Carbon, Graphene and Graphene Oxide Quantum Dots for Analytical Biochemistry Research

Graphene quantum dots (GQDs) have emerged rapidly as a new class of attractive fluorescence nano-probes ever since they were made from coal in 2013 by the Rice University lab of chemist James Tour [1,2]. They offer 2-10 nm size, good quantum yield, high photostability, tunable photoluminescence, flexible molecular structure, easy functionalization, excellent bio-compatibility, stable dispersion in water, and facile hydrothermal synthesis. A method for forming GQDs included adding an organic starting material to a vessel and heating the material to within 20°C of its boiling temperature for a time no longer than ten minutes [3]. Their chemical inertness and low toxicity have triggered numerous studies on their unique properties in interdisciplinary science and engineering research over the last several years [4]. Different sized GQDs with a narrow size distribution could be obtained via gel electrophoresis of the crude GQDs prepared through a photo-Fenton reaction of graphene oxide [5]. It was illustrated that the photoluminesce emissions of the well-defined GQDs originated mainly from the peripheral carboxylic groups and conjugated carbon backbone planes. GQDs enhanced sulfur/sulfide utilization, proving that C–S bonding could occur [6]. Graphene oxide quantum dots (GOQDs) exhibited photoluminescence color variation from orange-red to blue as their size was reduced, with electron transitions from the graphene π* orbital to the oxygen n orbital [7]. The electronic properties and mechanisms involved in quantumconfined photoluminescence can be applied to research in analytical biochemistry. Chiral nanostructures can greatly enrich the chemical property of graphene-based materials to facilitate their applications in biology. Covalent attachment of l/d-cysteine moieties to GQDs led to their helical buckling due to chiral interactions at the edges [8]. Exposure of liver HepG2 cells to the l/d-GQD stereoisomers revealed a noticeable difference in their toxicity. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than l-GQDs.