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This Letter reports the measurement of photoinduced electron-transfer rates from individual CdSe/CdS nanocrystals, or quantum dots (QDs), to methyl viologen acceptor molecules adsorbed on the QD surfaces, using time-resolved photoluminescence at the single-nanocrystal level. For each QD measured, the electron-transfer rate is constant over time, and the photoluminescence blinking dynamics are independent of the measured transfer rate. The total electron-transfer rate is distributed in discrete, constant increments, corresponding to discrete numbers of adsorbed molecules on each QD. The results thus validate previous assumptions that viologen molecules adsorb independently on QD surfaces and that the total electron-transfer rate from a single QD to multiple molecules on its surface is simply the sum of the transfer rates to the individual molecules. The measurement provides an optical method to count the number of active acceptor molecules bound to a single nanocrystal and opens up new possibilities for me...

Displacement of native octylphosphonate (OPA) ligands for methylthiophenolate (CH3-TP) on the surfaces of CdSe quantum dots (QDs) causes a moderate (up to 50 meV) decrease in the band gap (Eg) of the QD. Plots of the corresponding increase in apparent excitonic radius, ΔR, of the QDs versus the surface coverage of CH3-TP, measured by 1H NMR, for several sizes of QDs reveal that this ligand adsorbs in two distinct binding modes, (1) a tightly bound mode (Ka = 1.0 ± 0.3 × 104 M–1) capable of exciton delocalization, and (2) a more weakly bound mode (Ka = 8.3 ± 9.9 × 102 M–1) that has no discernible effect on exciton confinement. For tightly bound CH3-TP, the degree of delocalization induced in the QD is approximately linearly related to the fractional surface area occupied by the ligand for all sizes of QDs. Comparison of the dependence of ΔR on surface coverage of CH3-TP over a range of physical radii of the QDs, R = 1.1–2.4 nm, to analogous plots simulated using a 3D spherical potential well model yield a ...

This Perspective describes the mechanisms by which organic surfactants, in particular, phenyldithiocarbamates (PTCs), couple electronically to the delocalized states of semiconductor quantum dots (QDs). This coupling reduces the confinement energies of excitonic carriers and, in the case of PTC, the optical band gap of metal chalcogenide QDs by up to 1 eV by selectively delocalizing the excitonic hole. The reduction of confinement energy for the hole is enabled by the creation of interfacial electronic states near the valence band edge of the QD. The PTC case illuminates the general minimal requirements for surfactants to achieve observable bathochromic or hypsochromic shifts of the optical band gap of QDs; these include frontier orbitals with energies near the relevant semiconductor band edge, the correct symmetry to mix with the orbitals of the relevant band, and an adsorption geometry that permits spatial overlap between the orbitals of the ligand and those of the relevant band (Se 4p orbitals for CdSe, for example). The shift is enhanced by energetic resonance of frontier orbitals of the surfactant with a high density of states region of the band, which, for CdSe, is ∼1 eV below the band edge. The Perspective discusses other examples of strong-coupling surfactants and compares the orbital mixing mechanism with other mechanisms of surfactant-induced shifts in the QD band gap.

This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium-thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δr1, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δr1 for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing.

Unraveling how regulatory divergence contributes to species differences and adaptation requires identifying functional variants from among millions of genetic differences. Analysis of allelic imbalance (AI) reveals functional genetic differences in cis regulation and has demonstrated differences in cis regulation within and between species. Regulatory mechanisms are often highly conserved, yet differences between species in gene expression are extensive. What evolutionary forces explain widespread divergence in cis regulation? AI was assessed in Drosophila melanogaster–Drosophila simulans hybrid female heads using RNA-seq technology. Mapping bias was virtually eliminated by using genotype-specific references. Allele representation in DNA sequencing was used as a prior in a novel Bayesian model for the estimation of AI in RNA. Cis regulatory divergence was common in the organs and tissues of the head with 41% of genes analyzed showing significant AI. Using existing population genomic data, the relationship between AI and patterns of sequence evolution was examined. Evidence of positive selection was found in 30% of cis regulatory divergent genes. Genes involved in defense, RNAi/RISC complex genes, and those that are sex regulated are enriched among adaptively evolving cis regulatory divergent genes. For genes in these groups, adaptive evolution may play a role in regulatory divergence between species. However, there is no evidence that adaptive evolution drives most of the cis regulatory divergence that is observed. The majority of genes showed patterns consistent with stabilizing selection and neutral evolutionary processes.

A study of the adsorption equilibrium of solution-phase CdS quantum dots (QDs) and acid-derivatized viologen ligands (N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate, V(2+)) reveals that the structure of the surfaces of the QDs depends on their concentration. This adsorption equilibrium is monitored through quenching of the photoluminescence of the QDs by V(2+) upon photoinduced electron transfer. When modeled with a simple Langmuir isotherm, the equilibrium constant for QD-V(2+) adsorption, K(a), increases from 6.7 × 10(5) to 8.6 × 10(6) M(-1) upon decreasing the absolute concentration of the QDs from 1.4 × 10(-6) to 5.1 × 10(-8) M. The apparent increase in K(a) upon dilution results from an increase in the mean number of available adsorption sites per QD from 1.1 (for [QD] = 1.4 × 10(-6) M) to 37 (for [QD] = 5.1 × 10(-8) M) through desorption of native ligands from the surfaces of the QDs and through disaggregation of soluble QD clusters. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially distributed quantities is described. This model yields a concentration-independent value for K(a) of 8.7 × 10(5) M(-1) for the QD-V(2+) system and provides a convenient means for quantitative analysis of QD-ligand adsorption in the presence of competing surface processes.

Displacement of cadmium oleate (Cd(oleate)2) ligands for the exciton-delocalizing ligand 4-hexylphenyldithiocarbamate (C6-PTC) on the surfaces of CdS quantum dots (QDs) causes a decrease in the band gap (Eg) of the QD of ∼100 meV for QDs with a radius of 1.9 nm and ∼50 meV for QDs with a radius of 2.5 nm. The primary mechanism of this decrease in band gap, deduced in previous work, is a decrease in the confinement barrier for the excitonic hole. The increase in apparent excitonic radius of the QD that corresponds to this decrease in Eg is denoted ΔR. The dependence of ΔR on the surface coverage of C6-PTC, measured by (1)H NMR spectroscopy, appears to be nonlinear. Calculations of the excitonic energy of a CdS QD upon displacement of native insulating ligands with exciton-delocalizing ligands using a 3D spherical potential well model show that this response includes the contributions to ΔR from both isolated, bound C6-PTC ligands and groups of adjacent C6-PTC ligands. Fits to the experimental plots of ΔR vs surface coverage of C6-PTC with a statistical model that includes the probability of formation of clusters of bound C6-PTC on the QD surface allow for the extraction of the height of the confinement barrier presented by a single, isolated C6-PTC molecule to the excitonic hole. This barrier height is less than 0.6 eV for QDs with a radius of 1.9 nm and between 0.6 and 1.2 eV for QDs with a radius of 2.5 nm.

Enhancing visitor experiences is arguably the primary and most important goal for interpretation by many protected area managers and tourism business. However, little research has been conducted in Sarawak, Malaysia to directly quantify the effects of thematic interpretation has on tourist experiences. Drawing on the TORE-model of interpretation and through the inception of Park Guiding Training and Licensing System in Sarawak since 2007, this quantitative study examines the effectiveness of thematic interpretive guided tours delivered by park guides at Bako National Park, Sarawak, with the assumption that it will further enhance visitor experiences. A descriptive analysis and Pearson's product-moment correlation analysis of sub-indicators of the global evaluation of interpretation of site, and sub-indicators of elaboration surveyed from visitors of purposively sampled park guides revealed a strong measurement and correlation coefficients of visitors' overall quality of thematic intepretive guided tours effecting visitor satisfaction and experiences. These findings provide empirical evidence that good thematic interpretive guided tour makes a positive impacts on visitor experiences, thus making training of tourism businesses' employees as park guides as a good investment. The suggestions for further research in influencing visitor attitude and shaping visitor behaviour are offered.

This paper describes a method to control the quantum confinement, and therefore the energy, of excitonic holes in CdSe QDs through adsorption of the hole-delocalizing ligand phenyldithiocarbamate, PTC, and para substitutions of the phenyl ring of this ligand with electron-donating or -withdrawing groups. These substitutions control hole delocalization in the QDs through the energetic alignment of the highest occupied orbitals of PTC with the highest density-of-states region of the CdSe valence band, to which PTC couples selectively.

This paper describes unprecedented bathochromic shifts (up to 970 meV) of the optical band gaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R∼1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices. Facilitating carrier-specific electronic coupling across heterogeneous interfaces is especially important for nanostructured devices, which comprise a high density of such interfaces.

BACKGROUND: Cellulose and hemicellulose are quantitatively the most important structural carbohydrates present in ruminant diets. Rumen micro-organisms produce enzymes that catalyse their hydrolysis, but the complex network formed by structural carbohydrates and lignin reduces their digestibility and restricts efficient utilisation of feeds by ruminants. This study aimed to produce two enzymatic extracts, apply them in ruminant diets to determine the best levels for ruminal digestibility and evaluate their effects on in vitro digestibility. RESULTS: In experiment 1 a two-stage in vitro technique was used to examine the effects of different enzymatic levels of Aspergillus japonicus and Aspergillus terricola on tropical forages. Enzyme addition had minor effects on corn silage at the highest enzymatic level. In experiment 2 an in vitro gas production (GP) technique was applied to determine apparent in vitro organic matter digestibility and metabolisable energy. The addition of enzymes in GP showed interesting results. Good data were obtained using sugar cane and Tifton-85 hay supplemented with extracts of A. japonicus and A. terricola respectively. CONCLUSION: Overall, the study suggests that addition of crude extracts containing exogenous fibrolytic enzymes to ruminant diets enhances the effective utilisation of ruminant feedstuffs such as forages. Copyright © 2012 Society of Chemical Industry

Sarawak, a state of Malaysia has 30 national parks, six wildlife sanctuaries and eight nature reserves with 15 of the totally protected areas opened to the public. The national parks, nature reserves and wildlife centres draw tourists who are attracted to the nature for inspirational, educative, cultural and recreational purposes expecting a degree of adventure thus generating income to the local communities. As impact of tourism could degrade natural non-renewable resources, an environmental education for the park guides was included in the syllabus of the Certificate in Park Guiding in Sarawak's National Parks and Nature Reserves conducted by Universiti Teknologi MARA the training provider for Sarawak Forestry designed toward reflective-learning, experience and reflection based on thematic interpretation. The initiative is to enhance their professionalism by improving their awareness, sensitize them to environmental issues, and care about the places. The purpose of this study is to elucidate the effectiveness of learning Ecology using thematic interpretation approach and integrating their traditional ecological knowledge, local ecological knowledge and conventional science from the park guide perspective as adult learner. An analysis of the effectiveness of the course from indicated that all participants regardless of educational background, experience in terms of number of years in guiding and age showed significant differences. Although there was no significant correlation between written and practical assessments, there was a strong correlation of thematic interpretative skills and their cultural background and experiences rather than their educational qualification level.

Background RNA-seq is revolutionizing the way we study transcriptomes. mRNA can be surveyed without prior knowledge of gene transcripts. Alternative splicing of transcript isoforms and the identification of previously unknown exons are being reported. Initial reports of differences in exon usage, and splicing between samples as well as quantitative differences among samples are beginning to surface. Biological variation has been reported to be larger than technical variation. In addition, technical variation has been reported to be in line with expectations due to random sampling. However, strategies for dealing with technical variation will differ depending on the magnitude. The size of technical variance, and the role of sampling are examined in this manuscript. Results In this study three independent Solexa/Illumina experiments containing technical replicates are analyzed. When coverage is low, large disagreements between technical replicates are apparent. Exon detection between technical replicates is highly variable when the coverage is less than 5 reads per nucleotide and estimates of gene expression are more likely to disagree when coverage is low. Although large disagreements in the estimates of expression are observed at all levels of coverage. Conclusions Technical variability is too high to ignore. Technical variability results in inconsistent detection of exons at low levels of coverage. Further, the estimate of the relative abundance of a transcript can substantially disagree, even when coverage levels are high. This may be due to the low sampling fraction and if so, it will persist as an issue needing to be addressed in experimental design even as the next wave of technology produces larger numbers of reads. We provide practical recommendations for dealing with the technical variability, without dramatic cost increases.