Exciton-assisted electron tunnelling in van der Waals heterostructures.

The control of elastic and inelastic electron tunnelling relies on materials with well-defined interfaces. Two-dimensional van der Waals materials are an excellent platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here we use a tunnelling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride with an adjacent TMD monolayer and observe prominent resonant features in current-to-voltage measurements appearing at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunnelling pathway, we demonstrate that this tunnelling process does not require any charge injection into the TMD. The appearance of such optical modes in electrical transport introduces additional functionality towards van der Waals material-based optoelectronic devices.

Overbias Photon Emission from Light-Emitting Devices Based on Monolayer Transition Metal Dichalcogenides.

Tunneling light-emitting devices (LEDs) based on transition metal dichalcogenides (TMDs) and other two-dimensional (2D) materials are a new platform for on-chip optoelectronic integration. Some of the physical processes underlying this LED architecture are not fully understood, especially the emission at photon energies higher than the applied electrostatic potential, so-called overbias emission. Here we report overbias emission for potentials that are near half of the optical bandgap energy in TMD-based tunneling LEDs. We show that this emission is not thermal in nature but consistent with exciton generation via a two-electron coherent tunneling process.

Homomesoporous Metal-Organic Framework for High-Performance Electrochromatographic Separation.

Metal-organic frameworks (MOFs) have exhibited tremendous potential in the area of separation science. However, most of the developed MOF-based stationary phases contained only microporous structures and suffer from limited separation performance. Herein, homomesoporous MOFs with excellent mass transfer capability and strong thermodynamic interactions are first explored as the novel stationary phase for high-performance capillary electrochromatographic separations. As a proof of concept, noninterpenetrated mesoMOF-1 with uniform mesopore sizes (22.5 × 26.1 Å) and good stability was facilely grown on the inner surface of capillaries and applied as a homomesoporous MOF coating-based stationary phase for high-efficiency electrochromatographic separation. Seven types of analytes with different molecular dimensions were all baseline separated on a mesoMOF-1 coated column with high theoretical plate numbers and excellent repeatability, exhibiting significantly improved separation selectivity and column efficiency in comparison to a microporous HKUST-1 coated column. The maximum column efficiencies of the mesoMOF-1 coated column for substituted benzenes and halobenzenes reached up to 1.4 × 105 plates/m, and its mass loadability was also much higher than that of the HKUST-1 coated column. In addition, based on the analysis of adsorption kinetics and chromatographic retention behaviors, the interaction and retention mechanisms of different molecular-weight analytes on mesoMOF-1 coated stationary phases were systematically explored and disclosed in detail. These results indicate that the homomesoporous MOF-based stationary phase can effectively balance the kinetic diffusion (mass transfer capability) and thermodynamic interactions (the strength of adsorption interaction), having great potential for high-performance chromatographic separation.

Plastome phylogenomics provide new perspective into the phylogeny and evolution of Betulaceae (Fagales).

BACKGROUND: Betulaceae is a relatively small but morphologically diverse family, with many species having important economic and ecological values. Although plastome structure of Betulaceae has been reported sporadically, a comprehensive exploration for plastome evolution is still lacking. Besides, previous phylogenies had been constructed based on limited gene fragments, generating unrobust phylogenetic framework and hindering further studies on divergence ages, biogeography and character evolution. Here, 109 plastomes (sixteen newly assembled and 93 previously published) were subject to comparative genomic and phylogenomic analyses to reconstruct a robust phylogeny and trace the diversification history of Betulaceae. RESULTS: All Betulaceae plastomes were highly conserved in genome size, gene order, and structure, although specific variations such as gene loss and IR boundary shifts were revealed. Ten divergent hotspots, including five coding regions (Pi > 0.02) and five noncoding regions (Pi > 0.035), were identified as candidate DNA barcodes for phylogenetic analysis and species delimitation. Phylogenomic analyses yielded high-resolution topology that supported reciprocal monophyly between Betula and Alnus within Betuloideae, and successive divergence of Corylus, Ostryopsis, and Carpinus-Ostrya within Coryloideae. Incomplete lineage sorting and hybridization may be responsible for the mutual paraphyly between Ostrya and Carpinus. Betulaceae ancestors originated from East Asia during the upper Cretaceous; dispersals and subsequent vicariance accompanied by historical environment changes contributed to its diversification and intercontinental disjunction. Ancestral state reconstruction indicated the acquisition of many taxonomic characters was actually the results of parallel or reversal evolution. CONCLUSIONS: Our research represents the most comprehensive taxon-sampled and plastome-level phylogenetic inference for Betulaceae to date. The results clearly document global patterns of plastome structural evolution, and established a well-supported phylogeny of Betulaceae. The robust phylogenetic framework not only provides new insights into the intergeneric relationships, but also contributes to a perspective on the diversification history and evolution of the family.

Species divergence and phylogeography of Corylus heterophylla Fisch complex (Betulaceae): Inferred from molecular, climatic and morphological data.

Historical geo-climatic changes have shaped the geographical distributions and genetic diversity of numerous plant taxa in East Asia, which promote species divergence and ultimately speciation. Here, we integrated multiple approaches, including molecular phylogeography, ecological niche modeling, and morphological traits to examine the nucleotide diversity and interspecific divergence within Corylus heterophylla complex (C. heterophylla, C. kweichowensis, and C. yunnanensis). These three sibling taxa harbored similar high levels of nucleotide diversity at the species level. The molecular data (SCNG and cpDNA) unanimously supported the division of C. heterophylla complex into two major clades, with C. yunnanensis diverged earlier from the complex, whereas C. heterophylla and C. kweichowensis could hardly be separated. The split between the two clades (c. 12.89 Ma) coincided with the formation of Sichuan Basin in the middle Miocene, while the divergence among and within the five subclades (YUN1-YUN3, HK1-HK2) occurred from the late Miocene to the Pleistocene. C. heterophylla of northern China experienced glacial contraction and interglacial expansion during the Quaternary, whereas C. kweichowensis and C. yunnanensis of southern China presented population expansion even during the last glacial maximum. Despite of high levels of genetic admixture between C. heterophylla and C. kweichowensis, significant ecological and morphological discrepancy as well as incomplete geographic isolation indicated that adaptive evolution triggered by divergent selection may have played important roles in incipient ecological speciation.

Metformin Hydrochloride Mucosal Nanoparticles-Based Enteric Capsule for Prolonged Intestinal Residence Time, Improved Bioavailability, and Hypoglycemic Effect.

Metformin hydrochloride enteric-coated capsule (MH-EC) is a commonly used clinical drug for the treatment of type 2 diabetes. In this study, we described a metformin hydrochloride mucosal nanoparticles enteric-coated capsule (MH-MNPs-EC) based on metformin hydrochloride chitosan mucosal nanoparticles (MH-CS MNPs) and its preparation method to improve the bioavailability and hypoglycemic effect duration of MH-EC. In intestinal adhesion study, the residue rates of free drugs and mucosal nanoparticles were 10.52% and 67.27%, respectively after cleaned with PBS buffer. MH-CS MNPs could significantly improve the efficacy of MH and promote the rehabilitation of diabetes rats. In vitro release test of MH-MNPs-EC showed continuous release over 12 h, while commercial MH-EC released completely within about 1 h in intestinal environment (pH 6.8). Pharmacokinetic study was performed in beagle dogs compared to the commercial MH-EC. The durations of blood MH concentration above 2 µg/mL were 9 h for MH-MNPs-EC versus 2 h for commercial MH-EC. The relative bioavailability of MH-MNPs-EC was determined as 185.28%, compared with commercial MH-EC. In conclusion, MH-CS MNPs have good intestinal adhesion and can significantly prolong the residence time of MH in the intestine. MH-MNPs-EC has better treatment effect compared with MH-EC, and it is expected to be a potential drug product for the treatment of diabetes because of its desired characteristics.

Chiral Protein-Covalent Organic Framework 3D-Printed Structures as Chiral Biosensors.

Three-dimensional (3D) printing technology has attracted great attention for prototyping different electrochemical sensor devices. However, chiral recognition remains a crucial challenge for electrochemical sensors with similar physicochemical properties such as enantiomers. In this work, a magnetic covalent organic framework (COF) and bovine serum albumin (BSA) (as the chiral surface) functionalized 3D-printed electrochemical chiral sensor is reported for the first time. The characterization of the chiral biomolecule-COF 3D-printed constructure was performed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX). A tryptophan (Trp) enantiomer was chosen as the model chiral molecule to estimate the chiral recognition ability of the magnetic COF and BSA-based 3DE (Fe3O4@COF@BSA/3DE). We have demonstrated that the Fe3O4@COF@BSA/3DE exhibited excellent chiral recognition to l-Trp as compared to d-Trp. The chiral protein-COF sensing interface was used to determine the concentration of l-Trp in a racemic mixture of d-Trp and l-Trp. This strategy of on-demand fabrication of 3D-printed protein-COF-modified electrodes opens up new approaches for enantiomer recognition.

Nonlinear behavior in preparative liquid chromatography: A method-development case study for hydroxytyrosol purification.

Preparative liquid chromatography has become an important purification method owing to its advantages of high separation efficiency, good reproducibility, and low solvent consumption. Because overloading in preparative liquid chromatography must be performed to increase the throughput in a cycle, nonlinear chromatographic behavior is observed. Therefore, it is crucial to carefully study nonlinear chromatography for the purification of a given product, which facilitates the efficient optimization of the purification parameters. In this work, a method for the development of a purification method using preparative liquid chromatography based on nonlinear chromatography is proposed. Hydroxytyrosol was selected as the subject for method demonstration. Using methanol and ethanol as organic modifiers, the optimum flow rate was determined on three commercial columns entitled C8 TDE, C18 ME, and C18 TDE, respectively. The curves were fitted with the van Deemter equation, with thorough analysis of the A, B, and C terms. Adsorption isotherms were subsequently studied to explore the distribution of solutes between the stationary and mobile phases at equilibrium. C18 TDE, 5 vol% ethanol-water, and 0.2 mL/min were selected as the optimal separation material, elution solvent, and flow rate, respectively. Purification of hydroxytyrosol was tentatively confirmed on a C18 TDE column with 1.6% sample loading, 90.98% recovery, and 98.01% purity.

Covalently modified enzymatic 3D-printed bioelectrode.

Three-dimensional (3D) printing has showed great potential for the construction of electrochemical sensor devices. However, reported 3D-printed biosensors are usually constructed by physical adsorption and needed immobilizing reagents on the surface of functional materials. To construct the 3D-printed biosensors, the simple modification of the 3D-printed device by non-expert is mandatory to take advantage of the remote, distributed 3D printing manufacturing. Here, a 3D-printed electrode was prepared by fused deposition modeling (FDM) 3D printing technique and activated by chemical and electrochemical methods. A glucose oxidase-based 3D-printed nanocarbon electrode was prepared by covalent linkage method to an enzyme on the surface of the 3D-printed electrode to enable biosensing. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the glucose oxidase-based biosensor. Direct electrochemistry glucose oxidase-based biosensor with higher stability was then chosen to detect the two biomarkers, hydrogen peroxide and glucose by chronoamperometry. The prepared glucose oxidase-based biosensor was further used for the detection of glucose in samples of apple cider. The covalently linked glucose oxidase 3D-printed nanocarbon electrode as a biosensor showed excellent stability. This work can open new doors for the covalent modification of 3D-printed electrodes in other electrochemistry fields such as biosensors, energy, and biocatalysis.

Compact SQUID Realized in a Double-Layer Graphene Heterostructure.

2D systems that host 1D helical states are advantageous from the perspective of scalable topological quantum computation when coupled to a superconductor. Graphene is particularly promising for its high electronic quality, its versatility in van der Waals heterostructures, and its electron- and hole-like degenerate 0th Landau level. Here we study a compact double-layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by the independent gate control of the chemical potentials in both layers. Furthermore, both Josephson junctions show a skewed current-phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime, we measure a well-defined conductance plateau of 2e2/h indicative of counter-propagating edge channels in the two layers.

Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols via Anodically Generated Silyl Cations.

The first electrochemical hydrolysis of hydrosilanes to silanols under mild and neutral reaction conditions is reported. The practical protocol employs commercially available and cheap NHPI as a hydrogen-atom transfer (HAT) mediator and operates at room temperature with high selectivity, leading to various valuable silanols in moderate to good yields. Notably, this electrochemical method exhibits a broad substrate scope and high functional-group compatibility, and it is applicable to late-stage functionalization of complex molecules. Preliminary mechanistic studies suggest that the reaction appears to proceed through a nucleophilic substitution reaction of an electrogenerated silyl cation with H2 O.

Nanoscale Hierarchically Micro- and Mesoporous Metal-Organic Frameworks for High-Resolution and High-Efficiency Capillary Electrochromatographic Separation.

Metal-organic frameworks (MOFs) have been widely applied in a variety of fields. However, most of the developed MOFs are micrometer scale in crystal size and contain only micropores, which will limit the mass transport and diffusion of various analytes into their internal interaction sites, severely restricting the potential of MOFs in separation science. Herein, nanoscale hierarchically porous MOFs (NHP-MOFs) were first explored as a novel MOF-based stationary phase with excellent mass transfer performance and abundant accessible interaction sites for high-performance chromatographic separation. As a proof-of-concept demonstration, the nanoscale hierarchically micro- and mesoporous UiO-66 (NHP-UiO-66) was firmly immobilized on the capillary inner surface and utilized as the porous stationary phase for high-resolution and high-efficiency electrochromatographic separation. A wide range of low-, medium-, and high-molecular-weight analytes, including substituted benzenes, chlorobenzenes, polycyclic aromatic hydrocarbons, nucleosides, polypeptides, and proteins were all separated well on a NHP-UiO-66-coated column with excellent resolution and repeatability, exhibiting significantly improved column efficiency and separation ability compared to those of a microporous UiO-66-modified column. The maximum column efficiencies for all the six kinds of analytes reached up to 1.2 × 105 plates/m, and the relative standard deviations of the migration times of substituted benzenes for intraday, interday, and column-to-column were all lower than 5.8%. These results reveal that NHP-MOFs can effectively combine the advantages of the high specific surface area of microporous MOFs and the excellent mass transfer performance and abundant accessible interaction sites of NHP materials, possessing great prospect for high-performance chromatographic separation.

Mobility Enhancement in Graphene by in situ Reduction of Random Strain Fluctuations.

Microscopic corrugations are ubiquitous in graphene even when placed on atomically flat substrates. These result in random local strain fluctuations limiting the carrier mobility of high quality hBN-supported graphene devices. We present transport measurements in hBN-encapsulated devices where such strain fluctuations can be in situ reduced by increasing the average uniaxial strain. When ∼0.2% of uniaxial strain is applied to the graphene, an enhancement of the carrier mobility by ∼35% is observed while the residual doping reduces by ∼39%. We demonstrate a strong correlation between the mobility and the residual doping, from which we conclude that random local strain fluctuations are the dominant source of disorder limiting the mobility in these devices. Our findings are also supported by Raman spectroscopy measurements.

Preparation and evaluation of a molybdenum disulfide quantum dots embedded C18 mixed-mode chromatographic stationary phase.

Molybdenum disulfide quantum dots (MoS2 QDs) were chosen as a functional two-dimensional material to improve the separation performance of a traditional C18 column. In this work, MoS2 QDs were synthesized by the combination of sonication and solvothermal treatment of bulk MoS2. The prepared MoS2 QDs were characterized by transmission electron microscope (TEM), Zeta potential measurement, UV-visible absorption and fluorescence spectroscopy. Then, a novel MoS2 QDs embedded C18 (Sil-MoS2-C18) stationary phase was prepared for performing mixed-mode liquid chromatography. The results of elemental analysis (EA), thermogravimetric analysis (TGA), Fourier transform infrared spectrometry (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) measurements indicated the stationary phase was prepared successfully. Five types of compounds including alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), nucleosides and nucleobases, anilines and flavonoids were utilized to evaluate reversed phase, weak cation exchange and hydrophilic interaction of the new column. To a certain extent, the column could achieve separation for different properties of samples on one column, with less organic solvent and shorter time than conventional alkyl and amino columns. Furthermore, the mechanism for separation was studied by investigating effects of mobile phase composition and pH on retentions. In summary, the Sil-MoS2-C18 stationary phase was deemed able to serve the performance of various types of phases, which revealed the prepared mixed-mode column could be potentially applied for the analysis of complex samples. Graphical abstract.

Farnesal-loaded pH-sensitive polymeric micelles provided effective prevention and treatment on dental caries.

BACKGROUND: Farnesol is a sesquiterpene from propolis and citrus fruit that shows promising anti-bacterial activity for caries treatment and prevention, but its hydrophobicity limits the clinical application. We aimed to develop the novel polymeric micelles (PMs) containing a kind of derivative of farnesol and a ligand of pyrophosphate (PPi) that mediated PMs to adhere tightly with the tooth enamel. RESULTS: Farnesal (Far) was derived from farnesol and successfully linked to PEG via an acid-labile hydrazone bond to form PEG-hyd-Far, which was then conjugated to PPi and loaded into PMs to form the aimed novel drug delivery system, PPi-Far-PMs. The in vitro test about the binding of PPi-Far-PMs to hydroxyapatite showed that PPi-Far-PMs could bind rapidly to hydroxyapatite and quickly release Far under the acidic conditions. Results from the mechanical testing and the micro-computed tomography indicated that PPi-Far-PMs could restore the microarchitecture of teeth with caries. Moreover, PPi-Far-PMs diminished the incidence and severity of smooth and sulcal surface caries in rats that were infected with Streptococcus mutans while being fed with a high-sucrose diet. The anti-caries efficacy of free Far can be improved significantly by PPi-Far-PMs through the effective binding of it with tooth enamel via PPi. CONCLUSIONS: This novel drug-delivery system may be useful for the treatment and prevention of dental caries as well as the targeting therapy of anti-bacterial drugs in the oral disease.

In Situ Strain Tuning in hBN-Encapsulated Graphene Electronic Devices.

Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional quality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudomagnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices.

New Generation of Moiré Superlattices in Doubly Aligned hBN/Graphene/hBN Heterostructures.

The specific rotational alignment of two-dimensional lattices results in a moiré superlattice with a larger period than the original lattices and allows one to engineer the electronic band structure of such materials. So far, transport signatures of such superlattices have been reported for graphene/hBN and graphene/graphene systems. Here we report moiré superlattices in fully hBN encapsulated graphene with both the top and the bottom hBN aligned to the graphene. In the graphene, two different moiré superlattices form with the top and the bottom hBN, respectively. The overlay of the two superlattices can result in a third superlattice with a period larger than the maximum period (14 nm) in the graphene/hBN system, which we explain in a simple model. This new type of band structure engineering allows one to artificially create an even wider spectrum of electronic properties in two-dimensional materials.

Green synthesis of carbon dots using the flowers of Osmanthus fragrans (Thunb.) Lour. as precursors: application in Fe3+ and ascorbic acid determination and cell imaging.

In this work, dried flowers of Osmanthus fragrans Lour. were applied as green precursors to synthesize carbon dots (CDs) by a green hydrothermal method for the first time. The CDs showed strong blue fluorescence at 410 nm under 340-nm excitation with a quantum yield of approximately 18.53%. Furthermore, the CDs were applied for the sensitive detection of Fe3+. The linear response of Fe3+ ranged from 10 nM to 50 µM with a limit of detection as low as 5 nM. In addition, other ions were used as competitive substances to explore the selectivity of CDs for Fe3+. The fluorescence quenching effect of Fe3+ was much stronger, which demonstrated that the CDs had high selectivity for Fe3+ and they can be employed for the selective detection of Fe3+. The potential fluorescence quenching mechanism between CDs and Fe3+ was identified as the inner filter effect. The CDs were then used as a fluorescent sensor for the detection of Fe3+ in water samples and human serum; the recovery range was 93.76-113.80% (relative standard deviation less than 0.79%). These results indicate that the CDs can be applied for the sensitive and selective detection of Fe3+ in real samples. Moreover, on the basis of the redox reaction between Fe3+ and ascorbic acid (AA), the CD-Fe3+ system can be used as a fluorescent "off-on" sensor for the detection of AA with a limit of detection of 5 µM. What is more, because of their low toxicity and biocompatibility, the CDs can also be used for cell imaging and acted as a fluorescent probe for fluorescence imaging of Fe3+ and AA in living cells. These results demonstrate that the CDs have great potential for application in the fields of sensing, bioimaging, and even disease diagnosis.

Resolution of (R,S)-ibuprofen catalyzed by immobilized Novozym40086 in organic phase.

The enantioselective esterification of ibuprofen catalyzed by Novozym40086 was successfully conducted in organic solvent. Removing-water reagent was added into the reaction mixture to remove water produced in the esterification. The effects of temperature, n-hexanol concentration, ibuprofen concentration, and loading of enzymes were investigated. Under the condition of equilibrium, the thermodynamic equilibrium constant (K) of 7.697 and enantioselectivity (E) of 8.512 were obtained. The esterification reaction achieved its equilibrium in approximately 30 hours with conversion of 56% and eeS of 93.78%. The predicted values of X and eeS were 67.90% and 95.60%, respectively. The experimental value is approximately equal to the theoretical value, which indicates the feasibility of ideal models.

Extraction and determination of bioactive flavonoids from Abelmoschus manihot (Linn.) Medicus flowers using deep eutectic solvents coupled with high-performance liquid chromatography.

A highly efficient and ecofriendly extraction method using deep eutectic solvents was developed to extract bioactive flavonoids from Abelmoschus manihot (Linn.) Medicus flowers. First, a series of deep eutectic solvents using choline chloride as hydrogen bond acceptor with different hydrogen bond donors was successfully synthesized. Then, the types of deep eutectic solvents and the extraction conditions for bioactive flavonoids (hyperoside, isoquercitrin, and myricetin) were optimized based on the flavonoids extraction efficiencies. The optimized deep eutectic solvent for hyperoside and isoquercitrin extraction was composed of choline chloride and acetic acid with a molar ratio of 1:2. The optimized deep eutectic solvent for myricetin extraction was composed of one mole of choline chloride and two moles of methacrylic acid. The optimal extraction conditions were set as: solid to solvent ratio, 35:1 (mg/mL); extraction time, 30 min; extraction temperature, 30°C. Qualitative and quantitative analysis were performed using ultra high performance liquid chromatography with tandem mass spectrometry and high-performance liquid chromatography. And the extraction efficiencies of hyperoside, isoquercitrin, and myricetin under optimal extraction conditions were calculated as 11.57, 5.64, and 1.11 mg/g, much higher than those extracted by traditional extraction solvents. Therefore, the prepared deep eutectic solvents can be selected as alternative solvent to extract bioactive flavonoids.