Amino Acid Analogue-Conjugated BN Nanomaterials in a Solvated Phase: First Principles Study of Topology-Dependent Interactions with a Monolayer and a (5,0) Nanotube

Using density functional theory and an implicit solvation model, the relationship between the topology of boron nitride (BN) nanomaterials and the protonated/deprotonated states of amino acid analogues is investigated. In the solvated phase, the calculated results show distinct “physisorbed versus chemisorbed” conditions for the analogues of arginine (Arg)- and aspartic acid (Asp)-conjugated BN nanomaterials, including a monolayer (ML) and a small-diameter zigzag nanotube (NT). Such a distinction does not depend on the functional groups of amino acids but rather depends on the curvature-induced interactions associated with the tubular configuration. Arg and Asp interact with the BNML to form physisorbed complexes irrespective of the state of the amino acids in the solvated phase. For the NT, Arg and Asp form chemisorbed complexes, and the distinct nature of bonds between the donor electron moieties of N_(Arg) and O_(Asp) and the boron of the tubular surface is revealed by the natural bond orbital analysis; stronger s-type bonds for the deprotonated conjugated complexes and slightly weaker p-type dominated bonds for the protonated conjugated complexes. The interaction of neutral Trp with BN nanomaterials results in physisorbed configurations through π-stacking interactions with the indole ring of the Trp and BN nanomaterials. The calculated results form the basis for a theoretical study of more complex protein macromolecules interacting with nanomaterials under physiological conditions

Controlling the Performance of a Three-Terminal Molecular Transistor: Conformational versus Conventional Gating

The effect of conformational changes in the gate arm of a three-terminal device is investigated. In the ground state, the gate (triphenyl) arm is nonplanar, where the middle phenyl ring is approximately 30° out-of-plane relative to other two rings. At this geometry, the calculated tunnel current (<i>I</i>_d) as a function of external bias (<i>V</i>_ds) across the two D–A substituted arms exhibits a typical insulator-semiconductor behavior. Similar <i>I</i>_d–<i>V</i>_ds characteristics is calculated when planarity of the triphenyl arm is restored. However, a significant increase, by more than an order of magnitude, and a distinct variation in the current are predicted in its operational mode (<i>V</i>_ds > 1.5 V) when additional nonplanarity is induced in the triphenyl chain. Analysis of the results suggest that, unlike in “voltage” gating, neither the HOMO–LUMO gap nor the dipole moment of the system undergo significant changes due to pure conformational gating, as observed in this study. Instead, the observed conformational gating affects the current via localization/delocalization of the electronic wave function in the conduction channel. Furthermore, the tunneling current corresponding to conformational gating in two different directions appears to exhibit oscillatory nature with a phase factor of π/2 in the presence of the gate field. The current modulation is found to reach its maximum only under exclusive effect of voltage or conformational gating and diminishes when both of them are present

Hierarchical Self-Assembly of Noncanonical Guanine Nucleobases on Graphene

Self-assembly characterizes the fundamental basis toward realizing the formation of highly ordered hierarchical heterostructures. A systematic approach toward the supramolecular self-assembly of free-standing guanine nucleobases and the role of graphene as a substrate in directing the monolayer assembly are investigated using the molecular dynamics simulation. We find that the free-standing bases in gas phase aggregate into clusters dominated by intermolecular H-bonds, whereas in solvent, substantial screening of intermolecular interactions results in π-stacked configurations. Interestingly, graphene facilitates the monolayer assembly of the bases mediated through the base–substrate π–π stacking. The bases assemble in a highly compact network in gas phase, whereas in solvent, a high degree of immobilization is attributed to the disruption of intermolecular interactions. Graphene-induced stabilization/aggregation of free-standing guanine bases appears as one of the prerequisites governing molecular ordering and assembly at the solid/liquid interface. The results demonstrate an interplay between intermolecular and π-stacking interactions, central to the molecular recognition, aggregation dynamics, and patterned growth of functional molecules on two-dimensional nanomaterials

Can Single-Atom Change Affect Electron Transport Properties of Molecular Nanostructures such as C₆₀ Fullerene?

At the nanoscale, even a single atom change in the structure can noticeably alter the properties, and therefore, the application space of materials. We examine this critical behavior of nanomaterials using fullerene as a model structure by a first-principles density functional theory method coupled with nonequilibrium Green’s function formalism. Two different configurations, namely, (i) endohedral (B@C₆₀ and N@C₆₀), in which the doping atom is encapsulated inside the fullerene cage, and (ii) substitutional (BC₅₉ and NC₅₉), in which the doping atom replaces a C atom on the fullerene cage, are considered. The calculated results reveal that the conductivity for the doped fullerene is higher than that of the pristine fullerene. In the low-bias regime, the current (I) voltage (V) characteristic of the endohedral as well as the substitutional configurations are very similar. However, as the external bias increases beyond 1.0 V, the substitutional BC₅₉ fullerene exhibits a considerably higher magnitude of current than all other species considered, thus suggesting that it can be an effective semiconductor in <i>p</i>-type devices

Nature of Interaction between Semiconducting Nanostructures and Biomolecules: Chalcogenide QDs and BNNT with DNA Molecules

Interactions of DNA oligomers with two categories of semiconducting nanostructureschalcogenide quantum dots (QDs) and boron nitride nanotubes (BNNTs)owing to their widespread presence in bio-inspired processes are investigated using the first-principles density functional theory and continuum solvent model. The chalcogenide QDs interact strongly at their metal centers featuring electrostatic interaction with DNA oligomers at oxygen or nitrogen site, while BNNTs form covalent bonds with DNA oligomers at multiple surface sites. It is found that the different bonding nature leads to distinctly different response to the aqueous environment; the presence of solvent drastically reduces the binding strength of nucleobases with the QDs due to the strong electrostatic screening. This is not the case with BNNTs for which the covalent bonding is barely affected by the solvent. This study thus clearly shows how a solvent medium influences chemical interactions providing guidance for technological applications of bioconjugated systems

Real-Time Electrochemical Monitoring of Adenosine Triphosphate in the Picomolar to Micromolar Range Using Graphene-Modified Electrodes

We report on a competitive electrochemical detection system that is free of wash steps and enables the real-time monitoring of adenosine triphosphate (ATP) in a quantitative manner over a five-log concentration range. The system utilizes a recognition surface based on ATP aptamer (ATPA) capture probes prebound to electroactive flavin adenine dinucleotide (FAD) molecules, and a signaling surface utilizing graphene (Gr) and gold nanoparticle (AuNP) modified carbon paste electrode (Gr–AuNP–CPE) that is optimized to enhance electron-transfer kinetics and signal sensitivity. Binding of ATP to ATPA at the recognition surface causes the release of an equivalent concentration of FAD that can be quantitatively monitored in real time at the signaling surface, thereby enabling a wide linear working range (1.14 × 10^–10 to 3.0 × 10^–5 M), a low detection limit (2.01 × 10^–11 M using graphene and AuNP modified glassy carbon), and fast target binding kinetics (steady-state signal within 12 min at detection limit). Unlike assays based on capture probe-immobilized electrodes, this double-surface competitive assay offers the ability to speed up target binding kinetics by increasing the capture probe concentration, with no limitations due to intermolecular Coulombic interactions and nonspecific binding. We utilize the real-time monitoring capability to compute kinetic parameters for target binding and to make quantitative distinctions on degree of base-pair mismatch through monitoring target binding kinetics over a wide concentration range. On the basis of the simplicity of the assay chemistry and the quantitative detection of ATP within fruit and serum media, as demonstrated by comparison of ATP levels against those determined using a standard high-performance liquid chromatography (HPLC)-UV absorbance method, we envision a versatile detection platform for applications requiring real-time monitoring over a wide target concentration range

<i>In situ</i> Synthesis of Fluorescent Gold Nanoclusters by Nontumorigenic Microglial Cells

To date, the directed <i>in situ</i> synthesis of fluorescent gold nanoclusters (AuNCs) has only been demonstrated in cancerous cells, with the theorized synthesis mechanism prohibiting AuNC formation in nontumorigenic cell lines. This limitation hinders potential biostabilized AuNC-based technology in healthy cells involving both chemical and mechanical analysis, such as the direct sensing of protein function and the elucidation of local mechanical environments. Thus, new synthesis strategies are required to expand the application space of AuNCs beyond cancer-focused cellular studies. In this contribution, we have developed the methodology and demonstrated the direct <i>in situ</i> synthesis of AuNCs in the nontumorigenic neuronal microglial line, C8B4. The as-synthesized AuNCs form <i>in situ</i> and are stabilized by cellular proteins. The clusters exhibit bright green fluorescence and demonstrate low (<10%) toxicity. Interestingly, elevated ROS levels were not required for the <i>in situ</i> formation of AuNCs, although intracellular reductants such as glutamate were required for the synthesis of AuNCs in C8B4 cells. To our knowledge, this is the first-ever demonstration of AuNC synthesis in nontumorigenic cells and, as such, it considerably expands the application space of biostabilized fluorescent AuNCs

Growth of Large Single-Crystalline Two-Dimensional Boron Nitride Hexagons on Electropolished Copper

Hexagonal-boron nitride (h-BN) or “white graphene” has many outstanding properties including high thermal conductivity, high mechanical strength, chemical inertness, and high electrical resistance, which open up a wide range of applications such as thermal interface material, protective coatings, and dielectric in nanoelectronics that easily exceed the current advertised benefits pertaining to the graphene-based applications. The development of h-BN films using chemical vapor deposition (CVD) has thus far led into nucleation of triangular or asymmetric diamond shapes on different metallic surfaces. Additionally, the average size of the triangular domains has remained relatively small (∼0.5 μm²) leading to a large number of grain boundaries and defects. While the morphology of Cu surfaces for CVD-grown graphene may have impacts on the nucleation density, domain sizes, thickness, and uniformity, the effects of the decreased roughness of Cu surface to develop h-BN films are unknown. Here, we report the growth and characterization of novel large area h-BN hexagons using highly electropolished Cu substrate under atmospheric pressure CVD conditions. We found that the nucleation density of h-BN is significantly reduced while domain sizes increase. In this study, the largest hexagonal-shape h-BN domain observed is 35 μm², which is an order of magnitude larger than a typical triangular domain. As the domains coalesce to form a continuous film, the larger grain size offers a more pristine and smoother film with lesser grain boundaries induced defects