Large-scale aqueous synthesis of layered double hydroxide single-layer nanosheets

Interactions between Layered Double Hydroxide Nanoparticles and Egg Yolk Lecithin Liposome Membranes

The burgeoning need to study the applications of nanoparticles (NPs) in biomedical and pharmaceutical fields requires an understanding of their interactions with lipid membranes for further in vivo studies. In this paper, negatively charged egg yolk lecithin liposome (EYL) has been prepared and used as model lipid membranes. Positively charged Mg₃Al-layered double hydroxides (LDHs) are viewed as models of clay particles. The ability of the LDH NPs, a two-dimensional nanostructure with an average diameter of 100 nm (LDHs-100) or 500 nm (LDHs-500) to cross the membranes, has been thoroughly investigated via (high-resolution) transmission electron microscopy (TEM), optical microscopy (OM), scanning electron microscopy (SEM), confocal fluorescence microscopy (CLSM), and dynamic light scattering (DLS). The liposomes with an average diameter of 1.5 μm were prepared by the thin-film rehydration method followed by an extrusion technique. A calcein leakage assay and steady-state fluorescence measurement displayed the variation of membrane integrity and polarity of the pyrene-located microenvironment during the interaction between EYL and calcein-interacted LDH NPs (CE-LDHs) or LDH NPs, respectively. These results imply that not only spherical particles but also even more sophisticated nanostructured materials are able to effectively cross the lipid bilayers, thereby engineering new compounds that may be encapsulated for safe and potential use in biomedical applications.

Layered double hydroxide-based nanomaterials for biomedical applications

Against the backdrop of increased public health awareness, inorganic nanomaterials have been widely explored as promising nanoagents for various kinds of biomedical applications. Layered double hydroxides (LDHs), with versatile physicochemical advantages including excellent biocompatibility, pH-sensitive biodegradability, highly tunable chemical composition and structure, and ease of composite formation with other materials, have shown great promise in biomedical applications. In this review, we comprehensively summarize the recent advances in LDH-based nanomaterials for biomedical applications. Firstly, the material categories and advantages of LDH-based nanomaterials are discussed. The preparation and surface modification of LDH-based nanomaterials, including pristine LDHs, LDH-based nanocomposites and LDH-derived nanomaterials, are then described. Thereafter, we systematically describe the great potential of LDHs in biomedical applications including drug/gene delivery, bioimaging diagnosis, cancer therapy, biosensing, tissue engineering, and anti-bacteria. Finally, on the basis of the current state of the art, we conclude with insights on the remaining challenges and future prospects in this rapidly emerging field.

Synthesis of two-Dimensional layered double hydroxide: A systematic overview

Two-dimensional (2D) layered double hydroxides (LDH) are classic materials in fundamental research and practical application. 2D LDH have unique structural features, such as high aspect ratio, high specific surface area,...

Electrodeposition of Defect-Rich Ternary NiCoFe Layered Double Hydroxides: Fine Modulation of Co3+ for Highly Efficient Oxygen Evolution Reaction

The low-cost, high-abundance and durable layered double hydroxides (LDHs) have been considered as promising electrocatalysts for oxygen evolution reaction (OER). However, the easy agglomeration of lamellar LDHs in the aqueous phase limits their practical applications. Herein, a series of ternary NiCoFe LDHs were successfully fabricated on nickel foam (NF) via a simple electrodeposition method. The as-prepared Ni(Co_0.5 Fe_0.5 )/NF displayed an unique nanoarray structural feature. It showed an OER overpotential of 209 mV at a current density of 10 mA cm^-2 in alkaline solution, which was superior to most systems reported so far. As evidenced by the XPS and XAFS results, such excellent performance of Ni(Co_0.5 Fe_0.5 )/NF was attributed to the higher Co³⁺ /Co²⁺ ratio and more defects exposed, comparing with Ni(Co_0.5 Fe_0.5 )-bulk and Ni(Co_0.5 Fe_0.5 )-mono LDHs prepared by conventional coprecipitation method. Furthermore, the ratio of Co to Fe could significantly tune the Co electronic structure of Ni(Co_x Fe_1-x )/NF composites (x=0.25, 0.50 and 0.75) and affect the electrocatalytic activity for OER, in which Ni(Co_0.5 Fe_0.5 )/NF showed the lowest energy barrier for OER rate-determining step (from O* to OOH*). This work proposes a facile method to develop high-efficiency OER electrocatalysts.

Preparation and application of layered double hydroxide nanosheets

Layered double hydroxides (LDH) with unique structure and excellent properties have been widely studied in recent years. LDH have found widespread applications in catalysts, polymer/LDH nanocomposites, anion exchange materials, supercapacitors, and fire retardants. The exfoliated LDH ultrathin nanosheets with a thickness of a few atomic layers enable a series of new opportunities in both fundamental research and applications. In this review, we mainly summarize the LDH exfoliation methods developed in recent years, the recent developments for the direct synthesis of LDH single-layer nanosheets, and the applications of LDH nanosheets in catalyzing oxygen evolution reactions, crosslinkers, supercapacitors and delivery carriers.

Layered double hydroxides (LDHs) with unique structure and excellent properties have been widely studied in recent years.

Solvothermal synthesis of carbonate-type layered double hydroxide monolayer nanosheets: Solvent selection based on characteristic parameter matching criterion

Monolayer nanosheets of CO₃^2--type layered double hydroxides (LDHs) have many special applications, but their fabrication is challenging. Herein, Co₂Al-CO₃ and Co₂Fe-CO₃ LDH nanosheets were synthesized via a solvothermal method. 31 solvents with different characteristic parameters, including the surface free energy (γ) and solubility (δ) parameters were chosen, to explore the correlation between the formation of monolayer LDHs (ML-LDHs) and the characteristic parameters of solvents. The results reveal that when the solvents used have the characteristic parameters matching to those of the LDHs, CO₃^2--type ML-LDHs with a thickness of ca. 1 nm can be obtained. The mixed-solvent strategy can provide the effective solvents for the synthesis of ML-LDHs. The dispersions of CO₃^2--type ML-LDHs can be stable for at least six months without obvious precipitation. In addition, it is demonstrated that the δ parameters of LDHs can be calculated from the γ parameters via the molar volume-free γ-δ equations developed previously. Furthermore, a new parameter called "surface free energy distance" is introduced, which can be used for screening effective solvents for the synthesis of ML-LDHs. To the best of our knowledge, this is the first time to investigate the applicable of the characteristic parameter matching principle for the bottom-up synthesis of ML-LDHs. This work deepens the understanding on the feature of CO₃^2--type LDHs and provides a solvent selection strategy for the synthesis of CO₃^2--type ML-LDHs.

Effect of LDHs and Other Clays on Polymer Composite in Adsorptive Removal of Contaminants: A Review

Recently, the development of a unique class of layered silicate nanomaterials has attracted considerable interest for treatment of wastewater. Clean water is an essential commodity for healthier life, agriculture and a safe environment at large. Layered double hydroxides (LDHs) and other clay hybrids are emerging as potential nanostructured adsorbents for water purification. These LDH hybrids are referred to as hydrotalcite-based materials or anionic clays and promising multifunctional two-dimensional (2D) nanomaterials. They are used in many applications including photocatalysis, energy storage, nanocomposites, adsorption, diffusion and water purification. The adsorption and diffusion capacities of various toxic contaminants heavy metal ions and dyes on different unmodified and modified LDH-samples are discussed comparatively with other types of nanoclays acting as adsorbents. This review focuses on the preparation methods, comparison of adsorption and diffusion capacities of LDH-hybrids and other nanoclay materials for the treatment of various contaminants such as heavy metal ions and dyes.

Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO₃ layered double hydroxide (LDH) single-layer nanosheets dispersed in water were investigated in the presence of K⁺ salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σ_eff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. On the basis of the Stern model and the DLVO theory, the relationship of CCC with σ_eff and the ionic valences of salts (z_i) is theoretically analyzed, which can accurately describe the dependence of CCC on the σ_eff and z_i but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (E_sc) of anions within the Stern layer is developed. Especially, an empirical relationship of E_sc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO₃^2- and SO₄^2-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability but also provides useful information for the potential applications of LDH single-layer nanosheets.

Tetracycline Generated Red Luminescence Based on a Novel Lanthanide Functionalized Layered Double Hydroxide Nanoplatform

Considerable interest in using lanthanide complexes in optics have been well-known persisted for a long time. But such molecular-based edifices have been excluded from practical application because of their poor thermal or photo stabilities. Here a novel europium embedded layered double hydroxide (Mg-Al LDH-Eu) has been established and such an inorganic-organic framework demonstrates improved thermal performance due to hydrolysis and poly condensation of the trimethoxysilyl-unit. In addition, the incorporation of a functional building block such as ethylenediamine triacetic acid can significantly minimize the negative effects of hydroxyl groups. In the presence of tetracycline (Tc), the nanoprobe exhibits an "off-on" change in aqueous solution, and the red luminescence can be excited in the visible light range (405 nm). It provides a very sensitive signal response to Tc with an excellent linear relation in the range of 0.1 μM to 5.0 μM, and the detection limit of this probe is measured to be 7.6 nM. This nanoplatform exhibits low cytotoxicity during in vitro experiments and can be employed for the detection of tetracycline in 293T cells.

Hydrodynamic assembly of two-dimensional layered double hydroxide nanostructures

Formation mechanisms of two-dimensional nanostructures in wet syntheses are poorly understood. Even more enigmatic is the influence of hydrodynamic forces. Here we use liquid flow cell transmission electron microscopy to show that layered double hydroxide, as a model material, may form via the oriented attachment of hexagonal nanoparticles; under hydrodynamic shear, oriented attachment is accelerated. To hydrodynamically manipulate the kinetics of particle growth and oriented attachment, we develop a microreactor with high and tunable shear rates, enabling control over particle size, crystallinity and aspect ratio. This work offers new insights in the formation of two-dimensional materials, provides a scalable yet precise synthesis method, and proposes new avenues for the rational engineering and scalable production of highly anisotropic nanostructures.

While liquid-phase synthesis of 2D materials presents opportunities for large-scale production, achieving precise control over product quality, size and morphology remains challenging. Here, the authors show that hydrodynamic manipulation of nanoparticle assembly enables control over crystallinity, size and aspect ratio.

Synthesis of layered double hydroxide/poly(N-isopropylacrylamide) nanocomposite hydrogels with excellent mechanical and thermoresponsive performances

Nanocomposite (NC) hydrogels of positively charged layered double hydroxide (LDH) single-layer nanosheet (SLNS) cross-linked poly(N-isopropylacrylamide) (PNIPAM) were synthesized. Especially, the LDH SLNSs used here were pre-synthesized via an aqueous synthetic route without using organic solvents and modifiers. The obtained LDH/PNIPAM NC hydrogels were characterized using XRD, SEM, TEM, and DSC. The mechanical and thermoresponsive properties were determined using tensile, compression, and swelling/deswelling tests. Interestingly, different network structures are observed for the NC hydrogels along the horizontal and vertical directions; those along the horizontal direction exhibit a fine and uniform sponge-like network structure while those along the vertical direction exhibit a hierarchical layered architecture. Compared with the conventional N,N'-methylene bisacrylamide cross-linked PNIPAM hydrogel, the NC hydrogels exhibit extraordinary deformability and stretchability and obviously improved thermoresponsive swelling/deswelling characteristics. Furthermore, the fracture elongation observed here is obviously higher than those reported for negatively charged clay/PNIPAM NC hydrogels. With the increase in the LDH content from 0.8 to 2.0 wt%, the fracture strength and the compressive strength at an 85% strain increase from 23.5 to 37.2 kPa and from 0.15 to 0.57 MPa, respectively, while the fracture elongation decreases from 2689 to 2202%. The mechanism for the improved mechanical performances of the NC hydrogels is discussed. To the best of our knowledge, this is the first report on LDH/PNIPAM hydrogels. This work provides a green synthesis route for LDH-containing NC hydrogels. The new NC hydrogels may have great potential applications such as in tissue engineering, drug vehicles, and sorbents.

Synthesis of Water-Dispersible Single-Layer CoAl-Carbonate Layered Double Hydroxide

Despite extensive study on single-layer layered double hydroxides (SL-LDHs) with NO₃^- counterions, SL-LDHs with CO₃^2- counterions (CO₃^2- SL-LDHs) have never been prepared before. Herein, a CoAl-CO₃^2- SL-LDH which stays stable in water and powdery state is first synthesized using ethylene glycol as a reaction medium. The SL-LDH, with thickness of ∼0.85 nm, is composed of one Co(Al)O₆ layer sandwiched between two CO₃^2- layers. The SL-LDH powder shows high specific surface area (∼289 m²/g) and excellent electrocatalytic oxygen evolution efficiency. This work provides the first simple way to prepare CO₃^2- SL-LDHs and will open an avenue for synthesizing other SL-LDHs.

One-step synthesis of hierarchical Ni–Fe–Al layered double hydroxide with excellent sensing properties for NOx at room temperature

Hierarchical flower-like Ni–Al-layered and Ni–Fe–Al-layered double hydroxide intercalation compounds were synthesized and showed good sensing for NO_x at room temperature.