NMP is miscible with water, so is GO (not easily due to the sheets) as well due to the -COOH and -OH groups. I suggest you sonicate at 20-40 W for 4 h. Cheers!

Best way to get a well dispersion of GO or rGO is to do sonication for at least 15-20 mins in aqueous or TE buffer solutions.

Thank you very much

SO do you think sonicating the NMP dispersion of GO in 20 min needs high powers such as about 300w?? I have seen such power for CNT  in water in articles but does not this  high power cause cracks or totally fracture in the sheets structure?

I think it's better to sonicate longer than to do this at very high power