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Posted: Nov 07, 2012
Protective nanotechnology: Nanomaterials for capturing nerve agents
(Nanowerk Spotlight) Protection against nerve agents – such as tabun, sarin, soman, VX, and others – is a major terrorism concern of security experts. Nerve agents, which attack the nervous system of the human body, are clear and colorless or slightly colored liquids and may have no odor or a faint, sweetish smell. They evaporate at various rates and are denser than air. Current methods to detect nerve agents include surface acoustic wave sensors; conducting polymer arrays; vector machines; and the most simple: color change paper sensors. Most of these systems have have certain limitations including low sensitivity and slow response times.
Nanoporous materials (adsorbents) can remove highly toxic nerve agent vapors by physical adsorption. Unfortunately, the broad range of toxic agents, environmental conditions and types of carbonaceous material simply does not allow laboratory testing of every possible combination. In this regard, screening of candidate carbonaceous materials for efficient capturing of highly toxic nerve agents is of undoubted interest for both military and civilian protective applications.
Although respirators with a cartridge filled with carbonaceous adsorbents (i.e., activated charcoals, activated carbons, and activated carbon fibers) have been used since World War I, the impact of the internal porous structure on the protective time of respirators has been very poorly understood.
"Everyone can argue that the internal structure of nanoporous material affects the protective time of any respiratory device," Dr. Piotr Kowalczyk, a Senior Research Fellow at the Nanochemistry Research Institute at Curtin University of Technology in Australia, tells Nanowerk. "However, the fundamental question is how to optimize this structure to get longer protection against highly toxic agent molecules. Clearly, fundamental research is needed to answer this nontrivial question."
The slit-shaped carbon pores in carbonaceous nanoporous material are like stop signs for sarin, capturing highly toxic molecules as a 'fly trapped tape'. (Image: Dr. Kowalczyk)
"We propose an entirely new strategy for combined experimental and computational screening of candidate carbonaceous materials for capturing of highly toxic molecules inside nanopores," says Kowalczyk.
He notes that the proposed idea is actually pretty simple: "First, we determined the distribution of nanopore sizes for selected commercialized porous materials (nitrogen porosimitry, non-local density functional theory, and integral theory of adsorption). Next we computed Henry constants for sarin and DMMP (its common stimulant) adsorbed in model slit-shaped carbon nanopores at 298 K (Metropolis-Ulam Monte Carlo method, and force-field calculations).
"Finally, we averaged Henry constants with experimental pore size distribution (note that normalized pore size distribution is the probability distribution). Knowing the averaged Henry constant for studied carbonaceous materials and concentration of sarin/DMMP in the bulk phase, we were able to predict the exact mass of sarin/DMMP adsorption (simply, the captured mass of toxic agent per mass of material). This allowed us to select the optimal carbonaceous nanomaterial for potential use in protection devices."
According to the team's calculations, the optimal carbonaceous adsorbent – pitch-based P7 activated carbon fiber – adsorbed ∼100 µg per gram DMMP at 0.03 µg per cubic meter. Commercialized Norit activated carbon adsorbed only ∼20 µg per gram DMMP at 0.03 µg per cubic meter.
"However, what is more important, we discovered the strong relationship between the pore size and the efficacy of nerve agent capturing via physisorption," Kowalczyk points out. "In general, it appears that slit-shaped carbon pores with pore sizes around ∼0.5 nm are optimal for sarin and DMMP capture. This is because of the strong confinement of agent molecules adsorbed in small carbon ultramicropores."
He explains this process in more detail: "Consider the virtual thin layer of glue that covers the walls of pores. If pores are very small – i.e., their effective size is comparable with the size of agent molecules – the adsorbed molecules are very 'sticky' and they are 'captured by glue' – like insects on trapping tape. In wider pores, this 'virtual thin layer of glue' is not an effective capturing medium and agent molecules can move easily through the pores."
Thus the key question is: how do the adhesive properties of the 'virtual layer of glue' depend on the size of the nanopore? And here, the team discovered that this dependence is exponential. What this means, in other words, is that capturing of nerve agents is only very effective when the size of carbon pores is around ∼0.5 nm. Shrinking or widening the slit-shaped carbon pore width decreases the efficiency of sarin and DMMP capturing significantly.
"Some of my colleagues asked me if I believe in our theoretical results" says Kowalczyk. "The great physicist Paul Dirac used to say: 'This result is too beautiful to be false; it is more important to have beauty in one's equations than to have them fit experiment'."
"And I truly believe that our theoretical results have to be correct – within the assumed model of nanopores – because they are so simple and beautiful" he concludes.