Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO(3)/NaBH(4)/polyvinylpyrrolidone (PVP)/trisodium citrate (Na(3)CA)/H(2)O(2) solutions by monitoring time-dependent surface plasmon resonance (SPR) bands in the UV-vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na(3)CA, and H(2)O(2) were studied without addition of a reagent, with different timing of each reagent's addition, and with addition of H(2)O(2) to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H(2)O(2). The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H(2)O(2) addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H(2)O(2). Oxidative etching of spherical particles by H(2)O(2) is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cube-to-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH(4)/PVP/Na(3)CA/H(2)O(2) solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength.

The effect of localized electric fields on the photocurrent responses of phthalocyanine that was self-assembled on a gold nanoparticle film was investigated by comparing the conventional and the total internal reflection (TIR) experimental systems. In the case of photocurrent measurements, self-assembled monolayers (SAMs) of a thiol derivative of palladium phthalocyanine (PdPc) were prepared on the surface of gold-nanoparticle film that was fixed on the surface of indium-tin-oxide (ITO) substrate via a polyion (PdPc/AuP/polyion/ITO) or on the ITO surface (PdPc/ITO). Photocurrent action spectra from the two samples were compared by using the conventional spectrometer, and were found that PdPc/AuP/polyion/ITO gave considerably larger photocurrent signals than PdPc/ITO under the identical concentration of PdPc. In the case of the TIR experiments for the PdPc/AuP/polyion/ITO and the PdPc/AuP/Glass systems, incident-angle profiles of photocurrent and emission signals were correlated with each other, and they were different from that of the PdPc/ITO system. Accordingly, it was demonstrated that the photocurrent signals were certainly enhanced by the localized electric fields of the gold-nanoparticle film.

Colloidal gold nanorods (GNRs), which were passivated with cationic cerasome-forming lipids having triethoxysilyl groups, were obtained in the aqueous phase by sonication of the mixture of lipids and GNRs.

The two-photon Ionization spectra of perylene and naphthacene In hexane have been measured with tunable dye lasers. They have shown structure, which can be correlated with the onephoton absorption spectra. The signal-to-noise (S/N) ratio has been evaluated as functlons of the excitatlon wavelength and the laser pulse energy, and the detection limit depended on the excitation wavelength, the laser pulse energy, and the molar absorptivity. The lowest detection limits ( S I N = 3) of perylene and naphthacene were 50 pg/mL at 383 nm and 20 pg/mL at 407 nm, respectlvely; these dye laser resuits were superior to those obtained by nitrogen-laser excitation. Table I. Lasers Used in Two-Photon Ionization Experiments wavelength, nm dye solvent 337a 360-376 PBD ethanol/toluene, 50/50 ethanol/toluene, 50/50 p-dioxane 374-394 BBQ 396-412 DPS 412-424 POPOP p-dioxane 418-426 Bis-MSB p-dioxane 426-448 C-440 ethanol 444-472 (2-460 ethanol Nitrogen laser.Laser two-photon ionization in solution is a sensitive technique for detection of polynuclear aromatic hydrocarbons (1-8), drugs (3), quinones (5, 7), and vitamins (6). Experimental conditions such as laser power, bias voltage, irradiation position, and electrode configuration have to be optimized (9), and the design of a photoionization cell is very important for sensitive detection of the photoionization signal (1,2,4,5, 7, 8).The threshold for photoionization in solution is considerably lower than the ionization potential in the gas phase due to the polarization energy of the positive ion and the electron affinity of the solvent (1 0); differences are -1-3 eV for aromatic molecules (10-13). Thus, UV-vis lasers such as a nitrogen (1-3,5-8), excimer (4), or dye (3,8,12,13) laser should be able to ionize most aromatic molecules by two-photon processes. With the use of dye lasers, the two-photon ionization spectra (wavelength dependence of the photoionization signal at a constant photon flux of the laser pulse) were obtained for 9-methylanthracene (8), pyrene (121, fluoranthene (13), and N,N,N',"-tetramethyl-p-phenylenediamine (13,14). The results show that the spectrum has structure (8,12,13) and the signal tends to be larger for a shorter wavelength (8,(12)(13)(14). Since two-photon ionization is more efficient for molecules with larger molar absorptivities from the ground to the first excited states (5,7), wavelength dependence will be valuable in choosing an optimal wavelength for the twophoton ionization detection with the highest signal-to-noise

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Laser two-photon Ionization of several aromatic molecules In hexane has been carried out with the fourth harmonics (266 nm) of a Nd-YAG laser as an excitation source. The photolonlzatlon current Is quadratlcally proportional to the laser pulse energy, Indicating two-photon Ionization. There Is a large blank signal due to the solvent (hexane) Ionization, and H has been canceled with a differential technique. The effect of laser pulse energy fluctuation has been corrected by an on-line normalization procedure. The photolonlzatlon current also depends on the molar absorptivity of the molecule, and the first step to Ionization should be excitation at 266 nm above the ground state. The detection limit ranges from 3.8 ppb for benzene to 61 ppt for pyrene. Analytical curves were linear for at least 3 orders of magnitude above the detection limits.