The 2010 Nobel Prize in Physics has just been awarded for the discovery of graphene — atom-thin flakes of carbon — in part because of this material’s enormous device potential. Graphene’s quirky ‘chicken-wire’ framework gives electrons significant speed boosts compared to silicon circuits, and makes it mechanically stronger and stiffer than diamond. Recently, chemists have also begun to investigate graphene as water-soluble platforms for custom molecular transformations.

Kian Ping Loh and colleagues at the National University of Singapore1 have now found that a specially designed fluorescent dye called PNPB can attach to and stack parallel to graphene oxide. The interactions between these two compounds create a charge-transfer complex that quenches the PNPB dye’s fluorescence, leading to new applications in both biological sensing and optical safety.

The PNPB dye used in the study has two main components: a positively charged unit called pyridinium that links to the periphery of graphene oxide sheets through ion-exchange reactions, and a flat aromatic segment known as pyrene that positions the dye parallel to the graphene surface, quenching the fluorescent output.

“This design allows us to perform optical sensing, where the fluorescence of PNPB is switched on or off depending on whether it complexes more strongly with graphene oxide or other biomolecules,” says Loh.

Fig. 1: The fluorescence of PNPB dye is quenched when the dye is attached to graphene oxide sheets. DNA strands can efficiently disrupt this bonding and thus restore the dye’s fluorescence, resulting in a highly specific biosensing device.

The team first studied how the complex responded to the primary constituents of blood serum: DNA, RNA, proteins and glucose molecules. They found that the sensor was extremely selective towards DNA (Fig. 1): no matter which biomolecules were in the test mixture, DNA could be detected fluorescently with parts-per-million accuracy. Loh explains that only DNA has the ionic strength capable of separating the PNPB dye from the graphene oxide sheets and thus activating its fluorescence, making this non-toxic complex ideal for applications like identifying contaminant DNA in recombinant proteins.

Charge-transfer forces within the graphene oxide–dye complex also enable it to act as an ‘optical-limiting’ material. After excitation by a laser, charged sites in the complex create micro-sized plasmas that scatter intense laser light while transmitting low-intensity light, an effect that could be used in protective equipment like goggles and filters.