Jorge A. García-Macedo

Effect of chromophore–chromophore electrostatic interactions in the NLO response of functionalized organic–inorganic sol–gel materials ☆

Abstract In the last years, important non-linear optical (NLO) results on sol–gel and polymeric materials have been reported, with values comparable to those found in crystals. These new materials contain push–pull chromophores either incorporated as guest in a high T g polymeric matrix (doped polymers) or grafted onto the polymeric matrix. These systems present several advantages, however they require significant improvement at the molecular level—by designing optimized chromophores with very large molecular figure of merit, specific to each application targeted. Besides, it was recently stated in polymers that the chromophore–chromophore electrostatic interactions, which are dependent of chromophore concentration, have a strong effect into their NLO properties. This has not been explored at all in sol–gel systems. In this work, the sol–gel route was used to prepare hybrid organic–inorganic thin films with different NLO chromophores grafted into the skeleton matrix. Combining a molecular engineering strategy for getting a larger molecular figure of merit and by controlling the intermolecular dipole–dipole interactions through both: the tuning of the push–pull chromophore concentration and the control of tetraethoxysilane concentration, we have obtained a r 33 coefficient around 15 pm/V at 633 nm for the classical DR1 azo-chromophore and a r 33 around 50 pm/V at 831 nm for a new optimized chromophore structure.

Silver Core - Silver Oxide Shell Nanoparticles Embedded on Mesostructured Silica Films

Metallic silver particles in the nanometer size range were obtained in SiO2 matrix by the reduction of AgNO3 with the non-ionic diblock copolymer (Brij 58). Hexagonal mesostructured sol-gel films were synthesized by dip-coating method using the surfactant Brij58 to produce channels into the film, which house the silver nanoparticles. Optical properties of the metallic nanoparticles were studied by UV-Vis spectroscopy, TEM and HRTEM images. The experimental absorption spectrum of the metallic silver nanoparticles exhibits an absorption band located at 438 nm and a shoulder at longer wavelength. The TEM images show randomly distributed silver nanoparticles (Type I) along with some oriented as long line (Type II). Both distributions exhibit a silver oxide shell around of them. The second shell covering the silver core - silver oxide shell system is related to the surfactant. The optical absorption spectrum was modelled using the Gans theory. The fit shows two main contributions related to metallic silver nanoparticles with different axial ratios, and surrounding of a dielectric medium with high refractive index. Presence of the high refractive index silver oxide shell was confirmed by X-ray diffraction technique. The contributions of silver core and silver oxide shell play important roles in the optical properties of the films.

SERS on mesostructured thin films with silver nanoparticles

Mesostructured 2d-hexagonal sol-gel films were prepared by dip-coating method. Their structures were detected by X-ray diffraction. Silver nanoparticles reduced from Ag+ ion (silver nitrate) to Ag0 were deposited into the channels of the structure produced by the neutral surfactant Brij58. Surface enhanced raman spectroscopy technique was used to characterize the films. Spectra of films without metallic particles were compared to those with silver nanoparticles; the earliest exhibit an increased intensity on the 885, 955, 1061, 1129, 1230,1429, 1521, and 1796 cm-1 bands. This enhancement due to SERS is the result of the surface plasmon excitation inside the silver particles causing a reactivation of the Raman scattering from the molecules on the surface colloids. Photoconductivity studies were performed on mesostructured films with silver colloids. φl0 and φµτ parameters are bigger than those from photorefractive crystals KnbO3:Fe3+. The photovoltaic effect increases with AgNO3 concentration. Mesostructured film without silver colloids shows a small photovoltaic parameter.

Luminescence of Eu3+ incorporated into PZT tetragonal ceramics prepared by sol–gel

Luminescence of Eu3+ in Pb(ZrxTi1−x)O3 (PZT) polycrystalline tetragonal samples synthesized by sol–gel processing is reported. Studies by x-ray diffraction, energy dispersive spectroscopy and high resolution transmission electron microscopy demonstrate the incorporation of the dopant in the host. A broad charge transfer band centred around 266 nm was observed in the Eu3+ excitation spectra. Eu3+ is in a low symmetry site. Luminescence was lost between 673 and 1173 K. It was recovered after 1 h of heat treatment at 1273 K, when nanocrystallites were formed.

Synthesis, optical properties and modeling of silver core-silver oxide shell nanostructures in silica films

Sol-gel silica films containing silver ions were annealed in a rich-hydrogen atmosphere and subsequently in a rich-oxygen atmosphere. High resolution transmission electronic microscopy measurements shown unusual core-shell structures of silver-silver oxide nanoparticles in these films. The optical properties of these nanostructures exhibited red shift and damping in the UV-vis spectra. In order to calculate the optical properties were used the Mie theory using two approaches. The first one, corresponding to the core-shell model that includes a refractive index of the shell (ns) different to the one of the host matrix (nm). While in the second approach, we replaced in the Mie theory the refractive index of the environment (silica) by a local refractive index (nl) depending on the thickness of the silver oxide shell. Both calculations given the same results because they are equivalents.

Hybrid Organic-Inorganic Photodriven Nanoimpellers for Drug Release

Mesostructured SiO2 films functionalized with the azo-chromophore Disperse Red 1 were synthesized by sol-gel method. The mesostructured long-range order in the films was determined by X-Ray Diffraction. The azo-chromophores in the films work as nano-impellers through their photo-induced trans-cis reversible isomerization. When the films are doped, they are able to control the release of the dopant by all-optical processes. We used the dye laser Rhodamine 6G as dopant, its very distinctive luminescence around 550 nm allows to follow the release. Polarized green and infrared laser light were used as pump sources to direct the movement of the nano-impellers. 299 nm light was used as a probe to induce the Rhodamine 6G luminescence, which was measured as function of the pumping time with a photomultiplier coupled to a monochromator. The results corresponding to the green and to the infrared pumping sources are compared in order to determine the feasibility to photo-control the nano-impellers movement through a two-photon absorption process.

Photoconductivity on nanostructured sol-gel thin films with silver nanoparticles

2d-hexagonal nanostructured sol-gel thin films were prepared by dip-coating method. A neutral surfactant Brij58 was used as template to produced channels into the film. The structure was identified by X-ray diffraction and TEM. Silver nanoparticles were obtained by spontaneous reduction process of Ag+ ions to Ag0 at room temperature. A broad band located at 430 nm was detected by optical absorption; it corresponds to the surface plasmon. Photoconductivity studies were performed on films with ions and with silver nanoparticles to characterize their mechanisms of charge transport in the darkness and under illumination at 420 and 633 nm wavelengths. Straight lines showing an ohmic behavior fit the experimental data. Films without colloids possess normal photoconductivity behavior. But films with colloids present an abnormal response. Transport parameters were calculated. The films with silver nanoparticles exhibit a photovoltaic effect stronger than the films without nanoparticles, except to high concentrations. A theoretical model is proposed to predict these processes as function of the silver concentration.

Photocatalytic antibacterial effect of ZnO nanoparticles into coaxial electrospun PCL fibers to prevent infections from skin injuries

Antibacterial studies of inorganic nanoparticles (nps) have become important due to the increased bacterial resistance against antibiotics. We used Zinc oxide nanoparticles (ZnO nps), which possess excellent photocatalytic properties with a wide band gap (Eg), are listed as “generally recognized as safe” by the Food and Drug Administration (FDA) and have shown antibacterial activity (AA) against many bacterial strains. The AA of ZnO nps is partly attributed to the production of Reactive Oxygen Species (ROS) by photocatalysis. When ZnO nps in aqueous media are illuminated with an energy <Eg, electron-hole pairs are generated on nps surface reacting with water and Oxygen molecules to generate hydroxyl-radical (OH• ), superoxide-radical (O2 •- ) and hydrogen-peroxide (H2O2). These ROS induce cell membrane damage resulting in cell death. However, the application of inorganic nps in medical treatments is limited due to the possible long-term side effects of nps release. To prevent its release, ZnO nps were dispersed into Polycaprolactone (PCL) fibers obtained by electrospinning technique. To optimize the use of ZnO nps concentration, we developed coreshell coaxial electrospun fibers where the core corresponded to PCL and the shell to a mixture of ZnO nps/PCL. Thus, ZnO nps were only dispersed on the surface of the fibers increasing its superficial contact area. We evaluated the AA against E. coli of different electrospun ZnO nps/PCL fibers under two different conditions: UVA pre-illumination and darkness. Preliminary results suggest that the AA against E. coli is better when electrospun ZnO nps/PCL were preilluminated with UVA than under darkness conditions.

Photoconductivity and Stabilization of Dopamine embedded in sol-gel TiO2 matrix

This work reports the synthesis of amorphous TiO2 matrix by sol-gel method at atmospheric conditions. DA was encapsulated in a TiO2 matrix to reduce its chemical instability. To TiO2/DA sample was added the 15C5 to diminish the oxidation process. The stabilization process was followed by absorption spectra, colour change and infrared spectroscopy. Oxidation processes of the DA were identified by the presence of DA-quinone and DA-chrome. The TiO2/DA complex retarded the oxidation process for 30 days, while the TiO2/DA/15C5 complex this period was extended for 47 days. Photoconductivity studies were performed on both kinds of samples to analyze their charge transports. The experimental data were fitted with straight lines at darkness and under illumination at 320 nm, 400 nm, and 515 nm. This indicates an ohmic behavior. Transport parameters were calculated. The conductive effect is stronger under darkness than under illumination at 320 nm because the oxidation process in the darkness is less intense than under illumination.

Corona poling assisted second harmonic genertion in nanostructured polymethylmetacrylate films

Films made of Polymethylmetacrylate (PMMA) and doped with dipolar second order nonlinear optical chromophores were studied by UV-visible spectroscopy and transmitted resonant Second Harmonic Generation technique. The chromophores were non-centrosymmetrically oriented by a Corona poling field. The UV-visible spectra were measured in poled films at normal incidence as function of their poling temperatures (60, 80 and 100°C) and they were analyzed in terms of the Second Order Parameter (A2). The Second Harmonic Generation (SHG) signals in the films were measured in-situ as function of the poling time at several incidence angles for each poling temperature. The stability of the SHG signal was also determined, by turning off the Corona field but leaving the films at their original poling temperature. The films were of two different kinds: amorphous and nanostructured. All the films doped simultaneously with chromophores and surfactants showed long-range ordered nanostructures. Two kinds of surfactants were used during the synthesis of the films: ionic and neutral, both of them induced long-range order in the structure of the PMMA, but only some of the nanostructured films exhibited enough large SHG signals. The second order nonlinear optical response of the nanostructured films was compared with the corresponding response of the amorphous films. The long-range order in the films was detected by X-Ray Diffraction (XRD).