Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels: synthesis and their regulated optical and catalytic properties

Herein, we present the synthesis of Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels. The aim is to get both the surface plasmon resonance (SPR) and catalytic performance of the composite material can be changed in response to external stimuli. Ag@poly(N-isopropylacrylamide-co-3-methacryloxypro-pyltrimethoxysilane) (Ag@P(NIPAM-co-MAPTMS)) hybrid microgels were synthesized by seed-emulsion polymerization using Ag nanoparticles (NPs) as the core and NIPAM/MAPTMS as monomers. Ag–Au@P(NIPAM-co-MAPTMS) bimetallic hybrid microgels were prepared by a galvanic replacement (GR) reaction between Ag NPs and HAuCl₄, with the composition and structure of these bimetallic nanocomposites being determined by the amount of added HAuCl₄. The highly porous organic–inorganic microgel layer provided confined space for the GR reaction, effectively preventing the aggregation of Ag–Au NPs. The shell layer of P(NIPAM-co-MAPTMS) three-dimensional network chains not only enhanced nanocomposite dispersity and stability, but also provided highly porous gel microdomains that could increase the diffusion of the substrate and hence enhanced catalytic activity. Additionally, the SPR and catalytic properties of Ag–Au@P(NIPAM-co-MAPTMS) are reversibly sensitive to external temperature. With increase of temperature, the maximum absorption peak of bimetallic nanocomposites shifted to longer wavelengths, and the catalytic activity of these composites for the reduction of 4-nitrophenol by NaBH₄ remarkably increased. The features above mentioned are related to presence of the thermosensitive PNIPAM chains and the highly porous structure constructed by rigid MAPTMS segments intersected between NIPAM chains.

Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels allowed the mass transfer of reactants to be controlled by temperature modulation.     

Biosynthesis of Ag–Pd bimetallic alloy nanoparticles through hydrolysis of cellulose triggered by silver sulfate

We report a simple but efficient biological route based on the hydrolysis of cellulose to synthesize Ag–Pd alloy nanoparticles (NPs) under hydrothermal conditions. X-ray powder diffraction, ultraviolet-visible spectroscopy and scanning transmission electron microscopy-energy dispersive X-ray analyses were used to study and demonstrate the alloy nature. The microscopy results showed that well-defined Ag–Pd alloy NPs of about 59.7 nm in size can be biosynthesized at 200 °C for 10 h. Fourier transform infrared spectroscopy indicated that, triggered by silver sulfate, cellulose was hydrolyzed into saccharides or aldehydes, which served as both reductants and stabilizers, and accounted for the formation of the well-defined Ag–Pd NPs. Moreover, the as-synthesized Ag–Pd nanoalloy showed high activity in the catalytic reduction of 4-nitrophenol by NaBH₄.

We report a simple but efficient biological route based on the hydrolysis of cellulose to synthesize Ag–Pd alloy nanoparticles (NPs) under hydrothermal conditions.     

Preparation and characterization of Ag–Pd bimetallic nano-catalysts in thermosensitive microgel nano-reactor

Thermosensitive microgels consisting of a solid core of polystyrene and a shell of cross-linked poly(N-isopropylacrylamide) (PNIPA) were synthesized as nano-reactors, in which Ag–Pd bimetallic nanoparticles were prepared through simultaneous in situ reduction reaction. The spatial distribution of metallic nanoparticles in the microgels was analyzed by small angle X-ray scattering (SAXS) and the results indicated that metal nanoparticles were mainly located in the inner layer of microgels. The catalytic activity of Ag–Pd bimetallic nanoparticles was investigated using the reduction of p-nitrophenol to p-aminophenol by NaBH₄ as model reaction. The data demonstrated that Ag–Pd bimetallic nanoparticles showed enhanced catalytic activity compared to each monometallic nanoparticle alone and their catalytic activity was controllable by temperature due to the volume transition of PNIPA microgels.

Thermosensitive microgels with PS core and cross-linked PNIPA shell were synthesized as nano-reactor to prepare Ag–Pd bimetallic nanoparticles.     

Magnetic–plasmonic Ni@Au core–shell nanoparticle arrays and their SERS properties

In this paper, large-area magnetic–plasmonic Ni@Au core–shell nanoparticle arrays (NPAs) with tunable compositions were successfully fabricated by a direct laser interference ablation (DLIA) incorporated with thermal dewetting method. The magnetic properties of the Ni@Au core–shell NPAs were analyzed and the saturation magnetization (M_s) of the Ni₈₀@Au₂₀ nanoparticles was found to be higher than that of nickel-only nanoparticles with the same diameter. Using Rhodamine 6G (R6G) as a Raman reporter molecule, the surface enhanced Raman scattering (SERS) property of the Ni@Au core–shell NPAs with a grain size distribution of 48 ± 42 nm and a short-distance order of about 200 nm was examined. A SERS enhancement factor of 2.5 × 10⁶ was realized on the Ni₅₀@Au₅₀ NPA substrate, which was 9 times higher than that for Au nanoparticles with the same size distribution. This was due to the enhanced local surface plasmon resonance (LSPR) between the ferromagnetic Ni cores and the surface polariton of the Au shells of each nanoparticle. The fabrication of the Ni@Au core–shell NPAs with different compositions offers a new avenue to tailor the optical and magnetic properties of the nanostructured films for chemical and diagnostic applications.

In this paper, large-area magnetic–plasmonic Ni@Au core–shell nanoparticle arrays (NPAs) with tunable compositions were successfully fabricated by a direct laser interference ablation (DLIA) incorporated with thermal dewetting method.     

Pyroelectric synthesis of Au/Pt bimetallic nanoparticles–BaTiO3 hybrid nanomaterials

This paper introduces an approach to synthesize bimetallic nanoparticles under an alternating temperature field in aqueous solution. During the synthesis, pyro-catalytic barium titanate is used as the substrate to reduce the metallic ions dispersed in the solution due to the generated charges at the surface of pyro-materials under temperature oscillation. Chloroauric acid and potassium tetrachloroplatinate are used as precursors to produce gold/platinum bimetallic nanoparticles through a pyro-catalytic process. Transmission electron microscopy characterization, in combination with energy dispersive X-ray spectroscopy mapping, demonstrates that the bimetallic nanoparticle is composed of an Au core and Au/Pt alloy shell structure. Compared to the conventional approaches, the pyroelectric synthesis approach demonstrated in this work requires no toxic reducing agents and waste heat can be used as a thermal energy source in the synthesis. Hence, it offers a potential “green” synthetic method for bimetallic nanoparticles.

A “green” synthetic approach to Au/Pt bimetallic nanoparticles under an alternating temperature field.     

Structure analysis of precursor alloy and diffusion during dealloying of Ag–Al alloy

Nanoporous silver (NPS) was fabricated by dealloying Ag–Al alloy ribbons with nominal compositions of 30, 35 and 40 at% Ag (corresponding to hypoeutectic composition, eutectic composition and hypereutectic composition, respectively). The microstructures of the Ag–Al precursor and as-dealloyed samples were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM) as well as via focused ion beam (FIB) technique. We concluded that with the increase in Ag content from 30 to 40 at%, the diameter of ligament increased from 70 ± 15 nm to 115 ± 35 nm. Due to the method of crystalline solidification and the distribution of α-Al(Ag) and γ-Ag₂Al phases, the as-dealloyed Ag₃₅Al₆₅ alloy exhibited a homogeneous ligament/pore structure, whereas the microstructures of Ag₃₀Al₇₀ and Ag₄₀Al₆₀ showed thinner and coarser ligament structures, respectively.

Nanoporous silver (NPS) was fabricated by dealloying Ag–Al alloy ribbons with nominal compositions of 30, 35 and 40 at% Ag (corresponding to hypoeutectic composition, eutectic composition and hypereutectic composition, respectively).     

The promoted catalytic hydrogenation performance of bimetallic Ni–Co–B noncrystalline alloy nanotubes

A noncrystalline Ni–B alloy in the shape of nanotubes has demonstrated its superior catalytic performance for some hydrogenation reactions. Remarkable synergistic effects have been observed in many reactions when bimetallic catalysts were used; however, bimetallic noncrystalline alloy nanotubes are far less investigated. Here, we report a simple acetone-assisted lamellar liquid crystal approach for synthesizing a series of bimetallic Ni–Co–B nanotubes and investigate their catalytic performances. The dilution effect of acetone on liquid crystals was characterized by small-angle X-ray diffraction (SAXRD) and scanning electron microscopy (SEM). The Ni/Co molar ratio of the catalyst was varied to study the composition, porous structure, electronic interaction, and catalytic efficiency. In the liquid-phase hydrogenation of p-chloronitrobenzene, the as-prepared noncrystalline alloy Ni–Co–B nanotubes exhibited higher catalytic activity and increased stability as compared to Ni–B and Co–B alloy nanotubes due to electronic interactions between the nickel and cobalt. The excellent hydrogenation performance of the Ni–Co–B nanotubes was attributed to their high specific surface area and the characteristic confinement effects, compared with Ni–Co–B nanoparticles.

Ni–Co–B noncrystalline alloy nanotubes exhibited higher catalytic activity and better stability due to the synergistic interactions between nickel and cobalt.     

Plasmonic nanoprobes based on the shape transition of Au/Ag core–shell nanorods to dumbbells for sensitive Hg-ion detection

We report a sensitive and selective localized surface plasmon resonance (LSPR) nanoprobe for the detection of mercuric ions (Hg²⁺) using gold/silver core–shell nanorods as an optical nanosubstrate. Sulfide can quickly react with silver atoms to generate Ag₂S at room temperature in the presence of oxygen. The transformation from Ag shell to Ag₂S on the nanorod surface results in its LSPR absorption band shifting to a longer wavelength, which is attributed to their different refractive indices. Interestingly, the morphology also changed from a rod-like to dumbbell shape. However, in the presence of Hg²⁺, this morphology transformation is inhibited because the sulfide reacts with free Hg²⁺ prior to the Ag atoms. The amount of Ag₂S reduced with the increasing concentration of Hg²⁺, and the absorption band shift was also decreased. According to this “rod-like to dumbbell or not” shape change, a sensitive and selective LSPR nanoprobe was established, assisted by UV-Vis absorption spectroscopy. The detection limit of this probe for Hg²⁺ was as low as 13 nM. The efficiency of this probe in complex samples was evaluated by the detection of Hg²⁺ in spiked water samples.

Sensitive plasmonic nanoprobes for the sensitive detection of mercury ions based on a “rod-like to dumbbell or not” morphology transition of the Au/Ag core–shell hybrid nanorods.     

Deposition of Ag and Ag–Au nanocrystalline films with tunable conductivity at the water–toluene interface

Thin films of Au, Ag and Ag–Au alloy nanocrystals extending to areas of several square centimetres are obtained by deposition at the interface of water and toluene. Toluene containing chlorotris(triphenylphosphine)silver(i) and/or chlorotriphenylphosphine gold(i) is reacted with aqueous tetrakishydroxymethylphosphonium chloride to obtain nanocrystalline films adhered to the interfacial region. Alloying was induced by varying the composition of the toluene layer. The composition change results in regular and reproducible variation in the transport characteristics of the films, with the initially metallic deposits turning non-metallic with increased Au content. The films at the interface were transferred to different substrates and characterised using atomic force microscopy, UV-visible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy.

Thin films of Au, Ag and Ag–Au alloy nanocrystals extending to areas of several square centimetres are obtained by deposition at the interface of water and toluene.     

Green synthesis of biocompatible core–shell (Au–Ag) and hybrid (Au–ZnO and Ag–ZnO) bimetallic nanoparticles and evaluation of their potential antibacterial,antidiabetic, antiglycation and anticancer activities

The fabrication of bimetallic nanoparticles (BNPs) using plant extracts is applauded since it is an environmentally and biologically safe method. In this research, Manilkara zapota leaf extract was utilized to bioreduce metal ions for the production of therapeutically important core–shell Au–Ag and hybrid (Au–ZnO and Ag–ZnO) BNPs. The phytochemical profiling of the leaf extract in terms of total phenolic and flavonoid content is attributed to its high free radical scavenging activity. FTIR data also supported the involvement of these phytochemicals (polyphenols, flavonoids, aromatic compounds and alkynes) in the synthesis of BNPs. Whereas, TEM and XRD showed the formation of small sized (16.57 nm) spherical shaped core–shell Au–Ag BNPs and ZnO nano-needles with spherical AuNPs (48.32 nm) and ZnO nano-rods with spherical AgNP (19.64 nm) hybrid BNPs. The biological activities of BNPs reinforced the fact that they show enhanced therapeutic efficacy as compared to their monometallic components. All BNPs showed comparable antibacterial activities as compared to standard tetracycline discs. While small sized Au–Ag BNPs were most effective in killing human hepato-cellular carcinoma cells (HepG2) in terms of lowest cell viability, highest intracellular ROS/RNS production, loss of mitochondrial membrane potential, induction of caspase-3 gene expression and enhanced caspase-3/7 activity. BNPs also effectively inhibited advanced glycation end products and carbohydrate digesting enzymes which can be used as a nano-medicine for aging and diabetes. The most important finding was the permissible biocompatibility of these BNPs towards brine shrimp larvae and human RBCs, which suggests their environmental and biological safety. This research study gives us insight into the promise of using a green route to synthesize commercially important BNPs with enhanced therapeutic efficacy as compared to conventional treatment options.

Graphical demonstartion of the Manikara zapota-mediated biosynthesis of Bimetallic nanoparticles (BNPs) and evalution of their biological activities.     

Modified graphene supported Ag–Cu NPs with enhanced bimetallic synergistic effect in oxidation and Chan–Lam coupling reactions

Herein, well dispersed Ag–Cu NPs supported on modified graphene have been synthesized via a facile and rapid approach using sodium borohydride as a reducing agent under ambient conditions. Dicyandiamide is selected as an effective nitrogen source with TiO₂ as an inorganic material to form two kinds of supports, labelled as TiO₂–NGO and NTiO₂–GO. Initially, the surface area analysis of these two support materials was carried out which indicated that N-doping of GO followed by anchoring with TiO₂ has produced support material of larger surface area. Using both types of supports, ten nano-metal catalysts based on Ag and Cu were synthesized. Benefiting from the bimetallic synergistic effect and larger specific surface area of TiO₂–NGO, Cu@Ag–TiO₂–NGO is found to be a highly active and reusable catalyst out of other synthesized catalysts. It exhibits excellent catalytic activity for oxidation of alcohols and hydrocarbons as well as Chan–Lam coupling reactions. The nanocatalyst is intensively characterized by BET, SEM, HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS.

Cu@Ag–TiO₂–NGO prepared from modified graphene by simple methodology exhibits enhanced catalytic activity towards oxidation and Chan–Lam coupling due to the synergistic effect between Ag and Cu NPs.     

Preparation of core/shell organic–inorganic hybrid polymer nanoparticles and their application to toughening poly(methyl methacrylate)

On account of the utility of poly(methyl methacrylate) (PMMA) as a glass substitute, toughening of PMMA has attracted significant attention. Brittle failure can often be avoided by incorporating a small fraction of filler particles. Core–shell composite particles composed of a rubbery core and a glassy shell have recently attracted interest as a toughening agent for brittle polymers. Here, core/shell organic–inorganic hybrid polymer nanoparticles (Si-ASA HPNs) with a silicone-modified butyl acrylate copolymer (PBA) core and a styrene-acrylonitrile copolymer (SAN) shell were used to toughen PMMA. Silicone plays dual roles as a compatibilizer and a chain extender, and it not only improves interfacial adhesion between the PBA particles and SAN copolymer, but it also increases chain entanglement of poly(acrylonitrile-styrene-acrylate) (ASA). The mechanical properties of the PMMA/ASA alloys strongly depend on the Si content, and the impact strength and elongation at break greatly improve when silicone-modified ASA is added. However, this is accompanied by loss of rigidity. Specifically, the PMMA/ASA-2 composite exhibits a good balance between toughness and rigidity, indicating that ASA-2 with 5 wt% KH570 is the most suitable impact modifier. This research provides a facile and practical method to overcome the shortcomings of ASA and promote its application in a wider range of fields.

Core/shell organic–inorganic hybrid polymer nanoparticles are synthesized by micellar nucleation, core enlarged polymerization and a grafting reaction in the system.     

Cu–Cd–Zn–S/ZnS core/shell quantum dot/polyvinyl alcohol flexible films for white light-emitting diodes

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation. The PL spectra of Cu–Cd–Zn–S/ZnS core/shell quantum dots can cover the whole visible light region in the case of only two ratios of Cu/Cd/Zn. The emission wavelength of Cu–Cd–Zn–S/ZnS QDs can be conveniently tuned from 474 to 515 and 548 to 629 nm by adjusting the pH value when the ratios of Cu/Cd/Zn are fixed at 1 : 5 : 80 and 1 : 5 : 10, respectively. It is worth noting that under the condition of a constant Cu/Cd/Zn ratio, the UV-vis absorption spectra do not change with the fluorescence spectra, indicating that the band gap of QDs remains unchanged during the change of pH value. The photoluminescence (PL) quantum yield of the as-prepared QDs with yellow emission is up to 76%. The QDs also show excellent chemical stability after the deposition of the ZnS shell. Luminescent and flexible films are fabricated by combining Cu–Cd–Zn–S QDs with polyvinyl alcohol (PVA). The QD/PVA flexible hybrid films are successfully applied on top of a conventional blue InGaN chip for remote-type warm-white LEDs. As-fabricated warm-white LEDs exhibit a higher color rendering index (CRI) of about 89.2 and a correlated color temperature (CCT) of 4308 K.

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation.     

Counterion coupled (COCO) gemini surfactant capped Ag/Au alloy and Ag@Au core–shell nanoparticles for cancer therapy

Hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized. Ag/Au alloy and Ag@Au core–shell type NPs were prepared from Ag and Au with various ratios using the COCO gemini surfactant, 1,6-bis (N,N-hexadecyldimethylammonium) adipate (COCOGS), 16-6-16 as a stabilizer. The formation of the Ag/Au alloy and Ag@Au core–shell was confirmed by UV-visible absorption spectroscopy, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED) patterns. Depending on the composition of the Ag/Au alloy NPs, the λ_max values varied from 408 nm to 525 nm. FTIR measurements were used to evaluate the adsorption of the COCO gemini surfactant (16-6-16) on the Ag/Au alloy and Ag@Au core–shell surface. In this present work, we study how to achieve the stability and activity of the COCO gemini surfactant (16-6-16) capped Ag/Au alloy and Ag@Au core–shell NPs for developing novel anti-cancer agents by evaluating their potentials in the Hep-2 cell line model. Thus the developed core–shell NPs were possibly involved in inducing cytotoxicity followed by inhibition of cell proliferation to the cancer cells with apoptosis induction. The developed core–shell NPs might serve as highly applicable agents in the development of next-generation cancer chemotherapeutic agents.

In this work hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized and applied for anticancer evaluations and which is effectively involved in cancer cell apoptosis through DNA damage.     

X-ray powder diffraction to analyse bimetallic core–shell nanoparticles (gold and palladium; 7–8 nm)

A comparative X-ray powder diffraction study on poly(N-vinyl pyrrolidone) (PVP)-stabilized palladium and gold nanoparticles and bimetallic Pd–Au nanoparticles (both types of core–shell nanostructures) was performed. The average diameter of Au and Pd nanoparticles was 5 to 6 nm. The two types of core–shell particles had a core diameter of 5 to 6 nm and an overall diameter of 7 to 8 nm, i.e. a shell thickness of 1 to 2 nm. X-ray powder diffraction on a laboratory instrument was able to distinguish between a physical mixture of gold and palladium nanoparticles and bimetallic core–shell nanoparticles. It was also possible to separate the core from the shell in both kinds of bimetallic core–shell nanoparticles due to the different domain size and because it was known which metal was in the core and which was in the shell. The spherical particles were synthesized by reduction with glucose in aqueous media. After purification by multiple centrifugation steps, the particles were characterized with respect to their structural, colloid-chemical, and spectroscopic properties, i.e. particle size, morphology, and internal elemental distribution. Dynamic light scattering (DLS), differential centrifugal sedimentation (DCS), atomic absorption spectroscopy (AAS), ultraviolet-visible spectroscopy (UV-vis), high-angle annular dark field imaging (HAADF), and energy-dispersed X-ray spectroscopy (EDX) were applied for particle characterization.

A comparative X-ray powder diffraction study on poly(N-vinyl pyrrolidone) (PVP)-stabilized palladium and gold nanoparticles and bimetallic Pd–Au nanoparticles (both types of core–shell nanostructures) was performed.     

Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation. Initially, pure Ag and Cu NPs were produced individually using the laser ablation in liquid technique (with ∼50 femtosecond pulses at 800 nm) which was followed by laser irradiation of the mixed Ag and Cu NPs in equal volume. These Ag, Cu, and Ag–Cu NPs were characterised by UV-visible absorption, HRTEM and XRD techniques. The alloy formation was confirmed by the presence of a single surface plasmon resonance peak in absorption spectra and elemental mapping using FESEM techniques. Furthermore, the results from surface enhanced Raman scattering (SERS) studies performed for the methylene blue (MB) molecule suggested that Ag–Cu alloy NPs demonstrate a higher enhancement factor (EF) compared to pure Ag/Cu NPs. Additionally, SERS studies of Ag–Cu alloy NPs were implemented for the detection of explosive molecules such as picric acid (PA – 5 μM), ammonium nitrate (AN – 5 μM) and the dye molecule methylene blue (MB – 5 nM). These alloy NPs exhibited superiority in the detection of various analyte molecules with good reproducibility and high sensitivity with EFs in the range of 10⁴ to 10⁷.

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation.     

Monometallic and bimetallic Cu–Ag MOF/MCM-41 composites: structural characterization and catalytic activity

Monometallic and bimetallic MOF/MCM-41 composites (Cu, Ag and Cu–Ag) were synthesized via a solvothermal method. The synthesized composites were characterized by XRD, FTIR, SEM, EDX and BET surface area measurements. The acidity was determined through two techniques; potentiometric titration with n-butyl amine for determining the strength and the total number of acid sites and FTIR spectra of chemisorbed pyridine on the surface of MOFs for determining the type of acid sites (Brønsted and/or Lewis). All the prepared MOFs showed Lewis-acid sites and the higher acidity was observed for the bimetallic Cu–Ag MOF/MCM-41 composite. The catalytic activity was examined on the synthesis of 1-amidoalkyl-2-naphthol via the reaction of benzaldehyde, 2-naphthol and benzamide. The best yield (92.86%) was obtained in the least time (10 min) with a molar ratio 1.2 : 1.2 : 1.7 of benzaldehyde : β-naphthol : benzamide and 0.1 g bimetallic Cu–Ag MOF/MCM-41 composite under solvent-free conditions at 130 °C. Reuse of the catalysts showed that they could be used at least four times without any reduction in the catalytic activity.

Monometallic and bimetallic MOF/MCM-41 composites (Cu, Ag and Cu–Ag) were synthesized via a solvothermal method.     

Facile synthesis of high-performance SiO2@Au core–shell nanoparticles with high SERS activity

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs). The thicknesses of Au shells can be easily controlled, and the process of Au shell formation was completed within 5 min through sonication. The fabricated SiO₂@Au NPs with highly uniform size and SERS activity could be ideal SERS tags for SERS-based immunoassay.

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs).

The design and controlled fabrication of Au nanocomposites have attracted extensive attention because of their outstanding chemical and optical properties and wide applications in various fields, such as catalysis,¹ drug delivery,² photothermal cancer therapy,³ sensing,⁴ and surface-enhanced Raman scattering (SERS).⁵ However, small Au nanocomposites tend to aggregate, which seriously affects their stability and usability. The combination of silica nanoparticles (SiO₂ NPs) and Au shells provides a good alternative to Au nanocomposites.^6,7 These SiO₂ NPs are ideal core materials due to their high stability, easy preparation, uniform spherical shape, and large particle size range.^8,9Many synthesis methods have been explored for the fabrication of SiO₂@Au core–shell NPs; these methods include electroless plating,¹⁰ self-assembly,¹¹ layer-by-layer synthesis,¹² and seed growth.¹³ The seed growth method is the most commonly used to coat the Au shell on the surface of the SiO₂ core and involves two steps: deposition of nucleus seeds on the functionalized SiO₂ surface and Au shell growth. Although this method is beneficial for the synthesis of nanostructures with narrow size distribution, it exhibits two major shortcomings. First, the surface of SiO₂ NPs must be functionalized with various organosilanes containing amino (–NH₂) or mercapto (–SH) groups for adsorption or deposition of metal seeds on the SiO₂ NPs before subsequent growth of Au shells.^14,15 However, full surface amino/mercapto modification is often difficult to achieve; in this regard, dense metal seed layer formation on the surface of SiO₂ NPs cannot be achieved, eventually affecting the uniform and complete Au shell coating. Second, the formation of complete Au shell on the SiO₂ NPs is frequently achieved using a slow-growth approach through slow or multiple addition of HAuCl₄ to the seed-coated SiO₂ NPs suspension containing reducing agents.^16,17 The application of these slow-growth methods is restricted by its complex procedure and time-consuming preparation. Thus, a facile method must be developed for synthesis of Au coated SiO₂ NPs with controllable Au shell, good dispersibility, and fast preparation.In this work, we report a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs for the first time. Cationic polyethyleneimine (PEI) was used to form a cationic thin interlayer with numerous primary amine groups for easy adsorption of dense Au seeds on the silica surface and keeping the nanostructure stability during shell growth. Sonication was used instead of traditionally used mechanical stirring to shorten the reaction time. The entire reaction process for Au shell formation was completed within 5 min. Moreover, the thickness of the Au shell was easily controlled outside the silica cores of different sizes. To the best of our knowledge, the proposed method is the most convenient synthesis route for preparation of high-performance SiO₂@Au core–shell NPs to date. Our results further demonstrate that the fabricated SiO₂@Au NP could be an ideal SERS tag for SERS-based lateral flow immunoassay (LFA). The method was validated for detection of human immunoglobulin M (IgM) and showed a detection limit as low as 0.1 ng mL⁻¹. The details of the experiments including SiO₂@Au NPs preparation, SERS-based LFA strip preparation, SERS detection protocol, and sensitivity test were provided in the ESI^† section.The synthesis principle of monodisperse SiO₂@Au NPs is presented in Fig. 1a. SiO₂ NPs were first prepared by using a modified Stöber method as the core. The SiO₂ NPs were ultrasonically treated with PEI solution to form PEI-coated SiO₂ NPs (SiO₂@PEI). The positively charged PEI effectively attached to the negatively charged SiO₂ NPs and formed a stable polymer layer via electrostatic self-assembly. SiO₂–Au seed NPs were prepared by adsorbing small Au NPs (3–5 nm) on the PEI layer of SiO₂ NPs densely and firmly through covalent binding between the –NH₂ groups of PEI and Au NPs. Finally, monodisperse SiO₂@Au NPs were quickly obtained through the reduction of HAuCl₄ by hydroxylamine hydrochloride (NH₂OH·HCl) under the stabilization of PVP. The uniform Au shells outside the SiO₂ NPs were formed within 5 minutes through the isotropic growth of all Au seeds under sonication.Open in a separate windowFig. 1Synthesis principle of SiO₂@Au NPs (a). TEM images of (b) SiO₂ NPs, (c) SiO₂–Au seed NPs, (d) SiO₂@Au NPs and their corresponding elemental mapping in (g), (h), and (i) respectively. (e) HRTEM picture and (f) bright-field TEM image of a single SiO₂@Au NP.The morphology of the as-synthesized products in different stages were characterized through transmission electron microscopy (TEM). The as-prepared SiO₂ NPs were uniform in size and had a diameter of approximately 140 nm (Fig. 1b). After coating the SiO₂@PEI NPs with Au seeds, many small seeds homogeneously adhered to the surface of the silica core (Fig. 1c). The dense Au seeds acted as randomly oriented crystalline sites for subsequent seed-mediated growth of the Au shell. Fig. 1d and e show the low- and high-magnification TEM images of the final SiO₂@Au core–shell NPs, respectively. Continuous and rough edges were detected around the SiO₂@Au NPs. The HRTEM image (Fig. 1e) indicated that large adjacent Au NPs covered the entire surface of the SiO₂ NPs, forming a complete and rough Au shell. The average particle size increased from 140 nm to 190 nm after the Au shell formation, indicating that the thickness of the Au shell was approximately 25 nm. Additionally, the SEM images (Fig. S1^†) showed that the SiO₂@Au NPs were successfully fabricated on a large scale and exhibited a rough surface and uniform size. The elemental composition of SiO₂@Au NPs was also confirmed through X-ray mapping (Fig. 1f–i). The results indicated that a layer of Au shell was uniformly coated on the surface of the SiO₂ NPs. The zeta potentials of SiO₂, SiO₂@PEI, SiO₂–Au seeds, and SiO₂@Au NPs in aqueous solution were found to be −46.7, +41.9, −7.4, and −21.1 mV, respectively (Fig. S2^†). The significant change in the zeta potential revealed the successive completion of PEI coating, Au seed adsorption, and Au shell formation. Fig. 2a shows the typical XRD patterns of the as-synthesized SiO₂–Au seed (blue line) and SiO₂@Au NPs (red line). The specific XRD pattern of Au is characterized by five peaks positioned at 2θ values of 38.3°, 44.3°, 64.5°, 77.4°, and 81.6°, which correspond to the reflections of the (111), (200), (220), (311), and (222) crystalline planes of Au (JCPDS no. 04-0784), respectively.^18,19 The intensity of the diffraction peaks of SiO₂@Au NPs increased when the Au shells were coated. No peaks of SiO₂ and PEI were detected in the XRD pattern because of their amorphous form.²⁰Open in a separate windowFig. 2Typical XRD patterns (a) and UV-visible spectra (b) of the as-synthesized products. Fig. 2b illustrates the UV-vis spectra of the as-synthesized products dispersed in deionized water in different stages. SiO₂ and SiO₂@PEI NPs had no obvious absorption peaks in the UV-vis spectra (curves a and b). SiO₂–Au seed NPs displayed a clear absorption peak at about 568 nm (curve c), which confirms the formation of the Au seed layer onto the surface of SiO₂ NPs. As the Au shell formed, the UV-vis spectral peak obviously red-shifted, and the intensity increased significantly (curve d). This result could be due to the strong interaction between and the coupling of the large adjacent Au NPs of the Au shells outside the SiO₂ NPs.²¹The strategy for Au shell formation is essentially seed-mediated growth. Thus, the surface morphology of SiO₂@Au NPs can be easily controlled by adjusting the Au³⁺ concentration by using a constant amount of SiO₂–Au seed. Fig. 3a–d shows the representative TEM images of SiO₂@Au NPs synthesized with different concentrations of HAuCl₄ while the other parameters remained constant. As the concentration of the HAuCl₄ increased from 0.01 mM to 0.04 mM, the Au seeds absorbed outside the SiO₂ NPs gradually increased in size and finally intersected with each other and formed a continuous and Au shell of a different thickness.Open in a separate windowFig. 3TEM images of SiO₂@Au NPs synthesized with different HAuCl₄ concentrations: (a–d) 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄. (e) UV-vis spectra of SiO₂@Au synthesized with different HAuCl₄ concentrations: curves (a–e) 0, 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄ and the corresponding Raman spectra of DTNB (f). Fig. 3e shows the UV-vis spectra of the synthesized SiO₂–Au seed and SiO₂@Au NPs with different Au shell thicknesses. The absorption peak of the obtained products red shifted gradually from 568 nm to 700 nm, and the peak width became broader with increasing concentration of HAuCl₄. Thus, the absorption peak of SiO₂@Au NPs can also reflect the formation and thickness of the Au shell. Fig. 3f shows the SERS activity of SiO₂@Au NPs prepared with different HAuCl₄ concentrations. 5,5-Dithiobis-(2-nitrobenzoic acid) (DTNB) was used as Raman molecule because it contains a double sulfur bond, which can be chemically coupled to the Au shell to form Au–S chemical bond and could produce strong and concise SERS peaks located at 1062, 1148, 1331, and 1556 cm⁻¹.^22,23 Moreover, DTNB molecules can provide free carboxyl groups as sites to conjugate antibodies.²⁴ As shown in the Raman spectra in Fig. 3f, the SiO₂–Au seed showed fairly weak SERS ability (spectra a), whereas the SiO₂@Au NPs exhibited gradually enhanced SERS activity as the HAuCl₄ concentration increased (spectra b–d). However, the overgrowth of the Au shell decreased the SERS activity of SiO₂@Au NPs (spectra e). This phenomenon could be attributed to the fully continuous Au shell formation, which reduced the nanogaps and hot spots on the surface of SiO₂@Au NPs. Hence, we chose SiO₂@Au NPs prepared with 0.02 mM HAuCl₄ as the optimal material for SERS application because of their nearly complete Au shell and optimal enhancement effect.PEI can be self-assembled on the surface of SiO₂ NPs of any size under sonication conditions. Thus, our proposed PEI-assisted seed growth method is a general route for preparing monodisperse SiO₂@Au core–shell particles with different sizes, ranging from nanoscale to microscale levels. Fig. 4a–c shows the TEM images of single SiO₂–Au seed NPs with different sizes (70–300 nm), and Fig. 4d–f clearly shows their corresponding fabricated SiO₂@Au NPs, respectively. The TEM images of multiple SiO₂@Au NPs with different sizes are displayed in Fig. S3.^† All of the obtained SiO₂@Au NPs possess homogeneous nanostructure, uniform Au nanoshell, and good dispersity. We further examine the dependence of SERS activity on the SiO₂@Au NPs size up to 350 nm. Fig. S4^† shows a set of SERS spectra of DTNB (10⁻⁵ M) adsorbed on the SiO₂@Au NPs of different sizes. The SERS intensity presented in the figure is the average intensity from 10 spots for each sample. Obviously, all the SiO₂@Au NPs exhibited excellent SERS abilities, and the signal intensities were gradually enhanced as the particle size increased. In fact, the Au shells of SiO₂@Au NPs were made of large sized Au NPs. This experimental result indicates that the larger the size of the Au NPs of shell, the higher the SERS activity achieved.Open in a separate windowFig. 4(a–c) TEM images of single SiO₂–Au seed with different sizes: (a) 70, (b) 150, and (c) 300 nm and their corresponding fabricated SiO₂@Au NPs (d), (e), and (f), respectively.For the determination of the SERS sensitivity of the 80 nm SiO₂@Au NPs, a series of DTNB ethanol solution (with concentration ranging from 10⁻⁴ M to 10⁻¹¹ M) was prepared. Each tube of DTNB solution was mixed with 10 μL of SiO₂@Au NPs (1 mg mL⁻¹) and sonicated for 1 h. After separation and washing, the final precipitate was dropped on a Si chip and analyzed with Raman signals. The spectra and calibration curve of DTNB absorbed on the SiO₂@Au NPs are shown in Fig. 5a and b, respectively. The SERS signal significantly decreased as the concentration of DTNB decreased, and the main Raman peak at 1331 cm⁻¹ remained evident at DTNB concentrations as low as 10⁻¹⁰ M. Thus, the limit of detection (LOD) of DTNB is 10⁻¹⁰ M. These results indicate that the SiO₂@Au NPs have good potential to be active SERS substrate for greatly enhancing the SERS signal of molecules adsorbed on them.Open in a separate windowFig. 5(a) SERS spectra of DTNB measured with different concentrations on the SiO₂@Au NPs. (b) Calibration curve for DTNB at a concentration range of 10⁻⁴ M to 10⁻¹¹ M obtained using SERS intensity at 1331 cm⁻¹. The error bars represent the standard deviations from five measurements.Upon modification with Raman report molecules and detection antibodies, the monodisperse SiO₂@Au NPs must be efficient SERS tags for highly reproducible SERS immunoassays due to the integration of high SERS activity of the Au nanoshell and the homogeneity and stability of SiO₂ NPs (Fig. 6a). SERS-based LFA strip is a recently reported analytical technique to overcome the shortcomings of conventional lateral flow assay, such as poor sensitivity and semiquantitative ability on the basis of colorimetric analysis.^25–27 The fundamental principle of SERS-based strip is the use of functional SERS tags instead of Au NPs. High-sensitivity and quantitative detection can be achieved by Raman spectroscopy because the intensity of the SERS signal is directly proportional to the number of SERS tags on the test line.Open in a separate windowFig. 6(a) Synthesis route for SiO₂@Au SERS tags. (b) Schematic of SERS-based LFA strips for quantitative detection of human IgM. Fig. 6b represents the operating principle of the monodisperse SiO₂@Au NPs (80 nm) based SERS-LFA strip. Human IgM was selected as the model target antigen to explore the sensitivity of the proposed method. The representative SERS-LFA strip is composed of a sample loading pad, a conjugate pad, a NC membrane containing test line and control line, and an absorption pad. In our system, goat anti-human IgM antibody-labeled SiO₂@Au/DTNB NPs were dispensed onto the glass fiber paper as the conjugate pad, and the goat anti-human IgM antibody and donkey anti-goat immunoglobulin G (IgG) antibody were dispensed onto the NC membrane to form the test line and control line, respectively. When the sample solution containing the target human IgM passed through the conjugation pad, immunocomplexes (human IgM/SERS tags) were formed and continued migrating along the NC membrane until they reach the test line where they were captured by the previously immobilized anti-human IgM antibodies. Excess antibody-conjugated SiO₂@Au tags continued to migrate to the control line and were immobilized by the donkey anti-goat IgG antibody. Consequently, two dark bands appeared in the presence of the target human IgM (positive), whereas only the control line turned to a dark band in the absence of human IgM (negative). Quantitative detection of human IgM could be realized by detecting the SERS signal on the test line.Human IgM was diluted within 10 000 ng mL⁻¹ to 0.1 ng mL⁻¹ as the sample solution, and PBST solution (10 mM PBS, 0.05% Tween-20) was used as blank control. As shown in Fig. 7a, the color of SERS tags captured by the test line was visualized and gradually decreased with decreasing human IgM concentration. The LOD of colorimetric method for detection of human IgM was found to be 10 ng mL⁻¹. Quantitative analysis was also conducted by measuring the characteristic Raman signals of the SERS tags on the test lines, and the Raman spectra are displayed in Fig. 7b. The Raman spectra were analyzed by plotting the intensity at 1331 cm⁻¹ of DTNB as a function of the logarithm of the target human IgM concentration to generate a calibration curve (Fig. 7c). The LOD of the SERS-LFA strips based on the SiO₂@Au tags is 0.1 ng mL⁻¹, which was calculated as a 3 : 1 threshold ratio with respect to the blank control measurement. Using SiO₂@Au tags-based SERS LFA strip offers a 100-fold improvement in the detection limit compared with colorimetric analysis. Basing on these results, we demonstrated the high efficiency and great potential of monodisperse SiO₂@Au NPs as suitable SERS tags for SERS-based LFA strips. The specificity of the SERS-LFA strips was tested by a high concentration (1 μg mL⁻¹) of other proteins including human IgG and BSA. Fig. S5^† shows the result of the specificity test. IgG and BSA did not show significant interference signals both in visualization and Raman spectrum analyses, whereas 100 ng mL⁻¹ human IgM exhibited a strong signal. Hence, the SiO₂@Au tags-based SERS-LFA strip has good selectivity.Open in a separate windowFig. 7(a) Photographs of SERS-based LFA strips in the presence of different concentrations of human IgM. (b) SERS spectra measured in the corresponding test lines. (c) Plot of the Raman intensity at 1331 cm⁻¹ as a function of the logarithmic concentration of human IgM. The error bars represent the standard deviations from five measurements.In summary, this work proposes a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs with a complete Au shell. This method is a general route for preparing SiO₂@Au particles with sizes ranging from nanoscale to microscale levels. High-performance SiO₂@Au NPs were obtained from the intermediate product (SiO₂–Au seed) within 5 min through sonication. The obtained SiO₂@Au NPs were highly uniform in size and shape and exhibited satisfactory SERS activity. Hence, these NPs could be ideal SERS tags for various SERS based immunoassays. The small SiO₂@Au NPs (80 nm) with light weight and good dispersibility were also successfully applied to SERS-based LFA strip for human IgM rapid detection, with limit of detection as low as 0.1 ng mL⁻¹. We expect that high-performance SiO₂@Au NPs SERS tags can be used for actual detection.     

Colorimetric sensing of glucose and GSH using core–shell Cu/Au nanoparticles with peroxidase mimicking activity

The catalytic properties of bimetallic nanoparticles have been widely studied by researchers in many fields. In this paper, core–shell Cu/Au nanoparticles (Cu/Au NPs) were synthesized by a simple and mild one-pot method, and their peroxidase activity was proved by catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) with color change to blue. The change of solution color and absorbance strongly depends on the concentration of H₂O₂, so it can be used for direct detection of H₂O₂ and indirect detection of glucose. What''s more, GSH can efficiently react with the hydroxyl radicals from H₂O₂ catalyzed by core–shell Cu/Au NPs to inhibit the production of ox-TMB. Thus, the concentration of GSH can be determined by the decrease in the absorbance of the solution at 652 nm. The results showed that our proposed strategy had good detection range and detection limit for the detection of glucose and GSH. This method has been used in the detection of practical samples and has great application potential in environmental monitoring and clinical diagnosis.

Core–shell Cu/Au nanoparticles were synthesized by a one pot method, their peroxidase activity was proved by catalysing the oxidation of 3,3′,5,5′-tetramethylbenzidine with colour change to blue. Results showed a good range and limit for the detection of glucose and GSH.     

Preparation of biomimetic non-iridescent structural color based on polystyrene–polycaffeic acid core–shell nanospheres

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties. In this work, caffeic acid was introduced into the preparation of photonic band gap materials. By controlling the variables, a reasonable preparation method of polystyrene (PS) @polycaffeic (PCA)–Cu(ii) core–shell microspheres was achieved: 1 mmol L⁻¹ cupric chloride anhydrous (CuCl₂), 3 mmol L⁻¹ sodium perborate tetrahydrate (NaBO₃·4H₂O), 2 mmol L⁻¹ CA and 2 g L⁻¹ polystyrene (PS) were reacted at 50 °C for 10 min to prepare PS@PCA–Cu(ii) core–shell microspheres through rapid oxidative polymerization of CA coated PS of different particle diameters. The amorphous photonic crystal structure was self-assembled through thermal assisted-gravity sedimentation, resulting in structural color nanomaterials with soft and uniform color, no angle dependence, stable mechanical fastness and excellent UV resistance.

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties.