Continuous production of water dispersible carbon–iron nanocomposites by laser pyrolysis: Application as MRI contrasts

Journal of Colloid and Interface Science 313 (2007) 511–518 www.elsevier.com/locate/jcis

Continuous production of water dispersible carbon–iron nanocomposites by laser pyrolysis: Application as MRI contrasts Y. Leconte a , S. Veintemillas-Verdaguer b,∗ , M.P. Morales b , R. Costo b , I. Rodríguez c , P. Bonville a , B. Bouchet-Fabre a , N. Herlin-Boime a a CEA Saclay, 91191 Gif/Yvette cedex, France b Instituto de Ciencia de Materiales de Madrid, Cantoblanco, 28049 Madrid, Spain c Universidad Complutense de Madrid, Unidad de RMN, Paseo Juan XXIII, 1, 28040 Madrid, Spain

Received 2 February 2007; accepted 3 May 2007 Available online 22 May 2007

Abstract Carbon encapsulated iron/iron-oxide nanoparticles were obtained using laser pyrolysis method. The powders were processed to produce stable and biocompatible colloidal aqueous dispersions. The synthesis method consisted in the laser decomposition of an aerosol of ferrocene solution in toluene. This process generated, in a continuous way and in a single step, a nanocomposite formed by amorphous carbon nanoparticles of 50–100 nm size in which isolated iron based nanoparticles of 3–10 nm size are located. The effect of using different carriers and additives was explored in order to improve the efficiency of the process. The samples after purification by solid–liquid extraction with toluene, were oxidised in concentrated nitric acid solution of sodium chlorate, washed and finally ultrasonically dispersed in 1 mM tri-sodium citrate solutions. The dispersions obtained have hydrodynamic particle size less than 150 nm and are stable in the pH range of 2–11. Finally the shortening of the transversal relaxation time of water protons produced by the dispersed particles was studied in order to test the feasibility of these systems to be traced by magnetic resonance imaging techniques. © 2007 Elsevier Inc. All rights reserved. Keywords: Magnetic particles; Laser pyrolysis; Nanocomposites; Fe/C

1. Introduction Due to their high magnetic response, biocompatible dispersions of iron nanoparticles have high potential use in applications like contrast agents for magnetic resonance imaging (MRI) or magnetothermia. Additionally, they could be seen as magnetic platforms that after convenient functionalisation could be advantageously used in drug delivery, or biosensor technology. Unfortunately, iron nanoparticles with a large surface area are easily oxidised, i.e. they react vigorously with the oxygen present in the air and also between themselves forming aggregates. This fact prevents their use in favour of the less active but more stable iron oxides.

* Corresponding author.

E-mail address: [email protected] (S. Veintemillas-Verdaguer). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.05.010

The incorporation of iron nanoparticles into an organic or inorganic polymeric matrix to form a nanocomposite could ease the transportation, storage and use of these materials by preventing the oxidation [1]. Among the inorganic polymers considered for this purpose, carbon is preferred due to its biocompatibility and rich surface chemistry [2]. The dispersion procedure for producing stable aqueous dispersions in spite of the hydrophobic nature of the graphitic carbon by adsorption of amphiphilic copolymers has been reported [3]. Also the unspecific absorption capacity of the carbon could be profited for drug delivering procedures [4] or detoxification of biological fluids [5]. Carbon encapsulated magnetic nanoparticles (C@FE) have been produced by impregnation of active carbon with an iron precursor followed by particle synthesis by reduction [6]. Relatively big (D > 100 nm) C@FE nanoparticles could be produced by chemical reaction among hematite and carbon at 1200 ◦ C [7] and by iron segregation from transient FeC2 gen-

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erated in situ by reaction of FeCl2 and CaC2 in acetonitrile at 250 ◦ C [8]. Small Fe based nanoparticles encapsulated in a continuum carbon matrix could be obtained at large scale by cocarbonisation of 1,2,4,5-tetramethylbenzene (durene) and ferrocene [9] or, in a more controlled way, by heating a polyimide/ Fe/polyimide thin film [10]. In this case, the magnetic core and the protective coating are not disaggregated in the form of small nanoparticles, which is mandatory for the preparation of colloidal suspensions. The synthesis of C@FE nanocomposites suitable for the preparation of colloids can be achieved in a single process by carbon arc methods involving the evaporation of a metal doped graphite anode in an inert atmosphere [11–14] or by electric plasma discharge in a ultrasonic cavitation field in liquid ethanol [15]. The arc synthesis method produces encapsulated nanoparticles together with sootlike material, including carbon nanotubes and graphitic particles, which made their purification and separation difficult. More often the Chemical Vapour Condensation technique is used for the same purpose [16– 19]. This technique consisted in a controlled decomposition of an organometallic precursor in a furnace. The experimental conditions and the precursors are chosen to avoid the formation of soot material. Although the metal load of the powders reach the 20% or even higher their particle size distribution is broad with particle diameters ranging between 10 and 100 nm [17]. The particle size distribution of the metal nanoparticles was improved by the detonation-induced decomposition of ferrocene/picric acid mixtures reaching the range 5–20 nm size, but the process is not suitable for high or moderate productions and generates carbon nanotubes as by-products [20]. Recently a continuous synthesis of C@FE nanocomposites has been reported by spray pyrolysis using iron pentacarbonyl and ethanol as iron and carbon precursors. The obtained products had 30– 50 nm of particle size and a thin graphite coat of 3–6 nm. These ferromagnetic particles strongly coupled by magnetic forces as produced can hardly be used in the preparation of magnetic colloids, but they could have high potential in magnetic recording technologies [21]. To overcome the above mentioned limitations the laser pyrolysis technique has been employed in previous works for the preparation of carbon coated iron nanoparticles using iron pentacarbonyl as iron precursor and ethylene/acetylene mixtures [22,23] or toluene [24] as carbon donors. This process combined the fast laser heating of the gaseous precursor together with the fast cooling of the reaction products. Two kind of materials were obtained, homogeneous C@FE composites consisting of quite uniform iron nanoparticles (3–7 nm size) distributed in a continuous carbon layer [22] and rather inhomogeneous composites composed by a mixture of carbon based 18 nm size particles in which 3–6 nm size Fe3 C/α-Fe particles are distributed and bigger carbon coated Fe3 C particles of around 10–13 nm size [24]. It should be noted that in those works, a poisonous Fe precursor such as Fe(CO)5 was employed and no attempts for the preparation of water dispersions with these materials were made. The objective of the present work is to explore the possibilities of the laser pyrolysis technique for the production of stable

dispersions of magnetic C@FE nanoparticles for the production of new contrast agents in magnetic resonance imaging (MRI) and any other possible biological application. To achieve this goal, the combination of the laser synthesis of powdered samples and its chemical processing has been carried out to make stable aqueous dispersions at pH 7. Special care has been taken in using a less toxic iron precursor, such as ferrocene, instead of iron pentacarbonyl, and no polymeric surfactants in order to improve the safety of the process and the biocompatibility of the dispersions. 2. Experimental 2.1. Materials The chemicals used in this study and their sources were as follows: Ferrocene (98% purity), oleic acid (90% purity), glycidyl methacrylate (97% purity) and sodium chlorate (>99% purity): Sigma–Aldrich (USA). Tri-sodium citrate dihydrate (>99.5% purity): Fluka (Switzerland). Toluene (99% purity): Riedel-de Haën (Germany). Fumant nitric acid (90% purity): Carlo Erba (Italy). Analytical grade hydrochloric acid (99.99% purity): Panreac (Spain). Argon (99.999% purity), ethylene (99.9% purity) acetylene (>99% purity) and anhydrous ammonia (>99% purity): Praxair (USA). Cellulose dialysis tube 12000 D cut off: Sigma–Aldrich (USA). 2.2. Methods 2.2.1. Preparation of the iron/carbon nanoparticles The method of synthesis involved heating a flowing solution precursor in aerosol form with a CO2 laser which initiates and sustains the chemical reaction [25]. The laser employed in this work is the Trumpf TLF2400 with a maximum power of 2400 W and a spot size of 15 mm operating at pulse frequency of 20 kHz. The reactive flow of gas was confined to the flow axis by a coaxial argon stream. Additional argon flows were used in order to avoid the deposition of powder on the laser windows. Once generated the particles are carried to a filter by means of the argon flow. The solution precursor was nebulized by using a Pyrosol 721 system (RBI Company, Meylan, France) and the cloud formed was dragged by the carrier gas flow to the reaction zone. Due to the no-absorption of the precursor solutions at the laser wavelength (10.60 ± 0.05 µm), ethylene or ammonia were components of the carrier gas. In this work the influence of additives as oleic acid and glycidyl methacrylate was studied. Both substances have been previously used for maghemite nanoparticles coating, the oleic acid as biocompatible surface modifier [26] and the glycidyl methacrylate as the monomer for polymer encapsulation [27]. See Table 1 for the experimental conditions and sample identification. 2.2.2. Characterisation of the powders Thermogravimetric analysis of the samples was performed in a Sheiko, TG/DTA 320U thermobalance under air flow (100 ml/min) to produce the oxidation of the sample during heating. Alumina (11.76 mg) was used as reference material.

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Table 1 Experimental details and characterisation of C@FE composites prepared by laser pyrolysis after purification by solid–liquid extraction Sample Id.

Precursor composition

Carrier gas (sccm)

Production rate (g/min)

Composition (w%) C

H

N

O

Fe

d002 C (Å)

Size of the magnetic cores, TEM (nm)

No liquid precursor Toluene 11% ferrocene 89% toluene

400 C2 H4 415 Ar ” ”

0.13

94.5

2.1

0

3.4

0

3.593

−

0.27 0.29

94.6 92.4

2.0 1.5

0 0

3.4 4.9

0 1.2

3.593 3.542

CFE2

11% ferrocene 89% toluene

0.25

93.8

1.1

0

4.5

0.6

CFE3

11% ferrocene 89% toluene 11% ferrocene 11% oleic acid 77% toluene 11% ferrocene 11% GLYM 78% toluene

100 C2 H4 300 C2 H2 415 Ar 400 NH3 415 Ar 400 C2 H4 415 Ar

0.13

90.8

0.9

2.5

3.2

0.30

91.9

1.5

0

”

0.29

92.9

1.4

0

C1 T1 CFE1

CFE4

CFE5

Solid properties Particle size, BET (nm)

Saturation magnetisation (emu/g Fe)

91

−

− 6±3

111 70

− 124

3.590

4±2

67

190

2.2

3.549

6±4

48

70

5.0

1.6

3.527

7±3

71

111

4.0

1.7

3.549

6±2

68

100

Note. Other experimental conditions are: Incident laser power 850 W (15 mm spot diameter), coaxial flow 2500 sccm Ar, windows cleaning flow 3300 sccm Ar, precursor consumption rate 0.4 g/min. Average duration of the experiment 90 min, 10% of absorbed power and approximately 1100 ◦ C of temperature (Optical Pyrometer).

The initial weight of the sample was 15 mg. The samples were heated from 25 to 900 ◦ C at 10 ◦ C/min. The iron content of the sample was determined assuming that the residual weight that remains after the oxidation of the samples is haematite. Carbon, hydrogen and nitrogen were determined by elemental analysis (PERKIN ELMER 2400CHN). The oxygen present in the samples was estimated by difference. X-ray diffraction patterns (XRD) were recorded between 10 and 100◦ (2θ ) at 0.5 ◦ /min in an X-ray diffractometer (PHILIPS PW1710) with CuKα radiation. Particle size and the distribution of the individual magnetic crystals in the carbon matrix were determined with a 200 keV JEOL JEM-2000FX transmission electron microscope. For TEM examination, powders were ultrasonically dispersed in distilled water and a drop of this suspension was deposited onto a carbon coated copper grid. About one hundred particles were measured to evaluate the number averaged diameter of the particles and the standard deviation. The single point BET determination of the specific surface area S (m2 /g) of the material has been performed with nitrogen at 77 K in a Flowsorb II 2300 apparatus from Micromeritics. S was further used to estimate the particle size d BET (nm) (assuming that its density ρ is the same as the carbon 2 × 106 g m−3 ) according to d BET =

6 × 109 . ρS

(1)

For selected samples the micro- and meso-porosities were determined from the adsorption/desorption isotherms of N2 at 77 K obtained in a Micromeritics ASAP 2010 apparatus after out-gassing the samples 16 h at 573 K under high vacuum. Magnetic characterisation of the nanoparticles was carried out in an Oxford Instrument model MLVSM9 MagLab 9 T vibrating sample magnetometer, at room temperature and 5 K with a

maximum applied field of 5 T. The magnetisation values were normalised to the amount of iron to yield the specific magnetisation (emu/g Fe) and the saturation magnetisation (MS ) was evaluated by extrapolating to 1/H = 0 the M values in the high field region. Mössbauer data were recorded in a standard spectrometer worked with a maximum velocity of 10 mm/s using a 57 Co:Rh source at temperatures of 4.2 and 298 K. The measured spectra were fitted using a standard minimisation of χ 2 procedures, where the relative areas of the spectrum components gave the atomic fraction of the Fe atoms in the sample. 2.2.3. Colloid preparation In order to produce the colloidal suspensions in water using the as synthesised hydrophobic C@FE powders, the excess of ferrocene precursor was first removed by solid–liquid extraction with toluene using a Soxhlet apparatus. Chemical oxidation following a well established process described in the literature that generates carboxylate groups on the carbon surface [28] was subsequently performed. These groups favour the stabilisation of the water dispersions and provide reactive surface sites for further functionalisation of the nanoparticles with bio-molecules. In detail the oxidation–dispersion process is the following: 100 mg of the purified sample was treated with a mixture of 850 mg of sodium chlorate and 1 ml of 65% nitric acid. The mixing process must be taken very carefully and in an ice bath in order to avoid acid projections. In order to complete the reaction the mixture was heated in a water bath at 80 ◦ C for three days. After the oxidation the excess of reactants was removed by washing with hydrochloric acid 2 N separation by centrifugation and elimination of the supernatant solution three times. After a second washing process with distilled water, the solution was dialysed in front of 5 l distilled water for 24 h. The

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solution volume was adjusted to 15 ml and tri-sodium citrate dihydrate (1 mM) was added in order to adjust the pH to physiologic. Finally an ultrasonic dispersion procedure was made in order to reduce at maximum the particle size and generate the final product. The sonication was obtained by means of a Vibra Cell Probe (Sonics and Materials Inc.) employing 1 s duration pulses of 11 W during 10 min.

47/40 NMR spectrometer [30]. The T1 and T2 values were converted to relaxivity in s−1 /mM Fe by subtracting the buffer contribution and dividing by the Fe concentration. Finally the colloidal properties and the stability of the C@FE particles were studied by measuring the zeta potential of the particles as a function of pH. 3. Results and discussion

2.2.4. Colloid characterisation The iron concentration of the colloidal suspensions was studied by total reflection X-ray fluorescence (TXRF) using a Seifert EXTRA-II spectrometer. The hydrodynamic aggregate size in intensity (ZAVE ) and in volume (dVPCS ), as well as the zeta potential at various pH were determined by photon correlation spectroscopy (PCS) using a Malvern Zetasizer 3000HS apparatus. The degree of dispersion of the particles was evaluated by means of the average agglomeration number (AAN) defined as the average number of primary particles that form the agglomerate (Eq. (2)). The system is considered well dispersed if AAN < 15 [29],  PCS 3 dV AAN = (2) . d BET For the magnetic characterisation of the suspensions, 100 µl of them was sealed in a Teflon holder and frozen prior to the measure in order to keep the dispersion stable under the strong magnetic field. Eventually the experimental magnetisation curve was corrected by subtraction of the diamagnetic contribution of the sample holder and the water, which was comparable to the sample magnetisation at very high fields. The initial susceptibilities of powders and suspensions were measured in the field range ±100 Oe. T1 and T2 relaxation times of the dispersions were measured at 4.7 T (1 H frequency 200 MHz) using a Bruker BIOSPEC

3.1. Powder characterisation Different samples have been prepared by laser pyrolysis of ferrocene by changing the experimental conditions, as described in Table 1. Two of the samples (C1 and T1) prepared without iron precursor were used as reference. For the rest of the samples, the iron precursor proportion was kept constant, being the main difference between the samples the carrier gas composition and the presence of additives. The productivities obtained were high, but due to the low residence time in the laser (in the millisecond range), the decomposition was far to be complete and the extraction with toluene removes approximately 30% of the sample as unreacted ferrocene mainly. After purification the sample composition approaches to pure carbon, with low iron content of around 1–2% in all cases (see Table 1). The colour of the obtained powders was black reflecting the high carbon content of the samples. The XRD diffraction pattern enables us to identify the carbon phase as graphite hexagonal (data not shown). From the analysis of the single peak (002) we obtained the interplanar spacing of the closest packing plane of the structure (Table 1) and the Sherrer’s crystallite size. In all cases these latter sizes are in the range of 1.4–1.7 nm. The interplanar spacing of the (002) plane reported for graphite is 3.395 Å smaller than the one reported in this work. The increase of the interplanar spacing in our samples could be due to the disordered nature of

Fig. 1. TEM micrographs of samples CFE3 and T1.

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the graphite coating. This fact was confirmed by neutron scattering [31]. The identification of the magnetic phase by XRD is difficult because of the small size of the magnetic particles, combined with coincidence of the diffraction peaks of the carbon matrix, iron and iron carbides phases. Only a small peak at approximately 35◦ (2θ ) could be ascribed to iron oxides with a spinel structure such as maghemite or magnetite [32]. TEM microphotographs of sample produced by laser pyrolysis of pure toluene (T1) and of sample CFE3 produced in presence of ferrocene are shown in Fig. 1. It is remarkable the fact that the grain size of the carbon nanoparticles is much bigger in the pure carbon sample T1 (∼70 nm) than in the nanocomposite CFE3 (∼30 nm). The particle size of carbon in both cases is much larger than the crystal size calculated by XRD, reflecting the polycrystalline or even amorphous nature of the carbon coating. In Fig. 1, magnetic nanoparticles with higher density and therefore larger contrast in the image can be clearly distinguished encapsulated in the carbon matrix. Magnetic nanoparticles appear almost isolated, forming no-chains or other aggregation structures, in contrast to the heavily aggregated nanocomposites obtained by chemical vapour condensation or chemical vapour deposition [16–19], and are similar to the one obtained by the explosion method [20]. The particle

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size of the magnetic component obtained from the TEM micrographs ranged between 4 and 10 nm in all cases. Infrared spectra of the powders are similar to the previously reported for carbon nanopowders produced by laser pyrolysis [33,34]. They are composed of large superimposed bands on a continuum decreasing towards longer wavelengths. The continuum is attributed to the building of an extended aromatic network giving rise to an electronic contribution. In Fig. 2, the continuum has been subtracted. The IR spectra are dominated by the band characteristic of OH (3400 cm−1 ), the aromatic C–C bonds (1630 cm−1 ) and the broad signals at 1300 and 554 cm−1 related with the formation of an extended aromatic network [34]. Weak bands are showed by the aromatic CH stretching bonds (3055 cm−1 ) and aliphatic CH stretching bonds (2930 and 2850 cm−1 ). The clearly distinguishable group of three bands between 600 and 1000 cm−1 is characteristic of the aromatic CH out of plane bending modes. These latter modes are very sensitive to the ring substitution and give rise to three components according to the number of adjacent H, the lower mode frequency (754 cm−1 ) corresponding to three to

Fig. 2. Infrared spectra of the C@FE samples. Table 2 Mössbauer parameters of CFE1 and CFE3 samples Isomer shift 298 K (mm/s)/ (α-Fe mm/s)

Hyperfine field 4.2 K (kOe)

Fraction in % of Fe atoms

CFE1

γ -Fe α-Fe Fe3 C γ -Fe2 O3

0.02 0.12 0.31 0.40

0 346 248 463

20 20 45 15

CFE3

γ -Fe α-Fe Fe3+ magnetic unidentified

0.00 0.13 0.3 (2)

0 346 Distribution

31 6 64

Fig. 3. Mössbauer spectra at 4.2 K of some C@FE samples and their deconvolution into components: (1) γ -Fe2 O3 (Fe3 O4 ), (2) Fe3 C, (3) α-Fe, (4) γ -Fe and (5) unidentified magnetic Fe3+ compound. In the inset: histogram of hyperfine fields for the adjustment of the unidentified ferric component, the peak at 252 kOe could correspond to low crystalline or amorphous Fe3 C.

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four adjacent H, the 813 cm−1 broad band corresponding to two adjacent H and the 871 cm−1 being attributed to lone H [34]. The bands at 1099 and 467 cm−1 corresponding to primary amines group (≡C–NH2 ) [35] are present only in the sample CFE3 prepared using ammonia as sensitiser. The infrared spectrum of the ferrocene is also presented in Fig. 2 in order to test the absence of the acute ferrocene bands in our samples. In order to detect changes in the carbon surface after the dispersion process, we dried at 50 ◦ C the dispersion generated with the sample CFE1 and recorded their IR spectrum. As expected, the IR spectrum of this sample showed a band at 1711 cm−1 characteristic of the new carboxylic acid groups, generated by oxidation during the dispersion procedure. The specific surface area of the C@FE samples measured after the purification process showed values around 40 m2 /g, higher than the ones for the pure C samples obtained using ethylene (33 m2 /g) or toluene (27 m2 /g). In Table 1 the particle sizes of the carbon deduced from the specific area measurements are presented. Additionally the full isotherm of sample CFE1 was measured in order to test the porosity of carbon. The isotherm obtained (data not shown) is of type II with almost non existent hysteresis loop. The analysis of these loops indicated a mesopore (2–50 nm) volume of 0.063 cm3 /g and a micropore (<2 nm) volume of 0.01 cm3 /g. This means that in contrast with the carbon produced by conventional methods [6], the carbon coating produced by laser pyrolysis is approximately non-porous. The Mössbauer spectra of samples CFE1 and CFE3 (Table 2, Fig. 3) show the complex composition of the Fe containing phases of the nanocomposites and its dependence with the synthesis conditions. The main magnetic component is the iron carbide with secondary presence of α- and γ -iron. The magnetic iron oxides present in the samples were produced by oxidation post-synthesis, probably during the purification step. 3.2. Magnetic properties The magnetic behaviour of the samples CFE1 and CFE3 at room temperature shows approximate superparamagnetic behaviour with negligible coercivity and remnant magnetisation, only the sample CFE2 is truly superparamagnetic (data not shown). This clearly indicates that the magnetic particles are

very small, i.e. smaller than 10 nm in diameter and with a narrow particle size distribution. Additionally, the iron nanoparticles seem to be well isolated in the carbon matrix avoiding magnetic dipolar interactions, which would give rise to ferromagnetic behaviour at these sizes [36]. Magnetisation data were fitted with a Langevin function and the particle sizes obtained from the fittings were between 2 and 3 nm in all cases close to the ones determined by TEM. It should be mentioned that all the carbon coated iron particles reported in the bibliography showed coercivities of around 250–350 Oe, even for particles smaller than 10 nm, which could be attributed to a wide particle size distribution [37]. Saturation magnetisation values (MS ) for the samples are included in Table 1. The MS value for sample CFE2 is closed to the bulk value for α-Fe, that is 212 emu/g Fe, while the rest is lower and approaches the MS value of iron carbide, Fe3 C (140 emu/g Fe). Amorphous carbon is diamagnetic and therefore it is expected not to contribute to the magnetisation value. γ -Fe phase is also non-magnetic, but it contributes to lower the saturation magnetisation value. The lowest value was obtained for the sample prepared with ammonium with a higher content of γ -Fe phase as detected by Mössbauer spectroscopy. 3.3. Formation mechanism The laser pyrolysis process reported here have similarities with the explosion method reported in [20] and [38] in which mixtures of ferrocene and picric acid explode in an autoclave leading to C@FE nanocomposites similar to the one reported in this work. The mechanism of formation proposed is as follows [38]: (1) formation of small carbon clusters and Fe species; (2) condensation of Fe and C clusters with the formation of a (Fe, C) alloy; (3) growth of a pure graphite shell around the iron based nanoparticles during cooling. The formation of graphite being endothermic, it generates a local decrease in temperature stimulating further carbon precipitation. The smaller size of carbon nanoparticles generated in presence of iron (Fig. 2) is easily explained if we consider that the (Fe, C) alloy in the nanoparticles initially formed acts as nucleation centre for the carbon. In a pure hydrocarbon system, these centres do not exist and the condensation of carbon clusters takes place homogeneously

Table 3 Characterisation of C@FE nanocomposite dispersions Concentration (mM)

Hydrodynamic diameter (nm) (ZAVE PDI)

Fe

C

pH 3

pH 7

pH 11

CFE1

0.036

266

CFE2

0.034

383

CFE3

0.052

612

CFE4

0.065

305

CFE5

0.055

289

122 0.202 157 0.228 103 0.224 152 0.182 129 0.125

129 0.223 159 0.272 105 0.221 146 0.222 131 0.164

110 0.134 147 0.128 99 0.179 140 0.169 130 0.124

AAN

Relaxometric (s−1 /mM Fe) R1

R2

4

1.9

228

10

1.5

97

5

1.4

57

9

1.1

171

6

0.9

81

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from the few nuclei present in that case. This small amount of growth nuclei leads to larger final particle sizes. The presence of γ -Fe was reported previously in the laser pyrolysis of iron pentacarbonyl/ammonia mixtures at temperatures higher than 1100 ◦ C [39]. Consequently the increase of γ -Fe/α-Fe proportion in the sample CFE3 with respect to CFE1 indicates higher temperature of the former process. That circumstance is also reflected in the higher proportion of iron present, i.e. in the better efficiency of ferrocene decomposition (Table 1). The presence of iron carbides has been reported repeatedly when ferrocene is used as iron precursor, independently of the pyrolysis technique [16–20]. It is hypothesised that iron carbides are metastable at temperatures higher than 725 ◦ C [37], but even if this temperature is surpassed in laser pyrolysis experiments, the short reaction time prevents their transformation into metal phase and carbon. The overall structure of the magnetic cores obtained in this work, is probably close to that reported previously in the case of the pyrolysis of mixtures of ferrocene and C60 : an α-Fe core surrounded by the γ -Fe phase and finally covered by a Fe3 C layer, which should interface the metallic phases and the carbon [40]. 3.4. Aqueous dispersions The dispersion procedure successfully employed here changed the hydrophobic nature of the as synthesised material to hydrophilic. In all cases the samples could be considered as monodispersed but with rather large polydispersity indexes (see Table 3). This is related to the irregular fragmentation of the partially coalesced carbon particles during the oxidative dispersion. The values of the hydrodynamic diameter are correlated with the particle size determined by BET and the values obtained of the AAN (Table 3) indicate a good dispersion degree in all samples. It is expected that the small differences in dispersivities among the samples will be related with the nature of the carbon shell, which in turn should depend on the synthesis conditions. Nevertheless it is difficult at this point to relate the lower AAN numbers of CFE1, CFE5 and CFE3 with some specific characteristic of the carbon, with the exception of sample CFE3 that should be more polar (and dispersible) due to the existence of ≡C–NH2 groups mentioned previously. The samples do not settle in months due partially to the relatively low density of the composite and to the high absolute values of the ζ potentials of the dispersions (Fig. 4). The evolution of these potentials with the pH indicates that this stability is maintained in a wide pH interval. This is confirmed by the approximately constant values of hydrodynamic sizes measured under different pH’s (Table 3). The negative values of the ζ potentials are justified by the presence of carboxylic surface groups previously suggested by infrared spectroscopy. In spite of the partial dissolution of the magnetic particles during the dispersion procedure, the dispersed samples retain their superparamagnetic behaviour (data not shown). The relaxometric properties are related with the magnetic properties and consequently present important differences among the samples (Table 3). The sensitivity of the magnetic cores and the carbon coat in front of the oxidation is also variable. This justify that

Fig. 4. Comparison between the zeta potential dependence with pH of some C@FE dispersions in water.

the most magnetic sample CFE2 present only modest values of R2 (Table 3) and that the CFE1 dispersion have the highest value of R2 attained. This value is similar to those of the commercial contrast agents of SPIO type [41,42]. 4. Conclusions Iron–carbon nanocomposites suitable to be used as contrast agent for NMR imaging have been obtained by laser pyroly-

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sis of ferrocene/toluene solutions in a single and continuous process. The material consists of iron metal particles, whose size was controlled between 3 and 10 nm isolated in a carbon matrix. The introduction of additives has little effect on the properties of the nanocomposites. Increasing the reaction temperature by the use of ammonia instead of ethylene produces a better conversion of ferrocene into iron, but with lower magnetic and relaxometric properties due to the presence of γ -Fe instead of α-Fe. The dispersions generated using these nanoparticles are stable under a wide pH interval and have similar colloidal properties than the commercial MRI contrasts based in the dextran coating. Additionally in some cases the relaxometric properties are also similar, and their commercial use is conditioned at present by the improvement of the iron content of the samples. The principal advantage of the dispersions described in this work is the permanent coupling of the carbon coat to the magnetic core. This characteristic makes possible the attachment of small molecules or antibodies to the surface, without risk of detaching the primary coat, with the subsequent loss of functionality and agglomeration. Acknowledgments This work was supported by the Spanish Ministry of Education and Science (MEC) through the Projects No: HF20030283, MAT2005-03179, and by the Community of Madrid, Project No S-0505/MAT/0194. Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.05.010. References [1] D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770. [2] Z.P. Xu, Q.H. Zeng, G.Q. Lu, A.B. Yu, Chem. Eng. Sci. 61 (2006) 1027. [3] Y. Lin, T.W. Smith, P. Alexandridis, J. Dispersion Sci. Technol. 23 (2002) 539. [4] A.A. Kuznetsov, V.I. Pilippov, O.A. Kuznetsov, V.G. Gerlivanov, E.K. Dobrinsky, S.I. Malashin. J. Magn. Magn. Mater. 194 (1999) 22. [5] M.V. Kutushov, A.A. Kuznetshov, V.I. Filippov, O.A. Kuznetshov, in: U. Häfeli, W. Schütt, J. Teller, M. Zborowski (Eds.), Scientific and Clinical Applications of Magnetic Carriers, Plenum Press, New York, 1996, p. 391. [6] A.B. Fuertes, P. Tartaj, Chem. Mater. 18 (2006) 1675. [7] H. Tokoro, S. Fujii, T. Oku, Diamond Relat. Mater. 13 (2004) 1270. [8] K. Kosugi, M.J. Bushiri, N. Nishi, Appl. Phys. Lett. 84 (2004) 1753. [9] H. Song, X. Chen, Chem. Phys. Lett. 374 (2003) 400. [10] J.H. Kim, J. Kim, J.H. Park, C.K. Kim, C.S. Yoon, Y. Shon, Nanotechnology 18 (2007) 115609. [11] Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, K. Sumigawa, K. Suzuki, A. Kasuya, Y. Nishina, J. Phys. Chem. Solids 54 (1993) 1849. [12] E.M. Brunsman, R. Sutton, E. Bortz, S. Kirkpatrick, K. Midelfort, J. Wiliams, P. Smith, M.E. McHenry, S.A. Majetich, J.O. Artman, M.D. Graef, S.W. Staley, J. Appl. Phys. 75 (1994) 5882.

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