Direct Cross-Linking of Au/Ag Alloy Nanoparticles into Monolithic Aerogels for Application in Surface-Enhanced Raman Scattering

Self-Supported Ag/Pt/Pd Alloy Aerogels as High-Performance Bifunctional and Durable Electrocatalysts for Methanol and Ethanol Oxidation Reactions

Assembly of nanoparticles (NPs) into functional macrostructures is imperative for the development of NP-based devices. However, existing methods employ insulating organic ligands, polymers, and biomolecules as mediators for the NP assembly, which are detrimental for charge transport and interparticle coupling that impede the efficient integration of low-dimensional properties. Herein, we report a methodology for the direct self-supported assembly of Ag/Pt/Pd alloy NPs into high surface area (119.1 ± 3.9 to 140.1 ± 5.7 m2/g), mesoporous (19.7 ± 6.2 to 23.0 ± 1.6 nm), and conducting nanostructures (aerogels) that show superior electrocatalytic activity and stability in methanol (MOR) and ethanol (EOR) oxidation reactions. Ultrasmall (3.9 ± 1.3 nm) and quasi-spherical Ag/Pt/Pd alloy NPs were synthesized via stepwise galvanic replacement reaction (GRR) of glutathione (GSH)-coated Ag NPs. As-synthesized NPs were transformed into free-standing alloy hydrogels via chemical oxidation of the GSH ligands. The composition of alloy aerogels was tuned by varying the oxidant/thiolate molar ratio of the precursor NP sol that prompts Ag dealloying with in situ generated HNO3, selectively enriching the Pt and Pd catalytic sites on the aerogel surface. The highest-performing alloy aerogel (Ag0.449Pt0.480Pd0.071) demonstrates excellent mass activity for methanol (3179.5 mA/mg) and ethanol (2444.5 mA/mg) electro-oxidation reactions, which are ∼4-5 times higher than those of commercial Pt/C and Pd/C electrocatalysts. The aerogel also maintained high alcohol oxidation activity for 17 h at a constant potential of -0.3 V in an alkaline medium. The synergistic effects of noble metal alloying, high surface area and mesoporosity, and the pristine active surface of aerogels provide efficient interaction of analytes with the nanostructure surface, facilitating both MOR and EOR activity and improving tolerance for poisonous byproducts, enabling the Ag/Pt/Pd alloy aerogel a promising (electro)catalyst for a number of new technologies.

Structural investigations of Au-Ni aerogels: morphology and element distribution

The physical properties of nanomaterials are determined by their structural features, making accurate structural control indispensable. This carries over to future applications. In the case of metal aerogels, highly porous networks of aggregated metal nanoparticles, such precise tuning is still largely pending. Although recent improvements in controlling synthesis parameters like electrolytes, reductants, or mechanical stirring, the focus has always been on one particular morphology at a time. Meanwhile, complex factors, such as morphology and element distributions, are studied rather sparsely. We demonstrate the capabilities of precise morphology design by deploying Au-Ni, a novel element combination for metal aerogels in itself, as a model system to combine common aerogel morphologies under one system for the first time. Au-Ni aerogels were synthesized via modified one- and two-step gelation, partially combined with galvanic replacement, to obtain aerogels with alloyed, heterostructural (novel metal aerogel structure of interconnected nanoparticles and nanochains), and hollow spherical building blocks. These differences in morphology are directly reflected in the physisorption behavior, linking the isotherm shape and pore size distribution to the structural features of the aerogels, including a broad-ranging specific surface area (35-65 m2 g-1). The aerogels were optimized regarding metal concentration, destabilization, and composition, revealing some delicate structural trends regarding the ligament size and hollow sphere character. Hence, this work significantly improves the structural tailoring of metal aerogels and possible up-scaling. Lastly, preliminary ethanol oxidation tests demonstrated that morphology design extends to the catalytic performance. All in all, this work emphasizes the strengths of morphology design to obtain optimal structures, properties, and (performances) for any material application.

Leveraging Ligand and Composition Effects: Morphology-Tailorable Pt-Bi Bimetallic Aerogels for Enhanced (Photo-)Electrocatalysis

Metal aerogels (MAs) are emerging porous materials displaying unprecedented potential in catalysis, sensing, plasmonic technologies, etc. However, the lack of efficient regulation of their nano-building blocks (NBBs) remains a big hurdle that hampers the in-depth investigation and performance enhancement. Here, by harmonizing composition and ligand effects, Pt- and Bi-based single- and bimetallic aerogels bearing NBBs of controlled dimensions and shapes are obtained by facilely tuning the metal precursors and the applied ligands. Particularly, by further modulating the electronic and optic properties of the aerogels via adjusting the content of the catalytically active Pt component and the semiconducting Bi component, both the electrocatalytic and photoelectrocatalytic performance of the Pt-Bi aerogels can be manipulated. In this light, an impressive catalytic performance for electro-oxidation of methanol is acquired, marking a mass activity of 6.4-fold higher under UV irradiation than that for commercial Pt/C. This study not only sheds light on in situ manipulating NBBs of MAs, but also puts forward guidelines for crafting high-performance MAs-based electrocatalysts and photoelectrocatalysts toward energy-related electrochemical processes.

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.

Flexible, Strong and Multifunctional Anf@Ag Nanocomposite Film for Human Physiology and Motion Monitoring

To expand the application range of flexible pressure sensors, endowing them with multifunction capabilities becomes extremely important. Herein, a flexible, strong and multifunctional nanocomposite film was prepared by introducing silver nanoparticles (Ag NPs) into aramid nanofiber (ANF) film using a simple two-step vacuum filtration method. When the Ag content was 27.6 vol%, the electrical resistance of the resulting ANF@Ag nanocomposite film was as low as 1.63 Ω/cm2, and the water contact angle of the nanocomposite film reached 153.9 ± 1°. Compared to the ANF film, the tensile strength of the nanocomposite film increased from 55 MPa to 66.3 MPa with an increase of 20.5%. After being applied to the human body, the nanocomposite film served as a pressure sensor that was able to recognize different stimuli for healthcare monitoring. Based on the advantages, it may become a potential candidate for electronic skin, intelligent wearable devices and medical detection equipment.

SERS-based Au@Ag NPs Solid-phase substrate combined with chemometrics for rapid discrimination of multiple foodborne pathogens

In this study, a surface enhanced Raman scattering (SERS) sensor based on Au@Ag NPs solid-phase substrate combined with chemometrics was constructed for the discrimination of three pathogenic bacteria (Staphylococcus aureus, Escherichia coli and Listeria monocytogenes). The Au@Ag NPs were synthesized and self-assembled on filter paper using the dip-coating method. The good absorbency of the filter paper immobilized the bacteria on the substrate, increased the interaction between the bacteria and the substrate, and enhanced the SERS signal of the bacteria. The main peaks of the bacterial spectra were close to each other, but the relative intensities of the vibrational peaks were significantly different, and each strain exhibited unique Raman peaks. The combination of partial least squared discriminant analysis (PLS-DA) method with bacterial SERS allowed the effective identification of the three bacteria. Moreover, the method was applied for the quantitative detection of Staphylococcus aureus with a minimum detection concentration of 10⁴ cfu/mL.

Interstitial boron-triggered electron-deficient Os aerogels for enhanced pH-universal hydrogen evolution

Developing high-performance electrocatalysts for hydrogen evolution reaction (HER) is crucial for sustainable hydrogen production, yet still challenging. Here, we report boron-modulated osmium (B-Os) aerogels with rich defects and ultra-fine diameter as a pH-universal HER electrocatalyst. The catalyst shows the small overpotentials of 12, 19, and 33 mV at a current density of 10 mA cm⁻² in acidic, alkaline, and neutral electrolytes, respectively, as well as excellent stability, surpassing commercial Pt/C. Operando X-ray absorption spectroscopy shows that interventional interstitial B atoms can optimize the electron structure of B-Os aerogels and stabilize Os as active sites in an electron-deficient state under realistic working conditions, and simultaneously reveals the HER catalytic mechanisms of B-Os aerogels in pH-universal electrolytes. The density functional theory calculations also indicate introducing B atoms can tailor the electronic structure of Os, resulting in the reduced water dissociation energy and the improved adsorption/desorption behavior of hydrogen, which synergistically accelerate HER.

While noble metals can be active electrocatalysts for producing renewable H₂, there are relatively few works examining osmium materials. Here, the authors prepare boron-doped osmium aerogels for H₂ evolution electrocatalysis plus examine the mechanism using computational and in situ characterization.

Aerogels in drug delivery: From design to application

Aerogels are the lightest processed solid materials on Earth and with the largest empty volume fraction in their structure. Composition versatility, modularity, and feasibility of industrial scale manufacturing are behind the fast emergence of aerogels in the drug delivery field. Compared to other 3D materials, the high porosity (interconnected mesopores) and high specific surface area of aerogels may allow faster loading of small-molecule drugs, less constrained access to inner regions of the matrix, and more efficient interactions of the biological milieu with the polymer matrix. Processing in supercritical CO₂ medium for both aerogel production (drying) and drug loading (impregnation) has remarkable advantages such as absence of an oxidizing environment, clean manufacture, and easiness for the scale-up under good manufacturing practices. The aerogel solid skeleton dictates the chemical affinity to the different drugs, which in turn determines the loading efficiency and the release pattern. Aerogels can be used to increase the solubility of BCS Class II and IV drugs because the drug can be deposited in amorphous state onto the large surface area of the skeleton, which facilitates a rapid contact with the body fluids, dissolution, and release. Conversely, tuning the aerogel structure by functionalization with drug-binding moieties or stimuli-responsive components, application of coatings and incorporation of drug-loaded aerogels into other matrices may enable site-specific, stimuli-responsive, or prolonged drug release. The present review deals with last decade advances in aerogels for drug delivery. An special focus is paid first on the loading efficiency of active ingredients and release kinetics under biorelevant conditions. Subsequent sections deal with aerogels intended to address specific therapeutic demands. In addition to oral delivery, the physical properties of the aerogels appear to be very advantageous for mucosal administration routes, such as pulmonary, nasal, or transdermal. A specific section devoted to recent achievements in gene therapy and theranostics is also included. In the last section, scale up strategies and life cycle assessment are comprehensively addressed.

Noble Metal Aerogels

Noble metal-based nanomaterials have been a hot research topic during the past few decades. Particularly, self-assembled porous architectures have triggered tremendous interest. At the forefront of porous nanostructures, there exists a research endeavor of noble metal aerogels (NMAs), which are unique in terms of macroscopic assembly systems and three-dimensional (3D) porous network nanostructures. Combining excellent features of noble metals and the unique structural traits of porous nanostructures, NMAs are of high interest in diverse fields, such as catalysis, sensors, and self-propulsion devices. Regardless of these achievements, it is still challenging to rationally design well-tailored NMAs in terms of ligament sizes, morphologies, and compositions and profoundly investigate the underlying gelation mechanisms. Herein, an elaborate overview of the recent progress on NMAs is given. First, a simple description of typical synthetic methods and some advanced design engineering are provided, and then, the gelation mechanism models of NMAs are discussed in detail. Furthermore, promising applications particularly focusing on electrocatalysis and biosensors are highlighted. In the final section, brief conclusions and an outlook on the existing challenges and future chances of NMAs are also proposed.

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano-sized highly-active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser-generated inorganic-salt-stabilized metal nanoparticles, various impurity-free NMAs (Au, Pd, and Au-Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Multimeric Rhodamine Dye-Induced Aggregation of Silver Nanoparticles for Surface-Enhanced Raman Scattering

Isotopic variants of Rhodamine 6G (R6G) have previously been used as a method of multiplexed detection for Surface Enhanced Raman Spectroscopy (SERS), including protein detection and quantification. Challenges exist, however, with producing long-term stable SERS signals with exposure to silver or gold metal surfaces without the use of additional protective coatings of nanomaterials. Here, novel rhodamine "dimers" and "trimers" have been created that demonstrate a higher avidity for metal nanoparticles and induce aggregation to create plasmonic "hotspots" as indicated by enhanced Raman scattering in situ. These aggregates can be formed in a colloid, on surfaces, or membrane substrates such as poly(vinylidene fluoride) for applications in biosciences. The integrity of the materials and Raman signals are maintained for months of time on different substrates. These dye materials should provide avenues for simplified in situ generation of sensors for Raman-based assays especially in settings requiring highly robust performance.

Plasmonic and photo-electrochemical enhancements of the AuAg@Au/RGO–C3N4 nanocomposite for the detection of DA

Plasmonic photocatalyst has attracted significant attention due to its valuable theoretical study and promising practical applications in solar cells, functional composites, and sensors.

3D assembly of preformed colloidal nanoparticles into gels and aerogels: function-led design

Nanoparticle-based aerogels combine the properties of traditional aerogels with those of nanoparticles, and hold promise for various applications following a function-led design.