Ultra-high dose-rate (FLASH) radiotherapy: Generation of early, transient, strongly acidic spikes in the irradiated tumor environment.

Monte Carlo simulations of γ/fast electron-radiolysis of water show that the in situ formation of H3O+ temporarily renders each "native" isolated spur/track region very acidic. For pulsed (FLASH) irradiation with high dose rate, this early time, transient "acid-spike" response is shown to extend evenly across the entire irradiated volume. Since pH controls many cellular processes, this study highlights the need to consider these spikes of acidity in understanding the fundamental mechanisms underlying FLASH radiotherapy.

Solar photochemical and thermochemical splitting of water.

Artificial photosynthesis to carry out both the oxidation and the reduction of water has emerged to be an exciting area of research. It has been possible to photochemically generate oxygen by using a scheme similar to the Z-scheme, by using suitable catalysts in place of water-oxidation catalyst in the Z-scheme in natural photosynthesis. The best oxidation catalysts are found to be Co and Mn oxides with the e(1) g configuration. The more important aspects investigated pertain to the visible-light-induced generation of hydrogen by using semiconductor heterostructures of the type ZnO/Pt/Cd1-xZnxS and dye-sensitized semiconductors. In the case of heterostructures, good yields of H2 have been obtained. Modifications of the heterostructures, wherein Pt is replaced by NiO, and the oxide is substituted with different anions are discussed. MoS2 and MoSe2 in the 1T form yield high quantities of H2 when sensitized by Eosin Y. Two-step thermochemical splitting of H2O using metal oxide redox pairs provides a strategy to produce H2 and CO. Performance of the Ln0.5A0.5MnO3 (Ln = rare earth ion, A = Ca, Sr) family of perovskites is found to be promising in this context. The best results to date are found with Y0.5Sr0.5MnO3.

LIGHT-SABRE enables efficient in-magnet catalytic hyperpolarization.

Nuclear spin hyperpolarization overcomes the sensitivity limitations of traditional NMR and MRI, but the most general method demonstrated to date (dynamic nuclear polarization) has significant limitations in scalability, cost, and complex apparatus design. As an alternative, signal amplification by reversible exchange (SABRE) of parahydrogen on transition metal catalysts can hyperpolarize a variety of substrates, but to date this scheme has required transfer of the sample to low magnetic field or very strong RF irradiation. Here we demonstrate "Low-Irradiation Generation of High Tesla-SABRE" (LIGHT-SABRE) which works with simple pulse sequences and low power deposition; it should be usable at any magnetic field and for hyperpolarization of many different nuclei. This approach could drastically reduce the cost and complexity of producing hyperpolarized molecules.

Sunlight-promoted photocatalytic hydrogen gas evolution from water-suspended cellulose: a systematic study.

This work presents a systematic study of cellulose (CLS) as a sacrificial biomass for photocatalytic H2 evolution from water. The idea is indeed to couple a largely available and not expensive biomass, and water, with a renewable energy like solar radiation. An aqueous CLS suspension irradiated either at 366 nm (UV-A) or under sunlight in the presence of Pt/TiO2 behaves as a H2 evolving system. The effects of irradiation time, catalyst and CLS concentrations, pH and water salinity are studied. Addition of CLS to the sample significantly improved H2 evolution from water splitting, with yields up to ten fold higher than those observed in neat water. The mechanism of the photocatalytic process relies on the TiO2-mediated CLS hydrolysis, under irradiation. The polysaccharide depolymerisation generates water-soluble species and intermediates, among them 5-hydroxymethylfurfural (HMF) was identified. These intermediates are readily oxidized following the glucose photoreforming, thus enhancing water hydrogen ion reduction to give gas-phase H2. The formation of "colored" by-products from HMF self-polymerization involves a sort of "in situ dye sensitization" that allows an effective photoreaction even under solar light. The procedure is evaluated and successfully extended on cellulosic biomasses, i.e. rice husk and alfalfa (Medicago sativa) stems, not previously investigated for this application.

Mechanistic modeling of destratification in cryogenic storage tanks using ultrasonics.

Stratification is one of the main causes for vaporization of cryogens and increase of tank pressure during cryogenic storage. This leads subsequent problems such as cavitation in cryo-pumps, reduced length of storage time. Hence, it is vital to prevent stratification to improve the cost efficiency of storage systems. If stratified layers exist inside the tank, they have to be removed by suitable methods without venting the vapor. Sonication is one such method capable of keeping fluid layers mixed. In the present work, a mechanistic model for ultrasonic destratification is proposed and validated with destratification experiments done in water. Then, the same model is used to predict the destratification characteristics of cryogenic liquids such as liquid nitrogen (LN2), liquid hydrogen (LH2) and liquid ammonia (LNH3). The destratification parameters are analysed for different frequencies of ultrasound and storage pressures by considering continuous and pulsed modes of ultrasonic operation. From the results, it is determined that use of high frequency ultrasound (low-power/continuous; high-power/pulsing) or low frequency ultrasound (continuous operation with moderate power) can both be effective in removing stratification.

Optical absorption characteristics of nanometer and submicron a-Si:H solar cells with two kinds of nano textures.

The optical absorption properties of a-Si:H have acquired much attention in solar cell(SC) research. In this paper, we studied enhancement of light absorption in the a-Si:H(10%H) SCs with thicknesses from 31.25nm to 2µm and with nano textures of the column-shaped nanohole (CLNH) array and of the cone-shaped nanohole (CNNH) array, via the Finite Difference Time Domain (FDTD) simulation. For a given type of nano texture and film thickness, d, the ultimate efficiency, the ideal efficiency without considering carrier combinations, is optimized over array period, p, and filling fraction, f, and is defined as the optimized ultimate efficiency, η(0). The simulation results demonstrated that: even for the CLNH textured a-Si:H(10%H) SCs as thin as 62.5 nm,η(0) is 19.7%. When the a-Si:H(10%H) SC is thinner than a critical depth of about 250nm, the CLNH texture is more efficient than the CNNH texture, and vice versa. When the thicknesses of SCs are very thin, especially smaller than 100nm, the efficiencies of the a-Si:H(10%H) SCs are evidently higher than those of the c-Si SCs. For example, in the CLNH arrays, when d = 62.5nm, η(0)for the a-Si:H(10%H) SCs is higher than the c-Si SCs by a factor of approximate 2.3.

Graphene-based materials for hydrogen generation from light-driven water splitting.

Hydrogen production from solar water splitting has been considered as an ultimate solution to the energy and environmental issues. Over the past few years, graphene has made great contribution to improving the light-driven hydrogen generation performance. This article provides a comprehensive overview of the recent research progress on graphene-based materials for hydrogen evolution from light-driven water splitting. It begins with a brief introduction of the current status and basic principles of hydrogen generation from solar water splitting, and tailoring properties of graphene for application in this area. Then, the roles of graphene in hydrogen generation reaction, including an electron acceptor and transporter, a cocatalyst, a photocatalyst, and a photosensitizer, are elaborated respectively. After that, the comparison between graphene and other carbon materials in solar water splitting is made. Last, this review is concluded with remarks on some challenges and perspectives in this emerging field.

Recent advances in visible-light-responsive photocatalysts for hydrogen production and solar energy conversion--from semiconducting TiO2 to MOF/PCP photocatalysts.

The present perspective describes recent advances in visible-light-responsive photocatalysts intended to develop novel and efficient solar energy conversion technologies, including water splitting and photofuel cells. Water splitting is recognized as one of the most promising techniques to convert solar energy as a clean and abundant energy resource into chemical energy in the form of hydrogen. In recent years, increasing concern is directed to not only the development of new photocatalytic materials but also the importance of technologies to produce hydrogen and oxygen separately. Photofuel cells can convert solar energy into electrical energy by decomposing bio-related compounds and livestock waste as fuels. The advances of photocatalysts enabling these solar energy conversion technologies have been going on since the discovery of semiconducting titanium dioxide materials and have extended to organic-inorganic hybrid materials, such as metal-organic frameworks and porous coordination polymers (MOF/PCP).

Self-organization of ascending-bubble ensembles.

Self-organization of hydrogen bubbles generated by laser-treated areas of an aluminum plate etched in a basic aqueous solution of ammonia is studied experimentally and theoretically. The dynamics of the establishment of a stationary pattern of gas bubbles is experimentally is shown. In the theoretical model, the velocity field of liquid flows around an ensemble of several bubbles is obtained. Modeling of the process of self-organization of gas bubbles is performed on the basis of a continuum model of a bubble jet. Under certain assumptions, the pressure of a diluted system of bubbles is described by an equation similar to that for nonideal gas, which follows the van der Waals equation of state. The model predicts an alignment of gas bubbles along bisectors of the laser-treated area limited by a square, which is in good agreement with experimental observations. Further development of the model leads to an equation with a negative diffusion coefficient that may be responsible for symmetry breakdown and pattern formation.

Anomalous isotopic effect on electron-directed reactivity by a 3-µm midinfrared pulse.

We have theoretically studied the effect of nuclear mass on electron localization in dissociating H2⁺ and its isotopes subjected to a few-cycle 3-µm pulse. Our results reveal an anomalous isotopic effect in which the degree of electron-directed reactivity can be even higher for heavier isotopes in the intense midinfrared field. We show, for the first time, the pronounced electron localization can be established through the interferences among the multi-photon coupling channels. Due to the relative enhancement of higher-order coupling channels with growing mass, the interference maxima at different kinetic energy of the spectra gradually become in phase, ultimately resulting in the larger dissociation asymmetries of heavier isotopes.

Enhanced water splitting by Fe2O3-TiO2-FTO photoanode with modified energy band structure.

The effect of TiO2 layer applied to the conventional Fe2O3/FTO photoanode to improve the photoelectrochemical performance was assessed from the viewpoint of the microstructure and energy band structure. Regardless of the location of the TiO2 layer in the photoanodes, that is, Fe2O3/TiO2/FTO or TiO2/Fe2O3/FTO, high performance was obtained when α-Fe2O3 and H-TiNT/anatase-TiO2 phases existed in the constituent Fe2O3 and TiO2 layers after optimized heat treatments. The presence of the Fe2O3 nanoparticles with high uniformity in the each layer of the Fe2O3/TiO2/FTO photoanode achieved by a simple dipping process seemed to positively affect the performance improvement by modifying the energy band structure to a more favorable one for efficient electrons transfer. Our current study suggests that the application of the TiO2 interlayer, together with α -Fe2O3 nanoparticles present in the each constituent layers, could significantly contribute to the performance improvement of the conventional Fe2O3 photoanode.

Nanoscale strontium titanate photocatalysts for overall water splitting.

SrTiO(3) (STO) is a large band gap (3.2 eV) semiconductor that catalyzes the overall water splitting reaction under UV light irradiation in the presence of a NiO cocatalyst. As we show here, the reactivity persists in nanoscale particles of the material, although the process is less effective at the nanoscale. To reach these conclusions, Bulk STO, 30 ± 5 nm STO, and 6.5 ± 1 nm STO were synthesized by three different methods, their crystal structures verified with XRD and their morphology observed with HRTEM before and after NiO deposition. In connection with NiO, all samples split water into stoichiometric mixtures of H(2) and O(2), but the activity is decreasing from 28 µmol H(2) g(-1) h(-1) (bulk STO), to 19.4 µmol H(2) g(-1) h(-1) (30 nm STO), and 3.0 µmol H(2) g(-1) h(-1) (6.5 nm STO). The reasons for this decrease are an increase of the water oxidation overpotential for the smaller particles and reduced light absorption due to a quantum size effect. Overall, these findings establish the first nanoscale titanate photocatalyst for overall water splitting.

A model study on the possible effects of an external electrical field on enzymes having dinuclear iron cluster [2Fe-2S].

Hydrogenases which catalyze the H(2)↔ 2H(+) + 2e(-) reaction are metalloenzymes that can be divided into two classes, the NiFe and Fe enzymes, on the basis of their metal content. Iron-sulfur clusters [2Fe-2S] and [4Fe-4S] are common in ironhydrogenases. In the present model study, [2Fe-2S] cluster has been considered to visualize the effect of external electric field on various quantum chemical properties of it. In the model, all the cysteinyl residues are in the amide form. The PM3 type semiempirical calculations have been performed for the geometry optimization of the model structure in the absence and presence of the external field. Then, single point DFT calculations (B3LYP/6-31+G(d)) have been carried out. Depending on the direction of the field, the chemical reactivity of the model enzyme varies which suggests that an external electric field could, under proper conditions, improve the enzymatic hydrogen production.

Photochemical production of hydrogen from water using microporous porphyrin coordination lattices.

In this study, we investigated the photochemical production of hydrogen from water using bio-inspired heterogeneous microporous porphyrin coordination lattices (PCLs), [Ru2(MTCPP)BF4] (M = H2 (PCL-1), Zn (PCL-2); TCPP = Tetrakis(4-carboxyphenyl)porphyrin), under visible (380 nm <) and UV (320 nm <) light irradiations. In the presence of Na2EDTA (as a sacrificial donor) and MV2+ (methyl-vilologen; as a electron relay), PCLs exhibits photocatalytic activity for hydrogen evolution; the maximum amounts of turnover numbers (TONs) of PCL-1 and PCL-2 at 24 h irradiation were 20.8 and 29.9, respectively. In the catalytic reactions, the relation between PCLs and MV2+ was similar to the relation between a [cytochrome c3 hydrogenase] pair and lysine residues in enzymatic reactions. By using the hydrogen production rate and the MV+ (methyl-vilologen radical-cation) concentration, kinetic parameters such as affinities between MV+ and PCLs, maximum reaction rate, and total efficiency of the reaction are introduced using the Michaelis-Menten equation. These parameters indicated that PCLs are good artificial enzyme model catalysts. The stability of the PCLs after the catalytic reactions was confirmed by X-ray photoelectron spectroscopy and Fourier transform-infrared spectra. These results indicated that the frameworks of PCLs are stable for this catalytic reaction.

EELS characterization of radiolytic products in frozen samples.

Electron energy loss spectroscopy (EELS) was used to obtain information about the radiation chemistry of frozen aqueous specimens in the electron microscope by observing the hydrogen and oxygen K-edges. Measurements on frozen solutions of 30% hydrogen peroxide revealed the presence of molecular oxygen identified by a distinct 531-eV peak at the O K-edge even for electron doses below 100 e/nm². The molecular oxygen content of irradiated H2O2 solution was determined by least squares fitting of O K-edge reference spectra from water and gas-phase oxygen. It was found that the fraction of molecular oxygen to water oxygen was in the range 0.03-0.05. EELS from pure frozen water showed no features attributable to molecular oxygen or molecular hydrogen (K edge at ~13 eV) even at high electron doses above 105 e/nm². Spectra from frozen sucrose and protein solutions and their mixtures, however, did show evolution of a molecular hydrogen peak at ~13 eV for doses above 105 e/nm², consistent with previous measurements and indicative of hydrogen bubble formation. Molecular oxygen was not observed in any of the frozen solutions of organic compounds indicating that oxygen is not a major product of free radical decay, in contrast to molecular hydrogen formation.

The conical intersection dominates the generation of tropospheric hydroxyl radicals from NO2 and H2O.

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO(2) and H(2)O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO(2) irradiated at approximately 410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle of NO(2). In the former case, the conical intersection between B(2)B(1) and A(2)B(2) (CI ((2)B(2)/(2)B(1)) first funnels NO(2) out of the Franck-Condon region of B(2)B(1) and relaxes to the A(2)B(2) surface. Following the primary relaxation, the conical intersection between A(2)B(2) and X(2)A(1) (CI((2)B(2)/(2)A(1))) drives NO(2) to decay into highly vibrationally excited X(2)A(1) state that is more than 20,000 cm(-1) above zeroth-order |n(1),n(2),n(3) = 0 vibrational level. In the latter case, increasing the O-N-O angle leads NO(2) to relax to a minimum of B(2)B(1) with a linear O-N-O arrangement. This minimum point is also funnel region between B(2)B(1) and X(2)A(1) (CI((2)B(1)/(2)A(1))) and leads NO(2) to relax into a highly vibrationally excited X(2)A(1) state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor, producing OH ((2)Pi(3/2)) radicals. The collision between NO(2) and H(2)O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited X(2)A(1) states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO(2) and H(2)O in the troposphere.

Selective hydrogenation by polymer-encapsulated platinum nanoparticles prepared by an easy single-step sonochemical synthesis.

Polypyrrole-encapsulated platinum nanoparticles (PPy/Pt-NPs) prepared by an easy single-step sonochemical synthesis were used as catalysts for the liquid phase hydrogenation of substituted alkenes in methanol or methanol/water mixtures. Polypyrrole (PPy) coatings on the nanoparticles were able to act as nanoscopic filters for substrates molecules, and consequently substrate selectivity could be controlled in the catalytic processes.

Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins.

OBJECTIVE: The aim of this study was to compare the photobleaching rate of CQ in different dental resins. METHODS: The photodecomposition rate of CQ/amine system in bis-GMA/TEGDMA, bis-EMA and UDMA polymerizing monomers was evaluated at different light intensities. The photobleaching of the CQ was studied by monitoring the decrease in light absorption as a function of continuous irradiation time. The absorption changes were assessed by recording the transmitted light that passed through samples of monomers containing CQ/amine. RESULTS: Complete photobleaching of CQ was observed in all the monomer tested and the rate constant for the photobleaching was proportional to the radiation intensity. Hydrogen abstraction from amines by the excited CQ state via electron transfer and direct hydrogen abstraction from monomer structures were involved in the CQ photoreduction. CQ was photobleached in the absence of coinitiator in a dimethacrylate monomer containing a carbamate functional group (UDMA). This behavior was attributed to the presence of labile hydrogen atoms in the UDMA monomer. The CQ photobleaching rate constant in UDMA containing CQ/amine was similar to that in UDMA in the absence of amine. Moreover, the efficiency of CQ to photoinitiate the polymerization of UDMA in the absence of amine demonstrated that the radicals derived from the UDMA monomer via hydrogen abstraction are highly reactive toward double bonds. SIGNIFICANCE: CQ photoinitiates the polymerization of the UDMA monomer in the absence of amine and the efficiency of this process is comparable to that of traditional bis-GMA and bis-EMA monomers activated with CQ/amine.

Effect of low and zero magnetic field on the hyperpolarization lifetime in parahydrogenated perdeuterated molecules.

Hyperpolarization can be kept for times longer than T(1) if it is maintained in a singlet state, from which transitions are not allowed. Another, more direct, way to slower the relaxation process consists in the use of perdeuterated molecules. Here both methods have been applied and the hyperpolarization (induced by para-H(2)) decay rate has been measured at two different magnetic fields: earth field and zero field. While relaxation is very slow at earth field, it becomes faster at zero field: this rather unexpected finding has been explained on the basis of isotropic mixing between (1)H and (2)H.

Iron-doped Pt-TiO2 nanotubes for photo-catalytic water splitting.

In this work we report on the photo-catalytic performance of phase-pure and iron-doped anatase and rutile nanotubes, produced via a sol-gel process using pristine carbon nanotubes as templates. The encapsulated iron residues can be used to in situ dope the TiO(2) nanotubes without phase separation. The anatase and rutile nanotubes were further impregnated with platinum crystals with a uniform dispersion and an average size of approximately 2 nm. The materials showed dramatically improved activities for the photo-catalytic splitting of water compared to commercial TiO(2) with similar surface area (up to two orders of magnitudes), due to their higher illumination area, extended absorption range and reduced electron-hole recombination rate. The homogeneous dispersion of platinum nanoparticles further increased the hydrogen evolution rate for anatase nanotubes by a factor of seven in comparison to that for the pristine material, thus proving the great potential for commercial applications.