexternal field.

Acknowledgements

TB acknowledges financial support (Project No. Conv/162/NanoPr 2011) from the Centre for Research in Nanoscience andNanotechnology (CRNN), University of Calcutta. DS and RJacknowledge CRNN and CSIR, respectively, for research fellow-ship. CSG acknowledges the partial financial support fromCSC0404. CRNN is also acknowledged for the FTIR, DLS andVSM instrumental facilities.

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a facile method for the synthesis of co-core au-shell nanohybrid

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