Photocatalytic activity of silica and silica-silver nanocolloids based on photo-induced formation of reactive oxygen species.

Semiconductor nanomaterials are often proposed as photocatalysts for wastewater treatment; silica nanomaterials are still largely unexploited because their photocatalytic performances need improvements, especially under visible light. The present study is a proof-of-concept that amorphous silica colloids once submitted to the proper surface modifications change into an efficient photocatalyst even under low-energy illumination source. For this reason, silica-based colloidal nanomaterials, such as bare (SiO2 NPs), aminated (NH2-SiO2 NPs), and Ag NPs-decorated (Ag-SiO2 NPs) silica, are tested as photocatalysts for the degradation of 9-anthracenecarboxylic acid (9ACA), taken as a model aromatic compound. Interestingly, upon irradiation at 313 nm, NH2-SiO2 NPs induce 9ACA degradation, and the effect is even improved when Ag-SiO2 NPs are used. On the other hand, irradiation at 405 nm activates the plasmon of Ag-SiO2 NPs photocatalyst, providing a faster and more efficient photodegradation. The photodegradation experiments are also performed under white light illumination, employing a low-intensity fluorescent lamp, confirming satisfying efficiencies. The catalytic effect of SiO2-based nanoparticles is thought to originate from photo-excitable surface defects and Ag NP plasmons since the catalytic degradation takes place only when the 9ACA is adsorbed on the surface. In addition, the involvement of reactive oxygen species was demonstrated through a scavenger use, obtaining a yield of 17%. In conclusion, this work shows the applicability of silica-based nanoparticles as photocatalysts through the involvement of silica surface defects, confirming that the silica colloids can act as photocatalysts under irradiation with monochromatic and white light. Silica and Ag-decorated silica colloids photosensitize the formation of Reactive Oxygen Species with 17% efficiencies. ROS are able to oxidase aromatic pollutants chemi-adsorbed on the surface of the colloids. Silica-silver nanocomposites present a photocatalytic activity useful to degrade aromatic compounds.

Molecular-based upconversion in homo/heterogeneous liquids and in micro/nanostructured solid materials.

Radiation upconversion can be an elegant and efficient strategy to minimize waste in energy harvesting and storage processes. The upconversion based on triplet-triplet annihilation processes of molecular dyes is a very versatile approach, but it requires a systematic photophysical characterization of the systems to optimize the upconversion yields and develop materials for technological applications. This paper represents an overview of the work carried out in our laboratories for the study and characterization of a molecular dye pair, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(ii) (PtOEP) and 1,3,6,8-tetraphenylpyrene (TPPy), suitable as the sensitizer and emitter, respectively, in a triplet-triplet annihilation based upconversion process. The investigation has been carried out in various media with increasing complexity. First, we used the dye pair to characterize the UC-efficiencies in homogeneous solvents of different viscosities and in oil-in-water microemulsions; then we explored the possibility to achieve upconversion in solid materials, like nanostructured silica matrices and liquid filled microcapsules. The possibility to achieve upconversion emission even in confined and rigid media has been confirmed and can inspire further applications of the process.

In situ synthesis of Au-shelled Ag nanoparticles on PDMS for flexible, long-life, and broad spectrum-sensitive SERS substrates.

A simple and fast one-step fabrication method of silver nanoparticles (AgNPs) on a polydimethylsiloxane (PDMS) film and their improvement as highly sensitive surface enhanced Raman scattering (SERS) substrates via atomically thin Au coatings is demonstrated. The thin Au layer provides oxidation resistivity while maintaining the broad spectral range SERS sensitivity of Ag nanoparticles.

A novel method for in situ synthesis of SERS-active gold nanostars on polydimethylsiloxane film.

A novel one-step in situ synthesis of gold nanostars (AuNSs) on a pre-cured polydimethylsiloxane (PDMS) film is proposed for the fabrication of highly sensitive surface-enhanced Raman scattering (SERS) substrates. Plasmonic activity of the substrates was investigated by collecting SERS maps of 4-mercaptobenzoic acid (4-MBA). The applicability of these flexible substrates is further demonstrated by SERS-based pesticide detection on fruit skin.

Nano-biocomposite films with modified cellulose nanocrystals and synthesized silver nanoparticles.

Ternary nano-biocomposite films based on poly(lactic acid) (PLA) with modified cellulose nanocrystals (s-CNC) and synthesized silver nanoparticles (Ag) have been prepared and characterized. The functionalization of the CNC surface with an acid phosphate ester of ethoxylated nonylphenol favoured its dispersion in the PLA matrix. The positive effects of the addition of cellulose and silver on the PLA barrier properties were confirmed by reductions in the water permeability (WVP) and oxygen transmission rate (OTR) of the films tested. The migration level of all nano-biocomposites in contact with food simulants were below the permitted limits in both non-polar and polar simulants. PLA nano-biocomposites showed a significant antibacterial activity influenced by the Ag content, while composting tests showed that the materials were visibly disintegrated after 15 days with the ternary systems showing the highest rate of disintegration under composting conditions.

Ag/AgCl nanoparticle decorated layered double hydroxides: synthesis, characterization and antimicrobial properties.

A layered double hydroxide (LDH) surface was employed as a substrate for growing silver nanoparticles (NPs). An efficient method to produce stable silver/silver chloride nanoparticles supported on the ZnAl-LDH surface was developed. NPs of AgCl were grown on the ZnAl-LDH surface by using AgNO3 as the silver source. The ZnAl-LDH in chloride form acts as a nucleating agent, and depending on the pH of the LDH dispersion, AgClNPs with different dimensions were obtained. In particular AgClNPs with a diameter of 60 nm were formed at pH 5. The AgClNPs supported on LDH sheets were partially reduced by different reducing agents (NaBH4 and formaldehyde) resulting in a Ag/AgCl-LDH nanocomposite. The silver chloride and silver NP dimensions were evaluated by X-ray powder diffraction, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). UV-Vis spectra of the samples upon reduction showed a band centred at 415 nm due to the surface plasmon resonance of silver nanoparticles with a diameter of about 10 nm, in agreement with the TEM analysis. The AgCl-LDH and Ag/AgCl-LDH nanocomposites, subjected to antimicrobial tests, exhibited good antimicrobial activity against both Gram-negative (Pseudomonas aeruginosa) and Gram-positive (Staphylococcus epidermidis and S. aureus) bacteria and yeast (Candida albicans). The nanocomposites were also studied for their ability to release silver by obtaining release curves, under conditions of antibacterial assays. Finally, the nanocomposites antibacterial behavior, as a function of time, was investigated by performing time-kill experiments using S. aureus and Candida albicans.

PLGA/Ag nanocomposites: in vitro degradation study and silver ion release.

New nanocomposite films based on a biodegradable poly (DL-Lactide-co-Glycolide) copolymer (PLGA) and different concentration of silver nanoparticles (Ag) were developed by solvent casting. In vitro degradation studies of PLGA/Ag nanocomposites were conducted under physiological conditions, over a 5 week period, and compared to the behaviour of the neat polymer. Furthermore the silver ions (Ag(+)) release upon degradation was monitored to obtain information on the properties of the nanocomposites during the incubation. The obtained results suggest that the PLGA film morphology can be modified introducing a small percentage of silver nanoparticles that do not affect the degradation mechanism of PLGA polymer in the nanocomposite. However results clearly evinced the stabilizing effect of the Ag nanoparticles in the PLGA polymer and the mineralization process induced by the combined effect of silver and nanocomposite surface topography. The Ag(+) release can be controlled by the polymer degradation processes, evidencing a prolonged antibacterial effect.

Comprehensive photochemistry and photophysics of land- and marine-based beta-carbolines employing time-resolved emission and flash transient spectroscopy.

The photophysical and photochemical behavior of Norharmane (Norh), Harmane (Hara) and Harmine (Hari) and their cations have been examined as a function of the nature of the solvent. Time-resolved emission in nonprotic polar solvents showed fluorescence for all and also phosphorescence for Hari. All emissions were assigned as those of the neutral molecules. Norh and Hari showed fluorescence of both the neutral and the cation in methanol as well as phosphorescence of the neutral while Hari also had fluorescence of the zwitter ion. In ethanol, Norh and Hari displayed fluorescence and phosphorescence of the neutral. The ground-state cations of Norh and Hari exhibited fluorescences of the cation and Hari also had a phosphorescence (cation). The flash transient spectra in nonprotic solvents of all three carbolines had long-lived triplet transients only of the neutral. Triplet and singlet oxygen yields were quite high, 0.31-0.40. Direct excitation of any of the cations gave only the cation triplet. The triplet yields of the cations appear to be low (0.01-0.10 range). Theoretical calculations were done relative to location of triplet states. Some new information will be reported on other naturally occurring differently substituted marine-based beta-carbolines. The impact of all of the foregoing observations on the photosensitizing potential of all compounds is discussed.

Intercalation of the radical scavenger ferulic acid in hydrotalcite-like anionic clays.

Hydrotalcite is a biocompatible lamellar anionic clay formed by double hydroxide layers with a metal cation coordinating four OH groups. The different layers are held together by anionic hosts that can be replaced by a simple ion-exchange process. The synthetic Mg-Al-hydrotalcite was used to intercalate ferulic acid, a compound that shows antioxidant properties due to its free radical scavenger capacity. Analysis of the intercalated compound showed a good intercalation percentage (35.53%) accompanied by an increase of the interlayer space from 7.8A (chloride form) to 17.1A due to the presence of the ferulate. The intercalation product was stable in water, did not show any significant degradation after UV-irradiation, had a higher capacity of UV absorption in comparison to both the pure ferulic acid and ferulic acid-hydrotalcite chloride physical mixture. The intercalated compound was formulated in a siliconic cream and the ferulate in vitro release profiles determined.

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.

Photophysical properties and photobiological activity of the furanochromones visnagin and khellin.

The larger photobiological activity of visnagin (VI) versus khellin (KH) toward several living organisms, including fungi, viruses, yeasts and bacteria, induced a detailed investigation of the photophysical properties of these naturally occurring furanochromones, using laser-flash-photolysis, photoacoustic calorimetry and fluorescence (steady-state and time-resolved) techniques in solvents with different polarity and content of water, including micelles and vesicles. The results have shown that the magnitude of all the three rate constants out of S1 (radiative, kf; internal conversion, kic and intersystem crossing, kisc) for VI and KH strongly depend on the solvent, namely on its hydrogen bonding ability and polarity. The changes of kf and kisc are due to the solvent-assisted mixing and/or inversion of the two first singlet excited states (1n, pi and 1 pi, pi), while kic increases with a decrease of the S0-S1 energy gap. As a consequence, the quantum yield of triplet formation (phi T) strongly decreases from values of approximately 0.8 in dioxane to < 0.05 in water for both compounds. The magnitude of solvent polarity/hydrogen bonding ability required, at which the state order is inverted and phi T starts to decrease, is greater for VI than for KH and consequently phi T (VI) >> phi T (KH) over a broad range of water content including that appropriate to the environment of the compounds in a living system. These facts account for the larger photobiological activity of VI with respect to KH, regarding both the fungus Fusarium culmorum L. and the wild strain of Escherichia coli, studied by us.