Bactericidal Properties of Plasmonic Photocatalysts Composed of Noble Metal Nanoparticles on Faceted Anatase Titania.

Octahedral anatase particles (OAP) with eight equivalent {101} facets and decahedral anatase particles (DAP) with two additional {001} facets were modified with nanoparticles of noble metals (silver, copper, gold and platinum) by photodeposition, and applied for inactivation of Escherichia coli K12. XRD, DRS, XPS and STEM analyses confirmed the presence of noble metals nanoparticles (NPs) on the surface of faceted titania samples. Both OAP and DAP samples modified with silver and copper exhibited high bactericidal activities under visible light irradiation. It was also found that DAP under UV irradiation showed surprisingly high bactericidal activity, which could be attributed to efficient generation of reactive oxygen species, due to intrinsic properties of DAP, i.e., charge carriers' separation (migration of electrons and holes to {101} and {001} facets, respectively). However, an unexpected decrease in activity after DAP modification with gold and platinum NPs (mainly deposited on {101} facets) suggested that bacteria cells were directly decomposed on DAP surface. SEM images revealed that silver-modified samples caused severe damages of cell walls and membranes, due to antibacterial properties of silver (in the dark) and photocatalytic effect under visible and UV irradiation.

Noble metal-modified faceted anatase titania photocatalysts: Octahedron versus decahedron.

Octahedral anatase particles (OAP, with eight equivalent {101} facets) and decahedral anatase particles (DAP, with two additional {001} facets) were modified with nanoparticles of noble metals (Au, Ag, Cu). The titania morphology, expressed by the presence of different arrangements of exposed crystal facets, played a key role in the photocatalytic properties of metal-modified faceted titania. In the UV/vis systems, two-faceted configuration of DAP was more favorable for the reaction efficiency than single-faceted OAP because of an efficient charge separation described by the transfer of electrons to {101} facets and holes to {001} facets. Time-resolved microwave conductivity (TRMC) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) confirmed that distribution of electron traps (ET) and mobility of electrons were key-factors of photocatalytic activity. In contrast, metal-modified OAP samples had higher photocatalytic activity than metal-modified DAP and metal-modified commercial titania samples under visible light irradiation. This indicates that the presence of single type of facets ({101}) is favorable for efficient electron transfer via shallow ET, whereas intrinsic properties of DAP result in fast charge carriers' recombination when gold is deposited on {101} facets (migration of "hot" electrons: Au→{101}→Au).

Preparation of CdS and Bi2S3 quantum dots co-decorated perovskite-type KNbO3 ternary heterostructure with improved visible light photocatalytic activity and stability for phenol degradation.

CdS quantum dot-decorated KNbO3 composite photocatalysts co-modified with Bi2S3 QDs were designed and synthesized by a combination of the hydrothermal method with a linker-assisted adsorption route, using starch and thioglycolic acid as capping agents, which facilitated the attachment of modifiers to the surface of potassium niobate. The quantum dots were successfully deposited onto the surface of the perovskite-type KNbO3 with a good dispersion and a stable heterostructure was formed. The as-prepared photocatalysts were subsequently characterized by UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) specific surface area, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) emission spectroscopy. The obtained KNbO3-based composites showed greatly improved photocatalytic performance for the degradation of phenol in the aqueous phase under visible light irradiation (λ > 420 nm) over pristine KNbO3. The highest photocatalytic performance and enhanced stability were observed for the ternary 30% CdS-5%Bi2S3 quantum dot-decorated KNbO3 composite, which could be related to the enhanced visible-light absorption ability, efficient charge separation in the three-level electron transfer heterojunction, improved stability and appropriate amounts of composite components. The formation of a surface layer of CdO decreased the visible light photoactivity of the CdS QD-decorated KNbO3 photocatalysts. The main phenol oxidation intermediates were benzoquinone, catechol, hydroquinone, and 1,2,4-benzenetriol, which underwent further photooxidation to form non-cyclic organic acids. Action spectral analysis proved the better photocatalytic activity of the ternary CdS/B2S3 QDs co-decorated KNbO3 composite compared to the binary CdS QDs decorated KNbO3 sample and revealed that irradiation ranging from 420 to 520 nm was responsible for the visible light photoactivity.

Enhanced Photocatalytic and Antimicrobial Performance of Cuprous Oxide/Titania: The Effect of Titania Matrix.

A simple, low-cost method was applied to prepare hybrid photocatalysts of copper (I) oxide/titania. Five different TiO2 powders were used to perform the study of the effect of titania matrix on the photocatalytic and antimicrobial properties of prepared nanocomposites. The photocatalytic efficiency of such a dual heterojunction system was tested in three reaction systems: ultraviolet-visible (UV-Vis)-induced methanol dehydrogenation and oxidation of acetic acid, and 2-propanol oxidation under visible light irradiation. In all the reaction systems considered, the crucial enhancement of photocatalytic activity in relation to corresponding bare titania was observed. The reaction mechanism for a specific reaction and the influence of titania matrix were discussed. Furthermore, antimicrobial (bactericidal and fungicidal) properties of Cu2O/TiO2 materials were analyzed. The antimicrobial activity was found under UV, visible and solar irradiation, and even for dark conditions. The origin of antimicrobial properties with emphasis on the role of titania matrix was also discussed.

Noble metal-modified titania with visible-light activity for the decomposition of microorganisms.

Commercial titania photocatalysts were modified with silver and gold by photodeposition, and characterized by diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). It was found that silver co-existed in zero valent (core) and oxidized (shell) forms, whereas gold was mainly zero valent. The obtained noble metal-modified samples were examined with regard to antibacterial (Escherichia coli (E. coli)) and antifungal (Aspergillus niger (A. niger), Aspergillus melleus (A. melleus), Penicillium chrysogenum (P. chrysogenum), Candida albicans (C. albicans)) activity under visible-light irradiation and in the dark using disk diffusion, suspension, colony growth ("poisoned food") and sporulation methods. It was found that silver-modified titania, besides remarkably high antibacterial activity (inhibition of bacterial proliferation), could also decompose bacterial cells under visible-light irradiation, possibly due to an enhanced generation of reactive oxygen species and the intrinsic properties of silver. Gold-modified samples were almost inactive against bacteria in the dark, whereas significant bactericidal effect under visible-light irradiation suggested that the mechanism of bacteria inactivation was initiated by plasmonic excitation of titania by localized surface plasmon resonance of gold. The antifungal activity tests showed efficient suppression of mycelium growth by bare titania, and suppression of mycotoxin generation and sporulation by gold-modified titania. Although, the growth of fungi was hardly inhibited through disc diffusion (inhibition zones around discs), it indicates that gold does not penetrate into the media, and thus, a good stability of plasmonic photocatalysts has been confirmed. In summary, it was found that silver-modified titania showed superior antibacterial activity, whereas gold-modified samples were very active against fungi, suggesting that bimetallic photocatalysts containing both gold and silver should exhibit excellent antimicrobial properties.

Size-controlled gold nanoparticles on octahedral anatase particles as efficient plasmonic photocatalyst.

Octahedral anatase particles (OAPs), prepared by ultrasonication-hydrothermal reaction (US-HT), were modified with 2 wt% of gold by photodeposition. Conditions of US-HT process such as durations of US and durations of HT were varied to obtain OAPs products different by physicochemical and morphological properties. Au/OAPs samples were characterized by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The photocatalytic activity was tested under UV irradiation for decomposition of acetic acid (CO2 system) and dehydrogenation of methanol (H2 system) under aerobic and anaerobic conditions, respectively, and for oxidation of 2-propanol under visible light irradiation. Photodeposition of gold was very fast for all OAPs samples (0.5-10 min) under Ar atmosphere, and the clear correlation between the content of electron traps (ETs) and the induction period, during which nanoparticles (NPs) of gold are formed, indicates that ETs in titania samples are a key-factor for rapidity of gold photodeposition on titania surface. It was found that better morphology of titania (larger content of faceted particles) resulted in formation of larger gold NPs, while small gold NPs were deposited on structural defects. Modification of OAPs with gold NPs resulted in significant enhancement of photocatalytic activity, being e.g., 1.5 (CO2 system), 7.7 (H2 system), and even more than 40 under vis irradiation. It was found that both the properties of titania and gold are crucial for resultant photocatalytic activity, but a direct correlation between one structural/physical property and photocatalytic activity could not be obtained since all structural properties changed simultaneously when conditions of photocatalyst preparation (US-HT) were changed. Therefore, gold NPs of controlled sizes were deposited on OAPs product with the best morphology by modified photodeposition method. Clear correlation between photocatalytic activity under visible light and the size of gold NPs indicates that gold properties are decisive for visible light activity rather than titania properties. 3D-FDTD simulations confirm that an increase in the size of gold NPs results in extended surface areas with field enhancement.

The effect of anatase and rutile crystallites isolated from titania P25 photocatalyst on growth of selected mould fungi.

Antifungal properties of anatase and rutile crystallites isolated from commercial titania P25 photocatalyst were investigated by mycelium growth in the dark and under indoor light. Investigated fungi, i.e., Pseudallescheria boydii, Scedosporium apiospermum, Pseudallescheria ellipsoidea, Scedosporium aurantiacum, Aspergillus versicolor, Aspergillus flavus, Stachybotrys chartarum, Penicillium chrysogenum, Aspergillus melleus, were isolated from air and from moisture condensed on walls. Anatase and rutile were isolated from homogenized P25 (homo-P25) by chemical dissolution, and then purified by washing and thermal treatment. For comparison, homo-P25 was also thermally treated at 200 °C and 500 °C. Titania samples were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS) and scanning transmission electron microscopy (STEM). It was found that properties of titania, i.e., band-gap energy, impurities adsorbed on the surface, nanoparticle aggregation, and kind of fungal structure, highly influenced resultant antifungal activities. It is proposed that some fungi could uptake necessary water and nutrient from titania surface. It was also found that even when differences in mycelium growth were not significant, the sporulation and mycotoxin generation were highly inhibited by light and presence of titania.

Biosynthesis of the carbohydrate moieties of arabinogalactan proteins by membrane-bound ß-glucuronosyltransferases from radish primary roots.

A membrane fraction from etiolated 6-day-old primary radish roots (Raphanus sativus L. var hortensis) contained ß-glucuronosyltransferases (GlcATs) involved in the synthesis of the carbohydrate moieties of arabinogalactan proteins (AGPs). The GlcATs transferred [(14)C]GlcA from UDP-[(14)C]GlcA on to ß-(1 → 3)-galactan as an exogenous acceptor substrate, giving a specific activity of 50-150 pmol min(-1) (mg protein)(-1). The enzyme specimen also catalyzed the transfer of [(14)C]GlcA on to an enzymatically modified AGP from mature radish root. Analysis of the transfer products revealed that the transfer of [(14)C]GlcA occurred preferentially on to consecutive (1 → 3)-linked ß-Gal chains as well as single branched ß-(1 → 6)-Gal residues through ß-(1 → 6) linkages, producing branched acidic side chains. The enzymes also transferred [(14)C]GlcA residues on to several oligosaccharides, such as ß-(1 → 6)- and ß-(1 → 3)-galactotrioses. A trisaccharide, α-L-Araf-(1 → 3)-ß-Gal-(1 → 6)-Gal, was a good acceptor, yielding a branched tetrasaccharide, α-L-Araf-(1 → 3)[ß-GlcA-(1 → 6)]-ß-Gal-(1 → 6)-Gal. We report the first in vitro assay system for ß-GlcATs involved in the AG synthesis as a step toward full characterization and cloning.