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At least 217 records · Page 12Linked to original sources
PubMed2002

Recovery of dairy manure nutrients by benthic freshwater algae.

Harnessing solar energy to grow algal biomass on wastewater nutrients could provide a holistic solution to nutrient management problems on dairy farms. The production of algae from a portion of manure nutrients to replace high-protein feed supplements which are often imported (along with considerable nutrients) onto the farm could potentially link consumption and supply of on-farm nutrients. The objective of this research was to assess the ability of benthic freshwater algae to recover nutrients from dairy manure and to evaluate nutrient uptake rates and dry matter/crude protein yields in comparison to a conventional cropping system. Benthic algae growth chambers were operated in semi-batch mode by continuously recycling wastewater and adding manure inputs daily. Using total nitrogen (TN) loading rates of 0.64-1.03 g m(-2) d(-1), the dried algal yields were 5.3-5.5 g m(-2) d(-1). The dried algae contained 1.5-2.1% P and 4.9-7.1% N. At a TN loading rate of 1.03 g m(-2) d(-1), algal biomass contained 7.1% N compared to only 4.9% N at a TN loading rate of 0.64 g m(-2) d(-1). In the best case, algal biomass had a crude protein content of 44%, compared to a typical corn silage protein content of 7%. At a dry matter yield of 5.5 g m(-2) d(-1), this is equivalent to an annual N uptake rate of 1,430 kg ha(-1) yr(-1). Compared to a conventional corn/rye rotation, such benthic algae production rates would require 26% of the land area requirements for equivalent N uptake rates and 23% of the land area requirements on a P uptake basis. Combining conventional cropping systems with an algal treatment system could facilitate more efficient crop production and farm nutrient management, allowing dairy operations to be environmentally sustainable on fewer acres.

Animals↗
PubMed2001

The heart of photosynthesis in glorious 3D.

The input of solar energy into photosynthesis, and thence into the biosphere, occurs via chlorophyll-containing proteins known as reaction centres. There are two kinds of reaction centre in oxygenic photosynthesis: photosystem I (PSI) and photosystem II (PSII). The PSII reaction centre, alias the oxygen-evolving enzyme, the water-oxidizing complex or the water-plastoquinone photo-oxidoreductase, has now been crystallized and its structure solved to a resolution of 3.8 A.

Catalytic Domain↗
PubMed2003

Outdoor cultivation of a nitrogen-fixing marine cyanobacterium, Anabaena sp. ATCC 33047.

Optimization of conditions for outdoor production of the nitrogen-fixing cyanobacterium Anabaena sp. ATCC 33047 has been pursued. In open ponds operated under semi-continuous regime biomass productivity values achieved ranged from 9 g (dry weight) m(-2) per day, in winter, to over 20 g m(-2) per day, in summer, provided that key operation parameters, including cell density, were optimized. Under these conditions the efficiency of solar energy conversion by the cells was fairly constant throughout the year, with photosynthetic efficiency values higher than 2%. The cyanobacterial biomass was rich in high-value phycobiliproteins, namely allophycocyanin and phycocyanin, for which open cultures of marine Anabaena represent a most interesting production system. The performance of Anabaena cultures operated under continuous regime in a closed tubular reactor has also been assessed outdoors, in winter. Biomass productivity values similar to those obtained in the ponds have been recorded for the closed system. Additionally, under these conditions, the cells excreted to the medium large amounts of a previously characterized exopolysaccharide, at production rates as high as 35 g m(-2) per day (1.4 g l(-1) per day). Properly operated closed cultures of this strain of Anabaena appear most suitable for outdoor mass production of valuable extracellular polysaccharides.

Anabaena↗
PubMed2004

Structural changes in the photoactive site of proteorhodopsin during the primary photoreaction.

Proteorhodopsin (PR), found in marine gamma-proteobacteria, is a newly discovered light-driven proton pump similar to bacteriorhodopsin (BR). Because of the widespread distribution of proteobacteria in the worldwide oceanic waters, this pigment may contribute significantly to the global solar energy input in the biosphere. We examined structural changes that occur during the primary photoreaction (PR --> K) of wild-type pigment and two mutants using low-temperature FTIR difference spectroscopy. Several vibrations detected in the 3500-3700 cm(-1) region are assigned on the basis of H(2)O --> H(2)(18)O exchange to the perturbation of one or more internal water molecules. Substitution of the negatively charged Schiff base counterion, Asp97, with the neutral asparagine caused a downshift of the ethylenic (C=C) and Schiff base (C=N) stretching modes, in agreement with the 27 nm red shift of the visible lambda(max). However, this replacement did not alter the normal all-trans to 13-cis isomerization of the chromophore or the environment of the detected water molecule(s). In contrast, substitution of Asn230, which is in a position to interact with the Schiff base, with Ala induces a 5 nm red shift of the visible lambda(max) and alters the PR chromophore structure, its isomerization to K, and the environment of the detected internal water molecules. The combination of FTIR and site-directed mutagenesis establishes that both Asp97 and Asn230 are perturbed during the primary phototransition. The environment of Asn230 is further altered during the thermal decay of K. These results suggest that significant differences exist in the conformational changes which occur in the photoactive sites of proteorhodopsin and bacteriorhodopsin during the primary photoreaction.

Amino Acid Substitution↗
PubMed1997

A new pathway for transmembrane electron transfer in photosynthetic reaction centers of Rhodobacter sphaeroides not involving the excited special pair.

It is generally accepted that electron transfer in bacterial photosynthesis is driven by the first singlet excited state of a special pair of bacteriochlorophylls (P*). We have examined the first steps of electron transfer in a mutant of the Rhodobacter sphaeroides reaction center in which charge separation from P* is dramatically slowed down. The results provide for the first time clear evidence that excitation of the monomeric bacteriochlorophyll in the active branch of the reaction center (B(A)) drives ultrafast transmembrane electron transfer without the involvement of P*, demonstrating a new and efficient mechanism for solar energy transduction in photosynthesis. The most abundant charge-separated intermediate state probably is P+B(A)-, which is formed within 200 fs from B(A)* and decays with a lifetime of 6.5 ps into P+H(A)-. We also see evidence for the involvement of a B(A)+H(A)- state in the alternative pathway.

Bacteriochlorophylls↗
PubMedDate not supplied

Microalgal reactors: a review of enclosed system designs and performances.

One major challenge to industrial microalgal culturing is to devise and develop technical apparata, cultivation procedures and algal strains susceptible of undergoing substantial increases in efficiency of use of solar energy and carbon dioxide. Despite several research efforts developed to date, there is no such thing as "the best reactor system"- defined, in an absolute fashion, as the one able to achieve maximum productivity with minimum operation costs, irrespective of the biological and chemical system at stake. In fact, choice of the most suitable system is situation-dependent, as both the species of alga available and the final purpose intended will play a role. The need of accurate control impairs use of open-system configurations, so current investigation has focused mostly on closed systems. In this review, several types of closed bioreactors described in the technical literature as able to support production of microalgae are comprehensively presented and duly discussed, using transport phenomenon and process engineering methodological approaches. The text is subdivided into subsections on: reactor design, which includes tubular reactors, flat plate reactors and fermenter-type reactors; and processing parameters, which include gaseous transfer, medium mixing and light requirements.

Bioreactors↗
PubMed2002

Formation and identification of intermediates in the visible-light-assisted photodegradation of sulforhodamine-B dye in aqueous TiO2 dispersion.

The photodegradation of dye pollutants under visible light irradiation in TiO2 dispersions continues to draw considerable attention because of the greater effective utilization of solar energy and its potential application in treating wastewaters from textile and photographic industries. To get a better handle on the mechanistics details of this TiO2-assisted photodegradation of dyes with visible radiation, the process was examined by UV-visible spectroscopy, high-performance liquid chromatography, silica gel thin-layer chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometric techniques to separate and identify the N-de-ethylated intermediate products during the photodegradation of N,N,N,N-tetraethylsulforhodamine-B (SRB) in the absence and presence of iodide ions. Five intermediates, namely, N,N-diethyl-N'-ethyl-sulforhodamine, N,N-diethylsulforhodamine, N-ethyl-N'-ethylsulforhodamine, N-ethylsulforhodamine, and sulforhodamine were thus identified. They correspond to intermediate products having a different number of N-ethyl groups, which are removed sequentially from the SRB molecule. The reaction process was accompanied by the oxidation of I- to I3- in the presence of I- ions. Formation of radicals was assessed by spin-trap electron spin resonance spectrometry. The experimental results provided the basis for a more detailed description of the reaction mechanism(s).

Coloring Agents↗
PubMed2001

Dye sensitization of nanocrystalline titanium dioxide with square planar platinum(II) diimine dithiolate complexes.

A series of platinum-based sensitizers of the general type Pt(NN)(SS), where NN is 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) or 4,7-dicarboxy-1,10-phenanthroline (dcphen) and SS is ethyl-2-cyano-3,3-dimercaptoacrylate (ecda), quinoxaline-2,3-dithiolate (qdt), 1,2-benzenedithiolate (bdt), or 3,4-toluenedithiolate (tdt), that have various ground-state oxidation potentials has been synthesized and anchored to nanocrystalline titanium dioxide electrodes for light-to-electricity conversion in regenerative photoelectrochemical cells with an I(-)/I(-)(3) acetonitrile electrolyte. The intense mixed-Pt/dithiolate-to-diimine charge-transfer absorption bands in this series could be tuned from 440 to 580 nm by choosing appropriate dithiolate ligands, and the highest occupied molecular orbitals varied by more than 500 mV. Spectrophotometric titration of the Pt(dcphen)(bdt) complex exhibits a ground-state pK(a) value of 3.2 +/- 0.1, which can be assigned to the protonation of the carboxylate group of the dcphen ligand. Binding of Pt(dcbpy)(qdt) to porous nanostructured TiO(2) films was analyzed using the Langmuir adsorption isotherm model, yielding an adsorption equilibrium constant of 4 x 10(5) M(-1). The amount of dye adsorbed at the surface of TiO(2) films was 9.5 x 10(-8) mol/cm(2), which is ca. 50% lower than the full monolayer coverage. The resulting complexes efficiently sensitized TiO(2) over a notably broad spectral range and showed an open-circuit potential of ca. 600 mV with an impressive fill factor of > 0.70, making them attractive candidates for solar energy conversion applications. The visible spectra of the 3,4-toluenedithiol-based sensitizers showed an enhanced red response, but the lower photocurrent efficiency observed for these sensitizers stems in part from a sluggish halide oxidation rate and a fast recombination of injected electrons with the oxidized dye.

Journal Article↗
PubMed2003

Mimicking photosynthesis in a computationally designed synthetic metalloprotein.

While advances in protein design have made possible the construction of protein architectures with nativelike properties and predictable structures and function, there are as of yet no examples of functional, protein-based, solar energy conversion systems. This communication describes the design and characterization of an artificial reaction center (RC) protein that closely resembles the function of the natural photosynthetic RC. The synthetic protein, designed by the protein design program CORE, participates in multiple reduction/oxidation cycles with exogenous acceptors/donors following photoexcitation. The designed metalloprotein, aRC, consists of a tetrahelical bundle functionalized with two bis-histidine bound metal cofactors: a Ru(bpy)2 moiety and a heme group. Two distinct bis-histidine binding sites were engineered for each of these metal centers. Photoexcitation of aRC results in rapid ET from the RuII complex to the heme group (kET >/= 5 x 1010 s-1) yielding a long-lived (70 ns) charge-separated state (CSS), RuIII/FeII. This long-lived CSS participates in subsequent ET reactions with exogenous donors and acceptors in multiple photocycles, thus mimicking the basic function of native photosynthetic RCs. This study illustrates the successful design and construction of a protein-based functional charge separation device using a combination of automated computational protein design and knowledge of the engineering principles of biological electron tunneling extracted from natural electron-transfer systems. To our knowledge, this represents the first example of a functional protein-based artificial reaction center.

2,2'-Dipyridyl↗
PubMed2003

Synthesis of very small TiO2 nanocrystals in a room-temperature ionic liquid and their self-assembly toward mesoporous spherical aggregates.

In this Communication, we describe the synthesis of 2-3 nm sized titania nanocrystals in a room-temperature ionic liquid under mild conditions and their self-assembly toward mesoporous TiO2 spheres. The resulting structures combine the convenient handling of larger spheres with a considerable high surface area and narrow pore size distribution and are expected to have potential in solar energy conversion, catalysis, and optoelectronic devices.

Journal Article↗
PubMed2004

Efficient degradation of toxic organic pollutants with Ni2O3/TiO(2-x)Bx under visible irradiation.

To promote efficient use of solar energy, many studies have focused on the modification of TiO2 to extend its spectral response to visible region. Here we report a combined modification of TiO2 by two components: the nonmetal element boron and the metal oxide Ni2O3. The photocatalyst presents high photocatalytic activity in the visible region, which can efficiently degrade and mineralize toxic organic pollutants such as trichlorophenol (TCP), 2,4-dichlorophenol (2,4-DCP), and sodium benzoate. The dechlorination and mineralization results indicate the photocatalytic pathway via visible light excitation. The study demonstrates that the modification of TiO2 both to extend its spectral response to the visible region and to improve its catalytic efficiency can be achieved by doping with boron, a nonmetal, and Ni2O3, a metal oxide.

Journal Article↗
PubMed2005

Copper oxide nanocrystals.

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.

Journal Article↗
PubMed2005

Tailoring the optical property by a three-dimensional epitaxial heterostructure: a case of ZnO/SnO2.

Epitaxial growth, as a best strategy to attain a heterostructure with a well-defined and clean interface, usually takes place on a planar substrate. In this paper, using a ZnO/SnO2 core-shell heterostructure as an example, we demonstrate the possibility of establishing a three-dimensional epitaxial interface between two materials with different crystal systems for the first time and show possible tailoring optical properties by building the heteroepitaxial crystal interface. The characterization results of element mapping, high-resolution transmission electron microscopy, and selected area electric diffraction reveal that the as-prepared ZnO/SnO2 heterostructure has a tetrapod-like ZnO core and a SnO2 shell with 15-30 nm, and their special epitaxial relation is (010)SnO2//(010)ZnO and [100]SnO2//[0001]ZnO. Such three-dimensional epitaxy between the ZnO core and SnO2 shell is quite different from the usual planar epitaxy or three-dimensional epitaxy between materials having the same crystal structure. A rational model of such complicated epitaxy has been proposed through investigating the certain structural comparability between the wurtzite ZnO and rutile SnO2 crystals. The as-prepared T-ZnO/SnO2 epitaxial heterostructure exhibits unique luminescence properties in contrast with individual tetrapod ZnO and SnO2 nanostructures, in which the epitaxial interface induces new luminescence properties. This result may inspire great interest in exploring other complicated epitaxy systems and their potential applications in laser, gas sensor, solar energy conversion, photo catalysis, and nanodevices in the future.

Crystallization↗
PubMed2005

Bidirectional electron transfer in photosystem I: direct evidence from high-frequency time-resolved EPR spectroscopy.

Efficient charge separation occurring within membrane-bound reaction center proteins is the most important step of photosynthetic solar energy conversion. All reaction centers are classified into two types, I and II. X-ray crystal structures reveal that both types bind two symmetric membrane-spanning branches of potential electron-transfer cofactors. Determination of the functional roles of these pairs of branches is of fundamental importance. While it is established that in type II reaction centers only one branch functions in electron transfer, we present the first direct spectroscopic evidence that both cofactor branches are active in the type I reaction center, photosystem I.

Electron Spin Resonance Spectroscopy↗
PubMed2006

Photoinduced charge transfer between CdSe nanocrystal quantum dots and Ru-polypyridine complexes.

In this communication, we demonstrate a new approach to sensitization of Ru-polypyridine complexes by using semiconductor nanocrystal quantum dots (NQDs). When mixed in solution, the complexes functionalized by carboxylic groups adsorb onto the surface of the NQDs. Excitation of NQDs by 400 nm light leads to fast, 5 ps hole transfer from the photoexcited NQDs to the surface-adsorbed complexes. This result indicates that Ru complexes can be sensitized by CdSe NQDs, which opens interesting opportunities for designing new types of photocatalytic materials for solar energy conversion applications. These materials will take advantage of broad size-controlled absorption spectra and large extinction coefficients of NQDs as well as the unique property of NQDs to respond to absorption of a single photon by producing multiple electron-hole pairs.

Journal Article↗
PubMed2005

Zeolite membrane-based artificial photosynthetic assembly for long-lived charge separation.

Photochemically generated long-lived charge separation is the key step in processes that aim for conversion of solar energy into chemical energy. In this study, we focus on a Ru polypyridyl complex [(bpy)(2)Ru(II)L, bpy = bipyridine, L = 1,2-bis[4-(4(')-2,2(')-bipyridyl) ethene] encapsulated on the surface of a pinhole-free zeolite membrane by quaternization of L and surrounded with intrazeolitic bipyridinium ions (N,N'-trimethyl-2,2'-bipyridinium ion, 3DQ(2+)). Visible-light irradiation of the Ru complex side of the membrane in the presence of a sacrificial electron donor led to formation of PVS(-*) on the other side. Pore-blocking disilazane-based chemistry allows for Na(+) to migrate through the membrane to maintain charge balance, while keeping the 3DQ(2+) entrapped in the zeolite. These results provide encouragement that the zeolite membrane based architecture has the necessary features for not only incorporating molecular assemblies with long-lived charge separation but also for ready exploitation of the spatially separated charges to store visible light energy in chemical species.

Journal Article↗
PubMed2005

Quenching of semiconductor quantum dot photoluminescence by a pi-conjugated polymer.

In this communication we discuss the possibility of hole transfer between a photoexcited semiconductor quantum dot and a pi-conjugated polymer. This charge-transfer event will be investigated (exploited) on the basis of its implication toward a solar energy conversion scheme. Experimentally, we show that the steady-state photoluminescence (PL) of a solution of InP quantum dots is quenched by the introduction of solvated poly(3-hexylthiophene). Time-resolved PL experiments on these solutions are also presented. It was observed that the PL transients did not significantly change upon the addition of the conductive polymer. These new results indicate that said PL quenching is static in nature. This suggests that in solution, the quantum dot and the polymer exhibit a strong intermolecular interaction. As the two species encounter each other through diffusion, the polymer quenches the quantum dot photoluminescence without altering the population's PL lifetime. This new evidence suggests that the polymer and the quantum dot form a relatively stable complex.

Journal Article↗