Water‐Soluble Gold Nanoparticles: From Catalytic Selective Nitroarene Reduction in Water to Refractive Index Sensing

Water‐soluble gold nanoparticles (Au NPs) stabilized by a nitrogen‐rich poly(ethylene glycol) (PEG)‐tagged substrate have been prepared by reduction of HAuCl₄ with NaBH₄ in water at room temperature. The morphology and size of the nanoparticles can be controlled by simply varying the gold/stabilizer ratio. The nanoparticles have been fully characterized by TEM, high‐resolution (HR) TEM, electron diffraction (ED), energy‐dispersive X‐ray spectroscopy (EDS), UV/Vis, powder XRD, and elemental analysis. The material is efficient as a recyclable catalyst for the selective reduction of nitroarenes with NaBH₄ to yield the corresponding anilines in water at room temperature. Furthermore, the potential ability of the Au NPs as a refractive index sensor owing to their localized surface plasmon resonance (LSPR) effect has also been assessed.     

Fabrication of Au‐Nanoparticle/TiO2‐Nanotubes Electrodes Using Electrochemical Methods and Their Application for Electrocatalytic Oxidation of Hydroquinone

Gold nanoparticle (Au‐NPs)‐Titanium oxide nanotube (TiO₂‐NTs) electrodes are prepared by using galvanic deposition of gold nanoparticles on TiO₂‐NTs electrodes as support. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy results indicate that nanotubular TiO₂ layers consist of individual tubes of about 60–90 nm diameters and gold nanoparticles are well‐dispersed on the surface of TiO₂‐NTs support. The electrooxidation of hydroquinone of Au‐NPs/TiO₂‐NTs electrodes is investigated by different electrochemical methods. Au‐NPs/TiO₂‐NTs electrode can be used repeatedly and exhibits stable electrocatalytic activity for the hydroquinone oxidation. Also, determination of hydroquinone in skin cream using this electrode was evaluated. Results were found to be satisfactory and no matrix effects are observed during the determination of hydroquinone content of the “skin cream” samples.     

Organic–Inorganic Azafullerene‐Gold C59N‐Au Nanohybrid: Synthesis,Characterization, and Properties

Azafullerene (C₅₉N) was functionalized using a Mannich‐type reaction and then subsequently condensed with lipoic acid to yield dithiolane‐modified C₅₉N. In the following step, the extended dithiolane moiety from the C₅₉N core was utilized to decorate the azafullerene sphere with gold nanoparticles (Au NPs). The latter were initially stabilized with dodecanothiol (DT ? Au) and then integrated on azafullerene through a ligand exchange reaction with the dithiolane‐functionalized C₅₉N to produce the C₅₉N/DT ? Au nanohybrid. The nanohybrid was fully characterized by spectroscopy and microscopy, revealing the formation of spherical nanoparticles with a diameter in the range of 2–5 nm, as imaged by HR‐TEM. In the electronic absorption spectrum of C₅₉N/DT ? Au nanohybrid, the characteristic surface plasmon band (SPB) of Au NPs was observed, however, it was redshifted compared with that of DT ? Au. The redshift of the SPB is indicative of closer interparticle proximity of Au NPs, in accordance with the formation of aggregated NPs as observed by TEM, in C₅₉N/DT ? Au nanohybrid. Excited‐state interactions in C₅₉N/DT ? Au were probed by photoluminescence assays. It was found that the weak emission of C₅₉N at 819 nm was blueshifted by 14 nm in C₅₉N/DT ? Au, but was stronger in intensity, thus suggesting energy transfer to C₅₉N, within the organic–inorganic C₅₉N/DT ? Au nanohybrid. Finally, with the aid of pump–probe measurements and transient absorption spectroscopy, the formation of the singlet excited state of C₅₉N was identified.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

Synthesis and Characterization of MWNTs/Au NPs/HS(CH2)6Fc Nanocomposite: Application to Electrochemical Determination of Ascorbic Acid

In this article, a detailed electrochemical study of a novel 6‐ferrocenylhexanethiol (HS(CH₂)₆Fc) self‐assembled multiwalled carbon nanotubes‐Au nanoparticles (MWNTs/Au NPs) composite film was demonstrated. MWNTs/Au NPs were prepared by one‐step in situ synthesis using linear polyethyleneimine (PEI) as bifunctionalizing agent. HS(CH₂)₆Fc, which acted as the redox mediator, was self‐assembled to MWNTs/Au NPs via Au‐S bond. Transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), Fourier transformed infrared absorption spectroscopy (FT‐IR), UV‐visible absorption spectroscopy, and cyclic voltammetry were used to characterize the properties of the MWNTs/Au NPs/HS(CH₂)₆Fc nanocomposite. The preparation of the nanocomposite was very simple and effectively prevented the leakage of the HS(CH₂)₆Fc mediator during measurements. The electrooxidation of AA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique performance of MWNTs/Au NPs. The nanocomposite modified electrode exhibited excellent catalytic efficiency, high sensitivity, good stability, fast response (within 3 s) and low detection limit toward the oxidation of AA at a lower potential.     

pH‐Switchable Redox Reactions and Bioelectrocatalytic Processes Using Au Nanoparticles‐Modified Electrodes

Boronate ester complexes generated between methylene blue (MB⁺)‐functionalized Au nanoparticles (NPs) and electrode surfaces are implemented to stimulate the bioelectrocatalyzed reduction of H₂O₂ in the presence of horseradish peroxidase (HRP). Two kinds of Au NPs are prepared: Class I includes MB⁺/phenylboronic acid as a modifying layer, whereas Class II includes MB⁺/dithiothreitol as a mixed capping layer. The Class I or II NPs form boronate ester complexes with a dithiothreitol‐ or phenylboronic acid‐functionalized Au electrodes, respectively. By the cyclic loading of the NPs on the electrodes (pH 8.1), and the removal of the NPs (pH 1.5), switchable bioelectrocatalyzed reduction of H₂O₂ is demonstrated.     

Size‐Controlled Synthesis of Highly Monodisperse Gold Nanoparticles without a Size‐Selection and Long Range Ordered 2‐D Arrangement

A simple method is used to control the size of cetyltrimethylammoniumbromide‐protected Au nanoparticles by a reversal micelle in safe organic solvent. These Au nanoparticles can be evolved to highly monodisperse Au nanoparticles capped 1‐dodecanthiol in the 2, 3, and 5 nm diameter by refluxing at～160°C for 7 hours. Their ultraviolet visible spectroscopy (UV‐vis), x‐ray diffraction (XRD, transmission electron microscopy (TEM) showed that all the three different size gold nanoparticles(NPs) displayed high size homogenous properties and easy formed large areas of long ordered two‐dimensional arrangement at the air/solid interface.     

Preparation and Characterization of Pt/Co‐Core Au‐Shell Magnetic Nanoparticles

Pt/Co‐core Au‐shell nanoparticles were synthesized via a two‐step route using NaBH₄ as a reducing agent. The nanoparticles are characterized by UV‐vis spectroscopy, transmission electron microscopy (TEM) and powder X‐ray diffraction (XRD). The results indicate that the as‐synthesized Pt/Co‐core Au‐shell nanoparticles have a disordered face centered cubic (fcc) structure, whereas the annealed Pt/Co‐core Au‐shell nanoparticles exhibit an ordered face centered tetragonal (fct) structure. Superconducting quantum interference device (SQUID) studies reveal that the coercivity of the annealed Pt/Co‐core Au‐shell nanoparticles increases to 510 Oe after heat treatment at 500 °C for 2 h.     

Nanoelectrocatalyst Based on High‐Density Au/Pt Hybrid Nanoparticles Supported on a Silica Nanosphere

A high‐efficiency nanoelectrocatalyst based on high‐density Au/Pt hybrid nanoparticles supported on a silica nanosphere (Au‐Pt/SiO₂) has been prepared by a facile wet chemical method. Scanning electron microscopy, transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray photoelectron spectroscopy are employed to characterize the obtained Au‐Pt/SiO₂. It was found that each hybrid nanosphere is composed of high‐density small Au/Pt hybrid nanoparticles with rough surfaces. These small Au/Pt hybrid nanoparticles interconnect and form a porous nanostructure, which provides highly accessible activity sites, as required for high electrocatalytic activity. We suggest that the particular morphology of the Au‐Pt/SiO₂ may be the reason for the high catalytic activity. Thus, this hybrid nanomaterial may find a potential application in fuel cells.     

Activation and Deactivation of Gold/Ceria–Zirconia in the Low‐Temperature Water–Gas Shift Reaction

Gold (Au) on ceria–zirconia is one of the most active catalysts for the low‐temperature water–gas shift reaction (LTS), a key stage of upgrading H₂ reformate streams for fuel cells. However, this catalyst rapidly deactivates on‐stream and the deactivation mechanism remains unclear. Using stop–start scanning transmission electron microscopy to follow the exact same area of the sample at different stages of the LTS reaction, as well as complementary X‐ray photoelectron spectroscopy, we observed the activation and deactivation of the catalyst at various stages. During the heating of the catalyst to reaction temperature, we observed the formation of small Au nanoparticles (NPs; 1–2 nm) from subnanometer Au species. These NPs were then seen to agglomerate further over 48 h on‐stream, and most rapidly in the first 5 h when the highest rate of deactivation was observed. These findings suggest that the primary deactivation process consists of the loss of active sites through the agglomeration and possible dewetting of Au NPs.     

Label‐free Electrochemical Bisphenol A Aptasensor Based on Designing and Fabrication of a Magnetic Gold Nanocomposite

The present study focuses on designing and fabricating an electrochemical aptasensor for the label free detection of bisphenol A (BPA) using gold nanoparticles (Au NPs) immobilized on functional cupper magnetic nanoparticles (CuFe₂O₄‐SH) and multiwall carbon nanotubes (MWCNTs) modified with aptamer and 6‐mercapto‐1‐hexanol (MCH). A number of analysis techniques were used to characterize the nanocomposite, including Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, elemental mapping analysis and energy dispersive x‐ray diffraction. The results of the analyses revealed that the fabricated aptasensor had an acceptable linearity index (0.05‐9 nM) with an ultralow detection limit (25.2 pM) when used to determine BPA. Electrochemical experiments were conducted using a [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox system. The results of the electrochemical tests indicated that the existence of Au NPs along with magnetic nanoparticles and MWCNTs in nanocomposite led to a synergistic augmentation on the surface of the modified electrode, thus facilitating the efficient sensing of BPA. This method is highly selective, sensitive and environmentally friendly. Moreover, proposed aptasensor has valuable potential applications in medical diagnostics and food industries where a fast and reliable detection of BPA is of paramount importance for the health of the public.     

Podophyllotoxin extraction from Linum usitatissimum plant and its anticancer activity against HT‐29, A‐549 and MDA‐MB‐231 cell lines with and without the presence of gold nanoparticles

In recent years, gold nanoparticles (Au‐NPs) have been taken into consideration in nanomedicine due to their excellent biocompatibility, chemical stability and promising optical properties. In this research, podophyllotoxin conjugated with gold nanoparticles (Au‐NPs‐POT) was synthesized and the conjugation of POT with Au‐NPs was confirmed using scanning electron microscopy, mass spectrometry and Fourier transform infrared spectroscopy. The anticancer effects of the product on preclinical models of lung, colon and breast cancers were investigated using MTT test. The analyses showed a direct dose–response relationship. It was found that higher concentrations of POT have more positive effects on the inhibition of cancer cell growth. At POT concentrations of 1, 2.5, 5, 7.5, 10, 15 and 20 ng ml^?1, approximately 50% of the growth of colorectal, lung and breast cancer cell lines was inhibited, while similar results were obtained in the presence of 1, 2, 3, 4 and 5 μg ml^?1 Au‐NPs‐POT. Au‐NPs‐POT exhibited the lowest cytotoxicity due to the presence of POT. The anticancer feature of Au‐NPs‐POT proved the potential to develop better anticancer therapeutics and to open new avenues for treatment of cancers.     

Multiple plasmonic effect on photocurrent generation of metal‐loaded titanium dioxide composite/dye films on gold grating surface

We demonstrate the multiple plasmonic effect on the photocurrent properties of photoanodes containing Ag or Au nanoparticles (NPs) loaded onto titanium dioxide film (Ag–TiO₂ or Au–TiO₂) on Au grating surfaces. Ag–TiO₂ or Au–TiO₂ nanocomposite particles are prepared by a flame spray pyrolysis route. The structures and morphologies of the prepared products are characterized by high‐resolution transmission electron microscopy. The Ag–TiO₂ or Au–TiO₂ composite NPs are deposited by spin coating onto the Au grating surfaces. The photoanode electrode is a layered structure of blu‐ray disc‐recordable grating substrate/Au/Ag (or Au)–TiO₂/dye/electrolyte/indium‐tin oxide. The plasmonic effect is induced when Ag or Au NPs are located within the propagating surface plasmon (SP) field on the Au grating surface. The short‐circuit photocurrent is increased by exciting the grating‐coupled propagating SP on the Au gratings and is further enhanced by positioning the Ag or Au NPs within the grating‐coupled SP field. Copyright © 2014 John Wiley & Sons, Ltd.     

Ternary Ag/Polyaniline/Au nanocomposites: Preparation,characterization and electrochemical properties

Ternary Ag/Polyaniline/Au nanocomposites were synthesized successfully by immobilizing of Au nanoparticles (NPs) on the surface of Ag/Polyaniline (PANI) nanocomposites. Ag/PANI nanocomposites were prepared via in situ chemical polymerization of aniline in the presence of 4-aminothiophenol (4-ATP) capped silver colloidal NPs. Then, uniform gold (Au) NPs were assembled on the surface of resulted Ag/PANI nanocomposites through electrostatic interaction to get Ag/Polyaniline/Au nanocomposites. The nanocomposites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), ultraviolet visible spectroscopy (UV-Vis), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). Moreover, Ag/PANI/Au nanocomposites were immobilized on the surface of a glassy carbon electrode and showed enhanced electrocatalytic activity for the reduction of H₂O₂ compared with Ag/PANI.     

Large‐Sized Fabrication of Tunable Plasmonic Electrodes Via Electrodeposition

Electrodeposition method, a simple, cheap, and flexible approach, to fabricate gold nanoparticle (Au NPs) films with an area larger than 1 cm² on indium tin oxide (ITO) electrodes modified with (3‐mercaptopropyl) trimethoxysilane (MPTMS) was presented. Size‐controllable and high loading Au NPs were obtained, which were characterized by field‐emission scanning electron microscopic (FESEM) and UV‐vis spectroscopy. Our current method provides a versatile and facile pathway to fabricate large‐scale metal nanoparticles thin film, enhancing alternatives for academic investigation and industrial application.     

Surface‐Plasmon‐Enhanced Band Emission of ZnO Nanoflowers Decorated with Au Nanoparticles

A simple strategy was used to enhance band emission through the transfer of defect emission from ZnO to Au by using the energy match between the defect emission of ZnO and the surface plasmon absorbance of Au NPs through decorating the surface of ZnO nanoflowers with Au nanoparticles (Au NPs). The ZnO nanostructure, which was comprised of six nanorods that were attached on one side in a flower‐like fashion, was synthesized by using a hydrothermal method. The temperature‐dependent morphology and detailed growth mechanism were studied. The influence of the density of the Au NPs that were deposited onto the surface of ZnO on photoluminescence was investigated to optimize the configuration of the ZnO/Au system in terms of the maximum band emission. The sequential transfer of defect energy from ZnO to Au and electron transfer from excited Au to ZnO was proposed as a possible mechanism for the enhanced band emission.     

Polyethylenimine‐assisted Synthesis of Au Nanoparticles for Efficient Syngas Production

The ability to capture, store, and use CO₂ is important for remediating greenhouse‐gas emissions and combatting global warming. Herein, Au nanoparticles (Au‐NPs) are synthesized for effective electrochemical CO₂ reduction and syngas production, using polyethylenimine (PEI) as a ligand molecule. The PEI‐assisted synthesis provides uniformly sized 3‐nm Au NPs, whereas larger irregularly shaped NPs are formed in the absence of PEI in the synthesis solution. The Au‐NPs synthesized with PEI (PEI?Au/C, average PEI Mw=2000) exhibit improved CO₂ reduction activities compared to Au‐NPs formed in the absence of PEI (bare Au NPs/C). PEI?Au/C displays a 34 % higher activity toward CO₂ reduction than bare Au NPs/C; for example, PEI?Au/C exhibits a CO partial current density (j_CO) of 28.6 mA cm^?2 at ?1.13 V_RHE, while the value for bare Au NPs/C is 21.7 mA cm^?2; the enhanced j_CO is mainly due to the larger surface area of PEI?Au/C. Furthermore, the PEI?Au/C electrode exhibits stable performance over 64 h, with an hourly current degradation rate of 0.25 %. The developed PEI?Au/C is employed in a CO₂‐reduction device coupled with an IrO₂ water‐oxidation catalyst and a proton‐conducting perfluorinated membrane to form a PEI?Au/C|Nafion|IrO₂ membrane‐electrode assembly. The device using PEI?Au/C as the CO₂‐reduction catalyst exhibits a j_CO of 4.47 mA/cm² at 2.0 V_cell. Importantly, the resulted PEI?Au/C is appropriate for efficient syngas production with a CO ratio of around 30–50 %.     

Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three‐dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self‐assembly. Nanostructures consisting of one to 20 depositions of 13 nm‐diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection–absorption spectroscopy. Surface‐enhanced polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) is observed from Au NPs modified by the probe molecule 4‐hydroxythiophenol. The limitation of this kind of substrate for surface‐enhanced PM‐IRRAS is discussed. The surface‐enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au‐NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au‐NP depositions, and the absolute SERS intensities from different batches are reproducible within 20 %. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between ?800 and +200 mV (vs. Ag/AgCl/sat. Cl^?).     

Gold/Wüstite Core–shell Nanoparticles: Suppression of Iron Oxidation through the Electron‐Transfer Phenomenon

Plasmonic Au and magnetic Fe are coupled into uniform Au@Fe core–shell nanoparticles (NPs) to confirm that electron transfer occurred from the Au core to the Fe shell. Au NPs synthesized in aqueous medium are used as seeds and coated with an Fe shell. The resulting Au@Fe NPs are characterized by using various analytical techniques. X‐ray photoelectron spectroscopy and superconducting quantum interference device measurements reveal that the Fe shell of the Au@Fe NPs mainly consists of paramagnetic Wüstite with a thin surface oxide layer consisting of maghemite or magnetite. Electron transfer from the Au core to the Fe shell effectively suppresses iron oxidation from Fe²⁺ to Fe³⁺ near the interface between the Au and the Fe. The charge‐transfer‐induced electronic modification technique enables us to control the degree of iron oxidation and the resulting magnetic properties.     

One‐Dimensional Carbon Nanotube/SnO2/Noble Metal Nanoparticle Hybrid Nanostructure: Synthesis,Characterization, and Electrochemical Sensing

Herein we report a facile and efficient method for self‐assembling noble‐metal nanoparticles (NPs) to the surface of SnO₂‐coated carbon nanotubes (CNT@SnO₂) to construct CNT@SnO₂/noble metal NP hybrids. By using SnCl₄ as the precursor of the SnO₂ shell on the surface of CNTs, the hydrolysis speed of SnCl₄ was slowed down in ethanol containing a trace amount of urea and water. The coaxial nanostructure of CNT@SnO₂ was confirmed by using X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). It was found that the coating layer of SnO₂ was homogeneous with the mean thickness of 8 nm. The CNT@SnO₂/noble‐metal NP hybrids were obtained by mixing noble‐metal NPs with as‐prepared CNT@SnO₂ coaxial nanocables by means of a self‐assembly strategy. With the amino group terminated, the CNT@SnO₂ coaxial nanocable can readily adsorb the as‐prepared noble‐metal NPs (Au, Ag, Au? Pt, and Au? Pd NPs). The presence of an amino group at the surface of SnO₂ was proved by use of X‐ray photoelectron spectroscopy (XPS). In addition, H₂O₂ sensing by amperometric methods could serve as detection models for investigating the electrocatalytic ability of as‐prepared hybrid materials. It was found that wide linear ranges and low detection limits were obtained by using the enzyme‐free CNT@SnO₂@Au? Pt modified electrode, which indicated the potential utilizations of the hybrid based on CNT@SnO₂ for electrochemical sensing.