What about DLS measurement which is concentration sensitive? I don't expect it to be extremely accurate but I believe it can give you satisfying results.

http://www.horiba.com/scientific/products/particle-characterization/education/sz-100/particle-size-by-dynamic-light-scattering-resources/choosing-concentration-for-dls-size-measurement/

http://www.sciencedirect.com/science/article/pii/S2215016115000060

What about UV-Vis absorbance studies? Take the UV-Vis of the initial concentration and you know the absorbance, then compare the value with the final centrifuged solution, and find the concentration.

How about using a quartz crystal microbalance? Pipette a known volume onto the QCM crystal, allow to dry and check the mass increase. Assuming there's nothing else in the solution other than the exfolliated graphene oxide, or if there is, you know precisely what and at what concentration so you can subtract that, then you should be able to find the concentration of the graphene oxide.

UV-Vis can be used to estimate the concentration.  The maximum absorption (Abs) at around 230 nm.  cGO in mg/mL = Abs/0.47*0.01

To accurately determine the concentration, freeze drying or drying the supernatant in oven at 60-70 oC would be better. 

how if I want to determine concentration in "weight percent" ? 

Hi, you can find a method suitable for measuring low concentration of graphene oxide in our publication.

Best regards,

-Erkka

Article Measuring Synthesis Yield in Graphene Oxide Synthesis by Mod...

Dr. Ji Chen , I want to know about proper reference for your suggestive calculation. I have a calculation for graphene oxide concentration, but I dont know it is right. Please help me for graphene oxide concentration calculation. Also attach my calculation.