I don't think your problem has anything to do with your pre-treatment or cleaning of your electrode. The problem is that you are going so negative in potential that you are reducing the ITO... essentially you no longer have an oxide, you have a metallic tin electrode by the time you reach -1.3V. I cannot be 100% sure, as it depends what reference you are using (-1.3V vs what?) and the pH, but I'm pretty sure that ITO is not stable at the conditions you are using.

Dear Daniela,

Thanks for your answer.

The potential has been checked in bibliography prior to the selection and is versus SCE reference electrode. Before measure until -1.3 V, we also have checked the response in ferrycianide and we've obtained more than 200 mV of difrerence between the peak potential.

In my oppion the best way to do it is:

To clean with detergents and deionized water, ultrasonicated in isopropanol, dried,

and treated with oxygen plasma for 5 min.

On the other hand, I agree with Daniel: you are going so cathodic potencials that you are reducing the ITO.

Finally, look at this: http://www.delta-technologies.com/downloads/applicationnotes.pdf?C=8

At -1.3 V you have probably reduced the surface of your ITO. In which case, you may want to try an oxidizing treatment, such as oxygen plasma that Daniel had mentioned. You cal also try a range of wet chemistry oxidants, such as ammonium persulfate, hydrogen peroxide, potassium permanganate, or a pirahna solution (conc. H2SO4 and 30% H2O2 mixed 4:1). The last one is vicious enough to oxidize graphite, but it works well with oxide glasses; not sure about ITO.

Alternatively, if the color is due to reduced indium and tin, you may be able to dissolve it off with some nitric acid.

Then there is a question of what's in your cell and whether your cathode is reacting with your solution.

Don't bother with any acid treatment -- ITO dissolves in strong acids.

As some people pointed out you may be reducing the ITO. You can oxidize it again with O2 plasma (which also cleans the surface) or just thermal annealing in air on a hotplate.

If you don't have O2 plasma you can use "basic pirahna": conc NH3 + conc H2O2 + H2O (1:1:4 volume) heated to 80 degrees C. This is a very efficient cleaning without acidic conditions.

Recently i have used the acidic solution for cleaning ITO, it works and no change was observed in resistance of conducting surface.