Semiconducting SN2 monolayer with three-dimensional auxetic properties: a global minimum with tetracoordinated sulfurs.

Designing new two-dimensional (2D) materials, exploring their unique properties and diverse potential applications are of paramount importance to condensed matter physics and materials science. In this work, we predicted a novel 2D SN2 monolayer (S-SN2) by means of density functional theory (DFT) computations. In the S-SN2 monolayer, each S atom is tetracoordinated with four N atoms, and each N atom bridges two S atoms, thus forming a tri-sublayer structure with square lattice. The monolayer exhibits good stability, as demonstrated by the moderate cohesive energy, all positive phonon modes, and the structural integrity maintained through 10 ps molecular dynamics simulations up to 1000 K. It is an indirect-bandgap semiconductor with high hole mobility, and the bandgap can be tuned by changing the thickness and external strains (the indirect-bandgap to direct-bandgap transition occurs when the biaxial tensile strain reaches 4%). Significantly, it has large Young's modulus and three-dimensional auxetic properties (both in-plane and out-of-plane negative Poisson's ratios). Therefore, the S-SN2 monolayer holds great potential applications in electronics, photoelectronics and mechanics.

T1- and T2-weighted Magnetic Resonance Dual Contrast by Single Core Truncated Cubic Iron Oxide Nanoparticles with Abrupt Cellular Internalization and Immune Evasion.

Conventional T1- or T2-weighted single mode contrast-enhanced magnetic resonance imaging (MRI) may produce false results. Thereby, there is a need to develop dual contrast agents, T1- and T2-weighted, for more accurate MRI imaging. The dual contrast agents should possess high magnetic resonance (MR) relaxivities, targeted tumor linking, and minimum recognition by the immune system. We have developed nitrodopamine-PEG grafted single core truncated cubic iron oxide nanoparticles (ND-PEG-tNCIOs) capable of producing marked dual contrasts in MRI with enhanced longitudinal and transverse relaxivities of 32 ± 1.29 and 791 ± 38.39 mM-1 s-1, respectively. Furthermore, the ND-PEG-tNCIOs show excellent colloidal stability in physiological buffers and higher cellular internalization in cancerous cells than in phagocytic cells, indicating the immune evasive capability of the nanoparticles. These findings indicate that tNCIOs are strong candidates for dual contrast MRI imaging, which is vital for noninvasive real-time detection of nascent cancer cells in vivo and for monitoring stem cells transplants.

FeB6 Monolayers: The Graphene-like Material with Hypercoordinate Transition Metal.

By means of density functional theory (DFT) computations and global minimum search using particle-swarm optimization (PSO) method, we predicted three FeB6 monolayers, namely α-FeB6, ß-FeB6 and γ-FeB6, which consist of the Fe©Bx (x = 6, 8) wheels with planar hypercoordinate Fe atoms locating at the center of six- or eight-membered boron rings. In particular, the α-FeB6 sheet constructed by Fe©B8 motifs is the global minimum due to completely shared and well delocalized electrons. The two-dimensional (2D) boron networks are dramatically stabilized by the electron transfer from Fe atoms, and the FeB6 monolayers have pronounced stabilities. The α-FeB6 monolayer is metallic, while the ß-FeB6 and γ-FeB6 sheets are semiconductors with indirect band gaps and significant visible-light absorptions. Besides the novel chemical bonding, the high feasibility for experimental realization, and unique electronic and optical properties, render them very welcome new members to the graphene-like materials family.

Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes.

Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole- and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics.

Flexible structural and electronic properties of a pentagonal B2C monolayer via external strain: a computational investigation.

Inspired by the recent theoretical finding that penta-graphene, composed entirely of carbon pentagons, is dynamically and mechanically stable [Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 2372-2377], we computationally designed a new two-dimensional (2D) inorganic material, a pentagonal B2C monolayer (penta-B2C), in which each pentagon contains three boron and two carbon atoms, the C atom is four-coordinated with four B atoms, and all the B atoms are three-coordinated with two C atoms and one B atom, forming a buckled 2D network. The pentagonal B2C monolayer is semiconducting with a wide indirect band gap of 2.28 eV from HSE calculations. The absence of imaginary modes in its phonon spectrum, and the high melting point predicted by molecular dynamics (MD) simulations indicate its good stability. Interestingly, the buckled structure could be stretched to planar under 15% biaxial tensile strain, and the band gap will be strikingly reduced to 0.06 eV. The semiconducting properties of penta-B2C could also be switched to those of a metallic semiconductor under certain biaxial strains, while uniaxial strains could only tune the band gaps without changing the semiconducting characteristics.

An indicator-displacement assay for naked-eye detection and quantification of histidine in human urine.

A simple and efficient colorimetric method for the naked-eye detection and quantification of histidine in biological fluids was developed based on an indicator-displacement assay (IDA) and the Ni(2+)-histidine affinity pair. In this IDA approach, a commercially available dye, murexide, was used as the indicator and the selective detection of histidine was achieved based on the competition between indicator and histidine for the binding with Ni(2+). The competition of histidine with murexide for Ni(2+) resulted in an obvious color change of the solution from yellow to purple, and the permitted naked-eye detection of trace histidine. The developed bioassay allows the rapid, sensitive and selective detection of histidine in urine samples, and does not need complicated sample pretreatment. The detection limit was 0.4 µM with a linear range from 2 to 30 µM. The relative standard deviation for 11 replicate detections of 8 µM histidine was 2.0%. The developed sensor was successfully applied to the determination of histidine in human urine samples with recoveries from 97 to 105%.