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Highly stable antioxidant dispersions were designed on the basis of ring-opened ellagic acid (EA) intercalated into MgAl-layered double hydroxide (LDH) nanoparticles. The morphology of the composite was delicately modified with ethanolic washing to obtain EtOH-EA-LDH with a high specific surface area. The colloidal stability was optimized by surface functionalization with positively charged polyelectrolytes. Polyethyleneimine (PEI), protamine sulfate (PS) and poly(acrylamide-co-diallyl dimethyl ammonium chloride) (PAAm-co-DADMAC) was adsorbed onto the surface of the oppositely charged EtOH-EA-LDH leading to charge neutralization and overcharging at appropriate doses. Formation of adsorbed polyelectrolyte layers provided remarkable colloidal stability for the EtOH-EA-LDH. Modification with PEI and PAAm-co-DADMAC outstandingly improved the resistance of the particles against salt-induced aggregation with a critical coagulation concentration value above 1 M, while only limited stability was achieved by covering the nanoparticles with PS. The high antioxidant activity of EtOH-EA-LDH was greatly preserved upon polyelectrolyte coating, which was proved in the scavenging of radicals in the test reaction applied. Hence, an active antioxidant nanocomposite of high drug dose and remarkable colloidal stability was obtained to combat oxidative stress in systems of high electrolyte concentrations.

Three layered double hydroxides (MgAl-, CaAl- and CaGa-LDH) were obtained via co-precipitation with NaOH as base, and the effects of ageing in the slurry at elevated temperatures were under investigation. As expected, the crystallinity of the products improved after ageing, and the particle diameter of the Ca-containing samples grew as well. The experiments were carried out in the presence of either 1-hexanol, 1-heptanol or 1-octanol during the LDH formation. Each alcohol, functioning as a co-solvent in low concentration, had the same impact on the crystalline properties of the product: the morphology of all LDHs improved and the diameter of the CaM-LDH particles (M = Al, Ga) increased from the original 100–300 nm to 1–3 μm without ageing to an immense 5–10 μm after treatment at 60 °C. Small changes were detected in the X-ray traces, the basal spacing of the CaM-LDHs shrank. Considering the presence of large alcohol molecules in the interlamellar space, one may predict a rise in the basal spacing, however, it is also a function of the hydration degree of the interlayer space. Since alcohols with 6–8 carbon atoms are regarded as hydrophilic ones, we attribute the smaller interlayer distance to the partial exclusion of water from the interlamellar space. This phenomenon was confirmed via thermogravimetry. The difference in the size of alcohol and heat-treated CaM-LDHs to MgAl-LDH presumably arise from the stronger interaction of calcium ions with structural alcohol molecules due higher coordination number of the calcium ion (7) in the corresponding LDHs than that for the magnesium or aluminium ions (6).

Homoaggregation of dispersed particles, i.e., aggregation of particles of the same shape, charge, size, and composition, is a well-studied field and various theoretical and experimental approaches exist to understand the major phenomena involved in such processes. Besides, heteroaggregation of particles, i.e., aggregation of particles of different shape, charge, size, or composition, has attracted widespread interest due to its relevance in various biomedical, industrial, and environmental systems. For instance, heteroaggregation of plastic contaminant particles with naturally occurring solid materials in waters (e.g., clays, silica and organic polymers) plays an important role in the decontamination technologies. Moreover, nanofabrication processes involving heteroaggregation of particles to prepare novel composite materials are widely implemented in fundamental science and in more applied disciplines. In such procedures, stable particle dispersions are mixed and the desired structure forms owing to the presence of interparticle forces of various origins, which can be tuned by performing appropriate surface functionalization as well as altering the experimental conditions. These composites are widely used in different fields from sensing through catalysis to biomedical delivery. The present review summarizes the recent progresses in the field including new findings regarding the basic principles in particle heteroaggregation, preparation strategies of heteroaggregated structures of different morphology, and the application of the obtained hybrid composites. Such information will be very helpful to those involved in the design of novel composites consisting of different nano or colloidal particles.

A broad-spectrum reactive oxygen species (ROS)-scavenging hybrid material (CASCADE) was developed by sequential adsorption of heparin (HEP) and poly(L-lysine) (PLL) polyelectrolytes together with superoxide dismutase (SOD) and horseradish peroxidase (HRP) antioxidant enzymes on layered double hydroxide (LDH) nanoclay support. The synthetic conditions were optimized so that CASCADE possessed remarkable structural (no enzyme leakage) and colloidal (excellent resistance against salt-induced aggregation) stability. The obtained composite was active in decomposition of both superoxide radical anions and hydrogen peroxide in biochemical assays revealing that the strong electrostatic interaction with the functionalized support led to high enzyme loadings, nevertheless, it did not interfere with the native enzyme conformation. In vitro tests demonstrated that ROS generated in human cervical adenocarcinoma cells were successfully consumed by the hybrid material. The cellular uptake was not accompanied with any toxicity effects, which makes the developed CASCADE a promising candidate for treatment of oxidative stress-related diseases.

Counteracting reactive oxygen species (ROS, e.g., superoxide radical ion, H2O2 and hydroxyl radical) is an important task in fighting against oxidative stress-related illnesses and in improving product quality in industrial manufacturing processes. This review focuses on the recent advances on two-dimensional (2D) nanomaterials of antioxidant activity, which are designed for effective decomposition of ROS and thus, for reduction of oxidative stress. Some materials featured in this paper are of uni- or multi-lamellar structures modified with small molecular or enzymatic antioxidants. Others are enzyme-mimicking synthetic compounds (the so-called nanozymes) prepared without antioxidant additives. However, carbon-based materials will not be included, as they were extensively reviewed in the recent past from similar aspects. Given the landmark development around the 2D materials used in various bio-applications, sheet-like antioxidant compounds are of great interest in the scientific and technological communities. Therefore, the authors hope that this review on the recent progresses will be helpful especially for researchers working on novel developments to substantially reduce oxidative stress either in biological systems or industrial liquors.

The syntheses of nicotinate anion– and NAD coenzyme–layered double hydroxide (LDH) composites were performed with the aim of having the organic component among the layers. In-house prepared CaAl-LDHs were the host materials. Intercalation was attempted by direct ion exchange or by the dehydration-rehydration method applying aqueous solvent mixtures (containing ethanol, propanol, acetone, N,N-dimethylformamide). For structural characterization, beside X-ray diffractometry, X-ray photoelectron and IR spectroscopies, transmission and scanning electron microscopies as well as energy-dispersive X-ray analysis were used. Molecular modelling served for the visualization of the arrangements of the intercalated ions among the layers of the LDH samples. Although not all the intercalation methods and solvent mixtures led to intercalated composite materials, successful ones could be identified. The combination of spectroscopic methods helped in proposing sensible spatial arrangements for the intercalated anions. The NAD–CaAl-LDH composite proved to be an active catalyst in the oxidation of hydroquinone to 1,4-bezoquinoe in the presence of H2O2.

The delamination-restacking properties of MgAl-layered double hydroxide (MgAl-LDH) were studied in various solvents. The LDH samples were successfully delaminated in polar amides (formamide, N-methylformamide, N-methylacetamide). Usually, delamination was finalized by ultrasonic treatment. As rehydrating solutions, numerous Na-salts with single-, double- and triple-charged anions were used. Reconstruction was accomplished with anions of one or two negative charges, but triple-charged ones generally disrupted the rebuilding process, likely, because their salts with the metals of the LDH are very stable, and the thin layers can more readily transform to salts than the ordered materials. Samples and delamination-restacking processes were characterized by X-ray diffractometry (XRD), infrared spectroscopy (IR), dynamic light scattering (DLS), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX).

Host–guest composite materials were prepared applying the anionic forms of Ni(II)-amino acid (l-histidine, l-cysteine, and l-tyrosine) as the guests and CaAl-layered double hydroxide (CaAl-LDH) as the host. The syntheses were performed either by introducing the amino acid anions first and then constructing the metal ion–amino acid complexes or intercalating the pre-prepared complexes in anionic forms. The pristine as well as the composite LDH samples were structurally characterized by X-ray diffractometry, mid IR spectroscopy and scanning electron microscopy. The structural features of the interlayer complexes were studied by UV–Vis, inductively coupled plasma optical emission, mid and far IR and X-ray absorption spectroscopies as well as energy-dispersed X-ray analysis. On the basis of the acquired data, structural models were constructed. The composites were applied as catalysts in the liquid-phase oxidation of cyclohexene applying peracetic acid and the in situ formed iodosyl benzene as oxidants. Using peracetic acid afforded epoxide, while applying iodosyl benzene provided cis diol as the major or exclusive oxidation product. The catalysts displayed good recycling properties.

Photocatalytic decomposition of methylene blue was attempted over as-prepared Mn(II)Cr(III)-layered double hydroxide (LDH) containing Mn(II) and Cr(III) in 2:1 molar ratio. The LDH was prepared by the co-precipitation method, and was found to form at pH = 10 and at 80 °C following 24 h hydrothermal treatment. The Mn 2 Cr-LDH thus obtained was structurally characterized by X-ray diffractometry, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Mn 2 Cr-LDH displayed significant photocatalytic activity in the degradation of methylene blue under illumination with UV–vis light. The photocatalytic performance of the phase-pure and uncalcined Mn 2 Cr-LDH is practically identical to that of the commercially available Degussa P25 TiO 2 , and it was found to remain unaltered over five consecutive photocatalytic runs. The presence of the LDH structure in the composite is the prerequisite of the photocatalytic activity. Applying increasing calcination temperature, the LDH structure gradually collapses, and the Mn 2 Cr-LDH transforms to photocatalytically inactive double oxide.

Boron-containing CaAl-layered double hydroxides (LDHs) were prepared by milling, as well as the dehydration–rehydration and the anion-exchange methods. The substances obtained were structurally characterized by powder X-ray diffraction measurements, scanning electron microscopy and infrared spectroscopy. Various borate species could be introduced among the layers and/or the outer surface of the CaAl-LDH. The materials were tested as catalysts in the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam in the liquid phase, at 373 K. Two of them, the samples prepared by dehydration–rehydration and anion exchange, proved to be active, selective and recyclable catalysts, while the one made by the mechanochemical method was practically inactive. The catalytic activity was tied to intercalated and/or surface-bonded triborate and tetraborate species.

A highly stable nanocomposite of antioxidant activity was developed by immobilization of a superoxide dismutase-mimicking metal complex on copolymer-functionalized nanoclay. The layered double hydroxide (LDH) nanoclays were synthesized and surface modification was performed by adsorbing poly(vinylpyridine-b-methacrylic acid) (PVPMAA). The effect of the adsorption on the charging and aggregation properties was investigated and the copolymer dose was optimized to obtain stable LDH dispersions. The LDH-PVPMAA hybrid particles showed high resistance against salt-induced destabilization in aqueous dispersions. Copper(II)-histamine (Cu(Hsm)(2)) complexes were immobilized via the formation of dative bonds between the metal ions and the nitrogen atoms of the functional groups of the copolymer adsorbed on the particles. Changes in the coordination geometry of the complex upon immobilization led to higher superoxide radical anion scavenging activity than the one determined for the non-immobilized complex. Comparison of superoxide dismutase (SOD)-like activity of the obtained hybrid LDH-PVPMAA-Cu(Hsm)(2) with the nanoclay-immobilized SOD enzyme revealed that the developed composite maintained its activity over several days and was able to function at elevated temperature, while the immobilized native enzyme lost its activity under these experimental conditions. The developed nanocomposite is a promising antioxidant candidate in applications, where high electrolyte concentration and elevated temperature are applied. (C) 2019 Elsevier Inc. All rights reserved.

The combination of mechanochemical and ultrasonic treatment was applied to synthesize CaAlFe-layered triple hydroxides with carbonate or chloride anions in the interlamellar space. The optimal parameters of the preparation were explored by altering the initial ratio of the metal ions and the temperature of ultrasonic irradiation. The resulting triple hydroxides were characterized by X-ray diffractometry, infrared and X-ray absorption spectroscopies, thermogravimetric analysis and scanning electron microscopy. The products were close-to-phase-pure CaAlFe-layered triple hydroxides. Elevation of the temperature transformed the CaAlFe-Cl(-)-layered triple hydroxide to rare oxyhalides (Ca2FeO3Cl and Ca12Al14O32Cl2).

Intercalated composite materials were prepared with CaAl-layered double hydroxide as the host and Mn(II)-amino acid (l-cysteine, l-histidine and l-tyrosine) complex anions as the guest. Two methods (intercalation of the ligand first followed by constructing the complex; preforming the complex first, then introducing it among the layers of the host) and optimization of the auxiliary conditions were performed to arrive at composites having the complex exclusively among the layers. The obtained substances were structurally characterized by powder X-ray diffractometry, mid IR spectroscopy in diffuse reflectance mode and with ATR or photoacoustic detections, and scanning electron microscopy. The structural features of the intercalant (coordination number, coordination sites) were elucidated by classical chemical and energy dispersive X-ray analyses, EPR, X-ray absorption and far IR spectroscopies. Structural models are also given. The catalytic activities, selectivities and recycling abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and in situ formed iodosylbenzene as oxidants and allylic alcohol with peracetic acid, in the liquid phase. The intercalated substances proved to be efficient and highly selective (with peracetic acid: outstanding epoxide, with iodosylbenzene superior diol selectivities) catalysts with very good recycling abilities. (Less)

Prussian blue (PB) nanoparticles of intrinsic peroxidase and superoxide dismutase-like activities were prepared by the co-precipitation method and immobilized on amidine functionalized polystyrene latex (AL) particles. The interaction between the AL and PB particles and the colloidal stability of the resulting AL-PB hybrid composites were assessed at different mass ratios via determination of the charging and aggregation characteristics in the samples. The negatively charged PB nanoparticles strongly adsorbed on the oppositely charged AL particles resulting in a range of AL-PB composites of positive, neutral and negative overall charge, once the PB dose was increased. The AL-PB composite of a saturated PB layer on the surface of the AL particles formed considerably stable dispersions. Further, the morphology, structural and functional features of the AL-PB composites were explored by electron microscopy and enzymatic assays. The results revealed that the immobilization of PB nanoparticles not only provided a sustained catalytic surface but did not compromise the enzyme-like activities. The obtained stable composite is a promising agent in antioxidant therapies and wherever the aim is to reduce oxidative stress at laboratory or larger scales.

Ellagic acid (EA), a polyphenolic antioxidant of poor water solubility, was intercalated into biocompatible layered double hydroxide (LDH) nanoparticles by the coprecipitation method. Structural investigation of the composite revealed that the lactone bonds split under the synthetic experimental conditions, and EA was transformed to 4,4&prime, 5,5&prime, 6,6&prime, hexahydroxydiphenic acid during intercalation. To improve the surface properties of the EA-LDH composite, the samples were treated with different organic solvents. The antioxidant activity of the LDH hybrids was assessed in test reactions. Most of the obtained hybrids showed antioxidant activity comparable to the one of the free EA indicating that the spontaneous structural transformation upon immobilization did not change the efficiency in radical scavenging. Treatments with organic solvents influenced the activities of the materials remarkably. The main advantage of the immobilization procedure is that the products can be applied in aqueous samples in high concentrations overcoming the problem related to the low solubility of EA in water. The developed composites of high antioxidant content can be applied as efficient reactive oxygen species scavenging materials during biomedical treatments or industrial manufacturing processes.

Synthesis of intercalated composites were carried out using CaAl-LDH (layered double hydroxide) as host and the anionic form of Fe(III)-amino acid complexes as guest materials. Intercalation was attempted with two methods either introducing the preformed complexes or constructing the complex among the layers of the LDH. After optimization of the synthesis parameters, structural characterization was performed by X-ray diffractometry, scanning electron microscopy as well as mid and far infrared spectroscopies. Quantitative data about the intercalated complexes were collected by chemical analysis and X-ray absorption spectroscopy investigating the near edge region as well as the extended fine structure. Structural models based on characterization measurements are also given. Catalytic activities, selectivities and recycling abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and in situ formed iodosylbenzene as oxidants in the liquid phase. The catalysts were active in the Ullmann-type etherification coupling reaction as well. The intercalated substances were found to be efficient and highly selective catalysts with very good recycling abilities.

Titanium oxide particles of various morphologies have been prepared for applications of scientific or industrial interest in recent decades. Besides development of novel synthetic routes and solid-state characterization of the obtained particles, colloidal stability of titanium oxide dispersions was the focus of numerous research groups due to the high importance of this topic in applications in heterogeneous systems. The influence of dissolved ionic compounds, including monovalent salts, multivalent ions and polyelectrolytes, on the charging and aggregation behaviour of titanium oxide materials of spherical and elongated structures will be discussed in the present review.