Surface modification of two-dimensional layered double hydroxide nanoparticles with biopolymers for biomedical applications.

Layered double hydroxides (LDHs) are appealing nanomaterials for (bio)medical applications and their potential is threefold. One can gain advantage of the structure of LDH frame (i.e., layered morphology), anion exchanging property towards drugs with acidic character and tendency for facile surface modification with biopolymers. This review focuses on the third aspect, as it is necessary to evaluate the advantages of polymer adsorption on LDH surfaces. Beside the short discussion on fundamental and structural features of LDHs, LDH-biopolymer interactions will be classified in terms of the effect on the colloidal stability of the dispersions. Thereafter, an overview on the biocompatibility and biomedical applications of LDH-biopolymer composite materials will be given. Finally, the advances made in the field will be summarized and future research directions will be suggested.

Nanoclay-based sensor composites for the facile detection of molecular antioxidants.

The detection and quantification of antioxidant molecules is an important task in food science, the fine chemical industry and healthcare. Antioxidants help in preventing the deterioration of nutrition and healthcare products, while eliminating over-the-limit exogenic reactive species, which may lead to illnesses. In our contribution, an inexpensive and rapid method to determine the concentration of various molecular antioxidants was developed. The principle of the analysis relies on the cupric ion reducing antioxidant capacity (CuPRAC) method, which is based on the color-changing reduction of chelated Cu2+ ions. This complex was successfully immobilized on an alginate-functionalized layered double hydroxide (dLDH) nanosheet via electrostatic interactions. The synthesis conditions of alginate (NaAlg) and the cupric complex were optimized, and the optimized composite was fabricated on cellulose paper to obtain a sensing platform. The paper-based sensor was superior to the ones prepared without the dLDH support, as the limit of detection (LOD) values decreased, and the linearity ranges broadened. The results offer a single-point measurement to evaluate the antioxidant efficiency in a cuvette-based method. The superior ability of the sensor was assigned to the presence of solid dLDH particles, as they offer adsorption sites for the dissolved antioxidant molecules, which contributes significantly to the decrease of the diffusion limitation during the detection process.

Composite materials based on heteroaggregated particles: Fundamentals and applications.

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.

Nanocomposite-based dual enzyme system for broad-spectrum scavenging of reactive oxygen species.

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.

Layered double hydroxide-based antioxidant dispersions with high colloidal and functional stability.

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.

Layered Double Hydroxide Nanoparticles to Overcome the Hydrophobicity of Ellagic Acid: An Antioxidant Hybrid Material.

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',5,5',6,6'-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.

Self-Assembly of Protamine Biomacromolecule on Halloysite Nanotubes for Immobilization of Superoxide Dismutase Enzyme.

An antioxidant material composed of halloysite nanotubes (HNTs), protamine sulfate polyelectrolyte (PSP), and superoxide dismutase (SOD) enzyme was prepared by self-assembly of the PSP and SOD biomacromolecules on the nanoparticulate support. The structural, colloidal and biocatalytic features were assessed. Adsorption of PSP on the oppositely charged HNT surface at appropriate loadings gave rise to charge neutralization and overcharging, which resulted in unstable and stable dispersions, respectively. The formation of a saturated PSP layer on the HNT led to the development of positive surface charge and to remarkable resistance against salt-induced aggregation making the obtained HNT-PSP hybrid suitable for immobilization of negatively charged SOD. No enzyme leakage was observed from the HNT-PSP-SOD composite indicating sufficient structural stability of this material due to electrostatic, hydrophobic, and hydrogen bonding interactions taking place between the particles and the biomacromolecules. Enzymatic assays revealed that SOD kept its functional integrity upon immobilization and showed high activity in superoxide radical dismutation. In this way, stable antioxidant bionanocomposite dispersions were obtained, which can be used as antioxidants in heterogeneous samples.

Radical scavenging activity of plant extracts from improved processing.

Radical scavenging activity of extracts obtained from 16 plants harvested in South Hungary was assessed and compared to the activity of ascorbic acid standard. During extraction, a novel technique involving an ethanolic treatment at ambient temperature was used for advanced active component release. Although the procedure is time consuming, it serves as an efficient and harmless route to extract valuable antioxidant compounds from their natural sources. The as-prepared extracts consist of two phases (except Allium sativum), a clear solution and a thick suspension containing solid plant parts that separates in about 2 h. The samples were analysed by the antioxidant assay based on the scavenging of 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radicals. For most of the species, the solid phase retained considerable amount of available antioxidant agents, while the solution parts showed significant radical scavenging activity. The main exceptions were Nigella sativa, Hippophae rhamnoides and Linum usitatissimum, where the solid parts were less active. Overall, the extracts possessed remarkable antioxidant activity that were compared to published literature data and were found to be superior.

Highly stable enzyme-mimicking nanocomposite of antioxidant activity.

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.

Synthesis of high-quality, well-characterized CaAlFe-layered triple hydroxide with the combination of dry-milling and ultrasonic irradiation in aqueous solution at elevated temperature.

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).