Graphene in which solvent?

Hi,

I got a graphene dispersion on ebay but the graphene flakes cumulated at the bottom of the bottle. I'm trying to mix it and then solve in water using an ultrasonic cleaner but the flakes do not dissolve in water? How can I dissolve it so that I can obtain a large scale graphene by dropping on a surface and then drying. What should I use to solve graphene flakes?