Improving insecticides in a microwave

Cases of malaria have spiked alarmingly owing to COVID restrictions hampering control efforts. The good news, though, is that scientists from New York University have found that they can weaponise a common insecticide and make it twelve times more powerful - just by cooking it in the kitchen microwave, as published in the journal PNAS. The compound, called deltamethrin, is a neurotoxin that sits on surfaces as tiny crystals, and gets inside malaria-carrying mosquitoes through their feet. Heating it up and then allowing it to cool, causes the molecules to rearrange themselves into a different crystal structure that turns out to be much more potent. Phil Sansom heard how from author Bart Kahr…

Bart - If we change the arrangement of molecules in deltamethrin, then the crystals act much more quickly by a factor of 12. That's a factor of speed.

Phil - It was 12 times quicker?

Bart - 12 times quicker when we changed the crystals to the new form.

Phil - Can you give me some context, please? Deltamethrin, is that the compound to use when getting rid of mosquitoes?

Bart - Yes. The leading insecticides that are used today for combating malaria-transmitting mosquitoes are in the so-called class of pyrethroids. These are synthetic compounds that are derived from a compound that's found in chrysanthemum flowers.

Phil - What do people do with it?

Bart - It's incorporated into insecticide-treated bed nets. And the other way that deltamethrin is used is by spraying microcrystals on the interior walls of homes. And that's where the interaction between crystals and the feet of mosquitoes is most consequential.

Phil - And is this something that's working just fine at the moment?

Bart - It's not working just fine at the moment. It's working particularly bad at the moment. And for two reasons, firstly, over the last decade or so, mosquitoes have been turning up, having developed a chemical resistance to deltamethrin. It's also failing right now because Covid interruptions have caused malaria deaths in this very unusual year to skyrocket, because malaria prevention is really a house to house operation.

Phil - It's interesting that you're telling me this. Because you're not an insect person, or a public health person, right? And so what was it that you did?

Bart - We study the chemical and physical properties of crystals made out of molecules. And we discovered a new arrangement or what we call them, polymorphs.

Phil - What does the polymorph look like?

Bart - The old form tended to crystallise as isolated patches with kind of random organisations. Whereas the new form kind of looks like a starburst when it's grown in a thin film. But those two arrangements have different intrinsic energies. And so they will liberate their molecules at different rates when a mosquito should step upon them.

Phil - And how do you get there?

Bart - It's remarkably simple. The way to get from form one to form two is simply to melt form one and then allow the melt to cool and then recrystallise.

Phil - You're kidding. So just by heating and cooling this stuff, you've discovered a new form that's 12 times as effective.

Bart - Exactly. Just by heating and cooling. You can put it on a hot plate. You can put it in a microwave oven. You know, it's like the same temperature as boiling water.

Phil - What's the potential here?

Bart - It could extend the useful life of deltamethrin in the face of chemical resistance. We might be able to introduce enough of the compound, even into resistant mosquitoes, that we overwhelm the rates of the chemical resistance mechanisms.