Layered Double Hydroxide Nanosheets: Synthesis Strategies and Applications in the Field of Energy Conversion

In recent years, layered double hydroxides (LDH) nanosheets have garnered substantial attention as intriguing inorganic anionic layered clay materials. These nanosheets have captured the attention of researchers due to their unique physicochemical properties. This review aims to showcase the latest advancements in laboratory research concerning LDH nanosheets, with a specific emphasis on their methods of preparation. This review provides detailed insights into the factors influencing the anionic conductivity of LDH, along with delineating the applications of LDH nanosheets in the realm of energy conversion. Notably, the review highlights the crucial role of LDH nanosheets in the oxygen evolution reaction (OER), a vital process in water splitting and diverse electrochemical applications. The review emphasizes the significant potential of LDH nanosheets in enhancing supercapacitor technology, owing to their high surface area and exceptional charge storage capacity. Additionally, it elucidates the prospective application of LDH nanosheets as anion exchange membranes in anion exchange membrane fuel cells, potentially revolutionizing fuel cell performance through improved efficiency and stability facilitated by enhanced ion transport properties.

Hydrangea-like AuPtRu/ZnO-rGO Nanocomposites with Enhanced Peroxidase Mimiking Activity for Senitive Colorimetric Determination of H2O2

In this study, a facile and cost-effective hydrothermal synthesis method was used to synthesize zinc oxide nanoflowers modified by reduced graphene oxide, and subsequently, trimetallic AuPtRu nanoparticles(AuPtRuNPs) were supported via the reduction method for high-sensitivity colorimetric detection of H2O2 in weakly acidic solutions. Compared to monometallic and bimetallic nanoparticles, trimetallic nanoparticles exhibit significant synergistic effects and enhanced catalytic activity. After providing a three-dimensional structure with multiple pores by zinc oxide and enhancing electron transfer ability by reduced graphene, the trimetallic nanocomposites (AuPtRu/ZnO-rGO) exhibited excellent peroxidase-mimicking activity, which can effectively catalyze 3,3',5,5'-tetramethylbenzidine (TMB) to produce a blue oxidation product (oxTMB) in the presence of H2O2. Compared to horseradish peroxidase (HRP), AuPtRu/ZnO-rGO demonstrated significantly enhanced catalytic velocity (Vmax = 6.16 × 10-8 M/s) and affinity (Km = 0.02) for H2O2. The study of the catalytic mechanism showed that trimetallic Au, Pt, and Ru could effectively catalyze H2O2 to produce hydroxyl radicals (•OH) to accelerate the oxidation of TMB and enhance the peroxidase-mimicking activity of the AuPtRu/ZnO-rGO nanocomposites. The results showed that the as-synthesized hydrangea-like AuPtRu/ZnO-rGO nanocomposites showed enhanced peroxidase-mimicking activity. It could be used for the colorimetric detection of H2O2 in the range 5-1000 μM with a LOD of 3.0 μM (S/N = 3), and the recoveries are 93.0-101.7%. In addition, the AuPtRu/ZnO-rGO nanocomposites have good applicability for sensitive colorimetric determination of H2O2 in milk, and it has broad application prospects as a multifunctional sensing platform in the food processing industry.

Carbon-coated copper nanocrystals with enhanced peroxidase-like activity for sensitive colorimetric determination of 2,4-dinitrophenylhydrazine

Carbon-coated copper nanocrystals (CuNCs) with peroxidase-like activity were hydrothermally prepared by using copper acetate, citric acid (CA) and histidine (His) as the precursors. Various shaped CuNCs, including urchin-like, slab-like and spherical appearance were facilely prepared by addition of different amount of NaNO2 in the precursor solutions. When 3,3',5,5'-tetramethylbenzidine (TMB) was used as the substrate, the CuNCs with urchin-like appearance have greatest peroxidase-like activity and their Michaelis-Menten constant (Km) and the maximum rate constant (νmax) are respectively 8.8 and 1.2 times higher than that obtained from horseradish peroxidase (HRP). The production of reactive oxygen species (ROS) was confirmed by radical quenching and electron spin resonance (ESR) tests. Subsequent studies have found that the CuNCs catalyzed color reaction of TMB can be selectively quenched by the environmental pollutant 2,4-dinitrophenylhydrazine (2,4-DNPH). Thus a new colorimetric method for the determination of 2,4-DNPH with a linear range of 0.60-20 µM was developed and a limit of detection (LOD) as low as 0.166 µM was achieved. The results obtained not only reveal the tunability of the peroxidase-like activity of Cu-based nanomaterials, but also provide a new method for the sensitive determination of environmental contaminate.

Controlled molecular arrangement of easily aggregated deoxycholate with layered double hydroxide

Deoxycholate (DA) is a natural emulsifying agent involved in the absorption of dietary lipids. Due to the facial distribution of hydrophobic-hydrophilic region, DA easily aggregates under ambient conditions, and this property hinders the practical application of DA in clinical applications. In this study, we found that the molecular arrangement of DA molecules could be controlled by using layered double hydroxide (LDH) under a specific reaction condition. The effect of reaction methods such as co-precipitation, ion exchange and reconstruction on the molecular arrangement of DA was investigated by X-ray diffraction, Fourier-transform infrared spectroscopy, high-resolution transmission electron microscopy and differential scanning calorimetry. It was demonstrated that the self-aggregation of DA molecules could be suppressed by the oriented arrangement of DA between the gallery space of LDH. The DA moiety was well stabilized in the LDH layers due to the electrostatic interaction between DA molecules and LDH layers. The most ordered arrangement of DA molecules was observed when DA was incorporated into LDH via a reconstruction method. The DA molecules arranged in LDH via reconstruction did not show significant exothermic or endothermic behaviour up to 400°C, showing that the DA moiety lost its intermolecular attraction in between LDH layers.

A visualized sensor based on layered double hydroxides with peroxidase-like activity for sensitive acetylcholinesterase assay

Acetylcholinesterase (AChE) plays a crucial role in biological neurotransmission. The aberrant expression of AChE is associated with various neurodegenerative diseases. Therefore, it is of great significance to develop a simple and highly sensitive AChE analysis platform. Herein, a simple colorimetric sensor was constructed for sensitive detection of AChE based on the peroxidase-like catalytic activity of Ni/Co layered double hydroxides (Ni/Co LDHs). In this sensor, the fabricated Ni/Co LDHs possess high peroxidase-like activity, enabling rapid catalysis of o-phenylenediamine (OPD) to produce yellow oxOPD in the presence of H₂O₂. This peroxidase-like activity of Ni/Co LDHs was found to be effectively inhibited by the presence of AChE. It is speculated that the combination of AChE on the outer surface of Ni/Co LDHs through non-covalent interaction may cover the active sites and hinder their adsorption to the substrates, leading to the failure of OPD oxidation. As a result, the yellow color from oxOPD is related to the AChE concentration, enabling the direct AChE assay in an equipment-free manner. In addition, the fabricated Ni/Co LDHs could be modified on a paper surface to obtain a paper-based analytical device for visualized colorimetric detection of AChE. The as-proposed sensor shows high sensitivity to AChE with a detection limit down to 6.6 μU mL^-1. Therefore, this naked-eye paper-based sensor is capable of on-site and real-time detection of AChE, and has outstanding application prospects in clinical diagnosis and biomedical fields.

Platinum Nanoparticles Loaded Graphitic Carbon Nitride Nanosheets with Enhanced Peroxidase-like Activity for H2O2 and Oxidase-Based Sensing

Platinum nanoparticles (PtNPs) are classical peroxidase-like nanozyme; self-agglomeration of nanoparticles leads to the undesirable reduction in stability and catalytic activity. Herein, a hybrid peroxidase-like nanocatalyst consisting of PtNPs in situ growing on g-C₃N₄ nanosheets with enhanced peroxidase-mimic catalytic activity (PtNP@g-C₃N₄ nanosheets) was prepared for H₂O₂ and oxidase-based colorimetric assay. g-C₃N₄ nanosheets can be used as carriers to solve the problem of poor stability of PtNPs. We observed that the catalytic ability could be maintained for more than 90 days. PtNP@g-C₃N₄ nanosheets could quickly catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), and the absorbance of blue color oxidized TMB (oxTMB) showed a robust linear relationship with the concentration of H₂O₂ (the detection limit (LOD): 3.33 μM). By utilizing H₂O₂ as a mediator, this strategy can be applied to oxidase-based biomolecules (glucose, organophosphorus, and so on, that generate or consume hydrogen peroxide) sensing. As a proof of concept, a sensitive assay of cholesterol that combined PtNP@g-C₃N₄ nanosheets with cholesterol oxidase (ChOx) cascade catalytic reaction was constructed with an LOD of 9.35 μM in a widespread range from 10 to 800 μM (R² = 0.9981). In addition, we also verified its ability to detect cholesterol in fetal bovine serum. These results showed application prospect of PtNP@g-C₃N₄ nanosheets-based colorimetry in sensing and clinical medical detection.

Switchable Nanozyme Activity of Porphyrins Intercalated in Layered Gadolinium Hydroxide

In this study, organo-inorganic nanohybrids LHGd-MTSPP with enzyme-like activity were prepared by in situ intercalation of anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin and its complexes with Zn(II) and Pd(II) (MTSPP, M = 2H, Zn(II) and Pd(II)) into gadolinium layered hydroxide (LHGd). The combination of powder XRD, CHNS analysis, FT-IR, EDX, and TG confirmed the layered structure of the reaction products. The basal interplanar distances in LHGd-MTSPP samples were 22.3-22.6 Å, corresponding to the size of an intercalated tetrapyrrole molecule. According to SEM data, LHGd-MTSPP hybrids consisted of individual lamellar nanoparticles 20-50 nm in thickness. The enzyme-like activity of individual constituents, LHGd-Cl and sulfoporphyrins TSPP, ZnTSPP and PdTSPP, and hybrid LHGd-MTSPP materials, was studied by chemiluminescence analysis using the ABAP/luminol system in phosphate buffer solution. All the individual porphyrins exhibited dose-dependent antioxidant properties with respect to alkylperoxyl radicals at pH 7.4. The intercalation of free base TSPP porphyrin into the LHGd preserved the radical scavenging properties of the product. Conversely, in LHGd-MTSPP samples containing Zn(II) and Pd(II) complexes, the antioxidant properties of the porphyrins changed to dose-dependent prooxidant activity. Thus, an efficient approach to the design and synthesis of advanced LHGd-MTSPP materials with switchable enzyme-like activity was developed.

Advances in layered double hydroxide based labels for signal amplification in ultrasensitive electrochemical and optical affinity biosensors of glucose

Since the development of enzyme electrodes, the research area of glucose biosensing has seen outstanding progress and improvement. Numerous sensing platforms have been developed based on different immobilization techniques and improved electron transfer between the enzyme and electrode. Interestingly, these platforms have consistently used innovative nanostructures and nanocomposites. In recent years, layered double hydroxides (LDHs) have become key tools in the field of analytical chemistry owing to their outstanding features and benefits, such as facile synthesis, cost-effectiveness, substantial surface area, excellent catalytic performance, and biocompatibility. LDHs are often synthesized as nanomaterial composites or manufactured with specific three-dimensional structures. The purpose of this review is to illustrate the biosensing prospects of LDH-based glucose sensors and the need for improvement. First, various clinical and conventional approaches for glucose determination are discussed. The definitions, types, and various synthetic methodologies of LDHs are then explained. Subsequently, we discuss the various research studies regarding LDH-based electrochemical and optical assays, focusing on modified systems, improved electron transfers pathways (through developments in surface science), and different sensing designs based on nanomaterials. Finally, a summary of the current limitations and future challenges in glucose analysis is described, which may facilitate further development and applications.

Comparative Study of M(Ⅱ)Al (M=Co, Ni) Layered Double Hydroxides for Silicone Foam: Characterization, Flame Retardancy, and Smoke Suppression

To compare the different actions of the two representative transition metal cations of Co²⁺ and Ni²⁺ in layered double hydroxides (LDHs), CoAl-LDH and NiAl-LDH intercalated with CO₃²⁻ were synthesized, and the chemical structures, microstructures, and surface areas thereof were successfully characterized. Then, the two LDHs were utilized as flame retardants and smoke suppressants for silicone foam (SiF). The densities, flame retardancy, smoke suppression, thermal stabilities, and compressive strengths of the two SiF/LDHs nanocomposites were investigated. The introduction of LDHs slightly decreased the density of SiF due to the catalytic actions of Co and Ni during the foaming process of SiF. With respect to the flame retardancy, the addition of only 1 phr of either CoAl-LDH or NiAl-LDH could effectively improve the limiting oxygen index of SiF from 28.7 to 29.6%. Based on the results of vertical flame testing and a cone calorimeter test, the flame retardancy and fire safety of the SiF were effectively enhanced by the incorporation of LDHs. In addition, owing to the good catalytic action and large specific surface area (NiAl-LDH: 174.57 m² g⁻¹; CoAl-LDH: 51.47 m² g⁻¹), NiAl-LDH revealed higher efficiencies of flame retardancy and smoke suppression than those of CoAl-LDH. According to the results of energy-dispersive X-ray spectroscopy, Co and Ni participated in the formation of protective char layers, which inhibited the release of SiO₂ into the gas phase. Finally, the influences on the thermal decomposition and compressive strength for SiF resulting from the addition of LDHs are discussed.

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.

An Advanced Approach for MgZnAl-LDH Catalysts Synthesis Used in Claisen-Schmidt Condensation

Using organic-base tetramethylammonium hydroxide (TMAH) is a viable, cheap, and fast option for the synthesis of MgZnAl-LDH-type materials by both co-precipitation and mechano-chemical methods. TMAH presents several advantages, such as the smaller quantity of water required in the washing step compared to the use of inorganic alkalis, the prevention of LDH contamination with alkali cations, and its action as a template molecule in texture tailoring. It also has disadvantages, such as its presence in small quantities in the resulting layered materials. Regardless of the use of organic/inorganic bases and co-precipitation/mechano-chemical methods, zincite stable phase was found in all the synthesized solids. The basicity of catalysts followed the trend: mixed oxides > reconstructed > parent LDH. The memory effect of LDH was supported only by the presence of Mg and Al cations, while Zn remained as a zincite stable phase. The catalytic activities for Claisen–Schmidt condensation of benzaldehyde with cyclohexanone provided values higher than 90% after 2 h, with a total selectivity toward 2,6-dibenzylidenecyclohexanone, while self-condensation of cyclohexanone yielded no more than 7.29% after 5 h. These behaviors depended on catalyst basicity as well as on the planar rigidity of the compound.

Efficient and Fast Removal of Aqueous Tungstate by an Iron-Based LDH Delaminated in L-Asparagine

High concentrations of tungstate in aqueous systems pose a severe threat to the environment and human health. This study explored the potential of iron-based LDHs to remove tungstate from water. To improve its tungstate uptake capacity, environment-friendly L-asparagine was used to delaminate iron-based LDH synthesized via a coprecipitation method. The successful delamination was proved by AFM, revealing that the thickness of the obtained nanoparticles was approximately 1–2 times that of a single LDH layer. XRD, TEM, and XPS analyses confirmed that the delaminated LDHs were amorphous and ultrathin and had surface defects within their nanosheets that acted as active sites, leading to a very fast tungstate sorption rate and superior tungstate uptake capacity. Notably, the original layered structure of the L-asparagine-treated LDH was recovered upon its reaction with tungstate-bearing solutions, and therefore, the high availability of aqueous tungstate to the interlayer regions during the structural restoration of the delaminated iron-based LDH contributed to its excellent capability of tungstate removal as well. In addition, the tungstate uptake by the delaminated iron-based LDH was not affected substantially by the presence of coexisting anions, implying that the strong inner-sphere complexation between the tungstate and LDH layers with defects (i.e., Fe-O bonds) was the primary mechanism responsible for the tungstate removal. The delamination process described in this paper was validated to be an effective way to enhance the immobilization of tungstate by iron-based LDHs without inducing secondary pollutions, and delaminated iron-based LDHs are promising to be used extensively in the practice of treating tungstate-rich waters.

2D material-based peroxidase-mimicking nanozymes: catalytic mechanisms and bioapplications

The boom in nanotechnology brings new insights into the development of artificial enzymes (nanozymes) with ease of modification, lower manufacturing cost, and higher catalytic stability than natural enzymes. Among various nanomaterials, two-dimensional (2D) nanomaterials exhibit promising enzyme-like properties for a plethora of bioapplications owing to their unique physicochemical characteristics of tuneable composition, ultrathin thickness, and huge specific surface area. Herein, we review the recent advances in several 2D material-based nanozymes, such as carbonaceous nanosheets, metal-organic frameworks (MOFs), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and transition metal oxides (TMOs), clarify the mechanisms of peroxidase (POD)-mimicking catalytic behaviors, and overview the potential bioapplications of 2D nanozymes.

Facile Synthesis of Amorphous MoCo Lamellar Hydroxide for Alkaline Water Oxidation

To find an oxygen evolution reaction (OER) catalyst with satisfactory catalytic performance and affordable cost is of great importance to the development of many new energy devices. In this work, a simple and effective strategy was developed to synthesize a series of amorphous MoCo lamellar hydroxide through one-step chemical co-precipitation. Systematic investigations showed that different functional agents (2-methylimidazole, NaOH, NH₄ OH) in the fabrication process resulted in different micromorphology of the catalyst, thus influencing its electrocatalytic performance. Also, adding various amounts of Mo could influence the intrinsic catalytic properties. Samples synthesized with appropriate functional agent addition and optimized Mo addition exhibited amorphous nature and bent nanosheet morphology, as well as highest intrinsic catalytic activity, showing a low overpotential of 290 mV at 10 mA cm^-2 and a small Tafel slope of 55 mV dec^-1 in 1 m KOH solution. Additionally, the catalytic performance of the sample showed just small decay after 50 h chronopotentiometry test and 3000 cyclic voltammetry cycles, exhibiting the ultra-stable catalytic activity of the catalyst. This work provides a possible large-scale commercial production strategy of OER catalysts with promising performance and low fabrication cost.

A colorimetric detection strategy and micromotor-assisted photo-Fenton like degradation for hydroquinone based on the peroxidase-like activity of Co3O4–CeO2 nanocages

Co3O4–CeO2 micromotors were fabricated and the colorimetric detection and micromotor-assisted photodegradation capability were studied.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

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.

A review on optical sensors based on layered double hydroxides nanoplatforms

In recent years, significant efforts have been devoted towards the fabrication and application of layered double hydroxides (LDHs) due to their tremendous features such as excellent biocompatibility with negligible toxicity, large surface area, high conductivity, excellent solubility, and ion exchange properties. Most impressive, LDHs offer a favorable environment to attach several substances such as quantum dots, fluorescein dyes, proteins, and enzymes, which leads to strengthening the catalytic properties or increasing the sensing selectivity and sensitivity of the resulted hybrids. With the extensive ongoing research on the application of nanomaterials, many studies have led to remarkable achievements in exploring LDHs as sensing nanoplatforms. In optical sensors, for instance, many sensing strategies were tailored based on the enzyme-mimicking properties of LDHs, including colorimetric and chemiluminescence procedures. Meanwhile, others were designed based on intercalating some fluorogenic substrates on the LDHs, whereby the sensing signal can be acquired by quenching or enhancing their fluorescence after the addition of analytes. In this review, we aim to summarize the recent advances in optical sensors that use layered double hydroxides as sensing platforms for the determination of various analytes. By outlining some representative examples, we accentuate the change of spectral absorbance, chemiluminescence, and photoluminescence phenomena triggered by the interaction of LDH or functionalized-LDH with the indicators and analytes in the system. And finally, current limitations and possible future orientation in designing further LDHs-based optical sensors are presented. It is hoped that this review will be helpful in assisting the establishment of more improved sensors based on LDHs features. Optical sensors based on layered double hydroxides (LDHs) nanoplatforms were reviewed. The sensing system and detection approaches were rationally reviewed. Possible future orientations were highlighted.

Novel hierarchical CuNiAl LDH nanotubes with excellent peroxidase-like activity for wide-range detection of glucose

Novel hierarchical CuNiAl layered double hydroxide (CuNiAl LDH) nanotubes were prepared with in situ transformation of Al₂O₃ produced using the atomic layer deposition (ALD) method. Based on the characterizations using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrometry, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), CuNiAl LDH displays a typical nanotube-like structure consisting of uniform ultrathin nanoflakes. It is also confirmed that nitrate precursors play a crucial role in the formation of the LDH hierarchical structure. The unique hierarchical tube-like structure for CuNiAl LDH can supply more active sites and higher surface areas, leading to outstanding peroxidase mimicking property. The kinetic analyses indicate that the catalytic behavior of CuNiAl LDH follows classic Michaelis-Menten models and the affinity of CuNiAl LDH to the substrate is significantly higher than horseradish peroxidase. A simple and label-free method was developed for the colorimetric detection of glucose. As low as 2.9 μM of glucose can be detected with a broad linear range from 10 to 200 μM. The established method is also proved to be suitable for glucose detection in juice samples.

“Click” dendrimer-Pd nanoparticle assemblies as enzyme mimics: catalytic o-phenylenediamine oxidation and application in colorimetric H2O2 detection

“Click” dendrimer-Pd NPs as peroxidase enzyme mimics of H₂O₂ sensing using o-phenylenediamine oxidation by H₂O₂ to 2,3-diaminophenazine.

Ultrathin NiMn layered double hydroxide nanosheets with a superior peroxidase mimicking performance to natural HRP for disposable paper-based bioassays

Ultrathin NiMn LDH was synthesized as a nanozyme for disposable paper-based bioassays, and its active centers were identified as Mn sites.

In-situ Formation of Amorphous Co-Al-P Layer on CoAl Layered Double Hydroxide Nanoarray as Neutral Electrocatalysts for Hydrogen Evolution Reaction

Exploration of high-efficiency and inexpensive electrode catalysts is of vital importance for the hydrogen evolution reaction (HER). In this research, an amorphous Co-Al-P layer was constructed on the surface of CoAl layered double hydroxide (CoAl-LDH) via an in-situ wet phosphidation strategy. The core-shell CoAl-LDH@Co-Al-P on Ti mesh (CoAl-LDH@Co-Al-P/TM) as an active HER electrocatalyst demands an overpotential of 150 mV to achieve a current density of 10 mA cm-2 at neutral pH. Moreover, CoAl-LDH@Co-Al-P/TM also exhibits good electrochemical stability and a superior Faradic efficiency of nearly 100%.

2D CTAB-MoSe2 Nanosheets and 0D MoSe2 Quantum Dots: Facile Top-Down Preparations and Their Peroxidase-Like Catalytic Activity for Colorimetric Detection of Hydrogen Peroxide

We report the facile and economic preparation of two-dimensional (2D) and 0D MoSe2 nanostructures based on systematic and non-toxic top-down strategies. We demonstrate the intrinsic peroxidase-like activity of these MoSe2 nanostructures. The catalytic processes begin with facilitated decomposition of H2O2 by using MoSe2 nanostructures as peroxidase mimetics. In turn, a large amount of generated radicals oxidizes 3,3,5,5-tetramethylbenzidine (TMB) to produce a visible color reaction. The enzymatic kinetics of our MoSe2 nanostructures complies with typical Michaelis-Menten theory. Catalytic kinetics study reveals a ping-pong mechanism. Moreover, the primary radical responsible for the oxidation of TMB was identified to be Ȯ2- by active species-trapping experiments. Based on the peroxidase mimicking property, we developed a new colorimetric method for H2O2 detection by using 2D and 0D MoSe2 nanostructures. It is shown that the colorimetric sensing capability of our MoSe2 catalysts is comparable to other 2D materials-based colorimetric platforms. For instance, the linear range of H2O2 detection is between 10 and 250 μM by using 2D functionalized MoSe2 nanosheets as an artificial enzyme. Our work develops a systematic approach to use 2D materials to construct novel enzyme-free mimetic for a visual assay of H2O2, which has promising prospects in medical diagnosis and food security monitoring.

Colorimetric detection of H2O2 based on the enhanced peroxidase mimetic activity of nanoparticles decorated Ce2(WO4)3 nanosheets

In this paper, nanoparticles decorated Ce₂(WO₄)₃ nanosheets (CWNSs) with negative potential and large specific surface area were synthesized and developed as highly efficient peroxidase mimics for colorimetric detection of H₂O₂. CWNSs can efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to induce an obvious color variation. Kinetic analysis indicated that the catalytic behaviors of CWNSs obey the typical Michaelis-Menten mechanism. The peroxidase-like catalytic mechanism of CWNSs was proposed according to the active species trapping experiments, verifying that ·O₂^- radicals played primary roles in the catalytic reaction. Based on the strong and stable peroxidase-like catalytic activity of CWNSs, a simple, rapid, selective, and ultrasensitive method was successfully established for colorimetric detection of H₂O₂. The method has a good linear response ranging from 0.5 μM to 100 μM for H₂O₂ concentration with a lower detection limit of 0.15 μM. Benefitting from the sensitive response and good stability, the method is applied in real sample detection and shows a favorable reproducibility and feasibility. This work not only provides a novel enzymatic mimics with remarkable catalytic activities for biomedical and environmental analysis, but also extends the application area of Ce₂(WO₄)₃ materials.

Co-Induced Electronic Optimization of Hierarchical NiFe LDH for Oxygen Evolution

Developing efficient and stable non-noble electrocatalysts for the oxygen evolution reaction (OER) remains challenging for practical applications. While nickel-iron layered double hydroxides (NiFe-LDH) are emerging as prominent candidates with promising OER activity, their catalytic performance is still restricted by the limited active sites, poor conductivity and durability. Herein, hierarchical nickel-iron-cobalt LDH nanosheets/carbon fibers (NiFeCo-LDH/CF) are synthesized through solvent-thermal treatment of ZIF-67/CF. Extended X-ray adsorption fine structure analyses reveal that the Co substitution can stabilize the Fe local coordination environment and facilitate the π-symmetry bonding orbital in NiFeCo-LDH/CF, thus modifying the electronic structures. Coupling with the structural advantages, including the largely exposed active surface sites and facilitated charge transfer pathway ensured by CF, the resultant NiFeCo-LDH/CF exhibits excellent OER activity with an overpotential of 249 mV at 10 mA cm^-1 as well as robust stability over 20 h.

CoMoO4 nanobelts as efficient peroxidase mimics for the colorimetric determination of H2O2

CoMoO₄ materials were prepared through a simple hydrothermal method and developed as highly efficient peroxidase mimics for colorimetric determination of H₂O₂. Based on the different experimental conditions in the synthesis process, the CoMoO₄ materials present distinct morphologies, structures, surface properties, and peroxidase mimetic activities. Among them, CoMoO₄ nanobelts (NBs) display the best intrinsic peroxidase mimetic abilities due to the high-energy (100) facet exposed, more Co active sites at (100) facet, more negative potential, and larger specific surface area. It can efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to generate a blue oxide. In view of the excellent peroxidase mimetic catalytic activity of CoMoO₄ NBs, a rapid, convenient, and ultrasensitive method was successfully established for the visual and colorimetric determination of H₂O₂. The method exhibits good selectivity, practicability, stability, and reusability, and has a detection limit of 0.27 μM. The peroxidase mimetic catalytic mechanism of CoMoO₄ NBs was illustrated according to the kinetic and active species trapping experiments. The method has a good potential for rapid and sensitive determination of H₂O₂ for biomedical analysis. Graphical abstract Schematic presentation of the process of CoMoO₄ nanobelts catalyzing the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to generate a typical blue color, which can be applied in rapid and ultrasensitive detection of H₂O₂ visually.

Electronic coupling between molybdenum disulfide and gold nanoparticles to enhance the peroxidase activity for the colorimetric immunoassays of hydrogen peroxide and cancer cells

Peroxidase nanoenzymes exhibit a specific affinity toward substrates, thereby demonstrating application potential for realizing the colorimetric immunoassays of hydrogen peroxide (H₂O₂), which can be further used as a probe for imaging cancer cells. To enhance the intrinsic peroxidase activity of molybdenum sulfide (MoS₂) nanomaterials, gold (Au) nanoparticles with an average diameter of approximately 2.1 nm were modified on a MoS₂/carbon surface (denoted as MoS₂/C-Au₆₀₀) via ascorbic acid reduction. MoS₂/C-Au₆₀₀ can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to generate a blue oxidation product in the presence of H₂O₂; this product exhibits peroxidase-like activities, superior to those of most existing MoS₂-based nanoenzymes. Furthermore, MoS₂/C-Au₆₀₀ exhibits a high detection capability for H₂O₂ in the range of 1 × 10^-5 to 2 × 10^-4 mol/L (R² = 0.99), and the lowest detection limit is 1.82 µmol/L in a sodium acetate and acetic acid buffer solution. Steady state kinetics studies indicate that the catalytic mechanism is consistent with the ping-pong mechanism. Given its strong absorption peak at 652 nm in the visible region, MoS₂/C-Au₆₀₀ can be used to image cancer cells due to the enhanced permeability and retention effect. Our findings demonstrate that the synergistic electronic coupling between multiple components can enhance the peroxidase activity, which can facilitate the development of an effective, facile, and reliable method to perform colorimetric immunoassays of H₂O₂ and cancer cells.

Antioxidant Materials Based on 2D Nanostructures: A Review on Recent Progresses

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.

Nitrogen-doped graphene quantum dots anchored on NiFe layered double-hydroxide nanosheets catalyze the oxygen evolution reaction

N-GQDs/NiFe-LDH layered nanosheet structure has excellent OER catalytic performance.

In situ polymerization and covalent functionalisation of trithiocyanuric acid by MoS2 nanosheets resulting in a novel nanozyme with enhanced peroxidase activity

MoS₂ catalysed the polymerisation of trithiocyanuric acid, resulting in a network exhibiting peroxidase activity via a ping-pong mechanism.

Modulated transition metal-oxygen covalency in the octahedral sites of CoFe layered double hydroxides with vanadium doping leading to highly efficient electrocatalysts

The development of efficient and stable non-noble-metal electrocatalytic materials for the oxygen evolution reaction (OER) is a huge and important challenge at present. Herein, we report the prominent enhancement of OER activity via doping vanadium into CoFe-based layered double hydroxide (LDH) electrocatalysts. Electrochemical characterization shows that the Co₂Fe_0.5V_0.5 LDH grown on carbon papers (CPs) has an enormous electrochemical surface area (ECSA) and exhibits the smallest overpotential of 242 mV at 10 mA cm^-2, which only requires a small Tafel slope of 41.4 mV dec^-1 in 1 M KOH solution. The X-ray photoelectron spectroscopy (XPS) peak position of Co, Fe and O moves slightly to higher binding energy, elucidating the improved covalency of the metal-oxygen bond after V doping. DFT+U simulation indicates that the outstanding electrocatalytic activity of Co₂Fe_0.5V_0.5 could be ascribed to the increased metal-oxygen covalency in LDH after V element doping, and facilitates the charge-transfer from oxygen to the metal. This finely tuned strategy by V doping into the CoFe-based LDH matrix can adjust the covalency of metal-oxygen bridges and enhance its electrocatalytic activity for the OER. In this work, we also present a general method to study various highly efficient metal hydroxide catalysts for the OER.

Identifying the active site of ultrathin NiCo LDH as an efficient peroxidase mimic with superior substrate affinity for sensitive detection of hydrogen peroxide

Nanozymes have been extensively investigated to imitate protein enzymes in biomimetic chemistry and the identification of the active site is believed to be the pre-requisite before one can effectively regulate their activity. Herein, ultrathin NiCo LDH nanosheets are synthesized via a fast co-precipitation at room temperature and can be stably dispersed in water without any additives of surfactants or organic solvents. By tuning the ratio between Ni and Co in LDH nanosheets, the activity is tuned and their peroxidase-like activity is determined by Co sites that show higher affinity to both 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) due to the strong Lewis acidity of Co3+ and the low redox potential of Co3+/Co2+. Together with their small crystallite size, ultra-thin thickness and tunable composition, NiCo LDH is used as a nanozyme for highly sensitive colorimetric detection of H2O2 and the limit of detection (LOD) reaches 0.48 μM.

Incorporation of Glycine max Merrill Extract into Layered Double Hydroxide through Ion-Exchange and Reconstruction

We incorporated extract of Glycine max Merrill (GM), which is generally known as soybean, into a layered double hydroxide (LDH) nanostructure through two different methods, ion-exchange and reconstruction. Through X-ray diffraction, field-emission scanning electron microscopy, and zeta-potential measurement, GM moiety seemed to be simply attached on the surface of LDH by ion-exchange process, while the extract could be incorporated in the inter-particle pore of LDHs by reconstruction reaction. The quantification exhibited that both incorporation method showed comparable extract loading capacity of 15.6 wt/wt% for GM-LDH hybrid prepared by ion-exchange (GML-I) and 18.6 wt/wt% for GM-LDH hybrid by reconstruction (GML-R). On the other hand, bioactive substance in both GM-LDH hybrids, revealed that GML-R has higher daidzein content (0.0286 wt/wt%) compared with GML-I (0.0108 wt/wt%). According to time-dependent daidzein release, we confirmed that GML-R showed pH dependent daidzein release; a higher amount of daidzein was released in pH 4.5 physiological condition than in pH 7.4, suggesting the drug delivery potential of GML-R. Furthermore, alkaline phosphatase activity and collagen fiber formation on human osteoblast-like MG-63 cells displayed that GML-R had superior possibility of osteoblast differentiation than GML-I. From these results, we concluded that reconstruction method was more effective for extract incorporation than ion-exchange reaction, due to its pH dependent release property and alkaline phosphatase activity.

Intrinsic peroxidase-like activity of Cu2ZnSn(SxSe1-x)4 nanocrystals, and their application to the colorimetric detection of H2O2

Nanocrystals (NCs) of type Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) were prepared via a solvothermal approach. They are shown to be highly efficient peroxidase (POx) mimics for colorimetric detection of H₂O₂. By varying the molar ratio of S and Se during preparation, the NCs showed different crystal structures, morphologies, surface properties, and POx-like activities. Among them, the type CZTSSe-0.25 NCs exhibit the strongest POx-like activities towards the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂ to generate a blue product. The enhanced activity is attributed to its more negative potential and larger specific surface of the NCs. Based on these findings, a rapid and ultrasensitive method was developed for the visual and colorimetric determination of H₂O₂. The method is selective, and the NCs are reusable and long-term stable. The detection limit of H₂O₂ is 50 nM. Kinetic and active species trapping experiments were performed to elucidate the POx-like mechanism of the NCs. Graphical abstract Schematic presentation of the process of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals catalyzing the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to induce a typical blue color reaction, which can be applied in colorimetric detection of H₂O₂.

Atomic layer deposition-assisted growth of CuAl LDH on carbon fiber as a peroxidase mimic for colorimetric determination of H2O2 and glucose

An atomic-layer-deposited Al₂O₃-induced LDH growth strategy was proposed to prepare carbon fiber-supported ultrathin CuAl LDH nanosheets (CF@CuAl-LDH). The CF@CuAl-LDH exhibited superior peroxidase-like catalytic activity.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.