title: Chiral Graphene Quantum Dots.
abstract: Chiral nanostructures from metals and semiconductors attract wide interest as components for polarization-enabled optoelectronic devices. Similarly to other fields of nanotechnology, graphene-based materials can greatly enrich physical and chemical phenomena associated with optical and electronic properties of chiral nanostructures and facilitate their applications in biology as well as other areas. Here, we report that covalent attachment of l/d-cysteine moieties to the edges of graphene quantum dots (GQDs) leads to their helical buckling due to chiral interactions at the "crowded" edges. Circular dichroism (CD) spectra of the GQDs revealed bands at ca. 210-220 and 250-265 nm that changed their signs for different chirality of the cysteine edge ligands. The high-energy chiroptical peaks at 210-220 nm correspond to the hybridized molecular orbitals involving the chiral center of amino acids and atoms of graphene edges. Diverse experimental and modeling data, including density functional theory calculations of CD spectra with probabilistic distribution of GQD isomers, indicate that the band at 250-265 nm originates from the three-dimensional twisting of the graphene sheet and can be attributed to the chiral excitonic transitions. The positive and negative low-energy CD bands correspond to the left and right helicity of GQDs, respectively. Exposure of liver HepG2 cells to L/D-GQDs reveals their general biocompatibility and a noticeable difference in the toxicity of the stereoisomers. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than L-GQDs. Emergence of nanoscale chirality in GQDs decorated with biomolecules is expected to be a general stereochemical phenomenon for flexible sheets of nanomaterials.
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology , Ikoma, Nara 8916-5, Japan.
authors.forename: Nozomu
authors.initials: N
authors.lastname: Suzuki
authors.forename: Yichun
authors.initials: Y
authors.lastname: Wang
authors.forename: Paolo
authors.initials: P
authors.lastname: Elvati
authors.forename: Zhi-Bei
authors.initials: ZB
authors.lastname: Qu
authors.forename: Kyoungwon
authors.initials: K
authors.lastname: Kim
authors.forename: Shuang
authors.initials: S
authors.lastname: Jiang
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Baumeister
authors.affiliation: Department of Cogno-Mechatronics Engineering, Pusan National University , Miryang 627-706, Republic of Korea.
authors.forename: Jaewook
authors.initials: J
authors.lastname: Lee
authors.forename: Bongjun
authors.initials: B
authors.lastname: Yeom
authors.forename: Joong Hwan
authors.initials: JH
authors.lastname: Bahng
authors.affiliation: Department of Cogno-Mechatronics Engineering, Pusan National University , Miryang 627-706, Republic of Korea.
authors.forename: Jaebeom
authors.initials: J
authors.lastname: Lee
authors.forename: Angela
authors.initials: A
authors.lastname: Violi
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Kotov
mesh_terms: D002942:Circular Dichroism; D006108:Graphite / Q000737:chemistry*; D056945:Hep G2 Cells; D006801:Humans; D045663:Quantum Dots / Q000737:chemistry* / Q000648:ultrastructure*; D013237:Stereoisomerism
chemical_list: D006108:Graphite
keywords: biological activity; chiral excitons; chirality; circular dichroism; graphene quantum dots