Advances in Structural Modifications and Properties of Graphene Quantum Dots for Biomedical Applications.

Graphene quantum dots (GQDs) are carbon-based, zero-dimensional nanomaterials and unique due to their astonishing optical, electronic, chemical, and biological properties. Chemical, photochemical, and biochemical properties of GQDs are intensely being explored for bioimaging, biosensing, and drug delivery. The synthesis of GQDs by top-down and bottom-up approaches, their chemical functionalization, bandgap engineering, and biomedical applications are reviewed here. Current challenges and future perspectives of GQDs are also presented.

Rashba effect on finite temperature magnetotransport in a dissipative quantum dot transistor with electronic and polaronic interactions.

The Rashba spin-orbit coupling induced quantum transport through a quantum dot embedded in a two-arm quantum loop of a quantum dot transistor is studied at finite temperature in the presence of electron-phonon and Hubbard interactions, an external magnetic field and quantum dissipation. The Anderson-Holstein-Caldeira-Leggett-Rashba model is used to describe the system and several unitary transformations are employed to decouple some of the interactions and the transport properties are calculated using the Keldysh technique. It is shown that the Rashba coupling alone separates the spin-up and spin-down currents causing zero-field spin-polarization. The gap between the up and down-spin currents and conductances can be changed by tuning the Rashba strength. In the absence of a field, the spin-up and spin-down currents show an opposite behaviour with respect to spin-orbit interaction phase. The spin-polarization increases with increasing electron-phonon interaction at zero magnetic field. In the presence of a magnetic field, the tunneling conductance and spin-polarization change differently with the polaronic interaction, spin-orbit interaction and dissipation in different temperature regimes. This study predicts that for a given Rashba strength and magnetic field, the maximum spin-polarization in a quantum dot based device occurs at zero temperature.

Graphene nanofiber composites for enhanced neuronal differentiation of human mesenchymal stem cells.

Aim: To differentiate mesenchymal stem cells into functional dopaminergic neurons using an electrospun polycaprolactone (PCL) and graphene (G) nanocomposite. Methods: A one-step approach was used to electrospin the PCL nanocomposite, with varying G concentrations, followed by evaluating their biocompatibility and neuronal differentiation. Results: PCL with exiguous graphene demonstrated an ideal nanotopography with an unprecedented combination of guidance stimuli and substrate cues, aiding the enhanced differentiation of mesenchymal stem cells into dopaminergic neurons. These newly differentiated neurons were seen to exhibit unique neuronal arborization, enhanced intracellular Ca2+ influx and dopamine secretion. Conclusion: Having cost-effective fabrication and room-temperature storage, the PCL-G nanocomposites could pave the way for enhanced neuronal differentiation, thereby opening a new horizon for an array of applications in neural regenerative medicine.

A semi exact solution for a metallic phase in a Holstein-Hubbard chain at half filling with Gaussian anharmonic phonons.

The nature of phase transition from an antiferromagnetic SDW polaronic Mott insulator to the paramagnetic bipolaronic CDW Peierls insulator is studied for the half-filled Holstein-Hubbard model in one dimension in the presence of Gaussian phonon anharmonicity. A number of unitary transformations performed in succession on the Hamiltonian followed by a general many-phonon averaging leads to an effective electronic Hamiltonian which is then treated exactly by using the Bethe-Ansatz technique of Lieb and Wu to determine the energy of the ground state of the system. Next using the Mott-Hubbard metallicity condition, local spin-moment calculation, and the concept of quantum entanglement entropy and double occupancy, it is shown that in a plane spanned by the electron-phonon coupling coefficient and onsite Coulomb correlation energy, there exists a window in which the SDW and CDW phases are separated by an intermediate phase that is metallic.

Graphene Quantum Dot Oxidation Governs Noncovalent Biopolymer Adsorption.

Graphene quantum dots (GQDs) are an allotrope of carbon with a planar surface amenable to functionalization and nanoscale dimensions that confer photoluminescence. Collectively, these properties render GQDs an advantageous platform for nanobiotechnology applications, including optical biosensing and delivery. Towards this end, noncovalent functionalization offers a route to reversibly modify and preserve the pristine GQD substrate, however, a clear paradigm has yet to be realized. Herein, we demonstrate the feasibility of noncovalent polymer adsorption to GQD surfaces, with a specific focus on single-stranded DNA (ssDNA). We study how GQD oxidation level affects the propensity for polymer adsorption by synthesizing and characterizing four types of GQD substrates ranging ~60-fold in oxidation level, then investigating noncovalent polymer association to these substrates. Adsorption of ssDNA quenches intrinsic GQD fluorescence by 31.5% for low-oxidation GQDs and enables aqueous dispersion of otherwise insoluble no-oxidation GQDs. ssDNA-GQD complexation is confirmed by atomic force microscopy, by inducing ssDNA desorption, and with molecular dynamics simulations. ssDNA is determined to adsorb strongly to no-oxidation GQDs, weakly to low-oxidation GQDs, and not at all for heavily oxidized GQDs. Finally, we reveal the generality of the adsorption platform and assess how the GQD system is tunable by modifying polymer sequence and type.

Graphene Quantum Dots: Synthesis and Applications.

Graphene and its derivatives having at least one dimension in nanoscale range have attracted tremendous attention in recent years due to their unique electronic, optical, chemical, and mechanical properties. This chapter is about graphene quantum dots (GQDs) that are zero-dimensional graphene derivatives with one to few layers of graphene sheet having size range less than 20nm. This chapter is an overview of synthesis of GQDs by top-down and bottom-up approaches, as well as detailed methods of synthesis of GQDs by acidic oxidation of carbon fibers. Owing to their extremely small size, quantum confinement, edge effect, biocompatibility, low toxicity, photostability as well as water solubility they are excellent candidates for understanding biological systems and cellular processes at the molecular scale. These are also suitable nanomaterials to replace inorganic semiconducting nanoparticles (e.g., CdS, CdSe, ZnS, and Si) which are toxic to biological systems.