166
M. L. Fdez-Gubieda et al.
7.3 Magnetic Properties of Magnetosomes and
Magnetosome Chains in Magnetospirillum
gryphiswaldense
As introduced in Sect. 7.1, the morphology and size of the magnetosomes are
specific of each bacterial species. In particular, M. gryphiswaldense synthesize cubeoctahedral magnetite nanoparticles with an average diameter of ≈45 nm and a narrow size distribution ≈8 nm. Transmission electron microscopy (TEM) and cryotomography techniques reveal that the magnetosome presents faceted morphology
where the [111] axes define the growth directions of the hexagonal faces (Fig. 7.9a, b).
The bacteria organize the magnetosomes forming a chain where the hexagonal faces
are disposed face to face being the total magnetic moment oriented along the chain
axis. In this configuration the magnetosome chain behaves as a compass needle and
under the action of an external magnetic field, the magnetic torque reorients the
bacteria towards the field direction.
7.3.1 The Verwey Transition in the Magnetosomes
Electron micro-diffraction on a single magnetosome confirms that they are single
magnetite crystals, Fe 3 O 4 . Figure 7.6a shows the electron diffraction pattern of a
[111] axis of magnetite. The magnetic response of the cells also corresponds to
pure magnetite, as confirmed by the evolution of the magnetization as a function of
temperature (M(T )) measured at 5 mT (Fig. 7.6b). M(T ) curves display a marked
irreversibility in the whole analyzed temperature range, with the blocking temperature 300 K as expected owing to the large size of magnetosomes. The zero-field
cooled (ZFC) curve shows a sharp transition at T V = 107 K corresponding to the
well-known Verwey transition, a cubic-to-monoclinic crystallographic phase transition characteristic of magnetite. It should be noted that although this Verwey transition occurs at 120 K in bulk magnetite, lower values (between 102 and 117 K) are
found in magnetosomes [47–49]. The fact that the Verwey transition is so abrupt
reflects the homogenous stoichiometry of the magnetite of magnetosomes. At 30 K
the ZFC presents a shoulder attributed to the ordering of electron spins in magnetite
at low temperature [50–52]. The Verwey transition is also observed in the field cooled
(FC) curve. This time, the magnetization decreases only slightly below T V remaining
constant down to 5 K. In the FC curve the low-temperature transition is absent.
7.3.2 Magnetic Interactions in the Magnetosome Chain
The magnetosome chain inside the bacteria consists of ≈20 to 30 magnetic nanoparticles each of them surrounded by a lipid bilayer membrane with a thickness of 4 nm
M. L. Fdez-Gubieda et al.
7.3 Magnetic Properties of Magnetosomes and
Magnetosome Chains in Magnetospirillum
gryphiswaldense
As introduced in Sect. 7.1, the morphology and size of the magnetosomes are
specific of each bacterial species. In particular, M. gryphiswaldense synthesize cubeoctahedral magnetite nanoparticles with an average diameter of ≈45 nm and a narrow size distribution ≈8 nm. Transmission electron microscopy (TEM) and cryotomography techniques reveal that the magnetosome presents faceted morphology
where the [111] axes define the growth directions of the hexagonal faces (Fig. 7.9a, b).
The bacteria organize the magnetosomes forming a chain where the hexagonal faces
are disposed face to face being the total magnetic moment oriented along the chain
axis. In this configuration the magnetosome chain behaves as a compass needle and
under the action of an external magnetic field, the magnetic torque reorients the
bacteria towards the field direction.
7.3.1 The Verwey Transition in the Magnetosomes
Electron micro-diffraction on a single magnetosome confirms that they are single
magnetite crystals, Fe 3 O 4 . Figure 7.6a shows the electron diffraction pattern of a
[111] axis of magnetite. The magnetic response of the cells also corresponds to
pure magnetite, as confirmed by the evolution of the magnetization as a function of
temperature (M(T )) measured at 5 mT (Fig. 7.6b). M(T ) curves display a marked
irreversibility in the whole analyzed temperature range, with the blocking temperature 300 K as expected owing to the large size of magnetosomes. The zero-field
cooled (ZFC) curve shows a sharp transition at T V = 107 K corresponding to the
well-known Verwey transition, a cubic-to-monoclinic crystallographic phase transition characteristic of magnetite. It should be noted that although this Verwey transition occurs at 120 K in bulk magnetite, lower values (between 102 and 117 K) are
found in magnetosomes [47–49]. The fact that the Verwey transition is so abrupt
reflects the homogenous stoichiometry of the magnetite of magnetosomes. At 30 K
the ZFC presents a shoulder attributed to the ordering of electron spins in magnetite
at low temperature [50–52]. The Verwey transition is also observed in the field cooled
(FC) curve. This time, the magnetization decreases only slightly below T V remaining
constant down to 5 K. In the FC curve the low-temperature transition is absent.
7.3.2 Magnetic Interactions in the Magnetosome Chain
The magnetosome chain inside the bacteria consists of ≈20 to 30 magnetic nanoparticles each of them surrounded by a lipid bilayer membrane with a thickness of 4 nm