Effect of Crystal Growth on the Thermodynamic Stability and Oxygen Reduction Reaction Activity of Cu-Pt Nanoparticles.

Thermodynamically stable (ordered) platinum-based bimetallic nanoparticle (NP) catalysts are auspicious candidates for catalyzing the oxygen reduction reaction (ORR) in fuel cells. Although the cubic (L12) and tetragonal (L10) ordered phases have been extensively studied, very little is known about the cubic (D7) thermally stable/ordered CuPt7 with regard to its synthesis at room temperature and ORR activity. The typical synthetic approach to the ordered phase (L12 and L10) has been by thermal annealing of the disordered phase in an inert atmosphere. We demonstrate that by coordinating Cu2+ and Pt4+ ions to amino groups in aqueous polyethyleneimine (PEI) (precursor solution), slow crystal growth by a UV-light assisted photoreduction can be used to achieve ordered CuPt7 NPs at room temperature. Slow crystal growth produces a relatively expanded lattice (7.766 Å) of CuPt7 and a lesser ORR activity via a four-electron transfer pathway. Conversely, fast crystal growth through a NaBH4 assisted chemical reduction produces a disordered CuPt phase at room temperature and a contracted lattice (3.809 Å) that enhances the ORR activity of CuPt via a two-electron transfer pathway. Our comparative observations of CuPt and CuPt7 support the observation that lattice contraction is critical in the ORR activity of Cu-Pt nanoalloys.

Synthesis of FePt Nanoparticles by Photoreduction and Chemical Reduction in Poly(ethyleneimine).

Using poly(ethyleneimine) (PEI) polymer substrates, spherical FePt nanoparticles (NPs) with diameter <10 nm have been synthesized by photoreduction and chemical reduction in aqueous media at room temperature. In the photoreduction approach, PEvI acts as both the template into which the metal ions are coordinated and as a reductant when irradiated by ultraviolet light. In the chemical reduction method, PEI acts as only a template, with NaBH4 as the reductant. The as-prepared NPs were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). Starting from the same precursor state and relative concentrations, the as-prepared NPs from both methods are spherical, crystalline solid solutions with a chemically disordered face-centered cubic (fcc) structure. The as-prepared NPs from both methods are superparamagnetic with some contribution from a ferromagnetic phase. The photoreduced NPs have broad size distribution of (5 ± 1.0 nm), an expanded lattice (3.913 Å), and relatively lower magnetic moment (0.02 emu/g) compared to the narrower size distribution (4 ± 0.7 nm), shortened lattice (3.890 Å), and a dominant moment (15 emu/g) of the chemically reduced NPs. The difference in the rate of particle formation apparently leads to a low efficiency of FePt NP formation via photoreduction compared to chemical reduction.

The carotenoid Bixin found to exhibit the highest measured carotenoid oxidation potential to date consistent with its practical protective use in cosmetics, drugs and food.

The electrochemical oxidation potentials of cis bixin correspond to the production of the carotenoid radical cation, Car+ and dication Car++. The oxidation is a two-electron process with oxidation potentials at ~0.94 and ~1.14 V vs SCE (reference to ferrocene at 0.528 V) in THF. These potentials are higher than that of symmetrical canthaxanthin at 0.775 V and 0.972 V and for ß-carotene at 0.634 V and 0.605 V respectively. The second oxidation potential for canthaxanthin is higher by 0.20 V than the first. Similar difference is observed for bixin. In contrast, the second oxidation potential for ß-carotene is lower by 30 mV than that of the first. Reduction potentials were found to occur at ~-0.69 and ~-1.22 V vs SCE. The lifetime of the radical cation of cis bixin, Car+, is short and decays rapidly at ambient temperature. The suggested scavenging ability of cis bixin towards reactive oxidative oxygen species is estimated to be 44. On the other hand, that of ß-carotene, symmetrical canthaxanthin and the dicyano substituted carotenoid which exhibit oxidation potentials of 0.634 V, 0.775 V and 0.833 V vs SCE were measured to be 0.64, 1.96 and 23.60 respectively. The non-reversible electrochemical measurements suggest the tendency for bixin to react with trace amounts of reactive oxygen species (OH, O2-, OOH).

DFT and ENDOR Study of Bixin Radical Cations and Neutral Radicals on Silica-Alumina.

Bixin, a carotenoid found in annatto (Bixa orellana), is unique among natural carotenoids by being water-soluble. We stabilized free radicals from bixin on the surface of silica-alumina (Si-Al) and characterized them by pulsed electron-nuclear double resonance (ENDOR). DFT calculations of unpaired electron spin distribution for various bixin radicals predict the EPR hyperfine couplings. Least-square fitting of experimental ENDOR spectra by spectra calculated from DFT hyperfine couplings characterized the radicals trapped on Si-Al. DFT predicts that the trans bixin radical cation is more stable than the cis bixin radical cation by 1.26 kcal/mol. This small energy difference is consistent with the 26% trans and 23% cis radical cations in the ENDOR spectrum. The remainder of the ENDOR spectrum is due to several neutral radicals formed by loss of a H(+) ion from the 9, 9', 13, or 13' methyl group, a common occurrence in all water-insoluble carotenoids previously studied. Although carboxyl groups of bixin strongly affect its solubility relative to other natural carotenoids, they do not alter properties of its free radicals based on DFT calculations and EPR measurements which remain similar to typical water-insoluble carotenoids.

El Niño impact on mollusk biomineralization-implications for trace element proxy reconstructions and the paleo-archeological record.

Marine macroinvertebrates are ideal sentinel organisms to monitor rapid environmental changes associated with climatic phenomena. These organisms build up protective exoskeletons incrementally by biologically-controlled mineralization, which is deeply rooted in long-term evolutionary processes. Recent studies relating potential rapid environmental fluctuations to climate change, such as ocean acidification, suggest modifications on carbonate biominerals of marine invertebrates. However, the influence of known, and recurrent, climatic events on these biological processes during active mineralization is still insufficiently understood. Analysis of Peruvian cockles from the 1982-83 large magnitude El Niño event shows significant alterations of the chemico-structure of carbonate biominerals. Here, we show that bivalves modify the main biomineralization mechanism during the event to continue shell secretion. As a result, magnesium content increases to stabilize amorphous calcium carbonate (ACC), inducing a rise in Mg/Ca unrelated to the associated increase in sea-surface temperature. Analysis of variations in Sr/Ca also suggests that this proxy should not be used in these bivalves to detect the temperature anomaly, while Ba/Ca peaks are recorded in shells in response to an increase in productivity, or dissolved barium in seawater, after the event. Presented data contribute to a better understanding of the effects of abrupt climate change on shell biomineralization, while also offering an alternative view of bivalve elemental proxy reconstructions. Furthermore, biomineralization changes in mollusk shells can be used as a novel potential proxy to provide a more nuanced historical record of El Niño and similar rapid environmental change events.

Synthesis and characterization of CoPt nanoparticles prepared by room temperature chemical reduction with PAMAM dendrimer as template.

We describe an approach to synthesize monodisperse CoPt nanoparticles with dendrimer as template by a simple chemical reduction method in aqueous solution using NaBH4 as reducing agent at room temperature. The as-made CoPt nanoparticles buried in the dendrimer matrix have the chemically disordered fcc structure and can be transformed to the fct phase after annealing at 700 degrees C. This is the first report of dendrimer-mediated room temperature synthesis of monodisperse magnetic nanoparticles in aqueous solution.

Synthesis and electrocatalytic activity of photoreduced platinum nanoparticles in a poly(ethylenimine) matrix.

Monodisperse polymer-mediated platinum (Pt) nanoparticles (NPs) have been synthesized by photoreduction in the presence of poly(ethylenimine) (PEI), a hyperbranched polymer. The formation process of the Pt NPs is pursued by UV-vis spectroscopy, and the formation mechanism is discussed. The morphology and size of the Pt NPs were characterized by transmission electron microscopy (TEM). TEM imaging shows that the Pt NPs' average diameter is 2.88 +/- 0.53 nm. The PEI/Pt NPs were immobilized on glassy carbon electrodes, and the electrocatalytic activity of the catalysts was investigated by cyclic voltammetry. PEI/Pt NPs exhibit very high catalytic activity for a methanol oxidation reaction. PEI/Pt NPs on glassy carbon electrodes are robust, showing good tolerance to poisoning even after many cycles. The electrocatalytic activity of PEI/Pt NPs compares favorably with other polymer-mediated Pt NPs. The results indicate that PEI is an appropriate complexing reducing agent for the photochemical production of Pt NPs and a good capping agent, allowing immobilization of the NPs on the working electrode.