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. 2014 Feb 20;4:4104.
doi: 10.1038/srep04104.

Controlling Magnetotactic Bacteria Through an Integrated Nanofabricated Metallic Island and Optical Microscope Approach

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Free PMC article

Controlling Magnetotactic Bacteria Through an Integrated Nanofabricated Metallic Island and Optical Microscope Approach

Lina M González et al. Sci Rep. .
Free PMC article

Abstract

Herein, we demonstrate the control of magnetotactic bacteria through the application of magnetic field gradients with real-time visualization. We accomplish this control by integrating a pair of macroscale Helmholtz coils and lithographically fabricated nanoscale islands composed of permalloy (Ni₈₀Fe₂₀). This system enabled us to guide and steer amphitrichous Magnetospirillum magneticum strain AMB-1 to specific location via magnetic islands. The geometries of the islands allowed us to have control over the specific magnetic field gradients on the bacteria. We estimate that magnetotactic bacteria located less than 1 μm from the edge of a diamond shaped island experience a maximum force of approximately 34 pN, which engages the bacteria without trapping them. Our system could be useful for a variety of applications including magnetic fabrication, self-assembly, and probing the sensing apparatus of magnetotactic bacteria.

Figures

Figure 1
Figure 1. Integrated nanofabricated metallic island and optical microscope for control magnetic fields on magnetic particles and magnetotactic bacteria.
(a) Schematic of experimental set-up: Helmholtz coils integrated with nanofabricated permalloy (Ni80Fe20) islands. (b) Custom-built pair of Helmholtz coils to fit an inverted microscope for magnetizing the permalloy island. Inset (i) is a front view of the Helmholtz coils and the specimen holder. Inset (ii) is a side view of the Helmholtz coils (c) Characterization of the Helmholtz coils showing magnetic field strength versus applied current to the Helmholtz coils.
Figure 2
Figure 2. Fabrication of permalloy islands to make magnetic concentrators.
(a) The image reversal fabrication steps implemented to make permalloy islands of controlled geometries. (b) Bright field microscope images of different shapes of the permalloy islands fabricated using the image reversal method. Scale bar, 10 μm.
Figure 3
Figure 3. Characterization of the permalloy island and magnetic bead response.
(a) A two-dimensional field intensity profile of the application of magnetic fields on the permalloy islands through our Helmholtz coils simulated using finite element method magnetics (FEMM). The red circle represents the approximate area of homogenous magnetic field where the permalloy islands are placed in the simulation. (b) Simulation results of the islands with applied external magnetic fields from the Helmholtz coils of about 80 Oe (top). Experimental comparison of these concentrated magnetic field regions through tracking ferromagnetic bead movement with the applied magnetic field (bottom). Scale bar, 10 μm. Magnetic hysteresis loop of the ferromagnetic beads and permalloy film using an alternative gradient force magnetometer (AGFM). (c) AGFM hysteresis loop of the ferromagetic beads with a coercive field of 567 Oe and (d) AGM hysteresis loop of the permalloy films with a coercive field of 14 Oe. Both magnetic hysteresis curves are normalized by the moment.
Figure 4
Figure 4. Force versus distance results for the square- and diamond- shaped magnetic islands with an applied magnetic field.
The square and circles represent the data for fields equal to 40 G and 120 G, respectively. (a–b) Force due to square island with height equal to 30 nm and 120 nm. (c–d) Force due to the diamond island with height equal to 30 nm and 120 nm.
Figure 5
Figure 5. Forces on the magnetosomes from the islands and the effect on increasing the magnetosome chains size.
(a) The force vs. distance curve distance curves indicate the different sizes in the magnetosome chain of 0.5 (blue), 1 (cyan), 1.5 (magenta) and 2 (green) microns in length. The surface area is fixed and is 1.9 × 10−15 (50 nm in diameter). The forces on the magnetosome increase exponentially within a few microns from the edge of the island.
Figure 6
Figure 6. MTB interactions with notch and diamond islands.
(Left panel) MTB were tracked as they moved from the upstream region to the notch region. (a) Figure showing the different regions (upstream and notched regions). (b) Schematic showing the magnetic field lines near the notch island. (c) A schematic of the coordinates to track the deflection. Orientation of the MTB when they are upstream of the notch, nr (d) and as they approach the region above the notch, ur (e) when compared to vertical distance (y) away from the islands. The field corresponding to d–e is 3 Oe and the thickness of the island is 30 nm. (Right panel). The black symbols represent the bacteria when a field is applied and the blue symbols represent the bacteria when there is no external magnetic field. (Right panel) This panel shows the movement of MTB along the edge of the diamond island. (f) FEMM simulation of the diamond magnetic island showing the differences in the field between the “engaging” and “escape” zones. (g) A micrograph of the diamond island, which is 120 μm in thickness and the external magnetic field is 120 Oe. (Scale bar, 5 μm). (h–m) Image sequences of the magnetotactic bacteria moving along the edge of the diamond island. The bacterium (highlighted with a red circle) moves toward the engaging zone and glides along the edge (as in a “rail”) of this island until moving away from the island at the escape zone.

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