Electrodes get reactive: Electrophoresis without unwanted chemical reactions

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  • Published: Apr 18, 2011
  • Author: Jon Evans
  • Channels: Electrophoresis
thumbnail image: Electrodes get reactive: Electrophoresis without unwanted chemical reactions

Unwanted reactions

When you pass an electric current between two electrodes placed in an electrically-conducting liquid, otherwise known as an electrolyte, several things tend to happen. If there are any charged particles in the liquid then they will start to migrate towards the electrode with the opposite charge, while the liquid itself may start to move due to the bulk movement of ions. Both of these processes have proved of use to analytical scientists looking to separate or transport molecules and particles.

Another thing that can happen is that the electric current can induce chemical reactions at the electrodes, otherwise known as electrolysis. Now in some circumstances this can be very useful, such as if you're looking to split water into hydrogen and oxygen, but it can be much less useful if you're looking to separate or transport molecules. This is because the products formed by the chemical reaction, including hydrogen and oxygen, can interfere with the analytical process, by either stopping the electrodes working properly or reacting with the molecules being separated or transported.

So analytical scientists employing electrophoresis or electroosmosis try to dampen down these chemical reactions. This can be done by reducing the current passing through the electrolyte to a bare minimum, partitioning off the electrodes from the rest of the electrolyte or adding chemical buffers, all of which have their disadvantages. But now Per Elandsson and Nathaniel Robinson, two scientists at Linköping University in Sweden, have come up with another option: use electrodes made from a conducting polymer.

Oxidation and reduction

Unwanted they may be, but the chemical reactions are unfortunately necessary, because they are the mechanism by which an electrical current is able to pass through a liquid. In water, for instance, an oxidation reaction at the anode converts water into molecular oxygen and hydrogen ions (protons), which then migrate to the cathode and take part in a reduction reaction, combining with electrons to form molecular hydrogen. In this set-up, the hydrogen ions act as the charge carriers; if they weren't generated at the anode by an oxidation reaction and then reduced at the cathode by a reduction reaction you wouldn't be able to pass an electric current through water.

But although these oxidation and reduction reactions do need to take place, they don't need to take place in the electrolyte. The reason why they do is because electrodes have conventionally been made from metals such as platinum. While metal electrodes are very good at conducting electricity, they're not very good at taking part in oxidation or reduction reactions, which is why these reactions instead take place in the electrolyte.

What would happen, however, if you replaced the metal electrodes with electrodes made from a substance that could both conduct electricity and take part in oxidation and reduction reactions, such as a conducting polymer? Elandsson and Robinson decided to find out.

Switching polarity

They made electrodes from an electrically-conducting polymer called PEDOT:PSS and tested them in various salt solutions. In all cases, they found that the electrolytes were able to conduct an electric current very efficiently, but without any unwanted chemical reactions.

This is because the oxidation and reduction reactions are now taking place in the electrodes rather than the electrolyte. The PEDOT:PSS anode is being oxidised, causing it to release positively-charged ions, which then travel to the PEDOT:PSS cathode, where they are absorbed. This transfer of ions between the conducting polymers carries the charge, while the liquid doesn't get involved at all, merely acting as the medium through which the charge carriers migrate.

The only downside is that the PEDOT:PSS anode eventually releases all of its ions, at which point the oxidation and reduction reactions switch back to the electrolyte. However, the whole process can be started again by just switching the polarity of the electrodes, such that the cathode that absorbed all the positively-charged ions becomes the anode and vice-versa. This continues until the new anode has released all its ions and then the polarity is switched again. Elandsson and Robinson did this every two hours for 37 days without noticing any real drop in performance.

The scientists think this technique could be especially useful for lab-on-a-chip devices, which struggle with even small amounts of unwanted chemical products.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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