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A series of 1,3,5-triphenyl-2-pyrazoline (TPP) nanocrystals were prepared by the reprecipitation method. The electronic transitions of TPP nanocrystals have been studied using both absorption and fluorescence spectra. An absorption feature at about 400 nm gradually appeared with increasing nanocrystal size. This feature resulted from the formation of the charge-transfer (CT) exciton in nanocrystals. It was also supported by the longer decay time of the nanocrystal emission from CT compared with that of the solution emission from [S.sub.1] at 298 K. On the other hand, the different behavior of the molecular [pi]-[pi] * and n-[pi] * transitions originates from the different overlapping modes of the pyrazoline [pi] orbital and n-electron orbital, according to the molecular model calculations. The nanocrystal emission from both the [S.sub.1] and CT states was found at 77 K; moreover, the emission intensity redistributed from [S.sub.1] to CT with increasing nanocrystal size. In contrast, at 298 K, only the emission from the CT states was observed, since the thermal fluctuation easily relaxes the excited electrons into CT states through vibration. Key words: reprecipitation method, organic nanocrystals, charge-transfer exciton, size-dependent property.

Abstract: Silver nanoparticles have been prepared by a non-template method. Silver nitrate can be easily decomposed into silver nanosized materials. Small nanoparticles (less than 2 nm) can be formed in aqueous solution. Larger silver particles of about 100 nm can be formed in ethanol solution. By rationally adjusting the experimental conditions we finally obtain silver particles of about 20 nm with a relatively narrow distribution in ethanol solution. Differing from the previous reports, we find that silver nanoparticles can be formed by direct decomposition of AgNO3 under UV light irradiation. No catalyst like TiO2 is needed at all. We believe that it is a further step to precede the preparation of silver nanometer sized materials. [Copyright &y& Elsevier]

Transition-metal oxides — including tungsten oxide, molybdenum oxide, titanium dioxide, vanadium pentoxide, niobium pentoxide and zinc oxide — exhibit photochromism upon bandgap excitation. These oxides constitute an important group of inorganic chromogenic materials, which is of great significance in the perspective of science and technology. During the past several decades, great progresses have been made in the microstructure, photochromic behavior and mechanism of these oxides, specifically of tungsten oxide. These studies underscore the opportunity of using these materials in photonic applications. The details will be presented in this review. [ABSTRACT FROM AUTHOR]

A simple graphene quantum dots (GQDs)-based fluorescence sensing system was developed for rapid, sensitive and low-cost detection of ascorbic acid (AA) in the presence of copper ions. Due to the efficient electron-transfer between GQDs and Cu 2+ ions, the fluorescence of GQDs was greatly quenched by Cu 2+ ions. Upon addition of ascorbic acid, Cu 2+ was reduced to Cu + and the interaction of Cu 2+ -GQDs was destroyed, which inhibit the quenching and thus leading to fluorescence recovery of GQDs. The detection process is ultrafast and can be completed within 5 min. The linear response range of AA was obtained from 0.3 μM to 10 μM. The detection limit was as low as 94 nM, which was comparable to or even better than other reported assays. In addition, this sensing system also showed good selectivity for detecting AA in the presence of different physiological molecules and inorganic ions. Finally, the proposed sensing system was successfully applied to assay AA in real samples with satisfactory results. [ABSTRACT FROM AUTHOR]

A new photochromic film was prepared based on the Keggin type H3PW12O40 (PW12) with polyacrylamide (PAM) matrix. The microstructure, photochromic behavior and mechanism were investigated with FT-Infrared spectroscopy (FT-IR), atomic force microscopy (AFM), transmission electron microscopy (TEM), ultraviolet–visible absorption spectroscopy (UV–VIS), and electron resonance spectroscopy (ESR). FT-IR results showed that the Keggin geometry of PW12 was still preserved inside the matrix, and a charge-transfer bridge was built between PW12 and PAM via hydrogen bond. AFM images indicated that the surface topography of the polymer matrix changed after adding PW12. In the hybrid films, the PW12 particles had regular microstucture in nanoscale shown in TEM image. Under UV irradiation, the film was reduced photochemically to yield a blue species. With the increase of irradiation time, the photoreduction proceeded from one-electron to two-electron blue stage. The characteristic signals of W (V) and radical in ESR spectra indicated that the photoreduced process was in accordance with the radical mechanism. [Copyright &y& Elsevier]