Graphene quantum dots: It's all in the twist

Chiral nanoparticles are an exciting class of materials that show promise in areas such as optoelectronics, catalysis, bioanalysis and drug delivery. To date, research in chiral nanomaterials has overwhelmingly focused on inorganic (typically, plasmonic) systems. Although highly desirable, chirality in carbon-based semiconductor nanomaterials remains largely unexplored because of the lack of suitable synthesis methods. Now, Nicholas Kotov, Angela Violi and co-workers report in ACS Nano the synthesis, chiroptical properties and unique biological behaviour of chiral graphene quatum dots.

Credit: Image courtesy of P. Elvati and A. Violi, University of Michigan, USA.

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

To demonstrate biocompatibility, the researchers exposed liver cells to the chiral quantum dots. Remarkably, they found that the cells can discriminate between the different stereoisomers, leading to different degrees of cytotoxicity, with one stereoisomer entering the lipid bilayer with much greater ease than the other. As Violi explains: “The markedly different response of the cellular membrane to the structural chirality is truly surprising because it affects the graphene quantum dots when they are removed from the chiral centres of the lipid bilayer, and because the quantum dots do not assume a rigid chiral configuration but have a skewed distribution of conformations that gives them an ‘averagely’ chiral character.” This implies that even subtle chiral effects may have repercussions on the cellular life cycle.

This report of chiral graphene quantum dots opens up exciting directions for further exploration. “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”, says Violi. It is likely that the next phase of this research will provide greater mechanistic insight and be directed towards the development of such applications. Kotov adds: “We will aim in future work to gain a better understanding of the molecular mechanism of chiral interactions of graphene quantum dots and cells, and the development of new drug delivery agents.” Looking beyond graphene, it is expected that this method will be generally applicable to sheets of other nanomaterials.