Growth Of Ag Shells Research Articles

Plasma-assisted synthesis of gold-silver core-shell nanoparticles and their enhanced catalytic dye degradation and surface enhanced Raman spectroscopy performance

The plasma-liquid interaction (PLI) technique has been employed as a green approach for the synthesis of gold-silver core-shell nanoparticles (Au@AgNPs) using a two-step method. In the first step, the Au seed particles were grown by the PLI technique. Subsequently, the formation of Ag shell was achieved by adding AgNO3 to the solution containing Au seeds and placing it under the atmospheric pressure plasma. The growth of Ag shell on Au seeds was optimized and secondary nucleation was avoided by tuning the discharge current, precursor concentration, and stabilizer (d-fructose) concentration. The shell thickness of Ag was also changed by changing the concentration of AgNO3 in the solution. UV-Vis spectroscopy, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were used to analyze the synthesized nanoparticles. Furthermore, the particles were employed for catalysis and surface enhanced Raman spectroscopy (SERS)-based molecular trace detection. The catalytic efficiency of the NPs was investigated for the reduction of four molecules, i.e., 4-nitrophenol, methyl orange, Congo red, and rhodamine B. The SERS-based trace detection was studied for rhodamine 6G using the substrates of synthesized NPs. In both cases, core-shell nanoparticles showed far superior performance compared to the seed Au nanoparticles.

Particle Attachment Growth of Au@Ag Core–Shell Nanocuboids

In the templated synthesis of colloidal core-shell nanoparticles, the monomer attachment growth mechanism has been widely accepted to describe the growth process of shells. In this work, by using advanced transmission electron microscope techniques, we directly observe two alternative particle attachment growth pathways that dominate the growth of Au@Ag core-shell nanocuboids. One pathway involves the in situ reduction of AgCl nanoparticles attached to Au nanorods and the subsequent epitaxial growth of the Ag shell. The other pathway involves the adherence of Ag-AgCl Janus nanoparticles to Au nanorods with random orientations, followed by nanoparticle redispersion and the resulting formation of epitaxial Ag shells on the Au nanorods. The particle-mediated growth of Ag shells is accompanied by the redispersion of surface atoms, tending to form a uniform structure. The validation of the particle attachment growth processes at the atomic scale provides a new mechanistic understanding of core-shell nanostructure synthesis.

Plasmonic mesoporous core-shell Ag-Au@TiO2 photoanodes for efficient light harvesting in dye sensitized solar cells

In this study, plasmonic mesoporous core-shell Ag-Au@TiO2 nanocomposites were synthesized by seed-mediated growth of Ag shell onto Au decorated TiO2 and utilized as photoanodes to enhance the power conversion efficiency (PCE) of dye-sensitized solar cells. The enhanced preserved mesoporous characteristics of photoanodes, together with strong metal-dielectric interfacial interaction between Ag-Au shell, remarkably improved electron transfer ability onto TiO2, which caused to the enhancement of the light harvesting (η = 7.41% using Ag-Au@TiO2) 125% higher than bare TiO2 (η = 5.91%). These superior PCE value was mainly due to the shifting of the Fermi level close to the TiO2 conduction band thanks to the broad band localized surface plasmon resonance (LSPR) properties between Ag-Au and TiO2 interfaces. The significant outperforming in enhanced solar to electrical energy conversion efficiency using low amount of Ag-Au@TiO2 photoanode (0.0125 wt%) demonstrated a relative low-cost solar device from the economic perspective of renewable energy fabrication resources.

Enhanced colorimetric detection of norovirus using in-situ growth of Ag shell on Au NPs

Norovirus (NoV) is a leading cause of acute gastroenteritis. The low infectious dose and environmental stability of NoV facilitate its effective transmission through a variety of modes such as food, water and person-to-person. The available enzyme-linked immunosorbent assay (ELISA) for NoV detection has low sensitivity due to the low catalytic activity of the peroxidase used, and thus, a reliable ultrasensitive bioassay is needed. In this study, we apply the enhanced peroxidase-like activity of silver ion-incorporated gold nanoparticles (Au/Ag NPs) in a colorimetric bioassay for NoV detection. NoV was captured by anti-NoV genogroup II antibodies, which were immobilized on the surface of a 96-well microtiter plate and formed a sandwich structure among anti-NoV Ab, NoV and Ab-Au NP. Then, Ag ion-containing hydroquinone was added to form Au/Ag core/shell NPs. When H2O2/3,3′,5,5′-tetramethylbenzidine (TMB) solution was added to the wells, Ag ions were liberated from the surface of Au/Ag NPs and enhanced the oxidation of TMB. These reactions enhanced the oxidation of TMB and developed an intense blue color. The Au/Ag NPs were shown to exhibit higher affinity and catalytic efficiency for H2O2 and higher catalytic velocity based on the kcat of up to 7-fold compared with Au NPs. The bioassay was then optimized to detect clinically isolated NoV using NoV-like particles (NoV-LPs). NoV-LPs were detected with a limit of detection of 10.8 pg/mL, corresponding to 1000- and 100-fold higher sensitivity compared to the gold-immunoassay and horseradish peroxidase-based ELISA, respectively. Clinically isolated NoV GII.4 and NoV GII.3 were detected in the range of 102–106 copies of viral RNA/mL fecal solution with a detection limit of 13.2 copies/mL fecal solution for NoV GII.4, equivalent to 132 copies of viral RNA/g feces and indicating significantly higher sensitivity compared to commercial immunoassay kits. This bioassay represents a workable detection assay for low concentrations of NoV that is applicable for early-stage diagnosis for public hygiene.

Unravelling the Mechanisms of Gold-Silver Core-Shell Nanostructure Formation by in Situ TEM Using an Advanced Liquid Cell Design.

The growth of silver shells on gold nanorods is investigated by in situ liquid cell transmission electron microscopy using an advanced liquid cell architecture. The design is based on microwells in which the liquid is confined between a thin Si3N4 membrane on one side and a few-layer graphene cap on the other side. A well-defined specimen thickness and an ultraflat cell top allow for the application of high-resolution TEM and the application of analytical TEM techniques on the same sample. The combination of high-resolution data with chemical information is validated by radically new insights into the growth of silver shells on cetrimonium bromide stabilized gold nanorods. It is shown that silver bromide particles already formed in the stock solution play an important role in the exchange of silver ions. The Ag shell growth can be directly correlated with the layer-by-layer dissolution of AgBr nanocrystals, which can be controlled by the electron flux density via distinctly generated chemical species in the solvent. The derived model framework is confirmed by in situ UV-vis absorption spectroscopy evaluating the blue shift in the longitudinal surface plasmon resonance of anisotropic NRs in a complementary batch experiment.

Quantifying Photothermal and Hot Charge Carrier Effects in Plasmon-Driven Nanoparticle Syntheses.

The excitation of localized surface plasmon resonances in Au and Ag colloids can be used to drive the synthesis of complex nanostructures, such as anisotropic prisms, bipyramids, and core@shell nanoparticles. Yet, after two decades of research, it is challenging to paint a complete picture of the mechanisms driving such light-induced chemical transformations. In particular, whereas the injection of hot charge carriers from the metal nanoparticles is usually proposed as the dominant mechanism, the contribution of plasmon-induced heating can often not be neglected. Here, we tackle this uncertainty and quantify the contribution of different activation mechanisms using a temperature-sensitive synthesis of Au@Ag core@shell nanoparticles. We compare the rate of Ag shell growth in the dark at different temperatures with the one under plasmon excitation with varying laser intensities. Our controlled illumination geometry, coupled to numerical modeling of light propagation and heat diffusion in the reaction volume, allows us to quantify both localized and collective heating effects and determine their contribution to the total growth rate of the nanoparticles. We find that nonthermal effects can be dominant, and their relative contribution depends on the fraction of nanoparticle suspension under irradiation. Understanding the mechanism of plasmon-activated chemistry at the surface of metal nanoparticles is of paramount importance for a wide range of applications, from the rational design of novel light-assisted nanoparticle syntheses to the development of plasmonic nanostructures for catalytic and therapeutic purposes.

A New Type of SERS Tags: Au@Ag Core/Shell Nanorods with Embedded Aromatic Molecules

SERS tags are a new class of nanoprobes consisting of metal nanoparticles with adsorbed Raman active molecules. The brightness of a single tag depends on the Raman cross section of the reporter molecules and on the enhancing properties of plasmonic particles. Here, nine types of composite AuNR(core, 81 × 25 nm)@R@Ag(shell, 14 nm) nanorods (NRs) with thiolated aromatic molecules (R, nine different molecules) embedded between metal layers were synthesized. The Raman efficiency of reporters was evaluated by measuring normal Raman spectra in ethanol. The surface-enhanced Raman scattering (SERS) was evaluated for molecules adsorbed on Au nanorods before and after Ag shell growth. The coverage of the functionalized Au nanorods with an Ag shell results in an increase in SERS enhancement factor by two orders of magnitude, from 2.5 × 104 for the initial Au NRs to 8.5 × 106 for composite particles. Finite-difference time-domain (FDTD) simulations showed that the ultrahigh enhancement of Raman scattering inside composite nanorods can be explained by the enhancement of the EM field at the boundary of metal layers. Thus, for fabricated composite particles, we have a specific variant of EM SERS enhancement at the boundary of Au/Ag metals, which is different from the local field enhancement near plasmonic particles.

Asymmetric growth of Au-core/Ag-shell nanorods with a strong octupolar plasmon resonance and an efficient second-harmonic generation

Colloidal Au-core/Ag-shell nanorods with an asymmetric transverse cross-section and a strong octupolar plasmon resonance are synthesized by the controlled growth of Ag shells on one side of the Au cores. A largely enhanced second harmonic generation (SHG) from these asymmetric core–shell nanorods is demonstrated for the first time by tuning the dipolar and the octupolar plasmon modes to make them resonant with the fundamental and harmonic frequencies, respectively. The SHG intensity of the Au–Ag nanorods with dual-frequency resonances is enhanced by 21 times compared to that of the bare Au nanorods. The co-existence of the dipolar, quadrupolar, and octupolar radiations in the SHG is revealed. Additionally, the cellular uptake of the Au–Ag nanorods is monitored and the evolution of the membrane bleb is successfully observed by the SHG imaging. Our observations provide a strategy for enhancing the SHG of the colloidal metal nanoparticles and can have applications in chemical process monitoring and biological sensing.

SiO2/TiO2/Ag multilayered microspheres: Preparation, characterization, and enhanced infrared radiation property

SiO2/TiO2/Ag core–shell multilayered microspheres were successfully synthesized by the combination of anatase of TiO2 modification on the surfaces of SiO2 spheres and subsequent Ag nanoparticles deposition and Ag shell growth with face-centered cubic (fcc) Ag. The composites were characterized by TEM, FT-IR, UV–vis, Raman spectroscopy and XRD, respectively. The infrared emissivity values during 8–14μm wavelengths of the composites were measured. The results revealed that TiO2 thin layers with the thickness of ∼10nm were coated onto the SiO2 spheres of ∼220nm in diameter. The thickness of the TiO2 layers was controlled by varying the amount of TBOT precursor. Homogeneous Ag nanoparticles of ∼20nm in size were successfully deposited by ultrasound on the surfaces of SiO2/TiO2 composites, followed by complete covering of Ag shell. The infrared emissivity value of the SiO2/TiO2 composites was decreased than that of pure SiO2. Moreover, the introduction of the Ag brought the remarkably lower infrared emissivity value of the SiO2/TiO2/Ag multilayered microspheres with the lowest value down to 0.424. Strong chemical effects in the interface of SiO2/TiO2 core–shell composites and high reflection performance of the metal Ag are two decisive factors for the improved infrared radiation performance of the SiO2/TiO2/Ag multilayered microspheres.

Growth of silver-coated gold nanoshells with enhanced linear and nonlinear optical responses

Silver-coated gold nanoshells with 1,4-BDT molecules as the spacer (Ag/BDT/Au) were synthesized on the surface of SiO2 nanospheres. The surface plasmon resonance of Au/SiO2 and Ag/BDT/Au/SiO2 nanoparticles with single and double shells were tuned by adjusting the thickness of Au and Ag nanoshells. The enhanced local field in the gap of Au and Ag shells is demonstrated by measuring Raman scattering and nonlinear refraction. The results show that the Raman intensity is enhanced by 17 times and the nonlinear refractive index is enhanced by 30 % due to the growth of Ag shells.

Syntheses of Au@PdAg and Au@PdAg@Ag Core–Shell Nanorods through Distortion-Induced Alloying between Pd Shells and Ag Atoms over Au Nanorods

Noble Au@PdAg and Au@PdAg@Ag core–shell nanorods (NRs) having PdAg alloy shells were synthesized using Au@Pd NRs as seeds. Their crystal structures and growth mechanisms were examined using data from TEM, TEM–EDS, XRD, and UV–Vis–NIR extinction spectroscopy. First, cuboid or dumbbell-type Au@Pd NR seeds were prepared using Au NRs as seeds. In the second step, when Ag+ ions were reduced over these seeds, cuboid and dumbbell-type Au core PdAg alloy shell NRs were formed with maximum Ag content of about 16% and 24%, respectively, after heating at 60 °C. The driving force of alloying between Pd shells and Ag atoms was attributed to the distortion of Pd layers over Au NRs based on peak shifts and broadening of XRD data. The higher Ag content in dumbbell-type Au@PdAg NRs was explained by the greater distortion of Pd shells over Au NRs. When the Ag content exceeds its maximum solubility in Pd shells, the third Ag shells were grown over Au@PdAg NRs. For cuboid Au@PdAg NRs, the crystal growth of Ag shells over cub...

Morphology control and temporal growth of a continuous silver shell on core–shell spheres

We report the synthesis of Fe@TiO2@Ag core–shell-structured spheres with an extremely continuous and complete Ag shell (at least 50 nm in thickness). In particular, we investigated the Ag shell growth on the Fe@TiO2 spheres using an electroless deposition technique based on electrostatic attraction. In order to induce a well-defined Ag shell surface, we controlled various synthesis variables and found that the concentration of trisodium citrate, the molar ratio of NaOH/AgNO3, and the aging time of ammoniacal Ag solution all played a critical role in the formation and growth of the Ag shell. We investigated, for the first time, the temporal growth of the Ag shell at the early stage of the Ag growth reaction to examine how Ag islands grow and coalesce when trisodium citrate is used. In addition, we also investigated the reflectance of the resultant particles to evaluate the degree of continuity of the Ag shell.

Ag shell morphology on Au nanorod core: role of Ag precursor complex

Multicomponent nanostructures have substantial potential in a wide range of applications due to their unique chemical, optical, and magnetic properties, which arise from their architecture, composition, and associated heterojunctions. Colloidal approaches provide synthetic routes to fabricate these multicomponent metal nanostructures; however the complexity of processing parameters many times limits reproducibility and minimization of undesired secondary structures. By comparing the architecture across a diverse set of processing parameters for Ag shell growth on Au nanorods (NRs), we demonstrate that the composition and size of the in situ formed Ag precursor complex are the most critical contribution to the growth mechanism. Systematic control of these characteristics by varying hexadecyltrimethylammonium bromide (CTAB) concentration and aging time of the precursor-template solution, as well as pH and temperature of the final reaction solution, enables reproducible and continuous variation of the Ag shell architecture from conformal to rectilinear, and provides a unifying view of prior literature reports.

The shape evolution of gold seeds and gold@silver core–shell nanostructures

We report the seed-dependent shape evolution of gold@silver (Au@Ag) core–shellnanostructures with various morphologies through using pre-existing Au nanocrystals asnuclei in a polyvinylpyrrolidone (PVP)-assisted polyol reduction process. Aunanocrystalline seeds with different shapes such as cube, truncated-octahedron, octahedron,twinned hexagon and triangle, five-twinned decahedron and nanorod are firstly synthesizedby refluxing a 1,5-pentanediol solution containing Au precursors in the presence of PVP.The Au seeds obtained in this way then serve as the nuclei for further epitaxial growth ofAg shells by using Ag precursors via the same route. Scanning transmission electronmicroscope (STEM) characterization of the products obtained demonstrates thatthe morphological evolution of the Ag shells depends completely on the shapesof the Au seeds that are used. We have observed that the Au@Ag core–shellnanostructures formed with various regular shapes such as cube, bi-triangle, and nanorodwith five-twinned cross section, are mostly surrounded by {100}-type Ag crystallinefacets. Our findings provide new evidence and clear evolution routines from the Aucores with well-defined shapes to the corresponding Ag shells for the Au@Agcore–shell nanostructures by the family of the PVP-assisted polyol reduction methods.