Solvent effect on the electronic spectra of azine dyes under alkaline condition

Controllable corrosion-assisted fabrication of Au-Ag alloyed hollow nanocrystals for highly efficient and environmentally-stable SERS substrates

Surface enhanced Raman scattering (SERS) substrates with both high activity and long term chemical-stability have been expected in the practical application of the SERS-based detection. In this paper, Au-Ag bimetal nanocrystals are fabricated based on the template-etching reaction in the Ag nanocubes-contained cetylpyridinium chloride (CPC) aqueous solution via adding the HAuCl₄ solution. The obtained nanocrystals are Au-Ag alloyed and hollow in structure. Further, it has been found that with the increasing Au/Ag molar ratio, the shape of the alloyed nanocrystals evolve from the truncated nanocubes to the hollow boxes and then nanocages, showing the ever red-shifting surface plasmon resonance from the visible to the infrared region. The formation of the alloyed hollow nanocrystals is attributed to the preferential dissolution of the Ag nanocubes induced by CPC selective adsorption and the three to one galvanic replacement reaction between Ag and Au atoms. Importantly, such Au-Ag alloyed hollow nanocrystals, especially the ones with a low Au/Ag atomic ratio, show both high SERS activity and long term environmental stability compared with pure Ag or Au nanocrystals, and are the ideal candidate for the SERS substrate with practical application value. This work not only demonstrates the nanofabrication route to the alloyed hollow nanocrystals with controllable shapes and tunable optical properties in a large region, but also presents highly active and chemically-stable SERS substrates for the practical SERS-based detection.

In Vivo Synthesis of Nanocomposites Using the Recombinant Escherichia coli

Biogenic gold nanorod (AuNR)-Ag core-shell nanocomposites (NCs) are synthesized by using recombinant Escherichia coli to demonstrate in vivo synthesis of biogenic NCs for the first time. The chemically synthesized AuNRs are internalized into the E. coli, and Ag ions are reduced and grown on the surface of the AuNRs with the assistance of metal-binding proteins, producing biogenic core-shell AuNR-Ag NCs. The core-shell structure of the biogenic AuNR-Ag NC is confirmed by transmission electron microscopy and energy-dispersive X-ray analysis. The biogenic AuNR-Ag NCs exhibit good plasmonic effects. While the core-shell morphology of the AuNR and Ag NCs is due to the similar lattice of Au and Ag, the shape of the biogenic NCs composed of gold nanoparticles and Fe is aciniform, and that of Fe₃ O₄ NPs and Au/Ag is a network structure, demonstrating the controllability of biogenic nanosynthesis using diverse metal combinations with different NC morphologies.

Naked eye and optical biosensing of cysteine over the other amino acids using β-cyclodextrin decorated silver nanoparticles as a nanoprobe

A simple, highly selective and efficient sensing approach based on a β-CD AgNP colorimetric nanoprobe has been demonstrated, which permits quick and specific determination of Cys over other important amino acids.

Novel ratiometric surface-enhanced raman spectroscopy aptasensor for sensitive and reproducible sensing of Hg2

It is important to precisely monitor mercury (II) ions (Hg²⁺) for environment protection and human health monitoring. Although many strategies have been developed in the past decades, there still remains a challenge for developing an ultrasensitive, simple and reliable approach to detect Hg²⁺. Herein, we report a ratiometric surface-enhanced Raman scattering (SERS) aptasensor by employing aptamer-modified Au@Ag core-shell nanoparticles (Au@Ag NPs) as highly functional sensing probes, allowing for ultrasensitive detection of Hg²⁺. In principle, the thiolated 5'-Cy3 labeled aptamer probe (Cy3-aptamer) is firstly immobilized on the SERS substrate surface and then hybridizes with the 5'-Rox labeled complementary DNA (cDNA) to form a rigid double-stranded DNA (dsDNA), in which the Cy3 and Rox Raman labels are used to produce the ratiometric Raman signals. In the presence of Hg²⁺, the aptamer DNA turns into the thymine (T)-Hg²⁺-T mediated hairpin structure, leading to the dissociation of dsDNA. As a result, the Rox labels are away from the Au@Ag NP SERS substrate while Cy3 labels are close to it. Therefore, the intensity of SERS signal from Cy3 labels increases while that from Rox labels decreases. The ratio between the Raman intensities of Cy3 labels and Rox labels is linear with Hg²⁺ concentrations in the range from 0.001 to 1.0nM, and the limit of detection is estimated to be 0.4pM. The proposed strategy provides a new rapid, simple and reliable approach for sensitive detection of Hg²⁺ and may create a universal methodology for developing analogous aptasensors for a wide range of other analytes determination.

Rapid Surface Enhanced Raman Scattering (SERS) Detection of Sibutramine Hydrochloride in Pharmaceutical Capsules with a β-Cyclodextrin- Ag/Polyvivnyl Alcohol Hydrogel Substrate

Sibutramine hydrochloride (SH) is a banned weight-loss drug, but its illegal addition to health products is still rampant. This suggests a very urgent need for a fast and precise detection method for SH. Surface Enhanced Raman Scattering (SERS) is a promising candidate for this purpose, but the weak affinity between SH and bare metal limits its direct SERS detection. In the present work, β-cyclodextrin was capped in situ onto the surface of Ag nanoparticles to function as a scaffold to capture SH. The obtained Ag nanoparticles were encapsulated into polyvinyl alcohol (PVA) to fabricate a SERS active hydrogel with excellent reproducibility. A facile SERS strategy based on such substrate was proposed for trace SH quantification with a linear range of 7.0–150.0 µg·mL^–1, and a detection limit low to 3.0 µg·mL⁻¹. It was applied to analyze seven types of commercial slimming capsules with satisfactory results, showing good prospect for real applications.

Seedless synthesis of nanocomposites, optical properties, and effects of additives on their surface resonance plasmon bands

The work describes an easy seedless competitive chemical reduction method for the synthesis of Ag@Au/Ag bimetallic nanoparticles by mixing AgNO₃, HAuCl₄ and cysteine. Transmission electron microscope (TEM) images show that the large number of irregular, cross-linking, and aggregated Ag@Au/Ag are formed in a reaction mixture (HAuCl₄+AgNO₃+cysteine), whereas flower-like nanocomposites are obtained in presence of cetyltrimethylammonium bromide (CTAB), which acted as a shape-directing agent. Optical images reveal that the initially reaction proceeds through formation of purple color, which changes into dark brown color with the reaction time, indicating the formation of Ag@Au/Ag nanocomposites. The Ag⁺ has strong tendency to form complex with cysteine. Firstly, the reduction of Ag⁺ ions to Ag⁰ occurred by the HS group of the cysteine-Ag complex. Secondly, AuCl₄^- ions adsorbed on the positive surface of Ag⁰, which undergoes reduction by potential deposition, and leads to the formation of Ag@Au/Ag bimetallic nanoparticles. Inorganic electrolytes (NaCl, NaBr, NaNO₃ and Na₂SO₄) have significant impact on the stability and aggregation of Ag@Au/Ag nanocomposites.

Solid-Solution Alloying of Immiscible Ru and Cu with Enhanced CO Oxidation Activity

We report on novel solid-solution alloy nanoparticles (NPs) of Ru and Cu that are completely immiscible even above melting point in bulk phase. Powder X-ray diffraction, scanning transmission electron microscopy, and energy-dispersive X-ray measurements demonstrated that Ru and Cu atoms were homogeneously distributed in the alloy NPs. Ru_0.5Cu_0.5 NPs demonstrated higher CO oxidation activity than fcc-Ru NPs, which are known as one of the best monometallic CO oxidation catalysts.

Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

Core-shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core-shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core-shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core-shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core-shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

Leakage-free polypyrrole–Au nanostructures for combined Raman detection and photothermal cancer therapy

A novel PPy–Au nanostructure with the bifunctionality of Raman detection and photothermal therapy of cancer is reported.

Construction of CuS/Au Heterostructure through a Simple Photoreduction Route for Enhanced Electrochemical Hydrogen Evolution and Photocatalysis

An efficient Hydrogen evolution catalyst has been developed by decorating Au nanoparticle on the surface of CuS nanostructure following a green and environmental friendly approach. CuS nanostructure is synthesized through a simple wet-chemical route. CuS being a visible light photocatalyst is introduced to function as an efficient reducing agent. Photogenerated electron is used to reduce Au(III) on the surface of CuS to prepare CuS/Au heterostructure. The as-obtained heterostructure shows excellent performance in electrochemical H₂ evolution reaction with promising durability in acidic condition, which could work as an efficient alternative for novel metals. The most efficient CuS-Au heterostructure can generate 10 mA/cm² current density upon application of 0.179 V vs. RHE. CuS-Au heterostructure can also perform as an efficient photocatalyst for the degradation of organic pollutant. This dual nature of CuS and CuS/Au both in electrocatalysis and photocatalysis has been unveiled in this study.

Synthesis of Monometallic (Au and Pd) and Bimetallic (AuPd) Nanoparticles Using Carbon Nitride (C3N4) Quantum Dots via the Photochemical Route for Nitrophenol Reduction

In this study, we report the synthesis of monometallic (Au and Pd) and bimetallic (AuPd) nanoparticles (NPs) using graphitic carbon nitride (g-C₃N₄) quantum dots (QDs) and photochemical routes. Eliminating the necessity of any extra stabilizer or reducing agent, the photochemical reactions have been carried out using a UV light source of 365 nm where C₃N₄ QD itself functions as a suitable stabilizer as well as a reducing agent. The g-C₃N₄ QDs are excited upon irradiation with UV light and produce photogenerated electrons, which further facilitate the reduction of metal ions. The successful formation of Au, Pd, and AuPd alloy nanoparticles is evidenced by UV-vis, powder X-ray diffraction, X-ray photon spectroscopy, and energy-dispersive spectroscopy techniques. The morphology and distribution of metal nanoparticles over the C₃N₄ QD surface has been systematically investigated by high-resolution transmission electron microscopy (HRTEM) and SAED analysis. To explore the catalytic activity of the as-prepared samples, the reduction reaction of 4-nitrophenol with excellent performance is also investigated. It is noteworthy that the synthesis of both monometallic and bimetallic NPs can be accomplished by using a very small amount of g-C₃N₄, which can be used as a promising photoreducing material as well as a stabilizer for the synthesis of various metal nanoparticles.

Antibacterial properties and mechanisms of gold-silver nanocages

Despite the number of antibiotics used in routine clinical practice, bacterial infections continue to be one of the most important challenges faced in humans. The main concerns arise from the continuing emergence of antibiotic-resistant bacteria and the difficulties faced with the pharmaceutical development of new antibiotics. Thus, advancements in the avenue of novel antibacterial agents are essential. In this study, gold (Au) was combined with silver (Ag), a well-known antibacterial material, to form silver nanoparticles producing a gold-silver alloy structure with hollow interiors and porous walls (gold-silver nanocage). This novel material was promising in antibacterial applications due to its better biocompatibility than Ag nanoparticles, potential in photothermal effects and drug delivery ability. The gold-silver nanocage was then tested for its antibacterial properties and the mechanism involved leading to its antibacterial properties. This study confirms that this novel gold-silver nanocage has broad-spectrum antibacterial properties exerting its effects through the destruction of the cell membrane, production of reactive oxygen species (ROS) and induction of cell apoptosis. Therefore, we introduce a novel gold-silver nanocage that serves as a potential nanocarrier for the future delivery of antibiotics.

Photosynthesized silver–polyaminocyclodextrin nanocomposites as promising antibacterial agents with improved activity

Ag nanocomposites were prepared by photoreduction of ammoniacal silver acetate in the presence of poly-{6-[3-(2-(3-aminopropylamino)ethylamino)propylamino]}-(6-deoxy)-β-CD (amCD).

Oxidative esterification of furfural over Au–Pd/HAP-T and Au–Ag/HAP-T bimetallic catalysts supported on mesoporous hydroxyapatite nanorods

Core–shell (Au–Pd) and alloy (Au–Ag) architectured bimetallic mesoporous hydroxyapatite nano rods in the oxidative esterification of furfural.

Sub-100 nm anisotropic gold nanoparticles as surface-enhanced Raman spectroscopy substrates

This study describes a reliable preparation of relatively small Ag/Au-based anisotropic nanostructures possessing tunable absorption bands and their use as surface-enhanced Raman spectroscopy (SERS) substrates. These Au nanostructures were prepared via the seed growth process of small Ag-core-Au-shell-type nanoparticles that were obtained by the subsequent reduction of Ag and Au ions by NaBH(4) and L-ascorbic acid at room temperature. The presence of Ag during the transformation process of the Ag-Au core-shell nanoparticles under light irradiation led to the formation of various small anisotropic Au nanoparticles which clearly exhibited different structural and optical properties from those of nanoparticles prepared from typical Ag-Au alloy or bare Ag or Au seeds. As the optimal size of Au-based substrates for SERS applications was reported to be below 100 nm in diameter under a constant concentration, we tested our moderately small anisotropic nanoparticles (∼55 nm in diameter) as a SERS substrate to examine the signal enhancement of 4-nitrobenzenethiol. These nanoparticles exhibited a greatly increased SERS response compared to those of similar sizes of uniform Ag and Au nanoparticles, presumably because of the increased surface area due to the nanoparticles' anisotropic nature (i.e., chemical effect) and partial overlap of their absorption bands with the SERS excitation wavelength (i.e., electromagnetic effect). In addition, these nanoparticles have shown a suitable stability to prevent significant SERS signal fluctuations caused by unpredictable aggregations. Due to our simple synthetic and modification approaches, relatively small Au-based anisotropic nanostructures can be easily designed to serve as attractive SERS templates.

Optimized core-shell Au@Ag nanoparticles for label-free Raman determination of trace Rhodamine B with cancer risk in food product

A simple and reliable method based on surface-enhanced Raman scattering (SERS) with a portable Raman system is described for sensitive determination of trace levels of Rhodamine B (RB) in hot sauce samples. The sodium salt of phytic acid (IP6) stabilized Au@Ag core-shell bimetallic nanoparticles are constructed and used as SERS substrate, yielding high Raman enhancement of RB. The limit of detection for RB in water is 5 nM (2 ppb), which is below China Exit and Entry Inspection and Quarantine Bureau's tolerance level of 5 ppb. Also, the proposed easy assay of IP6-Au@Ag NPs combining with portable Raman system could be applied for on-site monitoring RB in hot sauce.

Nanoparticle properties and synthesis effects on surface-enhanced Raman scattering enhancement factor: an introduction

Raman spectroscopy has enabled researchers to map the specific chemical makeup of surfaces, solutions, and even cells. However, the inherent insensitivity of the technique makes it difficult to use and statistically complicated. When Raman active molecules are near gold or silver nanoparticles, the Raman intensity is significantly amplified. This phenomenon is referred to as surface-enhanced Raman spectroscopy (SERS). The extent of SERS enhancement is due to a variety of factors such as nanoparticle size, shape, material, and configuration. The choice of Raman reporters and protective coatings will also influence SERS enhancement. This review provides an introduction to how these factors influence signal enhancement and how to optimize them during synthesis of SERS nanoparticles.

Plasmonic nanorod arrays of a two-segment dimer and a coaxial cable with 1 nm gap for large field confinement and enhancement

Seeking plasmonic nanostructures with large field confinement and enhancement is significant for photonic and electronic nanodevices with high sensitivity, reproducibility, and tunability. Here, we report the synthesis of plasmonic arrays composed of two-segment dimer nanorods and coaxial cable nanorods with ∼1 nm gap insulated by a self-assembled Raman molecule monolayer. The gap-induced plasmon coupling generates an intense field in the gap region of the dimer junction and the cable interlayer. As a result, the longitudinal plasmon resonance of nanorod arrays with high tunability is obviously enhanced. Most interestingly, the field enhancement of dimer nanorod arrays can be tuned by the length ratio L1/L2 of the two segments, and the maximal enhancement appears at L1/L2 = 1. In that case, the two-photon luminescence (TPL) of dimer nanorod arrays and the Raman intensity in the dimer junction is enhanced by 27 and 30 times, respectively, under resonant excitation. In the same way, the Raman intensity in the gap region is enhanced 16 times for the coaxial cable nanorod arrays. The plasmonic nanorod arrays synthesized by the facile method, having tunable plasmon properties and large field enhancement, indicate an attractive pathway to the photonic nanodevices.

Synthesis of size-tunable chitosan encapsulated gold–silver nanoflowers and their application in SERS imaging of living cells

Monodispersed and biocompatible SERS tags are conveniently developed by one-pot synthesis and applied for cancer cell targeting and SERS imaging.

Molecularly imprinted electrochemical sensor based on an electrode modified with an imprinted pyrrole film immobilized on a β-cyclodextrin/gold nanoparticles/graphene layer

A novel molecularly imprinted electrochemical sensor was constructed for the sensitive detection of quercetin (Qu) based on a glassy carbon electrode (GCE) modified with β-cyclodextrin (β-CD), gold nanoparticles(AuNPs) and graphene (GR).

A cheap and efficient catalyst with ultra-high activity for reduction of 4-nitrophenol

Ni/SBA-15 catalyst with ultra-small particle size (7 nm), good dispersion, and ultra-high loading amount (57.4%) of Ni nanoparticles was prepared by a unique in situ thermal decomposition and reduction route. It exhibits excellent catalytic activity and stability for 4-nitrophenol reduction.

A novel glutathione-stabilized silver–gold nano-alloy/Cu2+ combination as a fluorescent switch probe for l-histidine

This paper reports the synthesis of glutathione-stabilized silver–gold nano-alloys and their use as a fluorescent switch probe for the detection of l-histidine.

Hydroxyl-rich C-dots synthesized by a one-pot method and their application in the preparation of noble metal nanoparticles

Hydro-rich C-dots were used as both the reducing and stabilizing agent in the preparation of noble metal nanoparticles (AuNPs, AgNPs and Au@AgNPs) for the detection of glucose.

In situ formation of gold/silver bi-metal nanodots on silica spheres and evaluation of their microbicidal properties

An environmentally benign synthetic method of seedless and one-step growth of 2–4 nm sized gold/silver bi-metal nanodots on preformed silica spheres and their microbicidal properties with different concentrations of Au and Ag are reported.

Au@Pd core–shell nanoparticles-decorated reduced graphene oxide: a highly sensitive and selective platform for electrochemical detection of hydrazine

An Au_core–Pd_shell-decorated reduced graphene oxide nanocomposite is successfully employed for the electrochemical detection of low-level hydrazine in an aqueous solution.

Improved catalytic activity and surface electro-kinetics of bimetallic Au–Ag core–shell nanocomposites

The Au_core–Ag_shell and hollow Ag–Au alloy nanostructures significantly improved (∼2 times) the reduction of 1,3-dinitrobenzene relative to the monometallic ones demonstrating the role of the synergistic effect of the Au–Ag interface in catalytic activity.

Selective and sensitive SERS sensor for detection of Hg2+ in environmental water base on rhodamine-bonded and amino group functionalized SiO2-coated Au–Ag core–shell nanorods

A novel SERS sensor for trace detection of Hg²⁺ using R6G-derive Schiff base bonded Au@Ag@SiO₂–NH₂ NRs was designed. The LOD is 0.33 pmol L⁻¹.

Core-size-dependent catalytic properties of bimetallic Au/Ag core-shell nanoparticles

Bimetallic core-shell nanoparticles have recently emerged as a new class of functional materials because of their potential applications in catalysis, surface enhanced Raman scattering (SERS) substrate and photonics etc. Here, we have synthesized Au/Ag bimetallic core-shell nanoparticles with varying the core diameter. The red-shifting of the both plasmonic peaks of Ag and Au confirms the core-shell structure of the nanoparticles. Transmission electron microscopy (TEM) analysis, line scan EDS measurement and UV-vis study confirm the formation of core-shell nanoparticles. We have examined the catalytic activity of these core-shell nanostructures in the reaction between 4-nitrophenol (4-NP) and NaBH4 to form 4-aminophenol (4-AP) and the efficiency of the catalytic reaction is found to be increased with increasing the core size of Au/Ag core-shell nanocrystals. The catalytic efficiency varies from 41.8 to 96.5% with varying core size from 10 to 100 nm of Au/Ag core-shell nanoparticles, and the Au100/Ag bimetallic core-shell nanoparticle is found to be 12-fold more active than that of the pure Au nanoparticles with 100 nm diameter. Thus, the catalytic properties of the metal nanoparticles are significantly enhanced because of the Au/Ag core-shell structure, and the rate is dependent on the size of the core of the nanoparticles.

High specific detection and near-infrared photothermal therapy of lung cancer cells with high SERS active aptamer-silver-gold shell-core nanostructures

Lung cancer is the leading cause of cancer death worldwide. Its early detection is of paramount importance for diagnosis, classification, treatment, and improvement of survivorship. However, current methods are not sensitive enough to detect lung cancer in its nascent stage. We reported an aptamer-Ag-Au shell-core nanostructure-based surface-enhanced Raman scattering (SERS) assay for sensitive and specific detection, and near-infrared (NIR) photothermal therapy of lung adenocarcinoma cells (A549 cells). The nanostructures target the cells with high affinity and specificity via the specific interaction between the aptamer (a 45-base oligonucleotide) and the cell, and distinguish A549 cells from other types of cancer cells (HeLa and MCF-7 cells) and subtypes of lung cancer cells (NCI-H157, NCI-H520, NCI-H1299, and NCI-H446 cells). The nanostructures have a high capability to absorb NIR irradiation and are able to perform photothermal therapy of the cells at a very low irradiation power density (0.20 W cm(-2)) without destroying the healthy cells and the surrounding normal tissues. In addition, the nanostructures exhibit a high SERS activity. Based on the SERS signal of the labeled Raman reporter (Rh6G molecules), we can specifically detect A549 cells at a very low abundance (~10 cells per mL) and monitor the therapy process of the cancer cells. Therefore, this nanostructure-based SERS assay has great potential in specific recognition, sensitive detection, and effective photothermal therapy of lung cancer.

Rapid simultaneous detection of multi-pesticide residues on apple using SERS technique

A rapid and straightforward method has been employed to simultaneously detect two pesticides (thiram and methamidophos (MTD)) on apple surface using surface enhanced Raman scattering (SERS) technique. In the experiment, ethanol was dropped onto the contaminated apple surface for pesticide extraction and then gold@silver core-shell nanorods (Au@Ag NRs) were added to generate the SERS signals of the pesticides. Under a laser excitation at 632.8 nm, prominent SERS peaks of blended contaminants were observed, which were chosen to characterize and quantify their concentration. It was found that the SERS intensity of these two peaks changed as a function of the concentration ratio of thiram to MTD. In addition, a better SERS enhancement performance of Au@Ag NRs was demonstrated compared with that of gold nanorods. Our experimental results show that the lowest detectable concentration on apple surfaces is ∼4.6 × 10(-7) M for thiram and ∼4.4 × 10(-4) M for MTD. This study provides a straightforward method for the simultaneous detection of multiple pesticides on fruit surfaces, which is important for food safety and human health.

Metal nanoparticles and supramolecular macrocycles: a tale of synergy

In this minireview, we summarize current research dealing with the combination of noble-metal nanoparticles and different families of supramolecular macrocycles (cyclodextrins, cucurbit[n]urils, calixarenes, and pillar[n]arenes). We intended to select relevant publications on the synthesis of noble-metal nanoparticles with macrocycles acting as capping agents or/and reducing agents, as well as on the post-synthetic metal-nanoparticle modification with macrocycles. We also discuss strategies in which supramolecular chemistry is applied to direct the self-assembly of nanoparticles and formation of polymer composites. We finally describe the main applications of these materials in various fields.