Noble Metal Aerogels

Confinement-enhanced electrochemiluminescence by Ru(dcbpy)32+-functionalized γ-CD-MOF@COF-LZU1 porous hybrid material as micro-reactor for CYFRA 21-1 detection

The improvement of electrochemiluminescence (ECL) performance relies on the electron transfer efficiency between luminophore and coreactant. An ultrasensitive ECL micro-reactor with confinement-enhanced performance was prepared by using the covalent organic framework-LZU1-functionalized metal-organic framework (MOF@COF-LZU1) as a platform to assemble enormous N,N-dibutyl-2-hydroxyethylamine (DBAE) and tris(4,4'-dicarboxylic acid-2,2'-bipyridyl) ruthenium(II) [Ru(dcbpy)32+] into its pore channels. Compared to individual substances of γ-CD-MOF and COF-LZU1, the synergistic effects can conduce to the enhancement of the intensity, durability and sensitivity of the micro-reactor. Besides, COF-LZU1 can provide a mild environment to accommodate a certain amount of DBAE by concentrating them from the aqueous solution into its hydrophobic cavities and boost the oxidation efficiency of DBAE to generate more DBAE●+ and profited the survival of DBAE●, leading to an improved reaction efficiency with the Ru(dcbpy)32+ intermediate. Thanks to the confinement-enhanced strategy, engineered as high-functioning luminescent materials, Ru@γ-CD-MOF@COF-LZU1 micro-reactors decorated with Au NPs can facilitate electron transfer and capture primary antibodies (Ab1). Moreover, Au-Pd-Pt noble metal aerogels (NMAs) functionalized MoS2 NFs (Au-Pd-Pt NMAs@MoS2 NFs) were chosen as base material due to its large specific surface areas, high porosity, and excellent electrical conductivity. Based on above merits, the sensor demonstrated a sensitive response to CYFRA 21-1 detection in a linear concentration gradient from 10 fg/mL to 50 ng/mL with a detection limit of 0.0055 pg/mL (S/N = 3). The COF-LZU1 decorated ECL micro-reactors were constructed based on the signal amplification strategies to realize accurate CYFRA 21-1 detection.

Research progress on polybenzoxazine aerogels: Preparation, properties, composites and hybrids fabrication, applications

The unremitting pursuit of high-performance and multifunctional materials has consistently propelled modern industries forward, stimulating research and motivating progress in related fields. In such materials, polybenzoxazine (PBz) aerogel, which combines the virtues of PBz and aerogel, has attracted salient attention recently, emerging as a novel research focus in the realm of advanced materials. In this review, the preparation scheme, microscopic morphology, and fundamental characteristics of PBz aerogels are comprehensively summarized and discussed in anticipation of providing a clear understanding of the correlation between preparation process, structure, and properties. The effective strategies for enhancing the performance of PBz aerogels including composite fabrication and hybridization are highlighted. Moreover, the applications of PBz-based aerogels in various domains such as adsorption (including wastewater treatment, CO2 capture, and microwave adsorption), thermal insulation, energy storage as well as sensors are covered in detail. Furthermore, several obstacles and potential directions for subsequent research are delineated with a view to surmounting the prevailing constraints and achieving a realization of the shift from experimental exploration to practical applications.

Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 μM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

Magnetic aerogels from FePt and CoPt3 directly from organic solution

Here the synthesis of magnetic aerogels from iron platinum and cobalt platinum nanoparticles is presented. The use of hydrazine monohydrate as destabilizing agent triggers the gelation directly from organic solution, and therefore a phase transfer to aqueous media prior to the gelation is not necessary. The aerogels were characterized through Transmission Electron Microscopy, Scanning Electron Microscopy, Powder X-Ray Diffraction Analysis and Argon Physisorption measurements to prove the formation of a porous network and define their compositions. Additionally, magnetization measurements in terms of hysteresis cycles at 5 K and 300 K (M-H-curves) as well as zero field cooled-field cooled measurements (ZFC-FC measurements) of the dried colloids and the respective xero- and aerogels were performed, in order to analyze the influence of the gelation process and the network structure on the magnetic properties.

Strain Effects in Ru-Au Bimetallic Aerogels Boost Electrocatalytic Hydrogen Evolution

To improve the sluggish kinetics of the hydrogen evolution reaction (HER), a key component in water-splitting applications, there is an urgent desire to develop efficient, cost-effective, and stable electrocatalysts. Strain engineering is proving an efficient strategy for increasing the catalytic activity of electrocatalysts. This work presents the development of Ru-Au bimetallic aerogels by a simple one-step in situ reduction-gelation approach, which exhibits strain effects and electron transfer to create a remarkable HER activity and stability in an alkaline environment. The surface strain induced by the bimetallic segregated structure shifts the d-band center downward, enhancing catalysis by balancing the processes of water dissociation, OH* adsorption, and H* adsorption. Specifically, the optimized catalyst shows low overpotentials of only 24.1 mV at a current density of 10 mA cm-2 in alkaline electrolytes, surpassing commercial Pt/C. This study can contribute to the understanding of strain engineering in bimetallic electrocatalysts for HER at the atomic scale.

Cu Aerogels with Sustainable Cu(I)/Cu(II) Redox Cycles for Sensitive Nonenzymatic Glucose Sensing

Developing functional nanomaterials for nonenzymatic glucose electrochemical sensing platforms is vital and challenging from the perspective of pathology and physiology. Accurate identification of active sites and thorough investigation of catalytic mechanisms are critical prerequisites for the design of advanced catalysts for electrochemical sensing. Herein, Cu aerogels are synthesized as a model system for sensitive nonenzymatic glucose sensing. The resultant Cu aerogels exhibit good catalytic activity for glucose electrooxidation with high sensitivity and a low detection limit. Significantly, in situ electrochemical investigations and Raman characterizations reveal the catalytic mechanism of Cu-based nonenzymatic glucose sensing. During the electrocatalytic oxidation of glucose, Cu(I) is electrochemically oxidized to generate Cu(II), and the resultant Cu(II) is spontaneously reduced back to Cu(I) by glucose, achieving the sustained Cu(I)/Cu(II) redox cycles. This study provides profound insights into the catalytic mechanism for nonenzymatic glucose sensing, which provides great potential guidance for a rational design of advanced catalysts in the future.

Element Distributions in Bimetallic Aerogels

ConspectusMetal aerogels assembled from nanoparticles have captured grand attention because they combine the virtues of metals and aerogels and are regarded as ideal materials to address current environmental and energy issues. Among these aerogels, those composed of two metals not only display combinations (superpositions) of the properties of their individual metal components but also feature novel properties distinctly different from those of their monometallic relatives. Therefore, quite some effort has been invested in refining the synthetic methods, compositions, and structures of such bimetallic aerogels as to boost their performance for the envisaged application(s). One such use would be in the field of electrocatalysis, whereby it is also of utmost interest to unravel the element distributions of the (multi)metallic catalysts to achieve a ratio of their bottom-to-up design. Regarding the element distributions in bimetallic aerogels, advanced characterization techniques have identified alloys, core-shells, and structures in which the two metal particles are segregated (i.e., adjacent but without alloy or core-shell structure formation). While an almost infinite number of metal combinations to form bimetallic aerogels can be envisaged, the knowledge of their formation mechanisms and the corresponding element distributions is still in its infancy. The evolution of the observed musters is all but well understood, not to mention the positional changes of the elements observed in operando or in beginning- vs end-of-life comparisons (e.g., in fuel cell applications).With this motivation, in this Account we summarize the endeavors made in element distribution monitoring in bimetallic aerogels in terms of synthetic methods, expected structures, and their evolution during electrocatalysis. After an introductory chapter, we first describe briefly the two most important characterization techniques used for this, namely, scanning transmission electron microscopy (STEM) combined with element mapping (e.g., energy-dispersive X-ray spectroscopy (EDXS)) and X-ray absorption spectroscopy (XAS). We then explain the universal methods used to prepare bimetallic aerogels with different compositions. Those are divided into one-step methods in which gels formed from mixtures of the respective metal salts are coreduced and two-step approaches in which monometallic nanoparticles are mixed and gelated. Subsequently, we summarize the current state-of-knowledge on the element distributions unraveled using diverse characterization methods. This is extended to investigations of the element distributions being altered during electrochemical cycling or other loads. So far, a theoretical understanding of these processes is sparse, not to mention predictions of element distributions. The Account concludes with a series of remarks on current challenges in the field and an outlook on the gains that the field would earn from a solid understanding of the underlying processes and a predictive theoretical backing.

Dry Synthesis of Pure and Ultrathin Nanoporous Metallic Films

Nanoporous metals possess unique properties attributed to their high surface area and interconnected nanoscale ligaments. They are mostly fabricated by wet synthetic methods that are not universal to various metals and not free from impurities due to solution-based etching processes. Here, we demonstrate that the plasma treatment of metal nanoparticles formed by physical vapor deposition is a general route to form such films with many metals including the non-noble ones. The resultant nanoporous metallic films are free of impurities and possess highly curved ligaments and nanopores. The metal films are ultrathin, yet remarkably robust and very well connected, and thus are highly promising for various applications such as transparent conducting electrodes.

Magnetic RuCo aerogels with enhanced peroxidase-like activity by regulation of boron and oxygen vacancies for colorimetric biosensing applications

Metallic aerogels (MAs) are self-supported porous nanomaterials with excellent catalytic activity, which could be a promising candidate for high-performance nanozymes. The interface regulation by heteroatom and vacancies is an effective strategy for boosting the enzyme-mimicking activity. Herein, magnetic RuCo aerogels with doping of boron and oxygen vacancies were prepared by a one-pot spontaneous NaBH₄ gelation method under a low temperature. The three-dimensional network structure with high specific surface area and interlinked pores of RuCo aerogels afford abundant active sites to facilitate the interaction with substrates. Moreover, the monolithic structure avoided conventional aggregation, thus enhancing stability during catalysis. Introducing elemtal boron and oxygen vacancies adjusted the electronic structure of RuCo aerogels to achieve enhanced enzyme-like performances. It is found that the RuCo aerogel nanozyme can mimic nature peroxidase, demonstrating their viable applications in the bioassay of H₂O₂ and glucose. The constructed glucose sensor possesses acceptable sensitivity and stability with a linear range of 0.002 ~ 5 mM and a low detection limit (1.66 μM). This work provides insights into the rational design of advanced nanozymes and paves the avenue for the applications of metallic aerogels in the bioassay field. A boron-doped RuCo bimetallic aerogel with rich oxygen vacancies was prepared by a facile self-assembly method under an ice bath. The unique physical and electronic structure of RuCo aerogel results in the improvement of the intrinsic peroxidaselike activity, and thus, a sensitive and robust colorimetric glucose sensor could be developed.

High-Entropy Alloy Aerogels: A New Platform for Carbon Dioxide Reduction

High-entropy alloy aerogels (HEAAs) combined with the advantages of high-entropy alloys and aerogels are prospective new platforms in catalytic reactions. However, due to the differences in reduction potentials and miscibility behavior of different metals, the realization of HEAAs with a single phase is still a great challenge. Herein, a series of HEAAs is fabricated via the freeze-thaw method as highly active and durable electrocatalysts for the carbon dioxide reduction reaction (CO₂ RR). Especially, the PdCuAuAgBiIn HEAAs can achieve Faradaic efficiency (FE) of C1 products almost 100% from -0.7 to -1.1 V versus reversible hydrogen electrode (V_RHE ), and a maximum FE for formic acid (FE_HCOOH ) of 98.1% at -1.1 V_RHE , outperforming PdCuAuAgBiIn high-entropy alloy particles (HEAPs) and Pd metallic aerogels (MAs). Specifically, the current density and FE_HCOOH are almost 200 mA cm^-2 and 87% in a flow cell. The impressive CO₂ RR performance of the PdCuAuAgBiIn HEAAs is attributed to the strong interactions between the different metals and the surface unsaturated sites, which can regulate the electronic structures of different metals and allow the optimal HCOO* intermediate adsorption and desorption onto the catalysts surface to enhance HCOOH production. The work not only provides a facile synthetic strategy to fabricate HEAAs, but also opens the avenue for development of efficient catalysts and beyond.

Pd Metallene Aerogels with Single-Atom W Doping for Selective Ethanol Oxidation

The development of advanced electrocatalysts with satisfactory C1 pathway selectivity for the ethanol oxidation reaction (EOR) is critical. Herein, a bubbling CO-induced gelation method is developed in acetic acid at 50 °C to construct single-atom W-doped Pd metallene aerogels (denoted as SA W-Pd MAs) within 1 h. In light of the metallene structural advantages of noble metal aerogels and single-atom W decoration, the resultant SA W-Pd MAs exhibit an outstanding EOR performance with high C1 pathway selectivity. Density functional theory calculations validate that the SA W-Pd MAs greatly improve the formation of the CH₃O intermediate and the transformation of poisonous CO species to CO₂, thus resulting in high C1 pathway selectivity. Therefore, this work not only offers an effective gelation method to fabricate noble metal aerogels with atomic-scale building blocks but also presents guidance to develop high-efficiency EOR electrocatalysts.

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Ultra-Low Content Bismuth-Anchored Gold Aerogels with Plasmon Property for Enhanced Nonenzymatic Electrochemical Glucose Sensing

Effective glucose surveillance provides a strong guarantee for the high-quality development of human health. Au nanomaterials possess compelling applications in nonenzymatic electrochemical glucose biosensors owing to superior catalytic performances and intriguing biocompatibility properties. However, it has been a grand challenge to accurately control the architecture and composition of Au nanomaterials to optimize their optical, electronic, and magnetic properties for further improving the performance of electrocatalytic sensing. Herein, ultra-low content Bi-anchored Au aerogels are synthesized via a one-step reduction strategy. Benefiting from the unique structure of aerogels as well as the synergistic effect between Au and Bi, the optimized Au₂₀₀Bi aerogels greatly boost the activity of glucose oxidation compared with Au aerogels. Under plasmon resonance excitation, bimetallic Au₂₀₀Bi aerogels with wider photics-dependent properties further show plasmon-promoted glucose electro-oxidation activity, which is derived from the photothermal and photoelectric effects caused by the local surface plasmon resonance. Thanks to the enhanced performance, a nonenzymatic electrochemical glucose biosensor is constructed to detect glucose with high sensitivity. This plasmon-promoted electrocatalytic activity through the synergetic strategy of bimetallic aerogels has potential applications in various research fields.

Amorphous metal-organic frameworks on PtCu hydrogels: Enzyme immobilization platform with boosted activity and stability for sensitive biosensing

Cell-free enzymatic catalysis (CFEC) is emerging biotechnology that simulates biological transformations without living cells. However, the high cost of separation and preparation of the enzyme has hindered the practical application of the CFEC. Enzyme immobilization technologies using solid supports to stabilize enzymes have been regarded as an efficient strategy to address this issue. Nevertheless, the activity and stability of the immobilized enzymes are still crucial challenges for working in vitro. Herein, an enzyme immobilization platform is developed by using PtCu hydrogels coated with amorphous metallic-organic frameworks (MOFs) as multifunctional carriers to encapsulate horseradish peroxidase (HRP). Specifically, PtCu hydrogels acting as a "reservoir of metal ions" can interact with the immobilized enzyme and facilitate electron transfer, leading to the boosted enzyme catalytic performances. Furthermore, amorphous MOFs on the surface of PtCu hydrogels serve as an "armor" to protect the internal enzymes from various perturbation environments. The resultant enzyme immobilization platform (PtCu@HRP@ZIF-8) not only shows an approximately 2.4-fold enhanced activity compared with free enzyme but also exhibits improved stability against harsh conditions. The PtCu@HRP@ZIF-8-based biosensor is constructed for sensitive sensing of organophosphorus pesticides (OPs). The proposed biosensor exhibits a favorable linear relationship with the concentration of paraoxon-ethyl from 6 to 800 ng/mL, with a low detection limit of 1.8 ng/mL. This work reveals the promising potential of our proposed enzyme immobilization platform in practical applications.

Ultrasmall Gold Nanoclusters-Enabled Fabrication of Ultrafine Gold Aerogels as Novel Self-Supported Nanozymes

Metal aerogels represent an emerging type of functional porous materials with promising applications in diverse fields, but the fabrication of metal aerogels with specific structure and property still remains a challenge. Here, the authors report a new approach to fabricate metal aerogels by using ultrasmall metal nanoclusters (NCs) as functional building blocks. By taking D-penicillamine-stabilized gold NCs (AuNCs) with a diameter of 1.4 nm as an example, Au aerogels with ultrafine ligament size (3.5 nm) and good enzyme-mimic properties are synthesized. Detailed characterization shows that the obtained Au aerogels possess typical 3D self-supported porous network structure with high gold purity and surface area. Time-lapse spectroscopic and microscopic monitoring of the gelation process reveal that these ultrasmall AuNCs first grow into large nanoparticles before fusion into nanowire networks, during which both pH and the precursor concentration are identified to be the determining factor. Owing to their highly porous structure and abundant metal nodes, these self-supported Au aerogels display excellent peroxidase-like properties. This work provides a strategy for fabricating advanced metal aerogels by taking ultrasmall-sized metal NCs as building blocks, which also opens new avenues for engineering the structure and properties of metal aerogels for further advancing their applications.

Expanding the Range: AuCu Metal Aerogels from H2O and EtOH

Due to their self-supporting and nanoparticulate structure, metal aerogels have emerged as excellent electrocatalysts, especially in the light of the shift to renewable energy cycles. While a large number of synthesis parameters have already been studied in depth, only superficial attention has been paid to the solvent. In order to investigate the influence of this parameter with respect to the gelation time, crystallinity, morphology, or porosity of metal gels, AuxCuy aerogels were prepared in water and ethanol. It was shown that although gelation in water leads to highly porous gels (60 m2g−1), a CuO phase forms during this process. The undesired oxide could be selectively removed using a post-washing step with formic acid. In contrast, the solvent change to EtOH led to a halving of the gelation time and the suppression of Cu oxidation. Thus, pure Cu aerogels were synthesized in addition to various bimetallic Au3X (X = Ni, Fe, Co) gels. The faster gelation, caused by the lower permittivity of EtOH, led to the formation of thicker gel strands, which resulted in a lower porosity of the AuxCuy aerogels. The advantage given by the solvent choice simplifies the preparation of metal aerogels and provides deeper knowledge about their gelation.

Identification of plasmon-driven nanoparticle-coalescence-dominated growth of gold nanoplates through nanopore sensing

The fascinating phenomenon that plasmon excitation can convert isotropic silver nanospheres to anisotropic nanoprisms has already been developed into a general synthetic technique since the discovery in 2001. However, the mechanism governing the morphology conversion is described with different reaction processes. So far, the mechanism based on redox reactions dominated anisotropic growth by plasmon-produced hot carriers is widely accepted and developed. Here, we successfully achieved plasmon-driven high yield conversion of gold nanospheres into nanoplates with iodine as the inducer. To investigate the mechanism, nanopore sensing technology is established to statistically study the intermediate species at the single-nanoparticle level. Surprisingly, the morphology conversion is proved as a hot hole-controlled coalescence-dominated growth process. This work conclusively elucidates that a controllable plasmon-driven nanoparticle-coalescence mechanism could enable the production of well-defined anisotropic metal nanostructures and suggests that the nanopore sensing could be of general use for studying the growth process of nanomaterials.

PdIr Aerogels with Boosted Peroxidase-like Activity for a Sensitive Total Antioxidant Capacity Colorimetric Bioassay

Metallic aerogels (MAs), imparting the active catalytic properties of nanostructured noble metals to macroscopic aerogels, draw tremendous interest in diverse fields owing to the unique features of three-dimensional interconnected channels, self-supported architectures, and pure metallic backbones. Moreover, flexible manipulation of compositions, high electrical conductivity, and abundant active sites of MAs contribute to the great potential to mimic natural enzymes. However, the cumbersome synthetic process takes a couple of hours to days, and unavoidable impurities usually impede surface electrons/mass transfer, posing the decrease of stability and enzyme-like activity of MAs. Here, a PdIr bimetallic aerogel prepared in the ethanol phase via spontaneous assembly and a surfactant-free strategy is reported. Gelation kinetics of PdIr aerogels in ethanol is increased with 2-4 orders of magnitude compared to the traditional preparation method in water. Owing to the intrinsic physicochemical properties, PdIr aerogels exhibit the high activity of peroxidase mimics using 3,3',5,5'-tetramethylbenzidine as a chromogenic probe. In addition, the PdIr aerogels maintain relatively high activity at an elevated temperature and pH of 3-7, demonstrating their good stability and survivability. Utilizing the exceptional peroxidase-like activity of PdIr aerogels, we realized the quantitative bioassay for H₂O₂ and total antioxidant capacity, indicating enormous potential in the quality evaluation of real samples.

Facile Synthesis of Unsupported Pd Aerogel for High Performance Formic Acid Microfluidic Fuel Cell

In this work, unsupported Pd aerogel catalysts were synthesized for the very first time by using microwaves as a heating source followed by a lyophilization drying process and used towards formic acid electro-oxidation in a microfluidic fuel cell. Aerogels were also made by heating in a conventional oven to evaluate the microwave effect during the synthesis process of the unsupported Pd aerogels. The performance of the catalysts obtained by means of microwave heating favored the formic acid electro-oxidation with H₂SO₄ as the electrolyte. The aerogels' performance as anodic catalysts was carried out in a microfluidic fuel cell, giving power densities of up to 14 mW cm^-2 when using mass loads of only 0.1 mg on a 0.019 cm² electrode surface. The power densities of the aerogels obtained by microwave heating gave a performance superior to the resultant aerogel prepared using conventional heating and even better than a commercial Pd/C catalyst.

Noble-Metal Nanorod Cryoaerogels with Electrocatalytically Active Surface Sites

Noble-metal-based electrocatalysts usually contain small nanoparticle building blocks to ensure a high specific surface area as the scene for the surface processes. Here, we show that relatively large noble-metal nanorods are also promising candidates to build up functional macrostructures with prominent electrocatalytic activity. After optimizing and upscaling the syntheses of gold nanorods and gold bipyramid-templated silver nanorods, cryoaerogels are fabricated on a conductive substrate via flash freezing and subsequent freeze drying. The versatile cryoaerogelation technique allows the formation of macrostructures with dendritic, open-pore structure facilitating the increase of the accessible nanorod surfaces. It is demonstrated via electrochemical oxidation and stripping test experiments that noble-metal surface sites are electrochemically active in redox reactions. Furthermore, gold nanorod cryoaerogels offer a platform for redox sensing, ethanol oxidation reaction, as well as glucose sensing. Compared to their simply drop-cast and dried counterparts, the noble-metal nanorod cryoaerogels offer enhanced activity due to the open porosity of the fabricated nanostructure while maintaining structural stability.

A Metallic Ion-Induced Self-Assembly Enabling Nanowire-Based Aerogels

The controlled assembly of nanowires is one of the key challenges in the development of a range of functional 3D aerogels with unique physicochemical properties for practical applications. However, the deep understanding of the dynamic assemble process for fabricating nanowire aerogels remains elusive. Herein, a facile strategy is presented for the metallic ion-induced assembly of nanowires into macroscopic aerogels via a solution-based process. This method enables the interconnecting between polymer-decorated nanowires via metallic coordination, resulting in plenty of nanowire bundles with the same orientation. Besides, the coordinated binding strength of nanowires with different metallic ions is also discussed. The assembly mechanism that the metallic ions induced dynamic behavior of nanowires is revealed via molecular dynamics theoretical evaluation. These findings benefit for constructing nanowire-based aerogels with unique structural features and multi-function, which pave new opportunities for other material systems.

B,N-Doped PdRu Aerogels as High-Performance Peroxidase Mimics for Sensitive Detection of Glucose

Among plentiful porous nanomaterials, noble metal aerogels taken as nanozymes attract broad attention in sensing applications with their distinct enzyme mimic functions. In the catalytic field, the heteroatom doping strategy is a kind of way with great promise in improving the enzyme mimic activity of noble metal aerogels. In this experiment, we find a type of creative materials that were prepared by the fast and simple method. Due to the unique porous structure and synergetic effect from doped atoms, PdRu aerogels co-doped with boron and nitrogen (B, N-PdRu aerogels) were prepared using NH₃BH₃ as a reductant, which present improved peroxidase mimicking activity. With the existence of H₂O₂, the oxidation of 3,3',5,5'-tetramethylbenzidine was catalyzed by B, N-PdRu aerogels fairly efficiently, whose solution would be a blue appearance at optimum absorption wavelength 652 nm. Thus, by the tandem reaction bound to the enzyme glucose oxidase, the B, N-PdRu aerogels can be used for the sensitive determination of glucose. The new method has a good linear detection effect for glucose in the range of 10 μM to 2 mM. The minimum limit of detection can reach as low as 6 μM. This work will contribute to research on the rational design of metal aerogels based on the heteroatomic doping strategy and enhance the corresponding performance for a variety of applications.

Immobilizing Enzymes on Noble Metal Hydrogel Nanozymes with Synergistically Enhanced Peroxidase Activity for Ultrasensitive Immunoassays by Cascade Signal Amplification

Enzyme immobilization plays an essential role in solving the problems of the inherently fragile nature of enzymes. Although prominent stability and reuse of enzymes can be achieved by enzyme immobilization, their bioactivity and catalytic efficiency will be adversely affected. Herein, PdCu hydrogel nanozymes with a hierarchically porous structure were used to immobilize horseradish peroxidase (HRP) to obtain PdCu@HRP. In addition to the improvement of stability and reusability, PdCu@HRP displayed synergistically enhanced activities than native HRP and PdCu hydrogels. Not only the specific interactions between PdCu hydrogel nanozymes and enzymes but also the enrichment of substrates around enzymes by electrostatic adsorption of hydrogels was proposed to expound the enhanced catalytic activity. Accordingly, by taking advantage of the excellent catalytic performance of the PdCu@HRP and the glucose oxidase encapsulated in zeolitic imidazolate framework-8, colorimetric biosensing of the carcinoembryonic antigen via catalytic cascade reactions for achieving signal amplification was performed. The obtained biosensor enhanced the detection sensitivity by approximately 6.1-fold as compared to the conventional HRP-based enzyme-linked immunosorbent assay, demonstrating the promising potential in clinical diagnosis.

Sub-Nanometer Pt Clusters on Defective NiFe LDH Nanosheets as Trifunctional Electrocatalysts for Water Splitting and Rechargeable Hybrid Sodium-Air Batteries

It is challenging to develop highly efficient and stable multifunctional electrocatalysts for improving the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) for sustainable energy conversion and storage systems such as water-alkali electrolyzers (WAEs) and hybrid sodium-air batteries (HSABs). In this work, sub-nm Pt nanoclusters (NCs) on defective NiFe layered double hydroxide nanosheets (Ni_xFe LDHs) are synthesized by a facile electrodeposition method. Due to the synergistic effect between Pt NCs and abundant atomic M(II) defects, along with hierarchical porous nanostructures, the Pt/Ni_xFe LDHs catalysts exhibit superior trifunctional electrocatalytic activity and durability toward the HER/OER/ORR. A WAE fabricated with Pt/Ni_xFe LDHs electrodes needs 1.47 V to reach a current density of 10 mA cm^-2, much lower than that of the mixed 20% Pt/C and 20% Ir/C catalysts. An HSAB assembled by Pt/Ni_xFe LDHs as a binder-free air cathode displays a high open-circuit voltage, a narrow overpotential gap, and remarkable rechargeability. This work provides a feasible strategy for constructing freestanding efficient trifunctional electrocatalysts for sustainable energy conversion and storage systems.