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Langmuir 2007, 23, 4026-4028
Synthesis and Coating of Cobalt Ferrite Nanoparticles: A First Step toward the Obtainment of New Magnetic Nanocarriers Giovanni Baldi,*,† Daniele Bonacchi,† Mauro Comes Franchini,*,‡ Denis Gentili,‡ Giada Lorenzi,† Alfredo Ricci,‡ and Costanza Ravagli† CERICOL, Colorobbia, Via Pietramarina 123, 50053 SoVigliana-Vinci, Firenze, Italy, and Dipartimento di Chimica Organica “A. Mangini”, UniVersita` di Bologna, V.Risorgimento 4, 40136 Bologna, Italy ReceiVed NoVember 7, 2006. In Final Form: January 19, 2007 Monodisperse and stable cobalt ferrite (CoFe2O4) nanoparticles (5.4 nm) have been produced, coated with monoand difunctional phosphonic and hydroxamic acids, and fully characterized (using thermogravimetric analysis (TGA), dynamic light scattering (DLS), IR spectroscopy, transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) measurements). Cobalt leakage of the coated nanoparticles has been also studied. Magnetic measurements show the possible applications in hyperthermia at low frequencies, and for this reason, water-soluble coated CoFe2O4 can be seen as a first step toward the obtainment of novel systems for biomagnetic applications.
Magnetic nanoparticles1 (MNp) have attracted great interest in the biomedical field due to the new perspectives that are forecasted in biomedicine,2 especially in magnetic targeted drug delivery3 and in magnetic fluid hyperthermia (MFH).4 Oxidebased spinel ferrites5 are very promising for hyperthermic treatment, and iron oxides are good candidates due to their wellknown biocompatibility. However, the use of different materials with larger magnetic anisotropy and larger magnetic moments is envisioned, since it could allow a significant improvement of the material efficiency for MFH. The spinel cobalt ferrite CoFe2O4 has already been proposed for biomedical applications,6 and it is known to have large anisotropy compared to other oxide ferrites.7 As a consequence, the magnetic moment of cobalt ferrite relaxes much slower than that in magnetite nanoparticles of similar size. This means that, in principle, smaller cobalt ferrite particles can be used instead of iron oxide, allowing the assembly of smaller biocompatible devices that are known to promote cellular uptake and to better avoid the reticuloendothelial system.8 Cobalt ferrite nanoparticle (NP) synthesis has been reported on a laboratory scale,9 but their use in medicine has not been possible because of numerous problems such as poor accessibility * To whom correspondence should be addressed. Telephone: 390571709758 (G.B.), 390512093654 (M.C.F.). Fax: 390571709875 (G.B.), 390512093654 (M.C.F.). E-mail:
[email protected] (M.C.F.). † CERICOL. ‡ Universita ` di Bologna. (1) Gao, I. Biofunctionalization of Nanomaterials. In Nanotechnologies for the life sciences; Kumar, C. S. S. R., Ed.; Wiley-VCH: New York, 2006; Vol. 1, Chapter 3. (2) Mornet, S.; Vasseir, S.; Grasset, F.; Duguet, E. J. Mater. Chem. 2004, 14, 2161. (3) Dobson, J. Drug DeV. Res. 2006, 67, 55. (4) (a) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (b) Jordan, A.; Scholz, R.; Wust, P.; Fahling, H.; Krause, J.; Wlodarczyk, W.; Sander, B.; Vogl, T.; Felix, R. J. Int. J. Hyperthermia 1997, 13, 587. (5) Gupta, A. K. Biomaterials 2005, 26, 3995. (6) Kuckelhaus, S.; Reis, S. C.; Carniero, M. F.; Todesco, A. C.; Oliveira, D. M.; Lima, E. C. D.; Morais, P. C.; Azevedo, R. B.; Lacava, Z. G. M. J. Magn. Magn. Mater. 2004, 2402, 272-276. (7) (a) Tung, L. D.; Kolesnichenko, V. L.; Caruntu, D.; Chou, N. H.; O’Connor, C. J.; Spinu, L. J. Appl. Phys. 2003, 93, 7486. (b) Hanh, N.; Quy, O. K.; Thuy, N. P.; Tung, L. D.; Spinu, L. Physica B 2003, 327, 382. (8) Brigger, I.; Dubernet, C.; Couvreur, P. AdV. Drug DeliVery ReV. 2002, 54, 631. (9) (a) Sun, S.; Zeng, H.; Robinson, D.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (b)Villai, V.; Shah, D. O. J. Magn. Magn. Mater. 1996, 163, 243.
of the surface due to the presence of surfactant,9b aggregation in solution, and the remarkable amount of cobalt release in aqueous solutions. In this article, we wish to present a new synthesis of stable and highly monodisperse cobalt ferrite NPs in diethylene glycol (DEG) without any surfactants and the investigation of the capability of mono- and difunctional organic ligands for decoration of the NP surfaces. The magnetic features of the resulting modified NPs have also been determined and examined for their potential applications for MFH. Moreover, since no studies have been reported on the stability of cobalt ferrite in water, we have examined the Co leakage of the ligand-modified cobalt ferrites. Cobalt ferrite nanoparticles (CoFe-1) have been produced via the following polyol method on a scale up to 1 kg:10 cobalt acetate (2.88 mol) and iron acetate (2 equiv) were solubilized in DEG (20 kg) at 110 °C for 1 h, and then the solution was heated to 180 °C. After 3 h, the product was air cooled to room temperature and then stored (Scheme 1). Transmission electron microscopy (TEM, 100 keV) of CoFe-1 showed a uniform dispersion of nanoparticles with a log-normal distribution (number) and with an average diameter of 5.4 nm (Figure 1). The dispersion remained stable for more than 1 year, as proved by dynamic light scattering (DLS). DLS also showed a uniform size distribution with an average (volume) diameter of 13.2 nm. For comparison with the TEM observations, we have used the calculations suggested by Thomas11 to transform a dynamic light scattering average diameter into the number mean diameter that would result from TEM observations. Using the transformation with the polydispersity index (PDI ) 0.19), we obtained 5.56 nm, which is in very good agreement with the TEM value. The X-ray diffraction spectra indicated the formation of a cobalt ferrite phase, and the Sherrer analysis of the (311) peak gave an average diameter of 5.5 nm for the crystallites, which is again in good agreement with the TEM observations (the Sherrer analysis assumes the particles as spherical, and this could explain the small differences). To synthesize magnetic-core ligand-shell nanostructures, the coating of CoFe-1 was performed using hydroxamic12 and (10) Baldi, G.; Barzanti, A.; Bitossi, M. PCT EP03/02281, 2002. (11) Thomas, J. C. J. Colloid Interface Sci. 1987, 117, 187. (12) Folkers, J. P.; Garman, C. B.; Laibinis, P. E.; Buchholz, S.; Nuzzo, R. G.; Whitesides, G. M. Langmuir 1995, 11, 813.
10.1021/la063255k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
Synthesis and Coating of CoFe2O4 Nanoparticles
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Scheme 1. Synthesis of CoFe-1 and Organic Graftings with 2-5
phosphonic acids,13 which are scarcely investigated but are very useful due to their ability to form a stable coating on metal oxides based on Al, Zr, Ti, and Fe. Ligands 2-5 (1.7 mmol), solubilized in ethanol (25 mL), were anchored to the metallic surface by slow addition to CoFe-1 (3% w/w in DEG, 10 g, 1.3 mmol) under vigorous stirring for 1 h at room temperature. Ferrites CoFe-2 and -3 were made lipophilic, were easily extracted in hexane, and were dried for the following analyses. In the cases of CoFe-4 and CoFe-5, water solubility was achieved. In all the cases, the same morphology was retained as shown in the TEM microphotograph of CoFe-2. DLS measurements indicated that all ligands produced stable and not aggregated dispersions. The DLS average diameters were shown to be 13.5 and 13.9 nm for CoFe-2 and -3, respectively, and the dispersions were almost the same. In the case of CoFe-4, we observed a rather large distribution of particles with a mean diameter of 37.8 nm. This may be due to the attraction of the ions to the charged surface significantly increasing the solvation sphere. In any case, a certain degree of aggregation is observed. In the case of CoFe-5, we observed a narrow size distribution with an average diameter of 15.7 nm. Transmission infrared spectroscopy (KBr pellet) studies were carried out to prove the coating of the surfaces and to confirm the selectivity of the capping functional groups CONHOH and PO(OH)2 with respect to the terminal NH2 and OH groups in 4 and 5. In the case of hydroxamic acid 2, we observed the disappearance of the two intense stretchings of OH at 3070 cm-1 and of NH at 3258 cm-1, which resulted in a broad band at 3431 cm-1 in CoFe-2. We also observed the broadening of the OH bending, which also switched from 1078 to 1075 cm-1 in the grafted CoFe-2 sample. The 3202 and 3033 cm-1 stretches of 4 (NH and OH, respectively) and the 3425 cm-1 band of NH2 became broadened to 3409 cm-1 in CoFe-4. The aliphatic region cannot be diagnostic as reported by Whitesides and Nuzzo,12 while below 2000 cm-1 the CdO stretching region could be used for monitoring the coating. Accordingly, we noticed the
Figure 1. TEM microphotographs of CoFe-1 and CoFe-2
disappearance of the two CO bands14 of 2 at 1663 and 1623 cm-1, which resulted in a shifted and broadened single band at 1596 cm-1 for CoFe-2, and we observed the same disappearance of the only strong band13 for the CO of 4 at 1605 cm-1, which shifted to 1576 cm-1 in CoFe-4. For the phosphonic acids, in the more diagnostic region,13 P-O stretching of the starting neat ligand 3 (1216 and 937 cm-1) disappeared as expected while the stretching of the PO3 group in 3 (1079 and 1005 cm-1) resulted in two broadened and shifted bands at 1122.6 and 1057 cm-1 for CoFe-3. As far as the aliphatic region is concerned, only small variations have been observed: the CH2 stretching peaks of the neat ligand 3 and the peaks of CoFe-3 are slightly shifted from 2959, 2922, and 2849 cm-1 to 2959, 2921, and 2848 cm-1, respectively. Also, the P-O stretching of 5 (1233 and 936 cm-1) disappeared while the stretching of the PO3 group (1162 and 1010 cm-1) resulted in two broadened and shifted bands at 1096 and 993 cm-1 for the grafted nanoparticles CoFe-5. The OH of 5 was moved and broadened from 3357 to 3445 cm-1 in CoFe-5, and the aliphatic stretchings of 5 and those of CoFe-5 were shifted also in this case by a few inverse centimeters, from 2917 and 2850 cm-1 to 2922 and 2848 cm-1, respectively. The ligand uptake was evaluated by thermogravimetric analysis (TGA, air/N2 mixture) by measuring the mass loss during heating up to 800 °C relative to the burning of the organic ligand. In the cases of 3 and 5, the final value was corrected due to the residual phosphate framework and this has also been confirmed by determination of the inorganic part content relative to the total mass of the dry sample by ICP-AES (ion-coupled plasma atomic emission spectroscopy). Ligands 3 and 5 gave the best uptakes with 34% for CoFe-3 and 35% for CoFe-5. Ligand 2 gave a 24% uptake in CoFe-2, while for ligand 4 an 18% uptake in CoFe-4 was calculated. These results indicate a strong interaction between the inorganic core of cobalt ferrite and the phosphonic moiety, showing, in particular, an increased uptake in the case of the difunctional hydroxyl ended particles of CoFe-5 that could be explained with a more dense surface packing of organic molecules due to the uncharged nature of the external moiety. This is especially true when compared with CoFe-4, which contains a charged amino group, and is in agreement with the rather broad size distribution of the particles observed with DLS. Superconducting quantum interference device (SQUID) measurements have been performed on samples immobilized in paraffin for CoFe-2 and -3 and in DEG for CoFe-4 and -5. Saturation magnetization (Msat) is not altered by ligand absorption (see entry 1 compared to entries 2-5 in Table 1). This indicates a marginal influence of the surface for particles having this (13) Higgins, S. F.; Magliocco, L. G.; Colthug, N. B. Appl. Spectrosc. 2006, 60, 279. (14) (a) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, Y.; Gedanken, A. Langmuir 1999, 15, 7111. (b) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Langumuir 1998, 14, 5826.
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Baldi et al.
Figure 2. Size distribution from dynamic light scattering (DLS) measurements of CoFe-2, -3, -4, and -5. Table 1. SQUID Measurements for Coated Cobalt Ferrites entry
sample
Msat at 2.5 K (emu/g)
1 2 3 4 5
CoFe-1 CoFe-2 CoFe-3 CoFe-4 CoFe-5
92.4 89.6 88 83.2 93.6
Msat at 300 K (emu/g)
ZFC maximum (K)
71.6 70.7 78.8 69.8 70.4
200 194 194 174 184
diameter and outlines an important feature, as the moment of the particles is directly correlated to the hyperthermic efficiency.15 The maximum of the zero cooled curve (ZFC) (entries 1-5) is much higher than that of iron oxide nanoparticles of the same dimension.16 The high maximum of the ZFC indicates a slower relaxation rate of magnetization, in agreement with the high crystalline anisotropy and with previous reports for particles of comparable size,7 and did not significantly change after the coating. This slow relaxation opens the possibility that a hyperthermic effect at the frequencies commonly used (100500 kHz) might take place in particles with smaller dimensions compared to iron oxides.15 To detect cobalt leakage with ICP-AES, in a comparative experiment, synthetic cobalt ferrite as the bulk material (10 µm) and CoFe-1 were dialyzed against water inside dialysis tubing (Visking Tubing, SiC) at neutral pH, introducing samples in a DEG solution. In the bulk material, cobalt concentrations in the permeate after 24 h were under the instrumental detection limit, whereas for CoFe-1 a total cobalt release of 31.2% in weight was found after 24 h. This is of paramount importance for medical applications, and for this reason, CoFe-4 and -5 were left for 24 h inside dialysis tubing with distilled water at neutral pH as for CoFe-1. After ultracentrifugation, ICP analysis of the water phase (15) Rosenweig, R. E. J. Magn. Magn. Mater. 2002, 252, 370. (16) Park, J.; Kwangjin, A.; Hwang, Y.; Park, J. G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891.
showed a cobalt dissolution of 8% for CoFe-4 and 1.2% for CoFe-5. The remarkable quantity of cobalt detected for CoFe-4 confirms poor ligand coverage, in line with the TGA analysis, while the phosphonic-based ligand 5 proved to be highly effective in preventing cobalt leakage. In summary, we have shown that well-defined monodisperse cobalt ferrite NPs can be prepared on a kilogram scale, grafted, and rendered both lipophilic17 and hydrophilic with suitable hydroxamic and phosphonic acids. We have shown that these coated cobalt ferrites are promising nanomaterials for hyperthermic applications since, due to their different magnetic properties, smaller particles, compared to magnetite, can be used. Moreover, since hyperthermic studies have been performed only on iron oxide NPs, they represent a valid alternative that can be used to prove classic hyperthermic models. The cobalt cession in water was investigated, and the phophonic ligand 5 appears to be suitable for the coating of nanoparticles and for future biomedical applications. Further developments of functionalized cobalt ferrite NPs are under investigation for hyperthermic applications in locoregional cancer therapy and as carriers of pharmaceutical compounds for magnetic drug targeting.18 Acknowledgment. The authors would like to thank Dr. C. Sangregorio and Dr. C. Innocenti of the University of Florence (Italy) for the magnetic measurements shown in the article. Supporting Information Available: Detailed procedures for the synthesis and the characterization of organic ligands 2, 4, and 5 and of the coated nanoparticles CoFe-2, -3, -4, and -5. This material is available free of charge via the Internet at http://pubs.acs.org. LA063255K (17) Also, CoFe-2 and CoFe-3 in conjunction with emulsifiers are under investigation for hyperthermic applications. (18) Alexiou, C.; Arnaold, W.; Klein, R. J.; Parak, F. G.; Hulen, P.; Mergemann, C.; Erhards, W.; Wagenpfeils, S.; Lubbe, A. S. Cancer Res. 2000, 60, 6641.
