In order to get more carboxylic acid groups on GO, you need to perform more than one oxidizing steps. You can use this paper:  Ind. Eng. Chem. Res. 2012, 51, 310–317.

The number of carboxylic acid groups depends on the oxidation conditions. The mechanism by how the carboxylic groups are formed include the aperture of double bonds by the action of KMnO4 and H2SO4. to form sulfonate termnated sheet. The residence time at low temperature will be an important factor to increase these groups, Thereafter the addition of water and the temperature ramps will determine the formation of carboxyl and hydroxyl groups. Check if the attached paper is useful for you. Regards

Article Graphene oxide powders with different oxidation degree, prep...

I suggest you to go for photo-oxidation using ATLAS UV Irradiation Chamber. This may be very useful in increasing oxidation. 

dear prof kushwaha, we were thinking on uv too, but do you ensure that uv oxidation leads mostly to cooh not to ether,or lactones for example? Or even rip the go sheets?

You can use some knowledge from the vastly explored field of activated carbons, I would go first for oxidation in liquid phase. See, for example, the classical paper by Figueiredo below.

http://www.sciencedirect.com/science/article/pii/S0008622398003339