Visible-light photocatalysis and charge carrier dynamics of elemental crystalline red phosphorus.

Elemental red phosphorus (red P) is a new class of photocatalysts with a desirable bandgap of ∼1.7 eV and has a strong visible-light response. Here, we show that the efficiency of red P is limited by severe electron trapping at deep traps that are intrinsic to the different crystal facets of the red P. To overcome this, we synthesized the red P/RGO (reduced graphene oxide) composite in a one-step ampoule chemical vapor deposition synthesis that formed a conducive interface between the red P photocatalyst and the RGO acceptor for efficient interfacial charge transport. As substantiated through photoelectrochemical characterization and ultrafast (femtoseconds) transient absorption spectroscopy, the interfacing with RGO provided a rapid pathway for the photocharges in red P to be interfacially separated, thereby circumventing the slower the charge trapping process. As a result, up to a sevenfold increase in the photocatalytic hydrogen production rate (apparent quantum yield = 3.1% at 650 nm) was obtained for the red P/RGO relative to the pristine red P.

Cascade reaction engineering on zirconia-supported mesoporous MFI zeolites with tunable Lewis-Brønsted acid sites: a case of the one-pot conversion of furfural to γ-valerolactone.

Catalytic cascade reactions are strongly desired as a potential means of combining multistep reactions into a single catalytic reactor. Appropriate catalysts composed of multi-reactive sites to catalyze cascade reactions in a sequential fashion are central to such efforts. Here, we demonstrate a bifunctional zeolite catalyst with close proximity of Brønsted and Lewis acid sites through the synthesis of a mesoporous ZrO2[Al]MFI nanosponge (NS). The unique mesopores of the MFI-NS allow the confinement of zirconium oxide clusters (Lewis acid sites, LA) within the few-unit-cell-thin MFI aluminosilicate zeolite wall (Brønsted acid sites, BA). Such a structure is clearly distinct from the conventional MFI zeolite, where the agglomeration of zirconium oxide clusters onto the external surface area within the crystal bulk is not possible, resulting in segregated BA and LA sites on the internal and external zeolite, respectively. By bringing the BA and LA within ZrO2[Al]MFI-NS 30, we uncovered a more efficient catalytic route for the conversion of furfural (100% within 2 h) to γ-valerolactone (GVL) (83%). This route is only evident when the long molecular diffusion path, in the most extreme case of physically mixed ZrO2-(LA) and Al-zeolites (BA) (45% of GVL yield), is eliminated. Unlike the bifunctional ZrO2-Al-beta (GVL yield of 75%), where the BA concentration is greatly compromised at the expense of LA formation, we also show that the ZrO2[Al]MFI-NS is able to maintain a high density and good stability of both types of acids.

Designing Photoelectrodes for Photocatalytic Fuel Cells and Elucidating the Effects of Organic Substrates.

Photocatalytic fuel cells (PFCs) are constructed from anodized photoanodes with the aim of effectively converting organic materials into solar electricity. The syntheses of the photoanodes (TiO2 , WO3 , and Nb2 O5 ) were optimized using the statistical 2(k) factorial design. A systematic study was carried out to catalog the influence of eleven types of organic substrate on the photocurrent responses of the photoanodes, showing dependence on the adsorption of the organic substrates and on the associated photocatalytic degradation mechanisms. Strong adsorbates, such as carboxylic acids, generated high photocurrent enhancements. Simple and short-chained molecules, such as formic acid and methanol, are the most efficient in the corresponding carboxylic acid and alcohol groups as a result of their fast degradation kinetics. The TiO2 -based PFC yielded the highest photocurrent and obtainable power, whereas the Nb2 O5 -based PFC achieved the highest open-circuit voltage, which is consistent with its most negative Fermi level.

Efficient photoelectrochemical water splitting over anodized p-type NiO porous films.

NiO photocathodes were fabricated by alkaline etching-anodizing nickel foil in an organic-based electrolyte. The resulting films have a highly macroporous surface structure due to rapid dissolution of the oxide layer as it is formed during the anodization process. We are able to control the films' surface structures by varying the anodization duration and voltage. With an onset potential of +0.53 V versus the reversible hydrogen electrode (RHE), the photocurrent efficiency of the NiO electrodes showed dependencies on their surface roughness factor, which determines the extent of semiconductor-electrolyte interface and the associated quality of the NiO surface sites. A maximum incident photon-to-current conversion efficiency (IPCE(max)) of 22% was obtained from NiO film with a roughness factor of 8.4. Adding an Al2O3 blocking layer minimizes surface charge recombination on the NiO and hence increased the IPCE(max) to 28%. The NiO/Al2O3 films were extremely stable during photoelectrochemical water splitting tests lasting up to 20 h, continuously producing hydrogen and oxygen in the stoichiometric 2:1 ratio. The NiO/Al2O3 and NiO films fabricated using the alkaline anodization process produced 12 and 6 times as much hydrogen, respectively, as those fabricated using commercial NiO nanoparticles.

Hierarchical growth of SnO2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties.

Direct hydrothermal growth of Sn(II)-doped SnO2 films on fluorine-doped tin oxide (FTO) substrates results in the formation of upstanding SnO2 nanosheet arrays covered by hierarchical SnO2 nanoflowers. The n-type semiconductor films show extended photoresponse in the visible spectrum arising from the coexistence of Sn(II) dopant ions and oxygen vacancies in these hierarchical SnO2 nanostructures, which leads to a narrowed bandgap. Photoluminescence spectroscopy revealed that the emission in the UV, blue and red spectral ranges is related to the evolution of Sn(II) dopants and oxygen vacancies with annealing temperature, whereas oxygen vacancies are mostly responsible for visible emission. The Sn(II)-doped SnO2 films show higher photocurrent when sensitized with narrow bandgap CdS nanoparticles, serving as efficient electron acceptors.

Labeling of cancer cells with magnetic nanoparticles for magnetic resonance imaging.

PURPOSE: The process of invasion and metastasis formation of tumor cells can be studied by following the migration of labeled cells over prolonged time periods. This report investigates the applicability of iron oxide nanoparticles as a magnetic resonance imaging (MRI) contrast agent for cell labeling. METHODS: γFe2 O3 nanoparticles prepared with direct flame spray pyrolysis are biofunctionalized with poly-l-lysine (PLL). The nanoparticles within the cells were observed with transmission electron microscopy, bright-field microscopy, and magnetorelaxometry. MRI of labeled cells suspended in agarose was used to estimate the detection limit. RESULTS: PLL-coated particles are readily taken up, stored in intracellular clusters, and gradually degraded by the cells. During cell division, the nanoparticle clusters are divided and split between daughter cells. The MRI detection limit was found to be 25 cells/mm(3) for R2*, and 70 cells/mm(3) for R2. The iron specificity, however, was higher for R2 images. Due to the degradation of intracellular γFe2 O3 to paramagnetic iron ions within 13 days, the R1, R2, and R2* contrast gradually decreased over this time period to approximately 50% of its initial value. CONCLUSIONS: These results suggest that PLL-coated γFe2 O3 nanoparticles can be used as an MRI contrast agent for long-term studies of cell migration. Magn Reson Med 71:1896-1905, 2014. © 2013 Wiley Periodicals, Inc.

Anti-angiogenic activity of heparin functionalised cerium oxide nanoparticles.

Cerium oxide nanoparticles (nanoceria) are widely reported to be non-cytotoxic and modulate intracellular reactive oxygen species (ROS). In this study, nanoceria (dxRD = 12 nm) were functionalised with either 130 or 880 molecules of unfractionated heparin using the organosilane linker, 3-aminopropyltriethoxysilane. Nanoceria with a low level of heparin functionalisation were found to scavenge intracellular ROS to the same extent as unfunctionalised nanoceria and significantly more than cells exposed to medium only. In contrast, nanoceria with the highest level of heparin functionalisation were not as effective at scavenging intracellular ROS. Nanoceria were localised predominantly in the cytoplasm, while heparin-nanoceria were localised in both the cytoplasm and lysosomes. Together these data demonstrated that the level of nanoceria surface functionalisation with heparin determined the intracellular localisation and ROS scavenging ability of these particles. Additionally, heparin-nanoceria were effective in reducing endothelial cell proliferation indicating that they may find application in the control of angiogenesis in cancer in the future.

Submicron and nano formulations of titanium dioxide and zinc oxide stimulate unique cellular toxicological responses in the green microalga Chlamydomonas reinhardtii.

The work investigates the eco-cytoxicity of submicron and nano TiO2 and ZnO, arising from the unique interactions of freshwater microalga Chlamydomonas reinhardtii to soluble and undissolved components of the metal oxides. In a freshwater medium, submicron and nano TiO2 exist as suspended aggregates with no-observable leaching. Submicron and nano ZnO undergo comparable concentration-dependent fractional leaching, and exist as dissolved zinc and aggregates of undissolved ZnO. Cellular internalisation of solid TiO2 stimulates cellular ROS generation as an early stress response. The cellular redox imbalance was observed for both submicron and nano TiO2 exposure, despite exhibiting benign effects on the alga proliferation (8-day EC50>100 mg TiO2/L). Parallel exposure of C. reinhardtii to submicron and nano ZnO saw cellular uptake of both the leached zinc and solid ZnO and resulting in inhibition of the alga growth (8-day EC50≥0.01 mg ZnO/L). Despite the sensitivity, no zinc-induced cellular ROS generation was detected, even at 100 mg ZnO/L exposure. Taken together, the observations confront the generally accepted paradigm of cellular oxidative stress-mediated cytotoxicity of particles. The knowledge of speciation of particles and the corresponding stimulation of unique cellular responses and cytotoxicity is vital for assessment of the environmental implications of these materials.

Induced adaptation of Bacillus sp. to antimicrobial nanosilver.

The natural ability of Bacillus sp. to adapt to nanosilver cytotoxicity upon prolonged exposure is reported for the first time. The combined adaptive effects of nanosilver resistance and enhanced growth are induced under various intensities of nanosilver-stimulated cellular oxidative stress, ranging from only minimal cellular redox imbalance to the lethal levels of cellular ROS stimulation. An important implication of the present work is that such adaptive effects lead to the ultimate domination of nanosilver-resistant Bacillus sp. in the microbiota, to which nanosilver cytotoxicity is continuously applied.

Cellular uptake and activity of heparin functionalised cerium oxide nanoparticles in monocytes.

Cerium oxide nanoparticles (nanoceria) are effective in scavenging intracellular reactive oxygen species (ROS). In this study nanoceria synthesized by flame spray pyrolysis (dXRD = 12 nm) were functionalised with heparin via an organosilane linker, 3-aminopropyltriethoxysilane. Nanoceria were functionalised with approximately 130 heparin molecules per nanoparticle as determined by thermo gravimetric analysis. Heparin functionalised nanoceria were more effectively internalised by the human monocyte cell line, U937, and U937 cells that had been activated with phorbol 12 myristate 13-acetate (PMA) than bare nanoceria. The heparin functionalised nanoceria were also more effective in scavenging ROS than nanoceria in both activated and unactivated U937 cells. Heparin coupled nanoceria were found to be biologically active due to their ability to bind fibroblast growth factor 2 and signal through FGF receptor 1. Additionally, the heparin-coupled nanoceria, once internalised by the cells, were found to be degraded by 48 h. Together these data demonstrated that heparin enhanced the biological properties of nanoceria in terms of cellular uptake and ROS scavenging, while the nanoceria themselves were more effective at delivering heparin intracellularly than exposing cells to heparin in solution.

A Perspective on the Flame Spray Synthesis of Photocatalyst Nanoparticles.

The synthesis of functional nanoparticles via one-step flame spray pyrolysis (FSP), especially those of catalytic nature, has attracted the interests of scientists and engineers, as well as industries. The rapid and high temperature continuous synthesis yields nanoparticles with intrinsic features of active catalysts, that is, high surface area and surface energetics. For these reasons, FSP finds applications in various thermally inducible catalytic reactions. However, the design and synthesis of photocatalysts by FSP requires a knowledge set which is different from that established for thermal catalysts. Unknown to many, this has resulted in frustrations to those entering the field unprepared, especially since FSP appears to be an elegant tool in synthesising oxide nanoparticles of any elemental construct. From simple oxide to doped-oxide, and mixed metal oxide to the in situ deposition of noble metals, this Perspective gives an overview on the development of photocatalysts made by FSP in the last decade that led to a better understanding of the design criteria. Various challenges and opportunities are also highlighted, especially those beyond simple metal oxides, which perhaps contain the greatest potential for the exploitation of photocatalysts design by FSP.

Cellular uptake and reactive oxygen species modulation of cerium oxide nanoparticles in human monocyte cell line U937.

Cerium oxide nanoparticles (nanoceria) are promising materials for intracellular oxygen free radical scavenging providing a potential therapy for reactive oxygen species (ROS)-mediated inflammatory processes. In this study rhombohedral-shaped nanoceria were synthesized by flame spray pyrolysis with tuneable particle diameters between 3 and 94 nm by changing the liquid precursor flow rate. Monocytes and macrophages are major players in inflammatory processes as their production of ROS species has important downstream effects on cell signalling. Therefore, this study examined the ability of the nanoceria to be internalised by the human monocytic cell line, U937, and scavenge intracellular ROS. U937 cells activated in the presence of phorbol 12-myristate 13-acetate (PMA) were found to be more responsive to the nanoceria than U937 cells, which may not be surprising given the role of monocyte/macrophages in phagocytosing foreign material. The smaller particles were found to contain more crystal lattice defects with which to scavenge ROS, however a greater proportion of both the U937 and activated U937 cell populations responded to the larger particles. Hence all nanoceria particle sizes examined in this study were equally effective in scavenging intracellular ROS.

Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors.

The field of heterogeneous photocatalysis has expanded rapidly in the last four decades, having undergone various evolutionary phases related to energy and the environment. The two most significant applications of photocatalysis are geared toward solar water splitting and the purification of air and water. Notably, the interdisciplinary nature of the field has increased significantly, incorporating semiconductor physics, surface sciences, photo and physical chemistry, materials science, and chemical engineering. Whereas this forms the basis on which the field continues to grow, adequate bridging of multidisciplinary knowledge remains essential. By recalling some of the classical fundamentals of photocatalysis, this Perspective provides contemporary views on heterogeneous photochemical conversion, encompassing charge transport characteristics, radical chemistry and organic degradation mechanisms, photocatalyst design, and photoreactor engineering.

Cytotoxic origin of copper(II) oxide nanoparticles: comparative studies with micron-sized particles, leachate, and metal salts.

The work investigates the source of toxicity of copper oxide nanoparticles (CuO NPs) with respect to its leaching characteristic and speciation. Complexation-mediated leaching of CuO NPs by amino acids was identified as the source of toxicity toward Escherichia coli, the model microorganism used in the current study. The leached copper-peptide complex induces a multiple-fold increase in intracellular reactive oxygen species generation and reduces the fractions of viable cells, resulting in the overall inhibition of biomass growth. The cytotoxicity of the complex leachate is however different from that of equivalent soluble copper salts (nitrates and sulfates). A pH-dependent copper speciation during the addition of copper salts gives rise to uncoordinated copper ions, which in turn result in greater toxicity and cell lysis, the latter of which was not observed for CuO NPs even at comparable pH. Since leaching did not occur with micrometer-sized CuO, no cytotoxicty effect was observed, thus highlighting the prominence of materials toxicity at the nanoscale.

Flame preparation of visible-light-responsive BiVO4 oxygen evolution photocatalysts with subsequent activation via aqueous route.

Visible-light-active BiVO(4) photocatalyst prepared by a one-step flame spray pyrolysis demonstrates the structural evolution from amorphous to crystalline scheelite-tetragonal and further to scheelite-monoclinic (the photocatalytic active phase). Up to 95% scheelite-monoclinic content, the rest being scheelite-tetragonal, can be achieved in situ by exposing the collection filter to higher flame temperature. Reasonable activity in terms of photocatalytic O(2) evolution was obtained with the increase in crystallinity and scheelite-monoclinic content. Although analogous postcalcination of BiVO(4) improves crystallization and phase transformation, it inevitably induces vacancy defects that are detrimental to the photocatalytic activity. Hence a facile aqueous acid treatment on the flame-made BiVO(4) is introduced, which in the presence of small addition of Bi and V promotes full transformation to scheelite-monoclinic and reduces charge trapping defects. As a result, the photocatalytic O(2) evolution activity was increased by a remarkable 5 folds compared to the best performing untreated flame-made BiVO(4).

Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication.

Combustion of appropriate precursor sprays in a flame spray pyrolysis (FSP) process is a highly promising and versatile technique for the rapid and scalable synthesis of nanostuctural materials with engineered functionalities. The technique was initially derived from the fundamentals of the well-established vapour-fed flame aerosols reactors that was widely practised for the manufacturing of simple commodity powders such as pigmentary titania, fumed silica, alumina, and even optical fibers. In the last 10 years however, FSP knowledge and technology was developed substantially and a wide range of new and complex products have been synthesised, attracting major industries in a diverse field of applications. Key innovations in FSP reactor engineering and precursor chemistry have enabled flexible designs of nanostructured loosely-agglomerated powders and particulate films of pure or mixed oxides and even pure metals and alloys. Unique material morphologies such as core-shell structures and nanorods are possible using this essentially one step and continuous FSP process. Finally, research challenges are discussed and an outlook on the next generation of engineered combustion-made materials is given.

Experimental validation of proton transverse relaxivity models for superparamagnetic nanoparticle MRI contrast agents.

Analytical models of proton transverse relaxation rate enhancement by magnetic nanoparticles were tested by making measurements on model experimental systems in a field of 1.4 T. Proton relaxivities were measured for five aqueous suspensions of iron oxide (maghemite) nanoparticles with nominal mean particle sizes of 6, 8, 10, 11, and 13 nm. Proton relaxivity increased with mean particle size ranging from 13 s(-1) mM Fe(-1) for the 6 nm sample, up to 254 s(-1) mM Fe(-1) for the 13 nm sample. A strong correlation between the measured and predicted values of the relaxivity was observed, with the predicted values being consistently higher than the measured values. The results indicate that the models give a reasonable agreement with experimental results and hence can be used as the basis for the design of new magnetic resonance imaging contrast and labelling agents.

Understanding photocatalytic metallization of preadsorbed ionic gold on titania, ceria, and zirconia.

A nonaqueous photodeposition procedure for forming Au nanoparticles on semiconducting supports (TiO(2), CeO(2), and ZrO(2)) was investigated. Intrinsic excitation of the support was sufficient to induce Au(0) nucleation, without the need for an organic hole-scavenging species. Photoreduction rates were higher over TiO(2) and ZrO(2) than over CeO(2), likely due to a lower rate of photogenerated electron recombination. Illumination resulted in metallization of the adsorbed Au species and formation of crystalline Au nanoparticles dispersed across the oxide surfaces. On the basis of transmission electron microscopy (TEM) evidence of a strong Au particle-metal oxide interaction, it is proposed that Au deposit formation proceeds via the nucleation of highly dispersed clusters which can diffuse and amalgamate at room temperature to form larger surface-defect-immobilized clusters, with the final particle size being significantly smaller than that achieved by conventional aqueous photodeposition. From this work, it is possible to draw several new fundamental insights, with regards to both the nonaqueous photodeposition process and the general mechanism by which dispersed metallic Au nanoparticles are formed from ionic precursors adsorbed upon metal oxide supports.