Síntesis y caracterización de las propiedades ópticas de puntos cuánticos de CdSe y CdSe/ZnS / Synthesis and characterization of optic properties of CdSe and CdSe/ZnS quantum dots / Síntese e caracterização das propriedades ópticas do pontos quânticos de CdSe e CdSe/ZnS

Resumen Se prepararon puntos cuánticos de CdSe y CdSe/ZnS (núcleo/capa) con ácido oleico como agente estabilizante en medio orgánico y se examinaron las propiedades ópticas de los nanocristales obtenidos. En la obtención de CdSe, se estudió la influencia del O2 en la cinética de crecimiento de los puntos cuánticos. Durante los primeros 90 s, el crecimiento de los nanocristales en presencia de O2 fue 1,6 veces mayor que en atmósfera inerte. A pesar de este rápido crecimiento, el O2 afectó las propiedades ópticas de los nanocristales, formando bandas de absorción anchas y espectros de fluorescencia de baja intensidad. En contraste, los puntos cuánticos de CdSe sintetizados en atmósfera inerte presentaron picos de absorción bien definidos y fluorescencia aguda e intensa. Estas propiedades se intensificaron con la formación de un 10% de la monocapa de ZnS: para un núcleo de 2,50 nm, el rendimiento cuántico de fluorescencia (ΦFl) en la región del verde se incrementó de 5,5 % a 42,3%. El procedimiento de síntesis de nanocristales de CdSe/ZnS desarrollado con baja concentración de Zn2+ y con un exceso de S2" puede emplearse en la obtención de materiales con excelentes propiedades fotoluminiscentes para aplicaciones como biomarcadores, sensores, catálisis y celdas solares.

Abstract CdSe and CdSe/ZnS (core/shell) quantum dots with oleic acid as stabilizing agent in organic medium were prepared and their optical properties were examined. For CdSe synthesis, the influence of O2 in the growth kinetics of quantum dots was determined. In the first 90 s, the nanocrystals growth was 1.6 higher in presence of O2 than when reaction was carried out in N2 atmosphere. However, the growth rate with O2 not favorable because the nanocrystal optical properties were affected: wider absorption band and lower fluorescence that those obtained in inert atmosphere. Properties of CdSe nanocrystals synthesized in inert atmosphere were intensified with 10% of monolayer. For a core with 2.5 nm diameter, the fluorescence quantum yield (ΦF1) in the green region increased from 5.5% to 42.3%. The synthesis process of CdSe/ZnS nanocrystals developed with low concentration of Zn2+ and an excess of S2- can be used to obtain materials with excellent photoluminescent properties for applications such as biomarkers, sensors, catalysis, and solar cells.

Resumo Pontos quânticos CdSe e CdSe/ZnS (núcleo/ couraça) com ácido oleico como um agente de estabilização em um meio orgânico foi preparado e as propriedades ópticas dos nanocristais obtidos são examinados Na obtenção de CdSe, a influência da O2 foi estudada em cinética de crescimento de pontos quânticos. Durante o primeiro crescimento de 90 s dos nanocristais na presença de O2 foi de 1,6 vezes mais elevado do que em atmosfera inerte. Apesar deste crescimento rápido, O2 afecta as propriedades ópticas dos nanocristais, formando bandas de absorção ampla e espectros de fluorescência de baixa intensidade. Em contraste, CdSe sintetizados em atmosfera inerte mostrou picos bem definidos de absorção e fluorescência nítida e intensa. Estas propriedades são intensificadas pela formação de 10% da monocamada de ZnS: para um núcleo de 2,50 nm, o rendimento quântico de fluorescência (ΦFl) na região do verde aumentou de 5,5% para 42,3%. O procedimento de síntese de nanocristais de CdSe/ZnS desenvolvidos com baixa concentração de Zn2+ e o excesso de S2" pode ser utilizada na obtenção de materiais fotoluminescentes com excelentes propriedades para aplicações como biomarcadores, sensores, catálise e células solares.

PHONON-EXCITON MECHANISM IN YBa2Cu3O7-δ SYSTEM / 药物分析学报

In this paper, based on the phonon-exciton mechanism, the superconductivity of yttrium barium copper oxide (YBCO) system is explained. The transition temperature (Tc) 's formula is derived by the extended BCS-Eliashberg theory, i.e. phonon-exciton mechanism and McMillan's energy gap function, and the Tc's expressions isreduced by using the Einstein spectrum. Last, the calculating method of element metal effective phonon spectrum is used to the high Tc cuprate YBCO system by using a crystal average atom approximation model. The theoretical analysis and calculational results show that the superconductivity of high Tc YBCO system maybe described by phonon-exciton model.

Cytochrome b6-f complex : The carburettor of exciton distribution in oxygenic photosynthesis.

Efficient oxygenic photosynthesis not only requires synchronous turover and operation of photosystem I (PS I) and photosystem II (PS II) but also the preferential turnover of PS I for cyclic photophosphorylation to maintain required ATP and NADPH ratio during carbon dioxide reduction. Ohe initial higher rate of turnover of PS II in viva is accounted by the fact that (i) PS I contains only about one-third of total chlorophylls, (ii) about 90% of light harvesting a/b protein (LAC) which accounts for about 50% of the total chlorophylls, remains associated with PS II as PS II-LHC II complexes (PS IIa) and (iii) the ratio of PS II/PS I is always greater than unity, in the range of 1-2 : 1 under different environmental regimes. Ohe initial preferential feeding of PS II, due to its larger antenna, is bound to result in faster rate of turn over of PS II than PS I, leading to higher rate of reduction of an intersystem carrier than the rate of its oxidation by PS I. Ohe light dependent phosphorylation of a 'mobile' and small pool (– 20%) of LHC II of PS IIα (possibly located at the edge of appressed regions of the membranes) increases the repulsive forces of LHC II resulting in its migration to non-appressed region associating itself with PS 1. Ohe phosphorylation itself is controlled by the redox state of an intermediate of electron transport. Several experimental approaches have provided evidence which suggest that (i) phosphorylation of LAC II involves interaction of cyt b5-f complex with LAC II kinase and the interaction of QA with cyt b5-f complex and (ii) different kinases may be involved in phosphorylation of LHC II versus PS II polypeptides. Ohe major purpose of light dependent LAC II phosphorylation and its consequent migration close to PS I appears to balance the rate of cyclic versus non-cyclic photophosphorylation. Ohe mechanism by which cyt b5-f complex controls the activation of LAC II is not known. Ohe role of membrane bound ealmodulin, electron transfer through cyt b6-f complex in activation of LAC II kinase should be explored.

Interaction of aromatic ionen oligomers with DNA.

A novel polycationic ionen was synthesized and fractionated on carboxymethyl- Sephadex using a salt gradient in 7M urea. A series of oligomers of discrete length were characterised by ultraviolet spectra. The ultraviolet spectra of oligomers revealed a new band centred at 232.5 nm which was probably due to exciton splitting. Thermal denaturation studies indicated both stabilization of the helix conformation and a higher degree of cooperativity in the melting of DNA (oligomers)n complex as compared to native calf thymus DNA. Ionen oligomers exhibited large extrinsic Cotton effect at 232.5 nm which could be attributed to exciton interaction.