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Graphene family materials (GFM) have large perspectives for drug delivery applications but their internalization in live cells is under investigation in a wide variety of studies in order to assess the best conditions for efficient cellular uptake. Here we show that mild oxidation of graphene nanoplatelets produces nano-graphene oxide (nGO) particles which are massively internalized into the cell cytoplasm. This remarkable uptake of nGO in NIH-3T3 cells has never been observed before. We performed vitality tests for demonstrating the biocompatibility of the material and analyzed the internalization mechanism under different oxidation degrees and concentrations. Moreover, we evaluated quantitatively, for the first time, the cell volume variation after nGO internalization in live cells through a label-free digital holographic imaging technique and in quasi real-time modality, thus avoiding the time consuming and detrimental procedures usually employed by electron-based microscopy. The results demonstrate that nGO formulations with a tailored balance between exposed surface and content of functional groups are very promising in drug delivery applications.