Making Air that Kills Bacteria

Sophie Chauveau
Mechanical Engineering
Stanford University
November 2002

I study the chemistry of excited air. Excitation is obtained by adding energy into air, which leads to the production of charged particles, and the air becomes a plasma (gas with a high number of charged particles). Air plasmas would provide a fast, non-polluting and safe way to remove bacteria from contaminated surfaces.

Plasma also called the fourth state of matter (the others are gas, liquid and solid), is a gas with a high density of charged particles (i.e. ions and electrons). Lightning and the "fire" created in front of the space shuttle during its reentry in the atmosphere are examples of plasmas.

Most plasmas are very hot gases, but my research group is looking at producing air plasmas at room temperature that contain a high density of electrons. Such plasmas can be used for medical sterilization, bio-decontamination of objects or human bodies (killing anthrax bacteria, for example) and toxic gas decomposition. Traditional methods of sterilization and biodecontamination use dry or moist heat that can damage the treated material or surface. Moreover, those methods typically take minutes or even hours. Room temperature air plasmas offer a faster and less damaging method to kill bacteria.

Currently, the key issue is to know how to generate these air plasmas. The air we breathe every day does not naturally contain electrons in a sufficient quantity. To produce larger electron number densities we need to add energy into air. This energy must be added only to the electrons because we don't want to heat the air. In our laboratory, we energize the electrons with an electrical discharge. We apply a high voltage between two electrodes which creates an electric field that accelerates only charged particles. The negatively charged electrons present in the air are very light so they are easily accelerated. But they also collide with other particles contained in the air and therefore modify its chemistry. For example, electrons can transfer a part of their energy to the molecules, and react with them. We then have two kinds of chemical reactions: those leading to the formation of new electrons, and unfortunately those tending to remove energy from electrons. The challenge of our work is to enhance the first kind of reactions while preventing the second kind.

My particular role within our research group is to guide and explain the experiments conducted in our laboratory by means of computer simulation. I can, for example, tell which discharge parameters (electric field and current) should be applied to produce a given number of electrons. This model is also necessary to understand all the chemical reactions involved in the production and destruction of electrons in the plasma. To develop such a tool, I need to know the characteristics of the various types of particles present in air (atoms, molecules) and the way each particle will react with the other particles. This is a complicated task because each atom or molecule is linked with the others by several potential reactions and if you consider all the atoms and molecules with their internal energy levels you have in air at least one million of reactions. However, once we understand this complex chemistry, we have at our disposal a key asset to efficiently generate such plasmas.