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ABSTRACT: Squids have used their tunable iridescence for camouflage and communication for millions of years; materials scientists have more recently looked to them for inspiration to develop new "biologically inspired" adaptive optics. Iridocyte cells produce iridescence through constructive interference of light with intracellular Bragg reflectors. The cell's dynamic control over the apparent lattice constant and dielectric contrast of these multilayer stacks yields the corresponding optical control of brightness and color across the visible spectrum. Here, we resolve remaining uncertainties in iridocyte cell structure and determine how this unusual morphology enables the cell's tunable reflectance. We show that the plasma membrane periodically invaginates deep into the iridocyte to form a potential Bragg reflector consisting of an array of narrow, parallel channels that segregate the resulting high refractive index, cytoplasmic protein-containing lamellae from the low-index channels that are continuous with the extracellular space. In response to control by a neurotransmitter, the iridocytes reversibly imbibe or expel water commensurate with changes in reflection intensity and wavelength. These results allow us to propose a comprehensive mechanism of adaptive iridescence in these cells from stimulation to color production. Applications of these findings may contribute to the development of unique classes of tunable photonic materials.
Proceedings of the National Academy of Sciences 01/2013; · 9.68 Impact Factor
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ABSTRACT: The way nature evolves and sculpts materials using proteins inspires new approaches to materials engineering but is still not completely understood. Here, we present a cell-free synthetic biological platform to advance studies of biologically synthesized solid-state materials. This platform is capable of simultaneously exerting many of the hierarchical levels of control found in natural biomineralization, including genetic, chemical, spatial, structural, and morphological control, while supporting the evolutionary selection of new mineralizing proteins and the corresponding genetically encoded materials that they produce. DNA-directed protein expression and enzymatic mineralization occur on polystyrene microbeads in water-in-oil emulsions, yielding synthetic surrogates of biomineralizing cells that are then screened by flow sorting, with light-scattering signals used to sort the resulting mineralized composites differentially. We demonstrate the utility of this platform by evolutionarily selecting newly identified silicateins, biomineralizing enzymes previously identified from the silica skeleton of a marine sponge, for enzyme variants capable of synthesizing silicon dioxide (silica) or titanium dioxide (titania) composites. Mineral composites of intermediate strength are preferentially selected to remain intact for identification during cell sorting, and then to collapse postsorting to expose the encoding genes for enzymatic DNA amplification. Some of the newly selected silicatein variants catalyze the formation of crystalline silicates, whereas the parent silicateins lack this ability. The demonstrated bioengineered route to previously undescribed materials introduces in vitro enzyme selection as a viable strategy for mimicking genetic evolution of materials as it occurs in nature.
Proceedings of the National Academy of Sciences 06/2012; 109(26):E1705-14. · 9.68 Impact Factor
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ABSTRACT: The circa-annual cycle of gametogenesis produces mature gametes at the spawning “season” for successful mass spawning of broadcast
corals. We develop a bioenergetic integrate-and-fire model that reveals how annual insolation rhythms can entrain the gametogenetic
cycles in tropical hermatypic corals to the appropriate spawning season, since photosynthate is their primary source of energy.
In the presence of short-term fluctuations in the energy input, a feedback regulatory mechanism is likely required to achieve
coherence of spawning times to within one lunar cycle, in order for subsequent signals such as lunar and diurnal light cycles
to unambiguously determine the “correct” night of spawning. The feedback mechanism can also provide robustness against population
heterogeneity that may arise due to genetic and environmental effects. We solve the integrate-and-fire bioenergetic model
numerically using the Fokker–Planck equation and use analytical tools such as rotation number to study entrainment.
KeywordsBioenergetic integrate-and-fire model–Broadcast spawning–Entrainment–Insolation–Coral reproduction –Biological rhythms–Computational biology
Theoretical Ecology 04/2012; 4(1):69-85. · 1.54 Impact Factor
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Journal of The Royal Society Interface 12/2011; · 4.40 Impact Factor
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ABSTRACT: Synchrony of spawning in many hermatypic corals, typically a few nights after the full moon, is putatively dependent on solar and lunar light cycles in conjunction with other possible cues such as tides and temperature. We analyze here the contributions of separate components of light dynamics, because the effects of twilight and lunar skylight on coral spawning synchrony have previously been conflated and the alternative hypothesis that these components have differential contributions as proximate cues has not been tested. Moonlight-dependent changes in spectra during twilight, rates of decreasing twilight intensities, and changes in lunar photoperiod were experimentally decoupled using programmed light-emitting diodes and compared for their separate effects on spawning synchrony in Acropora humilis. Effects on synchrony under the control of synthetic lunar cues were greatest in response to changes in lunar photoperiod; changes in light intensities and spectra had lesser influence. No significant differences among treatment responses were found at the circa-diel time scale. We conclude that spawning synchrony on a particular lunar night and specific time of night is a threshold response to differential periods of darkness after twilight that is primarily influenced by lunar photoperiod and secondarily by discrete optical components of early nocturnal illumination.
Biological Bulletin 06/2011; 220(3):161-73. · 1.70 Impact Factor
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Advanced Materials 05/2011; 23(20):2278-83. · 13.88 Impact Factor
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Advanced Materials 05/2011; 23(20):2256-9. · 13.88 Impact Factor
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Phys. Rev. B. 03/2011; 83(10).
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ABSTRACT: There are dramatic and physiologically relevant changes in both skylight color and intensity during evening twilight as the pathlength of direct sunlight through the atmosphere increases, ozone increasingly absorbs long wavelengths and skylight becomes increasingly blue shifted. The moon is above the horizon at sunset during the waxing phase of the lunar cycle, on the horizon at sunset on the night of the full moon and below the horizon during the waning phase. Moonlight is red shifted compared with daylight, so the presence, phase and position of the moon in the sky could modulate the blue shifts during twilight. Therefore, the influence of the moon on twilight color is likely to differ somewhat each night of the lunar cycle, and to vary especially rapidly around the full moon, as the moon transitions from above to below the horizon during twilight. Many important light-mediated biological processes occur during twilight, and this lunar effect may play a role. One particularly intriguing biological event tightly correlated with these twilight processes is the occurrence of mass spawning events on coral reefs. Therefore, we measured downwelling underwater hyperspectral irradiance on a coral reef during twilight for several nights before and after the full moon. We demonstrate that shifts in twilight color and intensity on nights both within and between evenings, immediately before and after the full moon, are correlated with the observed times of synchronized mass spawning, and that these optical phenomena are a biologically plausible cue for the synchronization of these mass spawning events.
Journal of Experimental Biology 03/2011; 214(Pt 5):770-7. · 3.00 Impact Factor
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ABSTRACT: The precipitation of crystals with stoichiometric and ordered arrangements of distinct metal cations often requires carefully designed molecular precursors and/or sufficient activation energy in addition to the necessary mass transport. Here, we study the formation of ordered double perovskite hydroxides, MnSn(OH)(6) and CoSn(OH)(6), of the generic chemical formula, BB'(OH)(6) (no A site), using kinetic control of aqueous hydrolysis from simple metal salt solutions. We find that the precipitation yields ordered compounds only when the B ion is Mn(II) or Co(II), and not when it is any other divalent transition metal ion, or Zn(II). The key step in forming the compounds is the prevention of rapid and uncontrolled hydrolysis of Sn(IV), and this is achieved by a fluoride counteranion. The two compounds, MnSn(OH)(6) and CoSn(OH)(6), are studied by high-resolution synchrotron X-ray diffraction and from the temperature dependence of magnetic behavior. From maximum entropy image restoration of the electron density and from Rietveld analysis, the degree of octahedral distortion and tilting and the small extent of anti-site disorder are determined. From the nonoverlapping electron density, we infer strongly ionic character of bonding. As the first magnetic study of such materials, we report simple paramagnetic behavior with no long-range magnetic order down to 2 K for the Mn(II) compound, while the cobalt compound presents uncompensated antiferromagnetic interactions, attributed to the single-ion anisotropy of octahedral Co(II).
Inorganic Chemistry 03/2011; 50(7):3003-9. · 4.60 Impact Factor
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ABSTRACT: Nanoparticulate Cd(1-x)Zn(x)O (x = 0, 0.05-0.26, 1) is synthesized in a simple two-step synthesis approach. Vapor-diffusion induced catalytic hydrolysis of two molecular precursors at low temperature induces co-nucleation and polycondensation to produce bimetallic layered hydroxide salts (M = Cd, Zn) as precursor materials which are subsequently converted to Cd(1-x)Zn(x)O at 400 °C. Unlike ternary materials prepared by standard co-precipitation procedures, all products presented here containing < 30 mol% Zn(2+) ions are homogeneous in elemental composition on the micrometre scale. This measured compositional homogeneity within the samples, as determined by energy dispersive spectroscopy and inductively coupled plasma spectroscopy, is a testimony to the kinetic control achieved by employing slow hydrolysis conditions. In agreement with this observation, the optical properties of the materials obey Vegard's Law for a homogeneous solid solution of Cd(1-x)Zn(x)O, where x corresponds to the values determined by inductively coupled plasma analysis, even though powder X-ray diffraction shows phase separation into a cubic mixed metal oxide phase and a hexagonal ZnO phase at all doping levels.
Dalton Transactions 02/2011; 40(6):1295-301. · 3.84 Impact Factor
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ABSTRACT: The circa-annual cycle of gametogenesis produces mature gametes at the spawning “season” for successful mass spawning of broadcast corals. We develop a bioenergetic integrate-and-fire model that reveals how annual insolation rhythms can entrain the gametogenetic cycles in tropical hermatypic corals to the appropriate spawning season, since photosynthate is their primary source of energy. In the presence of short-term fluctuations in the energy input, a feedback regulatory mechanism is likely required to achieve coherence of spawning times to within one lunar cycle, in order for subsequent signals such as lunar and diurnal light cycles to unambiguously determine the “correct” night of spawning. The feedback mechanism can also provide robustness against population heterogeneity that may arise due to genetic and environmental effects. We solve the integrate-and-fire bioenergetic model numerically using the Fokker–Planck equation and use analytical tools such as rotation number to study entrainment.
Theoretical Ecology 02/2011; 4(1):69-85. · 1.54 Impact Factor
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ABSTRACT: Cephalopods possess a sophisticated array of mechanisms to achieve camouflage in dynamic underwater environments. While active mechanisms such as chromatophore patterning and body posturing are well known, passive mechanisms such as manipulating light with highly evolved reflectors may also play an important role. To explore the contribution of passive mechanisms to cephalopod camouflage, we investigated the optical and biochemical properties of the silver layer covering the eye of the California fishery squid, Loligo opalescens. We discovered a novel nested-spindle geometry whose correlated structure effectively emulates a randomly distributed Bragg reflector (DBR), with a range of spatial frequencies resulting in broadband visible reflectance, making it a nearly ideal passive camouflage material for the depth at which these animals live. We used the transfer-matrix method of optical modelling to investigate specular reflection from the spindle structures, demonstrating that a DBR with widely distributed thickness variations of high refractive index elements is sufficient to yield broadband reflectance over visible wavelengths, and that unlike DBRs with one or a few spatial frequencies, this broadband reflectance occurs from a wide range of viewing angles. The spindle shape of the cells may facilitate self-assembly of a random DBR to achieve smooth spatial distributions in refractive indices. This design lends itself to technological imitation to achieve a DBR with wide range of smoothly varying layer thicknesses in a facile, inexpensive manner.
Journal of The Royal Society Interface 02/2011; 8(63):1386-99. · 4.40 Impact Factor
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ABSTRACT: We report here a robust, large-scale synthesis of BaTiO₃ nanopowders using a bioinspired process that first was developed on a much smaller scale. The most advantageous points of this protocol are that it takes place at nearly room temperature (25°C), overcomes many limitations encountered in other scale-up processes (such as the need for external drivers, e.g., heat, radiation or pressure), bypasses the use of surfactants and templates and does not necessitate pH adjustment. The use of a single-source, bimetallic alkoxide with the vapor diffusion of a hydrolytic catalyst (H₂O) provides the necessary conditions for facile crystallization and growth of small, well-defined BaTiO₃ nanoparticles at mild temperatures, yielding batches of up to 250 ± 5 g in a green process. Extension of this method to kilogram-scale production of BaTiO₃ nanocrystals in semicontinuous and continuous processes is feasible.
Nature Protocol 01/2011; 6(1):97-104. · 8.36 Impact Factor
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ABSTRACT: Microbial fuel cells (MFCs) represent an emerging technology for electricity generation from renewable biomass. Given the demand for a better understanding of the bio/inorganic interface that plays a key role in MFC energy production, small-scale MFCs are receiving considerable attention owing to their intrinsic advantages in both fundamental studies and applications as high-throughput platforms. Here, we present a brief review centered on the development of miniature MFCs at the milliliter to microliter scale. The principles, design motifs and experimental demonstrations of representative miniature MFC devices and systems are introduced, followed by a discussion of the key challenges and opportunities for realizing the exciting potentials of miniaturized MFCs.
Trends in Biotechnology 11/2010; 29(2):62-9. · 9.15 Impact Factor
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ABSTRACT: Effectively modifying an existing synthesis method for materials can prove as useful as developing a new one. In this work, we revisited a conventional sol−gel synthesis of lithium vanadium oxide, a promising electrode material for rechargeable lithium batteries. Employing a kinetically controlled, vapor diffusion strategy (in which ammonia vapor was slowly diffused into the solution), we modified the conventional method to obtain a thin, flaky, lithium vanadium oxide with an average thickness of 120 nm. In comparison, material prepared by the conventional sol−gel route (in which aqueous ammonia was dropwise added to the solution) exhibited an agglomeration of irregular particles with a typical size of 10 μm. When evaluated as cathode material for rechargeable lithium batteries, this flaky material displayed a stable, reversible capacity of 250 and 115 mAh/g at discharge rates of 0.1 C and 2 C, respectively, considerably better than the agglomerated sample. The reasons for this improved performance were investigated by evaluating the electrochemical reaction kinetics, morphological and structural stability using cyclic voltammetry, scanning electron microscopy, and X-ray diffraction.
10/2010;
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ABSTRACT: In many ostensibly crystalline materials, unit-cell-based descriptions do not
always capture the complete physics of the system due to disruption in
long-range order. In the series of cobalt hydroxides studied here,
Co(OH)$_{2-x}$(Cl)$_x$(H$_2$O)$_{n}$, magnetic Bragg diffraction reveals a
fully compensated N\'eel state, yet the materials show significant and open
magnetization loops. A detailed analysis of the local structure defines the
aperiodic arrangement of cobalt coordination polyhedra. Representation of the
structure as a combination of distinct polyhedral motifs explains the existence
of locally uncompensated moments and provides a quantitative agreement with
bulk magnetic measurements and magnetic Bragg diffraction.
10/2010;
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ABSTRACT: Ceria nanoparticles are synthesized from aqueous cerium nitrate by a facile, room temperature, vapor diffusion based process, resulting in an unusual evolution of particle morphology without exogenous surface capping agents. Transmission electron microscopy, powder X-ray diffraction, and UV−visible spectroscopy reveal that as the hydroxyl ion concentration of the reaction medium increases through progressive diffusion of ammonia vapor into the precursor solution, crystals grow from amorphous small seeds to 10 nm particles with unusual development of well-defined crystal morphologies. Quasi-spherical particles grow to form truncated octahedra that subsequently grow via face reconstruction to yield cubic nanocrystals with {100} surfaces. The slow progressive increase in hydroxyl concentration resulting from the controlled diffusion of ammonia into the reaction medium permits us to observe the sequence of evolutionary transformations and deduce a probable mechanism. We suggest that the increase in OH− concentration provides the driving force for the increased growth rate along the 111 direction, while the exposed and otherwise unstable, polar {100} surfaces are stabilized through OH− adsorption or deprotonation of bound water. The progressive increase in basicity also leads to internal cracking that results in lattice expansion in the final CeO2 particles. The tunable, size- and shape-selective formation of ceria nanoparticles offers advantages for catalytic applications.
09/2010;
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ABSTRACT: Structures of layered metal hydroxides are not well described by traditional crystallography. Total scattering from a synthesis-controlled subset of these materials, as described here, reveals that different cobalt coordination polyhedra cluster within each layer on short length scales, offering new insights and approaches for understanding the properties of these and related layered materials. Structures related to that of brucite [Mg(OH)(2)] are ubiquitous in the mineral world and offer a variety of useful functions ranging from catalysis and ion-exchange to sequestration and energy transduction, including applications in batteries. However, it has been difficult to resolve the atomic structure of these layered compounds because interlayer disorder disrupts the long-range periodicity necessary for diffraction-based structure determination. For this reason, traditional unit-cell-based descriptions have remained inaccurate. Here we apply, for the first time to such layered hydroxides, synchrotron X-ray total scattering methods-analyzing both the Bragg and diffuse components-to resolve the intralayer structure of three different alpha-cobalt hydroxides, revealing the nature and distribution of metal site coordination. The different compounds with incorporated chloride ions have been prepared with kinetic control of hydrolysis to yield different ratios of octahedrally and tetrahedrally coordinated cobalt ions within the layers, as confirmed by total scattering. Real-space analyses indicate local clustering of polyhedra within the layers, manifested in the weighted average of different ordered phases with fixed fractions of tetrahedrally coordinated cobalt sites. These results, hidden from an averaged unit-cell description, reveal new structural characteristics that are essential to understanding the origin of fundamental material properties such as color, anion exchange capacity, and magnetic behavior. Our results also provide further insights into the detailed mechanisms of aqueous hydrolysis chemistry of hydrated metal salts. We emphasize the power of the methods used here for establishing structure-property correlations in functional materials with related layered structures.
Chemistry 09/2010; 16(33):9998-10006. · 5.93 Impact Factor
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Macromolecular Chemistry and Physics 07/2010; 211(15):1701 - 1707. · 2.36 Impact Factor
