Au Microplates Research Articles

Hydroxyethylcellulose-Directed Synthesis of Gold Microplates with Hydrogen Peroxide in an Aqueous Phase.

In this study, we successfully synthesized well-defined polygonal gold microplates for the first time in an aqueous phase using hydroxyethylcellulose (HEC) in the presence of hydrogen peroxide (H2O2). HEC played a pivotal role during the synthesis, acting not only as a biotemplate but also as an in situ reduction site for the nucleation and growth of gold (Au) microplates. H2O2 played a crucial role in accelerating the growth of Au microplates from the Au nucleus. This methodology is ecofriendly and easy to operate and has potential applications in various fields, such as electronics, photonics, and biotechnology.

Tip-Enhanced Raman Analysis of Plasmonic and Photocatalytic Properties of Copper Nanomaterials.

Theoretical predictions suggest that, in addition to gold (Au) and silver (Ag), several other metals such as copper (Cu) and aluminum (Al) can be used as plasmonic materials. However, their plasmonic and photocatalytic properties remain poorly understood. In this contribution, we employed tip-enhanced Raman spectroscopy to examine photocatalytic properties of Cu nanowires and nanocubes (CuNWs and CuNCs). Our results show that both CuNWs and CuNCs demonstrate a far more efficient photocatalytic dimerization of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene than Au nano and microplates. We also found that CuNWs and CuNCs can neither reduce 4-mercaptobenzoic acid (4-MBA) to the corresponding aromatic alcohol nor dearboxylate it forming benzenethiol. We infer that this is due to a unique coordination of 4-MBA on Cu surfaces that was only rarely observed on Au and Ag nanomaterials. Finally, we found that Cu nanostructures can oxidize 4-mercapto-phenyl-methanol to 4-MBA, which was previously only observed on gold-platinum nanoplates.

Shape-controlled template-driven growth of large CuS hexagonal nanoplates

Ultrathin 2D metal sulphide nanostructures have revealed extraordinary properties, applications and thus, the subject matter of current research. However, synthesis of shape-controlled large-sized (approximately few µms) plates/sheets are not well known, specifically due to the colloidal synthesis process resulting in the distribution of shape/size. Here, we report the template-assisted growth of large-sized CuS nanoplates (~0.5–9 µm) by simple wet chemistry, where the Au microplates serve as a template. The Au microplates act as seed and facilitate the Au-Cu via epitaxial growth of Cu. Later, the Au-Cu acts as the source of Cu and in presence of aqueous S2−, the CuS hexagons form, however, the edge of the hexagons are parallel to the Au seed and thus, hinting to be controlled by the seed. Interestingly, the substrate independent Au-Cu can even provide the platform for the spiral nanoplates, by simply decorating the outer surface of the Cu with pillar-like features.

Thermal Reshaping of Gold Microplates: Three Possible Routes and Their Transformation Mechanisms.

The thermal stability of Au micro/nanomaterials (AuMNLs) has always been a hot topic because their physicochemical properties, like surface plasmon resonance and catalytic activity, are closely related to their morphology or exposed crystal planes which are heat-sensitive. In this study, we made careful and systematic investigation of thermal deformation and reshaping of individual Au microplates (AuMPs) using atomic force microscopy. We found that AuMPs could transform into walled AuMPs (WAuMPs) and concave AuMPs (CAuMPs) at just 300 °C, which are thermodynamically and kinetically favorable products, respectively. A small fraction of AuMPs, named invariable AuMPs (IAuMPs), remained intact. However, both CAuMPs and IAuMPs can be converted to WAuMPs if the temperature is further increased. We also showed that melting of AuMPs begins from vertices and the boundaries between the top and side plane, rather than only side crystal planes as envisaged before. Finally, we performed a series of electrochemical studies showing that WAuMPs exhibited substantially higher electrocatalytic conversion of methanol at lower formal potential compared to intact AuMPs. This work shows that thermal reshaping of Au is far more complicated as was expected before. It also demonstrates how thermal reshaping can be used to improve electrocatalytic performance of Au and potentially other MNLs.

Elucidation of Tip-Broadening Effect in Tip-Enhanced Raman Spectroscopy (TERS): A Cause of Artifacts or Potential for 3D TERS

The tip-broadening effect (TBE), which makes the apparent dimensions of the imaged objects larger than they truly are, is one of the major disadvantages of atomic force microscope-related technique, like tip-enhanced Raman spectroscopy (TERS). However, the TBE also provides an opportunity to achieve three-dimensional (3D) TERS if the plasmonically active zones located at the shaft of the used tip. Herein, we found that the side part of the p-nitrothiophenol (pNTP)-modified Au microplates (AuMPs) can be observed when TBE exists. Moreover, the 3D TERS feature was applied to monitor reduction species of pNTP catalyzed by Au microplates (AuMPs). Our results showed that the 3D TERS is capable of distinguishing signals from both the side and top part in a single piece of AuMP, corresponding to Au{100} or Au{110} with low catalytic activity and Au{111} with higher catalytic activity, respectively.

Shape-controllable gold nanostructures and their SERS enhancement

We report a new growth mechanism for the formation of gold (Au) nanoflowers (NFs) or Au nanoplates by simply tuning the solvent compositions. An aniline derivative N-(3-amidino)-aniline (NAAN) can reduce HAuCl4 to form AuNFs in water, while individual Au microplates were prepared in N-methyl-pyrrolidone (NMP)/water mixed solvents. Growth mechanism showed that incomplete coverage of Au (111) facets by oxidized product poly (N-(3-amidino)-aniline) (PNAAN) resulted in AuNFs, and improved coverage on Au (111) facet by NMP was favorable for individual Au microplate formation. The layered AuNFs showed 11 times SERS enhancement compared with randomly packed Au microplate film. As low as 10−12 M 4-mercaptobenzoic acid (4-MBA) can be detected using AuNF as a SERS substrate.

Unravelling the relationship between Raman enhancement and photocatalytic activity on single anisotropic Au microplates.

Anisotropic noble-metal structures are attracting increasing attention because of interesting size- and shape-dependent properties and have emerging applications in the fields of optics and catalysis. However, it remains a significant challenge to overcome chemical contributions and acquire molecular insight into the relationship between Raman enhancement and photocatalytic activity. This study gives visualized experimental evidence of the anisotropic spatial distribution of Raman signals and photocatalytic activity at the level of single nanometer-thin Au microtriangles and microhexagons. Theoretical simulations indicate an anisotropic spatial distribution and sharpness-dependent strength of the electric-field enhancement. Analysis by using statistical surface-enhanced Raman scattering (SERS) supports this view, that is, Raman enhancement is on the order of corner>edge>face for a single microplate, but SERS measurements at different depths of focus also imply a concentration-dependent feature of SERS signals, especially at the corners and edges. Similarly, the SERS signals of product molecules in plasmonic photocatalysis also exhibit asymmetrical strengths at different corners of the same microplate. However, by examining the variations in the relative intensities of the SERS peaks, the difference in the photocatalytic activities at the corners, edges, and faces has been successfully calculated and is highly consistent with electric-field simulations, thus indicating that an increased number of molecules adsorbed at specific sites does not necessarily lead to a higher conversion ratio in noble-metal photocatalysis. Our strategy weakens the assumed impact of plasmonic local heating and, to a certain extent, excludes the influence of concentration effects and chemical contributions in noble-metal photocatalysis, thus clearly profiling plasmon-related characteristics. This study also promises a new research direction to understand the enhancement mechanism of SERS-active structures.

Photoluminescence Plasmonic Enhancement of Single Quantum Dots Coupled to Gold Microplates

Optical properties of quantum dots can be drastically changed by surface plasmons excited in neighboring metallic nanostructures. Here we investigated the photoluminescence enhancement dependence of single quantum dots on the separation distance with Au microplates by a single particle spectroscopy. The quantum dot–Au microplate hybrid structures provided a photoluminescence enhancement of up to ∼16-fold when the separation distance between the surface of Au microplate and the center of QD was 18 ± 1.9 nm. Accordingly, the lifetime of those quantum dots was shortened compared with the ones on glass plates directly. Theoretical simulation revealed that the variation of photoluminescence enhancement was closely dependent on the separation distance between the QD and the Au microplate, in good agreement with the experimental results.

Aqueous Epitaxial Growth of ZnO on Single Crystalline Au Microplates

Epitaxial ZnO thin films were grown on single crystalline Au microplates from an aqueous solution at 90 °C. The composite structures were approximately 100 μm wide. The epitaxial ZnO films were smooth, continuous, and several micrometers thick. We examined the epitaxial relationship between the ZnO and Au through scanning electron microscopy, electron backscatter diffraction, high-resolution transmission electron microscopy, and electron diffraction and demonstrated control over the resulting film morphology through the kinetics of growth. An epitaxial relationship of ZnO[1100](0002)∥Au[211](111) was dominant in the structures.

Top-contacting molecular monolayers using single crystalline Au microplate electrodes

Integrating molecules in electronic devices to exploit their unique properties requires efficient methods to bring about their electrical contact in large arrays. In this study, chemically synthesized single crystalline Au microplates have been employed as top-contact electrodes in a micro-sandwich device where molecules could be sandwiched between the bottom Au film and the top microplate in capacitor geometry. The device has been tested employing many alkanemono- and -dithiols not only in as-adsorbed conditions but also under dynamic loading. Further, encapsulation using PDMS was carried out which enhanced its stability and shelf-life.

Molecule counting with alkanethiol and DNA immobilized on gold microplates for extended gate FET

Several molecule counting methods based on electrochemical characterization of alkanethiol and thiolated single-stranded oligonucleotide (HS-ssDNA) immobilized on gold microplates, which were used as extended gates of field effect transistors (FETs), have been investigated in this paper. The surface density of alkanethiol and DNA monolayers on gold microplates were quantitatively evaluated from the reductive desorption charge by using cyclic voltammetry (CV) and fast CV (FCV) methods in strong alkali solution. Typically, the surface density of 6-hydroxy-1-hexanethiol (6-HHT) was evaluated to be 4.639molecules/nm2, and the 28 base-pair dsDNA about 1.226–4.849molecules/100nm2 on Au microplates after post-treatment with 6-HHT. The behaviors on surface potential and capacitance of different aminoalkanethiols on Au microplates were measured in 0.1mol/L Na2SO4 and 10mmol/L Tris–HCl (pH=7.4) solutions, indicating that the surface potential increases and the double-layer capacitance decreases with the length of carbon chain increased for the thiol monolayers, which obey a physics relationship for a capacitor. Comparably, a simple sensing method based on the electronic signals of biochemical reaction events on DNA immobilization and hybridization at the Au surface of the extended gate FET (EGFET) was developed, with which the surface density of the hybridized dsDNA on the gold surface of the EGFET was evaluated to be 1.36 molecules per 100nm2, showing that the EGFET is a promising sensing biochip for DNA molecule counting.

A Real Time Microscopy Study of the Growth of Giant Au Microplates

A simple air-thermolysis of a precursor, (AuCl4)− phase transferred from aqueous medium and stabilized in toluene by tetraoctylammonium bromide (ToABr), is found to yield unusually large (∼12 000 μm2) hexagonal and triangular Au microplates. The optimal conditions for the growth of microplates with edge lengths over 100 μm are (AuCl4)−/ToABr ratio of 1:4.4 for phase transfer and thermolysis on a flat substrate such as Si at 130 °C for 24 h. Scanning electron microscopy performed using a heated stage showed nucleation of nanoparticles into dendrimeric structures and rudimentary shapes, which took several minutes to anneal into well-formed single crystalline triangles and hexagons. Amazingly quite often the plates were held vertical to the substrate surface during the growth even at large sizes, as evidenced by in situ optical microscopy. Three growth modes, namely, nucleation of tiny nanoparticles into dendrimeric structures which assemble into specific shapes, atom-by-atom addition from the precursor feedstock, and fusion of three-dimensional particles to the growing microplates, have been inferred from this study using real time microscopy.

Movable Au microplates as fluorescence enhancing substrates for live cells

Hexagonal and triangular Au microplates extending over an area of ~12,000 µm 2 with thickness in the range 30–1000 nm have been synthesized using a single step thermolysis of (AuCl4) – –tetraoctylammonium bromide complex in air. The microplates are self-supporting and can be easily manipulated using a sharp pin, a property which enables them to serve as substrates for living cells. The microplate surface is non-toxic to living cells and can enhance the fluorescence signal from fluorophores residing within the cell by an order of magnitude. In addition, the microplates are smooth and single-crystalline, and ideal as microscopy substrates and molecular electrodes. The growth of the microplates in the initial stages is interesting in that they seem to grow perpendicular to the substrate, as evidenced by in situ microscopy.

Gold microplates: Gold Microplates with Well‐Defined Shapes (Small 16/2010)

The frontispiece image shows the electron microscopy images of newly discovered Au microplates fabricated on the basis of a polyol process with variations during synthesis. These remarkable structures are formed through a simple process. It is believed that the introduction of obvious temperature variation in the early stage of nuclei formation is important for the growth of these structures. Simulations are also carried out to explain the experimental observations. Although there are still unresolved problems, the results represent a significant step in gaining a better understanding of the anisotropic growth of noble metals. Scientists can learn more about the physics and chemistry of how crystals grow simply by understanding the appearance of new structures; this knowledge has the potential to result in applications in microelectronic systems that are not yet foreseeable. For more information, please read the Full Paper “Gold Microplates with Well-Defined Shapes” by C. X. Kan and co-workers, beginning on page 1768.

Gold Microplates with Well‐Defined Shapes

The synthesis and growth mechanism of well-defined nanostructures are still challenging. In this study, gold microplates with starlike, shieldlike, and other polygonal shapes are successfully achieved in high yields on the basis of the polyol process. Structural studies demonstrate that these newly shaped Au plates are single-crystalline, several micrometers in lateral size, and tens of nanometers in thickness. It is believed that the introduction of temperature variation in the early stage of crystal growth is important for these products. The newly discovered Au microplates result from the growth of the {111} plane along the 211 and other high-index directions, in addition to the {111}-close-packed 110 directions. Simulations on the multiple-twin-induced crystal growth and surface energy are also carried out to explain the experimental observations. This work is valuable for anisotropic growth of newly shaped noble-metal nanostructures.

Synthesis of gold microplates using bovine serum albumin as a reductant and a stabilizer.

Gold microplates were synthesized in aqueous solutions by reducing HAuCl(4) with the hydroxyl groups in both serine and threonine of bovine serum albumin (BSA), which is a globular protein in its native state. In this article, we systematically investigated the effects of temperature, pH value, the concentration of BSA, and ionic species on the reduction kinetics and thus the size and morphology of the final product. The optimal experimental conditions for producing uniform Au microplates include the following: an elevated temperature in the range of 55-65 degrees C, an acidic solution with pH approximately 3, and the presence of NaCl (0.14 M). We found that if any one of these parameters was deviated from the optimal condition, Au microplates would not be formed in high yields. We also found that the surfaces of the as-synthesized Au microplates were covered by a dense array of BSA bumps.

Mechanistic Study of the Synthesis of Au Nanotadpoles, Nanokites, and Microplates by Reducing Aqueous HAuCl4 with Poly(vinyl pyrrolidone)

This article describes a simple approach to anisotropic Au nanostructures with various shapes by reducing HAuCl 4 with poly(vinyl pyrrolidone) (PVP) in aqueous solutions without the use of any additional capping agent or reductant. In this approach, the commercially available PVP servers as a mild reducing agent thanks to its hydroxyl (-OH) end groups, enabling kinetic control over both nucleation and growth. As the volume of HAuCl 4 solution added to the reaction was increased, the morphology of Au nanostructures evolved from nanotadpoles to nanokites and then triangular and hexagonal microplates. The slow reduction rate associated with the mild reducing power of PVP plays a critical role in forming nanoplates during nucleation as well as their growth into highly anisotropic nanostructures. Electron microscopy studies reveal that the nanotadpoles and nanokites are formed through the linear fusion of small Au particles (<10 nm) to the initially formed nanoplates, whereas the microplates result from the continuous addition of Au atoms to the side faces of nanoplates. Through this morphological control, the localized surface plasmon resonance peaks of these Au nanostructures can be tuned in the visible and near-IR regions.

PVP-capped twinned gold plates from nanometer to micrometer

PVP (poly(vinyl pyrrolidone))-capped micrometer-sized twinned gold (Au) plates with the shape of hexagon or triangle have been successfully synthesized in a large quantity by reducing hydrogen tetrachloroauric acid (HAuCl4·3H2O) using ethylene glycol in the presence of (PVP) molecules at 200°C under the extra condition of autogenous pressure. Clear twin boundaries in thus-obtained Au plates have been observed by scanning electron microscope (SEM). To further elucidate the mechanism of formation of the twinned Au plates, transmission electron microscopy (TEM) has been employed to characterize smaller Au nanoplates obtained simultaneously with the Au microplates, the occurrence of 1/3{422} forbidden reflections in selected area electron diffraction (SAED) can be believed to result from the presence of twinning boundary within the Au (111) plane normal to TEM electron beam. In addition, X-ray photoelectron spectroscopy (XPS) experiment has confirmed that PVP molecules exist on the surface of the Au cores, which may play a very important role in the formation and evolution of the twinned Au plates. A possible growth mechanism has been suggested to explain the Au plate evolution from nanometer to micrometer.

Watching the Transformation from Au Nanoparticles to Microplates

Abstract The transformation from Au nanoparticles to microplates was observed. Triangular and hexagonal Au microplates with canal-like stripes were finally formed. It was proposed that Au atoms or ions “flowed” along limited path on Au(111) surface toward edge of microplates.

Single-Crystalline Gold Microplates: Synthesis, Characterization, and Thermal Stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.