The Role of Serine/Threonine-Specific Protein Kinases in Cyanobacteria - SpkB Is Involved in Acclimation to Fluctuating Conditions in Synechocystis sp. PCC 6803.

Protein phosphorylation via serine/threonine protein kinases (Spk) is a widespread mechanism to adjust cellular processes toward changing environmental conditions. To study their role(s) in cyanobacteria, we investigated a collection of 11 completely segregated spk mutants among the 12 annotated Spks in the model cyanobacterium Synechocystis sp. PCC 6803. Screening of the mutant collection revealed that especially the mutant defective in SpkB encoded by slr1697 showed clear deviations regarding carbon metabolism, that is, reduced growth rates at low CO2 or in the presence of glucose, and different glycogen accumulation patterns compared to WT. Alterations in the proteome of ΔspkB indicated changes of the cell surface but also metabolic functions. A phospho-proteome analysis revealed the absence of any phosphorylation in two proteins, while decreased phosphorylation of the carboxysome-associated protein CcmM and increased phosphorylation of the allophycocyanin alpha subunit ApcA was detected in ΔspkB. Furthermore, the regulatory PII protein appeared less phosphorylated in the mutant compared to WT, which was verified in Western blot experiments, indicating a clearly delayed PII phosphorylation in cells shifted from nitrate-containing to nitrate-free medium. Our results indicate that SpkB is an important regulator in Synechocystis that is involved in phosphorylation of the PII protein and additional proteins.

Visualizing Ribbon-to-Ribbon Heterogeneity of Chemically Unzipped Wide Graphene Nanoribbons by Silver Nanowire-Based Tip-Enhanced Raman Scattering Microscopy.

Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, have gained tremendous attention due to their potential for next-generation nanoelectronic devices. The chemical unzipping of carbon nanotubes is one of the attractive fabrication methods to obtain single-layered GNRs (sGNRs) with simple and large-scale production.  The authors recently found that unzipping from double-walled carbon nanotubes (DWNTs), rather than single- or multi-walled, results in high-yield production of crystalline sGNRs. However, details of the resultant GNR structure, as well as the reaction mechanism, are not fully understood due to the necessity of nanoscale spectroscopy. In this regard, silver nanowire-based tip-enhanced Raman spectroscopy (TERS) is applied for single GNR analysis and investigated ribbon-to-ribbon heterogeneity in terms of defect density and edge structure generated through the unzipping process.  The authors found that sGNRs originated from the inner walls of DWNTs showed lower defect densities than those from the outer walls. Furthermore, TERS spectra of sGNRs exhibit a large variety in graphitic Raman parameters, indicating a large variation in edge structures. This work at the single GNR level reveals, for the first time, ribbon-to-ribbon heterogeneity that can never be observed by diffraction-limited techniques and provides deeper insights into unzipped GNR structure as well as the DWNT unzipping reaction mechanism.

The effect of elevated temperatures on excitonic emission and degradation processes of WS2 monolayers.

Controlled heating experiments in an inert environment have been performed on WS2 monolayers, in order to clarify the conflicting reports on the high-temperature photoluminescent response of 2D TMDs. We find that in contrast to some previous results on both WS2 and MoS2, the photoluminescent intensity shows a consistent reduction above room temperature. This is accompanied by an almost linear redshift of the peak maximum, and a nearly linear increase in the peak width, which is attributed to an enhanced interaction with optical phonons. Moreover, by fitting the photoluminescence integral intensity with an Arrhenius type dependence, we demonstrate that the center of the WS2 monolayer flake starts to undergo irreversible degradation at a temperature of 573 K in an inert environment. Regions close to flake edges in contrast, with a more intense room temperature PL response, remain stable. The macroscopic PL signal is largely recovered in these regions following subsequent cooling to room temperature.

The redox-sensitive R-loop of the carbon control protein SbtB contributes to the regulation of the cyanobacterial CCM.

SbtB is a PII-like protein that regulates the carbon-concentrating mechanism (CCM) in cyanobacteria. SbtB proteins can bind many adenyl nucleotides and possess a characteristic C-terminal redox sensitive loop (R-loop) that forms a disulfide bridge in response to the diurnal state of the cell. SbtBs also possess an ATPase/ADPase activity that is modulated by the redox-state of the R-loop. To investigate the R-loop in the cyanobacterium Synechocystis sp. PCC 6803, site-specific mutants, unable to form the hairpin and permanently in the reduced state, and a R-loop truncation mutant, were characterized under different inorganic carbon (Ci) and light regimes. Growth under diurnal rhythm showed a role of the R-loop as sensor for acclimation to changing light conditions. The redox-state of the R-loop was found to impact the binding of the adenyl-nucleotides to SbtB, its membrane association and thereby the CCM regulation, while these phenotypes disappeared after truncation of the R-loop. Collectively, our data imply that the redox-sensitive R-loop provides an additional regulatory layer to SbtB, linking the CO2-related signaling activity of SbtB with the redox state of cells, mainly reporting the actual light conditions. This regulation not only coordinates CCM activity in the diurnal rhythm but also affects the primary carbon metabolism.

Nanoscale Chemical Diversity of Coke Deposits on Nanoprinted Metal Catalysts Visualized by Tip-Enhanced Raman Spectroscopy.

Coke formation is the prime cause of catalyst deactivation, where undesired carbon wastes block the catalyst surface and hinder further reaction in a broad gamut of industrial chemical processes. Yet, the origins of coke formation and their distribution across the catalyst remain elusive, obstructing the design of coke-resistant catalysts. Here, the first-time application of tip-enhanced Raman spectroscopy (TERS) is demonstrated as a nanoscale chemical probe to localize and identify coke deposits on a post-mortem metal nanocatalyst. Monitoring coke at the nanoscale circumvents bulk averaging and reveals the local nature of coke with unmatched detail. The nature of coke is chemically diverse and ranges from nanocrystalline graphite to disordered and polymeric coke, even on a single nanoscale location of a top-down nanoprinted SiO2 -supported Pt catalyst. Surprisingly, not all Pt is an equal producer of coke, where clear isolated coke "hotspots" are present non-homogeneously on Pt which generate large amounts of disordered coke. After their formation, coke shifts to the support and undergoes long-range transport on the surrounding SiO2 surface, where it becomes more graphitic. The presented results provide novel guidelines to selectively free-up the coked metal surface at more mild rejuvenation conditions, thus securing the long-term catalyst stability.

Unusual Defect-Related Room-Temperature Emission from WS2 Monolayers Synthesized through a Potassium-Based Precursor.

Alkali-metal-based synthesis of transition metal dichalcogenide (TMD) monolayers is an established strategy for both ultralarge lateral growth and promoting the metastable 1T phase. However, whether this can also lead to modified optical properties is underexplored, with reported photoluminescence (PL) spectra from semiconducting systems showing little difference from more traditional syntheses. Here, we show that the growth of WS2 monolayers from a potassium-salt precursor can lead to a pronounced low-energy emission in the PL spectrum. This is seen 200-300 meV below the A exciton and can dominate the signal at room temperature. The emission is spatially heterogeneous, and its presence is attributed to defects in the layer due to sublinear intensity power dependence, a noticeable aging effect, and insensitivity to washing in water and acetone. Interestingly, statistical analysis links the band to an increase in the width of the A1g Raman band. The emission can be controlled by altering when hydrogen is introduced into the growth process. This work demonstrates intrinsic and intense defect-related emission at room temperature and establishes further opportunities for tuning TMD properties through alkali-metal precursors.

Spent Li-Ion Battery Graphite Turned Into Valuable and Active Catalyst for Electrochemical Oxygen Reduction.

Employing Li-ion batteries (LIBs) in portable electronics has become a necessity in the modern world but, due to the short application time for any given battery (1-3 years), the quantity of spent LIBs (SLIBs) waste is becoming substantial. Herein, a novel strategy for recycling SLIB graphite and reforming it as a valuable catalyst material for electrochemical oxygen reduction reaction was proposed. SLIB graphite has been used as a precursor material for graphite oxide, which was thereafter doped with nitrogen to prepare nitrogen-doped graphene (NG-Bat). The prepared NG-Bat was characterized by various physical characterization methods and the electrochemical properties of the resulting catalyst material were investigated in alkaline media. It was found that NG-Bat prepared from SLIB had superior physical and electrochemical properties in comparison to commercial nitrogen-doped graphene. The findings clearly demonstrate the importance of the recycling of SLIB graphite and its great potential to be re-applied for various applications.

Li@C60 thin films: characterization and nonlinear optical properties.

Organic materials have attracted considerable attention in nonlinear optical (NLO) applications as they have several advantages over inorganic materials, including high NLO response, and fast response time as well as low-cost and easy fabrication. Lithium-containing C60 (Li@C60) is promising for NLO over other organic materials because of its strong NLO response proven by theoretical and experimental studies. However, the low purity of Li@C60 has been a bottleneck for applications in the fields of solar cells, electronics and optics. In 2010, highly purified Li@C60 was finally obtained, encouraging further studies. In this study, we demonstrate a facile method to fabricate thin films of Li@C60 and their strong NLO potential for high harmonic generation by showing its comparatively strong emission of degenerate-six-wave mixing, a fifth-order NLO effect.

Graphene Meets Ionic Liquids: Fermi Level Engineering via Electrostatic Forces.

Graphene-based two-dimensional (2D) materials are promising candidates for a number of different energy applications. A particularly interesting one is in next generation supercapacitors, where graphene is being explored as an electrode material in combination with room temperature ionic liquids (ILs) as electrolytes. Because the amount of energy that can be stored in such supercapacitors critically depends on the electrode-electrolyte interface, there is considerable interest in understanding the structure and properties of the graphene/IL interface. Here, we report the changes in the properties of graphene upon adsorption of a homologous series of alkyl imidazolium tetrafluoroborate ILs using a combination of experimental and theoretical tools. Raman spectroscopy reveals that these ILs cause n-type doping of graphene, and the magnitude of doping increases with increasing cation chain length despite the expected decrease in the density of surface-adsorbed ions. Molecular modeling simulations show that doping originates from the changes in the electrostatic potential at the graphene/IL interface. The findings described here represent an important step in developing a comprehensive understanding of the graphene/IL interface.

Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates Using Diazonium Chemistry.

A network of self-assembled polystyrene beads was employed as a lithographic mask during covalent functionalization reactions on graphitic surfaces to create nanocorrals for confined molecular self-assembly studies. The beads were initially assembled into hexagonal arrays at the air-liquid interface and then transferred to the substrate surface. Subsequent electrochemical grafting reactions involving aryl diazonium molecules created covalently bound molecular units that were localized in the void space between the nanospheres. Removal of the bead template exposed hexagonally arranged circular nanocorrals separated by regions of chemisorbed molecules. Small molecule self-assembly was then investigated inside the resultant nanocorrals using scanning tunneling microscopy to highlight localized confinement effects. Overall, this work illustrates the utility of self-assembly principles to transcend length scale gaps in the development of hierarchically patterned molecular materials.

Silver nanowires for highly reproducible cantilever based AFM-TERS microscopy: towards a universal TERS probe.

Tip-enhanced Raman scattering (TERS) microscopy is a unique analytical tool to provide complementary chemical and topographic information of surfaces with nanometric resolution. However, difficulties in reliably producing the necessary metallized scanning probe tips has limited its widespread utilisation, particularly in the case of cantilever-based atomic force microscopy. Attempts to alleviate tip related issues using colloidal or bottom-up engineered tips have so far not reported consistent probes for both Raman and topographic imaging. Here we demonstrate the reproducible fabrication of cantilever-based high-performance TERS probes for both topographic and Raman measurements, based on an approach that utilises noble metal nanowires as the active TERS probe. The tips show 10 times higher TERS contrasts than the most typically used electrochemically-etched tips, and show a reproducibility for TERS greater than 90%, far greater than found with standard methods. We show that TERS can be performed in tapping as well as contact AFM mode, with optical resolutions around or below 15 nm, and with a maximum resolution achieved in tapping-mode of 6 nm. Our work illustrates that superior TERS probes can be produced in a fast and cost-effective manner using simple wet-chemistry methods, leading to reliable and reproducible high-resolution and high-sensitivity TERS, and thus renders the technique applicable for a broad community.

Facilitating Tip-Enhanced Raman Scattering on Dielectric Substrates via Electrical Cutting of Silver Nanowire Probes.

TERS is a powerful tool for nanoscale optical characterization of surfaces. However, even after 20 years of development, the parameters for optimal TERS tips are still up for debate. As a result, routine measurements on bulk or dielectric substrates remain exceptionally challenging. Herein we help to alleviate this by using electrical cutting to strategically modify silver nanowire TERS probes. Following cutting, the tips present a large, spherical apex and are often nanostructured with numerous nanoparticles, which we argue improve light collection and optical coupling. This doubles TERS signals on a highly enhancing, gap-mode substrate compared to our standard nanowire tips while maintaining a high reproducibility and resolution. More interestingly, on a dielectric substrate (graphene on SiO2) the tips give ∼7× higher signals than our standard tips. Further investigations point to the nonlocal nature of the enhancement using standard, smooth TERS probes without gap-mode, making such nanostructuring highly beneficial in these cases.

Self-Assembled Monolayers as Templates for Linearly Nanopatterned Covalent Chemical Functionalization of Graphite and Graphene Surfaces.

An approach for nanoscale covalent functionalization of graphite surfaces employing self-assembled molecular monolayers of n-alkanes as templating masks is presented. Linearly aligned aryl groups with a lateral periodicity of 5 or 7 nm are demonstrated utilizing molecular templates of different lengths. The key feature of this approach is the use of a phase separated solution double layer consisting of a thin organic layer containing template molecules topped by an aqueous layer containing aryldiazonium molecules capable of electrochemical reduction to generate aryl radicals which bring about surface grafting. Upon sweeping of the potential, lateral displacement dynamics at the n-alkane terminal edges acts in conjunction with electrochemical diffusion to result in templated covalent bond formation in a linear fashion. This protocol was demonstrated to be applicable to linear grafting of graphene. The present processing described herein is useful for the realization of rationally designed nanoscale materials.

Area-selective passivation of sp2 carbon surfaces by supramolecular self-assembly.

Altering the chemical reactivity of graphene can offer new opportunities for various applications. Here, we report that monolayers of densely packed n-pentacontane significantly reduce the covalent grafting of aryl radicals to graphitic surfaces. The effect is highly local in nature and on fully covered substrates grafting can occur only at monolayer imperfections such as interdomain borders and vacancy defects. Grafting partially covered substrates primarily results in the covalent modification of uncoated areas.

Tunable doping of graphene by using physisorbed self-assembled networks.

One current key challenge in graphene research is to tune its charge carrier concentration, i.e., p- and n-type doping of graphene. An attractive approach in this respect is offered by controlled doping via well-ordered self-assembled networks physisorbed on the graphene surface. We report on tunable n-type doping of graphene using self-assembled networks of alkyl-amines that have varying chain lengths. The doping magnitude is modulated by controlling the density of the strong n-type doping amine groups on the surface. As revealed by scanning tunneling and atomic force microscopy, this density is governed by the length of the alkyl chain which acts as a spacer within the self-assembled network. The modulation of the doping magnitude depending on the chain length was demonstrated using Raman spectroscopy and electrical measurements on graphene field effect devices. This supramolecular functionalization approach offers new possibilities for controlling the properties of graphene and other two-dimensional materials at the nanoscale.