High Protein Electronics: Students make Biological Nanowires

By David Rosenberg, Chemical Engineering, 2020

This summer, a team of students at the University of Kent in England developed a method for fabricating nanowires using bacteria. The project, presented at the International Genetically Engineered Machines Jamboree this September, aims to increase sustainability in nanowire manufacturing. It was inspired by recent research in functional amyloid, a class of self-assembling protein structures.

Amyloid proteins contain beta sheets, zigzagging amino acid chains linked in a grid by hydrogen bonds, and can be staked in a column or spiral to form strongly bound fibers. When a free amyloid unit comes into contact with a fiber, it can alter its shape and join the stack. This structure is physically and chemically very stable, making amyloid an attractive material because of its strength and ability to self-assemble.

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Representation of a beta sheet structure[/caption]

Kent students combined DNA from a yeast amyloid protein, a peptide signal, and a cytochrome protein. The yeast DNA encodes the self-polymerizing portion of a protein involved in transcription which deactivates itself by combining into strands. The peptide signal is part of E. coli’s own amyloid synthesis and causes the cell to assemble the amyloid on its surface before releasing it. A cytochrome from the electron transport chain attached to the amyloid protein attracts a current conducting heme molecule. The resulting strands could potentially replace expensive and less sustainable metal wires in electronics and research.

Typically, a nanowire is a material with two dimensions approaching the wavelength of its electrons. At this scale, quantum effects that have very little impact on larger wires become predominant. Combined with a very high surface area to volume ratio, this gives nanowires unique properties and challenges. Various types of nanowires have been prepared for efficient energy storage, molecule detection, and research. They have also been prepared to carry electricity in nanoscale circuits for computer chips, disk drives, and biological implants or transparent, flexible circuits and solar cells.

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Structural representation of a cytochrome, one of the components used to produce biological nanowires[/caption]

The wires developed at Kent are 5–10 nanometers in diameter and can resist temperatures up to 98°C, overcoming a major drawback of silver nanowires, although the ability of the wires to conduct electricity remains uncertain. The biological nature of the wires gives them unique advantages. “If you use the bacterial systems, then you can imagine that you can feed the bacteria with a waste product . . . and also the nanowire you are producing is a protein-based nanowire” said Dr. Wei-Feng Xue, an advisor on the project and a chemical biology professor at the University of Kent.

Nanowire-forming microbes could also potentially be used in living circuits and modified to export electricity generated through normal respiration, creating self-assembling, self-repairing electronics that generate their own power from the environment. In the nearer future, they could be used in a biological battery known as a microbial fuel cell (MFC).

‘Conventional’ nanowire manufacturing includes a wide array of techniques adapted to different wire shapes and compositions. Lithography and templating typically involve depositing molten or gaseous material on a polymer which is later removed. Alternately, wires can be chemically synthesized. A catalyst can be used to draw dissolved ions out of solution or to attract gaseous particles. Electrodeposition builds wires directly from an electrolyte solution by applying a current that converts ions to solid metal on microscopic anodes.

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A conventional nanowire is held in place under an electron microscope[/caption]

Microbial synthesis would replace unsustainable polymers used in lithographic techniques and templating with organic molecules and without the high economic and environmental expense of depositing gaseous metals, which requires operation in a vacuum at temperatures close to 500 degrees Celsius. It would also replace expensive and often toxic materials used as catalysts or solvents.

This development builds on work published in the Proceedings of the National Academy of Sciences in which the same protein fragment was modified to bind gold nanoparticles and produced in E. coli. Amyloid fibers assembled outside the cell from small fragments were coated with gold and silver. The paper mentions a simultaneous study of metal-plated DNA wires, which are less stable but easy to modify and adapt. The method developed at Kent eliminates the need for outside modification and expensive nanoparticles while generating much thinner wires.

Purely organic wires won’t be ready for production any time soon, however. Resistance measured over cultures of the modified bacteria was highly erratic and much greater than that of purely metallic or metal-coated nanowires, possibly in part due to a tendency of released fibers to form clumps that kept electrons from reaching the heme groups. Further research will also be needed to control the length of the wires.

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