thanks for your reponse, I will read article  and tel you what happend 

in the article they said "these were cleaned by a 5-min treatment with a mixture of H2O2 (25% v/v) and KOH (50mM)" but they didn't precise the ratio of KOH and H2O2,

ok thanks

 I agree with Prosper, probably you can use several CVs (number of cycles and potential window to be optimized) before starting the experiments. You can use chemical treatment at the beginning and then the electrochemical, or just one. This depends on the precision.

Hi Karima,

After cleaning your gold electrode with piranha. You shall clean your gold electrode by using electrochemical cleaning in 0.5M of H2SO4 till you obtain a stable CV (you should obtain a stable reduction peak arround 0.9 V). After that keep your electrode in ethanol for at least for 30min to dissolve the gold oxide.

In you question you said that your DPV is not stable...does the current peak increase or decrease?

Good luck

 thank you all for your help, I confirm after chemical cleanning with H2SO4 it becames stable.

But what is the difference between its increased or decreased?

Hello, yes I solved this problem by the method proposed in the answers, chemical and electrochemical cleanning but you shold optimise the cleanning time because its depend also of the thickness of your electrodes, tell me if its work