Single-Step Synthesis and Surface Plasmons of Bismuth-Coated Spherical to Hexagonal Silver Nanoparticles in Dichroic Ag:Bismuth Glass Nanocomposites

Here, we report for the first time the synthesis of bismuth-coated silver nanoparticles in dichroic bismuth glass nanocomposites by a novel and simple one-step melt quench technique without using any external reducing agent. The metallic silver nanoparticles (Ag NPs) were generated first, and subsequently, metallic bismuth was deposited on the Ag NPs and formed a thick layer. The reduction of Bi³⁺ to Bi^o and subsequently its deposition on the Ag NPs (which were formed earlier than Bi^o) in the K₂O–Bi₂O₃–B₂O₃ (KBB) glass system have been explained by their standard reduction potentials. The UV–vis absorption spectra show a prominent surface plasmon resonance (SPR) absorption band at 575 nm at lower concentrations (up to 0.01 wt%); three bands at 569, 624 and 780 nm at medium concentration (0.02–0.03 wt%); and two weak bands at 619 and 817 nm at highest concentration (0.06 wt%) of silver. They have been explained by the electrodynamics theories. TEM images reveal the conversion of spheroidal (5–15 nm) to hexagonal (10–35 nm) shaped Ag NPs with the increase in concentration of silver (up to 0.06 wt%). SAED pattern confirms the crystalline planes of rhombohedral bismuth and cubic silver. Thermal treatment at 360 °C, which is the glass transformation temperature (T _g) of the sample containing lower concentration of silver (0.007 wt%), shows red-shifted SPR band due to increase in size of NPs. Whereas the sample containing higher concentration (0.06 wt%) of silver under similar treatment exhibited changes in SPR spectral profile happened due to conversion to spherical NPs from hexagonal shape and reduction in size (10–20 nm) of NPs after heat treatment for 65 h. HRTEM images corroborate the different orientations of the NPs. FESEM images reveal hexagonal disk like structure having different orientations. Dichroic nature of the nanocomposites has been explained with the size and shape of Ag nanoparticles. We believe that this work will create new avenues in the area of nanometal–glass hybrid nanocomposites and the materials have significant applications in the field of optoelectronics and nanophotonics.     

Oligonucleotide-Mediated Au–Ag Core–Shell Nanoparticles

The bimetallic core–shell nanoparticles show unique plasmonic properties and their preparations and characterizations are currently under investigation. A new type of Au core–Ag shell (Au@Ag) nanoparticles is prepared by sandwiching the chemically attached Raman reporter molecules (RRMs) and a 12-base-long oligonucleotide between the 13 nm average size core-gold nanoparticles (AuNPs) and 9 nm and 21 nm average size of Ag shell. The synthesized Au@Ag nanoparticles are tested for their surface-enhanced Raman scattering (SERS) performance. It is found that the chemical attachment of the oligonucleotides along with the RRM improved the enhancement in Raman scattering more than one order of the magnitude with the Au@Ag nanoparticles with an average 9-nm shell thickness while the Au@Ag nanoparticles with 21 nm average shell thickness have poor SERS activity. A minimum enhancement factor of 1.0 × 10⁷ is estimated for the SERS active oligonucleotide-mediated Au@Ag nanoparticles. The approach may provide new routes for preparation of highly sensitive new generation of bimetallic core–shell nanoparticles.     

A Simple and Sensitive SPRS Method for Trace H2O2 Based on the TiO2+ Complex and Gold Nanoparticles Aggregation Reactions

In the medium of H₂SO₄ and in the presence of TiO²⁺, gold nanoparticles in size of 10 nm exhibited a weak surface plasmon resonance scattering (SPRS) peak at 775 nm. Upon addition of trace H₂O₂, the yellow complex [TiO(H₂O₂)]²⁺ formed that cause the gold nanoparticles aggregations to form bigger gold nanoparticle clusters in size of about 900 nm, and the SPRS intensity at 775 nm (I) enhanced greatly. The enhanced intensity ΔI was linear to the H₂O₂ concentration in the range of 0.025–48.7 μg/mL, with a detection limit of 0.014 μg/mL H₂O₂. This SPRS method was applied to determining H₂O₂ in water samples with satisfactory results.     

Optimization of Carbon‐ and Binder‐Free Au Nanoparticle‐Coated Ni Nanowire Electrodes for Lithium‐Oxygen Batteries

A Au nanoparticle‐coated Ni nanowire substrate without binder or carbon is used as the electrode (denoted as the Au/Ni electrode) for Li‐oxygen (Li‐O₂) batteries. A minimal amount of Au nanoparticles with sizes of <30 nm on a Ni nanowire substrate are coated using a simple electrodeposition method to the extent that maximum capacity can be utilized. This optimized, one body, Au/Ni electrode shows high capacities of 921 mAh g^?1_Au, 591 mAh g^?1_Au, and 359 mAh g^?1_Au, which are obtained at currents of 300 mAg^?1_Au, 500 mAg^?1_Au, and 1000 mAg^?1_Au respectively. More importantly, the Au/Ni electrode exhibits excellent cycle stability over 200 cycles.     

Gold nanorod production by cyanobacteria—a green chemistry approach

Intracellular bioconversion of auric ion (Au³⁺) to gold nanorod (Au⁰) by the cyanobacterium Nostoc ellipsosporum has been observed for the first time in laboratory condition. The nanorods were produced within the cell after exposing the healthy growing filaments to 15 mg L⁻¹ gold (III) solution (pH 4.5) for 48 h at 20°C. The gold nanoparticles were extracted with sodium citrate solution and were subjected to UV–Visible spectroscopy. The characteristic surface-multiple plasmon bands at 560, 610, and 670 nm were observed. The nature and size of the particles were determined by transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), and zeta potential studies. The nanorod size ranged from 137 to 209 nm in length and 33 to 69 nm in diameter. DLS study revealed the average hydrodynamic size as 435 nm and XRD study indicated the reduction of Au³⁺ to Au⁰. Methods of extraction and preservation of gold nanorod particles have also been studied.     

Amperometric biosensor for hydrogen peroxide based on horseradish peroxidase onto gold nanowires and TiO2 nanoparticles

An electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) was fabricated, based on the electrostatic immobilization of horseradish peroxidase (HRP) with one-dimensional gold nanowires (Au NWs) and TiO₂ nanoparticles (nano-TiO₂) on a gold electrode. The nano-TiO₂ can give a biocompatible microenvironment and compact film, and the Au NWs can provide fast electron transferring rate and greatly add the amount of HRP molecules immobilized on the electrode surface. Au NWs were characterized by ultraviolet–visible spectra and transmission electron microscope. The electrode modification process was probed by cyclic voltammetry and electrochemical impedance spectroscopy. Chronoamperometry was used to study the electrochemical performance of the resulting biosensor. Under optimal conditions, the linear range for the determination of H₂O₂ was from 2.3 × 10⁻⁶ to 2.4 × 10⁻³ M with a detection limit of 7.0 × 10⁻⁷ M (S/N = 3). Moreover, the proposed biosensor showed superior stability and high sensitivity.     

Amplification of blue emission of Tm3+ ions in Li2O-HfO2-SiO2 glass system by means of Au0 metallic particles

Li₂O-HfO₂-SiO₂-Tm₂O₃:Au₂O₃ glass samples (containing fixed content of Tm₂O₃ and different concentration of Au₂O₃) were prepared and characterized. Bearing of Au⁰ metallic particles (MPs) on improving blue emission of thulium ions (Tm³⁺) ions was explored. Optical absorption (OA) spectra exhibited multiple bands excited from ³H₆ of Tm³⁺. Additionally, a broad peak in the wavelength range 500–600 nm due to surface plasmon resonance (SPR) of Au⁰ MPs was noticed in the spectra. Photoluminescence (PL) spectra (of thulium free glasses) indicated a peak in the visible range due to sp → d electronic transition of Au⁰ MPs. Luminescence spectra of Tm³⁺ and Au₂O₃ co-doped glasses exhibited intense blue emission with substantial increase of intensity with increase of Au₂O₃ content. Bearing of Au⁰ MPs on the reinforcement of blue emission of Tm³⁺ was discussed in detail with kinetic rate equations.     

Gold Nanoparticles from Induced Au3+→Au0 Reaction in Polyvinyl Alcohol Molecules in Presence of Sucrose in Hot Water

In hot water (50–60°C), polyvinyl alcohol (PVA) molecules have coordination reaction with Au³⁺ cations, forming an Au³⁺-PVA polymer complex. In the proposed model reaction in small templates, the complex converts to Au⁰ capping in PVA molecules. Adding sucrose (5–10 times the PVA in mass) in a typical batch promotes Au³⁺→Au⁰ reaction, showing absorption coefficient α in Au⁰ surface plasmon band to be enhanced as much as 28 times the value in reaction with PVA. The band shifts at 547 nm from 566 nm (α  =  21.4 cm⁻¹ mol⁻¹) in the PVA sample. Drying Au⁰-PVA/sucrose (2–5 wt% Au⁰) colloid at 60–70°C and then heating at 450°C in air burns off the organic part, leaving behind a light ash colored powder with Au⁰ nanoprisms or nanofibrils (∼30 nm average width). X-ray diffractogram has six reflections, (111), (200), (220), (311), (222), and (400), of Fm3m fcc Au⁰ of lattice parameter a  =  0.4080 nm. The powder has photoluminescence in transversal and longitudinal Au⁰ plasmon bands of 535 and 585 nm, respectively.     

State-sensitive monitoring of gold nanoparticle sites on titania and the interaction of the positive Au site with O2 by Au Lα1-selecting X-ray absorption fine structure

The heterogeneity of gold sites in various Au/TiO₂ catalysts was studied by means of state-sensitive Au L₃-edge X-ray absorption fine structure (XAFS) combined with high energy-resolution X-ray fluorescence spectrometry. A series of Au/TiO₂ catalysts were prepared via deposition-precipitation method on anatase-type or mesoporous (amorphous) TiO₂ added with NaOH (lower Au loading) or urea (higher Au loading). The mean Au particle size ranged between 29 and 87 Å based on high-resolution TEM (transmission electron microscope) measurements. The Au Lα₁ emission peak energy for Au/mesoporous-TiO₂ in air and Au/anatase-TiO₂ in CO (5%) corresponded to Au⁰ state. The emission peak energy for Au/anatase-TiO₂ in air shifted toward that of Au^I state. For relatively greater Au particles (average 87 Å) dispersed on mesporous TiO₂, the major valence state discriminated by Au Lα₁-selecting XANES (X-ray absorption near-edge structure) spectrum tuned to Au Lα₁ emission peak top was Au⁰, but the Au^δ− state could be successfully monitored by Au Lα₁-selecting XANES tuned to the emission energy at 9707.6 eV, of which population was relatively small compared to the case of smaller Au particles (average 29 Å) on anatase-type TiO₂. On the other hand, negative charge transfer from Au 5d to support was demonstrated in Au^δ+-state sensitive XANES tuned to 9718.3-9718.7 eV. The Au^δ+-state sensitive XANES spectra resembled theoretically generated XANES for interface Au^δ+ sites model on TiO₂ in contact with surface Ti sites. Further charge transfer was demonstrated from Au to adsorbed O₂ for Au/anatase-TiO₂ catalyst.     

All-electron scalar relativistic calculation of water molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation was performed on Au _n H₂O (n = 1–13) clusters using density functional theory (DFT) with the generalized gradient approximation at PW91 level. The calculation results reveal that, after adsorption, the small gold cluster would like to bond with oxygen and the H₂O molecule prefers to occupy the single fold coordination site. Reflecting the strong scalar relativistic effect, Au _n geometries are distorted slightly but still maintain a planar structure. The Au–Au bond is strengthened and the H–O bond is weakened, as manifested by the shortening of the Au–Au bond-length and the lengthening of the H–O bond-length. The H–O–H bond angle becomes slightly larger. The enhancement of reactivity of the H₂O molecule is obvious. The Au–O bond-lengths, adsorption energies, VIPs, HLGs, HOMO (LUMO) energy levels, charge transfers and the highest vibrational frequencies of the Au–O mode for Au _n H₂O clusters exhibit an obvious odd-even oscillation. The most favorable adsorption between small gold clusters and the H₂O molecule takes place when the H₂O molecule is adsorbed onto an even-numbered Au _n cluster and becomes an Au _n H₂O cluster with an even number of valence electrons. The odd–even alteration of magnetic moments is observed in Au _n H₂O clusters and may serve as material with a tunable code capacity of “0” and “1” by adsorbing a H₂O molecule onto an odd or even-numbered small gold cluster.     

A comparative theoretical study of the catalytic activities of Au<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> and AuAg<Superscript>-</Superscript> dimers for CO oxidation

The detailed mechanisms of catalytic CO oxidation over Au₂^- and AuAg^- dimers, which represent the simplest models for monometal Au and bimetallic Au-Ag nanoparticles, have been studied by performing density functional theory calculations. It is found that both Au₂^- and AuAg^- dimers catalyze the reaction according to the similar mono-center Eley–Rideal mechanism. The catalytic reaction is of the multi-channel and multi-step characteristic, which can proceed along four possible pathways via two or three elementary steps. In AuAg^-, the Au site is more active than the Ag site, and the calculated energy barrier values for the rate-determining step of the Au-site catalytic reaction are remarkably smaller than those for both the Ag-site catalytic reaction and the Au₂^- catalytic reaction. The better catalytic activity of bimetallic AuAg^- dimer is attributed to the synergistic effect between Au and Ag atom. The present results provide valuable information for understanding the higher catalytic activity of Au-Ag nanoparticles and nanoalloys for low-temperature CO oxidation than either pure metallic catalyst.     

Properties Evaluation of a New MRI Contrast Agent Based on Gd-Loaded Nanoparticles

Nanosized materials of gadolinium oxide can provide high-contrast enhancement in magnetic resonance imaging (MRI). The aim of this research was to characterize a novel emulsion composed of a silicon-based nanocomposite polymer (NCP) and gadolinium (III) oxide (Gd₂O₃) nanoparticles. The size and morphological structure of this nanoparticle are determined by particle size analysis device (zeta sizer) and transmission electronic microscope. We determined composition of Gd₂O₃ nanoparticles with energy dispersive X-ray analysis (EDXA) and magnetic resonance signal by T ₁-weighted MRI. Cytotoxicity of Gd₂O₃ nanoparticles in SK-MEL-3 cancer cells was evaluated. Zeta sizer showed Gd₂O₃ nanoparticles to be 75 nm in size. EDXA indicated the two main chemical components of gadolinium-nanocomposite polymer emulsion: gadolinium and silicon and MRI also showed a significantly higher incremental relaxivity for Gd₂O₃ nanoparticles compared to Magnevist (conventional contrast agent). In such concentrations, the slope of R₁ relaxivity (1/T ₁) vs. concentration curve of Magnevist and Gd₂O₃ were 4.33, 7.98 s⁻¹ mM⁻¹. The slope of R₂ relaxivity (1/T ₂) vs. concentration curve of Magnevist and Gd₂O₃ were 5.06, 13.75 s⁻¹ mM⁻¹. No appreciable toxicity was observed with Gd₂O₃ nanoparticles. Gadolinium-nanocomposite polymer emulsion is well characterized and has potential as a useful contrast agent for magnetic resonance molecular imaging.     

DNA Detection Based on Localized Surface Plasmon Resonance Spectroscopy of Ag@Au Biocomposite Nanoparticles

Noble metal nanoparticles (NPs) have attracted much attention due to their unique physical and chemical properties such as tunable surface plasmonics, high-efficiency electrochemical sensing, and enhanced fluorescence. We produced two biosensor chips consisting of Ag@Au bimetallic nanoparticles (BNPs) on a carbon thin film by simple RF-sputtering and RF-plasma-enhanced chemical vapor co-deposition. We deposited Au NPs with average size of 4 nm (Au₁ NPs) or 11 nm (Au₂ NPs) on a sensor chip consisting of Ag NPs with mean size of 15 nm, and we investigated the effect of shell size (Au NPs) on the chemical activities of the resulting Ag@Au₁ BNPs and Ag@Au₂ BNPs. We estimated the average size and morphology of Ag@Au BNPs by scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. X-ray diffraction (XRD) patterns revealed that Ag NPs and Au NPs had face-centered cubic (FCC) structure. We studied aging of the biosensor chips consisting of Ag@Au BNPs by localized surface plasmon resonance (LSPR) spectroscopy for up to 3 months. UV–visible aging of the prepared samples indicated that Ag@Au₁ BNPs, which corresponded to Ag NPs covered with smaller Au NPs, were more chemically active than Ag@Au₂ BNPs. Furthermore, we evaluated changes in the LSPR absorption peaks of Ag@Au₁ BNPs and bare Ag NPs in the presence of a DNA primer decamer at fM concentrations, to find that Ag@Au₁ BNPs were more sensitive biosensor chips within a short response time as compared to bare Ag NPs.

     

A selective resonance scattering assay for immunoglobulin G using Cu(II)-ascorbic acid-immunonanogold reaction

In sodium acetate–acetic acid buffer solution, Au, Ag, Pt, Pd, Fe₃O₄, and Cu₂O nanoparticles have catalytic enhancement effect on the reduction of Cu²⁺ by ascorbic acid to form large copper particles that exhibit a strong resonance scattering peak at 610 nm. Those nanocatalytic reactions were studied by the resonance scattering spectral technique, and smaller nanogold exhibited stronger catalytic enhancement effect in pH 4.2 sodium acetate–acetic acid buffer solution. The resonance scattering intensity at 610 nm increased linearly with the concentrations of 0.02 to 1.60, 0.040 to 1.20, and 0.12 to 4.70 nM nanogold in sizes of 5, 10, and 15 nm with detection limits of 0.010, 0.030, and 0.10 nM, respectively. An immunonanogold-catalytic resonance scattering bioassay was established, combining the immunonanogold-catalytic effect on CuSO₄–ascorbic acid reaction with the resonance scattering detection technique. As a model, 0.03 to 7.5 ng ml⁻¹ immunoglobulin G can be assayed by this immunonanogold-catalytic resonance scattering bioassay with a detection limit of 0.015 ng ml⁻¹.     

Optimization of the Field Enhancement and Spectral Bandwidth of Single and Coupled Bimetal Core–Shell Nanoparticles for Few-Cycle Laser Applications

We have theoretically studied and optimized the field enhancement and temporal response of single and coupled bimetal Ag/Au core–shell nanoparticles (NPs) with a diameter of 160 nm and compared the results to pure Ag and Au NPs. Very high-field enhancements with an amplitude reaching 100 (with respect to the laser field centered at 800 nm) are found at the center of a 2-nm gap between Ag/Au core–shell dimers. We have explored the excitation of the bimetal core–shell particles by Fourier transform-limited few-cycle optical pulses and identified conditions for an ultrafast plasmonic decay on the order of the excitation pulse duration. The high-field enhancement and ultrafast decay makes bimetal core–shell particles interesting candidates for applications such as the generation of ultrashort extreme ultraviolet radiation pulses via nanoplasmonic field enhancement. Moreover, in first experimental studies, we synthesized small bimetal Ag/Au core–shell NPs and compared their optical response with pure Au and Ag NPs and numerical results.     

Green synthesis of gold nanoparticles using Nyctanthes arbortristis flower extract

The present study explores the reducing and capping potentials of ethanolic flower extract of the plant Nyctanthes arbortristis for the synthesis of gold nanoparticles. The extract at different volume fractions were stirred with HAuCl₄ aqueous solution at 80 °C for 30 min. The UV–Vis spectroscopic analysis of the reaction products confirmed successful reduction of Au³⁺ ions to gold nanoparticles. Transmission electron microscope (TEM) revealed dominant spherical morphology of the gold nanoparticles with an average diameter of 19.8 ± 5.0 nm. X-ray diffraction (XRD) study confirmed crystalline nature of the synthesized particles. Fourier transform infra-red (FTIR) and nuclear magnetic resonance (NMR) analysis of the purified and lyophilized gold nanoparticles confirmed the surface adsorption of biomolecules during preparation and caused long-term (6 months) stability. Low reaction temperature (25 °C) favored anisotropy. The strong reducing power of the flower extract can also be tested in the green synthesis of other metallic nanoparticles.     

A systematic search for the structures,stabilities, and electronic properties of bimetallic Ca<Subscript>2</Subscript>-doped gold clusters: comparison with pure gold clusters

The local meta-GGA exchange correlation density functional (TPSS) with a relativistic effective core potential was employed to systematically investigate the geometric structures, stabilities, and electronic properties of bimetallic Ca₂Au _n (n = 1–9) and pure gold Au _n (n ≤ 11) clusters. The optimized geometries show that the most stable isomers for Ca₂Au _n clusters have 3D structure when n > 2, and that one Au atom capping the Ca₂Au _n−1 structure for different-sized Ca₂Au _n (n = 1–9) clusters is the dominant growth pattern. The average atomic binding energies and second-order difference in energies show that the Ca₂Au₄ isomer is the most stable among the Ca₂Au _n clusters. The same pronounced even–odd alternations are found in the HOMO–LUMO gaps, VIPs, and hardnesses. The polarizabilities of the Ca₂Au _n clusters show an obvious local minimum at n = 4. Moreover, the inverse corrections to the polarizabilities versus the ionization potential and hardness were found for the gold clusters.     

SPR Label-Free Biosensor with Oxide-Metal-Oxide-Coated D-Typed Optical Fiber: a Theoretical Study

In this article, a surface plasmon resonance (SPR) biosensor based on D-typed optical fiber coated by Al₂O₃/Ag/Al₂O₃ film is investigated numerically. Resonance in near infrared with an optimized architecture is achieved. Refractive index sensitivity of 6558 nm/RIU (refractive index unit) and detection limit of 1.5 × 10⁻⁶ RIU, corresponding to 0.4357 nm/μM and detection limit of 23 nM in BSA (bovine serum albumin) concentration sensing, are obtained. The analysis of the performance of the sensor in gaseous sensing indicates that this proposed SPR sensor is much suitable for label-free biosensing in aqueous media.

     

Theoretical study of crown ethers with incorporated azobenzene moiety

A series of crown ethers containing the azobenzene moiety incorporated into crowns of various sizes [Cr(O₆)_, Cr(O₇) and Cr(O₈)] and their corresponding alkali metal cation (Li⁺, Na⁺, K⁺, Rb⁺) complexes have been studied theoretically. The density functional theory (DFT) method was employed to elucidate the stereochemical structural natures and thermodynamic properties of all of the target molecules at the B3LYP/6-31 G(d) and LANL2DZ level for the cation Rb⁺. The fully optimized geometries had real frequencies, thus indicating their minimum-energy status. In addition, the bond lengths between the metal cation and oxygen atoms, atomic torsion angles and thermodynamic energies for complexes were studied. Natural bond orbital (NBO) analysis was used to explore the origin of the internal forces and the intermolecular interactions for the metal complexes. The calculated results show that the most significant interaction is that between the lone pair electrons of electron-donating oxygens in the cis-forms of azobenzene crown ethers (cis-ACEs) and the LP* (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Li⁺, Na⁺, K⁺ and Rb⁺). The electronic spectra for the cis-ACEs [cis-Cr(O₆), cis-Cr(O₇) and cis-Cr(O₈)] are obtained by the time-dependent density functional theory (TDDFT) at the B3LYP/6-31 G(d) level. The spectra of the cis-isomers show broad π → π* (S₀ → S₂) absorption bands at 310–340 nm but weaker n → π* (S₀ → S₁) bands at 480–490 nm. The calculated results are in good agreement with the experimental results.     

Plasmonic Coupling in Er3+:Au Tellurite Glass

In this paper, we report on luminescence and absorbance effects of Er⁺³:Au-doped tellurite glasses synthesized by a melting-quenching and heat treatment technique. After annealing times of 2.5, 5.0, 7.5, and 10.0 h, at 300 °C, the gold nanoparticles (GNP) effects on the Er⁺³ are verified from luminescence spectra and the corresponding levels lifetime. The localized surface plasmon resonance around 800 nm produced a maximum fluorescence enhancement for the band ranging from 800 to 840 nm, corresponding to the transitions ⁴H_11/2 → ⁴I_13/2 (805 nm) and ⁴S_3/2 → ⁴I_13/2 (840 nm), with annealing time till 7.5 h. The measured lifetime of the levels ⁴H_11/2 and ⁴S_3/2 confirmed the lifetime reduction due to the energy transfer from the GNP to Er⁺³, causing an enhanced photon emission rate in these levels.