@MIT Bacterial Cells produce Biofilms incorporating Nonliving Materials: Gold Nanoparticles and Quantum Dots

bacterial-cellCambridge, MA (Scicasts) – Inspired by natural materials such as bone — a matrix of minerals and other substances, including living cells — MIT engineers have coaxed bacterial cells to produce biofilms that can incorporate nonliving materials, such as gold nanoparticles and quantum dots.

These “living materials” combine the advantages of live cells, which respond to their environment, produce complex biological molecules, and span multiple length scales, with the benefits of nonliving materials, which add functions such as conducting electricity or emitting light.


An artist’s rendering of a bacterial cell engineered to produce amyloid nanofibers that incorporate particles such as quantum dots (red and green spheres) or gold nanoparticles. Image: Yan Liang

The new materials represent a simple demonstration of the power of this approach, which could one day be used to design more complex devices such as solar cells, self-healing materials, or diagnostic sensors, says Timothy Lu, an assistant professor of electrical engineering and biological engineering. Lu is the senior author of a paper describing the living functional materials in the March 23 issue of Nature Materials.

“Our idea is to put the living and the nonliving worlds together to make hybrid materials that have living cells in them and are functional,” Lu says. “It’s an interesting way of thinking about materials synthesis, which is very different from what people do now, which is usually a top-down approach.”

The paper’s lead author is Allen Chen, an MIT-Harvard MD-PhD student. Other authors are postdocs Zhengtao Deng, Amanda Billings, Urartu Seker, and Bijan Zakeri; recent MIT graduate Michelle Lu; and graduate student Robert Citorik.

Self-assembling materials

Lu and his colleagues chose to work with the bacterium E. coli because it naturally produces biofilms that contain so-called “curli fibres” — amyloid proteins that help E. coli attach to surfaces. Each curli fibre is made from a repeating chain of identical protein subunits called CsgA, which can be modified by adding protein fragments called peptides. These peptides can capture nonliving materials such as gold nanoparticles, incorporating them into the biofilms.

By programming cells to produce different types of curli fibres under certain conditions, the researchers were able to control the biofilms’ properties and create gold nanowires, conducting biofilms, and films studded with quantum dots, or tiny crystals that exhibit quantum mechanical properties. They also engineered the cells so they could communicate with each other and change the composition of the biofilm over time.

First, the MIT team disabled the bacterial cells’ natural ability to produce CsgA, then replaced it with an engineered genetic circuit that produces CsgA but only under certain conditions — specifically, when a molecule called AHL is present. This puts control of curli fiber production in the hands of the researchers, who can adjust the amount of AHL in the cells’ environment. When AHL is present, the cells secrete CsgA, which forms curli fibers that coalesce into a biofilm, coating the surface where the bacteria are growing.

The researchers then engineered E. coli cells to produce CsgA tagged with peptides composed of clusters of the amino acid histidine, but only when a molecule called aTc is present. The two types of engineered cells can be grown together in a colony, allowing researchers to control the material composition of the biofilm by varying the amounts of AHL and aTc in the environment. If both are present, the film will contain a mix of tagged and untagged fibres. If gold nanoparticles are added to the environment, the histidine tags will grab onto them, creating rows of gold nanowires, and a network that conducts electricity.

‘Cells that talk to each other’

The researchers also demonstrated that the cells can coordinate with each other to control the composition of the biofilm. They designed cells that produce untagged CsgA and also AHL, which then stimulates other cells to start producing histidine-tagged CsgA.

“It’s a really simple system but what happens over time is you get curli that’s increasingly labelled by gold particles. It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time,” Lu says. “Ultimately, we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals.”

To add quantum dots to the curli fibres, the researchers engineered cells that produce curli fibers along with a different peptide tag, called SpyTag, which binds to quantum dots that are coated with SpyCatcher, a protein that is SpyTag’s partner. These cells can be grown along with the bacteria that produce histidine-tagged fibres, resulting in a material that contains both quantum dots and gold nanoparticles.

These hybrid materials could be worth exploring for use in energy applications such as batteries and solar cells, Lu says. The researchers are also interested in coating the biofilms with enzymes that catalyze the breakdown of cellulose, which could be useful for converting agricultural waste to biofuels. Other potential applications include diagnostic devices and scaffolds for tissue engineering.

“I think this is really fantastic work that represents a great integration of synthetic biology and materials engineering,” says Lingchong You, an associate professor of biomedical engineering at Duke University who was not part of the research team.

The research was funded by the Office of Naval Research, the Army Research Office, the National Science Foundation, the Hertz Foundation, the Department of Defense, the National Institutes of Health, and the Presidential Early Career Award for Scientists and Engineers.

The original article was written by Anne Trafton, MIT News Office.

Publication: Synthesis and patterning of tunable multiscale materials with engineered cells. Allen Y. Chen, Zhengtao Deng, Amanda N. Billings, Urartu O. S. Seker, Michelle Y. Lu, Robert J. Citorik, Bijan Zakeri, Timothy K. Lu. Nature Materials (March 23, 2014):http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3912.html


Quantum Dots from a Familiar Energy Source, Coal: Video

201306047919620The prospect of turning coal into fluorescent particles may sound too good to be true, but the possibility exists, thanks to scientists at Rice University.

The Rice lab of chemist James Tour found simple methods to reduce three kinds of coal into graphene quantum dots (GQDs), microscopic discs of atom-thick graphene oxide that could be used in medical imaging as well as sensing, electronic and photovoltaic applications.

Coal yields production of graphene quantum dots

Band gaps determine how a semiconducting material carries an electric current. In quantum dots, band gaps are responsible for their fluorescence and can be tuned by changing the dots’ size. The process by Tour and company allows a measure of control over their size, generally from 2 to 20 nanometers, depending on the source of the coal.


An illustration shows the nanostructure of bituminous coal before separation into graphene quantum dots. Courtesy of the Tour Group

There are many ways to make GQDs now, but most are expensive and produce very small quantities, Tour said. Though another Rice lab found a way last year to make GQDs from relatively cheap carbon fiber, coal promises greater quantities of GQDs made even cheaper in one chemical step, he said.

“We wanted to see what’s there in coal that might be interesting, so we put it through a very simple oxidation procedure,” Tour explained. That involved crushing the coal and bathing it in acid solutions to break the bonds that hold the tiny graphene domains together.

“You can’t just take a piece of graphene and easily chop it up this small,” he said.

Tour depended on the lab of Rice chemist and co-author Angel Martí to help characterize the product. It turned out different types of coal produced different types of dots. GQDs were derived from bituminous coalanthracite and coke, a byproduct of oil refining.

Graphene quantum dots

An electron microscope image shows the stacking layer structure of graphene quantum dots extracted from anthracite. The scale bar equals 100 nanometers. Courtesy of the Tour Group.

The coals were each sonicated in nitric and sulfuric acids and heated for 24 hours. Bituminous coal produced GQDs between 2 and 4 nanometers wide. Coke produced GQDs between 4 and 8 nanometers, and anthracite made stacked structures from 18 to 40 nanometers, with small round layers atop larger, thinner layers. (Just to see what would happen, the researchers treated graphite flakes with the same process and got mostly smaller graphite flakes.)

Tour said the dots are water-soluble, and early tests have shown them to be nontoxic. That offers the promise that GQDs may serve as effective antioxidants, he said.

Medical imaging could also benefit greatly, as the dots show robust performance as fluorescent agents.

“One of the problems with standard probes in fluorescent spectroscopy is that when you load them into a cell and hit them with high-powered lasers, you see them for a fraction of a second to upwards of a few seconds, and that’s it,” Martí said. “They’re still there, but they have been photo-bleached. They don’t fluoresce anymore.”

Testing in the Martí lab showed GQDs resist bleaching. After hours of excitation, Martí said, the photoluminescent response of the coal-sourced GQDs was barely affected.

Rice University chemist James Tour, left, and graduate student Ruquan Ye show the source and destination of graphene quantum dots extracted from coal in a process developed at Rice. Tour said the fluorescent particles can be drawn in bulk from coal in a one-step process. Photo by Jeff Fitlow

That could make them suitable for use in living organisms. “Because they’re so stable, they could theoretically make imaging more efficient,” he said.

A small change in the size of a quantum dot – as little as a fraction of a nanometer – changes its fluorescent wavelengths by a measurable factor, and that proved true for the coal-sourced GQDs, Martí said.

Low cost will also be a draw, according to Tour. “Graphite is $2,000 a ton for the best there is, from the U.K.,” he said. “Cheaper graphite is $800 a ton from China. And coal is $10 to $60 a ton.

“Coal is the cheapest material you can get for producing GQDs, and we found we can get a 20 percent yield. So this discovery can really change the quantum dot industry. It’s going to show the world that inside of coal are these very interesting structures that have real value.”

Co-authors of the work include graduate students Ruquan Ye, Changsheng Xiang, Zhiwei Peng, Kewei Huang, Zheng Yan, Nathan Cook, Errol Samuel, Chih-Chau Hwang, Gedeng Ruan, Gabriel Ceriotti and Abdul-Rahman Raji and postdoctoral research associate Jian Lin, all of Rice. Martí is an assistant professor of chemistry and bioengineering. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science.

The Air Force Office of Scientific Research and the Office of Naval Research funded the work through their Multidisciplinary University Research Initiatives.

For more:  http://news.rice.edu/2013/12/06/coal-…

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Nanotechnology triples solar efficiency

By | December 11, 2012, 7:49 PM PST

Nanotechnology traps light for significantly greater solar efficiency.

Nanotechnology traps light for significantly greater solar efficiency.

Princeton University recently announced a new nanotechnology that has demonstrated the ability to triple the efficiency of solar cells by eliminating two of the primary reasons why light is reflected or lost. This breakthrough was achieved by applying a “nano-mesh” to plastics, which would make way for inexpensive, flexible devices, or even greatly improve the efficiency of standard photovoltaic panels, the researchers say.

The nano-mesh is designed to dampen reflection and trap light to be converted into electrical energy (existing technologies cannot fully capture light that enters the cell). Only 4 percent of light is reflected, and as much as 96 percent is absorbed, a press release noted. Its overall efficiency in converting light to energy is 52 percent higher than conventional cells in direct sunlight and up to 175 percent greater on cloudy days with less sun.

For reference, North Carolina’s Semprius Inc., a Siemens backed venture, revealed a prototype of what it called the world’s best solar efficiency at 33.9 percent earlier this year. Princeton didn’t reveal its overal efficiency.

Princeton’s findings were first reported in the November 2nd edition of the journal Optics Express, and exceeded the scientists’ expectations, according to project lead Dr. Stephen Chou. The research was funded by the Defense Advanced Research Projects Agency, the Office of Naval Research and the National Science Foundation. Chou said that the technology would become even more efficient with more experimentation.

Outside of the lab, U.S. PV maker ecoSolargy has already used nanotechnology to boost solar efficiency by an estimated 35 percent over a 20-year period by filling tiny holes that can accumulate dirt, dust, or water. Other approaches that are being taken to improve solar efficiency have been inspired by nature.

A team of researchers at the University of Wisconsin-Madison recently created a design that emulates how sunflowers move to maximize light exposure through an adaptation called heliotropism. One could imagine that any combination of these technologies would constitute another leap forward for solar power.

(Illustration by Dimitri Karetnikov/Chou Lab)QDOTS imagesCAKXSY1K 8