Magnetic Colloids from Magnetotactic Bacteria: Chain Formation and Colloidal Stability

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Original entry: William Bonificio, AP 225, Fall 2009

Information

Magnetic Colloids from Magnetotactic Bacteria: Chain Formation and Colloidal Stability. Albert P. Philipse and, Diana Maas. Langmuir 2002 18 (25), 9977-9984


Soft matter keywords

Magnetotactic Bacteria, paramagnetism, iron oxide, colloids, debye length.

Summary

The purpose of this study was to investigate the interactions and colloidal stability of the single domain magnetic nanoparticles produced by magnetotactic bacteria. Magnetotactic bacteria are a really cool type of bacteria that create these iron oxide paramagnetic nanoparticles within their cell walls that align with the earth's magnetic field which in turn direct the the bacteria down towards the ocean floor, to anaerobic conditions where they thrive. These researchers wanted to look at how these magnetic particles would effect the interactions between different bacteria, and to investigate the properties of the magnetic colloids when extracted from the bacteria.

Soft matter discussion

Top and bottom images show a magnetotactic bacteria with magnetic colloids formed as chains. Middle image shows a magnetotactic bacteria with extracted magnetic colloids.


All different really cool formations and aggregations that spontaneously arose from extracted magnetic colloids.


Magnetic (left) and topographic (right) images of the extracted magnetic colloids.

The academic group conducting this research was interested in single domain iron oxide colloids. They noted that the size of the colloids in magnetotactic bacteria (50nm) was much larger than anything that would form spontaneously form ferrofluids (10nm). Because they are so large they are expected to have much stronger dipole forces, and should form different patterns such as chains and rings if removed from the bacteria. The group wants to look at the properties of these colloids when removed from the bacteria, and this is their first study examining the magnetic properties of the bacteria themselves.

The first thing they did was cultivate the bacteria, after this was performed they took some awesome pictures as shown. They also removed the magnetic colloids and let them spontaneously interact. Not surprisingly, they aggregated and formed some interesting structures, such as loops, chains and 'handles' (a combination between a loop and a chain). They noted that for these interactions to occur the colloids must be very magnetic and have large dipole energies. They measured the magnetism and it was in line with there estimates. They also noticed that the ring and handle formation never formed within bacteria. They attributed this to the thought that maybe the bacteria somehow shielded the effects of the magnet so that the colloids could only form the chains they were produced in.

To experiment with this thought they put different magnetostatic bacteria together to see their interaction. They calculated the debye screening length and noted that it should not cause any repulsion between two side by side bacteria. They then calculated the van der waals forces between two bacteria to measure the attraction they should feel towards each other. They then did force measurements to see if the bacteria were at all attracted to each other due to the magnetic dipole force, and the results showed they were not attracted to one another. The magnetic attraction, which is large enough to feel the earth's magnetic field, is also well screened so that it does not see any of its neighbors in other cells. They did some modeling of the magnetic colloids within bacteria to show that the interactions with other colloids does not in any way effect colloidal stability.

FInally, they did sedimentation experiments that showed the bacteria did not aggregate at, again expected based on the previous results. This could have evolutionary advantages basd on competition for nutrients if the bacteria formed large clumps, or their magnetic poles effected one another. This was a neat article!