Maybe someone can offer a more insightful analysis, but chemically prepping a GC surface to be more hydrophilic comes to mind. You should be able to do that by adding oxide groups to the surface, and there is a host of ways to do that:

-high temperature (~500 deg C) in an oxidizing environment

-anodic oxidation in sulfuric acid, or a hydroxide

-oxidation in an aggressive chemistry: pyranha solution, H2SO4/HNO3, etc.

In my experience, either of the last two methods works well on freshly polished graphite. Glassy carbon should behave a bit differently, but probably not by much.

I've also seen suggestions for prepping GC by polishing it with very fine alumina abrasive (water based). The consensus there seemed to be that finer polishing media produces GC with higher surface oxygen content and higher reactivity. But full disclaimer: I've not tried this, so take it for what it's worth.

Good luck.

Thank you, Pavel! In fact I made this experiment after keeping GC in concentrated sulfuric acid for 24 h with  following sonication in acetone. I suppose sulfuric acid (98%wt.) should be a suitable oxidizing agent. And I also tried polishing with alumina.

Do you think it's all about GS's surface or it can be also a content of Nafion in the ink?

I'm not sure. I haven't worked much with Nafion in this context. It generally goes well with water, so I imagine that isn't your source of hydrophobicity.

I would try playing with the GC surface. Not sure how much oxidation you can get from H2SO4 by itself. I know that graphite felts have been activated in H2SO4 after several hours of boiling, so only after a long time and at high temperature. On the other hand, H2SO4 with HNO3, in ratio 10:1 visibly affect graphitic carbons in a matter of seconds. Of course, this is a matter of intercalation of H2SOf/HSO4-/SO4-- with HNO3 driving the reaction, so this may not be applicable to GC, but you can give that a shot.