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.

Aggregation of Halloysite Nanotubes in the Presence of Multivalent Ions and Ionic Liquids.

Colloidal stability was investigated in two types of particle systems, namely, with bare (h-HNT) and polyimidazolium-functionalized (h-HNT-IP-2) alkali-treated halloysite nanotubes in solutions of metal salts and ionic liquids (ILs). The valence of the metal ions and the number of carbon atoms in the hydrocarbon chain of the IL cations (1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), 1-butyl-3-methylimidazolium (BMIM+), and 1-hexyl-3-methylimidazolium (HMIM+)) were altered in the measurements. For the bare h-HNT with a negative surface charge, multivalent counterions destabilized the dispersions at low values of critical coagulation concentration (CCC) in line with the Schulze-Hardy rule. In the presence of ILs, significant adsorption of HMIM+ took place on the h-HNT surface, leading to charge neutralization and overcharging at appropriate concentrations. A weaker affinity was observed for MIM+, EMIM+, and BMIM+, while they adsorbed on the particles to different extents. The order HMIM+ < BMIM+ < EMIM+ < MIM+ was obtained for the CCCs of h-HNT, indicating that HMIM+ was the most effective in the destabilization of the colloids. For h-HNT-IP-2 with a positive surface charge, no specific interaction was observed between the salt and the IL constituent cations and the particles, i.e., the determined charge and aggregation parameters were the same within experimental error, irrespective of the type of co-ions. These results clearly indicate the relevance of ion adsorption in the colloidal stability of the nanotubes and thus provide useful information for further design of processable h-HNT dispersions.

Influence of adsorption of ionic liquid constituents on the stability of layered double hydroxide colloids.

The influence of ionic liquid (IL) anions and cations on the charging and aggregation properties of layered double hydroxide (LDH) nanoparticles was systematically studied. Surface charge characteristics were explored using zeta potential measurements, while aggregation processes were followed in dynamic light scattering experiments in aqueous IL solutions. The results revealed that the aggregation rates of LDHs were sensitive to the composition of ILs leading to IL-dependent critical coagulation concentration (CCC) values being obtained. The origin of the interparticle forces was found to be electrostatic, in line with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, as the experimental aggregation kinetics were in good agreement with the predicted data. The ion specific adsorption of IL anions led to different surface charge densities for LDHs, which decreased in the order Cl- > Br- > DCA- > SCN- > NO3- for counterions and BMIM+ > BMPYR+ > BMPY+ > BMPIP+ in the case of coions resulting in weaker electrical double layer repulsion in these sequences. Since van der Waals forces are always present and their strength does not depend significantly on the ionic strength, the CCC values decreased in the above order. The present results shed light on the importance of the interfacial arrangement of the IL constituent ions on the colloidal stability of particle dispersions and provide important information on the design of stable or unstable particle-ionic liquid systems.

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.

Reduction of intracellular oxidative stress with a copper-incorporated layered double hydroxide.

Biocompatible Cu(II)-doped layered double hydroxide (CMA) nanoparticles were developed to combat reactive oxygen species. The 2-dimensional nanozymes showed both superoxide dismutase- and catalase-like activities in chemical assays, while proving as efficient antioxidants in the reduction of intracellular oxidative stress. The results indicate the great promise of CMA in antioxidant therapies.

Molecular Antioxidants Maintain Synergistic Radical Scavenging Activity upon Co-Immobilization on Clay Nanoplatelets.

Unbalanced levels of reactive oxygen species (ROS) result in oxidative stress, affecting both biomedical and industrial processes. Antioxidants can prevent ROS overproduction and thus delay or inhibit their harmful effects. Herein, activities of two molecular antioxidants (gallic acid (GA), a well-known phenolic compound, and nicotinamide adenine dinucleotide (NADH), a vital biological cofactor) were tested individually and in combination to assess possible synergistic, additive, or antagonistic effects in free radical scavenging and in redox capacity assays. GA was a remarkable radical scavenger, and NADH exhibited moderate antioxidant activity, while their combination at different molar ratios led to a synergistic effect since the resulting activity was superior to the sum of the individual GA and NADH activities. Their coimmobilization was performed on the surface of delaminated layered double hydroxide clay nanoplatelets by electrostatic interactions, and the synergistic effect was maintained upon such a heterogenization of these molecular antioxidants. The coimmobilization of GA and NADH expands the range of their potential applications, in which separation of antioxidant additives is important during treatments or manufacturing processes.

Confinement of Triple-Enzyme-Involved Antioxidant Cascade in Two-Dimensional Nanostructure.

Application of antioxidant enzymes in medical or industrial processes is limited due to their high sensitivity to environmental conditions. Incorporation of such enzymes in nanostructures provides a promising route to obtain highly efficient and robust biocatalytic system to scavenge reactive oxygen species (ROS). Here, this question was addressed by confinement of superoxide dismutase (SOD), horseradish peroxidase (HRP), and catalase (CAT) enzymes into nanostructures containing polyelectrolyte building blocks (alginate (Alg) and trimethyl chitosan (TMC)) and delaminated layered double hydroxide (dLDH) nanoparticle support. The nanocomposite possessed excellent structural and colloidal stability, while antioxidant tests revealed that the enzymes remained active upon immobilization and the developed composite greatly reduced intracellular oxidative stress in two-dimensional cell cultures. Moreover, it effectively prevented hydrogen peroxide-induced double stranded DNA breaks, which is a common consequence of oxidative stress. The results provide important tools to design complex nanostructures with multienzymatic antioxidant activities for ROS scavenging.

Molecular orientation rules the efficiency of immobilized antioxidants.

Tannic acid (TA) and glutathione (GSH) are important molecular antioxidants against reactive oxygen species. Their efficiency is limited by low solubility and high sensitivity, which may be solved by confinement in composite materials. Here, effect of immobilization of these antioxidants on their radical scavenging activity was investigated using layered double hydroxide (LDH) nanoparticles as hosts. Different preparation methods were applied to build composite systems leading to variations in the molecular orientation of both TA and GSH on the surface or among the layers of LDHs. Systematic combination of spectroscopy (FT-IR, Raman, UV-VIS-NIR-DRS), diffraction (XRD) and microscopy (SEM) methods revealed perpendicular or parallel orientation of TA on the surface of LDH depending on the preparation approach applied. Immobilization of GSH protected the antioxidant molecules from degradation. Radical scavenging tests evidenced that the activity of the antioxidants strongly depends on the molecular orientation. The LDH supported GSH and TA proved as durable and reusable antioxidant agents to be applied as radical scavengers in medical therapies or in industrial processes.

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.

The Influence of Solvents and Colloidal Particles on the Efficiency of Molecular Antioxidants.

The radical scavenging activity of three molecular antioxidants (trolox, rutin and ellagic acid) was investigated in different solvents with and without added polymer-based colloidal particles (SL-IP-2). Rutin and ellagic acid showed poor solubility in water, preventing the accurate measurement of the effective antioxidant concentration values, which were determined in ethanol/water (EtOH/H2O) mixtures. The presence of trolox and rutin changed neither the surface charge properties nor the size of SL-IP-2 in these solvents, while significant adsorption on SL-IP-2 was observed for ellagic acid leading to overcharging and rapid particle aggregation at appropriately high antioxidant concentrations in EtOH/H2O. The differences in the radical scavenging capacity of trolox and ellagic acid that was observed in homogeneous solutions using water or EtOH/H2O as solvents vanished in the presence of the particles. Rutin lost its activity after addition of SL-IP-2 due to the larger molecular size and lower exposure of the functional groups to the substrate upon interaction with the particles. The obtained results shed light on the importance of the type of solvent and particle-antioxidant interfacial effects on the radical decomposition ability of molecular antioxidants, which is of crucial importance in industrial processes involving heterogeneous systems.

Antioxidant colloids via heteroaggregation of cerium oxide nanoparticles and latex beads.

Antioxidant colloids were developed via controlled heteroaggregation of cerium oxide nanoparticles (CeO2 NPs) and sulfate-functionalized polystyrene latex (SL) beads. Positively charged CeO2 NPs were directly immobilized onto SL particles of opposite surface charge via electrostatic attraction (SL/Ce composite), while negatively charged CeO2 NPs were initially functionalized with poly(diallyldimethylammonium chloride) (PDADMAC) polyelectrolyte and then, aggregated with the SL particles (SPCe composite). The PDADMAC served to induce a charge reversal on CeO2 NPs, while the SL support prevented nanoparticle aggregation under conditions, where the dispersions of bare CeO2 NPs were unstable. Both SL/Ce and SPCe showed enhanced radical scavenging activity compared to bare CeO2 NPs and were found to mimic peroxidase enzymes. The results demonstrate that SL beads are suitable supports to formulate CeO2 particles and to achieve remarkable dispersion storage stability. The PDADMAC functionalization and immobilization of CeO2 NPs neither compromised the peroxidase-like activity nor the radical scavenging potential. The obtained SL/Ce and SPCe artificial enzymes are foreseen to be excellent antioxidant agents in various applications in the biomedical, food, and cosmetic industries.

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.

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.

Papain Adsorption on Latex Particles: Charging, Aggregation, and Enzymatic Activity.

The effect of papain adsorption on the surface charge properties and aggregation mechanism of sulfate-functionalized polystyrene latex particles was studied. The positively charged enzyme possessed a high affinity to the oppositely charged particles, giving rise to charge neutralization and charge reversal at appropriate papain concentrations. The tendency in the particle aggregation rates at different enzyme doses revealed that the colloidal stability of the samples is governed by interparticle forces of electrostatic origin. The aggregation mechanism was qualitatively described within the classical DLVO theory, and unstable dispersions were detected near the charge neutralization point, while particle aggregation was not observed at low and elevated papain concentrations. The relatively high dispersion stability of the bare latex particles was maintained upon the formation of an enzyme layer on the surface, and the obtained latex-papain composite showed notable resistance against salt-induced aggregation. Remarkable hydrolytic and antioxidant activities of the immobilized enzyme were observed in probe reactions; therefore, the obtained hybrid can be considered as a multifunctional biocatalytic system with great promise in applications in industrial manufacturing processes.