Carbon-coated iron oxide nanoparticles as contrast agents in magnetic resonance imaging
© Bae et al; licensee Springer. 2012
Received: 1 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Coprecipitated ferrite nanoparticles were coated with carbon using a hydrothermal method. From transmission electron microscope pictures, we could see that the coated iron oxide nanoparticles were spherical in shape with an average diameter of 90 nm. The strong bonding of carbon on the nanoparticle surfaces was checked by noting the C = O and C = C vibrations in Fourier transform infrared spectra. The spin-lattice relaxation process [T1] and spin-spin relaxation process [T2] relaxivities of hydrogen protons in the aqueous solution of coated nanoparticles were determined to be 1.139 (mM·s)-1 and 1.115 (mM·s)-1, respectively. These results showed that the carbon-coated iron oxide nanoparticles are applicable as both T1 and T2 contrast agents in magnetic resonance imaging.
PACS: 81.05.y; 76.60.Es; 61.46; 75.50.k; 87.61.
Keywordsiron oxide nanoparticles carbon-coated nanoparticles relaxivity MRI
Nanostructured materials have attracted a great deal of attention in the development of biotechnology and medicine [1–3]. Among these nanostructured materials, carbon-coated metal oxide nanoparticles such as MgO, CaO, ZnO, TiO2, Al2O3, and Fe2O3 are now extensively studied because of their high application potential [4–9]. Recently, much research interest has been expended on the ferromagnetic iron oxide materials generally used for magnetic data storage as magnetic toners in xerography, and on the ferrofluids, used as contrast agents in magnetic resonance imaging [MRI] [10–13].
The carbon coating provides an effective oxidation barrier and prevents corrosion in magnetic core materials. Hydrophilic carbon coating on iron oxide nanoparticle cores endows better dispersibility and stability than those shown by bare iron oxide nanoparticles. In general, different approaches have been employed for the synthesis of carbon coatings, for example, electric arc discharge, catalytic pyrolysis of organic compounds, and the hydrothermal methods .
In this paper, we report the synthesis of carbon coating on iron oxide (Fe3O4) nanoparticles by a hydrothermal method proposed by Zhang et al.  with some modifications. We evaluated these coated particles as potential spin-lattice relaxation process [T1] and spin-spin relaxation process [T2] contrast agent in MRI. We studied the T1 and T2 relaxations of hydrogen protons in water molecules in an aqueous solution of carbon-coated iron oxide nanoparticles. We found that the T1 and T2 relaxivities for the aqueous solution of carbon-coated iron oxide nanoparticles were 1.139 and 1.115 (mM·s)-1, respectively. The ratio of these two relaxivities is close to unity. This result demonstrates that carbon-coated iron oxide nanoparticles are suitable as both T1 and T2 contrast agents in MRI.
Materials and fabrication
Carbon-coated iron oxide nanoparticles were synthesized by hydrothermal synthetic processes. Bare iron oxide nanoparticles were formed using the coprecipitation method, in which NaOH solution was slowly added to a mixed solution of ferric and ferrous chlorides in a glove box filled with argon gas. During this process, iron oxide nanoparticles were precipitated. These magnetic particles were separated by a magnet and were washed out by methanol, acetone, and DI [deionized] water. The collected nanoparticles were dried in a vacuum oven to obtain a powder sample of nanoparticles for coating. The dried nanoparticles were dispersed in a solution of 0.5 M glucose for 5 h with sonification. During this process, carbon was coated onto the surfaces of the nanoparticles. The solution was dried in a vacuum oven filled with argon gas for 4 h. The dried, coated nanoparticles were redispersed in DI water and filtered through a 100-nm filter paper several times.
The particle size distribution and structure of the carbon-coated nanoparticles were checked using a TEM microscope (TEM, H-7600, Hitachi High-Tech, Minato-ku, Tokyo, Japan). The hydrodynamic diameter and diffusion constant of the coated nanoparticles in water were measured with a dynamic light scattering [DLS] particle size analyzer (ELSZ-2, Otsuka Electronics Co., Ltd., Osaka, Japan). The bonding of carbon onto the iron oxide particles were confirmed by using Fourier transform infrared spectroscopy [FTIR]. For the relaxivity measurements, aqueous solutions of various nanoparticle concentrations were prepared. The concentration of nanoparticles in the aqueous solution was measured with an inductively coupled plasma [ICP] spectrophotometer (IRISAP, Thermo Jarrell Ash, Franklin, MA, USA). The T1 and T2 relaxation times of hydrogen protons in the aqueous solution of the coated nanoparticles were measured using an MR scanner (1.5 T Scanner, GE Medical System, Saskatchewan, Canada).
Results and discussion
In MRI, the signal recorded during the scan is related to the magnetic relaxation processes of the nuclear spins of the protons in the water molecules in the area of interest. Under a given external magnetic field (B), the nuclear spins of the protons align with the field, giving rise to a net magnetization, M. If a radio frequency [rf] pulse is perpendicularly applied to B, the nuclear spins are excited and start precessing in the plane perpendicular to B. Upon removal of the rf pulse, the nuclear spins gradually recover to their equilibrium state parallel to B. The recovery of the equilibrium takes place via two different relaxation mechanisms: the spin-lattice relaxation process (T1), or in other words, the recovery of the magnetization along the B direction, and the spin-spin relaxation process (T2), i.e., the loss of signal in the perpendicular plane. The presence of nanoparticles in the area of interest creates an additional magnetic field (B1) which induces local field inhomogeneities that significantly increase the speed of the proton transverse relaxation (decrease of T2), leading to a negative contrast or a darkening of the image.
where i = 1 or 2, and represents the relaxivity of nuclear spins with no nanoparticle contrast agent. Also, R i is the relaxivity of nuclear spins per mM of nanoparticles, and C represents the concentration of nanoparticles in the aqueous solution.
We synthesized highly water-dispersible carbon-coated iron oxide nanoparticles for use as contrast agents in MRI. The coated nanoparticles were observed to be spherical with a core-shell structure in the TEM images, and they showed a uniform size distribution with an average diameter of 90 nm. The strong bonding of carbon on the nanoparticle surfaces was checked by noting the C = O and C = C vibrations in FTIR spectra. The T1 and T2 relaxation times of hydrogen protons were measured using an MRI scanner in the aqueous solutions of various concentrations of nanoparticles ranging from 0.427 to 4.27 mM. The T1 and T2 relaxivities were 1.139 and 1.115 (mM·s)-1, respectively. The ratio of these two relaxivities was close to unity. This result shows that carbon-coated iron oxide nanoparticles are suitable as both T1 and T2 contrast agents in MRI.
dynamic light scattering
Fourier transform infrared
inductively coupled plasma
magnetic resonance imaging
transmission electron microscope.
This work was supported by the National Research Foundation of Korea (2010-0021315) and was also supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF) grant funded by the Korean government (MEST; 2009-0072413) and the Nuclear R&D Program (grant code: 20090081817) of the NRF funded by MEST.
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