14-09-2018 | | By Rob Coppinger
Gold and semiconductors are to be assembled into nano-structures using a genetically engineered virus that was originally found in the Escherichia coli bacteria, which is typically found in digestive tracts.
The shape of the virus combined with the atoms’ properties, which lead them to form stable crystals, enables the self-assembly of tiny structures that can be useful for electronics. The modified virus is called the M13 bacteriophage and it is used because of the wide range of shapes it can form. Viruses exist in a multitude of shapes and their surfaces have different receptors that can bind to molecules. This virus binding, atom crystal forming procedure has produced nanostructures that could be used for tiny battery electrodes, supercapacitors, sensors, photocatalytic materials, and photovoltaics.
“We’re working on combining semiconductors and gold on the same virus,” says Elaine Haberer, an associate professor in the University of California’s electrical and computer engineering department. Haberer and her colleagues have altered a virus to arrange gold atoms into spheroids measuring a few nanometres in diameter. The self-assembly of atoms into shapes is possible with a virus because of the proteins on the outside.
Those protein receptors can bind with gold and other elements. A virus consists of its genetic core, the deoxyribonucleic acid, or DNA, and a shell of protein called a capsid. The proteins consist of long strings of amino acids. There are 20 different amino acids and they can form peptides which are short sequences of five to 15 amino acids. Longer sequences of amino acids become proteins. By modifying the DNA, the proteins become better at binding to ions, charged particles, of metals. Ions of gold or other elements are introduced into a solution containing the viruses and they bind together. Chloroform is then used as a catalyst to advance the self-assembly.
A four step process is illustrated here showing how the gold is mixed in solution with the viruses. The reducing agent is the organic solvent, which is chloroform. Credit: University of California Riverside/Tam-Triet Ngo-Duc
The solution is also heated and the temperature and the time will determine what shape structure will be produced. The basic shape that can be produced is a filament. The genetically engineered M13 bacteriophages can form gold nanowires. “The virus starts out as a filament, a micron long and only six to seven nanometres in diameter. We [use] organic solvent [chloroform] and it retains its structural stability, but it shrinks,” explained Haberer. “It shrinks to a 200-300nm [long] rod and gets a bit fatter, so its tens of nanometres in diameter. If it goes further, it goes to a sub-100nm sphere.” However, the process can be stopped at the rod stage by lowering the solutions temperature.
The blue circle is the virus and in these four images, the gold ions attach to the virus until the shape that is formed is a spheroid. The reducing agent is the chloroform. Credit: University of California Riverside/Tam-Triet Ngo-Duc
The researchers are also using gold nanobeads they can make with this process to clean wastewater where, the use of light as a catalyst, allows the gold nanobeads to bind with the pollutants. Haberer’s team’s work has been supported by the United States Navy’s Office of Naval Research since 2014. Habere said the research began a year earlier.
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