From photons to biomass and biofuels: evaluation of different strategies for the improvement of algal biotechnology based on comparative energy balances

Microalgal based biofuels are discussed as future sustainable energy source because of their higher photosynthetic and water use efficiency to produce biomass. In the context of climate CO₂ mitigation strategies, algal mass production is discussed as a potential CO₂ sequestration technology which uses CO₂ emissions to produce biomass with high-oil content independent on arable land. In this short review, it is presented how complete energy balances from photon to harvestable biomass can help to identify the limiting processes on the cellular level. The results show that high productivity is always correlated with high metabolic costs. The overall efficiency of biomass formation can be improved by a photobioreactor design which is kinetically adapted to the rate-limiting steps in cell physiology. However, taking into account the real photon demand per assimilated carbon and the energy input for biorefinement, it becomes obvious that alternative strategies must be developed to reach the goal of a real CO₂ sequestration.     

Application of Plasmonics in Solar Cell Efficiency Improvement: a Brief Review on Recent Progress

We need electrical energy for our various daily life activities, and the existing fossil fuel will serve the purpose for a limited period only. Therefore, we need to look at alternate energy resources for long-term sustainable growth. Solar energy has been considered as one of the potential viable alternate sources of non-conventional energy. Solar photon harvesting is a technique where solar photon energy is being converted to electrical energy using solar cell devices. However, the solar power conversion efficiency is usually seen to be poor limiting the applications to large extent. Improving solar cell efficiency has, thus, been a great challenge to researchers who are actively working in this field. Various technological aspects are being looked at to improve efficiency, and out of them, plasmonic light trapping technique has been considered as one of the promising techniques. In the past years, considerable research efforts are being made towards the design and execution of the solar cell devices and to understand the mechanisms of performance enhancement. The present review outlines briefly the recent progress of plasmonic solar cell efficiency improvement that has happened, especially, for last 5–6 years. Plasmonic solar cells made of Si (silicon), GaAs (gallium arsenide); plasmonic dye-sensitized solar cells; plasmonic polymer (organic) solar cells and perovskite solar cells are mainly discussed along with the critical aspects that may influence the device performance.

     

A surface energy spectral study on the bone heterogeneity and beam obliquity using the flattened and unflattened photon beams

Aim

Using flattened and unflattened photon beams, this study investigated the spectral variations of surface photon energy and energy fluence in the bone heterogeneity and beam obliquity.

Background

Surface dose enhancement is a dosimetric concern when using unflattened photon beam in radiotherapy. It is because the unflattened photon beam contains more low-energy photons which are removed by the flattening filter of the flattened photon beam.

Materials and methods

We used a water and bone heterogeneity phantom to study the distributions of energy, energy fluence and mean energy of the 6 MV flattened and unflattened photon beams (field size = 10 cm × 10 cm) produced by a Varian TrueBEAM linear accelerator. These elements were calculated at the phantom surfaces using Monte Carlo simulations. The photon energy and energy fluence calculations were repeated with the beam angle turned from 0° to 15°, 30° and 45° in the water and bone phantom.

Results

Spectral results at the phantom surfaces showed that the unflattened photon beams contained more photons concentrated mainly in the low-energy range (0–2 MeV) than the flattened beams associated with a flattening filter. With a bone layer of 1 cm under the phantom surface and within the build-up region of the 6 MV photon beam, it is found that both the flattened and unflattened beams had slightly less photons in the energy range <0.4 MeV compared to the water phantom. This shows that the presence of the bone decreased the low-energy photon backscatters to the phantom surface. When both the flattened and unflattened photon beams were rotated from 0° to 45°, the number of photon and mean photon energy increased. This indicates that both photon beams became more hardened or penetrate when the beam angle increased. In the presence of bone, the mean energies of both photon beams increased. This is due to the absorption of low-energy photons by the bone, resulting in more beam hardening.

Conclusions

This study explores the spectral relationships of surface photon energy and energy fluence with bone heterogeneity and beam obliquity for the flattened and unflattened photon beams. The photon spectral information is important in studies on the patient''s surface dose enhancement using unflattened photon beams in radiotherapy.     

Photon beam energy dependent single-arc volumetric modulated arc optimization

PurposeThe purpose of this work was to present a new single-arc mixed photon (6&18MV) VMAT (SAMP) optimization framework that concurrently optimizes for two photon energies with corresponding partial arc lengths.Methods and materialsOwing to simultaneous optimization of energy dependent intensity maps and corresponding arc locations, the proposed model poses nonlinearity. Unique relaxation constraints based on McCormick approximations were introduced for linearization. Energy dependent intensity maps were then decomposed to generate apertures. Feasibility of the proposed framework was tested on a sample of ten prostate cancer cases with lateral separation ranging from 34 cm (case no.1) to 52 cm (case no.6). The SAMP plans were compared against single energy (6MV) VMAT (SE) plans through dose volume histograms (DVHs) and radiobiological parameters including normal tissue complication probability (NTCP) and equivalent uniform dose (EUD).ResultsThe contribution of higher energy photon beam optimized by the algorithm demonstrated an increase for cases with a lateral separation >40 cm. SAMP–VMAT notably improved bladder and rectum sparing in large size cases. Compared to single energy, SAMP–VMAT plans reduced bladder and rectum NTCP in cases with large lateral separation. With the exception of one case, SAMP–VMAT either improved or maintained femoral heads compared to SE–VMAT. SAMP–VMAT reduced the nontarget tissue integral dose in all ten cases.ConclusionsA single-arc VMAT optimization framework comprising mixed photon energy partial arcs was presented. Overall results underline the feasibility and potential of the proposed approach for improving OAR sparing in large size patients without compromising the target homogeneity and coverage.     

Induction of incomplete and complex chromosome aberrations by 30 kVp X rays

X rays of 26-30 kVp are routinely used for mammography screening. For radioprotection purposes, a quality factor (Q) of 1 is assumed for all photon energies, but it is thought that the relative biological effectiveness (RBE) increases as the photon energy decreases. The analysis of radiation-induced chromosome aberrations is one of the most widely used methods to study the interaction between radiation and DNA. Here we present a FISH study on metaphases from peripheral blood samples irradiated with three different X-ray energies (30, 80 and 120 kVp). The study comprises two FISH approaches: one using pantelomeric and pancentromeric probes to evaluate the induction of incomplete chromosome aberrations and the other using mFISH to evaluate the induction of complex chromosome aberrations. The results indicate that exposure to 30 kVp X rays resulted in a modest increase in the induction of incomplete elements and complex aberrations compared to 80 and 120 kVp X rays.     

A Silicon Ratchet to Produce Power from Below‐Bandgap Photons

This paper computationally demonstrates a new photovoltaic mechanism that generates power from incoherent, below‐bandgap (THz) excitations of conduction band electrons in silicon. A periodic sawtooth potential, realized through elastic strain gradients along a 100 nm thick Si slab, biases the oscillatory motion of excited electrons, which preferentially jump and relax into the adjacent period on the right to generate a net current. The magnitude of the ratchet current increases with photon energy (20, 50, and 100 meV) and irradiance (≈MW cm^?2), which control the probability of photon scattering, and peaks as a function of the well depth of the ratchet potential, and the dominant mode of energy loss (the 62 meV intervalley phonon). The internal power conversion efficiency of the ratchet has a maximum of 0.0083% at a photon energy of 100 meV, due to inefficiencies caused by isotropic scattering. This new photovoltaic mechanism uses wasted below‐bandgap absorptions to enhance the directional diffusion of charge carriers and could be used to augment the efficiency of traditional photovoltaics.     

Contribution of indirect action to radiation-induced mammalian cell inactivation: dependence on photon energy and heavy-ion LET

The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/microm to 440 keV/microm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/microm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly constant for photon inactivation, while for the heavy ions, the rate increased with increasing LET, suggesting a reaction with the densely produced OH radicals by high-LET ions. Using the protectable fraction, the cell killing was separated into two components, one due to indirect action and the other due to direct action. The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested. A significant contribution of direct action was also found for the increased RBE in the low photon energy region.     

"Either-or" two-slit interference: stable coherent propagation of individual photons through separate slits

In quantum theory, nothing that is observable, be it physical, chemical, or biological, is separable from the observer. Furthermore, ". all possible knowledge concerning that object is given by its wave function" (Wigner, E. 1967. Symmetries and Reflections. Indiana University Press, Bloomington, IN), which can only describe probabilities of future events. In physical systems, quantum mechanical probabilistic events that are microscopic must, in turn, account for macroscopic events that are associated with a greater degree of certainty. In biological systems, probabilistic statistical mechanical events, such as secretion of microscopic synaptic vesicles, must account for macroscopic postsynaptic potentials; probabilistic single-channel events sum to produce a macroscopic ionic current across a cell membrane; and bleaching of rhodopsin molecules (responsible for quantal potential "bumps") produces a photoreceptor generator potential. Among physical systems, a paradigmatic example of how quantum theory applies to the observation of events concerns the interactions of particles (e.g., photons, electrons) with the two-slit apparatus to generate an interference pattern from a single common light source. For two-slit systems that use two independent laser sources with brief (<1 ms) intervals of mutual coherence (Paul, H. 1986. Rev. Modern Phys. 58:209-231), each photon has been considered to arise from both beams and has a probability amplitude to pass through each of the two slits. Here, a single laser source two-slit interference system was constructed so that each photon has a probability amplitude to pass through only one or the other, but not both slits. Furthermore, all photons passing through one slit could be distinguished from all photons passing through the other slit before their passage. This "either-or" system produced a stable interference pattern indistinguishable from the interference produced when both slits were accessible to each photon. Because this system excludes the interaction of one photon with both slits, phase correlation of photon movements derives from the "entanglement" of all photon wave functions due to their dependence on a common laser source. Because a laser source (as well as Young's original point source) will have stable time-averaged spatial coherence even at low intensities, the "either-or" two-slit interference can result from distinct individual photons passing one at a time through one or the other slit-rather than wave-like behavior of individual photons. In this manner, single, successive photons passing through separate slits will assemble over time in phase-correlated wave distributions that converge in regions of low and high probability.     

The Dominant Energy Transport Pathway in Halide Perovskites: Photon Recycling or Carrier Diffusion?

Photon recycling and carrier diffusion are the two plausible processes that primarily affect the carrier dynamics in halide perovskites, and therefore the evaluation of the performance of their photovoltaic and photonic devices. However, it is still challenging to isolate their individual contributions because both processes result in a similar emission redshift. Herein, it is confirmed that photon recycling is the dominant effect responsible for the observed redshifted emission. By applying one‐ and two‐photon confocal emission microscopy on Ruddlesden–Popper type 2D perovskites, of which interplane carrier diffusion is strictly suppressed, the substantial PL redshift (72 meV) is well reproduced by the photon transport model. A comparison of 3D bulk CH₃NH₃PbBr₃ single crystal to 2D perovskite by depth‐resolved two‐photon PL spectra reveals the contribution of carrier diffusion on energy transport at a distance beyond diffusion length is constantly negligible, though the carrier diffusion indeed exists in the 3D crystal. The investigation resolves the fundamental confusion and debate surrounding the issue and provides significant insights into carrier kinetics in perovskites, which is important for future developments in solar cells and other optoelectronic devices.     

Epidermal diffusive conductance as related to leaf water potential and photon flux density

Response of epidermal diffusive conductance to simultaneous changes in leaf water potential and photon flux density was studied in primary bean leaves. Values of epidermal conductance corresponding to every photon flux density decreased with decreasing leaf water potential below - 6.9 x 10⁵Pa; slight deorease was followed by a rapid one at water potential ranging from - 8.0 to -10.5 x 10⁵ Pa. In the leaves with water potential lower than -10.5 x 10⁵ Pa neither the saturated photon flux density (1200 [xeinstein m^-2s^-1) induced photoactive stomatal opening. Negative influence of one factor could be partially compensated by positive influence of the other. These results were in good agreement with the considered mechanism of action of leaf water potential and photon flux density on epidermal conductance.     

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (ω) and momentum (k) and the probability to find them in a particular state with given ω and k is described by the spectral function A(k, ω). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He³ cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr₂RuO₄ and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.     

Learning continuous potentials from smFRET

Potential energy landscapes are useful models in describing events such as protein folding and binding. While single-molecule fluorescence resonance energy transfer (smFRET) experiments encode information on continuous potentials for the system probed, including rarely visited barriers between putative potential minima, this information is rarely decoded from the data. This is because existing analysis methods often model smFRET output assuming, from the onset, that the system probed evolves in a discretized state space to be analyzed within a hidden Markov model (HMM) paradigm. By contrast, here, we infer continuous potentials from smFRET data without discretely approximating the state space. We do so by operating within a Bayesian nonparametric paradigm by placing priors on the family of all possible potential curves. As our inference accounts for a number of required experimental features raising computational cost (such as incorporating discrete photon shot noise), the framework leverages a structured-kernel-interpolation Gaussian process prior to help curtail computational cost. We show that our structured-kernel-interpolation priors for potential energy reconstruction from smFRET analysis accurately infers the potential energy landscape from a smFRET binding experiment. We then illustrate advantages of structured-kernel-interpolation priors for potential energy reconstruction from smFRET over standard HMM approaches by providing information, such as barrier heights and friction coefficients, that is otherwise inaccessible to HMMs.     

Cage opening and fragmentation of C60 fullerene induced by an ultrashort laser pulse

Response of C₆₀ fullerene to a 40 fs full-width at half-maximum laser pulse with a photon energy of 2.0 eV and different laser intensities is studied by semiclassical dynamics simulation technique. The simulation results show that soon after the irradiation with a strong laser pulse, many C–C bonds abruptly break but no fragments are produced. The breaking of multiple C–C bonds induces a quick increase in the kinetic energy and potential energy and a decrease in electronic energy. These results suggest that the opening of the C₆₀ cage is an effective channel for the conversion of electronic energy to kinetic energy for the electronically excited C₆₀ fullerene.     

A conformational two-state peptide model system containing an ultrafast but soft light switch

Combining an azobenzene chromophore with the bis-cysteinyl active-site sequence of the protein disulfide isomerase (PDI) we constructed a simple but promising model for allosteric conformational rearrangements. Paralleling cellular signaling events, an external trigger, here absorption of a photon, leads to a structural change in one part of the molecule, namely the azobenzene-based chromophore. The change in geometry translates to the effector site, in our case the peptide sequence, where it modifies covalent and nonbonded interactions and thus leads to a conformational rearrangement. NMR spectroscopy showed that the trans-azo and cis-azo isomer of the cyclic PDI peptide exhibit different, but well-defined structures when the two cystine residues form a disulfide bridge. Without this intramolecular cross-link conformationally more variable structural ensembles are obtained that again differ for the two isomeric states. Ultrafast UV/Vis spectroscopy confirmed that the rapid isomerization of azobenzene is not significantly slowed down when incorporated into the cyclic peptides, although the amplitudes of ballistic and diffusive pathways are changed. The observation that most of the energy of an absorbed photon is dissipated to the solvent in the first few picoseconds when the actual azo-isomerization takes place is important. The conformational rearrangement is weakly driven due to the absence of appreciable excess energy and can be described as biased diffusion similar to natural processes.     

Impact of photon cross section uncertainties on Monte Carlo-determined depth-dose distributions

This work studies the impact of systematic uncertainties associated to interaction cross sections on depth dose curves determined by Monte Carlo simulations. The corresponding sensitivity factors are quantified by changing cross sections by a given amount and determining the variation in the dose. The influence of total and partial photon cross sections is addressed. Partial cross sections for Compton and Rayleigh scattering, photo-electric effect, and pair production have been accounted for. The PENELOPE code was used in all simulations. It was found that photon cross section sensitivity factors depend on depth. In addition, they are positive and negative for depths below and above an equilibrium depth, respectively. At this depth, sensitivity factors are null. The equilibrium depths found in this work agree very well with the mean free path of the corresponding incident photon energy. Using the sensitivity factors reported here, it is possible to estimate the impact of photon cross section uncertainties on the uncertainty of Monte Carlo-determined depth dose curves.     

Zeeman-Stark modeling of the RF EMF interaction with ligand binding

The influence of radiofrequency electromagnetic exposure on ligand binding to hydrophobic receptor proteins is a plausible early event of the interaction mechanism. A comprehensive quantum Zeeman-Stark model has been developed which takes into account the energy losses of the ligand ion due to its collisions inside the receptor crevice, the attracting nonlinear endogenous force due to the potential energy of the ion in the binding site, the out of equilibrium state of the ligand-receptor system due to the basal cell metabolism, and the thermal noise. The biophysical "output" is the change of the ligand binding probability that, in some instances, may be affected by a suitable low intensity exogenous electromagnetic "input" exposure, e.g., if the depth of the potential energy well of a putative receptor protein matches the energy of the radiofrequency photon. These results point toward both the possibility of the electromagnetic control of biochemical processes and the need for a new database of safety standards.     

Monte Carlo determination of a nanoDot OSLD response using quality index for diagnostic kilovoltage X-ray beams

PurposeThis study aims to investigate the energy response of an optically stimulated luminescent dosimeter known as nanoDot for diagnostic kilovoltage X-ray beams via Monte Carlo calculations.MethodsThe nanoDot response is calculated as a function of X-ray beam quality in free air and on a water phantom surface using Monte Carlo simulations. The X-ray fluence spectra are classified using the quality index (QI), which is defined as the ratio of the effective energy to the maximum energy of the photons. The response is calculated for X-ray fluence spectra with QIs of 0.4, 0.5, and 0.6 with tube voltages of 50–137.6 kVp and monoenergetic photon beams. The surface dose estimated using the calculated response is verified by comparing it with that measured using an ionization chamber.ResultsThe nanoDot response in free air for monoenergetic photon beams (QI = 1.0) varies significantly at photon energies below 100 keV and reaches a factor of 3.6 at 25–30 keV. The response differs by up to approximately 6% between QIs of 0.4 and 0.6 for the same half-value layer (HVL). The response at the phantom surface decreases slightly owing to the backscatter effect, and it is almost independent of the field size. The agreement between the surface dose estimated using the nanoDot and that measured using the ionization chamber for assessing X-ray beam qualities is less than 2%.ConclusionsThe nanoDot response is indicated as a function of HVL for the specified QIs, and it enables the direct surface dose measurement.     

Dual-color photon counting histogram analysis of mRFP1 and EGFP in living cells

We investigate the potential of dual-color photon counting histogram (PCH) analysis to resolve fluorescent protein mixtures directly inside cells. Because of their small spectral overlap, we have chosen to look at the fluorescent proteins EGFP and mRFP1. We experimentally demonstrate that dual-color PCH quantitatively resolves a mixture of EGFP and mRFP1 in cells from a single measurement. To mimic the effect of protein association, we constructed a fusion protein of EGFP and mRFP1 (denoted EGFP-mRFP1). Fluorescence resonant energy transfer within the fusion protein alters the dual-channel brightness of the fluorophores. We describe a model for fluorescence resonant energy transfer effects on the brightness and incorporate it into dual-color PCH analysis. The model is verified using fluorescence lifetime measurements. Dual-color PCH analysis demonstrated that not all of the expressed EGFP-mRFP1 fusion proteins contained a fluorescent mRFP1 molecule. Fluorescence lifetime and emission spectra measurements confirmed this surprising result. Additional experiments show that the missing fluorescent fraction of mRFP1 is consistent with a dark state population of mRFP1. We successfully resolved this mixture of fusion proteins with a single dual-color PCH measurement. These results highlight the potential of dual-color PCH to directly detect and quantify protein mixtures in living cells.     

External exposure of deciduous tooth enamel to photons: dose conversion coefficients for standard radiation fields

Dose conversion coefficients for teeth of children were computed for external photon sources by means of Monte Carlo methods using a modified MIRD-type mathematical phantom of a 5-year-old child. The tooth region is separated into eight smaller regions that represent incisors, canines, first and second molars. Each of these sub-regions is separated into enamel and dentin parts. Dose conversion coefficients were computed as ratio of absorbed dose in the enamel and air kerma. They are given for unidirectional (AP, PA, RLAT, LLAT), rotational (ROT) and isotropic (ISO) photon sources in the energy range from 10 keV to 10 MeV. All computations were performed with the MCNP4 code including coupled electron-photon transport. The computed coefficients demonstrate a significant non-linearity versus photon energy, which is more pronounced than that observed for adult phantoms. Due to this non-linearity, use of the EPR-measured doses in human teeth requires information on the incident photon fluence spectra. The data presented can be used for assessment of public exposure.