In Situ Structural Characterization of Ferric Iron Dimers in Aqueous Solutions: Identification of μ-Oxo Species

Dimerization of Fe(III) Ion in an Aqueous Medium: Mechanistic Modelling and Effects of Ligands

Aqueous iron solutions generally undergo spontaneous hydrolysis followed by aggregation resulting in the precipitation of nanocrystalline oxyhydroxide minerals. The mechanism of nucleation of such multinuclear oxyhydroxide clusters are unclear due to limited experimental evidence. Here, we investigate the mechanistic pathway of dimerization of Fe(III) ions using density functional theory (DFT) in aqueous medium considering effects of other ligands. Two hydrolyzed monomeric Fe(III) ions in aqueous medium may react to form two closely related binuclear products, the μ-oxo and the dihydroxo Fe2 dimer. Our studies indicate that the water molecules in the second coordination sphere and those co-ordinated to the Fe(III) ion, both participate in the dimerization process. The proposed mechanism effectively explains the formation of dihydroxo and μ-oxo Fe2 dimers with interconversion possibilities, for the first time. Results show, with only water molecules present in the second co-ordination sphere, dihydroxo Fe2 dimer is the thermodynamically and kinetically favored product with a low activation free energy. We calculated the step-wise reaction free energies of dimerization in the presence of nitrate ions in the first and second coordination sphere of Fe(III) ion separately, which shows that with nitrate ions in the second co-ordination sphere, the μ-oxo Fe2 dimer is the kinetically favored product.

Computational study of the interactions of tetravalent actinides (An = Th-Pu) with the α-Fe13 Keggin cluster

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe13 Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th-Pu onto a Na-decorated α-Fe13 Keggin cluster using DFT at the PBE0 level. The optimised Pu-O and Pu-Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest ΔrG° being for Th and Pu at -335.7 kJ mol-1 and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δrε0R2 = 0.97 and ΔrG° R2 = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th-Pu in these systems, supported by analysis of the occupied Kohn-Sham molecular orbitals where enhanced An(5f)-O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.

Homogeneous Carbon Dot-Anchored Fe(III) Catalysts with Self-Regulated Proton Transfer for Recyclable Fenton Chemistry

Fenton chemistry has been widely studied in a broad range from geochemistry, chemical oxidation to tumor chemodynamic therapy. It was well established that Fe³⁺/H₂O₂ resulted in a sluggish initial rate or even inactivity. Herein, we report the homogeneous carbon dot-anchored Fe(III) catalysts (CD-COOFe^III) wherein CD-COOFe^III active center activates H₂O₂ to produce hydroxyl radicals (^•OH) reaching 105 times larger than that of the Fe³⁺/H₂O₂ system. The key is the ^•OH flux produced from the O-O bond reductive cleavage boosting by the high electron-transfer rate constants of CD defects and its self-regulated proton-transfer behavior probed by operando ATR-FTIR spectroscopy in D₂O and kinetic isotope effects, respectively. Organic molecules interact with CD-COOFe^III via hydrogen bonds, promoting the electron-transfer rate constants during the redox reaction of CD defects. The antibiotics removal efficiency in the CD-COOFe^III/H₂O₂ system is at least 51 times large than the Fe³⁺/H₂O₂ system under equivalent conditions. Our findings provide a new pathway for traditional Fenton chemistry.

Characterization of a Hexanuclear Plutonium(IV) Nanostructure in an Acetate Solution via Visible-Near Infrared Absorption Spectroscopy, Extended X-ray Absorption Fine Structure Spectroscopy, and Density Functional Theory

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) were combined. The presence of Pu₆O₄(OH)₄(CH₃COO)₁₂ species in solution was first detected by vis-NIR and EXAFS spectroscopy. To confirm unambiguously this structure, EXAFS spectra were simulated from ab initio calculations. Debye-Waller factors and structural parameters were derived from DFT calculations. A large number of 5f electrons were treated as valence or core electrons using small- and large-core relativistic effective pseudopotentials. It is possible to reproduce accurately the EXAFS spectrum of the octahedral hexamer cluster at both levels of calculations. Further DFT and EXAFS calculations were performed on clusters of lower or higher nuclearities and of different geometries using the 5f-core approximation. The result shows that trimer, tetramer, flat hexamer, and even 16-mer clusters exhibit different EXAFS patterns and confirm the very specific octahedral hexanuclear EXAFS signature.

Hyaluronan self-agglomerating nanoparticles for non-small cell lung cancer targeting

Background

Owing to the limited amount of research, there are no nanoparticle-based anticancer agents that use hydrophilic drugs. Therefore, we developed irinotecan-loaded self-agglomerating hyaluronan nanoparticles (ISHNs). While irinotecan has high hydrophilicity, the resulting nanoparticle should possess high anticancer drug-loading capacity and allow selective targeting of the cluster of differentiation 44 (CD44) protein, which is overexpressed on the surface of tumor cells.

Results

The ISHNs were successfully made with hyaluronan (HA) as a targeting moiety, FeCl3 as a binder, and D-glutamic acid (GA) as a stabilizer. The ISHNs self-agglomerated via chelating bonding and were lyophilized using a freeze dryer. The particle diameter and zeta potential of the ISHNs were 93.8 ± 4.48 nm and − 36.3 ± 0.28 mV, respectively; a relatively narrow size distribution was observed. The drug fixation yield and drug-loading concentration were 58.3% and 1.75 mg/mL, respectively. Affinity studies revealed a tenfold stronger targeting to H23 (CD44+) non-small-cell lung cancer (NSCLC) cells, than of A549 (CD44−) cells.

Conclusion

We developed irinotecan-loaded ISHNs, which comprised irinotecan hydrochloride as a water-soluble anticancer agent, HA as a targeting moiety, FeCl3 as a binder for self-agglomeration, and GA as a stabilizer; HA is a binding material for CD44 in NSCLC cells. Owing to their ease of manufacture, excellent stability, non-cell toxicity and CD44-targeting ability, ISHNs are potential nanocarriers for passive and active tumor targeting.

Binding of As3+ and As5+ to Fe(III) Oxyhydroxide Clusters and the Influence of Aluminum Substitution: A Molecular Perspective

Fe(III) oxides and oxyhydroxides play a very important role in contaminant cycling and mobility in the environment through numerous sorption mechanisms owing to their nanoparticulate nature. Generally coprecipitated from mixtures of metal ions in natural environments, Fe(III) oxyhydroxides are often doped by various impurity metal ions to a certain degree. These dopant/impurity ions then play a crucial role in the geochemical cycling of toxic contaminants like arsenic via modified adsorption energetics on Fe(III) oxyhydroxide nanoparticles. Aluminum (Al) commonly coexists with ferric salts and minerals in nature and affects the arsenic (As) binding abilities of Fe(III) oxyhydroxides. We use electronic structure studies to model the As binding potential of Al-doped Fe(III) oxyhydroxide clusters, using a "bottom-up" molecular approach to understand their role in As fixation. We start from small Al-doped Fe(III) oxyhydroxide clusters, like dimers and trimers, and gradually study larger clusters including the δ-Fe₁₃ Keggin cluster, evaluating their As binding potential with respect to pure undoped Fe(III) oxyhydroxide clusters at each step. The calculated reaction free energies clearly show that Al doping into Fe(III) oxyhydroxide clusters reduces their As³⁺ binding potential, whereas the As⁵⁺ binding is not affected much due to Al doping.

Aqueous Iron(IV)-Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (Fe_aq^IVO²⁺) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that Fe_aq^IVO²⁺ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out Fe_aq^IVO²⁺ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of Fe_aq^IVO²⁺, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of Fe_aq^IVO²⁺ in Fe_aq²⁺ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of Fe_aq^IVO²⁺ in Fe_aq²⁺-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Fe_aq²⁺ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of Fe_aq^IVO²⁺ reactivity from an engineering viewpoint are proposed.

What are the inorganic nanozymes? Artificial or inorganic enzymes!

The research on inorganic nanozymes remains very active since the first paper on the “intrinsic peroxidase-like properties of ferromagnetic nanoparticles” was published in Nature Nanotechnology in 2007. However, there is...

On the singularity of scandium

Hydrogen-bonding interactions of the [Sc2(OH)2(OH2)10]4+ dimer containing 7-coordinate Sc(iii).

Aggregating ability of ferric chloride in the presence of phosphate ligand

Complexing anions such as phosphate or silicate play an ambivalent role in the performance of hydrolyzing metal coagulants: On one hand, they significantly interfere with the hydrolytic pathway of conventional iron or aluminum coagulants, the associated destabilization mechanism remaining rather elusive; on the other hand, they have been shown to be key ingredients in the formulation of innovative coagulant solutions exhibiting improved removal efficiency, their action mechanism at the molecular scale being presently poorly understood. In this paper, we explore the effect of small additions of phosphate ligand on the chemical coagulation of silica nanoparticles with ferric chloride. Transmission Electron Microscopy-Energy Dispersed X-ray Spectroscopy (TEM-EDXS) combined with Extended X-ray absorption Fine Structure Spectroscopy (EXAFS) at the Fe K-edge are used to provide an insight into the nature of coagulant species, whereas jar-tests, laser diffraction, Small Angle X-ray Scattering (SAXS), and electrophoretic mobility, are used to investigate the aggregation dynamics of silica particles in the presence of phosphate ligand. We show that, in spite of a slight increase in the consumption of iron coagulant, the addition of phosphate significantly improves the formation of silica aggregates provided that the elemental Fe/P ratio remains above 7. Such effects originate from both a large increase in the overall number of coagulant species, the binding of a phosphate ligand terminating the growth of polymeric chains of edge-sharing Fe octahedra, and a change in the nature of the coagulant species that evolves with the Fe/P ratio, small polycations built-up from Fe-oligomers linked by phosphate tetrahedra being eventually formed. Those non-equilibrium nanosize Fe-P coagulant species assemble the silica nanoparticles to form hetero-aggregates whose structure is consistent with a Diffusion-Limited Cluster Aggregation mechanism.

Hydration of ferric chloride and nitrate in aqueous solutions: water-mediated ion pairing revealed by Raman spectroscopy

Iron is the most abundant transition metal in the earth's crust and is important for the proper functioning of many technological and natural processes. Despite the importance, a complete microscopic understanding of the hydration of ferric ions and water mediated ion pairing has not been realized. Hydrated Fe(iii) is difficult to study due to the process of complexation to the anion and hydrolysis of the hydrating water molecules leading to a heterogeneous solution with diverse speciation. Here, ferric chloride and nitrate aqueous solutions are studied using polarized Raman spectroscopy as a function of concentration and referenced to their respective sodium salt or mineral acid. Perturbed water spectra (PWS) were generated using multivariate curve resolution-alternating least squares (MCR-ALS) to show the residual spectral response uniquely attributable to the hydration of ferric speciation. The hydrogen bonding network associated with the hydrating water molecules in ferric chloride solutions are found to be more similar to hydrochloric acid solutions, whereas in ferric nitrate solutions, the network behaves more similar to sodium nitrate, despite increased acidity. Thus, in the FeNO3 and FeCl3 solutions, ion pairing and coordination, respectively, are significantly influencing the hydration spectra signature. These results further reveal concentration dependent changes to the hydrogen bonding network, hydrating water symmetry, and changes to the relative abundance of solvent shared ion pairs that are governed primarily by the ferric salt identity.

Hydrolysis of Phosphate Esters Catalyzed by Inorganic Iron Oxide Nanoparticles Acting as Biocatalysts

Phosphorus ester hydrolysis is one of the key chemical processes in biological systems, including signaling, free-energy transaction, protein synthesis, and maintaining the integrity of genetic material. Hydrolysis of this otherwise kinetically stable phosphoester and/or phosphoanhydride bond is induced by enzymes such as purple acid phosphatase. Here, I report that, as in previously reported aged inorganic iron ion solutions, the iron oxide nanoparticles in the solution, which are trapped in a dialysis membrane tube filled with the various iron oxides, significantly promote the hydrolysis of the various phosphate esters, including the inorganic polyphosphates, with enzyme-like kinetics. This observation, along with those of recent studies of iron oxide, vanadium pentoxide, and molybdenum trioxide nanoparticles that behave as mimics of peroxidase, bromoperoxidase, and sulfite oxidase, respectively, indicates that the oxo-metal bond in the oxide nanoparticles is critical for the function of these corresponding natural metalloproteins. These inorganic biocatalysts challenge the traditional concept of replicator-first scenarios and support the metabolism-first hypothesis. As biocatalysts, these inorganic nanoparticles with enzyme-like activity may work in natural terrestrial environments and likely were at work in early Earth environments as well. They may have played an important role in the C, H, O, S, and P metabolic pathway with regard to the emergence and early evolution of life. Key Words: Enzyme-Hydrolysis-Iron oxide-Nanoparticles-Origin of life-Phosphate ester. Astrobiology 18, 294-310.

Microscopic insight into precipitation and adsorption of As(V) species by Fe-based materials in aqueous phase

The mechanism of As(V) removal from the drinking water and industrial effluents by iron materials remains unclear at the molecular level. In this work, the association of Fe-based materials with As(V) species was explored using density functional theory and ab initio calculations. Solvent separated ion pair structures of [FeH₂AsO₄]²⁺_aq species may be dominant in an acidic solution of FeAs complex. The association trend of H₂AsO₄^- species by Fe³⁺_aq is found to be quite weak in the aqueous solution, which may be attributed to the strong hydration of Fe³⁺_aq and [FeH₂AsO₄]²⁺ species. However, the association of H₂AsO₄^- species by colloidal clusters is quite strong, due to the weakened hydration of Fe(III) in colloidal structures. The hydrophobicity of Fe-based materials may be one of the key factors for their As(V) removal efficiency in an aqueous phase. When the number of OH^- coordinated with Fe(III) increases, the association trend of As(V) by colloidal ferric hydroxides weakens accordingly. This study provides insights into understanding the coprecipitation and adsorption mechanisms of arsenate removal and revealing the high efficiency of arsenate removal by colloidal ferric hydroxides or iron salts under moderate pH conditions.

Characterization and quantification of chromate adsorption by layered porous iron oxyhydroxide: An experimental and theoretical study

The inner structure of iron oxyhydroxide agglomerates (IOAs) prepared from hydrolysis of ferric chloride was characterized and correlated to surface complexation of hexavalent chromium, Cr(VI), in a broad range of pH (3-12) and ionic strengths (0.0-5.0M). Evolution of particle size, morphology, and surface activity, combined with density functional theory (DFT) calculations, support the condensation reaction initiated formation of IOAs in three levels: iron nanoparticles to nanolayers to agglomerates. This agglomeration process led to a layered porous structure for aqueous-phase IOAs resulting in a rapid and high removal of Cr(VI) in batch tests. By integrating adsorption results, thermodynamic modeling, and quantum chemical calculations for the adsorption reactions, a quantitative distribution profile for each surface coordination of Cr(VI) ions (i.e., monodentate, bidentate, and hydrogen-bonding) was established. Results of this study are important to understand the fundamental mechanism of IOAs formation in aqueous phase and the intrinsic nature of surface complexations at the mineral-water interface for optimal Cr(VI) removal in hypersaline waste streams.

Fabrication of Holey Graphene: Catalytic Oxidation by Metalloporphyrin-Based Covalent Organic Framework Immobilized on Highly Ordered Pyrolytic Graphite

We report a facile chemical method for fabricating holey graphene by catalytic oxidation of highly ordered pyrolytic graphite (HOPG) using an FeIII porphyrin complex-based covalent organic framework (COF) as a bifunctional surface catalyst-template. We demonstrate regular hole formation after oxidation with H2 O2 and NaOCl, COF removal, and HOPG exfoliation.

Detection of the electronic structure of iron-(iii)-oxo oligomers forming in aqueous solutions

The electronic structure of the small iron-oxo oligomers forming in iron-(iii) aqueous solutions is determined from liquid jet photoelectron spectroscopy.

Ferrihydrite Formation: The Role of Fe13 Keggin Clusters

Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for cleanup of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In situ SAXS measurements supported by ex situ TEM indicate that initially Fe13 Keggin clusters (radius ∼ 0.45 nm) form in solution at pH 0.12-1.5 and are persistent for at least 18 days. The Fe13 clusters begin to aggregate above ∼ pH 1, initially forming highly linear structures. Above pH ∼ 2 densification of the aggregates occurs in conjunction with precipitation of low molecular weight Fe(III) species (e.g., monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (∼3 nm) in which the Fe13 Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core-shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.

Fe-doped cryptomelane synthesized by refluxing at atmosphere: Structure, properties and photocatalytic degradation of phenol

Fe-doped cryptomelanes were synthesized by refluxing at ambient pressure, followed by characterization with multiple techniques and test in photocatalytic degradation of phenol. The introduction of Fe(III) into the structure of cryptomelane results in a decrease in particle size and the contents of Mn and K(+), and an increase in the Mn average oxidation state (AOS), specific surface area and UV-vis light absorption ability. Mn and Fe K-edge extended X-ray absorption fine structure spectroscopy analysis indicates that some Fe(III) is incorporated into the framework of cryptomelane by replacing Mn(III) while the remaining Fe(3+) is adsorbed in the tunnel cavity. These Fe-doped cryptomelanes have significantly improved the photocatalytic degradation rate of phenol, with the sample of ∼3.04 wt.% Fe doping being the most reactive and achieving a degradation rate of 36% higher than that of the un-doped one. The enhanced reactivity can be ascribed to the increase in the coherent scattering domain size of the crystals, Mn AOS and light absorption, as well as the presence of sufficient K(+) in the tunnel. The results imply that metal doping is an effective way to improve the performance of cryptomelane in pollutants removal and has the potential for modification of Mn oxide materials.