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Physicochemical characterization, and relaxometry studies of micro-graphite oxide, graphene nanoplatelets, and nanoribbons.

Paratala BS, Jacobson BD, Kanakia S, Francis LD, Sitharaman B - PLoS ONE (2012)

Bottom Line: The chemistry of high-performance magnetic resonance imaging contrast agents remains an active area of research.In this work, we demonstrate that the potassium permanganate-based oxidative chemical procedures used to synthesize graphite oxide or graphene nanoparticles leads to the confinement (intercalation) of trace amounts of Mn(2+) ions between the graphene sheets, and that these manganese intercalated graphitic and graphene structures show disparate structural, chemical and magnetic properties, and high relaxivity (up to 2 order) and distinctly different nuclear magnetic resonance dispersion profiles compared to paramagnetic chelate compounds.The results taken together with other published reports on confinement of paramagnetic metal ions within single-walled carbon nanotubes (a rolled up graphene sheet) show that confinement (encapsulation or intercalation) of paramagnetic metal ions within graphene sheets, and not the size, shape or architecture of the graphitic carbon particles is the key determinant for increasing relaxivity, and thus, identifies nano confinement of paramagnetic ions as novel general strategy to develop paramagnetic metal-ion graphitic-carbon complexes as high relaxivity MRI contrast agents.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Stony Brook University, Stony Brook, New York, United States of America.

ABSTRACT
The chemistry of high-performance magnetic resonance imaging contrast agents remains an active area of research. In this work, we demonstrate that the potassium permanganate-based oxidative chemical procedures used to synthesize graphite oxide or graphene nanoparticles leads to the confinement (intercalation) of trace amounts of Mn(2+) ions between the graphene sheets, and that these manganese intercalated graphitic and graphene structures show disparate structural, chemical and magnetic properties, and high relaxivity (up to 2 order) and distinctly different nuclear magnetic resonance dispersion profiles compared to paramagnetic chelate compounds. The results taken together with other published reports on confinement of paramagnetic metal ions within single-walled carbon nanotubes (a rolled up graphene sheet) show that confinement (encapsulation or intercalation) of paramagnetic metal ions within graphene sheets, and not the size, shape or architecture of the graphitic carbon particles is the key determinant for increasing relaxivity, and thus, identifies nano confinement of paramagnetic ions as novel general strategy to develop paramagnetic metal-ion graphitic-carbon complexes as high relaxivity MRI contrast agents.

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Plot of Magnetization (M) v/s Field strength (H) for (a) micro-graphite, (b) oxidized graphite (c) Oxidized Graphene nanoplatelets (d) Reduced Graphene nanoplatelets at 30 K, 150 and 300 K between −50,000 to 50,000 Oe (Inset shows plot between −5000 and 5000 Oe at 300 K), (e) ZFC and FC magnetization plots of reduced graphene nanoplatelets.
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pone-0038185-g001: Plot of Magnetization (M) v/s Field strength (H) for (a) micro-graphite, (b) oxidized graphite (c) Oxidized Graphene nanoplatelets (d) Reduced Graphene nanoplatelets at 30 K, 150 and 300 K between −50,000 to 50,000 Oe (Inset shows plot between −5000 and 5000 Oe at 300 K), (e) ZFC and FC magnetization plots of reduced graphene nanoplatelets.

Mentions: The structural, chemical and elemental analysis of oxidized graphite, oxidized graphene nanoplatelets, reduced graphene nanoplatelets and graphene nanoribbons are presented in the text S1 section 1 and 2 and Figure S1, S2, S3 and S4. Figure 1 shows the SQUID magnetic characterization of oxidized graphite, oxidized graphene nanoplatelets and reduced graphene nanoplatelets. Analytical grade micro-graphite used as the starting material for the preparation of these particles was the control in these experiments. Figure 1a shows the plot of magnetization (M) versus magnetic field strength (H) for the analytical grade micro-graphite (control) between −50,000 Oe and 50,000 Oe for three temperatures (30 K, 150 K, and 300 K). The negative slope indicates a decrease in the value of magnetic moments with increase in applied magnetic field, which is characteristic of diamagnetic behavior. Figure 1b and c shows the M versus H plot for oxidized graphite and oxidized graphene nanoplatelets, respectively. The plots show a linear increase in the value of the magnetic moments with field strength indicating paramagnetic behavior for both oxidized graphite and oxidized graphene nanoplatelets. The change to paramagnetism upon oxidation of graphite can be attributed to the presence of the paramagnetic Mn2+ ions present in the sample. Figure 1d shows the M versus H plot of reduced graphene nanoplatelets. The plot displays a ferromagnetic hysteresis curve at the lower temperature (30 K) indicating superparamagnetic behavior (inset of Figure 1d) at room temperature (300 K). Room temperature superparamagnetism has been widely reported in nanoparticle clusters (<30 nm) [23], [24], and is a size dependent phenomenon, wherein, the thermal energy of the nanoparticle is sufficient to allow flips in the magnetic spin direction, and insufficient to overcome the spin-spin exchange coupling energy. As a result, in the absence of a magnetic field, the net magnetization measured is zero, and the M versus H curve assumes an ‘S’ shape instead of a hysteresis loop. The zero field cooling (ZFC) and field cooling (FC) curves for the reduced graphene nanoplatelets at uniform field strength of 500 Oe and between 10 K and 300 K are shown in Figure 1e. The peak in the ZFC curve reveals a blocking temperature (TB) of 40 K indicating a transition between ferromagnetic and superparamagnetic states. The remnant magnetization of the hysteresis curve at 30 K is 12.47 emu/g and the coercivity is 6298.68 Oe and could be attributed to the single domain nature, and high shape anisotropy of the sample [25]. The results for reduced graphene nanoplatelets exhibit sharp resemblance with that of hausmannite [25]. Room temperature magnetism has been reported in carbon nanomaterials such as fullerenes, carbon nanotubes, carbon nanofoams, graphene, nanodiamonds and graphite [16], [17], [26]. The magnetic characteristic of these materials include spin-glass-like paramagnetic or ferromagnetic behavior attributed either to the presence of metal impurities or presence of defects in the graphite lattice structure. In case of the oxidized graphite, oxidized graphene nanoplatelets and reduced graphene nanoplatelets, the defects created in graphitic lattice structure during the oxidation or exfoliation process may contribute to the observed magnetic behavior. However, theoretical and experimental studies show the defects in graphitic structures induce very weak magnetic behavior with saturation magnetic moment values of approximately 10−3–10−6 emu/g [27]. Thus, the observed magnetic behavior reported above should be mainly due to the presence of manganese.


Physicochemical characterization, and relaxometry studies of micro-graphite oxide, graphene nanoplatelets, and nanoribbons.

Paratala BS, Jacobson BD, Kanakia S, Francis LD, Sitharaman B - PLoS ONE (2012)

Plot of Magnetization (M) v/s Field strength (H) for (a) micro-graphite, (b) oxidized graphite (c) Oxidized Graphene nanoplatelets (d) Reduced Graphene nanoplatelets at 30 K, 150 and 300 K between −50,000 to 50,000 Oe (Inset shows plot between −5000 and 5000 Oe at 300 K), (e) ZFC and FC magnetization plots of reduced graphene nanoplatelets.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3369907&req=5

pone-0038185-g001: Plot of Magnetization (M) v/s Field strength (H) for (a) micro-graphite, (b) oxidized graphite (c) Oxidized Graphene nanoplatelets (d) Reduced Graphene nanoplatelets at 30 K, 150 and 300 K between −50,000 to 50,000 Oe (Inset shows plot between −5000 and 5000 Oe at 300 K), (e) ZFC and FC magnetization plots of reduced graphene nanoplatelets.
Mentions: The structural, chemical and elemental analysis of oxidized graphite, oxidized graphene nanoplatelets, reduced graphene nanoplatelets and graphene nanoribbons are presented in the text S1 section 1 and 2 and Figure S1, S2, S3 and S4. Figure 1 shows the SQUID magnetic characterization of oxidized graphite, oxidized graphene nanoplatelets and reduced graphene nanoplatelets. Analytical grade micro-graphite used as the starting material for the preparation of these particles was the control in these experiments. Figure 1a shows the plot of magnetization (M) versus magnetic field strength (H) for the analytical grade micro-graphite (control) between −50,000 Oe and 50,000 Oe for three temperatures (30 K, 150 K, and 300 K). The negative slope indicates a decrease in the value of magnetic moments with increase in applied magnetic field, which is characteristic of diamagnetic behavior. Figure 1b and c shows the M versus H plot for oxidized graphite and oxidized graphene nanoplatelets, respectively. The plots show a linear increase in the value of the magnetic moments with field strength indicating paramagnetic behavior for both oxidized graphite and oxidized graphene nanoplatelets. The change to paramagnetism upon oxidation of graphite can be attributed to the presence of the paramagnetic Mn2+ ions present in the sample. Figure 1d shows the M versus H plot of reduced graphene nanoplatelets. The plot displays a ferromagnetic hysteresis curve at the lower temperature (30 K) indicating superparamagnetic behavior (inset of Figure 1d) at room temperature (300 K). Room temperature superparamagnetism has been widely reported in nanoparticle clusters (<30 nm) [23], [24], and is a size dependent phenomenon, wherein, the thermal energy of the nanoparticle is sufficient to allow flips in the magnetic spin direction, and insufficient to overcome the spin-spin exchange coupling energy. As a result, in the absence of a magnetic field, the net magnetization measured is zero, and the M versus H curve assumes an ‘S’ shape instead of a hysteresis loop. The zero field cooling (ZFC) and field cooling (FC) curves for the reduced graphene nanoplatelets at uniform field strength of 500 Oe and between 10 K and 300 K are shown in Figure 1e. The peak in the ZFC curve reveals a blocking temperature (TB) of 40 K indicating a transition between ferromagnetic and superparamagnetic states. The remnant magnetization of the hysteresis curve at 30 K is 12.47 emu/g and the coercivity is 6298.68 Oe and could be attributed to the single domain nature, and high shape anisotropy of the sample [25]. The results for reduced graphene nanoplatelets exhibit sharp resemblance with that of hausmannite [25]. Room temperature magnetism has been reported in carbon nanomaterials such as fullerenes, carbon nanotubes, carbon nanofoams, graphene, nanodiamonds and graphite [16], [17], [26]. The magnetic characteristic of these materials include spin-glass-like paramagnetic or ferromagnetic behavior attributed either to the presence of metal impurities or presence of defects in the graphite lattice structure. In case of the oxidized graphite, oxidized graphene nanoplatelets and reduced graphene nanoplatelets, the defects created in graphitic lattice structure during the oxidation or exfoliation process may contribute to the observed magnetic behavior. However, theoretical and experimental studies show the defects in graphitic structures induce very weak magnetic behavior with saturation magnetic moment values of approximately 10−3–10−6 emu/g [27]. Thus, the observed magnetic behavior reported above should be mainly due to the presence of manganese.

Bottom Line: The chemistry of high-performance magnetic resonance imaging contrast agents remains an active area of research.In this work, we demonstrate that the potassium permanganate-based oxidative chemical procedures used to synthesize graphite oxide or graphene nanoparticles leads to the confinement (intercalation) of trace amounts of Mn(2+) ions between the graphene sheets, and that these manganese intercalated graphitic and graphene structures show disparate structural, chemical and magnetic properties, and high relaxivity (up to 2 order) and distinctly different nuclear magnetic resonance dispersion profiles compared to paramagnetic chelate compounds.The results taken together with other published reports on confinement of paramagnetic metal ions within single-walled carbon nanotubes (a rolled up graphene sheet) show that confinement (encapsulation or intercalation) of paramagnetic metal ions within graphene sheets, and not the size, shape or architecture of the graphitic carbon particles is the key determinant for increasing relaxivity, and thus, identifies nano confinement of paramagnetic ions as novel general strategy to develop paramagnetic metal-ion graphitic-carbon complexes as high relaxivity MRI contrast agents.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Stony Brook University, Stony Brook, New York, United States of America.

ABSTRACT
The chemistry of high-performance magnetic resonance imaging contrast agents remains an active area of research. In this work, we demonstrate that the potassium permanganate-based oxidative chemical procedures used to synthesize graphite oxide or graphene nanoparticles leads to the confinement (intercalation) of trace amounts of Mn(2+) ions between the graphene sheets, and that these manganese intercalated graphitic and graphene structures show disparate structural, chemical and magnetic properties, and high relaxivity (up to 2 order) and distinctly different nuclear magnetic resonance dispersion profiles compared to paramagnetic chelate compounds. The results taken together with other published reports on confinement of paramagnetic metal ions within single-walled carbon nanotubes (a rolled up graphene sheet) show that confinement (encapsulation or intercalation) of paramagnetic metal ions within graphene sheets, and not the size, shape or architecture of the graphitic carbon particles is the key determinant for increasing relaxivity, and thus, identifies nano confinement of paramagnetic ions as novel general strategy to develop paramagnetic metal-ion graphitic-carbon complexes as high relaxivity MRI contrast agents.

Show MeSH