Porous Au Research Articles

Effect of Random Nanostructured Metallic Environments on Spontaneous Emission of HITC Dye.

We have studied emission kinetics of HITC laser dye on top of glass, smooth Au films, and randomly structured porous Au nanofoams. The observed concentration quenching of luminescence of highly concentrated dye on top of glass (energy transfer to acceptors) and the inhibition of the concentration quenching in vicinity of smooth Au films were in accord with our recent findings. Intriguingly, the emission kinetics recorded in different local spots of the Au nanofoam samples had a spread of the decay rates, which was large at low dye concentrations and became narrower with increase of the dye concentration. We infer that in different subvolumes of Au nanofoams, HITC molecules are coupled to the nanofoams weaker or stronger. The inhibition of the concentration quenching in Au nanofoams was stronger than on top of smooth Au films. This was true for all weakly and strongly coupled subvolumes contributing to the spread of the emission kinetics. The experimental observations were explained using theoretical model accounting for change in the Förster radius caused by the strong energy transfer to metal.

Molecular imprinting of polymer films on 2D silica inverse opal via thermal graft copolymerization for bisphenol A detection

Bisphenol A (BPA), which is extensively used in the plastic industry, is a known endocrine disrupting chemical whose release into the environment from industrial products is detrimental to human health. In this study, to characterize BPA-detectable imprinted films on quartz crystals (QCs), sensitive molecularly imprinted quartz crystal microbalance (MIP-QCM) sensors were fabricated. The MIP films were synthesized on 2D porous SiO2 and Au pinholes that were generally observed from replica structures prepared using a polystyrene colloidal monolayer as an insulating mold. For MIP film formation, surface-initiated atom transfer radical polymerization was precisely controlled to adjust the MIP film growth on initiator-modified porous structures. Note that three BPA-imprinted poly(4-VP-co-EGDMA) films, i.e., MIP(8-h), MIP(16-h), and MIP(24-h), under reaction time control were successfully prepared. Furthermore, all imprinted patterned surfaces were characterized via atomic force microscopy (AFM) and scanning electron microscopy (SEM). The MIP(24-h) film demonstrated the highest imprinting factor (i.e., IF ≈ 2.4) and sensitivity (i.e., 1.219 ± 0.079 Hz nM−1), although the maximum binding capacity (Qmax) and binding capacity (Qe) at 1-h rebinding equilibrium was relatively lesser than those of the MIP(8-h) and MIP(16-h) owing to the differentiation of the deposited MIP mass. Using the Freundlich isotherm model, the binding affinity of the BPA imprinted MIP films measured in an aqueous solution of BPA whose concentration ranged between 45 and 225 nM was evaluated. Finally, the selectivity of the MIP-QCM sensors for BPA detection was investigated in aqueous solutions in the presence of other analogous chemicals such as bisphenol F (BPF), bisphenol AP (BPAP), and bisphenol B (BPB). The MIP(8-h) film showed extremely high recovery percentage (≈ 94.54 %) owing to the relatively thinner film formation, which was significantly comparable with that of other MIP films, i.e., 68.37 % for MIP(16-h) and 61.99 % for MIP(24-h). However, the MIP(24-h) film appeared to have higher selective features with selectivity coefficients and relative selectivity (k* and k’ ≈ 2.5, respectively) despite measurement in a limited period (i.e., 1-h rebinding process).

Colloidal Porous AuAg Alloyed Nanoparticles for Enhanced Photoacoustic Imaging.

Colloidal porous AuAg alloyed nanoparticles (pAuAgNPs) were synthesized by galvanic replacement reaction from Ag nanocubes. pAuAgNPs have a 50 nm exterior diameter and half of their inner space consists of voids that have a bimodal size distribution with peaks at 21 and 8.3 nm. pAuAgNPs showed a plasmonic peak at 750 nm, which was exploited for photoacoustic (PA) imaging. Gold nanorods (AuNRs) were prepared and used as the control; they have a strong plasmonic peak at 720 nm. In in vitro experiments at respective plasmonic peak excitations, pAuAgNPs gave stronger PA signals than AuNRs by 8.9 times per particle and 11.7 times per dosage by exogenous atom. The high surface area per volume as a result of the inner voids amplified the PA signals by efficient thermoacoustic conversion. In experiments of chicken-tissue phantoms, pAuAgNPs showed PA signals through 4.5 cm thick tissue, whereas AuNRs gave no detectable signal. In whole-body in vivo experiments, pAuAgNPs injected into the body showed 2.7 times stronger PA signals than AuNRs. Coating the pAuAgNPs with a silica layer additionally increased their PA signal by 1.8 times when compared to the uncoated ones.

Efficient and green electrochemical synthesis of 4-aminophenol using porous Au micropillars

The development of new, efficient, chemoselective, and seemingly stable catalysts to rapidly convert 4-nitrophenol into 4-aminophenol, which is a particularly valuable chemical within multiple industries, is highly required. The use of non-toxic and non-corrosive chemicals for 4-aminophenol synthesis presents further challenges. We show a simple and scalable shape-controlled electrodeposition using a dual-template method for the synthesis of well-defined porous Au micropillar array electrodes to enhance electrocatalyst stability, efficiency, and durability during the electroreduction of 4-nitrophenol. Consequently, we avoid the use of toxic and aggressive chemical reducing agents and minimize the production of residues. Surface areas up to 55.2 m2 g−1 are achieved, which represent a substantial improvement over non-architectured Au film electrodes. Porous Au micropillars exhibit excellent yields (̴100 %), chemoselectivity (other byproducts are not detected), and a high kinetic constant of 2.5 × 10-2 min−1, which is impressively higher than those reported recently for Au thin films or Au-based electrocatalysts.

A facile periodic porous Au nanoparticle array with high-density and built-in hotspots for SERS analysis

Porous noble metal nanoparticles have attracted great attention in surface enhanced Raman scattering (SERS) applications due to the presence of high-density and built-in hotspots. Herein, we develop a facile route to fabricate large-area, periodic porous Au nanoparticle (PGNP) array based on ultra-thin alumina membrane (UTAM) incorporated with annealing and dealloying techniques. The size and ligament of the PGNP can be easily tailored by changing the thickness of the deposited Au/Ag film and the dealloying time. The PGNP array exhibits excellent uniformity and high sensitivity in SERS analysis due to the periodic structure and the enormous built-in hotspots. A large Raman enhancement factor (EF) of 1.4 × 107 is achieved with a relative standard deviation (RSD) less than 6.6%. Furthermore, experimental results and numerical simulations demonstrate that this PGNP array substrate exhibit polarization independence. The facile and controllable fabrication strategy makes the substrate promising for practical applications in many SERS-based qualitative and quantitative analyses.

Syngas Evolution from CO2 Electroreduction by Porous Au Nanostructures.

Electrocatalytic reduction of CO2 recently emerged as a viable solution in view of changing the common belief and considering carbon dioxide as a valuable reactant instead of a waste product. In this view, we herein propose the one-step synthesis of gold nanostructures of different morphologies grown on fluorine-doped tin oxide electrodes by means of pulsed-laser deposition. The resulting cathodes are able to produce syngas mixtures of different compositions at overpotentials as low as 0.31 V in CO2-presaturated aqueous media. Insights into the correlation between the structural features/morphology of the cathodes and their catalytic activity are also provided, confirming recent reports on the remarkable sensitivity toward CO production for gold electrodes exposing undercoordinated sites and facets.

Overvoltage reduction in membrane Bunsen reaction for hydrogen production by using a radiation-grafted cation exchange membrane and porous Au anode

An electrochemical membrane Bunsen reaction using a cation exchange membrane (CEM) is a key to achieving iodine-sulfur (IS) thermochemical water splitting for the mass-production of hydrogen. In this study, we prepared a radiation-grafted CEM with a high ion exchange capacity (IEC) and a highly-porous Au-electroplated anode, and then used them for the membrane Bunsen reaction to reduce cell overvoltage. The high ionic content of our CEM led to low resistivity for proton transport, while the high porosity of the electrode led to a large effective surface area for anodic SO2 oxidation. The cell overvoltage for the membrane Bunsen reaction was significantly reduced to 0.21 V at 200 mA/cm2, one-third of that achieved using a commercial CEM and non-porous anode. From the analysis of the current-voltage characteristics, the grafted CEM was demonstrated to play a dominant role in the overvoltage reduction compared to the porous Au anode.

Optimized Resistive Switching in TiO2 Nanotubes by Modulation of Oxygen Vacancy Through Chemical Reduction

The resistive switching behavior of 1-D TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotube-based resistive random access memory (ReRAM) is discussed in this article. Highly oriented TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes were synthesized by anodic oxidation method on a Ti substrate, which was used as the bottom electrode. To modulate the oxygen vacancy (VO) in TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes, hydrazine hydrate reduction was employed in the temperature range 60 °C-100 °C. After charactering the morphological, elemental, and crystallographic properties, the level of reduction in different TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes array was estimated by Raman, photoluminescence, and X-ray photoelectron spectroscopies. Au/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes/Ti devices were fabricated by using TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes with various levels of reductions where thin and porous Au top electrode was used to make the resistive switching faster. TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Nanotubes array, reduced at 80 °C showed promising resistive switching performance with SET/RESET voltages of 2 V/1.8 V, R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> /R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> of 19 at a read voltage of 0.5 V (25 °C) and stable endurance behavior after the 100th cycle. Interestingly, reduction temperature at 60 °C and 100°C, offered degraded resistive switching within the same voltage range. All the devices showed electroforming free bipolar resistive switching. Efforts were devoted to establish the role of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> and its tuning to improve the resistive switching behavior in 1-D TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes. This article systematically showcases the efficacy of 1-D metal oxide for potential ReRAM application and establishes an easy but efficient approach to improve the resistive switching by modulating oxygen vacancy in it.

Morphological and compositional analysis of alluvial gold: The Fresnedoso gold placer (Spain)

The Fresnedoso creek gold placer (FGP) in the Moraleja Basin (W Iberian Massif) has been investigated in terms of morphological, textural and compositional evolution of gold particles. A mixture of two populations has been identified being coherent with primary sources located at a proximal (2.5–10 km), and distal (20–50 km) range. Shape and compositional evidences suggest recycling of paleoplacers (tertiary) has to be considered to some extend. Primary laminar morphologies point to lode deposits in small-flat veins hosted in the metasediments of the Schist Greywacke Complex. All these features suggest the Fresnedoso creek gold placer is a deposit constituted by with mono- and polycyclic particles. Transport distance – Flattening indexes (CFI, Shilo) models have been tested and result in useful information for exploration of undiscovered deposits, even with a limited dataset. Compositional analysis of gold particle morphotypes has revealed that the Fresnedoso gold is a AuAg bimetallic alloy. Textural and compositional groups include: type 1 (Au1 = Au88-94Ag12-6), type 2 (Au2 = Au99 Ag1) and type 3 (Au3 ~ Au99Ag1). Variations in composition of type 1 (Particle's cores) could reflect differences in primary composition, spatial dispersion of sources or secondary processes, but fit within the compositional range of orogenic gold deposits. Secondary processes have been explored to explain compositional heterogeneity in the particles. Gold type 2 (rim) and 3 (micro-aggregates) represents two different de-alloying stages, from initial Ag-leaching at the rim and/or through grain-boundaries and microfractures (Au2), to complete resetting of primary chemical imprint, pervasive porous texture and Au re-precipitation with Fe-OOH and clays (Au3). A model is proposed which combines: Ag de-alloying enhanced in a Cl-Fe rich environment (lateritic) and the deformation/recrystallization processes related to mechanical cold-work in the river load-bed during transport.

Facile synthesis of porous hollow Au nanoshells with enhanced catalytic properties towards reduction of p-nitrophenol

We presented a facile synthesis of porous hollow Au nanoshells (PHASs) with excellent catalytic properties by galvanic replacement and dealloying process. The addition amount of HAuCl4 during galvanic replacement played the important role in controlling structure, catalytic and optical properties of PHASs. The PHASs exhibited shorter induction time and highly catalytic rate constant towards reduction of p-nitrophenol by NaBH4. The catalytic rate constant of PHASs (17.2 × 10−3 s−1) was 25 times that of sacrificial Ag particles, 4.2 times that of Ag@Au with dense Au shell and 1.5 times that of Ag-Au alloys at same particle concentration. The normalized rate constant of PHASs (143 s-1 mmol−1) was 5.3 times that of Ag@Au and 1.6 times that of Ag-Au alloys. The as-synthesized PHASs with unique structures and tunable properties have fantastic potential applications in catalysis, biosensors, drug delivery and cancer therapy.

Hierarchically porous Au nanostructures with interconnected channels for efficient mass transport in electrocatalytic CO2 reduction

Electrocatalytic CO2 reduction is a promising way to provide renewable energy from gaseous CO2 The development of nanostructures improves energy efficiency and selectivity for value-added chemicals, but complex nanostructures limit the CO2 conversion rates due to poor mass transport during vigorous electrolysis. Herein, we propose a three-dimensional (3D) hierarchically porous Au comprising interconnected macroporous channels (200-300 nm) and nanopores (∼10 nm) fabricated via proximity-field nanopatterning. The interconnected macropores and nanopores enable efficient mass transport and large active areas, respectively. The roles of each pore network are investigated using reliable 3D nanostructures possessing controlled pore distribution and size. The hierarchical nanostructured electrodes show a high CO selectivity of 85.8% at a low overpotential of 0.264 V and efficient mass activity that is maximum 3.96 times higher than that of dealloyed nanoporous Au. Hence, the systematic model study shows the proposed hierarchical nanostructures have important value in increasing the efficiency of expensive Au.

Direct electron transfer-type bioelectrocatalysis of FAD-dependent glucose dehydrogenase using porous gold electrodes and enzymatically implanted platinum nanoclusters

The direct electron transfer (DET)-type bioelectrocatalysis of flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase (GDH) from Aspergillus terreus (AtGDH) was carried out using porous gold (Au) electrodes and enzymatically implanted platinum nanoclusters (PtNCs). The porous Au electrodes were prepared by anodization of planar Au electrodes in a phosphate buffer containing glucose as a reductant. Moreover, PtNCs were generated into AtGDH by an enzymatic reduction of hexachloroplatinate (IV) ion. The modification was confirmed by native polyacrylamide gel electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses. The AtGDH-adsorbed porous Au electrode showed a DET-type bioelectrocatalytic wave both in the presence and absence of PtNCs; however, the current density with PtNCs (~1 mA cm−2 at 0 V vs. Ag|AgCl|sat. KCl) was considerably higher than that without PtNCs. The kinetic and thermodynamic analysis of the steady-state catalytic wave indicated that inner PtNCs shortened the distance between the catalytic center of AtGDH (=FAD) and the conductive material, and improved the heterogeneous electron transfer kinetics between them.

Ionic Liquid-Modulated Synthesis of Porous Worm-Like Gold with Strong SERS Response and Superior Catalytic Activities.

Porous gold with well-defined shape and size have aroused extensive research enthusiasm due to their prominent properties in various applications. However, it is still a great challenge to explore a simple, green, and low-cost route to fabricate porous gold with a “clean” surface. In this work, porous worm-like Au has been easily synthesized in a one-step procedure from aqueous solution at room temperature under the action of ionic liquid tetrapropylammonium glycine ([N3333][Gly]). It is shown that the as-prepared porous worm-like Au has the length from 0.3 to 0.6 μm and the width of approximately 100–150 nm, and it is composed of lots of small nanoparticles about 6–12 nm in diameter. With rhodamine 6G (R6G) as a probe molecule, porous worm-like Au displays remarkable surface enhanced Raman scattering (SERS) sensitivity (detection limit is lower than 10−13 M), and extremely high reproducibility (average relative standard deviations is less than 2%). At the same time, owing to significantly high specific surface area, various pore sizes and plenty of crystal defects, porous worm-like Au also exhibits excellent catalytic performance in the reduction of nitroaromatics, such as p-nitrophenol and p-nitroaniline, which can be completely converted within only 100 s and 150 s, respectively. It is expected that the as-prepared porous worm-like Au with porous and self-supported structures will also present the encouraging advances in electrocatalysis, sensing, and many others.

Efficient electrochemiluminescence of perylene nanocrystal entrapped in hierarchical porous Au nanoparticle-graphene oxide film for bioanalysis based on one-pot DNA amplification

Although the perylene (Pe) affords the high quantum efficiency, its electrochemiluminescence (ECL) depends on organic solvents because of the serious aggregation-caused quenching effect in aqueous media, which impedes its application in ECL biological analysis. In this work, the hierarchical porous Au nanoparticle-graphene oxide nanocomposites(Au-GO) were prepared by using oleylamine as a reductant to generate Au nanoparticles which are intercalant to extend the intersheets distance of graphene oxide. Then the Au-GO could act as matrix for perylene nanocrystals (PeNCs) to load via π-π stacking which could prevent PeNCs from the self-aggregation to achieve an intense ECL of PeNCs in aqueous media. Consequently, a novel ternary ECL system was established with the perylene nanocrystals entrapped in Au-GO as ECL emitter, H2O2 as coreactant and hemin/G-quadruplex as a coreaction accelerator. Furthermore, combined with the one-pot target-catalyzed hairpin assembly induced strand displacement amplification strategy, we proposed a sensitive sensing method to achieve microRNA (miRNA) promising detection with a limit of detection down to 3.3 aM and a wide detection range from 0.01 fM to 10 pM. Importantly, this work provides a new direction for solving the self-aggregation of organic ECL luminophores.

Robust Synthesis of Gold-Based Multishell Structures as Plasmonic Catalysts for Selective Hydrogenation of 4-Nitrostyrene.

A robust self-template strategy is used for facile and large-scale synthesis of porous multishell gold with controllable shell number, sphere size, and in situ surface modification. The process involved the rapid reduction of novel Au-melamine colloidal templates with a great amount of NaBH4 in presence of poly(sodium-p-styrenesulfonate) (PSS). After soaking the templates in other metal salt solution, the obtained bimetallic templates could also be generally converted into bimetallic multishell structures by same reduction process. In the hydrogenation of 4-nitrostyrene using NH3 BH3 as a reducing agent, the porous triple-shell Au with surface modification (S-PTSAu) exhibited excellent selectivity (97 %) for 4-aminostyrene in contrast with unmodified triple-shell Au. Furthermore, it also showed higher enhancement of catalytic activity under irradiation of visible light as compared to similar catalysts with fewer shells.

Highly-conductive porous poly(ether ether ketone) electrolyte membranes for flexible electrochromic devices with variable infrared emittance

Electrolytes with superior ion conductivity are highly demanded for infrared variable emittance devices (IR-VEDs) to realize fast response time and good IR emittance modulation ability. Herein, a poly(ether ether ketone) (PEEK) membrane featured by well-connected pore structure and high porosity is proposed for the first time as the electrolyte material for the IR-VEDs. In contrast to the conventionally used gel electrolytes, the newly designed porous PEEK membrane is capable of obtaining high liquid electrolyte loading while maintaining good membrane integrity, resulting in high conductivity (6.8 × 10−3 S cm−1) and simple device assembly process. Flexible IR-VEDs have been fabricated by employing polyaniline films deposited on porous Au electrode as the IR regulation layer. By controlling the loading of liquid electrolyte in porous electrode, continuously high-speed ion transfer channels could be constructed between the PEEK electrolyte membrane and the Au electrode in the devices taking advantage of their porous structure. The resultant IR-VED exhibits an excellent overall performance integrating superior IR emittance adjustment ability (the emittance change in the wavelength range of 2.5–25 μm is 0.47), fast switching time (<10 s) and good cycling stability, indicating promising applications in dynamic thermal control fields.

Miniature Fuel cell with Monolithically Fabricated Si Electrode -First prototype with Au-Pd-Pt multilayer catalyst-

Our first fuel cell prototype with a novel Au-Pd-Pt catalyst successfully demonstrated power generation. In order to reduce amount of platinum group metals and obtain high CO tolerance, electrochemical atomic layer depositions of Pd and Pt were applied on the porous Au. 5 ML Pd and sub-ML Pt deposition was attempted and amounts of Pd and Pt were electrochemically estimated to be 125 μg/cm2 and 6 μg/cm2, respectively. No catalyst poisoning to 100 ppm CO was observed during the power generation. Though the peak power was poor around 100 mW/cm2, results were satisfying for the first prototype.

Vibrational response of Au-Ag alloy and porous Au single nanowires probed by ultrafast pump-probe spectroscopy

We report on the ultrafast vibrational response of single Au-Ag alloy and Au porous nanowires obtained by the template method. The oscillations of the sample reflectivity are correlated with eigenmodes (EM) confined in the nanowire. Taking advantage of a free standing geometry, we are able to detect few harmonics. Coupling electron microscopy and pump-probe investigations with a very high spectral resolution performed on the same wire, we demonstrate that both longitudinal and transverse sound velocities could be obtained for variable Au concentrations. An additional quadrupolar mode is also observed and reproduced by finite element simulation. Finally, porous nanowires are investigated.

Hierarchical porous carbon material restricted Au catalyst for highly catalytic reduction of nitroaromatics.

In this study, four kinds of porous carbon materials were used as supports to anchor gold nanoparticles (AuNPs) for catalytic reduction of nitroaromatics and 4-nitrophenol (4-NP) was employed as a model material. Results identified that carbon black (CB) restricted-Au catalyst (Au/CB) provided large specific surface area, small AuNPs size, and low cost, which showed highly catalytic activity for 4-NP reduction. Besides, with the increase of Au loadings, the catalytic activity of Au/CB was enhanced and the 1.2 wt% of Au loading exhibited the best catalytic activity with the high rate of 0.8302 min-1 and the turnover frequency of 492.50 h-1. Universality and real water application demonstrated that the as-prepared Au/CB catalyst was promising candidate for other phenols and azo dyes reduction and had great potential for practical application. Furthermore, after ten cycles, Au/CB still retained satisfying stability and activity. These results suggested that the larger specific surface area and smaller particle size attributing to the porosity of CB were conducive to improving the catalytic activity of Au catalysts. This design shows high potential of hierarchical porous carbon materials for highly catalytic reaction in many fields, especially the water purification.

Improved direct electron transfer-type bioelectrocatalysis of bilirubin oxidase using porous gold electrodes

Porous gold (Au) electrodes prepared by anodization of Au electrodes in buffer solutions containing glucose were utilized as scaffolds for direct electron transfer (DET)-type bioelectrocatalysis of bilirubin oxidase (BOD). The three-dimensional porous structure appeared to promote DET-type bioelectrocatalysis of BOD. The current density of DET-type bioelectrocatalysis of BOD increased with the formation of a three-dimensional porous structure at the beginning of anodization; however, it showed saturation characteristics with a further increase in the non-Faradaic current. Moreover, the glucose concentration during anodization affected the activity of DET-type bioelectrocatalysis of BOD. Scanning electron microscopy of the prepared porous Au electrodes showed that the structure of the electrode surface depended on the glucose concentration. Achieving a pore size distribution suitable for substrate transport and enzyme orientation seemed to be dependent on the glucose concentration.