Graphene as a good material for sensing single small molecules is hardly believed to identify bio-molecules via electrical currents. This is because bio-molecules tend to bind to graphene through non-covalent bonds, such as pi-pi stacking interaction, which is not customarily considered to induce a clear perturbation of the graphene electronic structure. In contrast to these expectations, we demonstrate that oxygen in nucleobases adsorbed on graphene with pi-pi stacking interaction can clearly alter the electric current even in water at room temperature. This property allows us to devise the strategies employing graphene as material of choice in bio-sensorics, bio-chips. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681579]