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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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Room temperature EPR spectra of aqueous solutions of (a) oxidized micro-graphite, (b) oxidized graphene nanoplatelets, (c) reduced graphene nanoplatelets and (d) graphene nanoribbons.
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pone-0038185-g004: Room temperature EPR spectra of aqueous solutions of (a) oxidized micro-graphite, (b) oxidized graphene nanoplatelets, (c) reduced graphene nanoplatelets and (d) graphene nanoribbons.

Mentions: Figure 4a–d show the EPR spectra of aqueous solutions of oxidized micro-graphite, oxidized graphene nanoplatelets, reduced graphene nanoplatelets and graphene nanoribbons, respectively (the blank EPR spectrum of the quartz EPR tube and the EPR spectrum of the DPPH is shown in in the Figure S5). The g values, EPR line widths at half heights (ΔH1/2, Gauss), hyperfine coupling constant, and electron relaxation time (T2e) of each EPR spectra are listed in Table 2. All the four samples show 6-line EPR characteristic of an electron coupled to Mn-55 nucleus with spin I = 5/2. The EPR spectra of graphene nanoribbons also show a narrow EPR line at the center with g∼2.007, and line width of 1.2 Gauss due to the presence of free radicals. The observed g values are very close to the free electron spin value, and suggest the absence of spin-orbit coupling in the ground state of manganese ions present in all four samples. The manganese hyperfine coupling (AMn) of approximately 95 Gauss in these samples are very close to that of aqua ions of manganese, Mn(H2O)6. The large hyperfine coupling indicates octahedral coordination in the manganese species of all four samples. The four aqueous samples also show similar narrow line width (ΔH1/2) values between 29.2–31.5 Gauss indicative of long electron relaxation time (T2e). The calculated T2e values were between 2.08–2.25 ns. The free radical species present in the graphene nanoribbons have an order of magnitude longer electron relaxation time (T2e) of 55 ns. It should be noted that the EPR spectra only shows the Mn(II) ions. The spectra did not show presence of Mn(III) ions or other oxidation states of manganese even though, the Raman spectrum of at least reduced graphene nanoplatelets show the presence of Mn(III) ions. A possible reason of this non-detection could be that all the EPR measurements were done at room temperature. Mn(III) ions or other oxidation states of Manganese have very short electron relaxation times, and require very low sample temperatures (∼77 K) to obtain an EPR spectra. Thus, low temperature measurements were also carried out on all the four samples. However, the EPR spectra (results not shown) was dominated by Mn (II) contributions, and the presence of other oxidation states of manganese could not be confirmed, suggesting that most of the manganese ions present in the four samples are present in Mn(II) state.


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)

Room temperature EPR spectra of aqueous solutions of (a) oxidized micro-graphite, (b) oxidized graphene nanoplatelets, (c) reduced graphene nanoplatelets and (d) graphene nanoribbons.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3369907&req=5

pone-0038185-g004: Room temperature EPR spectra of aqueous solutions of (a) oxidized micro-graphite, (b) oxidized graphene nanoplatelets, (c) reduced graphene nanoplatelets and (d) graphene nanoribbons.
Mentions: Figure 4a–d show the EPR spectra of aqueous solutions of oxidized micro-graphite, oxidized graphene nanoplatelets, reduced graphene nanoplatelets and graphene nanoribbons, respectively (the blank EPR spectrum of the quartz EPR tube and the EPR spectrum of the DPPH is shown in in the Figure S5). The g values, EPR line widths at half heights (ΔH1/2, Gauss), hyperfine coupling constant, and electron relaxation time (T2e) of each EPR spectra are listed in Table 2. All the four samples show 6-line EPR characteristic of an electron coupled to Mn-55 nucleus with spin I = 5/2. The EPR spectra of graphene nanoribbons also show a narrow EPR line at the center with g∼2.007, and line width of 1.2 Gauss due to the presence of free radicals. The observed g values are very close to the free electron spin value, and suggest the absence of spin-orbit coupling in the ground state of manganese ions present in all four samples. The manganese hyperfine coupling (AMn) of approximately 95 Gauss in these samples are very close to that of aqua ions of manganese, Mn(H2O)6. The large hyperfine coupling indicates octahedral coordination in the manganese species of all four samples. The four aqueous samples also show similar narrow line width (ΔH1/2) values between 29.2–31.5 Gauss indicative of long electron relaxation time (T2e). The calculated T2e values were between 2.08–2.25 ns. The free radical species present in the graphene nanoribbons have an order of magnitude longer electron relaxation time (T2e) of 55 ns. It should be noted that the EPR spectra only shows the Mn(II) ions. The spectra did not show presence of Mn(III) ions or other oxidation states of manganese even though, the Raman spectrum of at least reduced graphene nanoplatelets show the presence of Mn(III) ions. A possible reason of this non-detection could be that all the EPR measurements were done at room temperature. Mn(III) ions or other oxidation states of Manganese have very short electron relaxation times, and require very low sample temperatures (∼77 K) to obtain an EPR spectra. Thus, low temperature measurements were also carried out on all the four samples. However, the EPR spectra (results not shown) was dominated by Mn (II) contributions, and the presence of other oxidation states of manganese could not be confirmed, suggesting that most of the manganese ions present in the four samples are present in Mn(II) state.

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