Water Oxidation by Cobalt Centers on Various Oxide Surfaces: The Effects of Oxide Surface Acidity and Oxygen Atom Affinity on Catalysis

Single-atom cobalt centers on various oxide surfaces (TiO₂, MgO, SBA-15, AlPO, and Y-Zeolite) were prepared and evaluated as water oxidation catalysts by photochemical water oxidation experiments. Superior catalytic rates were observed for cobalt sites on basic supporting oxides (TiO₂ and MgO) relative to those on acidic oxides (Y-Zeolite, AlPO, and SiO₂). Per-atom turnover frequencies of ca. 0.04 s^–1 were achieved, giving initial rates 100 times greater than that of a surface atom of a Co₃O₄ nanoparticle. Contrary to expectations based on theoretical work, no apparent correlation was observed between the catalytic rates and the oxygen atom affinities of the supporting oxides

Fungal colonization levels on dentures and palate tissue in rats inoculated with <i>C</i>. <i>albicans</i> biofilm deficient strains.

<p>Equilibrated rats were weaned onto gel diet and fitted with dentures. Rats were given broad-spectrum antibiotics in the drinking water for 4 days prior to inoculation. Rats were inoculated 3x at 3-day intervals with 1 x 10⁹ <i>C</i>. <i>albicans</i> DAY185, <i>efg1</i>^<i>-/-</i> or <i>bcr1</i>^<i>-/-</i> strain. Swab samples of both the palate (A) and denture (B) were taken weekly for a period of 8 weeks post-inoculation. Fungal burdens were assessed from swab suspension fluid. Figure represents cumulative results from 2 independent experiments with 4–5 animals per group. Data were analyzed using Repeated Measures ANOVA (longitudinal data for each group) and the unpaired Student’s <i>t</i> test (individual time points (weeks), experimental vs. control). *, <i>P</i> < 0.05; **, <i>P</i> < 0.01; ***, <i>P</i> < 0.001.</p

<i>In vitro</i> biofilm formation on denture materials in human saliva.

<p>Pre-sterilized denture material was inoculated with 1 x 10⁶ <i>C</i>. <i>albicans</i> DAY185 (A, F, K), <i>efg1</i>^<i>-/-</i> (B, G, L), <i>efg1</i>-reconstituted (C, H, M), <i>bcr1</i>^<i>-/-</i> (D, I, N) or <i>bcr1</i>-reconstituted (E, J, O) strain and incubated in saliva for 24h at 37°C to allow biofilm growth. The denture materials were then processed for SEM (A-E) or stained with calcofluor white (blue; stains fungal chitin in the cell wall) or Concanavalin A-Texas Red conjugate (red; stains mannose in the cell wall and ECM) and examined by fluorescent confocal microscopy to visualize biofilms in XY (F-J) and XYZ (K-O) views. Each panel shows a representative image of 3 repeats. Scale bar = 50 μm.</p

Mucosal damage in rats fitted with dentures inoculated with <i>C</i>. <i>albicans</i> biofilm deficient strains.

<p>Equilibrated rats were fitted with dentures, weaned onto gel diet, given antibiotics in the drinking water, and inoculated 3 times at 3 day intervals with 1 x 10⁹ <i>C</i>. <i>albicans</i> DAY185, <i>efg1</i>^<i>-/-</i> or <i>bcr1</i>^<i>-/-</i> or reconstituted mutants. Swab samples of the palate were taken weekly for a period of 4 weeks post-inoculation. LDH levels were assessed from swab suspension fluid. The results represent cumulative data from 2 independent experiments at 1–4 weeks post-inoculation with 4–5 animals per group. Data were analyzed using the Kruskal-Wallis test followed by the post hoc Mann-Whitney U test. n.s., not significant; **, <i>P</i> < 0.01; ***, <i>P</i> < 0.001.</p

The role of <i>C</i>. <i>albicans</i> EFG1 and BCR1 in biofilm formation on palate tissue <i>in vivo</i>.

<p>Equilibrated rats (n = 4/group) were weaned onto gel diet and fitted with dentures. Rats were given broad-spectrum antibiotics in the drinking water for 4 days prior to inoculation. Rats were inoculated 3x at 3-day intervals with 1x10⁹ CFU <i>C</i>. <i>albicans</i> DAY185 (A, F, K), <i>efg1</i>^<i>-/-</i> (B, G, L), <i>efg1</i>-reconstituted (C, H, M), <i>bcr1</i>^<i>-/-</i> (D, I, N) or <i>bcr1</i>-reconstituted (E, J, O) strain. Palate tissues were excised from inoculated rats at 4 weeks post-inoculation. The tissue samples were then processed for SEM (A-E) or stained with calcofluor white (blue; <i>C</i>. <i>albicans</i>) or Concanavalin A-Texas Red conjugate (red; ECM) and examined by fluorescent confocal microscopy to visualize biofilms in XY (F-J) and XYZ (K-O) views. Each panel shows a representative image of 2–3 animals. Scale bar = 50 μm.</p

Body weight change over time in rats fitted with dentures inoculated with <i>C</i>. <i>albicans</i> biofilm deficient strains.

<p>Equilibrated rats were fitted with dentures, weaned onto gel diet, given antibiotics in the drinking water, and inoculated 3x with 1 x 10⁹ <i>C</i>. <i>albicans</i> DAY185, <i>efg1</i>^<i>-/-</i> or <i>bcr1</i>^<i>-/-</i>. Rats were weighed bi-weekly for a period of 8 weeks post-inoculation using 3–4 animals per group and data are shown as (A) absolute weights and (B) % weight change (% weight change = weight at time point/weight at week 0 prior to inoculation). Data for individual weight changes per group were analyzed using the Repeated Measures ANOVA. Weight changes between groups at specific time points (experimental vs. control, actual weight or % change) were analyzed by the unpaired Student’s <i>t</i> test. * <i>P</i> < 0.05.</p

Quantification of biofilm thickness on dentures and palate tissue <i>in vivo</i>.

<p>Equilibrated rats were weaned onto gel diet and fitted with dentures. Rats were given broad-spectrum antibiotics in the drinking water for 4 days prior to inoculation. Rats were inoculated 3x at 3-day intervals with 1 x 10⁹ <i>C</i>. <i>albicans</i> DAY185, <i>efg1</i>^<i>-/-</i> or <i>bcr1</i>^<i>-/-</i> strain. (A) Dentures and (B) palate tissues were removed from inoculated rats at 4 weeks post-inoculation. Samples were stained with calcofluor white (blue; stains fungal chitin in the cell wall) or Concanavalin A-Texas Red conjugate (red; stains mannose in the cell wall and ECM) and examined by fluorescent confocal microscopy to visualize biofilms. Cross-sectional images of biofilms were visualized by confocal microscopy at 600X magnification, and the depths of biofilms were measured using the Fluoview software. Figure represents cumulative results from 2 independent experiments with 2–3 animals per group and assessment of 5 random areas per animal. Data were analyzed using a one-way ANOVA followed by the Tukey’s post hoc multiple comparison test. *, <i>P</i> < 0.05; **, <i>P</i> < 0.01 compared to the WT control.</p

Removal of Ca²⁺ from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn₄O₅ Core Structure: A Polarized Mn X‑ray Absorption Spectroscopy Study

Ca²⁺-depleted and Ca²⁺-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca²⁺ ion in the Mn₄O₅Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca²⁺-depleted S₁ (S₁′) and S₂ (S₂′) states, the S₂′Y_Z^• state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca²⁺-reconstituted S₁ state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca²⁺-depleted PS II investigated show only minor changes in distances and orientations of the Mn–Mn vectors compared to the Ca²⁺-containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca²⁺ ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca²⁺ ion in the OEC is not critical for structural maintenance of the cluster, at least in the S₁ and S₂ states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca²⁺ removal are discussed, attributing to the Ca²⁺ ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to Y_Z^• (D1-Tyr161)

High-Performance Overall Water Splitting Electrocatalysts Derived from Cobalt-Based Metal–Organic Frameworks

The design of active, robust, and nonprecious electrocatalysts with both H₂ and O₂ evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize porous Co-P/NC nanopolyhedrons composed of CoP_<i>x</i> (a mixture of CoP and Co₂P) nanoparticles embedded in N-doped carbon matrices as electrocatalysts for overall water splitting. The Co-P/NC catalysts were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) followed by phosphidation. Benefiting from the large specific surface area, controllable pore texture, and high nitrogen content of ZIF (a subclass of metal–organic frameworks), the optimal Co-P/NC showed high specific surface area of 183 m² g^–1 and large mesopores, and exhibited remarkable catalytic performance for both HER and OER in 1.0 M KOH, affording a current density of 10 mA cm^–2 at low overpotentials of −154 mV for HER and 319 mV for OER, respectively. Furthermore, a Co-P/NC-based alkaline electrolyzer approached 165 mA cm^–2 at 2.0 V, superior to that of Pt/IrO₂ couple, along with strong stability. Various characterization techniques including X-ray absorption spectroscopy (XAS) revealed that the superior activity and strong stability of Co-P/NC originated from its 3D interconnected mesoporosity with high specific surface area, high conductivity, and synergistic effect of CoP_<i>x</i> encapsulated within N-doped carbon matrices

Tunable Photoluminescent Core/Shell Cu⁺‑Doped ZnSe/ZnS Quantum Dots Codoped with Al³⁺, Ga³⁺, or In³⁺

Semiconductor quantum dots (QDs) with stable, oxidation resistant, and tunable photoluminescence (PL) are highly desired for various applications including solid-state lighting and biological labeling. However, many current systems for visible light emission involve the use of toxic Cd. Here, we report the synthesis and characterization of a series of codoped core/shell ZnSe/ZnS QDs with tunable PL maxima spanning 430−570 nm (average full width at half-maximum of 80 nm) and broad emission extending to 700 nm, through the use of Cu⁺ as the primary dopant and trivalent cations (Al³⁺, Ga³⁺, and In³⁺) as codopants. Furthermore, we developed a unique thiol-based bidentate ligand that significantly improved PL intensity, long-term stability, and resilience to postsynthetic processing. Through comprehensive experimental and computational studies based on steady-state and time-resolved spectroscopy, electron microscopy, and density functional theory (DFT), we show that the tunable PL of this system is the result of energy level modification to donor and/or acceptor recombination pathways. By incorporating these findings with local structure information obtained from extended X-ray absorption fine structure (EXAFS) studies, we generate a complete energetic model accounting for the photophysical processes in these unique QDs. With the understanding of optical, structural, and electronic properties we gain in this study, this successful codoping strategy may be applied to other QD or related systems to tune the optical properties of semiconductors while maintaining low toxicity