A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Monolayered boron nitride (BN) quantum dots (QDs; lateral size ≈10 nm) are fabricated using a novel method. Unlike monolayered BN sheets, these BN QDs exhibit blue-green luminescence due to defects formed during preparation. This optical behavior adds significant functionality to a material that is already receiving much attention. It is further shown that the QDs are nontoxic to biological cells and well suited to bio-imaging.

The enzyme protochlorophyllide oxidoreductase (POR) catalyses a lightdependent step in chlorophyll biosynthesis that is essential to photosynthesis and ultimately all life on Earth. 1-3 POR, which is one of three known light-dependent enzymes, 4,5 catalyzes reduction of the photosensitizer and substrate protochlorophyllide to form the pigment chlorophyllide. Despite its biological importance, a structural basis for POR photocatalysis has remained elusive. Here, we report crystal structures of cyanobacterial PORs from Thermosynechococcus elongatus and Synechocystis sp. in their free forms, and in complex with nicotinamide coenzyme. Our structural models and simulations of the ternary protochlorophyllide-NADPH-POR complex have identified multiple interactions in the POR active site that are important for protochlorophyllide binding, photosensitization and photochemical conversion to chlorophyllide. We demonstrate the importance of active-site architecture and protochlorophyllide structure in experiments using POR variants and protochlorophyllide analogues. These studies reveal how the POR active site facilitates light-driven reduction of protochlorophyllide by localized hydride transfer from NADPH and long-range proton transfer along structurally defined proton-transfer pathways. As the light-driven step in the chlorophyll biosynthetic pathway (Fig. 1), the POR reaction acts as the trigger for the germination of seedlings =in plants and provokes a marked change in the morphological development of the plant. 2,3 Given this crucial biological role, POR has been the focus of numerous mechanistic and biophysical investigations. A combination of time-resolved (at the femtosecond-to-second scale) and cryogenic spectroscopy methods have provided some understanding of the mechanism of POR photocatalysis in a range of photosynthetic organisms, including cyanobacteria and plants. Picosecond excited-state dynamics in the protochlorophyllide (Pchlide) molecule are thought to result in excited state interactions between the substrate and active-site residues that are necessary to trigger the subsequent reaction chemistry. 6-12 This involves sequential transfer of a hydride equivalent from NADPH and a proton transfer from either an active site residue or solvent. Proton transfer is reliant on solvent dynamics and an implied network of extended protein motions that occur on the microsecond timescale. 13-17 Hydride transfer from NADPH is not concerted, but occurs in a stepwise manner that involves

The reference range for nonpregnant women can be used for the assessment of pregnant women at 4 to 6 weeks of gestation. The upper limit of serum TSH in the first trimester was much higher than 2.5 mIU/L in Chinese pregnant women.