Infection Microenvironment-Responsive Coating on Titanium Surfaces for On-Demand Release of Therapeutic Gas and Antibiotic.

Aseptic loosening and bacterial infection pose significant challenges in the clinical application of titanium (Ti) orthopedic implants, which are primarily caused by insufficient osseointegration and bacterial contamination. To address these issues, a responsive coating on Ti surface is constructed, which achieves enhanced osseointegration and infection elimination by on-demand release of therapeutic gas hydrogen sulfide (H2S) and antibiotic. TiO2 nanotubes (TNT) are anodized on the Ti surface to enhance its bioactivity and serve as reservoirs for the antibiotic. An infection microenvironment-responsive macromolecular H2S donor layer is coated on top of TNT to inhibit premature leakage of antibiotic. This layer exhibits a sustained release of low-dosage H2S, which is capable of promoting the osteogenic differentiation and migration of cells. Moreover, the compactness of the macromolecular H2S donor layer could be broken by bacterial invasion, leading to rapid antibiotic release thus preventing infection. In vitro antibacterial experiments validates significant antibacterial activity of the coating against both Gram-negative (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus). Crucially, this coating effectively suppresses implant-associated infection with 98.7% antibacterial efficiency in a rat femoral bone defect model, mitigates inflammation at the defect site and promotes osseointegration of the Ti orthopedic implant.

Intelligent polymeric hydrogen sulfide delivery systems for therapeutic applications.

Hydrogen sulfide (H2S) plays an important role in regulating various pathological processes such as protecting mammalian cell from harmful injuries, promoting tissue regeneration, and regulating the process of various diseases caused by physiological disorders. Studies have revealed that the physiological effects of H2S are highly associated with its concentrations. At relatively low concentration, H2S shows beneficial functions. However, long-time and high-dose donation of H2S would inhibit regular biological process, resulting in cell dysfunction and apoptosis. To regulate the dosage of H2S delivery for precision medicine, H2S delivery systems with intelligent characteristics were developed and a variety of biocompatibility polymers have been utilized to establish intelligent polymeric H2S delivery systems, with the abilities to specifically target the lesions, smartly respond to pathological microenvironments, as well as real-timely monitor H2S delivery and lesion conditions by incorporating imaging-capable moieties. In this review, we focus on the design, preparation, and therapeutic applications of intelligent polymeric H2S delivery systems in cardiovascular therapy, inflammatory therapy, tissue regenerative therapy, cancer therapy and bacteria-associated therapy. Strategies for precise H2S therapies especially imaging-guided H2S theranostics are highlighted. Since H2S donors with stimuli-responsive characters are vital components for establishing intelligent H2S delivery systems, the development of H2S donors is also briefly introduced.

Core-Cross-Linking of Polymeric Micelles by Di-para-Substituted S-Aroylthiooximes as Linkers for Controlled H2S Release.

As one of the gasotransmitters, the therapeutic effects of hydrogen sulfide (H2S) were reported widespread in recent years. Considering the short physiological half-life and significant dose-dependent effects of H2S, it is vital to achieve controlled H2S delivery for biomedical applications. Polymeric micelles have been explored to regulate H2S delivery. However, the dilution-induced dissociation of micelles in physiological conditions limits their therapeutic effects. The circulation stability of polymeric micelles could be improved through core-cross-linking, but reduced H2S releasing efficiency is usually unavoidable. To solve these problems, we developed di-para-substituted S-aroylthiooximes (p-diSATOs) as linkers, which integrated cross-linking of micelle core and conjugation of H2S donors through one simple reaction. Compared with SATO-bearing non-cross-linked micelle, the core-cross-linked micelle (CCM) prepared through this method exhibited initial rapid H2S release owing to the electron-withdrawing effect of p-diSATOs, and subsequently, a sustained release could last for a long period of time. Considering the characteristic H2S releasing behavior of CCM, it may accelerate wound healing through initial efficient and subsequent prolonged pro-healing effects. As a proof of concept, we explored the therapeutic potential of CCM using a murine burn wound model, which exhibited pro-healing effect on burn wounds.

High-Throughput Experimental Study of Wurtzite Mn1-x Zn x O Alloys for Water Splitting Applications.

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn1-x Zn x O wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn1-x Zn x O thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of ZnO in MnO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn1-x Zn x O alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn1-x Zn x O compositions above x = 0.4. The wurtzite Mn1-x ZnxO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 µA cm-2 for 673 nm-thick films. These Mn1-x Zn x O films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn1-x Zn x O materials with Ga dramatically increases the electrical conductivity of Mn1-x Zn x O up to ∼1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott-Schottky and UPS/XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of mid-gap surface states. Overall, this study demonstrates that Mn1-x Zn x O alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.

Anisotropic Conductivity at the Single-Molecule Scale.

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation-dependent conductance is expected. Here, we fabricated single-molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room-temperature single-molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either "ON" or "OFF" conductance states.

Potential-Induced High-Conductance Transport Pathways through Single-Molecule Junctions.

Employing single molecules as electronic circuit building blocks is one promising approach to electronic device miniaturization. We report single-molecule junction formation where the orientation of molecules can be controlled externally by the working electrode potential. The scanning tunneling microscopy break junction (STM-BJ) method is used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic acid (TPA) molecules between the Au(111) electrode and the STM tip to measure the single-molecule conductance through the junction. When the Au(111) electrode is at negative potentials (with respect to the zero-charge potential), a highly ordered and flat-oriented superstructure forms, allowing for direct contact between the π system of the benzene ring of the molecules and the Au(111) electrode, leading to junction formation with no anchoring group involvement. Our first-principles nonequilibrium Green's function (NEGF) computation shows a flat configuration yields a conductance that is 3 orders of magnitude larger than for a molecule vertically connected to the electrodes via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ± 0.02 G0 are experimentally measured with the flat configurations of TFTPA and TPA, respectively. These values are at least 2 orders of magnitude higher than the experimental values previously reported for the conductance of TPA bridged through carboxylic acid anchoring groups (3.8 × 10-4-3.2 × 10-3 G0). In contrast, a positively charged surface triggers an order-disorder transition eliminating the high-conductance states, most likely because the formation of the flat-oriented junction is prevented. The dependence of TFTPA conductance on the electrode potential (electrode Fermi level) suggests a LUMO mediated transport mechanism. Calculation confirms the lack of an effect of the addition of an electron-withdrawing group are investigated.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Redox properties of birnessite from a defect perspective.

Birnessite, a layered-structure MnO2, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys.

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.

Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers.

We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by >60% in the imaginary part and >20% in the real part around exciton resonances using complementary metal-oxide-semiconductor (CMOS) compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of the refractive index. This work may pave the way toward the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.

Understanding band gaps of solids in generalized Kohn-Sham theory.

The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. However, the gap in the band structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density-functional theory. Here, we give a simple proof of a theorem: In generalized KS theory (GKS), the band gap of an extended system equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from metageneralized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential. The theorem also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules, solid aluminum arsenide, and solid argon provide numerical illustrations.

Accurate first-principles structures and energies of diversely bonded systems from an efficient density functional.

One atom or molecule binds to another through various types of bond, the strengths of which range from several meV to several eV. Although some computational methods can provide accurate descriptions of all bond types, those methods are not efficient enough for many studies (for example, large systems, ab initio molecular dynamics and high-throughput searches for functional materials). Here, we show that the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) within the density functional theory framework predicts accurate geometries and energies of diversely bonded molecules and materials (including covalent, metallic, ionic, hydrogen and van der Waals bonds). This represents a significant improvement at comparable efficiency over its predecessors, the GGAs that currently dominate materials computation. Often, SCAN matches or improves on the accuracy of a computationally expensive hybrid functional, at almost-GGA cost. SCAN is therefore expected to have a broad impact on chemistry and materials science.

Communication: Near-locality of exchange and correlation density functionals for 1- and 2-electron systems.

The uniform electron gas and the hydrogen atom play fundamental roles in condensed matter physics and quantum chemistry. The former has an infinite number of electrons uniformly distributed over the neutralizing positively charged background, and the latter only one electron bound to the proton. The uniform electron gas was used to derive the local spin density approximation to the exchange-correlation functional that undergirds the development of the Kohn-Sham density functional theory. We show here that the ground-state exchange-correlation energies of the hydrogen atom and many other 1- and 2-electron systems are modeled surprisingly well by a different local spin density approximation (LSDA0). LSDA0 is constructed to satisfy exact constraints but agrees surprisingly well with the exact results for a uniform two-electron density in a finite, curved three-dimensional space. We also apply LSDA0 to excited or noded 1-electron densities, where it works less well. Furthermore, we show that the localization of the exact exchange hole for a 1- or 2-electron ground state can be measured by the ratio of the exact exchange energy to its optimal lower bound.

Effect of extended strain fields on point defect phonon scattering in thermoelectric materials.

The design of thermoelectric materials often involves the integration of point defects (alloying) as a route to reduce the lattice thermal conductivity. Classically, the point defect scattering strength follows from simple considerations such as mass contrast and the presence of induced strain fields (e.g. radius contrast, coordination changes). While the mass contrast can be easily calculated, the associated strain fields induced by defect chemistry are not readily predicted and are poorly understood. In this work, we use classical and first principles calculations to provide insight into the strain field component of phonon scattering from isoelectronic point defects. Our results also integrate experimental measurements on bulk samples of SnSe and associated alloys with S, Te, Ge, Sr and Ba. These efforts highlight that the strength and extent of the resulting strain field depends strongly on defect chemistry. Strain fields can have a profound impact on the local structure. For example, in alloys containing Ba, the strain fields have significant spatial extent (1 nm in diameter) and produce large shifts in the atomic equilibrium positions (up to 0.5 Å). Such chemical complexity suggests that computational assessment of point defects for thermal conductivity depression should be hindered. However, in this work, we present and verify several computational descriptors that correlate well with the experimentally measured strain fields. Furthermore, these descriptors are conceptually transparent and computationally inexpensive, allowing computation to provide a pivotal role in the screening of effective alloys. The further development of point defect engineering could complement or replace nanostructuring when optimizing the thermal conductivity, offering the benefits of thermodynamic stability, and providing more clearly defined defect chemistry.

KAg11(VO4)4 as a candidate p-type transparent conducting oxide.

For a material to be a good p-type transparent conducting oxide (TCO), it must simultaneously satisfy several design principles regarding its bulk and defect phase thermochemistry, its optical absorption spectrum, and its electric transport properties. Recently, we predicted Ag3VO4 to be p-type but with low conductivity and an optical band gap not large enough for transparency. To improve on the transport and optical properties of Ag3VO4, we searched an extended material space including quaternary compounds based on Ag, V, O, and an additional atom for a new candidate p-type TCO. From this set of quaternary materials, we selected KAg11(VO4)4, a known oxide with a crystal structure related to that of Ag3VO4. Notably, one could expect a possible enhancement of the concentration of hole producing Ag-vacancy defects in KAg11(VO4)4 due to its different local geometries of Ag atoms (2- and 3-fold coordinated) with respect to the 4-fold coordinated Ag atoms in Ag3VO4. By performing first-principles calculations, we found that KAg11(VO4)4 is an intrinsic p-type conductor and can be synthesized under conditions similar to those predicted for the synthesis of Ag3VO4. However, we predict that the intrinsic hole content in KAg11(VO4)4 is similar to that in Ag3VO4 even though KAg11(VO4)4 contains 2- and 3-fold coordinated Ag, hole producing sites with a lower defect formation energy than the 4-fold coordinated one. Our calculation demonstrates that the advantage from lower coordination number of the Ag atom in KAg11(VO4)4 can be offset by the change in the range of Ag chemical potential in which synthesis is allowed due to the oxide phases that Ag forms with K and that energetically compete with KAg11(VO4)4.

Origin and enhancement of hole-induced ferromagnetism in first-row d0 semiconductors.

The origin of ferromagnetism in d;{0} semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d;{0} semiconductors and can be attributed to the localized nature of the 2p states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.