Graphene oxide nanosheet (GOns) with sharp edges was synthesized using controlled pyrolysis of citric acid. Scanning electron, as well as atomic force microscopy of the sample confirmed the formation of multilayered GOns with an average sheet length of 150 nm. X-ray diffraction pattern and Raman spectra also confirmed the formation of GOns. Furthermore, GOns was successfully functionalized (FGOns) by cross-linking with a small protein bovine α-lactalbumin (BLA). The crosslinking of protein with GOns in FGOns was confirmed by infrared spectroscopy, and the conformational change of BLA was observed by fluorescence, as well as circular dichroism spectroscopy. When applied to human erythrocytes, GOns demonstrated profound hemolysis; however, such hemolytic effect was drastically reduced by FGOns. To evaluate the potential biomedical application of FGOns, the cytotoxicity of the sample was also assessed. The administration of both GOns and FGOns in breast cancer cells MCF-7 and MDAMB-231 demonstrated more than 88% cell death within 24 h and such cytotoxicity against cancer cells was caused due to the generation of reactive oxygen species (ROS), as revealed from the N-acetyl-L-cysteine (NAC, a ROS-inhibitor)-based assay. FGOns demonstrated excellent biocompatibility against normal cells such as HaCaT and 3T3 compared to GOns that demonstrated dose-dependent toxicity. Moreover, FGOns demonstrated more efficient cellular uptake than GOns by cancer cells. Therefore, our present study demonstrated that the functionalization of GOns using small protein could improve its biocompatibility multifold and such strategy might represent wide opportunity to use GO like nanomaterial safely in various biomedical applications.