Noble-metal nanoparticles labeled with fluorescent molecules are used in a variety of applications requiring the measurement of size and diffusion properties of single nanoprobes. We have successfully used intrinsic surface-plasmon-induced photoluminescence (SPPL) signatures of monodispersed bare gold and silver nanoparticles in water to detect and measure their precise diffusion coefficient, concentration and hydrodynamic radius by fluorescence correlation spectroscopy (FCS). Measurement of the effective hydrodynamic radius confirms particle size to be 80 ± 8 and 64 ± 14 nm for gold and silver, respectively, which is in excellent agreement with scanning electron microscopic measurements made on the same particles. Detection of bare gold and silver nanoparticles at the single-molecule level with moderately high value of "per particle brightness" (PPB) confirms those particles to be used as fluorescent probes in biological research and in different medical and biotechnology applications where fluorescence detection plays a vital role. Additionally, these results demonstrate an alternative method for measuring hydrodynamic properties, particularly the size-distribution of bare noble-metal nanoparticles in solution using data-fitting algorithm for FCS based on the maximum entropy method (MEMFCS).
Surface morphology and photochemical isomerization properties of monolayers of anthrancene acrylic acid derivative with cholesterol (a new class of bistable compound), cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate (CAE), transferred onto quartz substrates were studied. The spectroscopic and photochromic behavior of CAE on solid substrates and in solution are compared keeping in mind the possible application of CAE in constructing molecular electronic devices. Monolayers of the trans(E)-isomer of CAE transferred from the airÀwater interface onto quartz plates show regular distribution of "holes" in the film, whereas similar monolayers of the cis(Z)-isomer of CAE (∼96%) show very smooth surfaces, free from any definite structures. The surface pressureÀarea (πÀA) isotherms of both monolayers at the airÀwater interface are found to be irreversible, indicating formation of 2D/3D aggregates for both isomers. The surface potentialÀarea (ΔVÀA) isotherms of the two isomers predict the orientation of their molecular dipoles to be different. The fluorescence peak intensity of the E-isomer of CAE in transferred monolayers shows a sharp decrease upon irradiation with 405 nm light, indicating the successful E-to-Z isomerization in the monolayer. Fluorescence excitation and emission polarization studies on the solid substrate also confirm the change of molecular orientation resulting from the E-to-Z isomerization. The isomerization rate is found to be faster in solid substrates than that in the solution phase. Six alternate monolayers of E-CAE and triplet sensitizer (liphophilic porphyrin) film shows 5% efficiency of Z-to-E isomerization upon exciting on 550 nm, where porphyrin has substantial absorbance where as film of 24 monolayers of mixture solution of the E-isomer of CAE (1 mM) and liphophilic porphyrin (1 mM) in chloroform increases 5-fold efficiency of Z-to-E conversion. These results suggest that the E-CAE has the potential to be used in making optical data storage devices employing the transÀcisÀtrans isomerization process.
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