Georgia C. Papaefthymiou

Fe3O4 precipitation in magnetotactic bacteria

Abstract Using Mossbauer resonance spectroscopy of 57Fe, we have determined the nature and distribution of major iron compounds in the magnetotactic bacterium Aquaspirillum magnetotacticum. In addition to magnetite (Fe3O4), cells contained a low-density hydrous ferric oxide, a high-density hydrous ferric oxide (ferrihydrite), and ferrous iron. Analysis at different temperatures of whole cells harvested early and late in growth, of mutant cells unable to synthesize magnetite, and of cell fractions enriched in 57Fe indicated that Fe3O4 precipitation resulted from partial reduction of the high-density hydrous ferric oxide precursor.

Magnetic properties of iron oxide nanoclusters within microdomains of block copolymers

Abstract The magnetic properties of γ-Fe 2 O 3 nanoclusters grown within the microdomains of optically transparent block copolymer films were investigated. SQUID magnetometry and 57 Fe Mossbauer spectroscopy were employed with characteristic measurement times of 100 s and 10 −8 s, respectively. The observed magnetism of the ca. 5 nm diameter particles was dominated by quantum-size effects, collective magnetic excitations, and superparamagnetic relaxation processes associated with the sub-magnetic domain γ-Fe 2 O 3 nanoclusters. The combination of these two techniques allowed determination of the values of the magnetic anisotropy constant ( K = 1.58 × 10 5 J/m 3 ) and the pre-exponential factor ( τ 0 = 4.2 × 10 −12 s) which determine the magnetic behavior of the nanocomposites. At low temperatures, the nanocomposite films exhibited hysteresis with the saturated coercivity, H c 0 , equal to 530 Oe.

Magnetic properties of CoFe2O4 nanoparticles synthesized through a block copolymer nanoreactor route

The development of self-assembled magnetic CoFe2O4 nanoparticles within polymer matrices at room temperature is reported. Diblock copolymers consisting of poly (norbornene) and poly (norbornene-dicarboxcylic acid) (NOR/NORCOOH) were synthesized. The self-assembly of the mixed metal oxide within the NORCOOH block was achieved at room temperature by processing the copolymer nanocomposite using wet chemical methods. Morphology and magnetic properties were determined by superconducting quantum interference device magnetometry, transmission electron microscopy, wide angle x-ray diffraction, and 57Fe Mossbauer spectroscopy. The CoFe2O4 nanoparticles are uniformly dispersed within the polymer matrix, and have an average radius of 4.8±1.4 nm. The nanocomposite films are superparamagnetic at room temperature and ferrimagnetic at 5 K.

Synthesis and characterization of submicron single-crystalline Bi2Fe4O9 cubes

Single-crystalline, submicron-sized Bi2Fe4O9 cubes of reproducible shape have been successfully prepared using a facile, large-scale solid-state reaction employing a molten salt technique in the presence of a nonionic surfactant. The role of surfactant as well as alterations in the molar ratio of Bi3+ to Fe3+ precursors have been examined under otherwise identical reaction conditions and correlated with the predictive formation of different shapes of Bi2Fe4O9 products. Extensive structural characterization of as-prepared samples has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Mossbauer spectroscopy, and X-ray diffraction (XRD). Magnetic measurements were obtained using a superconducting quantum interference device (SQUID).

Novel γ-Fe2O3/SiO2 magnetic nanocomposites via sol-gel matrix-mediated synthesis

Magnetic single-domain γ-Fe2O3 nanoclusters have been prepared by (i) coprecipitation of ferrous and ferric salts encapsulated within sol-gel derived silica (SiO2), and (ii) oxidation of ferrous cations incorporated in a sulfonated, ion-exchange porous silica matrix. In the first method, a SiO2 coating was formed on the Fe2O3 nanoclusters by hydrolysis and condensation of tetraethoxysilane. In the second route, sulfite (SO−3) functionalized silica was synthesized from a modified sol-gel precursor resulting in a porous gel with a narrow pore size distribution. The SiO2 coating and sulfonated SiO2 matrix provide the means for homogeneous dispersion of Fe2O3 clusters. The SiO2-coated Fe2O3 nanoclusters are spherical with 4–5 nm diameters. Acicular crystalline Fe2O3 nanoclusters with diameters of ∼4 nm and lengths of 10–20 nm were found in the porous sulfonated gel matrix. Magnetization and Mossbauer studies indicated that the γ-Fe2O3/SiO2 nanocomposites are superparamagnetic. The magnetic properties can be manipulated via the matrix structure and synthesis conditions.

Functionalization of Graphene Oxide and its Biomedical Applications

Graphene oxide (GO) offers interesting physicochemical and biological properties for biomedicine due to its versatility, biocompatibility, small size, large surface area, and its ability to interact with biological cells and tissues. GO is a two-dimensional material of exceptional strength, unique optical, physical, mechanical, and electronic properties. Ease of functionalization and high antibacterial activity are two major properties identified with GO. Due to its excellent aqueous processability, amphiphilicity, surface functionalization capability, surface enhanced Raman scattering (SERS), and fluorescence quenching ability, GO chemically exfoliated from oxidized graphite is considered a promising material for biological applications. In addition, due to π-π* transitions, a low energy is required for electron movement, a property important in Biosensor and Bioimaging applications of GO. In this article, we present an overview of current advances in GO applications in biomedicine and discuss future perspectives. We conclude that GO is going to play a vital role in Biomedical applications in the near future.

A thermally stable {FeNO}8 complex: properties and biological reactivity of reduced MNO systems

Reduced nitrogen oxide ligands such as NO−/HNO or nitroxyl participate in chemistry distinct from nitric oxide (NO). Nitroxyl has been proposed to form at heme centers to generate the Enemark–Feltham designated {FeNO}8 system. The synthesis of a thermally stable {FeNO}8 species namely, [Co(Cp*)2][Fe(LN4)(NO)] (3), housed in a heme-like ligand platform has been achieved by reduction of the corresponding {FeNO}7 complex, [Fe(LN4)(NO)] (1), with decamethylcobaltocene [Co(Cp*)2] in toluene. This complex readily reacts with metMb, resulting in formation of MbNO via reductive nitrosylation by the coordinated HNO/NO−, which can be inhibited with GSH. These results suggest that 3 could serve as a potential HNO therapeutic. Spectroscopic, theoretical, and structural comparisons are made to 1 and the {CoNO}8 complex, [Co(LN4)(NO)] (2), an isoelectronic analogue of 3.

Binding of Fe2+ by mammalian ferritin

An average of 140 Fe2+ ions bind to mammalian ferritin that has an average core content of 1876 Fe3+ ions per molecule. The Fe2+ ions enter the protein interior and exchange electrons with the core Fe3+ ions. A non-homogeneous model for the iron-containing core of ferritin is proposed.

Magnetosome dynamics in magnetotactic bacteria

Diffusive motions of the magnetosomes (enveloped Fe3O4 particles) in the magnetotactic bacterium Aquaspirillum magnetotacticum result in a very broad-line Mössbauer spectrum (T approximately 100 mm/s) above freezing temperatures. The line width increases with increasing temperature. The data are analyzed using a bounded diffusion model to yield the rotational and translational motions of the magnetosomes as well as the effective viscosity of the material surrounding the magnetosomes. The results are [theta 2] l/2 less than 1.5 degrees and [x2] 1/2 less than 8.4 A for the rotational and translational motions, respectively, implying that the particles are fixed in whole cells. The effective viscosity is 10 cP at 295 K and increases with decreasing temperature. Additional Fe3+ material in the cell is shown to be associated with the magnetosomes. Fe2+ material in the cell appears to be associated with the cell envelope.

Nature of iron deposits on the cardiac walls in β-thalassemia by Mössbauer spectroscopy

An identification of the nature and an estimation of the particle size distribution of the iron deposits on thalassemic heart tissue is carried out by variable temperature Mössbauer spectroscopy. Comparison of Mössbauer spectra obtained for the thalassemic heart tissue (I) with those of normal heart tissue (II) and of horse spleen ferritin (III) identifies the iron deposits to be small, superparamagnetic particles of ferritin and/or hemosiderin, two closely related iron storage proteins containing an iron core of (FeOOH)8(FeO x OPO3H2). The dependence of the superparamagnetic relaxation time, tau, of magnetically ordered fine particles on their volume V via the magnetic anisotropy constant K of the material and the condition tau > tauL, the Larmor precession time of the nuclear magnetic moment of 57Fe about an effective magnetic field, for observation of hyperfine structure are used in analyzing the Mössbauer data to yield the particle size distribution. Particle diameters are estimated to be 74 +/- 12 A.