Flow cytometric evaluation of nanoparticles using side-scattered light and reactive oxygen species-mediated fluorescence-correlation with genotoxicity

We recently clarified that the side-scatter(ed) light (SSC) of flow cytometry (FCM) could be used as a guide to measure the uptake potential of nanoparticles [ Suzuki et al. Environ. Sci. Technol. 2007 , 41 , 3018 - 3024 ]. In this paper, the method was improved so as to be able to determine simultaneously the uptake potential of nanoparticles and the production of reactive oxygen species (ROS), and correlations with genotoxicity were evaluated. In the FCM analysis, SSC and fluorescence of 6-carboxy-2,7'-diclorodihydrofluorescein diacetate, di(acetoxy ester) based on ROS production were concurrently detected after treatments with ZnO, CuO, Fe(3)O(4), TiO(2), and Ag nanoparticles. The ZnO and CuO nanoparticles caused high ROS production, which was more significant in the cells with higher SSC intensity. The increase of SSC intensity was more significant for TiO(2) than ZnO and CuO, whereas ROS production was higher for ZnO and CuO than TiO(2), suggesting that the extent of ROS production based on the uptake of nanoparticles differed with each kind of nanoparticle. ROS production was correlated with generation of the phosphorylated histone H2AX (γ-H2AX), a marker of DNA damage, and an antioxidant, n-acetylcysteine, could partially suppress the γ-H2AX. This method makes it possible to predict not only uptake potential but also genotoxicity.     

Root uptake and phytotoxicity of ZnO nanoparticles

Increasing application of nanotechnology highlights the need to clarify nanotoxicity. However, few researches have focused on phytotoxicity of nanomaterials; it is unknown whether plants can uptake and transport nanoparticles. This study was to examine cell internalization and upward translocation of ZnO nanoparticles by Lolium perenne (ryegrass). The dissolution of ZnO nanoparticles and its contribution to the toxicity on ryegrass were also investigated. Zn2+ ions were used to compare and verify the root uptake and phytotoxicity of ZnO nanoparticles in a hydroponic culture system. The root uptake and phytotoxicity were visualized by light scanning electron, and transmission electron microscopies. In the presence of ZnO nanoparticles, ryegrass biomass significantly reduced, root tips shrank, and root epidermal and cortical cells highly vacuolated or collapsed. Zn2+ ion concentrations in bulk nutrient solutions with ZnO nanoparticles were lower than the toxicity threshold of Zn2+ to the ryegrass; shoot Zn contents under ZnO nanoparticle treatments were much lower than that under Zn2+ treatments. Therefore, the phytotoxicity of ZnO nanoparticles was not directly from their limited dissolution in the bulk nutrient solution or rhizosphere. ZnO nanoparticles greatly adhered on to the rootsurface. Individual ZnO nanoparticles were observed present in apoplast and protoplast of the root endodermis and stele. However, translocation factors of Zn from root to shoot remained very low under ZnO nanoparticle treatments, and were much lower than that under Zn2+ treatments, implying that little (if any) ZnO nanoparticles could translocate up in the ryegrass in this study.     

Impact of aggregate size and structure on the photocatalytic properties of TiO2 and ZnO nanoparticles

Aggregation of photocatalytic semiconductors was determined to reduce the generation of free hydroxyl radicals in aqueous suspensions in a fashion dependent on aggregate size and structure. Static light scattering measurements were used to follow temporal changes in the fractal dimension of aggregating TiO(2) and ZnO nanoparticles. At length scales comparable to nanoparticle size, the structure of aggregated TiO(2) nanoparticles was independent of particle stability and the associated aggregation rate, consistent with the fused nature of TiO(2) primary particles in the initial suspension. In contrast, ZnO aggregates were characterized by smaller fractal dimensions when ionic strength, and the resulting aggregation rate, were increased. The photocatalytic activity of ZnO and TiO(2) in generating free hydroxyl radicals varied with aggregate structure and size, consistent with theory that predicts reduced reactivity as aggregates become larger and more dense.     

Recognition and uptake of free and nanoparticle-bound betalactoglobulin--a food allergen--by human monocytes

Scope : To improve our understanding of the interaction of food allergens with cells of the immune system, the endocytosis by human monocytes of bovine β‐lactoglobulin (BLG) and ovomucoid (OM) – two major food allergens – and human serum albumin (HSA) was studied. Methods and results : BLG was covalently conjugated to dextran‐coated magnetic nanoparticles (MNPs) without affecting its structure and immunoreactivity. BLG‐conjugated MNPs were taken up by human monocytes much more efficiently than non‐conjugated MNPs, allowing easy magnetic separation of cells that had adsorbed the allergen. BLG, OM, and HSA were conjugated to MNPs also labeled with a fluorescent probe. The uptake of these materials by human monocytes was monitored through flow cytometry, and compared with fluorescent MNPs and the free fluorescently labeled proteins, confirming higher uptake of the BLG‐conjugated MNPs versus non‐conjugated MNPs. OM but not HSA conjugation to particles enhanced uptake of the MNPs. Confocal microscopy provided direct evidence of the actual internalization of BLG–MNP conjugates into the cytoplasm. Conclusions : These results contribute to the current understanding of the interaction between food allergens and antigen‐presenting cells, and demonstrate that the BLG is readily endocytosed by monocytes both as the single protein and as a conjugate.     

Photocatalytic degradation of polystyrene plastic under fluorescent light

Plastic is used widely all over the world, due to the fact that it is low cost, is easily processable, and has lightweight properties. However, the hazard of discarding waste plastic, so-called "white pollution", is becoming more and more severe. In this paper, solid-phase photocatalytic degradation of polystyrene (PS) plastic, one of the most common commercial plastics, over copper phthalocyanine (CuPc) sensitized TiO2 photocatalyst (TiO2/CuPc) has been investigated under fluorescent light irradiation in the air. UV-vis spectra show that TiO2/CuPc extends its photoresponse range to visible light, contrasting to only UV light absorption of pure TiO2. The PS photodegradation experiments exhibit that higher PS weight loss rate, lower PS average molecular weight, less amount of volatile organic compounds, and more CO2 can be obtained in the system of PS-(TiO2/CuPc), in comparison with the PS-TiO2 system. Therefore, PS photodegradation over TiO2 CuPc composite is more complete and efficient than over pure TiO2, suggesting the potential application of dye-sensitized TiO2 catalyst in the thorough photodegradation of PS plastic under fluorescent light. During the photodegradation of PS plastic, the reactive oxygen species generated on TiO2 or TiO2/CuPc particle surfaces play important roles in chain scission. The present study demonstrates that the combination of polymer plastic with dye-sensitized TiO2 catalyst in the form of thin film is a practical and useful way to photodegrade plastic contaminants in the sunlight.     

Toxicity and internalization of CuO nanoparticles to prokaryotic alga Microcystis aeruginosa as affected by dissolved organic matter

This is the first study investigating the toxicity of nanoparticles (NPs) to algae in the presence of dissolved organic matter (DOM). Suwannee river fulvic acid (SRFA), a type of DOM, could significantly increase the toxicity of CuO NPs to prokaryotic alga Microcystis aeruginosa. Internalization of CuO NPs was observed for the first time in the intact algal cells using high resolution transmission electron microscopy (HRTEM), and the cell uptake was enhanced by SRFA. A fast Fourier transformation (FFT)/inversed FFT (IFFT) process revealed that a main form of intracellular NPs was Cu(2)O, and an intracellular environment may reduce CuO into Cu(2)O. The internalization behavior alone did not seem to pose a hazard to membrane integrity as shown from the flow cytometry data. Elevated CuO nanotoxicity by SRFA was related to a combination of a lesser degree of aggregation, higher Cu(2+) release, and enhanced internalization of CuO NPs.     

The state of nano-sized titanium dioxide (TiO2) may affect sunscreen performance

In the past several years, there has been a trend in the sunscreen/cosmetics industry to replace micron-sized titanium dioxide (TiO(2)) particles with nanoscale materials. The increased use of nanoscale TiO(2) has resulted in questions about these and other nanoproducts. This study examines the effects of using nanoscale TiO(2) on ultraviolet (UV) attenuation in simple to complex sunscreen formulations. UV light attenuation, product stability, and potential damage to the skin barrier were examined with both nanoscale and microscale TiO(2) particles. Results indicate that none of the formulations decreased the barrier function of the skin and the best UV attenuation occurs when the TiO(2) particles are stabilized with a coating and evenly distributed such as with non-agglomerated coated nanoscale materials. This indicates that nanoscale TiO(2) may have better efficacy while lacking toxicity.     

Recognition and effective degradation of 17beta-estradiol by anti-estradiol-antibody-immobilized TiO(2) nanoparticles

Polyelectrolyte polyacrylic acid (PAA), used in the chemical modification of titanium dioxide (TiO(2)) nanoparticles, allows TiO(2) nanoparticles to remain in suspension at neutral pH. The anti-17beta-estradiol (E2) antibody was immobilized on PAA-modified TiO(2) (PAA-TiO(2)) nanoparticles via covalent bonding between the carboxylic acid of PAA and the amino group of the antibody. The anti-E2-antibody-immobilized TiO(2) (E2Ab-PAA-TiO(2)) nanoparticles can form a suspension at neutral pH, with a particle size of less than 100 nm. The E2-PAA-TiO(2) nanoparticles caused the photocatalytic degradation of a typical TiO(2) substrate, methylene blue. The anti-E2 antibody immobilized on the TiO(2) surface recognized and bound E2 in the solution, thereby improving the efficiency of E2 degradation compared with that of PAA-TiO(2) nanoparticles. These results demonstrate that the E2Ab-PAA-TiO(2) nanoparticles developed in this study can be used in water treatment technology. Furthermore, this strategy of immobilizing proteins on nanoscale TiO(2) particles creates new applications not only in the treatment of environmental waste, but also in medical and public sanitation processes.     

金属离子掺杂对纳米TiO_2光催化降解造纸废水的影响

本文以廉价的无机盐四氯化钛为原料,通过在基材中加入金属离子,采用溶胶-凝胶法制备出纳米TiO2光催化剂。以河北省某造纸厂废水为研究对象,探讨了掺杂铜、铁、锰三种过渡金属离子,改变热处理温度和热处理时间等对废水降解率的影响。研究表明:同等条件下掺杂三种过渡金属元素的TiO2粉末光催化性能的排序为锰>铁>铜,热处理温度和时间对掺杂锰的TiO2粉末光催化性能有一定的影响。     

Formation and biological effects of protein corona for food-related nanoparticles

The rapid development of nanoscience and nanoengineering provides new perspectives on the composition of food materials, and has great potential for food biology research and applications. The use of nanoparticle additives and the discovery of endogenous nanoparticles in food make it important to elucidate in vivo safety of nanomaterials. Nanoparticles will spontaneously adsorb proteins during transporting in blood and a protein corona can be formed on the nanoparticle surface inside the human body. Protein corona affects the physicochemical properties of nanoparticles and the structure and function of proteins, which in turn affects a series of biological reactions. This article reviewed basic information about protein corona of food-related nanoparticles, elucidated the influence of protein corona on nanoparticles properties and protein structure and function, and discussed the effect of protein corona on nanoparticles in vivo. The effects of protein corona on nanoparticles transport, cellular uptake, cytotoxicity, and immune response were reviewed, and the reasons for these effects were also discussed. Finally, future research perspectives for food protein corona were proposed. Protein corona gives food nanoparticles a new identity, which makes proteins bound to nanoparticles undergo structural transformations that affect their recognition by receptors in vivo. It can have positive or negative impacts on cellular uptake and toxicity of nanoparticles and even trigger immune responses. Understanding the effects of protein corona have potential in evaluating the fate of the food-related nanoparticles, providing physicochemical and biological information about the interaction between proteins and foodborne nanoparticles. The review article will help to evaluate the safety of protein coronas formed on nanoparticles in food, and may provide fundamental information for understanding and controlling nanotoxicity.     

Exposure of engineered nanoparticles to human lung epithelial cells: influence of chemical composition and catalytic activity on oxidative stress

The chemical and catalytic activity of nanoparticles has strongly contributed to the current tremendous interest in engineered nanomaterials and often serves as a guiding principle for the design of functional materials. Since it has most recently become evident that such active materials can enter into cells or organisms, the present study investigates the level of intracellular oxidations after exposure to iron-, cobalt-, manganese-, and titania-containing silica nanoparticles and the corresponding pure oxides in vitro. The resulting oxidative stress was quantitatively measured as the release of reactive oxygen species (ROS). The use of thoroughly characterized nanoparticles of the same morphology, comparable size, shape, and degree of agglomeration allowed separation of physical (rate of particle uptake, agglomeration, sedimentation) and chemical effects (oxidations). Three sets of control experiments elucidated the role of nanoparticles as carriers for heavy metal uptake and excluded a potential interference of the biological assay with the nanomaterial. The present results indicate that the particles could efficiently enter the cells by a Trojan-horse type mechanism which provoked an up to eight times higher oxidative stress in the case of cobalt or manganese if compared to reference cultures exposed to aqueous solutions of the same metals. A systematic investigation on iron-containing nanoparticles as used in industrial fine chemical synthesis demonstrated that the presence of catalytic activity could strongly alter the damaging action of a nanomaterial. This indicates that a proactive development of nanomaterials and their risk assessment should consider chemical and catalytic properties of nanomaterials beyond a mere focus on physical properties such as size, shape, and degree of agglomeration.     

Plasma surface modified TiO2 nanoparticles: improved photocatalytic oxidation of gaseous m-xylene

Titanium dioxide (TiO(2)) is a preferred catalyst for photocatalytic oxidation of many air pollutants. In an effort to enhance its photocatalytic activity, TiO(2) was modified by pulsed plasma treatment. In this work, TiO(2) nanoparticles, coated on a glass plate, were treated with a plasma discharge of hexafluoropropylene oxide (HFPO) gas. By appropriate adjustment of discharge conditions, it was discovered that the TiO(2) particles can be either directly fluorinated (Ti-F) or coated with thin perfluorocarbon films (C-F). Specifically, under relatively high power input, the plasma deposition process favored direct surface fluorination. The extent of Ti-F formation increased with increasing power input. In contrast, at lower average power inputs, perfluorocarbon films are deposited on the surface of the TiO(2) particles. The plasma surface modified TiO(2) nanoparticles were subsequently employed as catalysts in the photocatalytic oxidation of m-xylene in air, as carried out inside a batch reactor with closed loop constant gas circulation. Both types of modified TiO(2) were significantly more catalytically active than that of the unmodified particles. For example, the rate constant of m-xylene degradation was increased from 0.012 min(-1) with untreated TiO(2) to 0.074 min(-1) with fluorinated TiO(2). Although it is not possible to provide unequivocal reasons for this increased photocatalytic activity, it is noted that the plasma surface treatment converted the TiO(2) from hydrophilic to highly hydrophobic, which would provide more facile catalyst adsorption of the xylene from the flowing air. Also, based on literature reports, the use of fluorinated TiO(2) reduces electron-hole recombination rates, thus increasing the photocatalytic activity.     

Factors influencing luxury uptake of phosphorus by microalgae in waste stabilization ponds

Phosphorus removal in waste stabilization ponds (WSP) is highly variable, but the reasons for this are not well understood. Luxury uptake of phosphorus by microalgae has been studied in natural systems such as lakes but not under the conditions found in WSP. This work reports on the effects of phosphate concentration, light intensity, and temperature on luxury uptake of phosphorus by WSP microalgae in continuous culture bioreactors. Increasing temperature had a statistically significant "positive effect" on intracellular acid-insoluble polyphosphate concentration. It is likely that elevated temperature increased the rate of polyphosphate accumulation, but because the biomass was not starved of phosphate, the stored acid-insoluble polyphosphate was not utilized. Increasing light intensity had no effect on acid-insoluble polyphosphate but had a "negative effect" on the acid-soluble polyphosphate. A possible explanation for this is that the faster growth rate at high light intensity results in this form of polyphosphate being utilized by the cells for synthesis of cellular constituents at a rate that exceeds replenishment. The variability in the phosphorus content of the microalgal biomass shows that with this new understanding ofthe luxury uptake mechanism there is the potential to optimize WSP for biological phosphorus removal.     

Photocatalytic oxidation of arsenite over TiO2: is superoxide the main oxidant in normal air-saturated aqueous solutions?

TiO(2) photocatalytic oxidation (PCO) of As(III) in the normal air-saturated aqueous solutions has been widely studied. Yet no consensus has been achieved on the mechanism whether superoxide is the main oxidant, although many approaches have been taken. (Photo)electrochemical method can minimize changes to TiO(2) surface and could therefore not alter the normal mechanism. In this Article, both this approach and As(III) oxidation kinetic measurements were performed to clarify the disputed mechanism. Under a sufficient cathodic bias potential, the dark oxidation of As(III) by superoxide could occur, but both the reaction rate and the columbic efficiency were rather low, suggesting that it is a weak oxidant. However, under UV light, both the reaction rate and the columbic efficiency were greatly enhanced even at potentials negative enough to eliminate photohole participation, indicating that more efficient oxidants than superoxide were produced. Under UV illumination and enough positive potential where superoxide was absent, the As(III) oxidation was the most highly efficient. The columbic efficiency of photoholes was much higher than that of superoxide. In the normal aerated aqueous solutions and at open circuit, although the total contribution of superoxide and its derivates to the PCO of As(III) was considerably high (up to 43%), it was not more than that of photohole (57%). In addition, the reported various approaches taken to elucidate the mechanism were discussed, and the resulting disputes can be clarified by these findings. It was demonstrated that (photo)electrochemical method could provide direct and undisputed evidence to reveal the truth mechanics issues.     

Titanium dioxide (P25) produces reactive oxygen species in immortalized brain microglia (BV2): implications for nanoparticle neurotoxicity

Concerns with the environmental and health risk of widely distributed, commonly used nanoparticles are increasing. Nanosize titanium dioxide (TiO2) is used in air and water remediation and in numerous products designed for direct human use and consumption. Its effectiveness in deactivating pollutants and killing microorganisms relates to photoactivation and the resulting free radical activity. This property, coupled with its multiple potential exposure routes, indicates that nanosize TiO2 could pose a risk to biological targets that are sensitive to oxidative stress damage (e.g., brain). In this study, brain microglia (BV2) were exposed to a physicochemically characterized (i.e., dispersion stability, particle size distribution, and zeta potential) nanomaterial, Degussa P25, and cellular expressions of reactive oxygen species were measured with fluorescent probes. P25's zeta potentials, measured in cell culture media and physiological buffer were -11.6 +/- 1.2 mV and -9.25 +/- 0.73 mV, respectively. P25 aggregation was rapid in both media and buffer with the hydrodynamic diameter of stable P25 aggregates ranging from 826 nm to 2368 nm depending on the concentration. The biological response of BV2 microglia to noncytotoxic (2.5-120 ppm) concentrations of P25 was a rapid (<5 min) and sustained (120 min) release of reactive oxygen species. The time course of this release suggested that P25 not only stimulated the immediate "oxidative burst" response in microglia but also interfered with mitochondrial energy production. Transmission electron microscopy indicated that small groups of nanosized particles and micron-sized aggregates were engulfed bythe microglia and sequestered as intracytoplasmic aggregates after 6 and 18 h exposure to P25 (2.5 ppm). Cell viability was maintained at all test concentrations (2.5-120 ppm) over the 18 h exposure period. These data indicate that mouse microglia respond to Degussa P25 with cellular and morphological expressions of free radical formation.     

Suppression of dioxin emission in co-incineration of poly(vinyl chloride) with TiO2-encapsulating polystyrene

This paper presents an approach to the suppression of the emission of dioxin and its precursors as a result of the co-incineration of poly (vinyl chloride) (PVC) with TiO2-encapsulating polystyrene (TEPS), in which TiO2 nanoparticles with the capacity to adsorb dioxin and its precursors are encapsulated without significant agglomeration. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to show that spherical PS particles encapsulating a uniform dispersion of TiO2 nanoparticles are obtained through the dispersion polymerization of styrene in an aqueous ethanol medium. To facilitate the encapsulation, the surface of the TiO2 nanoparticles was modified with a polymerizable organic silane linker priorto polymerization. For comparison purposes, experiments were performed in which PVC was co-incinerated with neat PS (PVC/PS), TiO2-encapsulating PS (PVC/TEPS), and mechanically mixed TiO2/PS (PVC/PS-MTiO2). Qualitative and quantitative investigations of the suppression of the emission of a model dioxin and its precursors were performed by analyzing the exhaust gases from the co-incinerations using gas chromatography/ mass spectrometry (GC/MS). The results show that the addition of TiO2 nanoparticles into co-incineration systems reduces the concentration of the dioxin and its precursors in exhaust gases. Moreover, the quantitative removal efficiencies for PVC/TEPS and PVC/PS-MTiO2 indicate that the suppression is successfully enhanced by the TiO2-encapsulation: increases in the dispersity of the nanoparticles result in improved adsorption of the dioxin and its precursors.     

Comparative toxicity of nanoparticulate ZnO, bulk ZnO, and ZnCl2 to a freshwater microalga (Pseudokirchneriella subcapitata): the importance of particle solubility

Metal oxide nanoparticles are finding increasing application in various commercial products, leading to concerns for their environmental fate and potential toxicity. It is generally assumed that nanoparticles will persist as small particles in aquatic systems and that their bioavailability could be significantly greater than that of larger particles. The current study using nanoparticulate ZnO (ca. 30 nm) has shown that this is not always so. Particle characterization using transmission electron microscopy and dynamic light scattering techniques showed that particle aggregation is significant in a freshwater system, resulting in flocs ranging from several hundred nanometers to several microns. Chemical investigations using equilibrium dialysis demonstrated rapid dissolution of ZnO nanoparticles in a freshwater medium (pH 7.6), with a saturation solubility in the milligram per liter range, similar to that of bulk ZnO. Toxicity experiments using the freshwater alga Pseudokirchneriella subcapitata revealed comparable toxicity for nanoparticulate ZnO, bulk ZnO, and ZnCl2, with a 72-h IC50 value near 60 microg Zn/ L, attributable solely to dissolved zinc. Care therefore needs to be taken in toxicity testing in ascribing toxicity to nanoparticles per se when the effects may be related, at least in part, to simple solubility.     

Enhanced degradation in nanocomposites of TiO2 and biodegradable polymer

Nanocomposites of titamium dioxide (TiO2) particles and biodegradable poly (butylene succinate) (PBS) were fabricated by melt-blending using a high-shear extruder. TiO2 particles were highly dispersed in the PBS matrix by high-shear processing, and the addition of TiO2 particles into PBS did not decrease its mechanical strength. The photocatalytic decomposition and biodegradable properties of the nanocomposites were evaluated by UV irradiation or enzymatic degradation methods in vitro. It was found that both the esterase enzyme and UV irradiation decomposed the nanocomposites. Photocatalytic decomposition of PBS clearly depended on the size and dispersibility of TiO2 particles in PBS polymer. Higher dispersibility and smaller size of TiO2 particles were effective on the photocatalytic oxidation of PBS. In addition, decomposition rate under a simultaneous UV irradiation treatment and immersion in an enzyme solution was higher than those under UV irradiation or immersion in an enzyme solution. These results indicate that the nanocomposites can easily be decomposed not only by an enzyme in soil or compost, but also by photocatalytic oxidation of TiO2 under sunlight.     

Development of TiO2 powder-coated food packaging film and its ability to inactivate Escherichia coli in vitro and in actual tests

Titanium dioxide (TiO2) has attracted a great deal of attention as a photocatalytic disinfecting material in the food and environmental industry. TiO2 has been used to inactivate a wide variety of microorganisms in many applications. In the present study, we aimed to develop a TiO2 powder-coated packaging film and clarify its ability to inactivate Escherichia coli both in vitro and in actual tests, using two different particle sizes and two types of illumination at different intensities. No inhibition effect of the testing method itself on the growth of E. coli was observed. The cells of E. coli were found to have decreased 3 log CFU/ml after 180 min of illumination by two 20 W black-light bulbs (wavelength of 300-400 nm) on TiO2-coated oriented-polypropylene (OPP) film, while E. coli decreased 1 log CFU/m with black-light illumination of uncoated OPP film. The results showed that both ultraviolet A (UVA; wavelength of 315-400 nm) alone and TiO2-coated OPP film combined with UVA reduced the number of E. coli cell in vitro, but that the reduction of E. coli cell numbers was greater by TiO2-coated OPP film combined with UVA. The antimicrobial effect of TiO2-coated film is dependent on the UVA light intensity (0, <0.05 and 1 mW/cm2) and the kind of artificial light (black-light and daylight fluorescent bulbs), but it is independent of the particle size of TiO2 coating on the surface of OPP film. The surviving cell numbers of E. coli on TiO2-coated film decreased 3 log and 0.35 log CFU/ml after 180 min of illumination by two 20 W black bulbs and two 20 W daylight fluorescent bulbs, respectively. Despite the lesser efficacy of the photocatalytic method with fluorescent lights, the survival of E. coli cells using this method was 50% of that using fluorescent lights alone. In the actual test, the number of E. coli cells from cut lettuce stored in a TiO2-coated film bag irradiated with UVA light decreased from 6.4 on Day 0 to 4.9 log CFU/g on Day 1, while that of an uncoated film bag irradiated with UVA light decreased from 6.4 to 6.1 log CFU/g after 1 day of storage. The result shows that the TiO2-coated film could reduce the microbial contamination on the surface of solid food products and thus reduce the risks of microbial growth on fresh-cut produce.     

Integrating plasmonic nanoparticles with TiO₂ photonic crystal for enhancement of visible-light-driven photocatalysis

Aimed at enhancing photocatalysis through intensifying light harvesting, a new photocatalyst was fabricated by infiltrating Au nanoparticles into TiO(2) photonic crystals (TiO(2) PC/Au NPs). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images showed that the Au NPs with average diameter around 15 nm were dispersed uniformly into the porous TiO(2) material. The results of the transmittance spectra demonstrated that the light absorption by Au NPs was amplified after they were infiltrated into TiO(2) 240, which was fabricated from 240 nm polystyrene spheres. In the photocatalytic experiments of 2,4-dichlorophenol degradation under visible light (λ > 420 nm) irradiation, the kinetic constant using TiO(2) 240/Au NPs was 2.3 fold larger than that using TiO(2) nanocrystalline/Au NPs (TiO(2) NC/Au NPs). The excellent photocatalysis benefited from the cooperatively enhanced light harvesting owing to the localized surface plasmon resonance of Au NPs, which extended the light response spectra and the photonic effect of the TiO(2) 240 which intensified the plasmonic absorption by Au NPs. The hydroxyl radicals originated from the electroreduction of dissolved oxygen with photogenerated electrons via chain reactions were the main reactive oxygen species responsible for the pollutant degradation.