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.
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.
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.