3D Gold Research Articles

Theoretical and Experimental Insights into the Origin of the Catalytic Activity of Subnanometric Gold Clusters: Attempts to Predict Reactivity with Clusters and Nanoparticles of Gold

Particle size is one of the key parameters determining the unexpected catalytic activity of gold, with reactivity improving as the particle gets smaller. While this is valid in the 1-5 nm range, chemists are now investigating the influence of particle size in the subnanometer regime. This is due to recent advances in both characterization techniques and synthetic routes capable of stabilizing these size-controlled gold clusters. Researchers reported in early studies that small clusters or aggregates of a few atoms can be extremely active in some reactions, while 1-2 nm nanoparticles are catalytically more efficient for other reactions. Furthermore, the possibility that small gold clusters generated in situ from gold salts or complexes could be the real active species in homogeneous gold-catalyzed organic reactions should be considered. In this Account, we address two questions. First, what is the origin of the enhanced reactivity of gold clusters on the subnanometer scale? And second, how can we predict the reactions where small clusters should work better than larger nanoparticles? Both geometric factors and electronic or quantum size effects become important in the subnanometer regime. Geometric reasons play a key role in hydrogenation reactions, where only accessible low coordinated neutral Au atoms are needed to dissociate H2. The quantum size effects of gold clusters are important as well, as clusters formed by only a few atoms have discrete molecule-like electronic states and their chemical reactivity is related to interactions between the cluster's frontier molecular orbitals and those of the reactant molecules. From first principles calculations, we predict an enhanced reactivity of small planar clusters for reactions involving activation of CC multiple bonds in alkenes and alkynes through Lewis acid-base interactions, and a better catalytic performance of 3D gold nanoparticles in redox reactions involving bond dissociation by oxidative addition and new bond formation by reductive elimination. In oxidation reactions with molecular O2, initial dissociation of O2 into basic oxygen atoms would be more effectively catalyzed by gold nanoparticles of ∼1 nm diameter. In contrast, small planar clusters should be more active for reactions following a radical pathway involving peroxo or hydroperoxo intermediates. We have experimentally confirmed these predictions for a series of Lewis acid and oxidation reactions catalyzed by gold clusters and nanoparticles either in solution or supported on solid carriers.

CMOS Neurotransmitter Microarray: 96-Channel Integrated Potentiostat With On-Die Microsensors

A 8 × 12 array of integrated potentiostats for on-CMOS neurotransmitter imaging is presented. Each potentiostat channel measures bidirectional redox currents proportional to the concentration of a neurochemical. By combining the current-to-frequency and the single-slope analog-to-digital converter (ADC) architectures a total linear dynamic range of 95 dB is achieved. A 3.8 mm × 3.1 mm prototype fabricated in a 0.35 μm standard CMOS technology was integrated with flat and 3D on-die gold microelectrodes and an on-chip microfluidic network. It is experimentally validated in in-situ recording of neurotransmitter dopamine.

Highly sensitive 3D gold nanotube ensembles: Application to electrochemical determination of metronidazole

Three-dimensional gold nanoelectrode ensembles (3D GNE) have proven to be promising nanoelectrodes by representing much higher sensitivity compared to both their 2D nanostructures and bulk counterparts. The sensitivity of 3D gold nanotubes (GNTs) fabricated through electrodeposition inside the pores of polycarbonate templates, was examined toward metronidazole (MTZ) as one of their pharmaceutical applications. The electrochemical behavior of MTZ at the 3D GNT-modified electrode was discussed in detail through cyclic voltammetry (CV) which suggested an irreversible reduction of nitro group to the corresponding hydroxylamine and a subsequently reversible redox peak for the corresponding product. This latter redox peak was selected for MTZ determination which has not been reported, so far. Square wave voltammetry (SWV) was used as a powerful tool for detection of MTZ in different concentrations. Under the optimal conditions, the highly sensitive 3D GNT showed a wide linear range from 1nM to 2μM with a sufficiently low detection limit of 0.1nM demonstrating promising results for the future usage in clinical applications.

Direct Experimental Evidence of Nucleation and Kinetics Driven Two-Dimensional Growth of Core–Shell Structures

We report the appearance of different anisotropic one-dimensional (1D) and two-dimensional (2D) gold (Au) core–silver (Ag) shell nanostructures in a single reaction environment. High-resolution transmission electron microscopy (HRTEM) images taken from the core region of a core–shell nanostructure revealed the prominent presence of the hexagonal-shaped gold seed leading to formation of a triangular final particle. The HRTEM studies provide the direct experimental evidence of the “silver halide” model proposed by Sigmund et al. (Lofton, C.; Sigmund, W. Adv. Funct. Mater.2005, 15, 1197–1208) to explain the kinetic growth mechanism behind their formation. It is important that this information cannot be identified from a single composite nanoparticle due to the lack of atomic number (Z) contrast difference. We have studied energy-dispersive X-ray line profile spectra and elemental mapping using the high-angle annular dark-field scanning/transmission electron microscopy (STEM-HAADF) technique corresponding to the Au-M and Ag-K energies from the similar core–shell structures. This confirms the composition of the core to be made of gold and that of the shell of silver. The line profile along the relative thickness map of a hexagonal and triangular nanoplate obtained using energy-filtered TEM indicates the formation of nearly uniform 2D structures. The nearly equal thicknesses of the core and outside shell of a core–shell nanoplate measured using electron energy loss spectroscopy in STEM mode also confirm the 2D growth of a gold seed forming a triangular core–shell nanoplate.

In vivo characterization of tumor vasculature using iodine and gold nanoparticles and dual energy micro-CT

Tumor blood volume and vascular permeability are well established indicators of tumor angiogenesis and important predictors in cancer diagnosis, planning and treatment. In this work, we establish a novel preclinical imaging protocol which allows quantitative measurement of both metrics simultaneously. First, gold nanoparticles are injected and allowed to extravasate into the tumor, and then liposomal iodine nanoparticles are injected. Combining a previously optimized dual energy micro-CT scan using high-flux polychromatic x-ray sources (energies: 40 kVp, 80 kVp) with a novel post-reconstruction spectral filtration scheme, we are able to decompose the results into 3D iodine and gold maps, allowing simultaneous measurement of extravasated gold and intravascular iodine concentrations. Using a digital resolution phantom, the mean limits of detectability (mean CNR = 5) for each element are determined to be 2.3 mg mL−1 (18 mM) for iodine and 1.0 mg mL−1 (5.1 mM) for gold, well within the observed in vivo concentrations of each element (I: 0–24 mg mL−1, Au: 0–9 mg mL−1) and a factor of 10 improvement over the limits without post-reconstruction spectral filtration. Using a calibration phantom, these limits are validated and an optimal sensitivity matrix for performing decomposition using our micro-CT system is derived. Finally, using a primary mouse model of soft-tissue sarcoma, we demonstrate the in vivo application of the protocol to measure fractional blood volume and vascular permeability over the course of five days of active tumor growth.

Size-controlled synthesis of gold bipyramids using an aqueous mixture of CTAC and salicylate anions as the soft template

One-dimensional (1D) gold (Au) bipyramids are successfully synthesized through a facile seed-mediated method using cetyltrimethylammonium chloride (CTAC), Au seed nanoparticles, Ag(+) ions, and ascorbic acid. The length and optical properties of the synthesized Au bipyramids are controlled with precision by varying the amount of salicylate anions (Sal(-)) added during the synthesis. The micelles formed from CTA(+)-Sal(-) mixtures in aqueous solutions act as effective templates for the size-controlled synthesis of 1D nanocrystals.

Nanoengineering gold particle composite fibers for cardiac tissue engineering

Gold nanostructures can be incorporated into macroporous scaffolds to increase the matrix conductivity and enhance the electrical signal transfer between cardiac cells. Here we report a simple approach for fabricating 3-dimensional (3D) gold nanoparticle (NP)-based fibrous scaffolds, for engineering functional cardiac tissues generating a strong contraction force. A polycaprolactone-gelatin mixture was electrospun to obtain fibrous scaffolds with an average fiber diameter of 250 nm. In a facile method, gold NPs were evaporated on the surface of the fibers, creating nanocomposites with a nominal gold thickness of 2, 4, and 14 nm. Compared to pristine scaffolds, cardiac cells seeded on the nano-gold scaffolds assembled into more elongated and aligned tissues. The gold NPs on the fibers were able to maintain the ratio of cardiomyocytes to fibroblasts in the culture, to encourage the growth of cardiomyocytes with significantly higher aspect ratio, and promote massive cardiac sarcomeric actinin expression. Finally, engineering cardiac tissues within gold NP-based scaffolds exhibited significantly higher contraction amplitudes and rates, as compared to scaffolds without gold. We envision that cardiac tissues engineered within these gold NP scaffolds can be used to improve the function of the infarcted heart.

Spectroscopic Properties of Multilayered Gold Nanoparticle 2D Sheets

We report the fabrication technique and optical properties of multilayered two-dimensional (2D) gold nanoparticle sheets ("Au nanosheet"). The 2D crystalline monolayer sheet composed of Au nanoparticles shows an absorption peak originating from a localized surface plasmon resonance (LSPR). It was found that the absorption spectra dramatically change when the monolayers are assembled into the multilayers on different substrates (quartz or Au). In the case of the multilayers on Au thin film (d = 200 nm), the LSPR peak is shifted to longer wavelength at the near-IR region by increasing the number of layers. The absorbance also depends on the layer number and shows the nonlinear behavior. On the other hand, the multilayers on quartz substrate show neither such LSPR peak shift nor nonlinear response of absorbance. The layer number dependence on metal surfaces can be interpreted as the combined effects between the near-field coupling of the LSPR and the far-field optics of the stratified metamaterial films, as proposed in our previous study. We also report the spectroscopic properties of hybrid multilayers composed of two kinds of monolayers, i.e., Au nanosheet and Ag nanosheet. The combination of the different metal nanoparticle sheets realizes more flexible plasmonic color tuning.

A Glucose Biosensor Based on a 3D Nanostructured Gold Electrode

In this study, we investigated a simple glucose biosensor based on a 3D nanostructured gold electrode. The nanostructured gold electrode was fabricated by electrochemical deposition using bulk gold dissolved in aqua regia as the nanogold precursor. Potassium ferricyanide and glucose oxidase were then sequentially coated onto the prepared nanostructure for glucose detection. For the nanostructured electrode, the polarization overpotential for glucose detection was 0.17 V, which is much smaller than that for a plane electrode. An applied potential of 0.7 V was sufficient to generate peak cathodic currents. The peak cathodic and anodic currents for the nanostructured electrode are three orders of magnitude larger than those for a plane gold electrode. The nanostructured electrode shows a fast response time of 1 s.

Large‐Area 2D Gold Nanorod Arrays Assembled on Block Copolymer Templates

Various large-area 2D gold nanorod arrays are achieved on plasma-etched block copolymer templates. With the help of capillary forces, aqueous gold nanorods assembled on the templates show good position selectivity and high coverage of the grooves. Furthermore, these nanorod arrays can transform into gold nanowires by a post-etching process.

Multi-scale Plasmonic Nanoparticles and the Inverse Problem.

This Perspective describes how multi-scale plasmonic structures with two or more length scales (fine, medium, coarse) provide an experimental route for addressing the inverse problem. Specific near-field and far-field optical properties can be targeted and compiled into a plasmon resonance library by taking advantage of length scales spanning three orders of magnitude, from 1 nm to greater than 1000 nm, in a single particle. Examples of multi-scale 1D, 2D, and 3D gold structures created by nanofabrication tools and templates are discussed, and unexpected optical properties compared to those from their smaller counterparts are emphasized. One application of multi-scale particle dimers for surface-enhanced Raman spectroscopy is also described.

Symmetry-Dependent Plasmonic Properties of Three-Dimensional Hybrid Metallic Nanostructure Arrays

We demonstrate the successful fabrication of various well-ordered 3D gold nanostructure arrays using nanosphere lithography method, and further reveal the strong dependence of the optical responses on their in-plane symmetry. For the concentrically stacked ring-cap array, its optical absorption behavior is similar to that of a ring array with the same dimension because they have the same in-plane symmetry. However, for the nonconcentrically stacked hole-cap array, the broken in-plane symmetry results in the appearance of crescent-shaped nanogaps at the interfaces and thus leads to a novel strong plasmon resonance mode. The finite-difference time-domain simulation shows that charge mainly assembles to the sharp edges of the nanogaps at the resonant wavelength and remarkable electric field enhancement is achieved around the sharp edges. Furthermore, the strongest resonance modes of the ring-cap array and hole-cap array show large red shift as the nanostructure size increases. The presented 3D nanostructure arrays may offer a spectrum of applications in sensing.

Density functional theory study of Aun (n = 1–5) clusters supported on montmorillonite

AbstractThe adsorption and nucleation of gold clusters Aun (n = 1–5) on montmorillonite (MMT) is studied using the density functional theory. All the calculations were performed using the full‐potential linearized augmented‐plane‐wave method as implemented in the WIEN2k code. We constructed a MMT supercell of formula Si16Al6Mg2O40(OH)8 containing four unit cells and the first moments of the nucleation process were studied by adding the gold atoms one by one on the MMT. The results show that the interaction energies between the gold clusters and the MMT are negative indicating that the Aun–MMT complexes are stable. In the Aun–MMT systems (n = 3–5), two gold atoms maintain the coordination with basal oxygens and the AuO distance was about 2.0–2.3 Å. The AuAu average distance is 2.6 Å in the supported gold clusters. The formation of the second layer of gold atoms occurs upon the arrival of the third gold atom. We are reporting that the transition of 2D → 3D gold cluster structures is located from Au3 to Au4 on MMT. The total density of states of clusters Au, Au2, Au3, Au4, and Au5 on MMT allows us to affirm that (i) gold atoms are the main contributors to the states closest to the Fermi level and (ii) in Au4–MMT and Au5–MMT systems, the main contributors to the states closest to the Fermi level are the most external gold atoms, and therefore these atoms are probably the most susceptible to interact with adsorbates and the most active sites. Odd–even oscillations in the values of the energy gap and interaction energy were found: the odd‐numbered supported clusters, Au3 and Au5, have larger energy gaps and more negative interaction energies. © 2012 Wiley Periodicals, Inc.

DNA–Gold Conjugates: Formation of 1D and 2D Gold Nanoparticle Arrays by Divalent DNA–Gold Nanoparticle Conjugates (Small 15/2012)

DNA–Gold Conjugates: Formation of 1D and 2D Gold Nanoparticle Arrays by Divalent DNA–Gold Nanoparticle Conjugates (Small 15/2012)

Gold Nanoparticle-Embedded Porous Graphene Thin Films Fabricated via Layer-by-Layer Self-Assembly and Subsequent Thermal Annealing for Electrochemical Sensing

A uniform three-dimensional (3D) gold nanoparticle (AuNP)-embedded porous graphene (AuEPG) thin film has been fabricated by electrostatic layer-by-layer assembly of AuNPs and graphene nanosheets functionalized with bovine serum albumin and subsequent thermal annealing in air at 340 °C for 2 h. Scanning electron microscopy (SEM) investigations for the AuEPG film indicate that an AuNP was embedded in every pore of the porous graphene film, something that was difficult to achieve with previously reported methods. The mechanism of formation of the AuEPG film was initially explored. Application of the AuEPG film in electrochemical sensing was further demonstrated by use of H(2)O(2) as a model analyte. The AuEPG film-modified electrode showed improved electrochemical performance in H(2)O(2) detection compared with nonporous graphene-AuNP composite film-modified electrodes, which is mainly attributed to the porous structure of the AuEPG film. This work opens up a new and facile way for direct preparation of metal or metal oxide nanoparticle-embedded porous graphene composite films, which will enable exciting opportunities in highly sensitive electrochemical sensors and other advanced applications based on graphene-metal composites.

Realization of thermally durable close-packed 2D gold nanoparticle arrays using self-assembly and plasma etching

Realization of thermally and chemically durable, ordered gold nanostructures using bottom-up self-assembly techniques are essential for applications in a wide range of areas including catalysis, energy generation, and sensing. Herein, we describe a modular process for realizing uniform arrays of gold nanoparticles, with interparticle spacings of 2 nm and above, by using RF plasma etching to remove ligands from self-assembled arrays of ligand-coated gold nanoparticles. Both nanoscale imaging and macroscale spectroscopic characterization techniques were used to determine the optimal conditions for plasma etching, namely RF power, operating pressure, duration of treatment, and type of gas. We then studied the effect of nanoparticle size, interparticle spacing, and type of substrate on the thermal durability of plasma-treated and untreated nanoparticle arrays. Plasma-treated arrays showed enhanced chemical and thermal durability, on account of the removal of ligands. To illustrate the application potential of the developed process, robust SERS (surface-enhanced Raman scattering) substrates were formed using plasma-treated arrays of silver-coated gold nanoparticles that had a silicon wafer or photopaper as the underlying support. The measured value of the average SERS enhancement factor (2 × 105) was quantitatively reproducible on both silicon and paper substrates. The silicon substrates gave quantitatively reproducible results even after thermal annealing. The paper-based SERS substrate was also used to swab and detect probe molecules deposited on a solid surface.

Surface-Enhanced Fluorescence: Mapping Individual Hot Spots in Silica-Protected 2D Gold Nanotriangle Arrays

Localized surface plasmon resonance (LSPR) is of particular interest to enhance the limit of detection for spectroscopic techniques such as Raman and fluorescence via a surface enhancement from metallic nanostructures and it is a key for single-molecule detection. In this study, using nanosphere lithography (NSL), a series of gold nanostructures over glass surfaces are prepared and modified to detect the position and the density of the individual confined hot spots. Once protected with an ultrathin layer of SiO2, the gold nanostructures are investigated using scanning confocal fluorescence microscopy to detect the fluorescence from a dye solution deposited over the SiO2-protected gold platforms. The fluorescence originates from an assembly of confined and homogeneously distributed hot spots. This clearly demonstrates that the dimensions of particles, the interparticle distance, and the thickness of the SiO2 protection layer are critical parameters to obtain the optimum electromagnetic enhancement. Herein, we show that the NSL-fabricated nanotriangle arrays made with particle sizes with dimensions closer to the excitation wavelength display the most intense hot spots for each bow tie assembly oriented along the polarization direction of the impinging light.

Catalytic Oxygen Activation on Helical Gold Nanowires

Oxygen activation on one-dimensional (1D) gold nanotube T(6,0) and nanowire W6-1 was investigated using first principles density functional methods (DFT). It is found that the oxygen adsorption strength on T(6,0) is unexpectedly weak, whereas W6-1 with inner Au–Au bonds is more active for O2 adsorption. To further promote the catalyzed oxygen activation, we proposed and evaluated two working strategies, including imposing a small strain along the axial direction and doping alien transition metal (TM) atoms. Specifically, 4% compression or doping Pt atoms can decrease the O2 dissociation barriers from 0.85 eV to a more desired 0.6 eV. The analysis of electronic structures and charge density difference revealed that the inner Au–TM bond can decrease the filling degree of the Au-dz2 orbital and, thus, improve the oxygen activity.

Novel 3D arrays of gold nanostructures on suspended platinum-coated carbon nanotubes as surface-enhanced Raman scattering substrates

We report electrochemically controlled fabrications and the corresponding enhancement in Raman scattering of the three-dimensional (3D) gold arrays prepared on the template of platinum-coated carbon nanotubes (CNTs) by electrochemical deposition (ECD). The topography of deposited gold was varied from nanoparticles to nanowires by ECD conditions. The increasing surface area of 3D arrays of gold nanoparticles contributed to increase significantly the sensitivity of surface enhanced Raman scattering (SERS). For the same 3D arrays consisting of gold nanowires but with different surface roughness, much higher SERS activity was obtained when the surface of nanowires was rough.

Formation of 1D and 2D Gold Nanoparticle Arrays by Divalent DNA–Gold Nanoparticle Conjugates

Divalent DNA-AuNP (gold nanoparticle) conjugates comprising two DNA strands at diametrically opposed positions are prepared. Highly linear 1D and tetragonal lattice-like 2D AuNP arrays are constructed using the conjugates and DNA assemblies based on T- and double-crossover motifs and the Holliday junction.