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Picosecond to millisecond tracking of a photocatalytic decarboxylation reaction provides direct mechanistic insights
Get PDFThe photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. However, the sequential nature of this multi-step reaction renders the mechanism challenging to probe. Here, we employ a 100 kHz mid-infrared probe in a transient absorption spectroscopy experiment to track the decarboxylation of cyclohexanecarboxylic acid in acetonitrile-d3 over picosecond to millisecond timescales using a photooxidant pair (phenanthrene and 1,4-dicyanobenzene). Selective excitation of phenanthrene at 256 nm enables a diffusion-limited photoinduced electron transfer to 1,4-dicyanobenzene. A measured time offset in the rise of the CO2 byproduct reports on the lifetime (520 ± 120 ns) of a reactive carboxyl radical in solution, and spectroscopic observation of the carboxyl radical confirm its formation as a reaction intermediate. Precise clocking of the lifetimes of radicals generated in situ by an activated C-C bond fission will pave the way for improving the photocatalytic selectivity and turnover
Gravity-driven syneresis in model low-fat mayonnaise
Get PDFLow-fat food products often contain natural, edible polymers to retain the desired mouth feel and elasticity of their full-fat counterparts. This type of product, however, can suffer from syneresis: densification due to the expulsion of fluid. Gaining insight into the physical principles governing syneresis in such soft hybrid dispersions remains a challenge from a theoretical perspective, as experimental data are needed to establish a basis. We record non-accelerated syneresis in a model system for low-fat mayonnaise: a colloid polymer mixture, consisting of oil in water emulsion with starch in the aqueous phase. We find the flow rate of expelled fluid to be proportional to the difference in hydrostatic pressure over the system. The osmotic pressure of the added starch, while being higher than the hydrostatic pressure, does not prevent syneresis because the soluble starch is lost to the expelled fluid. From these findings, we conclude that forced syneresis in these systems can be described as a gravity-driven porous flow through the densely packed emulsion, explainable with a model based on Darcy's law
Author Correction: Formation of helical membrane tubes around microtubules by single-headed kinesin KIF1A
Get PDFCorrection to: Nature Communications 10.1038/ncomms9025, published online 13 August 2018.
This Article contains an error in the first sentence of the ‘KIF1A purification and labelling’ section of the Methods, which incorrectly reads ‘A construct containing the first 382 residues of KIF1A with a His-tag and a Cys residue in the N-terminal, was kindly provided by N. Hirokawa (University of Tokyo, Japan; see ref. 13).’ This should say ‘C-terminus’ instead of ‘N-terminus’. The error has not been fixed in the PDF or HTML versions of the Article
Charge Carriers Are Not Affected by the Relatively Slow-Rotating Methylammonium Cations in Lead Halide Perovskite Thin Films
Get PDFRecently, several studies have investigated dielectric properties as a possible origin of the exceptional optoelectronic properties of metal halide perovskites (MHPs). In this study we investigated the temperature-dependent dielectric behavior of different MHP films at different frequencies. In the gigahertz regime, dielectric losses in methylammonium-based samples are dominated by the rotational dynamics of the organic cation. Upon increasing the temperature from 160 to 300 K, the rotational relaxation time, τ, decreases from 400 (200) to 6 (1) ps for MAPb-I3 (-Br3). By contrast, we found negligible temperature-dependent variations in τ for a mixed cation/mixed halide FA0.85MA0.15Pb(I0.85Br0.15)3. From temperature-dependent time-resolved microwave conductance measurements we conclude that the dipolar reorientation of the MA cation does not affect charge carrier mobility and lifetime in MHPs. Therefore, charge carriers do not feel the relatively slow-moving MA cations, despite their great impact on the dielectric constants
Directed nanoscale metal deposition by the local perturbation of charge screening at the solid–liquid interface†
Get PDFUnderstanding and directing electrochemical reactions below the micrometer is a long-standing challenge in electrochemistry.
Conning reactions to nanoscale areas paradoxically requires both isolation and communication with the bulk electrolyte in
terms of potential and access of ions, respectively. Here, we demonstrate the directed electrochemical deposition of copper
nanostructures by using an oscillating nanoelectrode operated with an atomic force microscope (AFM). Strikingly, the writing is
only possible in highly dilute electrolytes and for a particular combination of AFM and electrochemical parameters. We propose
a mechanism based on cyclic charging and discharging of the electrical double layer (EDL). The extended screening length and
slower charge dynamics in dilute electrolytes allows the nanoelectrode to operate inside, and disturb, the EDL even for large
oscillation amplitudes (~100 nm). Our unique approach can not only be used for controlled additive nano-fabrication but also
provides insights into ion behavior and EDL dynamics at the solid-liquid interface.Understanding and directing electrochemical reactions below the micrometer scale is a long-standing challenge in electrochemistry. Confining reactions to nanoscale areas paradoxically requires both isolation from and communication with the bulk electrolyte in terms of electrochemical potential and access of ions, respectively. Here, we demonstrate the directed electrochemical deposition of copper nanostructures by using an oscillating nanoelectrode operated with an atomic force microscope (AFM). Strikingly, the writing is only possible in highly dilute electrolytes and for a particular combination of AFM and electrochemical parameters. We propose a mechanism based on cyclic charging and discharging of the electrical double layer (EDL). The extended screening length and slower charge dynamics in dilute electrolytes allow the nanoelectrode to operate inside, and disturb, the EDL even for large oscillation amplitudes (∼100 nm). Our unique approach can not only be used for controlled additive nano-fabrication but also provides insights into ion behavior and EDL dynamics at the solid–liquid interface
Tumor classification with MALDI-MSI data of tissue microarrays: A case study
Get PDFWith mass spectrometry imaging (MSI) on tissue microarrays (TMAs) a large number of biomolecules can be studied for many patients at the same time, making it an attractive tool for biomarker discovery. Here we investigate whether lymph node metastasis can be predicted from MALDI-MSI data. Measurements are performed on TMAs and then filtered based on spectral intensity and the percentage of tumor cells, after which the resulting data for 122 patients is further preprocessed. We assume differences between patients with and without metastasis are expressed in a limited number of features. Two univariate feature selection methods are applied to reduce the dimensionality of the MALDI-MSI data. The selected features are then used in combination with three classifiers. The best classification scores are obtained with a decision tree classifier, which classifies about 72% of patients correctly. Almost all the predictive power comes from a single peak (m/z 718.4). The sensitivity of our classification approach, which can be generically used to search for biomarkers, is investigated using artificially modified data
Nanoscale Relative Emission Efficiency Mapping Using Cathodoluminescence g(2) Imaging
Get PDFCathodoluminescence (CL) imaging spectroscopy provides two-dimensional optical excitation images of photonic nanostructures with a deep-subwavelength spatial resolution. So far, CL imaging was unable to provide a direct measurement of the excitation and emission probabilities of photonic nanostructures in a spatially resolved manner. Here, we demonstrate that by mapping the cathodoluminescence autocorrelation function g(2) together with the CL spectral distribution the excitation and emission rates can be disentangled at every excitation position. We use InGaN/GaN quantum wells in GaN nanowires with diameters in the range 200–500 nm as a model system to test our new g(2) mapping methodology and find characteristic differences in excitation and emission rates both between wires and within wires. Strong differences in the average CL intensity between the wires are the result of differences in the emission efficiencies. At the highest spatial resolution, intensity variations observed within wires are the result of excitation rates that vary with the nanoscale geometry of the structures. The fact that strong spatial variations observed in the CL intensity are not only uniquely linked to variations in emission efficiency but also linked to excitation efficiency has profound implications for the interpretation of the CL data for nanostructured geometries in general.
Keywords: autocorrelation function; cathodoluminescence; nano-optics; SEM; Semiconductor; spectroscop
Energy Relaxation and Structural Dynamics of Protons in Water/DMSO Mixtures
Get PDFWe investigate the structure and dynamics of proton solvation structures in mixed water/dimethyl sulfoxide (DMSO) solvents using two-color mid-infrared femtosecond pump-probe spectroscopy. At a water fraction below 20%, protons are mainly solvated as (DMSO-H)(+) and (DMSO-H)(+) -H2O structures. We find that excitation of the OH-stretch vibration of the proton in (DMSO-H)(+)-H2O structures leads to an ultrafast contraction of the hydrogen bond between (DMSO-H)(+) and H2O. This excited state relaxes rapidly with T-1 = 95 +/- 10 fs and leads in part to a strong local heating effect and in part to predissociation of the protonated cluster into (DMSO-H)(+) and water monomers
