Fabrication of Au@Ag core-shell nanoparticles using polyelectrolyte multilayers as nanoreactors

Catalytic Membrane Nanoreactor with Cu-Agx Bimetallic Nanoparticles Immobilized in Membrane Pores for Enhanced Catalytic Performance

A catalytic membrane nanoreactor (CMNR) with Cu-Ag_x (where x is the millimolar concentration of AgNO₃) bimetallic catalysts immobilized in membrane pores has been fabricated via coupling flowing synthesis and replacement reaction. Surface characterization by transmission electron microscopy (TEM) gives obvious evidence of the formation of Cu-Ag bimetallic core-shell nanostructures with Ag islands deposited on the Cu core metal. An apparent high shift phenomenon for the Cu element and a low shift phenomenon for the Ag element was determined by X-ray photoelectron spectroscopy (XPS), indicating a close interaction with the transfer of electron density from the Cu atom to the Ag atom. The hydrogenation catalysis of p-nitrophenol (p-NP) was tested to evaluate the catalytic performance. During the catalytic process, the Cu core acts as an electron-deficient site to adsorb and activate the -NO₂ group for p-NP, and the Ag shell is beneficial for enhancing active H spilling to the Cu surface and then performing hydrogenation. A volcano-shaped apparent reaction rate constant can be achieved, which rises initially with the increasing Ag content and subsequently drops with a further increase in the Ag content. The highest value of 1071 min^-1 can be achieved for CMNR immobilized with Cu-Ag₂ owing to the suitable adsorption activation behavior and the best hydrogen spillover behavior.

Influence of pH on interaction of silver nanoparticles - protein: Analyses by spectroscopic and thermodynamic ideology

To investigate the interaction between bovine serum albumin (BSA) and silver nanoparticles (AgNPs) at five different pHs (below (3.0 and 4.0), above (7.4 and 9.2) and at the isoelectric point (4.7) of BSA) by spectroscopic (viz., UV-vis, fluorescence, circular dichroism (CD)), microscopic (viz., atomic force microscopy (AFM), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM)) and thermodynamic (viz., isothermal titration calorimetry (ITC)) methods. The fluorescence quenching spectra provided binding constants via Stern-Volmer plot, quenching constant (K_sv) and rate constant (K_q) were calculated. From the CD spectra, it is clear that the α-helix decreases by increasing the AgNP's concentration. However, at isoelectric point (pH = 4.7), BSA shows more helicity in the presence of AgNPs, which indicates that the structures of BSA become more ordered and stable, and aggregation occurs at strong acidic (3.0), and basic medium (9.2) Fluorescence spectra also indicate the aggregation of the protein at strong acidic (pH = 3.0) and basic medium (pH = 9.2). Furthermore, the morphological and topographical evolute ion upon the interaction was examined using TEM, FESEM, and AFM. The studies conclude the effect of the pH in the medium and behavior of AgNPs with BSA by using different spectroscopic and microscopic techniques.

Synergistic effect of Pd content and polyelectrolyte multilayer structure on nitrobenzene hydrogenation in a microreactor

In this study, we proposed a Pd–polyelectrolyte multilayer (PEM) hybrid film grafted on the polydopamine coated interior wall of a microreactor for nitrobenzene hydrogenation. Here, Pd nanoparticles were in situ synthesized in the PEMs consisting of poly(diallyldimethylammonium chloride) and poly(styrene sulfonate) via a two-stage ion-exchange and reduction process. The preparation process was monitored by UV-vis spectroscopy, which confirmed the formation of Pd in the PEM film. In addition, SEM and ICP-OES results indicated that the Pd content in the PEM film could be controlled by the number of the ion exchange and reduction cycles. Experimental results also showed that the prepared Pd–PEM hybrid film was active for the hydrogenation of nitrobenzene. The microreactor with the Pd–PEM hybrid film via multiple times had the increased catalyst loading, leading to a high yield of aniline and much better durability. In addition, it was also found that the NaCl concentration in the polyelectrolyte solution could affect the structure of the PEM film and therefore the Pd loading and catalytic performance.

In this study, we developed a Pd–PEMs hybrid film grafted on the polydopamine coated interior wall of a microreactor for nitrobenzene hydrogenation.

Colorimetric paper sensor for sensitive detection of explosive nitroaromatics based on Au@Ag nanoparticles

Rapid, reliable, onsite approaches for detection trace level of trinitrotoluene (TNT) is a pressing necessity for both homeland security and environmental protection. To this end, hydrophilic amine(-NH₂) protected Au@Ag nanoparticles (NPs) were developed and fabricated as colorimetric paper sensor for delicate detection of TNT. The as-developed nanoprobe selectively reacts with TNT through classic Meisenheimer complex formation by means of charge transfer process from an electron-rich NH₂ group of β-cysteamine to an electron-deficient nitro group on TNT. Due to the absence of this particular interaction of other nitroaromatics, the proposed probe is highly selective for TNT detection with a better linear range (0-20 μg/mL) and limit of detection (LOD) of 0.35 μg/mL. The present work provides a novel and facile strategy to fabricate colorimetric paper sensors with rapid and selective recognition ability for label free analysis of TNT.

Colorimetric and visual detection of mercury(II) based on the suppression of the interaction of dithiothreitol with agar-stabilized silver-coated gold nanoparticles

A colorimetric and visual method is described for the determination of mercury(II) ion. A gel consisting of agar-stabilized silver-coated gold nanoparticles (Au@Ag NPs) was prepared. The reaction with dithiothreitol (DTT) via thiol-Ag chemistry results in an orange to purple color change of the gel. However, in the presence of Hg(II), the reaction of DTT with the silver shells is suppressed due to the strong thiophilicity of Hg(II). The color of the gel changes from purple to red to orange in the presence of increasing concentrations of Hg(II). The Au@Ag NPs therefore are a viable optical probe for Hg(II) which can be detected in concentration as low as 78 nM via dual-wavelength ratiometric absorbance (A₃₉₀/A₅₂₀), and at 1 μM levels with bare eyes. The use of agar as a support is mandatory to prevent the aggregation of the NPs and also improves selectivity. The method was applied to the analysis of spiked samples, and recoveries ranged between 96.3 and 104%. The assay is easy, inexpensive, and in our perception represents an attractive tool for on-site visual detection of Hg(II). Graphical abstract Schematic of the assay. With increasing concentrations of Hg(II), the oxidative etching of silver shells caused by dithiothreitol (DTT) is gradually inhibited, and the color of agar-stabilized Au@Ag NP gel varies from purple to red, and finally to orange. This can be used for visual detection of Hg(II).

Catalytic Membrane Reactor Immobilized with Alloy Nanoparticle-Loaded Protein Fibrils for Continuous Reduction of 4-Nitrophenol

A catalytic membrane reactor, which contains a membrane matrix and a catalytic film of alloy nanoparticle-loaded β-lactoglobulin fibrils (NPs@β-LGF), was developed for the continuous-flow reduction of 4-nitrophenol (4-NP). The Cu-Ag and Cu-Ag-Au alloy NPs were synthesized using β-LGF as a scaffold and stabilizing agent. In this process, the Cu nanoclusters were formed in the initial stage and were able to promote the synthesis of Ag⁰, which acts as a reducing agent for the rapid formation of Au⁰. Furthermore, a catalytic membrane reactor was constructed by depositing the NPs@β-LGFs on a membrane matrix. The catalytic activity of the Cu-Ag-Au alloy NPs was higher than that of the Cu-Ag alloy NPs, using the reduction of 4-NP to 4-AP as a model reaction. The observed rate constant in the continuous-flow system is also higher than that in the batch system. In addition, these catalytic membrane reactors had good operating stability and antibacterial activity.

The effect of guanidinium functionalization on the structural properties and anion affinity of polyelectrolyte multilayers

Poly(allylamine hydrochloride) (PAH) is chemically functionalized with guanidinium (Gu) moieties in water at room temperature. The resulting PAH-Gu is used to prepare polyelectrolyte multilayers (PEMs) with poly(sodium 4-styrene sulfonate) (PSS) via layer-by-layer deposition. The polyelectrolyte (PE) adsorption processes are monitored real-time by optical reflectometry and a quartz crystal microbalance with dissipation monitoring (QCM-D). Compared to the reference PSS/PAH PEMs, the PSS/PAH-Gu PEMs show a lower amount of deposited PE materials, lower wet thickness, higher stability under alkaline conditions and higher rigidity. These differences are rationalized by the additional Gu-SO3(-) interactions, also affecting the conformation of the PE chains in the PEM. The interactions between the PEMs and various sodium salts (NaCl, NaNO3, Na2SO4 and NaH2PO4) are also monitored using QCM-D. From the changes in the frequency, dissipation responses and supportive Reflection Absorption Infrared Spectroscopy it is concluded that Gu-functionalized PEMs absorb more H2PO4(-) compared to the Gu-free reference PEMs. This can be understood by strong interactions between Gu and H2PO4(-), the differences in the anion hydration energy and the anion valency. It is anticipated that compounds like the presented Gu-functionalized PE may facilitate the further development of H2PO4(-) sensors and ion separation/recovery systems.

The Ag shell thickness effect of Au@Ag@SiO2 core–shell nanoparticles on the optoelectronic performance of dye sensitized solar cells

The LSPR effect of Au@Ag@SiO₂ core–shell–ultra-thin shell nanoparticles is finely tailored and tuned by varying the Ag shell thickness. The growth of silver shell onto Au NPs led to color changes from different tones of red to orange.

A colorimetric agarose gel for formaldehyde measurement based on nanotechnology involving Tollens reaction

Gold nanoparticles (Au NPs) coupled with Tollens reagent were used for measuring formaldehyde. Au@Ag core-shell NPs were formed along with distinct color changes from pink to deep yellow. This colorimetric system was further immobilized into an agarose gel, which was used for monitoring of gaseous formaldehyde.

Fabrication of Au@Ag core/shell nanoparticles decorated TiO2 hollow structure for efficient light-harvesting in dye-sensitized solar cells

Improving the light-harvesting properties of photoanodes is promising way to enhance the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). We synthesized Au@Ag core/shell nanoparticles decorated TiO2 hollow nanoparticles (Au@Ag/TiO2 HNPs) via sol-gel reaction and chemical deposition. The Au@Ag/TiO2 HNPs exhibited multifunctions from Au@Ag core/shell NPs (Au@Ag CSNPs) and TiO2 hollow nanoparticles (TiO2 HNPs). These Au@Ag CSNPs exhibited strong and broadened localized surface plasmon resonance (LSPR), together with a large specific surface area of 129 m(2) g(-1), light scattering effect, and facile oxidation-reduction reaction of electrolyte from TiO2 HNPs, which resulted in enhancement of the light harvesting. The optimum PCE of η = 9.7% was achieved for the DSSCs using photoanode materials based on TiO2 HNPs containing Au@Ag/TiO2 HNPs (0.2 wt % Au@Ag CSNPs with respect to TiO2 HNPs), which outperformed by 24% enhancement that of conventional photoanodes formed using P25 (η = 7.8%).

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.

Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing

We synthesize Au@Ag core/shell nanoparticles (NPs) using a Au NP assisted Tollens reaction. The as-synthesized NPs are used for the colorimetric cyanide sensing with a detection limit of 0.4 μM. The bimetallic NPs are immobilized into agarose gels as portable "test strips".

Incredible antibacterial activity of noble metal functionalized magnetic core-zeolitic shell nanostructures

Functionalized magnetic core-zeolitic shell nanostructures were prepared by hydrothermal and coprecipitation methods. The products were characterized by Vibrating Sample Magnetometer (VSM), X-ray powder diffraction (XRD), Fourier Transform Infrared (FTIR) spectra, nitrogen adsorption-desorption isotherms, and Transmission Electron Microscopy (TEM). The growth of mordenite nanoparticles on the surface of silica coated nickel ferrite nanoparticles in the presence of organic templates was also confirmed. Antibacterial activity of the prepared nanostructures was investigated by the inactivation of Escherichia coli as a gram negative bacterium. A new mechanism was proposed for inactivation of E. coli over the prepared samples. In addition, the Minimum Inhibitory Concentration (MIC) and reuse ability were studied. TEM images of the destroyed cell wall after the treatment time were performed to illustrate the inactivation mechanism. According to the experimental results, the core-shell nanostructures which were modified by organic agents and then functionalized with noble metal nanoparticles were the most active. The interaction of the noble metals with the organic components on the surface of nanostructures was studied theoretically and the obtained results were used to interpret the experimental results.

Synthesis and visible photocatalytic activities of a Au@Ag@ZnO triple layer core–shell nanostructure

Visible light induced photocatalytic mechanism of Au@Ag@ZnO core–shell nanostructure and kinetics of methyl orange (MO) photo degradation by ZnO, Au@ZnO, Au@Ag@ZnO and TiO₂.

Rapid hydrogenation: perfect quasi architecture (Ag@SiO2NPs) as a substrate for nitrophenol reduction

Spherical nanoparticles with core-frame architecture are a viable route to combine multiple functionalities on a nanoscopic scale.

Size tunable Au@Ag core-shell nanoparticles: synthesis and surface-enhanced Raman scattering properties

We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core-shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core-shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core-shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core-shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core-shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core-shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

Synthesis of hollow Ag-Au bimetallic nanoparticles in polyelectrolyte multilayers

Ag nanoparticles of ~20 nm size and rather uniform size distribution were synthesized in polyelectrolyte multilayers (PEMs) via an ion-exchange/reduction process in two stages (seeding and growth), which were used as sacrificial templates to fabricate Ag-Au bimetallic hollow nanoparticles via galvanic replacement reaction. The reaction process was monitored by UV-vis spectroscopy. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy, which confirmed the formation of hollow Ag-Au bimetallic nanoparticles. UV-vis absorbance spectroscopy and TEM results indicated that both size and optical properties of the Ag nanoparticles in the PEM can be controlled by manipulating ion content in the PEM and the number of the ion-exchange/reduction cycle, whereas that of Ag-Au bimetallic nanoparticles were dependent on size of the Ag templates and the replacement reaction kinetics. The hollow Ag-Au bimetallic nanoparticles exhibited a significant red shift in the surface plasmon resonance to the near-infrared region. The strategy enables facile preparation of hollow bimetallic nanoparticles in situ in polymer matrixes.