1. Characterization of Magnetite Nanoparticles Coating Density
R. Ahamadi a, H.R. Madaah Hosseini a, A. Sohrabi b a Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box , Tehran, Iran
b Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box , Tehran, Iran
Abstract
Materials and Methods
MR Imaging
In this work, some stabilized ferrofluids based on magnetite nanoparticles (mean core and its coating size about 25 and 1 nm, respectively) were synthesized via co-precipitation method. Cysteine was used as surfactant because of suitable structure and proper conjunction to magnetite nanoparticles surface. Synthesized Ferrofluids were characterized using TEM, TGA, DLS and FT-IR techniques and finally used as MRI contrast agent. In this research an indirect approach is proposed for evaluation of coating density using TEM and TGA results.
Co-precipitation procedure performed using FeCl3.6H2O and FeCl2.4H2O as Fe resources, NaOH as reducing agent and Cysteine as surfactant. Optimizing experimental conditions such as temperature, pH, molar ratio of reactants, ultrasonicating frequency and power, etc. magnetite Cysteine capped nanoparticles were synthesized. Some suspension samples were prepared for DLS and TEM tests, whereas TGA and FT-IR samples were dried powders.
TEM images from prepared sample taken at accelerating voltage of 200 and 80 KV and related diffraction pattern are shown in Figure 2-a, 2-b, and 2-c, respectively. Spinel structure of magnetite can be upheld from Figure 2-c in which calculations using rd=Lλ equation will lead to measurement in agreement with crystallographic planes distances of Fe3O4. Figure 2-a show 200 KV TEM image in which mean particles diameter is about 15 nm. Particles shape lies mainly between spherical and cubic pattern. No surfactant layer can be recognized in this high voltage. Attempts done for this reorganization were not efficacious. On the other hand, a dim ring can be seen in lower voltage, 80 KV (Figure 2-b). This uniform ring surrounded all particles and has a thickness of about 1 nm. Agglomerate size in this image is about 140 nm, which is in good coincidence with DLS digram (Figure 3). This dimension of agglomerates is appropriate for imaging or drug delivery to organs such as liver and spleen, however individual nanoparticles can accumulate at lymph nodes according to their size [3].
a) b) c)
Fig. 2- TEM image of synthesized nanopartcles at a) 200 KV and b) 80 KV, c) related diffraction pattern showing spinel structure of magnetite
Fig. 3- DLS diagram of synthesized nanoparticles with mean hydrodynamic diameter of about 110 nm
MR imaging performed with a 1.5 T (GE medical system) by using a knee coil for transmission and reception of the signal. Mouse was anaesthetized by pentobarbital sodium at the dose of 40 mg/kg body weight. Synthesized ferrofluid was administered in rat intravenously via lateral tail vein. MRI scan was performed 24 h after contrast agent administration at a dose of 2.5 mg (‏Fe)/kg body weight (Figure 6 ). As is explained by theory [3], larger and aggregated particles are mainly accumulated in tissues such as liver and spleen, however smaller ones (20-40 nm) are phogositosed by macrophages of lymphatic system and enhance image contrast of these tissues through substantial shortening of T2 relaxation times leading to hypoitence signal intensity of related tissues on MR images.
Fig. 6- MRI of rat 24h after IV injection
FT-IR pattern confirms presence of cysteine on the magnetite nanoparticles surface(Figure 4).
Characterization
Fig. 4- FT-IR spectrum of nanoparticles
According to TGA diagram, weight loss percent due to decomposition of probably organic phase is about 7% which is considered above 100 oC (Figure 5). Some simplifying assumptions can be accomplished to calculate coating density: First, magnetic core is pure magnetite. This assumption is in good agreement with diffraction pattern (Figure 2-c) and FT-IR diagram which shows only Fe-O band belongs to Oxygen and Iron ions covalent band in Fe3O4 chemical structure [4]. Thus, core density can be assumed equal to 5.15 gr.cm-3 [5]. Second, magnetic core has a nearly spherical shape. Thus core volume can be simply calculated. Third, average coating thickness surrounding all particles is about 1 nm. TEM image (Figure 2-c) confirms this assumption. Forth, mean magnetite core size is about 25 nm (TEM Figures 2-a and 2-b). Using these assumptions, coating density is simply calculated to be about 1.54 gr.cm-3. Comparing with solid L-Cysteine density of 1.67 gr.cm-3[5], this means that a rather high density coating is formed that satisfies fundamental conditions of formation of a stable ferrofluid. This was in good agreement with this fact that synthesized ferrofluids in this work were stable for more than 2 months.
Introduction
Ferrofluids containing magnetic nanoparticles, have potential applications in drug delivery, cancer therapy, magnetic resonance imaging (MRI) contrast enhancement agent, etc [1, 2]. One of the fundamental preconditions for mentioned medical applications is high stability of synthesized ferrofluid with time and environmental variations. Among various parameters affect this stability, surfactant layer density plays an important role. Formation of a high density coating will prevent agglomeration or sedimentation of solid phase. Besides, the nature and composition of surfactant is an important factor. Cysteine with three functional groups (Figure 1-a) have a certain binding affinity to metal atoms specially Fe atoms, by a proposed conjunction arrangement showing schematically in Figure 1-b.
a) b)
Fig. 1-a) Structure of Cysteine, b) Proposed structure in the interaction between Cysteine and Fe3O4 nanoparticle surface
It seems that the most accurate and reliable method of measuring coating thickness and density is direct observation of coating layer via electron microscopy. Rutherford TEM equation can be presented as:
in which is probability or fraction of scattering phenomenon from phase of Z molecular weight at accelerating voltage of V. According to this equation, TEM study of organic compounds of Cysteine type containing low atomic weight elements such as Hydrogen and Carbon should be performed at low voltages. Besides, TEM study at intermediate should be performed with low illumination, i.e. low dose of electron density due to beam damage. In order to detect surfactant layer directly and measure its thickness, synthesized nanopaticles were studied at two various levels of accelerating voltage in this work. Comparison of measured thickness by TGA and DLS results provides possibility of evaluation of coating density via some simple calculations. This approach can be used in similar cases.
Conclusion
In summary, stable ferrofluids were synthesized using an ultrasonic-assisted co-precipitation method. Characterization was performed using TEM, DLS, FT-IR and TGA techniques. In These samples, mean magnetic core size and coating thickness were about 25 and 1 nm, respectively. Besides, synthesized ferrofluids show the capability of use as MRI contrast agent. A direct approach for estimating coating density is proposed mainly based on TEM and TGA techniques that can be used in similar cases.
A. K. Guptaa & M. Gupta, Biomaterials 26 (2005) 3995–4021.
E. H. Kim, Y. Ahn & H. S. Lee, Journal of Alloys and Compounds 434–435 (2007) 633–636.
Leslie LaConte, Nitin Nitin & Gang Bao, Materials Today, 8 (2005) 32.
4. R. Y. Hong, T.T. Pan, H.Z. Li, J. Magn. Magn. Mater. 303 (2006) 60.
5. R. C. Weast, CRC Handbook of Chemistry and Physics, 70th edition, 1989.
Refrences
Fig. 5- TGA diagram of synthesized nanoparticles