Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia

Pseudomonas aeruginosa PAO1 Preferentially Grows as Aggregates in Liquid Batch Cultures and Disperses upon Starvation

In both natural and artificial environments, bacteria predominantly grow in biofilms, and bacteria often disperse from biofilms as freely suspended single-cells. In the present study, the formation and dispersal of planktonic cellular aggregates, or ‘suspended biofilms’, by Pseudomonas aeruginosa in liquid batch cultures were closely examined, and compared to biofilm formation on a matrix of polyester (PE) fibers as solid surface in batch cultures. Plankton samples were analyzed by laser-diffraction particle-size scanning (LDA) and microscopy of aggregates. Interestingly, LDA indicated that up to 90% of the total planktonic biomass consisted of cellular aggregates in the size range of 10–400 µm in diameter during the growth phase, as opposed to individual cells. In cultures with PE surfaces, P. aeruginosa preferred to grow in biofilms, as opposed to planktonicly. However, upon carbon, nitrogen or oxygen limitation, the planktonic aggregates and PE-attached biofilms dispersed into single cells, resulting in an increase in optical density (OD) independent of cellular growth. During growth, planktonic aggregates and PE-attached biofilms contained densely packed viable cells and extracellular DNA (eDNA), and starvation resulted in a loss of viable cells, and an increase in dead cells and eDNA. Furthermore, a release of metabolites and infective bacteriophage into the culture supernatant, and a marked decrease in intracellular concentration of the second messenger cyclic di-GMP, was observed in dispersing cultures. Thus, what traditionally has been described as planktonic, individual cell cultures of P. aeruginosa, are in fact suspended biofilms, and such aggregates have behaviors and responses (e.g. dispersal) similar to surface associated biofilms. In addition, we suggest that this planktonic biofilm model system can provide the basis for a detailed analysis of the synchronized biofilm life cycle of P. aeruginosa

On-Surface Synthesis of Sandwich Molecular Nanowires on Graphene

We demonstrate a new synthesis route for the growth of organometallic sandwich molecular nanowires, taking the example of Eu-cyclooctatetraene (EuCot), a predicted ferromagnetic semiconductor. We employ simultaneous exposure of Cot molecules and Eu vapor in ultrahigh vacuum to an inert substrate, such as graphene. Using a Cot excess under temperature conditions of a finite residence time of the molecule, the reactand diffusion confined to two dimensions results in a clean product of ultralong wires. In situ scanning tunneling microscopy reveals not only their molecular structure but also a rich and intriguing growth morphology. The new on-surface synthesis permits experimental access to a largely unexplored class of one-dimensional organometallic systems with potential for exciting electronic and magnetic properties

Detailed growth curves established by automated OD determination.

<p>The OD in identically incubated batch cultures of aggregating <i>Pseudomonas aeruginosa</i> PAO1 (panel A), and of non-aggregating strain <i>Comamonas testosteroni</i> KF-1 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005513#pone.0005513-Schleheck3" target="_blank">[42]</a> for comparison (panel B), were monitored in 10-min intervals (solid circles) in a flow-through OD photometric system (see inset in panel B). At longer time intervals, growth was also followed by manual sampling and determination of standard OD (open circles) and carbon substrate concentration (open squares; succinate). Time points of starvation are indicated by dashed lines.</p

Correlation of increased release of active bacteriophage with carbon starvation.

<p>Aggregates and aggregate dispersion after glucose depletion (dashed line) was monitored by OD (open circle) and glucose determination (solid circle), and the release of active bacteriophage into the culture supernatant (square) determined as plaque formation units (pfu) after drop-plating serial dilutions of cell-free culture supernatant onto soft-agar plates with <i>P. aeruginosa</i> PAO1 ΔPf4 lawn, indicative of infective phage production.</p

Effect of the inhibition of cAMP synthesis on the dispersal of PAO1 WT biofilms.

<p>The effect of inhibition of cAMP synthesis on dispersal of biofilms grown under continuous flow conditions in M9 medium (100 µM CaCl₂) or M9 medium containing 7 mM atropine (Ap) as assessed photometrically (A) and percentage of biomass remaining after 1 day of glucose starvation with and without 7 mM Ap (<i>p</i><0.05) (B) were determined. Intracellular cAMP levels in WT, <i>cyaA</i> mutant and Ap-treated <i>P. aeruginosa</i> planktonic cells were determined (C). Effect of CCCP and arsenate treatment on intracellular levels of cAMP in <i>P. aeruginosa</i> grown in planktonic culture (D). Two sets of samples were prepared on different days, and were measured in triplicate (n = 6). Error bars indicate standard error.</p

Functional annotation of differentially expressed proteins.

<p>Proteins that were differentially expressed in <i>P. aeruginosa</i> starved biofilms were compared to unstarved biofilm (A) and dispersal cells (B). The functional categories are according to <i>Pseudomonas</i> Community Annotation Project (<a href="http://www.pseudomonas.com" target="_blank">www.pseudomonas.com</a>).</p

Biofilm formation and response to glucose starvation.

<p><i>P. aeruginosa</i> PAO1 WT (grey) and <i>cyaA</i> mutant (black) biofilm formation and response to starvation was determined by biofilm-opacity measurement (A). Percentage of of <i>cyaA</i> biomass remaining after 1 day of glucose starvation (<i>p</i><0.05) as determined by the biofilm-opacity monitoring system (B); error bars represent standard error. Biofilm formation and response to glucose starvation of <i>P. aeruginosa</i> PAO1 WT (grey) and complemented <i>cyaA</i> mutant (black) as determined by biofilm-opacity measurement (C). Glucose starvation was induced at the time shown by the vertical line.</p

Biofilm formation and response to glucose starvation of <i>P. aeruginosa</i> PAO1 WT and mutant strains.

<p>Strains mutated in (A) <i>nirS</i>, (B) <i>bdlA</i>, (C) Pf4 phage, (D) <i>lasRrhlR</i>, (E) <i>rpoS</i> and (F) <i>vfr</i> were grown under continuous flow conditions in M9 medium (100 µM CaCl₂). Glucose starvation was induced at the time indicated by the vertical line. Data represent single experiments.</p