I can think of a couple of ways:

1. Try use an sonic-bath dipping the electrode in Acetone and then water each for 10 min. if its is organic contaminants you can always try dipping it in Piranha solution for a few minutes. 

2. polish with a finer paper.

3. electro-cleaning by cycling in 0.1 M H2SO4

Dear Dr. Noyhouzer, 

Thanks for your kind answer. Regarding the step number 2... Some colleagues have recommended me to never touch the Pt tip (15 micrometers) but indeed I found that if I polish the tip softly with alumina, I obtain an appropriate electrochemical response. What do you think about this?

So each time you polish the tip you can:

A. break it = kill the electrode

B. damage your RG. We use 3-4 RG UMEs so when you polish them you increase your RG. Some commercial electrodes have an RG>100 and polishing wont significantly change the RG. 

If you can clean the electrode without polishing its much better 

Electrochemical cleaning, possibly according to the linked document, seemingly might be found of some help: http://courses.chem.psu.edu/chem524/Pt_electrode_expt0.pdf

Dear Dr. Araújo and Dr. Noyhouzer,

Thanks for your kind answers, I followed your recommendations and my Pt microelectrode is working well now.

Best Regards,

Dear Dr. Carlos Araujo Queiroz

Since I could not download the pdf you shaerd could you please kindly share it again?

Dear Dr. Tomer Noyhouzer

Is it posible to share a short vido of polishing the microelectrode ?

Thank you so much

Regards