Graphene is the fundamental building element of many carbon allotropes including graphite, charcoals, carbon nanotubes, buckminsterfullerene, and other buckyballs, etc. Graphene comprises a single layer of six-atom rings in a honeycombed network and can be conceptually considered as a true planar aromatic macromolecule. Graphene oxide (GO) exhibits interesting optical properties.4 Because the disordered oxygenated functional groups on GO confine p electrons within the sp2-carbon nanodo-mains, GO can fluoresce in a wide range of wavelengths. Interestingly, although GO is itself fluorescent, it is also capable of quenching fluorescence. The chemical versatility and tunable function groups on the graphene oxide (GO) surface make GO an ideal platform to immobilize a variety of biomolecules including nucleic acids, peptides, proteins, etc. Since GO has intrinsic fluorescence, GO-based biosensors have been designed by altering its intrinsic fluorescence in the presence of target biomolecules including protein, peptide, DNA/RNA, and virus.