They synthesized these new chiral nanomaterials by covalently attaching either L- or D-cysteine to the graphene sheets; as Kotov explains, “we hypothesized that carbon quantum dots could be produced in chiral forms as mirror images of each other using edge modification with amino acids”. The optical activity of the graphene sheets was confirmed by circular dichroism. For both L- and D-cysteine-modified sheets, the spectrum featured two distinct bands. Using different computational methods, they were able to assign the bands and ascertain the origin of chirality; one band was attributed to the amino acid ligands attached to dot edges, whereas the other was attributed to the 3D twisting of the graphene sheet (with the sign of the band depending on the chirality of the cysteine used). The complementary results from experiment and simulation led Kotov and co-workers to the conclusion that, “intermolecular interactions of chiral edge ligands cause out-of-plane buckling of the graphene sheets, resulting in twisting of the nanosheets in which the helicity of the twist depends on the handedness of the attached ligands”. Hence, the nature of symmetry breaking in this system is fundamentally different from that in metal and semicoductor nanoparticles or carbon nanotubes.
The ability of cells to discriminate between chiral graphene quantum dots and the flexibility afforded in the chemical composition of the groups attached to the quantum dots offer great potential for biomedical applications
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