Photonic devices can be developed, and their working principle can be understood only by considering the phenomena taking place at the nanoscale level. Optical properties of plasmonic structures depend on their geometric parameters and are sensitive to them. Recently, many advanced methods for the preparation of nanostructures have been proposed; however still, the geometric parameters are inaccurate. Numerical simulations provide a powerful tool for the analysis of plasmonic nanostructures. To the best of our knowledge, there are not many papers on near-field and far-field properties of single nanoprism and nanoprism dimer, the so-called bowtie, with rounded edges. For this purpose, Finite Integration Technique implemented to the CST Microwave Studio was used. Besides the edge rounding, an additional modification of the resonance modes was investigated, achieved by placement of a spherical nanoparticle in the gap between the prisms. Results of numerical simulations indicate that the radius of the curvature edges strongly affects the plasmon peak localization, and this effect cannot be neglected in plasmonic device design. Increase in the radius of edge curvature causes main extinction cross-section peak blueshift in all cases analyzed. Moreover, our calculations imply that the nanoparticle in the gap between prisms strongly influences the dependence of spectral properties on the radius curvature.
Gold nanorods deserve special attention as they exhibit tunable longitudinal localized surface plasmon resonances (LSPRs). In our study, gold nanorods of the aspect ratio of 2.25 (maximum of LSPR band at 660 nm) and of controllable SiO 2 thickness in the range of 6−14 nm were mixed with pheophorbide (chlorophyll derivative) in order to create a hybrid system. Energy transfer and singlet oxygen generation were studied for different SiO 2 thicknesses of the nanorod shell. The spectral properties of the hybrid mixture were characterized, and the overlapping of the pheophorbide fluorescence and the longitudinal LSPR band of nanorods on the fluorescence emission, energy transfer, and generation of singlet oxygen were studied. Two independent approaches were used to determine the quantum yield and enhanced factor of singlet oxygen generation. For a certain thickness of the SiO 2 shell and for certain concentrations of gold nanorods, the effect of the plasmon-enhanced singlet oxygen production was observed. Moreover, the enhanced of singlet oxygen yield enhancement was correlated with the far-field optical properties of the gold nanorods. The results obtained indicate the significance of further studies of dye-photosensitizers in hybrid mixtures, taking into account the spectral overlap between dye emission and longitudinal LSPR bands as well as the character of coatings (type and thickness) and scattering yields of gold nanorods.
Photosynthetic energy conversion competes with the formation of chlorophyll triplet states and the generation of reactive oxygen species. These may, especially under high light stress, damage the photosynthetic apparatus. Many sophisticated photoprotective mechanisms have evolved to secure a harmless flow of excitation energy through the photosynthetic complexes. Time-resolved laser-induced optoacoustic spectroscopy was used to compare the properties of the T states of pheophytin a and its metallocomplexes. The lowest quantum yield of the T state is always observed in the Mg complex, which also shows the least efficient energy transfer to O . Axial coordination to the central Mg further lowers the yield of both T and singlet oxygen. These results reveal the existence of intrinsic photoprotective mechanisms in chlorophylls, embedded in their molecular design, which substantially suppress the formation of triplet states and the efficiency of energy transfer to O , each by 20-25 %. Such intrinsic photoprotective effects must have created a large evolutionary advantage for the Mg complexes during their evolution as the principal photoactive cofactors of photosynthetic proteins.
In a hybrid mixture of organic (dye) and inorganic (metallic nanoparticles) components, the optical properties of a dye can be easily controlled by tailoring the shape or the concentration of the noble metal nanoparticles (NPs). The influences of multiexcitation (multiwavelength excitation) of photosensitizers (pheophorbide a and hematoporphyrin) on the interactions with pegylated Au-NPs and on the photophysical parameters of the dyes are studied. Detailed, systematic fluorescence quenching studies were performed in the mixtures of different contents of Au-NPs, and interpreted together with the results of quantum singlet oxygen yield examinations. According to the results, the fluorescence of the two dyes studied was effectively quenched in the presence of Au-NPs, mainly because of the resonance energy transfer between the donor (dye) and the acceptor (Au-NPs). Stern-Volmer quenching constants were determined by a few orders of magnitude higher than those describing the photochemical quenching process. In hybrid mixtures analyzed, the mechanism of energy transfer between the donor and the acceptor was nanometal surface energy transfer. Furthermore, different behavior of the mixtures on excitation with the wavelengths from the Soret and Q bands of the dyes and with those corresponding to the surface plasmon resonance band of Au-NPs was analyzed. Moreover, for certain concentrations of Au-NPs and for certain excitation wavelengths, an increase in singlet oxygen generation was observed. The results obtained indicate the significance of further studies of photosensitizers in hybrid mixtures with NPs.
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