Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.

Growth of nanostructures by cluster deposition : a review

This paper presents a comprehensive analysis of simple models useful to analyze the growth of nanostructures obtained by cluster deposition. After detailing the potential interest of nanostructures, I extensively study the first stages of growth (the submonolayer regime) by kinetic Monte-Carlo simulations. These simulations are performed in a wide variety of experimental situations : complete condensation, growth with reevaporation, nucleation on defects, total or null cluster-cluster coalescence... The main scope of the paper is to help experimentalists analyzing their data to deduce which of those processes are important and to quantify them. A software including all these simulation programs is available at no cost on request to the author. I carefully discuss experiments of growth from cluster beams and show how the mobility of the clusters on the surface can be measured : surprisingly high values are found. An important issue for future technological applications of cluster deposition is the relation between the size of the incident clusters and the size of the islands obtained on the substrate. An approximate formula which gives the ratio of the two sizes as a function of the melting temperature of the material deposited is given. Finally, I study the atomic mechanisms which can explain the diffusion of the clusters on a substrate and the result of their mutual interaction (simple juxtaposition, partial or total coalescence...)Comment: To be published Rev Mod Phys, Oct 99, RevTeX, 37 figure

Diet composition and feeding periodicity of the seahorse Hippocampus barbouri reared in illuminated sea cages

The zooplankton prey composition and feeding periodicity of juvenile and adult seahorses Hippocampus barbouri reared in illuminated and non-illuminated sea cages were compared. Mean frequency of occurrence (%FO), prey composition (%N), and gut fullness of seahorses were calculated from analyses of gut contents. Compared with juvenile seahorses, adults consumed more variety of prey consisting of copepods, larvae of decapods, polychaetes and fish, and euphausid shrimps. Calanoid copedods were found in the gut of more juvenile (%FO = 47) and adult (%FO = 64) seahorses in illuminated cages but harpacticoid copepods were ingested by more juvenile fish (%FO = 50) in non-illuminated cages. Decapod larvae (%N = 66) in illuminated cages dominated the diet of juvenile seahorses, whereas in non-illuminated cages harpacticoid copepods (%N = 59) did. Calanoid copepods and decapod larvae (%N = 91–97) comprised the bulk of ingested prey among adult seahorses in all experimental cages. The gut of caged seahorses was generally full during daytime but declined in the evening, becoming almost empty at midnight, particularly among juveniles. Cage illumination commencing at midnight increased the number of filled guts at dawn (0400 h) among juvenile and adult seahorses. Unlike adult seahorses over a 24-h period, the overall incidence of filled guts among juveniles was not different between those in non-illuminated and illuminated cages. These results provide an alternative to growing caged H. barbouri on cultured live food, particularly copepods attracted by night illumination.This study was funded by the Southeast Asian Fisheries Development Center/Aquaculture Department (SEAFDEC/AQD) under Study Code Nr-04-F99T and, in part, by the Natural Sciences Research Institute (NSRI) of the University of the Philippines (UP Diliman, Quezon City) under Project Code BIO-09-2-06 and BIO-11-2-07. The assistance of the staff of SEAFDEC/AQD's Igang Marine Station, A.V. Labador, and A.L. Selisana of the Institute of Biology of UP Diliman is gratefully acknowledged. The comments of three anonymous reviewers improved earlier drafts of the manuscript

Feeding selectivity of the seahorse, Hippocampus kuda (Bleeker), juveniles under laboratory conditions

This study examined the feeding selectivity of Hippocampus kuda juveniles under captive conditions and evaluates different food organisms that could be used to improve hatchery-rearing of this species. Newly born H. kuda were reared for 10 days in 60-L capacity tanks and fed rotifers (Brachionus rotundiformis), zooplankton (mostly Pseudodiaptomus annandalei and Acartia tsuensis) alone or both food sources. The size and amount of food ingested increased as seahorses grew. Selective feeding of seahorses appeared to change as they develop, preferring copepod adults over nauplii and rotifers. A. tsuensis was highly selected by juveniles over P. annandalei. Specific growth rate in terms of body weight (SGR-BW, 15% day–1) was the highest and mortality rate (9% at day 10) the lowest in seahorses fed a mixed food sources. Slowest growth rate (0.3% day–1) and highest mortality rate (60% at day 7) were observed in seahorses fed rotifers alone. These results indicate that copepods are suitable food for seahorse juveniles, but a mixture of food organisms in the rearing tank environment enhances survivorship and growth of H. kuda, thus potentially providing a source of cultured rather than wild specimens for characterizing the life history of this threatened species.This study was funded by SEAFDEC Aquaculture Department (study code Br-04-F95T of GV Hilomen-Garcia)

Self-Assembly of Triton X-100 in Water Solutions: A Multiscale Simulation Study Linking Mesoscale to Atomistic Models

A multiscale scheme is proposed and validated for Triton X-100 (TX-100), which is a detergent widely employed in biology. The hybrid particle field formulation of the model allows simulations of large-scale systems. The coarse-grained (CG) model, accurately validated in a wide range of concentrations, shows a critical micelle concentration, shape transition in isotropic micellar phase, and appearance of hexagonal ordered phase in the experimental ranges reported in the literature. The fine resolution of the proposed CG model allows one to obtain, by a suitable reverse mapping procedure, atomistic models of micellar assemblies and of the hexagonal phase. In particular, atomistic models of the micelles give structures in good agreement with experimental pair distance distribution functions and hydrodynamic measurements. The picture emerging by detailed analysis of simulated systems is quite complex. Polydisperse mixtures of spherical-, oblate-, and prolate-shaped aggregates have been found. The shape and the micelle behavior are mainly dictated by the aggregation number (Nagg). Micelles with low Nagg values (∼40) are spherical, while those with high Nagg values (∼140 or larger) are characterized by prolate ellipsoidal shapes. For intermediate Nagg values (∼70), fluxional micelles alternating between oblate and prolate shapes are found. The proposed model opens the way to investigations of several mechanisms involving TX-100 assembly in protein and membrane biophysics

Tolerance of spermatogonia to oxidative stress is due to high levels of Zn and Cu/Zn superoxide dismutase.

BACKGROUND: Spermatogonia are highly tolerant to reactive oxygen species (ROS) attack while advanced-stage germ cells such as spermatozoa are much more susceptible, but the precise reason for this variation in ROS tolerance remains unknown. METHODOLOGY/PRINCIPAL FINDINGS: Using the Japanese eel testicular culture system that enables a complete spermatogenesis in vitro, we report that advanced-stage germ cells undergo intense apoptosis and exhibit strong signal for 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage marker, upon exposure to hypoxanthine-generated ROS while spermatogonia remain unaltered. Activity assay of antioxidant enzyme, superoxide dismutase (SOD) and Western blot analysis using an anti-Copper/Zinc (Cu/Zn) SOD antibody showed a high SOD activity and Cu/Zn SOD protein concentration during early spermatogenesis. Immunohistochemistry showed a strong expression for Cu/Zn SOD in spermatogonia but weak expression in advanced-stage germ cells. Zn deficiency reduced activity of the recombinant eel Cu/Zn SOD protein. Cu/Zn SOD siRNA decreased Cu/Zn SOD expression in spermatogonia and led to increased oxidative damage. CONCLUSIONS/SIGNIFICANCE: These data indicate that the presence of high levels of Cu/Zn SOD and Zn render spermatogonia resistant to ROS, and consequently protected from oxidative stress. These findings provide the biochemical basis for the high tolerance of spermatogonia to oxidative stress