Facile Fabrication of SERS Substrates by the Electrodeposition Method to Detect Pesticides with High Enhancement Effect and Long-Term Stability.

In this study, surface-enhanced Raman scattering substrates were investigated by the electrodeposition method to detect low concentrations of pesticides via the electrodeposition method with different agents from silver and gold precursors on APTES-modified ITO glass. A dual-potential method supplied three electrodes and was performed with a nucleation potential of 0.7 V for 2 s and a growth potential of -0.2 V for 500 s. The Ag film produced by the electrodeposition approach has great surface uniformity and good SERS signal amplification for the thiram insecticide at low concentrations. Interestingly, the ITO/APTES/Ag substrate extensively increased the sensitivity than the other investigated ones, thanks to the adequate assistance of amino groups of APTES in the denser and hierarchical deposition of Ag NPs. These observations were additionally elucidated via finite-difference time-domain (FDTD) calculation. For thiram, the detection was set at 10-8 M with an enhancement factor of up to 3.6 × 107 times. Comparing the SERS spectra of thiram at concentrations of 10-3, 10-4, and 10-5 M with a relative standard deviation (RSD) of less than 7.0% demonstrates excellent reproducibility of the ITO/APTES/Ag substrate. In addition, the special selectivity of the ITO/APTES/Ag substrate for thiram demonstrates that these nanostructures can identify pesticides with extreme sensitivity.

Boosting Urea-Assisted Natural Seawater Electrolysis in 3D Leaf-Like Metal-Organic Framework Nanosheet Arrays Using Metal Node Engineering.

Metal node engineering, which can optimize the electronic structure and modulate the composition of poor electrically conductive metal-organic frameworks, is of great interest for electrochemical natural seawater splitting. However, the mechanism underlying the influence of mixed-metal nodes on electrocatalytic activities is still ambiguous. Herein, a strategic design is comprehensively demonstrated in which mixed Ni and Co metal redox-active centers are uniformly distributed within NH2-Fe-MIL-101 to obtain a synergistic effect for the overall enhancement of electrocatalytic activities. Three-dimensional mixed metallic MOF nanosheet arrays, consisting of three different metal nodes, were in situ grown on Ni foam as a highly active and stable bifunctional catalyst for urea-assisted natural seawater splitting. A well-defined NH2-NiCoFe-MIL-101 reaches 1.5 A cm-2 at 360 mV for the oxygen evolution reaction (OER) and 0.6 A cm-2 at 295 mV for the hydrogen evolution reaction (HER) in freshwater, substantially higher than its bimetallic and monometallic counterparts. Moreover, the bifunctional NH2-NiCoFe-MIL-101 electrode exhibits eminent catalytic activity and stability in natural seawater-based electrolytes. Impressively, the two-electrode urea-assisted alkaline natural seawater electrolysis cell based on NH2-NiCoFe-MIL-101 needs only 1.56 mV to yield 100 mA cm-2, much lower than 1.78 V for alkaline natural seawater electrolysis cells and exhibits superior long-term stability at a current density of 80 mA cm-2 for 80 h.

Dopant-Induced Charge Redistribution on the 3D Sponge-like Hierarchical Structure of Quaternary Metal Phosphides Nanosheet Arrays Derived from Metal-Organic Frameworks for Natural Seawater Splitting.

Dopant-induced electron redistribution on transition metal-based materials has long been considered an emerging new electrocatalyst that is expected to replace noble-metal-based electrocatalysts in natural seawater electrolysis; however, their practical applications remain extremely daunting due to their sluggish kinetics in natural seawater. In this work, we developed a facile strategy to synthesize the 3D sponge-like hierarchical structure of Ru-doped NiCoFeP nanosheet arrays derived from metal-organic frameworks with remarkable hydrogen evolution reaction (HER) performance in natural seawater. Based on experimental results and density functional theory calculations, Ru-doping-induced charge redistribution on the surface of metal active sites has been found, which can significantly enhance the HER activity. As a result, the 3D sponge-like hierarchical structure of Ru-NiCoFeP nanosheet arrays achieves low overpotentials of 52, 149, and 216 mV at 10, 100, and 500 mA cm-2 in freshwater alkaline, respectively. Notably, the electrocatalytic activity of the Ru-NiCoFeP electrocatalyst in simulated alkaline seawater and natural alkaline seawater is nearly the same as that in freshwater alkaline. This electrocatalyst exhibits superior catalytic properties with outstanding stability under a high current density of 85 mA cm-2 for more than 100 h in natural seawater, which outperforms state-of-the-art 20% Pt/C at high current density. Our work provides valuable guidelines for developing a low-cost and high-efficiency electrocatalyst for natural seawater splitting.

Highly Efficient and Stable Hydrogen Evolution from Natural Seawater by Boron-Doped Three-Dimensional Ni2P-MoO2 Heterostructure Microrod Arrays.

The rational design of highly active and stable electrocatalysts toward the hydrogen evolution reaction (HER) is highly desirable but challenging in seawater electrolysis. Herein we propose a strategy of boron-doped three-dimensional Ni2P-MoO2 heterostructure microrod arrays that exhibit excellent catalytic activity for hydrogen evolution in both alkaline freshwater and seawater electrolytes. The incorporation of boron into Ni2P-MoO2 heterostructure microrod arrays could modulate the electronic properties, thereby accelerating the HER. Consequently, the B-Ni2P-MoO2 heterostructure microrod array electrocatalyst exhibits a superior catalyst activity for HER with low overpotentials of 155, 155, and 157 mV at a current density of 500 mA cm-2 in 1 M KOH, 1 M KOH + NaCl, and 1 M KOH + seawater, respectively. It also exhibits exceptional performance for HER in natural seawater with a low overpotential of 248 mV at 10 mA cm-2 and a long-lasting lifetime of over 100 h.

Promoting High-Oxidation-State Metal Active Sites in a Hollow Ternary Metal Fluoride Nanoflake Array for Urea Electrolysis.

The adsorption ability of hydrogen, hydroxide, and oxygenic intermediates plays a crucial role in electrochemical water splitting. Electron-deficient metal-active sites can prompt electrocatalytic activity by improving the adsorption ability of intermediates. However, it remains a significant challenge to synthesize highly abundant and stable electron-deficient metal-active site electrocatalysts. Herein, we present a general approach to synthesizing a hollow ternary metal fluoride (FeCoNiF2) nanoflake array as an efficient and robust bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). We find that the F anion withdraws electrons from the metal centers, inducing an electron-deficient metal center catalyst. The rationally designed hollow nanoflake array exhibits the overpotential of 30 mV for HER and 130 mV for UOR at a current density of 10 mA cm-2 and superior stability without decay events over 150 h at a large current density of up to 100 mA cm-2. Remarkably, the assembled urea electrolyzer using a bifunctional hollow FeCoNiF2 nanoflake array catalyst requires cell voltages of only 1.352 and 1.703 V to afford current densities of 10 and 100 mA cm-2, respectively, which are 116 mV less compared with that required for overall water splitting.

Inductive Effect of Lewis Acidic Dopants on the Band Levels of Perovskite for a Photocatalytic Reaction.

Band-edge modulation of halide perovskites as photoabsorbers plays significant roles in the application of photovoltaic and photochemical systems. Here, Lewis acidity of dopants (M) as the new descriptor of engineering the band-edge position of the perovskite is investigated in the gradiently doped perovskite along the core-to-surface (CsPbBr3-CsPb1-xMxBr3). Reducing M-bromide bond strength with an increase in hardness of acidic M increases the electron ability of basic Br, thus strengthening the Pb-Br orbital coupling in M-Pb-Br, noted as the inductive effect of dopants. Especially, the highly hard Lewis acidic Mg localized in the outer position of the perovskite induces the increase of work function and then shifts band edge upward along the core-to-surface of the perovskite. Thus, charge separation driven by the dopant-induced internal electric field induces the slow annihilation of the excited holes, improving the slow aromatic Csp3-H dissociation in the photocatalytic oxidation process by ∼211% (491.39 µmol g-1 h-1) enhancements, compared with undoped nanocrystals.

Coupling Amorphous Ni Hydroxide Nanoparticles with Single-Atom Rh on Cu Nanowire Arrays for Highly Efficient Alkaline Seawater Electrolysis.

Exploring efficient catalysts for alkaline seawater electrolysis is highly desired yet challenging. Herein, coupling single-atom rhodium with amorphous nickel hydroxide nanoparticles on copper nanowire arrays is designed as a new active catalyst for the highly efficient alkaline seawater electrolysis. We found that an amorphous Ni(OH)2 nanoparticle is an effective catalyst to accelerate the water dissociation step. In contrast, the single-atom rhodium is an active site for adsorbed hydrogen recombination to generate H2. The NiRh-Cu NA/CF catalyst shows superior electrocatalytic activity toward HER, surpassing a benchmark Pt@C. In detail, the NiRh-Cu NA/CF catalyst exhibits HER overpotentials as low as 12 and 21 mV with a current density of 10 mA cm-2 in fresh water and seawater, respectively. At high current density, the NiRh-Cu NA/CF catalyst also exhibits an outstanding performance, where 300 mA cm-2 can be obtained at an overpotential of 155 mV and shows a slight fluctuation in the current density over 30 h.

Efficient ammonia synthesis via electroreduction of nitrite using single-atom Ru-doped Cu nanowire arrays.

Here, we report the highly active and selective electrocatalytic reduction of NO2- ions to value-added NH3 over a single-atom Ru-modified Cu nanowire array on three-dimensional copper foam (Ru-Cu NW/CF) under ambient conditions. The obtained Ru-Cu NW/CF catalyst exhibited a maximum faradaic efficiency of 94.1% and an NH3 yield up to 211.73 mg h-1 cm-2 (0.732 mmol h-1 cm-2), which was approximately five times higher than that of the Cu NW/CF catalyst.

Restructuring highly electron-deficient metal-metal oxides for boosting stability in acidic oxygen evolution reaction.

The poor catalyst stability in acidic oxidation evolution reaction (OER) has been a long-time issue. Herein, we introduce electron-deficient metal on semiconducting metal oxides-consisting of Ir (Rh, Au, Ru)-MoO3 embedded by graphitic carbon layers (IMO) using an electrospinning method. We systematically investigate IMO's structure, electron transfer behaviors, and OER catalytic performance by combining experimental and theoretical studies. Remarkably, IMO with an electron-deficient metal surface (Irx+; x > 4) exhibit a low overpotential of only ~156 mV at 10 mA cm-2 and excellent durability in acidic media due to the high oxidation state of metal on MoO3. Furthermore, the proton dissociation pathway is suggested via surface oxygen serving as proton acceptors. This study suggests high stability with high catalytic performance in these materials by creating electron-deficient surfaces and provides a general, unique strategy for guiding the design of other metal-semiconductor nanocatalysts.

Earth-Abundant Transition-Metal-Based Bifunctional Electrocatalysts for Overall Water Splitting in Alkaline Media.

The depletion of fossil fuels has accelerated the search for clean, sustainable, scalable, and environmentally friendly alternative energy sources. Hydrogen is a potential energy carrier because of its advantageous properties, and the electrolysis of water is considered as an efficient method for its industrial production. However, the high-energy conversion efficiency of electrochemical water splitting requires cost-effective and highly active electrocatalysts. Therefore, researchers have aimed to develop high-performance electrode materials based on non-precious and abundant transition metals for conversion devices. Moreover, to further reduce the cost and complexity in real-world applications, bifunctional catalysts that can be simultaneously active on both the anodic (i.e., oxygen evolution reaction, OER) and cathodic (i.e., hydrogen evolution reaction, HER) sides are economically and technically desirable. This Minireview focuses on the recent progress in transition-metal-based materials as bifunctional electrocatalysts, including several promising strategies to promote electrocatalytic activities for overall water splitting in alkaline media, such as chemical doping, defect (vacancy) engineering, phase engineering, facet engineering, and structure engineering. Finally, the potential for further developments in rational electrode materials design is also discussed.

Porosity-Engineering of MXene as a Support Material for a Highly Efficient Electrocatalyst toward Overall Water Splitting.

The use of 2 D transition metal carbide MXenes as support materials to incorporate catalytically active compounds is of interest because of their unique properties. However, the preparation of well-dispersed catalytic phases on the inter-connected porous MXene network is challenging and has been rarely explored. This work focuses on the synthesis of basal-plane-porous titanium carbide MXene (ac-Ti3 C2 ) that is used subsequently as an effective host for the incorporation of a known catalytically active phase (IrCo) as an effective bifunctional electrocatalyst toward water splitting. The porous ac-Ti3 C2 with abundant macro/meso/micropores is prepared by a wet chemical method at room temperature and provides ideal anchor sites for intimate chemical bonding with alien compounds. The resulting IrCo@ac-Ti3 C2 electrocatalyst exhibits an excellent reactivity (220 mV at 10 mA cm-2 ) towards the oxygen evolution reaction in 1.0 m KOH, which surpasses that of the benchmark RuO2 , a low voltage cell of 1.57 V (@ 10 mA cm-2 ) and good long-term durability. Our work demonstrates the effectiveness of porosity engineering in MXene nanosheets as a support material to shorten ion migration pathways, to increase electrolyte accessibility between inter-sheets and to overcome inherited re-stacking and aggregation issues.

Intertwined Titanium Carbide MXene within a 3 D Tangled Polypyrrole Nanowires Matrix for Enhanced Supercapacitor Performances.

The exploration of the rational design and synthesis of unique and robust architectured electrodes for the high capacitance, rate capability, and stability of supercapacitors is crucial to the future of energy storage technology. Herein, an in situ synthesis of multilayered titanium carbide MXene tightly caging within a 3 D conducting tangled polypyrrole (PPy) nanowire (NW) network is proposed as an effective strategy to prevent the aggregation of MXene, profoundly enhancing the electrochemical performance of the supercapacitor. Owing to the beneficial effects of an ideal 3 D interconnected porous structure and high electrical conductivity, the obtained electrode exhibits fast charge and ion transport kinetics as well as full usage of active material. As expected, the 3 D Ti3 C2 Tx @PPY NW exhibits a specific capacitance five times higher than that of pristine MXene (610 F g-1 ), a good rate capability up to a current density of 25 A g-1 , and excellent stability with 100 % retention after 14 000 cycles at 4 A g-1 , outperforming the known state-of-the-art MXene-based supercapacitor. Our work provides a facile method for enhancing the performance of MXene-based energy storage devices.

Enrichment of Pyrrolic Nitrogen by Hole Defects in Nitrogen and Sulfur Co-Doped Graphene Hydrogel for Flexible Supercapacitors.

The effect of the doping configuration and concentration of nitrogen (N) and sulfur (S) on the electrochemical performance of 3 D N and S co-doped hole defect graphene hydrogel (NS-HGH) electrodes is investigated. Surprisingly, by introducing a hole defect on the graphene surface, the difference in the doping concentrations of N and S can be used to effectively modulate the electrochemical behavior of the NS-HGH. The hole defects provide a rapid ion diffusion path. Finally, we showed that the intriguing specific capacitance (536 F g(-1) ) of the NS-HGH could enhance the overall performance of the pseudocapacitance and electric double layer capacitance. The rational design of the NS-HGH-based flexible solid state supercapacitor results in not only outstanding electrochemical performance with a maximum energy density of 14.8 Wh kg(-1) and power density of 5.2 KW kg(-1) but also in extraordinary mechanical flexibility and excellent cycle stability.

Design of Advanced MnO/N-Gr 3D Walls through Polymer Cross-Linking for High-Performance Supercapacitor.

Three-dimensional, vertically aligned MnO/nitrogen-doped graphene (3D MnO/N-Gr) walls were prepared through facile solution-phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross-links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as-prepared 3D MnO/N-Gr hybirdes provide a large surface area (91.516 m(2) g(-1)) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g(-1) at 0.25 A g(-1) and an excellent charge/discharge stability (93.7% capacity retention after 8000 cycles) in aqueous 1 m Na2 SO4 solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N-Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg(-1) and a power density of 437.5 W kg(-1).

Impact of salinity-tolerant MCM6 transgenic tobacco on soil enzymatic activities and the functional diversity of rhizosphere microbial communities.

The development of genetically modified plants for agriculture has provided numerous economic benefits, but has also raised concern over the potential impact of transgenic plants upon the environment. The rhizosphere is the soil compartment that is directly under the influence of living roots; it constitutes a complex niche likely to be exploited by a wide variety of bacteria potentially influenced by the introduction of transgenes in genetically modified plants. In the present study, the impact of overexpression of the salinity stress-tolerant minichromosome maintenance complex subunit 6 (MCM6) gene upon functional diversity and soil enzymatic activity in the rhizosphere of transgenic tobacco in the presence and absence of salt stress was examined. The diversity of culturable bacterial communities and soil enzymes, namely, dehydrogenases and acid phosphatases, was assessed and revealed no significant (or only minor) alterations due to transgenes in the rhizosphere soil of tobacco plants. Patterns in principal components analysis showed clustering of transgenic and non-transgenic tobacco plants according to the fingerprint of their associated bacterial communities. However, the presence of MCM6 tobacco did not cause changes in microbial populations, soil enzymatic activities or the functional diversity of the rhizosphere soil microbial community.

Plant MCM proteins: role in DNA replication and beyond.

Mini-chromosome maintenance (MCM) proteins form heterohexameric complex (MCM2-7) to serve as licensing factor for DNA replication to make sure that genomic DNA is replicated completely and accurately once during S phase in a single cell cycle. MCMs were initially identified in yeast for their role in plasmid replication or cell cycle progression. Each of six MCM contains highly conserved sequence called "MCM box", which contains two ATPase consensus Walker A and Walker B motifs. Studies on MCM proteins showed that (a) the replication origins are licensed by stable binding of MCM2-7 to form pre-RC (pre-replicative complex) during G1 phase of the cell cycle, (b) the activation of MCM proteins by CDKs (cyclin-dependent kinases) and DDKs (Dbf4-dependent kinases) and their helicase activity are important for pre-RC to initiate the DNA replication, and (c) the release of MCMs from chromatin renders the origins "unlicensed". DNA replication licensing in plant is, in general, less characterized. The MCMs have been reported from Arabidopsis, maize, tobacco, pea and rice, where they are found to be highly expressed in dividing tissues such as shoot apex and root tips, localized in nucleus and cytosol and play important role in DNA replication, megagametophyte and embryo development. The identification of six MCM coding genes from pea and Arabidopsis suggest six distinct classes of MCM protein in higher plant, and the conserved function right across the eukaryotes. This overview of MCMs contains an emphasis on MCMs from plants and the novel role of MCM6 in abiotic stress tolerance.

Promoter of a salinity and cold stress-induced MCM6 DNA helicase from pea.

The eukaryotic hetrohexameric mini-chromosome maintenance (MCM2-7) proteins complex provides DNA unwinding function during the DNA replication. The complex also functions as DNA replication licensing factor which ensures that the DNA in genome is replicated only once per cell division cycle. Recently, a single subunit MCM6 from pea has been shown to contain helicase and ATPase activities in vitro. Recently, the transcript of a single subunit was reported to be upregulated in pea plant in response to high salinity and cold stress and not with ABA, drought and heat stress. The first direct evidence that overexpression of single subunit MCM6 confers salinity stress tolerance without yield loss has also been reported. Here we report the promoter of the pea MCM6 single subunit that contains stress responsive elements which may be responsible for regulating the MCM6 under abiotic stress conditions.

A single subunit MCM6 from pea promotes salinity stress tolerance without affecting yield.

The eukaryotic pre-replicative complex (Pre-RC), including heterohexameric minichromosome maintenance (MCM2-7) proteins, ensures that the DNA in genome is replicated only once per cell division cycle. The MCMs provide DNA unwinding function during the DNA replication. Since MCM proteins play essential role in cell division and most likely are affected during stress conditions therefore their overexpression in plants may help in stress tolerance. But the role of MCMs in abiotic stress tolerance in plants has not been reported so far. In this study we report that: a) the MCM6 transcript is upregulated in pea plant in response to high salinity and cold stress and not with ABA, drought and heat stress; b) MCM6 overexpression driven by a constitutive cauliflower mosaic virus-35S promoter in tobacco plants confers salinity tolerance. The T(1) transgenics plants were able to grow to maturity and set normal viable seeds under continuous salinity stress, without yield penalty. It was observed that in salt-grown T(1) transgenic plants the Na(+) ions is mostly accumulated in mature leaves and not in seeds of T(1) transgenic lines as compared with the wild-type (WT) plants. T(1) transgenic plants exhibited better growth status under salinity stress conditions in comparison to WT plants. Furthermore, the T(1) transgenic plants maintained significantly higher levels of leaf chlorophyll content, net photosynthetic rate and therefore higher dry matter accumulation and yield with 200 mM NaCl as compared to the WT plants. Tolerance index data showed better salt tolerance potential of T(1) transgenic plants in comparison to WT. These findings provide first direct evidence that overexpression of single subunit MCM6 confers salinity stress tolerance without yield loss. The possible mechanism of salinity tolerance is discussed. These findings suggest that DNA replication machinery can be exploited for promoting stress tolerance in crop plants.

Inhibition of unwinding and ATPase activities of pea MCM6 DNA helicase by actinomycin and nogalamycin.

Pea mini-chromosome maintenance 6 (MCM6) single subunit (93 kDa) forms homohexamer (560 kDa) and contains an ATP-dependent and replication fork stimulated 3' to 5' DNA unwinding activity along with intrinsic DNA-dependent ATPase and ATP-binding activities [Plant Mol. Biol. 2010; DOI: 10.1007/s11103-010-9675-7]. Here, we have determined the effect of various DNA-binding agents, such as actinomycin, nogalamycin, daunorubicin, doxorubicin, distamycin, camptothecin, cyclophosphamide, ellipticine, VP-16, novobiocin, netropsin, cisplatin, mitoxantrone and genistein on the DNA unwinding and ATPase activities of the pea MCM6 DNA helicase. The results show that actinomycin and nogalamycin inhibited the DNA helicase (apparent Ki values of 10 and 1 µM, respectively) and ATPase (apparent Ki values of 100 and 17 µM, respectively) activities. Although, daunorubicin and doxorubicin also inhibited the DNA helicase activity of pea MCM6, but with less efficiency; however, these could not inhibit the ATPase activity. These results suggest that the intercalation of the inhibitors into duplex DNA generates a complex that impedes translocation of MCM6, resulting in the inhibitions of the activities. This study could be useful in our better understanding of the mechanism of plant nuclear DNA helicase unwinding.

A single subunit MCM6 from pea forms homohexamer and functions as DNA helicase.

The initiation of DNA replication starts from origins and is controlled by a multiprotein complex, which involves about twenty protein factors. One of the important factors is hetrohexameric minichromosome maintenance (MCM2-7) protein complex which is evolutionary conserved and functions as essential replicative helicase for DNA replication. Here we report the isolation and characterization of a single subunit of pea MCM protein complex, the MCM6. The deduced amino acid (827) sequence contains all the known canonical MCM motifs including zinc finger, MCM specific Walker A and Walker B and arginine finger. The purified recombinant protein contains ATP-dependent 3'-5' DNA helicase, ATP-binding and ATPase activities. The helicase activity was stimulated by replication fork like substrate and anti-MCM6 antibodies curtail all the enzyme activities of MCM6 protein. In vitro it self-interacts and forms a homohexamer which is active for DNA helicase and ATPase activities. The complete protein is required for self-interaction as the truncated MCM6 proteins were unable to self-interact. Western blot analysis and in vivo immunostaining followed by confocal microscopy showed the localization of MCM6 both in the nucleus and cytosol. These findings provide first direct evidence that single subunit MCM6 contains DNA helicase activity which is unique to plant MCM6 protein, as this activity was only reported for heteromultimers of MCM proteins in animal system. This discovery should make an important contribution to a better understanding of DNA replication in plants.