I'm not totally sure but it's impossible you find gold oxide in pure gold.

So good luck.

You could try cleaning by oxygen plasma etching. Try 50% O2 for 2 mins at 80 W, 100 mTorr and 20 C.

When electropolishing in sulphuric acid, you can remove any residual gold oxide by chemical reduction in ethanol, or by electrochemical stripping in sulphuric acid. This method worked for me, although i was working on macroelectrodes not microelectrodes.

It is indeed very difficult to polish any microelectrode for that matter especially when using microfluidic devices