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Posted: Jan 16, 2009

Morphology control of organic structures leads to better nanotechnology bomb sniffers

(Nanowerk Spotlight) The thing with technology is that the science behind it isn't good or bad but the way it is used could make all the difference. There are plenty of examples throughout history. Military nanotechnology is a good present day example. And we don't even need to talk about the weird, far-out stuff like nanobots and the Casimir force (see our Spotlight Nanotechnology, the mysterious Casimir Force, and interstellar spaceships). Take explosives. While on one side there is research going on to improve high explosives through nanoscale structuring, other research teams are putting a lot of effort into detecting ever smaller trace amounts of explosives and building the ultimate nanoscale bomb dog.
A recent example has been demonstrated by scientists in China, who obtained organic super-nanostructures via a simple solution process, and, for the first time, demonstrated that such structures can be applied as chemosensors to detect explosives.
"The scientific core of our findings is that not only can we control the morphology of organic nanostructures through self-assembly in solution, but also the optoelectronic properties of the organic semiconductors can be tuned by their morphologies," Jian Wang a professor at the Institute of Polymer Optoelectronic Materials and Devices at South China University of Technology in Guangzhou, tells Nanowerk. "We successfully fabricated three different crystalline structures, self-assembled from a single organic molecule through a simple solution dropcasting process, and found that differences in the crystal structures and in the film morphologies led to dramatic enhancements of the explosive detection speed."
Wang, together with collaborators from the Key Laboratory of Bioorganic Chemistry and Molecular Engineering at Peking University, demonstrated that by simply drop casting oligoarene derivative solutions in different solvents, three different well-controllable organic nanostructures, including 1D microbelts and 3D flower-shaped supernanostructures, are easily self-assembled.
different crystalline nanostructures
Top row: Microbelt; middle row: flower A, and bottom row: flower B self-assembled from 1,4-dioxane,THF, and n-decane solutions, respectively. All the three structures were self-assembled from 40 µL of solution with a concentration of 1 mg/mL through drop casting onto glass substrates. (Reprinted with permission from the American Chemical Society)
Wang explains that the flower-like super-nanostructures with 1D nanometer-size pedals exhibit two distinct characteristics leading to higher explosive detection sensitivity.
"First, flower-shaped super-nanostructures have larger surface areas exposed to the analytes than their one-dimensional microbelt counterparts, due to the efficient distributions of their nanosized petals in space, resembling natural flowers" Wang says. "Second, in terms of film morphology, the flower-shaped super-nanostructures actually form films with higher porosity than microbelts do, which favors the absorption and the diffusion of the analytes' molecules. The combination of these two distinct structural properties contributes to the better sensitivity of explosive detection by the flower-shaped super-nanostructures than that by the microbelts."
The Chinese team's investigation showed the morphology control through self-assembly of the aromatic molecules by solution process adds a new dimension in nanotechnology material design, which could widen the applications of such organic crystalline super-nanostructures in chemosensors, optoelectronics, biosensors, bioelectronics, etc.
According to Wang, this work advances the field in both process and application. In terms of process, the scientists have successfully tuned the crystalline structures from 1D microbelts to 3D flower-shaped super-nanostructures from a single organic molecule in different solvents.
"This is the first time that a facile solution drop-casting process was utilized to realize complicated structure and morphology control," says Wang. "The solution drop-casting process in our work is not only simple and cheap (no need for high temperature, ultra vacuum, or complicated templates) but also involves no chemical reactions – thereby no impurity is introduced during the structures' self-assembly – which ensures the purity of the self-assembled micro- and nanostructures".
In terms of application, the successful utilization of these super-nanostructures in chemosensing leads to dramatic improvements of DNT/TNT detection abilities.
Wang says that, with the evolution of structures from netted 1D microbelts to 3D flower-shaped super-nanostructures, the detection speed of the chemosensors for 2,4-dinitrotoluene (DNT) and trinitrotoluene (TNT) improved by more than 700 times.
"The operation mechanism of the chemosensors, for the detection of vaporous explosives such as DNT and TNT, is mainly based on the electron transfer from the electron-rich organic materials to those electron-deficient nitroaromatic compounds leading to the fluorescence quenching of the organic materials," Wang explains.
Because the optoelectronic properties of organic semiconductors in the solid state are highly correlated with their structural forms at every structural hierarchy, morphology control plays a key role in optimizing their optoelectronic performance.
The team plans to conduct more work with regard to examining the correlation between optoelectronic properties and structures. Another direction will be the application of these organic super-nanostructures in new areas such as biosensors and bioelectronics.
Wang points out that one particular challenge is how to identify the crystalline structures of the supernanostructures because it is very difficult to obtain enough information utilizing current experimental methods, such as XRD and TEM, to disclose the secrets of the high-level organic crystalline assemblies. "To analyze the organic crystalline structures, we have to develop advanced experimental tools and methods" says Wang.
The team has reported their findings in the December 31, 2008 online edition of Langmuir ("Organic Supernanostructures Self-Assembled via Solution Process for Explosive Detection").
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