Aggregates of Au nanoparticles have been extremely easily obtained on glass substrates by physical sputtering under primary vacuum. With such a protocol, we demonstrate that it is possible to control the surface plasmon band absorption. Surface enhanced Raman spectroscopy (SERS) experiments were performed with methylene blue, zinc octacarboxyphthalocyanine, 4-aminothiophenol and cysteamine. The correlation between the absorption band and the wavelength giving the highest SERS intensity is clearly observed for methylene blue, in accordance with the electromagnetic enhancement theory. For the other molecules, effects of the chemical enhancement are also observed. In addition, we noticed a strong influence of the nature of the adsorbed molecule on the enhancement factor for a given wavelength. The origin of this feature is discussed in terms of resonant effects or multipolar surface plasmon modes.

The ecological success of diatoms is emphasized by regular blooms of many different species in all aquatic systems, but the reason behind their success is not fully understood. A special feature of the diatom cell is the frustule, a nano-patterned cell encasement made of amorphous biosilica. The optical properties of a cleaned single valve (one half of a frustule) from the diatom Coscinodiscus centralis were studied using confocal micro-spectroscopy. A photonic crystal function in the frustule was observed, and analysis of the hyperspectral mapping revealed an enhancement of transmitted light around 636 and 663 nm. These wavelengths match the absorption maxima of chlorophyll a and c, respectively. Additionally, we demonstrate that a highly efficient light trapping mechanism occurred, resulting from strong asymmetry between the cribrum and foramen pseudo-periodic structures. This effect may prevent transmitted light from being backscattered and in turn enhance the light absorption. Based on our results, we hypothesize that the multi-scaled layered structure of the frustule improves photosynthetic efficiency by these three mechanisms. The optical properties of the frustule described here may contribute to the ecological success of diatoms in both lentic and marine ecosystems, and should be studies further in vivo.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Optical properties of single diatom frustule valves from two different Coscinodiscus species (C. Wailesii and C. Centralis) are studied by transmission confocal hyperspectral imaging and numerical calculations. Light convergence, concentration and trapping effects are observed and depend on both the wavelength and the valve orientation. These effects seem to occur independently of the incident light angle. From our results, a wavelength dependent multifocal lens behavior can be explained by light diffraction related to the radial symmetry of the multi-scaled 3D nanostructure.

A multilayer model of a single coated nanoparticle has been refined through finite elements method based simulations and resulted in a successful matching of the experimental UV–visible spectra of ligand-coated silver nanoparticles. The computational matching of the surface plasmon resonance (SPR) band reveals both a ligand-type dependence of the effective plasma frequency and a size dependence of the SPR damping effect within the modeled nanoparticle. The observed differences of effective plasma frequency between thiol and amine-coated nanoparticles are consistent with the already known stronger bonding of thiols on silver compared to amines. The significant increase of the damping effect at the surface of the nanoparticle when increasing their size suggests an inverse relation between the ligand packing density and the nanoparticle size, which is supported by the expected influence of the surface curvature radius on the ligand packing.