Dear Sir. Concerning your issue about how to determine the concentration of reduced graphene oxide in aqueous solution. Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL–1. Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m2 g–1 in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL–1. I think the following below links  may help you in your analysis:

http://pubs.acs.org/doi/abs/10.1021/la4029904

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.687.6747&rep=rep1&type=pdf

https://arxiv.org/ftp/arxiv/papers/1210/1210.1469.pdf

Thanks

Thank you so much Dr.Isam

Simplest way to measure the concentration of GO or rGO is to measure the weight of a known volume of filtered rGO dispersion. If you know the weight of filter paper and the weight of filter paper after filtration and drying of rGO, the concentration of graphene can be easily calculated. Alternatively, UV-Vis spectra will also give out the concentration of graphene if the absorption co-efficient is known. In case of graphene the absorption co-efficient is 2,460 L g-1 m-1 at 660 nm. 

Thank you so much Dr.Khaled

Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL–1. Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m2 g–1 in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL–1.