Control of Tyrosyl Radical Stabilization by {SiO2@Oligopeptide} Hybrid Biomimetic Materials.

Tyrosine radicals are notoriously short-lived/unstable in solution, while they present an impressive degree of stability and versatility in bioenzymes. Herein, we have developed a library of hybrid biomimetic materials (HBMs), which consists of tyrosine-containing oligopeptides covalently grafted on SiO2 nanoparticles, and studied the formation, lifetime, and redox properties of tyrosyl radicals. Using electron paramagnetic resonance spectroscopy, we have studied the radical-spin distribution as a probe of the local microenvironment of the tyrosyl radicals in the HBMs. We find that the lifetime of the tyrosyl radical can be enhanced by up to 6 times, by adjusting three factors, namely, a proximal histidine, the length of the oligopeptide, and the interface with the SiO2 nanomatrix. This is shown to be correlated to a significant lowering of E1/2 from +736 mV, in free tyrosine, to +548 mV in the {12-peptide}@SiO2 material. Moreover, we show that grafting on SiO2 lowers the E1/2 of tyrosine radicals by ∼50 mV in all oligopeptides. Analysis of the spin-distribution by EPR reveals that the positioning of a histidine at a H-bonding distance from the tyrosine further favors tyrosine radical stabilization.

Lattice Defects Engineering in W-, Zr-doped BiVO4 by Flame Spray Pyrolysis: Enhancing Photocatalytic O2 Evolution.

A flame spray pyrolysis (FSP) method has been developed, for controlled doping of BiVO4 nanoparticles with W and Zr in tandem with the oxygen vacancies (Vo) of the BiVO4 lattice. Based on XPS and Raman data, we show that the nanolattice of W-BiVO4 and Zr-BiO4 can be controlled to achieve optimal O2 evolution from H2O photocatalysis. A synergistic effect is found between the W- and Zr-doping level in correlation with the Vo-concentration. FSP- made W-BiVO4 show optimal photocatalytic O2-production from H2O, up to 1020 µmol/(g × h) for 5%W-BiVO4, while the best performing Zr-doped achieved 970 µmol/(g × h) for 5%Zr-BiVO4. Higher W-or Zr-doping resulted in deterioration in photocatalytic O2-production from H2O. Thus, engineering of FSP-made BiVO4 nanoparticles by precise control of the lattice and doping-level, allows significant enhancement of the photocatalytic O2-evolution efficiency. Technology-wise, the present work demonstrates that flame spray pyrolysis as an inherently scalable technology, allows precise control of the BiVO4 nanolattice, to achieve significant improvement of its photocatalytic efficiency.

Layer-by-Layer Assembly of Clay-Carbon Nanotube Hybrid Superstructures.

Much of the research effort concerning layered materials is directed toward their use as building blocks for the development of hybrid nanostructures with well-defined dimensions and behavior. Here, we report the fabrication through layer-by-layer deposition and intercalation chemistry of a new type of clay-based hybrid film, where functionalized carbon nanotubes are sandwiched between nanometer-sized smectite clay platelets. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized in a single step with phenol groups, via 1,3-dipolar cycloaddition, to allow for stable dispersion in polar solvents. For the production of hybrid thin films, a bottom-up approach combining self-assembly with Langmuir-Schaefer deposition was applied. Smectite clay nanoplatelets act as a structure-directing interface and reaction media for grafting functionalized carbon nanotubes in a bidimensional array, allowing for a controllable layer-by-layer growth at a nanoscale. Hybrid clay/SWCNT multilayer films deposited on various substrates were characterized by X-ray reflectivity, Raman, and X-ray photoelectron spectroscopies, as well as atomic force microscopy.

Molybdena deposited on titania by equilibrium deposition filtration: structural evolution of oxo-molybdenum(vi) sites with temperature.

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoOx)n/TiO2 catalysts. In situ Raman spectroscopy in the temperature range of 25-450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo-Mo(VI) species on TiO2 with increasing temperature as well as the influence of the supported (MoOx)n species on the photo-generation of electrons and holes of TiO2. This study concerns (MoOx)n/TiO2 samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm(2), thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.

Stabilization of Phenolic Radicals on Graphene Oxide: An XPS and EPR Study.

A graphene oxide-gallic acid hybrid material was synthesized by the immobilization of gallic acid (3,4,5-trihydroxobenzoic acid) on graphene oxide. The grafting was achieved via the formation of amide bonds between the amine groups on the organofunctionalized graphite oxide surface and the carboxyl groups of the gallic acid molecules. The EPR signal of the gallic acid radicals in this hybrid material remained almost unaltered over at least 500 days, with less than 3% signal decay over that period, pointing to the truly remarkable stability of these radicals. The produced material was characterized by Fourier transform infrared, X-ray photoelectron, and electron paramagnetic resonance spectroscopies as well as by thermogravimetric analysis and the Kaiser test. The stability of the radicals in the material was studied in powder form and in aqueous solution vs pH. We demonstrate that in the graphene oxide-gallic acid hybrid material a radical is favorably stabilized on the ring-O while the oxidation of the second OH is precluded, and this results in long-term stabilization of the gallic acid radicals in solid hybrid material. Thus, in applications where it will be used under O2-free and humidity-free conditions, the graphene oxide-gallic acid hybrid material is a reliable spintronics scaffold.

Interfacial Hydrogen Atom Transfer by nanohybrids based on Humic Acid Like Polycondensates.

Novel nanohybrid materials were prepared by covalent grafting of a polyphenolic polymer [Humic Acid Like Polycondensate (HALP)] on SiO2 nanoparticles. Four nanohybrids were so-produced, using four different types of SiO2 i.e. three Aerosil flame-made nanoparticles with nominal specific surface area of 50, 90 and 300 m(2)/g, herein codenamed OX50, A90, A300 respectively, plus a colloidal SiO2[S300] with SSA=300 m(2)/g. The antioxidant activity of the SiO2-HALP nanohybrids was evaluated by assessing their kinetics for Hydrogen Atom Transfer [HAT] to DPPH radicals. When normalized per same HALP concentration, bigger NPs SiO2[OX50]-HALP NPs can scavenge 280 µmoles of DPPH radicals per gram of HALP, while [A90]-HALP and [A300]-HALP NPs can scavenge 514 and 832 µmoles of DPPH radicals per gram of HALP, respectively. The colloidal SiO2[S300]-HALP can scavenge fewer DPPH radicals (252 µmoles) per gram of HALP. Based on detailed kinetic data it is shown that (i) surface grafted HALPs perform 300% better HAT than non-grafted HALP in solution. (ii) By controlling the particle type and grafting-loading, we can control/optimize the HAT performance: when grafted on the appropriate SiO2 surface the HALP macromolecules are able to quench up to 0.8 mmoles of DPPH-radical per gram of HALP.

Data mining with molecular design rules identifies new class of dyes for dye-sensitised solar cells.

A major deficit in suitable dyes is stifling progress in the dye-sensitised solar cell (DSC) industry. Materials discovery strategies have afforded numerous new dyes; yet, corresponding solution-based DSC device performance has little improved upon 11% efficiency, achieved using the N719 dye over two decades ago. Research on these dyes has nevertheless revealed relationships between the molecular structure of dyes and their associated DSC efficiency. Here, such structure-property relationships have been codified in the form of molecular dye design rules, which have been judiciously sequenced in an algorithm to enable large-scale data mining of dye structures with optimal DSC performance. This affords, for the first time, a DSC-specific dye-discovery strategy that predicts new classes of dyes from surveying a representative set of chemical space. A lead material from these predictions is experimentally validated, showing DSC efficiency that is comparable to many well-known organic dyes. This demonstrates the power of this approach.

Humic acid-inspired hybrid materials as heavy metal absorbents.

Three SiO(2)-based materials were prepared via covalent immobilization of carboxyl groups (COOH), phenolic groups (GA), or humic acid on an SiO(2) surface. Their sorbing properties were evaluated for removal of heavy metals (Pb(2+), Cd(2+), Cu(2+), Zn(2+), and Mg(2+)) from aqueous solution. The data show a significant improvement for metal uptake, compared to unmodified silica that can be attributed to the adsorption of metals to the deprotonated form of functional groups (COOH, GA, HA). The metal-uptake capacity of the SiO(2)-HA material was 10 times higher that those of the other two materials. The present data provide direct experimental proof that HA can be viewed and modeled as a combination of -COO and R-OH functional groups.

Mechanism of heavy metal uptake by a hybrid MCM-41 material: surface complexation and EPR spectroscopic study.

A novel hybrid MCM-41-based material was synthesized by incorporation of AEDTC [N-(2-aminoethyl)dithiocarbamate] in the MCM-41 pores. The derived MCM-41 x AEDTC material possesses high AEDTC loading 35% [w:w], and a well-defined array of regular mesopores with a specific surface area of 632 m(2)/g. Heavy metal, Cd, Pb, Cu, and Zn, uptake was studied in detail at physiological pH values 6-8, by a combination of analytical and electron paramagnetic resonance (EPR) spectroscopic techniques. The analytical data show a significant improvement, i.e., 200-500%, for Pb, Cu, and Zn uptake by the MCM-41 x AEDTC hybrid vs the unmodified MCM-41. In contrast, Cd shows an exceptional behavior: (a) Cd uptake by MCM-41 x AEDTC is very low. (b) Competitive metal uptake experiments reveal that Cd ions cause a characteristic inhibition of Cu or Pb uptake by the MCM-41 x AEDTC while Cd binding itself always remained low. The present findings are analyzed by a combination of surface complexation modeling and EPR spectroscopy. Accordingly, in the MCM-41 x AEDTC the sulfur atoms of AEDTC provide strong binding sites for metal binding, with a stoichiometry [S(AEDTC)]:[Metal] = 1:1. Cd inhibits accessibility of Cu or Pb ions in the AEDTC sites.

Interfacial impregnation chemistry in the synthesis of cobalt catalysts supported on titania.

The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H(2)O)(6)](2+) on the "titania/electrolyte solution" interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner-sphere complexes on deposition of the [Co(H(2)O)(6)](2+) ions at the "titania/electrolyte solution" interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co(II) surface concentrations to be determined. It was found that the interface speciation depends on the Co(II) surface concentration. Mononuclear complexes are formed at the compact layer of the "titania/electrolyte solution" interface for low and medium Co(II) surface concentrations. Formation of mono-hydrolyzed Ti(2)O-TiO and the dihydrolyzed TiO-TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H(2)O)(6)](2+) ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H(2)O)(6)](2+) ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co(II) surface concentrations.

Isothermal decomposition of hydroxylamine and hydroxylamine nitrate in aqueous solutions in the temperature range 80-160 degrees C.

Hydroxylamine (HA) and hydroxylamine nitrate (HAN) have been involved independently in several tragic accidents, which incurred numerous fatalities and injuries. Following these incidents, adiabatic calorimetry and computational chemistry research was conducted on those compounds, suggesting potential reaction pathways of their decomposition, but the mechanism of their unstable behavior, still have not been completely understood. In the present work, isothermal decomposition tests were performed accompanied with HPLC, ion chromatography and UV analyses in the temperature range 80-160 degrees C. Condition-dependent autocatalytic decompositions were demonstrated for HA and HAN, and an intermediate formation has been observed that is most likely responsible for their autocatalytic behavior. These findings corroborate previously reported computational chemistry results.

Physicochemical study of amino-functionalized organosilicon cubes intercalated in montmorillonite clay: H-binding and metal uptake.

Two organic-modified montmorillonite clays were prepared by embedding organosilanes bearing different chelating amino-functional groups [Apteos] (3-amino-propyltriethoxysilane), and [Edaptmos] (3-(2-aminoethylamino)propyltrimethoxysilane), in the interlayer space of a Zenith montmorillonite. XRD and FTIR spectroscopic data show that the amino organosilanes are intercalated into the interlamelar space forming cube-like structures bearing one polymanino tail at each cube apex. The intercalated cubes cause an increase of the interlayer spacing of the clay sheets by 6.6 A in [Zenith-Apteos] and by 7.1 A in [Zenith-Edaptmos]. The H-binding properties of the intercalated polyamino organosilanes were studied by potentiometric titration. The Cu-, Cd-, and Pb-binding capacity of [Zenith-Apteos] and [Zenith-Edaptmos] were evaluated in aqueous solution as a function of the pH. Both [Zenith-Apteos] and [Zenith-Edaptmos] showed improvement vs Zenith for metal binding in the order Cu > Pb > Cd. [Zenith-Edaptmos] showed the most important results vs Zenith. Theoretical analysis of the pH edge, achieved by a surface complexation model, shows that (a) the amino-functionalized cube-like structures constitute high affinity metal-binding sites; and (b) the metal ions are bound in a monodendate mode with the amino group of the cube, thus resulting in a maximization of metal-binding efficiency.

Physicochemical study of novel organoclays as heavy metal ion adsorbents for environmental remediation.

Four organic-modified clays based on a SWy-2 montmorillonite were prepared by embedding ammonium organic derivatives with different chelating functionalities (NH(2), COOH, SH or CS(2)) in the interlayer space of montmorillonite. Organic molecules such as (a) hexamethylenediamine, (b) 2-(dimethylamino)ethenethiol, (c) 5-aminovaleric acid and (d) hexamethylenediamine-dithiocarbamate were used for the clay modification in order to study the effect of the chelating functionality on heavy metal ions binding from aqueous solutions. The organoclays were characterized by powder X-ray diffraction (XRD), infrared (FTIR) and NMR spectroscopies. The experimental data showed that the organic molecules are intercalated into the interlamelar space with the long dimension parallel to the clay sheets. Their sorbing properties were evaluated for the removal of heavy metals, Pb, Cd and Zn, from aqueous solutions as a function of the pH. When compared with the unmodified SWy-2 montmorillonite, the modified clays show significant improvement in terms of sorbing selectivity as well as of metal loading capacity. The fit to adsorption data by a Surface Complexation Model shows that the intercalated molecules act as specific binding sites in the clay. These contribute additional sorption capacity which is additive to the variable charge edge-sites of the clay in competition with the permanent charge sites.

Effects of dissolved carbonates and carboxylates on the sorption of thiuram disulfide pesticides on humic acids and model surfaces.

The sorption of a hydrophobic pesticide, thiram, on humic acid (HA) occurs via a specific pH-dependent binding of thiram at the deprotonated carboxylates of humic acid, forming a species thiram-[HACOO-] with K = 0.69. Similarly, thiram was sorbed by two model polycarboxylate-{SiO2COOH} materials via the formation of a surface species thiram-{SiO2COO-} with K = 0.45 between thiram and the eprotonated carboxylates grafted on SiO2 particles. In all cases, allowance of presence of bicarbonate at natural concentration caused severe inhibition of thiram's sorption. Oxalate and formate mimic the inhibitive effect of bicarbonate. Theoretical fit of the data showed that the inhibitive effect of HCO3- is due to the formation of the anionic species [thiram-HCO3](-1) (with K = 0.90) which is water soluble and competes with the bound species thiram-{HACOO-}. The same phenomena were observed for the sorption of disulfiram. The specific interaction phenomena reported here bear relevance to the sorption properties of thiram and disulfiram on real soils and, therefore, may determine their environmental fate.

Adsorption and radical stabilization of humic-Acid analogues and Pb2+ on restricted phyllomorphous clay.

Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.

Effects of dissolved carboxylates and carbonates on the adsorption properties of thiuram disulfate pesticides.

The adsorption of thiram and disulfiram onto alpha-Al2O3 and montmorillonite clay has been studied in the presence of small carboxylate anions, bicarbonate, formate, and oxalate. At natural concentrations, HCO3- enhances dramatically the adsorption of both pesticides on alpha-Al2O3 and clay. An analogous significant enhancement of pesticide adsorption is also observed in the presence of formate and oxalate. Density functional theory calculations demonstrate that in solution a stable molecular complex between one molecule of thiram and one molecule of HCO3- is formed with interaction energy -35.6 kcal/mol. In addition, two H20 molecules further stabilize it by an interaction energy of -3.6 kcal/mol. This clustering [thiram- HCO3- -2H2O] leads to a change of the electronic structure and the ultraviolet-visible spectrum of thiram that is observed experimentally. Surface complexation modeling shows that the molecular cluster [thiram-HCO3- -2H2O], which bears a total net charge of -1, is responsible for the observed enhanced adsorption on the charged surface of alumina and clay at pH below their points of zero surface charge. The results reveal a novel pervasive role of carboxylate anions and particularly HCO3- on the adsorption of dithiocarbamate pesticides in natural waters.