Sepiolite promotes photodegradation of pyrene under visible light.

Mechanochemistry and photocatalysis are emergent technologies for the remediation of polycyclic aromatic hydrocarbons (PAHs) in soils. In this work, mechanochemistry and photocatalysis are combined for pyrene degradation. The photodegradation of pyrene, when in contact with sepiolite under pressure application, is studied. The mechanical treatment leads to a pyrene crystal phase transformation. In this new phase, pyrene undergoes a fast photodegradation in the 320-420 nm range. We show that sepiolite is superior as a photocatalyst in pyrene degradation to TiO2, the most exploited photocatalyst. A broad physicochemical characterization is carried out to propose a mechanism in which the photoexcitation of mechanically altered pyrene leads to an electron transfer to sepiolite matrix, which triggers the PAH degradation. Finally, we want to highlight that the pyrene/sepiolite combination is a simplified system to shed light on how PAH photodegradation may occur in soils.

Fano-Like Resonance from Disorder Correlation in Vacancy-Doped Photonic Crystals.

By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defects.

Colored Surfaces Made of Synthetic Eumelanin.

The polymerization of 3,4-dihydroxy-L-phenylalanine leads to a carboxylic acid-rich synthetic melanin-like material (poly-L-DOPA). Synthetic melanin most resembles natural eumelanin in chemical structure. However, its deposition on surfaces leading to colored surfaces by interference is not as easy to accomplish as in the case of the preparation of colored surfaces by dopamine hydrochloride polymerization. This study deals with the preparation of new colored surfaces made from poly-L-DOPA displaying vivid colors by interference. These surfaces were obtained by depositing thin films of poly-L-DOPA on a reflective silicon nitride substrate. A high ionic strength in the polymerization medium was essential to accomplish the coating. The effect of ionic strength on the resulting surfaces was studied via reflectance, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The refractive index was determined by ellipsometry, and was nearly constant to 1.8 when λ > 650 nm. In the visible spectral region, the imaginary part of the refractive index becomes relevant. The refractive index in the visible wavelength range (400-600 nm) was in the range 1.7-1.80.

Surface Wettability of a Natural Rubber Composite under Stretching: A Model to Predict Cell Survival.

We report the stress-strain effect of a stretchable natural rubber (NR)-calcium phosphate composite on the surface wettability (SW) using an innovative approach coupling a uniaxial tensile micromachine, goniometer, and microscope. In situ contact angle measurements in real time were performed during mechanical tension. Our results show that SW is guided by the stress-strain relationship with two different characteristics, depending on the static or dynamic experiments. The results evidenced the limits of the classical theory of wetting. Furthermore, based on the mechanically tunable SW of the system associated with the cytocompatibility of the NR composite, we have modeled such a system for application as a cell support. From the experimental surface energy value, our proposed 3D modeling numerical simulation predicted a window of opportunities for cell-NR survival under mechanical stimuli. The presented data and the thermodynamics-based theoretical approach enable not only accurate correlation of SW with mechanical properties of the NR composite but also provide huge potential for future cell supportability in view of tissue engineering.

Vacancies in Self-Assembled Crystals: An Archetype for Clusters Statistics at the Nanoscale.

Complex systems involving networks have attracted strong multidisciplinary attention since they are predicted to sustain fascinating phase transitions in the proximity of the percolation threshold. Developing stable and compact archetypes that allow one to experimentally study physical properties around the percolation threshold remains a major challenge. In nanoscale systems, this achievement is rare since it is tied to the ability to control the intentional disorder and perform a vast statistical analysis of cluster configurations. Here, a self-assembly method to fabricate perfectly ordered structures where random defects can be introduced is presented. Building binary crystals from two types of dielectric nanospheres and selectively removing one of them creates vacancies at random lattice positions that form a complex network of clusters. Vacancy content can be easily controlled and raised even beyond the percolation threshold. In these structures, the distribution of cluster sizes as a function of vacancy density is analyzed. For moderate concentrations, it is found to be homogeneous throughout the structure and in good agreement with the assumption of a random vacancy distribution.

Color Engineering of Silicon Nitride Surfaces to Characterize the Polydopamine Refractive Index.

A simple methodology to generate polydopamine (PDA) surfaces featured with color due to thin-film interference phenomena is presented. It is based on depositing ultra-thin films of polydopamine on a Si/Si3 N4 wafer that exhibits an interferential reflectance maximum right at the visible/UV boundary (∼400 nm). Therefore, a small deposit of PDA modifies the optical path, in such manner that the wavelength of the maximum of reflectance red shifts. Because the human eye is very sensitive to any change of the light spectral distribution at the visible region, very small film thickness changes (∼30 nm) are enough to notably modify the perceived color. Consequently, a controlled deposit of PDA, tune the color along the whole visible spectrum. Additionally, good quality of PDA deposits allowed us to determine the refractive index of polydopamine by ellipsometry spectroscopy. This data can be crucial in confocal skin microscopic techniques, presently used in diagnosis of skin tumors.

Magnetic modulation of mid-infrared plasmons using Giant Magnetoresistance.

We propose a novel approach to generate active mid IR plasmonic systems by incorporating giant magnetoresistance as the driving mechanism. The magnetic field induced change in resistivity in Ni81Fe19/Au multilayers directly affects the diagonal elements of the dielectric tensor of the system, mainly in the mid IR range, via the magnetorefractive effect. With magnetic fields as small as 50 Oe, we postulate the possibility to modulate the response of such continuous and patterned structures in a contactless, easy to implement fashion. The potential application impact of this modulation concept is analyzed.

Hydrogen Spillover between Single Gold Nanorods and Metal Oxide Supports: A Surface Plasmon Spectroscopy Study.

We used dark field spectroscopy to monitor the dissociation of hydrogen on single gold nanoparticles embedded in metal oxide supports. Individual gold nanorods were monitored in real time to reveal the peak position, the full width at half-maximum, and the relative intensity of the surface plasmon resonances during repeated N2-H2-N2 and air-H2-air cycles. Shifts in the spectra are shown to be due to changes in electron density and not to refractive index shifts in the environment. We demonstrate that hydrogen does not dissociate on gold nanorods (13 nm × 40 nm) at room temperature when in contact with silica and that electrons or hydrogen atoms migrate from Pt nanoparticles to Au nanoparticles through the supporting metal oxide at room temperature. However, this spillover mechanism only occurs for semiconducting oxides (anatase TiO2 and ZnO) and does not occur for Au and Pt nanoparticles embedded in silica. Finally, we show that hydrogen does dissociate directly on anatase surfaces at room temperature during air-H2-air cycles. Our results show that hydrogen spillover, surface dissociation of reactants, and surface migration of chemical intermediates can be detected and monitored in real time at the single particle level.

Dielectric behavior of ceramic-graphene composites around the percolation threshold.

Al2O3/graphene and BaTiO3/graphene composites with different concentrations of the conductive second phase, both below and above the percolation threshold, were prepared by the traditional ceramic processing route followed by spark plasma sintering. It is shown that the addition of graphene pins the grain growth of the ceramic matrix grains, leading to a change of the microstructure at low filler concentrations. As a consequence, the composites exhibit two percolation thresholds and their dielectric properties are not only determined by the dielectric properties of the constituents and their relative fractions but also the microstructure of the composite must be considered. Additionally, a giant increase of the dielectric constant has been found around the percolation thresholds in barium titanate-graphene composites. In particular, values of the dielectric constant up to 45,000 and 15,000 were found at 1 kHz in composites containing 0.4 and 0.6 wt. % graphene, respectively.

Influence of the close sphere interaction on the surface plasmon resonance absorption peak.

Effects of agglomeration in optical properties of noble metal colloids (gold) have been calculated by an integral equation formalism valid for small (around 6 nm) nanoparticles in the spectral region corresponding to surface plasmon resonance (SPR). These calculations are characterized by their accuracy and for the generality of their solutions, which do not depend on the nature of the materials nor the wavelength of the incident electric field. Chains of two (dimers), three (linear trimers) and four (linear tetramers) spheres have been taken as geometric models. Results indicate that for distances between spheres larger than 0.3 times the diameter, only dipolar interactions are relevant. For smaller distances between spheres, multipolar interactions generate low frequency modes that transform into a continuous band in the limiting case of touching spheres, band asymmetry and SPR broadening. Additionally, strong and sharp oscillation corresponding to surface charge density and electric field appear in the close neighborhood of sphere contact. These features are very difficult to determine by ad-hoc simulations but they should be taken into account when dealing with nearly touching nanosphere systems.

On the transparency of nanostructured alumina: Rayleigh-Gans model for anisotropic spheres.

A light scattering model under the Rayleigh-Gans-Debye approximation has been developed for polycristalline alumina. The model states that transmittance of dense alumina ceramics basically depends not only on the maximum grain size but also on the preferential orientation of their c-axis, or texture. The effect of texture in transparency has been experimentally measured on several dense alumina samples with different grain size and compared to that obtained from x-ray Rietveld refinements with a very good agreement. The Rayleigh-Gans-Debye approximation also allows to represent optical data in a very simple way (logarithm of transmittance vs. the inverse of the wavelength square). Using these variables, a straight line is obtained for the Rayleigh-Gans-Debye approximation, its slope being proportional to the maximum grain size and textural parameter. Deviation from this law implies the presence of pores or grain of extremely large size.