Nanotechnology: The Art of Molecular Carpet-Weaving

Stable two-dimensional networks of organic molecules are important components in various nanotechnology processes. However, producing these networks, which are only one atom thick, in high quality and with the greatest possible stability currently still poses a great challenge. Scientists from the Excellence Cluster Nanosystems Initiative Munich (NIM) have now successfully created just such networks made of boron acid molecules. The current issue of the scientific journal ACSnano reports on their results. 

Scanning electron microscopy image with a superimposed molecular model [Credit: TUM]

Even the costliest oriental carpets have small mistakes. It is said that pious carpet-weavers deliberately include tiny mistakes in their fine carpets, because only God has the right to be immaculate. Molecular carpets, as the nanotechnology industry would like to have them are as yet in no danger of offending the gods. A team of physicists headed by Dr. Markus Lackinger from the Technische Universität München (TUM) und Professor Thomas Bein from the Ludwig-Maximilians-Universität München (LMU) has now developed a process by which they can build up high-quality polymer networks using boron acid components. 

The "carpets" that the physicists are working on in their laboratory in the Deutsches Museum München consist of ordered two-dimensional structures created by self-organized boron acid molecules on a graphite surface. By eliminating water, the molecules bond together in a one-atom thick network held together solely by chemical bonds -- a fact that makes this network very stable. The regular honey-comb-like arrangement of the molecules results in a nano-structured surface whose pores can be used, for instance, as stable forms for the production of metal nano-particles. 

The molecular carpets also come in nearly perfect models; however, these are not very stable, unfortunately. In these models the bonds between the molecules are very weak -- for instance hydrogen bridge bonds or van der Waals forces. The advantage of this variant is that faults in the regular structure are repaired during the self-organization process -- bad bonds are dissolved so that proper bonds can form. 

However, many applications call for molecular networks that are mechanically, thermally and/or chemically stable. Linking the molecules by means of strong chemical bonds can create such durable molecule carpets. The down side is that the unavoidable weaving mistakes can no longer be corrected due to the great bonding strength. 

Markus Lackinger and his colleagues have now found a way to create a molecular carpet with stable covalent bonds without significant weaving mistakes. The method is based on a bonding reaction that creates a molecular carpet out of individual boron acid molecules. It is a condensation reaction in which water molecules are released. If bonding takes place at temperatures of a little over 100°C with only a small amount of water present, mistakes can be corrected during weaving. The result is the sought after magic carpet: molecules in a stable and well-ordered one-layer structure. 

Markus Lackinger's laboratory is located in the Deutsches Museum München. There he is doing research at the Chair of Prof. Wolfgang Heckl (TUM School of Education, TU München). Prof. Bein holds a Chair at the Department of Chemistry at the LMU. The research was conducted in collaboration with Prof. Paul Knochel's work group (LMU) and Physical Electronics GmbH, with funding by the Excellence Cluster Nanosystems Initiative Munich (NIM) and the Bavarian Research Foundation (BFS). 

Source: Technische Universitaet Muenchen [December 29, 2011]

Read more »

Researchers create living 'neon signs' composed of millions of glowing bacteria

In an example of life imitating art, biologists and bioengineers at UC San Diego have created a living neon sign composed of millions of bacterial cells that periodically fluoresce in unison like blinking light bulbs. 

Thousands of fluorescent E. coli bacteria make up a biopixel [Credit: Hasty Lab, UC San Diego]

Their achievement, detailed in this week's advance online issue of the journal Nature, involved attaching a fluorescent protein to the biological clocks of the bacteria, synchronizing the clocks of the thousands of bacteria within a colony, then synchronizing thousands of the blinking bacterial colonies to glow on and off in unison. 

A little bit of art with a lot more bioengineering, the flashing bacterial signs are not only a visual display of how researchers in the new field of synthetic biology can engineer living cells like machines, but will likely lead to some real-life applications. 

Using the same method to create the flashing signs, the researchers engineered a simple bacterial sensor capable of detecting low levels of arsenic. In this biological sensor, decreases in the frequency of the oscillations of the cells' blinking pattern indicate the presence and amount of the arsenic poison. 

The smaller chips contain about 500 blinking bacterial colonies or biopixels [Credit: Hasty Lab, UC San Diego]

Because bacteria are sensitive to many kinds of environmental pollutants and organisms, the scientists believe this approach could be also used to design low cost bacterial biosensors capable of detecting an array of heavy metal pollutants and disease-causing organisms. And because the senor is composed of living organisms, it can respond to changes in the presence or amount of the toxins over time unlike many chemical sensors. 

"These kinds of living sensors are intriguing as they can serve to continuously monitor a given sample over long periods of time, whereas most detection kits are used for a one-time measurement," said Jeff Hasty, a professor of biology and bioengineering at UC San Diego who headed the research team in the university's Division of Biological Sciences and BioCircuits Institute. "Because the bacteria respond in different ways to different concentrations by varying the frequency of their blinking pattern, they can provide a continual update on how dangerous a toxin or pathogen is at any one time." 

"This development illustrates how basic, quantitative knowledge of cellular circuitry can be applied to the new discipline of synthetic biology," said James Anderson, who oversees synthetic biology grants at the National Institutes of Health's National Institute of General Medical Sciences, which partially funded the research. "By laying the foundation for the development of new devices for detecting harmful substances or pathogens, Dr. Hasty's new sensor points the way toward translation of synthetic biology research into technology for improving human health." 

The larger chips contain about 13,000 biopixels [Credit: Hasty Lab, UC San Diego]

The development of the techniques to make the sensor and the flashing display built on the work of scientists in the Division of Biological Sciences and School of Engineering, which they published in two previous Nature papers over the past four years. In the first paper, the scientists demonstrated how they had developed a way to construct a robust and tunable biological clock to produce flashing, glowing bacteria. In the second paper, published in 2010, the researchers showed how they designed and constructed a network, based on a communication mechanism employed by bacteria, that enabled them to synchronize all of the biological clocks within a bacterial colony so that thousands of bacteria would blink on and off in unison. 

"Many bacteria species are known to communicate by a mechanism known as quorum sensing, that is, relaying between them small molecules to trigger and coordinate various behaviors," said Hasty, explaining how the synchronization works within a bacterial colony. "Other bacteria are known to disrupt this communication mechanism by degrading these relay molecules." 

But the researchers found the same method couldn't be used to instantaneously synchronize millions of bacteria from thousands of colonies. 

Tiny microfluidic chips allow the researchers to synchronize the bacteria to fluoresce or blink in unison [Credit: Hasty Lab, UC San Diego]

"If you have a bunch of cells oscillating, the signal propagation time is too long to instantaneously synchronize 60 million other cells via quorum sensing," said Hasty. But the scientists discovered that each of the colonies emit gases that, when shared among the thousands of other colonies within a specially designed microfluidic chip, can synchronize all of the millions of bacteria in the chip. "The colonies are synchronized via the gas signal, but the cells are synchronized via quorum sensing. The coupling is synergistic in the sense that the large, yet local, quorum communication is necessary to generate a large enough signal to drive the coupling via gas exchange," added Hasty. 

Graduate students Arthur Prindle, Phillip Samayoa and Ivan Razinkov designed the microfluidic chips, which for the largest ones, contain 50 to 60 million bacterial cells and are about the size of a paper clip or a microscope cover slip. The smaller microfluidic chips, which contain approximately 2.5 million cells, are about a tenth of the size of the larger chips. 

Each of the blinking bacterial colonies comprise what the researchers call a "biopixel," an individual point of light much like the pixels on a computer monitor or television screen. The larger microfluidic chips contain about 13,000 biopixels, while the smaller chips contain about 500 pixels. 

Hasty said he believes that within five years, a small hand-held sensor could be developed that would take readings of the oscillations from the bacteria on disposable microfluidic chips to determine the presence and concentrations of various toxic substances and disease-causing organisms in the field.

Source: University of California - San Diego [December 18, 2011]

Read more »