Chemically combating chemical combat

As we have witnessed all too recently, chemical warfare is a very tangible concern for many people of the world. The latest large scale attack occurred in Syria only a few months ago with the use of Sarin gas. The news from Syria makes it all the more urgent that scientists find ways to combat the use of chemical weapons. A group of engineers at UC San Diego have done just that using a new kind of structure called “micromachines”.

Many chemical weapon agents used in warfare belong to a class of chemicals broadly labeled “organophosphates”. This label is given to molecules that have a generically organic part and a phosphate part. Here is the molecule sarin, with its organic and phosphate parts highlighted:

Nerve gasses like sarin are dangerous because of the way the molecules interact with our nervous system. This interaction is highly dependent on the shape, or conformation of the molecule. That is, if we could somehow break up the atoms in the sarin molecule, they would no longer elicit the same deleterious effect on our bodies.

This is precisely the aim of chemical weapon decontamination methods. Typically hydrogen peroxide is added to the vat of the chemical weapon along with a strong base to raise the pH of the solution. After vigorous stirring for sometimes several hours, the organic and phosphate parts of the molecule have been separated, and the chemical is no longer “weaponized”.

The San Diego scientists want to make the decontamination process easier. Their idea is to use self-propelled micromachines to do the stirring for them. The “machine” is a tiny, tubular polymer structure coated with platinum metal on the inside. So how does this little tube help dismantle chemical weapons?

In basic solutions, hydrogen peroxide is “activated” to its highly reactive anionic form which easily breaks apart organophosphates like sarin. But that’s not all. Hydrogen peroxide also decomposes into water and oxygen gas under alkaline (basic) conditions. The decomposition of hydrogen peroxide occurs on the inside of the polymer-platinum tube of the micromachine. Consequently, oxygen bubbles collect inside the tube until they have nowhere else to go but out.  The escape of gas bubble from one end of the tube propels the micromachine forward, almost like a rocket blasting off.

You can imagine that thousands of self-propelled micromachines buzzing around a tank of chemical weapon would cause quite a bit of motion in the solution. So now, instead of waiting and stirring, all you have to do is wait. The scientists found that their micromachines could cause motion in solutions equivalent to conventional stirring at around 200 rpm and simultaneously dismantle the offending molecules at a higher rate. They conclude that using micromachines “leads to a higher decontamination efficiency while using significantly shorter reaction times and lower peroxide concentrations” than the usual methods. Science-1, potentially fatal chemical weapons-0. 

Sources and further reading:

Jahir Orozco et al. Micromotor-based high-yielding fast oxidative detoxification of chemical threats. Angew. Chem. Int. Ed. 2013, DOI : 10.1002/anie.201308072

Pop. Sci. article: http://www.popsci.com/article/technology/help-bubbles-tiny-motors-neutralize-chemical-weapons 

Wei Gao, Sirilak Sattayasamitsathit Joseph Orozco and Joseph Wang. Efficient bubble propulsion of polymer-based microengines in real-life environments. Nanoscale, 2013,5, 8909-8914.

Background photo from: http://chemistry.about.com/od/healthsafety/ig/Laboratory-Safety-Signs/Chemical-Weapon-Symbol.htm