Organometallic halide perovskites CH 3 NH 3 PbX 3 (X = I, Br, Cl) have quickly become one of the most promising semiconductors for solar cells, with photovoltaics made of these materials reaching power conversion efficiencies of near 20%. Improving our ability to harness the full potential of organometal halide perovskites will require more controllable syntheses that permit a detailed understanding of their fundamental chemistry and photophysics. In this manuscript, we systematically synthesize CH 3 NH 3 PbX 3 (X = I, Br) nanocrystals with different morphologies (dots, rods, plates or sheets) by using different solvents and capping ligands. CH3NH3PbX3 nanowires and nanorods capped with octylammonium halides show relatively higher photoluminescence (PL) quantum yields and long PL lifetimes. CH3NH3PbI3 nanowires monitored at the single particle level show shape-correlated PL emission across whole particles, with little photobleaching observed and very few off periods. This work highlights the potential of low-dimensional organometal halide perovskite semiconductors in constructing new porous and nanostructured solar cell architectures, as well as in applying these materials to other fields such as light-emitting devices and single particle imaging and tracking.Keywords organometal halide perovskites, nanocrystals, preferred orientation, morphology control, size control, single particle photoluminescence Disciplines Chemistry CommentsReprinted (adapted) with permission from ACS Nano 9 (2015) 15,16 respectively, as well as high absorption coefficients. Critically, organolead perovskites have very long electronÀ hole carrier diffusion lengths, exceeding 1 μm in CH 3 NH 3 PbI 3-x Cl x , and 100 nm in CH 3 NH 3 PbI 3 , which in principle allows for the development of several solar cell architectures including perovskite-sensitized solar cells, planar heterojunction solar cells, and meso-and nanostructured solar cells. 17 Building on the dramatic improvement of solar cell performance using the solid hole conductor spiro-OMeTAD instead of a liquid electrolyte (spiro-OMeTAD stands for 2,2 0 -7,7 0 -tetrakis(N,N-di-p-methoxy-phenylamine)-9,9 0 -spirobifluorene), 18 the energy conversion efficiency of photovoltaics made from these intensely absorbing, visible-active semiconductors has risen from 3.8% to near 20% in only four years. 19,20 Photovoltaic performance depends critically on perovskite composition, crystallinity and morphology. 21À23 Higher perovskite film uniformity leads to lower recombination rates in planar heterojuction solar cells. 24,25 Film uniformity is affected by factors such as precursor composition, annealing temperature and, if applicable, solvent used during the vapor-assisted or spin coating deposition process. 6,24,26À32 Highly efficient mesostructured solar cells are produced by a twostep deposition process. 33À35 Vapor-assisted methods and additives provide the means * Address correspondence to jwp@iastate.edu, esmith1@iastate.edu, vela@iastate.edu.Received for review December 9, 2014 a...

Peri-operative SARS-CoV-2 infection increases postoperative mortality. The aim of this study was to determine the optimal duration of planned delay before surgery in patients who have had SARS-CoV-2 infection. This international, multicentre, prospective cohort study included patients undergoing elective or emergency surgery during October 2020. Surgical patients with pre-operative SARS-CoV-2 infection were compared with those without previous SARS-CoV-2 infection. The primary outcome measure was 30-day postoperative mortality. Logistic regression models were used to calculate adjusted 30-day mortality rates stratified by time from diagnosis of SARS-CoV-2 infection to surgery. Among 140,231 patients (116 countries), 3127 patients (2.2%) had a pre-operative SARS-CoV-2 diagnosis. Adjusted 30-day mortality in patients without SARS-CoV-2 infection was 1.5% (95%CI 1.4-1.5). In patients with a pre-operative SARS-CoV-2 diagnosis, mortality was increased in patients having surgery within 0-2 weeks, 3-4 weeks and 5-6 weeks of the diagnosis (odds ratio (95%CI) 4.1 (3.3-4.8), 3.9 (2.6-5.1) and 3.6 (2.0-5.2), respectively). Surgery performed ≥ 7 weeks after SARS-CoV-2 diagnosis was associated with a similar mortality risk to baseline (odds ratio (95%CI) 1.5 (0.9-2.1)). After a ≥ 7 week delay in undertaking surgery following SARS-CoV-2 infection, patients with ongoing symptoms had a higher mortality than patients whose symptoms had resolved or who had been asymptomatic (6.0% (95%CI 3.2-8.7) vs. 2.4% (95%CI 1.4-3.4) vs. 1.3% (95%CI 0.6-2.0), respectively). Where possible, surgery should be delayed for at least 7 weeks following SARS-CoV-2 infection. Patients with ongoing symptoms ≥ 7 weeks from diagnosis may benefit from further delay.

Small silver clusters that form with short oligonucleotides are distinguished by their strong fluorescence. Previous work showed that red and blue/green emitting species form with the cytosine oligonucleotide dC12. To understand how the bases and base sequence influence cluster formation, the blue/green emitting clusters that form with the thymine-containing oligonucleotides dT12, dT4C4T4, and dC4T4C4 are discussed. With dT12 and dT4C4T4, variations in the solution pH establish that the clusters associate with the N3 of thymine. The small clusters are bound to the larger DNA template, as demonstrated by fluorescence anisotropy, circular dichroism, and fluorescence correlation spectroscopy (FCS) studies. For dT4C4T4, FCS studies showed that approximately 50% of the strands are labeled with the fluorescent clusters. Absorption spectra and the gas dependence of the fluorescence show that nonfluorescent clusters also form following the reduction of the silver cation – oligonucleotide conjugates. Fluorescent cluster formation is favored by oxygen, thus indicating that the DNA-bound clusters are partially oxidized. To elaborate the sequence dependence of cluster formation, dC4T4C4 was studied. Cluster formation depends on the oligonucleotide concentration, and higher concentrations favor a red emitting species. A blue/green emissive species dominates at lower concentrations of dC4T4C4, and it has spectroscopic, physical, and chemical properties that are similar to those of the clusters that form with dT12 and dT4C4T4. These results suggest that cytosine- and thymine-containing oligonucleotides stabilize a preferred emissive silver cluster.

Fluorescence lifetimes of single Rhodamine 6G molecules on silica surfaces were measured with pulsed laser excitation, time-correlated single photon counting, and near-field scanning optical microscopy (NSOM). The fluorescence lifetime varies with the position of a molecule relative to a near-field probe. Qualitative features of lifetime decreases are consistent with molecular excited state quenching effects near metal surfaces. The technique of NSOM provides a means of altering the environment of a single fluorescent molecule and its decay kinetics in a repeatable fashion.

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We recently developed an inorganic shell approach for suppressing blinking in nanocrystal quantum dots (NQDs) that has the potential to dramatically improve the utility of these fluorophores for single-NQD tracking of individual molecules in cell biology. Here, we consider in detail the effect of shell thickness and composition on blinking suppression, focusing on the CdSe/CdS core/shell system. We also discuss the blinking mechanism as understood through profoundly altered blinking statistics. We clarify the dependence of blinking behavior and photostability on shell thickness, as well as on interrogation times. We show that, while the thickest-shell systems afford the greatest advantages in terms of enhanced optical properties, thinner-shell NQDs may be adequate for certain applications requiring relatively shorter interrogation times. Shell thickness also determines the sensitivity of the NQD optical properties to aqueous-phase transfer, a critical step in rendering NQDs compatible with bioimaging applications. Lastly, we provide a proof-of-concept demonstration of the utility of these unique NQDs for fluorescent particle tracking. High-resolution image of an ultra-thick-shell ‘giant’ nanocrystal quantum dot (left). Suppressed blinking behaviour afforded by this class of semiconductor nanocrystal yields new statistical relationships in the probability densities of fluorescence on- and off-time distributions (right).

We report a method for tracking individual quantum dot (QD) labeled proteins inside of live cells that uses four overlapping confocal volume elements and active feedback once every 5 milliseconds to follow three dimensional molecular motion. This method has substantial advantages over 3D molecular tracking methods based upon CCD cameras, including increased Z tracking range (10 μm demonstrated here), substantially lower excitation powers (15 μW used here), and the ability to perform time-resolved spectroscopy (such as fluorescence lifetime measurements or fluorescence correlation spectroscopy) on the molecules being tracked. In particular, we show for the first time fluorescence photon anti-bunching of individual QD labeled proteins in live cells and demonstrate the ability to track individual dye labeled nucleotides (Cy5-dUTP) at biologically relevant transport rates. To demonstrate the power of these methods for exploring the spatio-temporal dynamics of live cells, we follow individual QD-labeled IgE receptors both on and inside rat mast cells. Trajectories of receptors on the plasma membrane reveal three dimensional, nano-scale features of the cell surface topology. During later stages of the signal transduction cascade, clusters of QD labeled IgE-FcεRI were captured in the act of ligand-mediated endocytosis and tracked during rapid (~950 nm/s) vesicular transit through the cell. KeywordsQuantum Dot; Single Molecule; Fluorescence; Tracking; Microscopy; Cell The direct observation of individual biological molecules in motion can transform our view of important biophysical and cellular processes. 1 For example, single molecule tracking has shed significant light on cellular membrane dynamics 2-4 , motor protein kinetics 5,6 , and gene regulation 7 . Advantages of a single molecule approach include the ability to observe dynamic, stochastic behavior (such as compartmentalized diffusion 2, 4 ) that would be masked in ensemble measurements and the ability for localization of molecules with a precision well below the diffraction limit of light 5,6 . To date the field has primarily relied * Corresponding Author: jwerner@lanl.gov. 8,16 or follow the Z position with multiple cameras or image planes 9, 14 . While these camera-based techniques can capture 3D molecular motion, they are generally limited in their Z-tracking range in cells to approximately plus or minus one μm from a fixed focal plane 8,10,14,17 , limited by the shallow depth of field of high numerical aperture microscope objectives needed for single molecule work. We point out the obvious: many cells are substantially thicker than two microns and different methods and techniques are required to follow single molecules throughout entire three dimensional cell volumes. In addition to its quite limiting Z-tracking range, CCD-based tracking approaches are also bounded in temporal resolution by the CCD frame rate (~1 ms for fast EM-CCDs), and must illuminate an entire cell slice at relatively large excitation intensities (~40 W/cm 2 ).In contrast to ...