Diffusion Kinetics of Adsorbed Species on Pyrite Surfaces

Surface diffusion can bring redox pairs closer together, which is necessary for electron transfer to happen, thereby strongly influencing the overall kinetics. However, little is known about the diffusion kinetics of redox-active species on pyrite and other sulfides, which aid in catalyzing many redox reactions. Here, we calculate the diffusion of oxidant (UO22+) and reductant (Fe2+, HS–) species on pyrite {100} surfaces using quantum-mechanical calculations. Energy curves along different diffusion paths are derived for both inner- and outer-sphere complexes by moving the species in small increments (0.05–0.25 Å). The diffusion path along the molecular ridges formed by disulfide groups on the uppermost pyrite surface has the lowest energy barrier for the diffusion of all species tested. Single-particle diffusion coefficients along their optimal diffusion pathways are derived from diffusion energy barriers and attempt frequencies. Calculations are performed on flat defect-free pyrite surfaces, while on actual surfaces, diffusion is affected by defects, steps, and impurities. Calculated mobilities of the outer-sphere complexed uranyl and ferrous iron are about 4–5 times faster than their inner-sphere ones. Although the results here focus on single-particle diffusion, UO22+-HS– was used as an example for interdependent multiple-particle diffusion on the pyrite surface. Interactions between diffusing species (uranyl vs HS–), and to a limited degree jump correlations, were derived quantum-mechanically. Interactions are a combination of electronic interactions underneath the mineral surface and through the aqueous near-surface region; their interdependent diffusion can be approximated by apparent Coulomb interactions (with a dielectric constant of ∼7.7) for processing in subsequent Monte Carlo simulations

Emotion malleability beliefs matter in emotion regulation: a comprehensive review and meta-analysis

Individuals’ beliefs about the malleability of emotions have been theorised to play a role in their psychological distress by influencing emotion regulation processes, such as the use of emotion regulation strategies. We conducted a meta-analysis to test this idea across studies with a focus on the relationships between emotion malleability beliefs and five distinct emotion regulation strategies: cognitive reappraisal, suppression, avoidance, rumination, and acceptance. Further, using two-stage meta-analytic structural equation modelling (TSSEM), we examined whether the emotion regulation strategies mediate the cross-sectional relationship between emotion malleability beliefs and psychological distress across studies. Thirty-seven studies were included in the meta-analyses and 55 cross-sectional studies were included in the TSSEM. Results demonstrated that, across studies, emotion malleability beliefs were significantly associated with greater use of putatively helpful strategies (particularly with cognitive reappraisal) and less use of putatively unhelpful strategies (particularly with avoidance). The use of cognitive reappraisal and avoidance partially mediated the relationship between emotion malleability beliefs and psychological distress. These results highlight the importance of considering beliefs about the malleability of emotions in the context of emotion regulation. These findings suggest the potential role of emotion malleability beliefs in interventions for individuals with emotion regulation-related difficulties and psychological distress.</p

Self-Assembly of Polydeoxyadenylic Acid Studied at the Single-Molecule Level

The investigation on the self-assembly of polydeoxyadenylic acid (poly(dA)) is highly important to fully understand its biological function and for its application in the field of nanotechnology. Using the fluorescence resonance energy transfer (FRET) technique, we report investigations for the self-assembly of adenine oligomers induced by pH and coralyne binding at the single-molecule level and in the bulk phase. Results presented here show that A-motif 1 (Alexa488-5′-(dA)20-3′-Cy5-5′-(dA)20-3′-Alexa488) forms the wire-type duplex at acidic pH, whereas the same conformation of A-motif 2 (Alexa488-5′-(dA)20-3′-Cy5-3′-(dA)20-5′-Alexa488) is induced by coralyne binding at neutral pH. These results indicate that poly(dA) at acidic pH forms a right-handed helical duplex with parallel-mannered chains, whereas the coralyne–poly(dA) binding induces a stable antiparallel duplex. Furthermore, we found that the antiparallel duplex of poly(dA) formed by coralyne binding has a rather extended and less twisted structure as compared to the parallel duplex of poly(dA) formed at acidic pH. On the other hand, from dilution experiments, we found that the parallel duplex formed at acidic pH is converted to “S-form”, which has the single-stranded structure with short intramolecular double-stranded regions formed by intramolecular A:A base pairing, while the A-motif–coralyne assembly is dissociated into single strands below a certain concentration. The formation of S-form with a short intramolecular double-stranded region formed at acidic pH and very low concentration is confirmed by the quantitative analysis of FCS curve to measure the hydrodynamic radius of a molecule

Actinyl Adsorption and Reduction on Pyrite Surfaces: Insights from DFT Calculations

Interactions of actinides with pyrite surfaces are highly important in catalyzing their reductive immobilization, thereby controlling the movement of these species in the environment. Here, surface adsorption and subsequent reduction of aqueous actinyl­(VI) on pyrite surfaces were explored using density functional hybrid theory (DFT-B3LYP) combined with a first hydration sphere of water molecules and a dielectric continuum for solvation effects. Adsorption of cationic (AnO2(H2O)5)2+(An = U, Np, Pu) and neutral AnO2(OH)2(H2O)3 actinyl onto a small pyrite cluster (Fe4S8) and the effect of coadsorption on the energetics and electron transfer are evaluated by adding either hydroquinone, H2Q (reduced), or quinone, Q (oxidized). The pyrite surface instantaneously transfers an electron to the adsorbed cationic actinyl. Unpaired electron atomic spin densities confirm the electron transfer from the pyrite surface to An atoms. For the neutral actinyl adsorption, electron transfer is confirmed for neptunyl and plutonyl but not for uranyl. Several factors control the overall adsorption energetics and kinetics, such as the nature of the coadsorbate (H2Q/Q), pyrite surface, actinyl, and charge or protonation state (cationic or neutral). The surface-mediated reduction of adsorbed actinyl occurs by receiving electrons either directly from the sulfide or from the coadsorbed H2Q through the sulfide. In the direct reduction case, an H+ ion is added to the surface-bound cationic actinyl, and the mineral surface acts as an electron donor. In contrast, in the proton-coupled electron transfer (PCET) reduction, the surface mediates the electrons through the surface by synergistically aligning relevant orbitals in line. This results in the less soluble and stable An­(IV). Our results indicate that the pyrite surface promotes a faster PCET reaction for the actinyl reduction under circumneutral (pH 4–7) conditions

Photochemistry of Singlet Oxygen Sensor Green

To detect singlet oxygen (¹O₂), the commercially available fluorescent sensor named Singlet Oxygen Sensor Green (SOSG) has been the most widely used from material studies to medical applications, for example, photodynamic therapy. In light of the previous studies, SOSG is a dyad composed of fluorescein and anthracene moieties. In the present study, we carried out quantitative studies on photochemical dynamics of SOSG for the first time, such as the occurrence of intramolecular photoinduced electron transfer (PET), ¹O₂ generation, and two-photon ionization. It was revealed that these relaxation pathways strongly depend on the irradiation conditions. The visible-light excitation (ex. 532 nm) of SOSG induced intramolecular PET as a major deactivation process (<i>k</i>_PET = 9.7 × 10¹¹ s^–1), resulting in fluorescence quenching. In addition, intersystem crossing occurred as a minor deactivation process that gave rise to ¹O₂ generation via the bimolecular triplet–triplet energy transfer (<i>k</i>_q = 1.2 × 10⁹ M^–1 s^–1). Meanwhile, ultraviolet-light excitation (355 nm) of SOSG caused the two-photon ionization to give a SOSG cation (Φ_ion = 0.003 at 24 mJ cm^–2), leading to SOSG decomposition to the final products. Our results clearly demonstrate the problems of SOSG, such as photodecomposition and ¹O₂ generation. In fact, these are not special for SOSG but common drawbacks for most of the fluorescein-based sensors

Mechanistic Study of Wettability Changes on Calcite by Molecules Containing a Polar Hydroxyl Functional Group and Nonpolar Benzene Rings

Oil extraction efficiency strongly depends on the wettability status (oil- vs water-wet) of reservoir rocks during oil recovery. Aromatic compounds with polar functional groups in crude oil have a significant influence on binding hydrophobic molecules to mineral surfaces. Most of these compounds are in the asphaltene fraction of crude oil. This study focuses on the hydroxyl functional group, an identified functional group in asphaltenes, to understand how the interactions between hydroxyl groups in asphaltenes and mineral surfaces begin. Phenol and 1-naphthol are used as asphaltene surrogates to model the simplest version of asphaltenes. Adsorption of oil molecules on the calcite {101̅4} surface is described using static quantum-mechanical density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. DFT calculations indicate that adsorption of phenol and 1-naphthol occurs preferentially between their hydroxyl group and calcite step edges. 1-Naphthol adsorbs more strongly than phenol, with different adsorption geometries due to the larger hydrophobic part of 1-naphthol. MD simulations show that phenol can behave as an agent to separate oil from the water phase and to bind the oil phase to the calcite surface in the water/oil mixture. In the presence of phenol, partial separation of water/oil with an incomplete lining of phenol at the water/oil boundary is observed after 0.2 ns. After 1 ns, perfect separation of water/oil with a complete lining of phenol at the water/oil boundary is observed, and the calcite surface becomes oil-wet. Phenol molecules enclose decane molecules at the water–decane boundary preventing water from repelling decane molecules from the calcite surface and facilitate further accumulation of hydrocarbons near the surface, rendering the surface oil-wet. This study indicates phenol and 1-naphthol to be good proxies for polar components in oil, and they can be used in designing further experiments to test pH, salinity, and temperature dependence to improve oil recovery

Effects of Hydroxyl and Carboxyl Functional Groups on Calcite Surface Wettability Using Atomic Force Microscopy and Density Functional Theory

Surface-active compounds, primarily in asphaltene fractions of crude oil, are responsible for binding the nonpolar oil components to mineral surfaces and, therefore, control wettability changes on reservoir rock/mineral surfaces. Surface wettability changes occur mainly through polar functional groups in these compounds, such as hydroxyl, carboxyl, or carbonyl. By using crude oil with its asphaltene fraction removed, so-called maltenes, we investigate the effect of hydroxyl and carboxyl functional groups on wettability changes of calcite surfaces. Atomic force microscopy (AFM) images show significantly increased adsorption of maltenes on calcite samples treated with two asphaltene surrogates (phenol with a hydroxyl group and benzoic acid with a carboxyl one) than that on water-treated samples. However, the adsorbate patterns are different between those two asphaltene surrogates, suggesting different aggregation mechanisms. In addition, we observed the formation of larger surface-adsorbed droplets on the phenol or benzoic acid-treated calcite samples even for relatively short exposure times (<30 min) to maltenes. Quantum-mechanical calculations show more favorable adsorption for benzoic acid onto the calcite surface both on terraces and step edges. However, when a model oil molecule adsorbs onto those two preadsorbed asphaltene surrogates, nonpolar oil molecules preferentially adsorb onto phenol on terrace sites and benzoic acid on step edges. Overall, benzoic acid changes the calcite surface wettability more significantly than phenol

Charge Carrier Dynamics in TiO₂ Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation

There is a great concern about black TiO₂ prepared by H₂ treatment because of its ability to enhance light harvesting of TiO₂. Black TiO₂ shows different photocatalytic activities compared with white TiO₂. However, the mechanism of photocatalytic reaction has not been clearly understood. Here, femtosecond time-resolved diffuse reflectance (fs-TRDR) spectroscopy and single-particle photoluminescence measurements were applied to gain better understanding about the relation between oxygen vacancy, charge transfer, lifetime of photogenerated charge, and photocatalytic activity. We prepared reduced TiO₂ mesocrystals (R-TMC) through simple solid-state chemical reduction at moderate temperature 350 °C. R-TMC has nearly two times higher photocatalytic activity for H₂ production under solar light irradiation. The presence of oxygen vacancies and Ti³⁺ was studied by electro paramagnetic resonance and X-ray photoelectron spectroscopy. This work confirms the findings of previous studies that enhancement of light absorption by formation of surface defects does not always lead to high photocatalytic activity. Femtosecond time-resolved diffuse reflectance (fs-TRDR) spectra reveal that proper concentration of oxygen vacancies enhances the charge separation of the photogenerated carriers, leading to high photocatalytic activity

Far-Red Fluorescence Probe for Monitoring Singlet Oxygen during Photodynamic Therapy

Singlet oxygen (¹O₂), molecular oxygen in the lowest excited state, has a critical role in the cell-killing mechanism of photodynamic therapy (PDT). Although ¹O₂ phosphorescence measurement has been mainly used to monitor ¹O₂ formation during PDT, its intensity is far insufficient to obtain two-dimensional images of intracellular ¹O₂ with the subcellular spatial resolution using the currently available near-IR detector. Here, we propose a new far-red fluorescence probe of ¹O₂, namely, Si-DMA, composed of silicon-containing rhodamine and anthracene moieties as a chromophore and a ¹O₂ reactive site, respectively. In the presence of ¹O₂, fluorescence of Si-DMA increases 17 times due to endoperoxide formation at the anthracene moiety. With the advantage of negligible self-oxidation by photoirradiation (Φ_Δ < 0.02) and selective mitochondrial localization, Si-DMA is particularly suitable for imaging ¹O₂ during PDT. Among three different intracellular photosensitizers (Sens), Si-DMA could selectively detect the ¹O₂ that is generated by 5-amino­levulinic acid-derived protoporphyrin IX, colocalized with Si-DMA in mitochondria. On the other hand, mitochondria-targeted KillerRed and lysosomal porphyrins could not induce fluorescence change of Si-DMA. This surprising selectivity of Si-DMA response depending on the Sens localization and photosensitization mechanism is caused by a limited intracellular ¹O₂ diffusion distance (∼300 nm) and negligible generation of ¹O₂ by type-I Sens, respectively. For the first time, we successfully visualized ¹O₂ generated during PDT with a spatial resolution of a single mitochondrial tubule

pH-Induced Intramolecular Folding Dynamics of i-Motif DNA

Using the combination of fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) technique, we investigate the mechanism and dynamics of the pH-induced conformational change of i-motif DNA in the bulk phases and at the single-molecule level. Despite numerous studies on i-motif that is formed from cytosine (C)-rich strand at slightly acidic pH, its detailed conformational dynamics have been rarely reported. Using the FRET technique to provide valuable information on the structure of biomolecules such as a protein and DNA, we clearly show that the partially folded species as well as the single-stranded structure coexist at neutral pH, supporting that the partially folded species may exist substantially in vivo and play an important role in a process of gene expression. By measuring the FCS curves of i-motif, we observed the gradual decrease of the diffusion coefficient of i-motif with increasing pH. The quantitative analysis of FCS curves supports that the gradual decrease of diffusion coefficient (D) associated with the conformational change of i-motif is not only due to the change in the intermolecular interaction between i-motif and solvent accompanied by the increase of pH but also due to the change of the shape of DNA. Furthermore, FCS analysis showed that the intrachain contact formation and dissociation for i-motif are 5–10 times faster than that for the open form. The fast dynamics of i-motif with a compact tetraplex is due to the intrinsic conformational changes at the fluorescent site including the motion of alkyl chain connecting the dye to DNA, whereas the slow intrachain contact formation observed from the open form is due to the DNA motion corresponding to an early stage interaction in the folding process of the unstructured open form