Gold Nanoplates Research Articles

Experimental and Theoretical Studies on Sustainable Synthesis of Gold Sol Displaying Dichroic Effect.

Gold nanoparticles depending on their shape and mixtures of multiple shapes can exhibit peculiar optical properties, including the dichroic effect typical of the Lycurgus cup, which has puzzled scientists for a long time. Such optical properties have been recently exploited in several fields such as paint technology, sensors, dichroic polarizers, display (LCD) devices, laser applications, solar cells and photothermal therapy among others. In this article, we have demonstrated a simple room temperature one-pot synthesis of gold sol displaying a dichroic effect using a slow reduction protocol involving only trisodium citrate as a reducing agent. We found that the dichroic gold sol can be easily formed at room temperature by reducing gold salt by trisodium citrate below a certain critical concentration. The sol displayed an orangish-brown color in scattered/reflected light and violet/blue/indigo/purple/red/pink in transmitted light, depending on the experimental conditions. With minor changes such as the introduction of a third molecule or replacing a small amount of water in the reaction mixture with ethanol, the color of the gold sol under transmitted light changed and a variety of shades of red, pink, cobalt blue, violet, magenta and purple were obtained. The main advantage of the proposed method lies in its simplicity, which involves the identification of the right ratio of the reactants, and simple mixing of reactants at room temperature with no other requirements. TEM micrographs displayed the formation of two main types of particles viz. single crystal gold nanoplates and polycrystalline faceted polyhedron nanoparticles. The mechanism of growth of the nanoplates and faceted polyhedron particles have been described by an enhanced diffusion limited aggregation numerical scheme, where it was assumed that both trisodium citrate and the gold ions in solution undergo a stochastic Brownian motion, and that the evolution of the entire system is regulated by a principle of energy minimization. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct.

Gold nanoplates with superb photothermal efficiency and peroxidase-like activity for rapid and synergistic antibacterial therapy.

Gold nanoplates (AuNPTs) exhibit outstanding photothermal conversion efficiency (68.5%) and peroxidase-like activity. The combination of the very low H2O2 concentration (0.1 mM) and the low AuNPT dosage (50 μg mL-1) with 808 nm laser irradiation (1 W cm-2, 3 min) shows excellent synergistic antibacterial ability and healing of MRSA-infected wounds in vivo.

Visual detection of different metal ions based on the tug of war between triangular Au nanoparticles and metal ions against mercaptans.

Herein, a new colorimetric sensor array was developed for the first time, which can rapidly recognize 9 types of metal ions (e.g., Ni2+, Zn2+, Cd2+, Cu2+, Cr3+, Fe2+, Se2+, Mn2+, and Mg2+). The colorimetric characteristics of the sensor array were closely related to the oxidation etching of the triangular gold nanoplates (AuNPLs) by hydrogen peroxide (H2O2), catalyzed by horseradish peroxidase (HRP). In the design, two types of thiols (glutathione (GSH) and cysteine (Cys)) as recognition elements were employed to construct the sensor units (AuNPLs/GSH and AuNPLs/Cys) and adjust the etching degrees of AuNPLs in the presence of various metal ions. The differential binding affinities between metal ions and thiols will lead to different degrees of oxidation etching of AuNPLs with hydrogen peroxide, exhibiting characteristic colors, which can be visually distinguished by the naked eye. Thus, the colorimetric sensor array provides a new way for the discrimination of various metal ions, thereby simplifying the water quality analysis.

Impact of microfabrication processing temperatures on the resonant wavelength of gold nanoplates

We show that low temperature processing (100–400 °C) of gold nanoplates (GNPs) permanently shifts their resonant wavelength peak from the infrared to visible (860–650 nm) after an hour at 350 °C. The mechanism is melting-induced shape change; as sharp triangular corners become round the smaller effective GNP diameter produces shorter resonant wavelengths. These wavelength shifts are relevant to GNP-driven microelectromechanical systems, where temperatures of 150–350 °C arise during thin film processing. Strategies to accommodate the observed thermal shifts include starting with particles that have a longer resonant wavelength, ensuring that nanoplates are well-dispersed, and using sub-200 °C processing methods.

Microwave irradiation-based synthesis of anisotropic gold nanoplates using Staphylococcus hominis as reductant and its optimization for therapeutic applications

The pivotal aim of this investigation was to synthesize anisotropic gold nanoplates using Staphylococcus hominis strain MANF2 as reducing agent and optimize statistically its production for therapeutic applications. Initially, gold nanoparticles were synthesized using strain MANF2 via microwave irradiation method which reduced Au3+ ions to Au+ ions, as evident by change in the colour of solution to purple, followed by observation of characteristic surface plasmon resonance peak (λmax) at 530 nm in UV–vis spectrum. Box-Behnken Design-based optimization (N = 17) estimated maximum production of nanoparticles (λmax – 3.18) under the conditions of 1 mM of auric chloride, 1 mL of reducing agent, and irradiation period of 45 s. Data obtained were modelled and trained using multilayer perceptron and Marquardt-Levenberg algorithm of artificial neural networking, respectively which showed the constructed neural network topology “3-8-1” neurons in input, hidden, and output layers, depicting the best fitness function. The optimized nanoparticles were characterized using varied analytical tools viz. UV–vis spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, Zeta potential analyzer, Dynamic light scattering (DLS), X-ray diffraction (XRD), and Energy-Dispersive X-ray Spectroscopy (EDX). Scanning electron microscopy (SEM) analysis revealed pentagonal and square-shaped gold nanoplates of 100 nm in size with sharp edges. Anti-tubercular activities of gold nanoplates were assessed using luciferase reporter phase (LRP) assay which exhibited relative light unit reduction of 38.2 ± 0.3–92.6 ± 0.4 % at varied concentrations. Anticancer activity of gold nanoplates was demonstrated against HT-29 (colon cancer) cell line using MTT assay. Results showed promising anticancer traits of gold nanoplates with IC50 value of 41.3 μg/mL against HT-29 cells. In conclusion, S. hominis-based synthesized anisotropic/polygonal gold nanoplates can certainly be implied as auspicious anti-tubercular and anticancer agents for designing ideal therapeutic drugs in future.

A multicolor colorimetric assay for sensitive detection of sulfide ions based on anti-etching of triangular gold nanoplates

This study presents a viable colorimetric assay for sensitive sensing of sulfide ions (S2−) using triangular gold nanoplates (AuNPLs) as nanoprobes. The sensing mechanism is based on regulating the etching process of AuNPLs upon the addition of different concentration of S2−. In the presence of Cu2+ and I−, the AuNPLs would be etched by triiodide ions (I3−) produced from the redox reaction between Cu2+ and I−, resulting in the transformation of the shape of AuNPLs from triangular into sphere and blue shifted localized surface plasmon resonance (LSPR). However, in the presence of S2−, a preferential reaction between Cu2+ and S2− occurred, and the inhibition of the formation of I3− prevented the AuNPLs from being etched. Correspondingly, a red shift of LSPR can be observed with an increase of the concentration of S2−, accompanying a readily distinguishable color transition that goes from pink to purplish red, purple, bluish purple and blue. The color alterations allow for a visual estimation of the concentrations of S2−, and the lowest visual concentration through naked-eyes was 0.1 μM. A good linear relationship (R2 = 0.997) between the peak shifts and the concentration of S2− was obtained in the range of 0.02–1.5 μM, and the limit of detection (3 s/k) by UV–vis spectroscopy was as low as 16 nM. The as-established assay displayed excellent selectivity, which was successfully applied for analysis of S2− in environmental water samples.

Gate-tunable Casimir equilibria with transparent conductive oxides

The active control of small particles due to the Casimir force is of great interest for noncontact and low-friction micro- and nanoelectromechanical systems (MEMS and NEMS). In this work, we study theoretically the Casimir force between a nanoplate and a metal-oxide-semiconductor (MOS) substrate immersed in a liquid environment. The MOS structure consists of layers indium-tin-oxide (ITO)/Teflon/Au, where a gate electrode is connected between the ITO film and the gold substrate. Our results suggest that the Casimir equilibria are determined, not only by the layer thicknesses in MOS structures but also by the carrier densities of ITO (gate-controllable). Under proper conditions, a gold nanoplate can be stably trapped due to the Casimir forces. Moreover, we show that the trapping state can be released by decreasing the carrier density in the active layer of ITO, e.g., from ${10}^{21}$ to ${10}^{19}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The switching between stable trapping and the release state of a suspended Teflon nanoplate is also demonstrated. Our findings can be useful in designing ultrafast switchable devices for applications in MEMS/NEMS.

One‐Step Preparation of Biocompatible Gold Nanoplates with Controlled Thickness and Adjustable Optical Properties for Plasmon‐Based Applications

AbstractThe ability to synthesize plasmonic nanomaterials with well‐defined structures and tailorable size is crucial for exploring their potential applications. Gold nanoplates (AuNPLs) exhibit appealing structural and optical properties, yet their applications are limited by difficulties in thickness control. Other challenges include a narrow range of tunability in size and surface plasmon resonance, combined with a synthesis conventionally involving cytotoxic cetyltrimethylammonium (CTA) halide surfactant. Here, a one‐step, high‐yield synthesis of single‐crystalline AuNPLs is developed, based on the combined use of two structure‐directing agents, methyl orange and FeBr3, which undergo preferential adsorption onto different crystalline facets of gold. The obtained AuNPLs feature high shape homogeneity that enables mesoscopic self‐assembly, broad‐range tunability of dimensions (controlled thickness from ≈7 to ≈20 nm, accompanied by modulation of the edge length from ≈150 nm to ≈2 µm) and plasmonic properties. These merits, coupled with a preparation free of CTA‐halide surfactants, have facilitated the exploration of various uses, especially in bio‐related areas. For example, they are demonstrated as biocompatible photothermal agents for cell ablation in NIR I and NIR II windows. This work paves the way to the innovative fabrication of anisotropic plasmonic nanomaterials with desired attributes for wide‐ranging practical applications.

Elucidation of Photocatalytic Properties of Gold–Platinum Bimetallic Nanoplates Using Tip-Enhanced Raman Spectroscopy

Plasmonic catalysis is based on a unique property of noble metal nanostructures to harvest electromagnetic radiation converting it into hot carriers. These high energy species can catalyze chemical reactions in molecules located in the vicinity to surfaces of the plasmonic nanostructures. However, traditional plasmonic metals such as gold (Au) are useful only for a limited number of chemical reactions. The spectrum of chemical reactions can be broadened up by coupling plasmonic and catalytic metals, such as platinum (Pt), in one nanostructure. In this study, we use tip-enhanced Raman spectroscopy (TERS) to probe photocatalytic properties of gold–platinum nanoplates (Au@PtNPs). We found that Au@PtNPs highly efficiently proto-reduced 4-nitrobenzenethiol (4-NBT) to p,p′-dimercaptoazobisbenzene (DMAB) with the yield similar to the one observed for gold nanoplates (AuNPs). These Au@PtNPs could be used to proto-oxidize 4-aminothiophenol (4-ATP) first to 4-NBT and then 4-NBT to DMAB, whereas photo-oxidation of 4-ATP on the surface of AuNPs directly proceeds to DMAB.

Gap-Mode Tip-Enhanced Raman Scattering on Au Nanoplates of Varied Thickness.

Gold nanoplates (AuNPLs) enable the gap-mode configuration of tip-enhanced Raman spectroscopy (TERS). This allows for low-concentration molecular sensing and high-resolution imaging. Compared with non-gap-mode TERS, the gap plasmon provides significantly higher enhancement factors. In addition, AuNPLs exhibit a lightning rod or edge effect, further enhancing the laser field and increasing the spectroscopic sensitivity. In this study, we investigate the relationship between the thickness of AuNPLs and the intensity of the spontaneous Raman signal produced by 4-nitrobenzenethiol, a reporter molecule used in TERS. Our experimental and theoretical results show that the intensity of TERS spectra increases with an increase in the thickness of the AuNPLs. This study of the thickness dependence of AuNPL allows us to find a configuration with maximal nanoplasmonic effects. Moreover, the electromagnetic interaction of the AuNPL with the tip, positioned near the AuNPL's edge, results in a plasmonic nanoantenna configuration for field enhancement, with important promise for future applications to nanobioimaging and biosensing.

Tunable Casimir equilibria with phase change materials: From quantum trapping to its release

A stable suspension of nanoscale particles due to the Casimir force is of great interest for many applications such as sensing, noncontract nanomachines. However, the suspension properties are difficult to change once the devices are fabricated. Vanadium dioxide (${\mathrm{VO}}_{2}$) is a phase change material, which undergoes a transition from a low-temperature insulating phase to a high-temperature metallic phase around a temperature of 340 K. In this work, we study Casimir forces between a nanoplate (gold or Teflon) and a layered structure containing a ${\mathrm{VO}}_{2}$ film. It is found that stable Casimir suspensions of nanoplates can be realized in a liquid environment, and the equilibrium distances are determined, not only by the layer thicknesses but also by the matter phases of ${\mathrm{VO}}_{2}$. Under proper designs, a switch from quantum trapping of the gold nanoplate (``on'' state) to its release (``off'' state) as a result of the metal-to-insulator transition of ${VO}_{2}$, is revealed. On the other hand, the quantum trapping and release of a Teflon nanoplate is found under the insulator-to-metal transition of ${\mathrm{VO}}_{2}$ . Our findings offer the possibility of designing switchable devices for applications in micro and nanoelectromechanical systems.

Sequential colorimetric sensing of cupric and mercuric ions by regulating the etching process of triangular gold nanoplates.

Atriangular gold nanoplate (AuNPL)-based colorimetric assay is presentedfor ultrasensitive determination of cupric ions (Cu2+) and mercuric ions (Hg2+) in sequence. AuNPLs were found to be etched efficiently when producing triiodide ions (I3-) by a redox reaction between Cu2+ and iodide ions (I-), leading to a change ofthe shape of AuNPLs from triangular to sphere along with a color change from blue to pink. Inthe presence of Hg2+ the etching of AuNPLs was suppressed due to the consumption of I- by the formation of HgI2. With an increase of the concentration of the Hg2+ a transformation from sphere to triangular in the shape of AuNPLs occurredwith a color change from pink to blue. The evolution of AuNPLs from etching to anti-etching state by sequential addition of Cu2+ and Hg2+ was accompanied with color variations and band shifts of localized surface plasmon resonance (LSPR), allowing for visual and spectroscopicdetermination of Cu2+ and Hg2+ successively within 15min. In the range 0.01-1.5μM for Cu2+ and 0.02-3.0μM for Hg2+, the linear relationship between the band shift values and the target ions concentration was found good (R2 > 0.996). The limit of detections (3S/k) was 19nM for Cu2+ and 9nM for Hg2+, respectively. The lowest visual estimation concentration was 80nM for both Cu2+ and Hg2+ through the distinguishable color changes. This system exhibited desirable selectivity for Cu2+ and Hg2+ over other common ions tested. The method has been successfully applied tosequential determinationof Cu2+ and Hg2+ in real water and food samples. Graphical abstract Scheme 1 Schematic illustration for sequential detection of Cu2+ and Hg2+ based on etching of AuNPLs.

Strengthening Fano resonance on gold nanoplates with gold nanospheres.

Plasmonic Fano resonance has attracted extensive attention due to its many applications, including plasmonic sensing, electromagnetically induced transparency, light trapping and stopping, due to its narrow linewidth and asymmetric spectral shape. However, many metal nanostructures are designed with complex geometries to generate Fano resonance and few of them can support a deep Fano dip. Herein we report on the strengthening of the Fano resonance on silicon-supported Au nanoplates through the formation of (Au nanosphere)-(Au nanoplate) heterodimers. The deposition of the Au nanosphere on the top can greatly strengthen the substrate-induced Fano resonance of the Au nanoplate with a deep dip. We also observe that the replacement of the Au nanosphere with a Au nanocube can suppress the excitation of the Fano resonance in the heterodimer. When the sharp corners and edges of the nanocubes gradually become rounded, the Fano resonance appears again with increasing asymmetry. Both the dip depth and wavelength of the Fano resonance can be independently tailored by varying the nanosphere diameter and the nanoplate thickness, respectively. We believe that our results provide an attractive and facile platform for modulating Fano dips and constructing Fano resonance-based devices.

Synthesis of fluorescent, triangular gold nanoplates through surface capping by a cationic diacetylene

In this work triangular gold nanoplates (AuNTPs) are prepared by spontaneous conversion of spherical-like gold precursors induced by the shape-directing properties of the diacetylene N,N,N-trimethylpentacosa-10,12-diyn-1-ammonium bromide (PCD_ABr), employed as capping agent. In detail, thin AuNTPs with edge length below 200 nm are obtained by incubating premade isotropic gold nanoparticles protected with chitosan (Chit@AuNPs) in PCD_ABr solutions through a standard ligand-exchange reaction (LER) carried out in water at room temperature. Control of the AuNTPs size as well as the reaction shape-yield up to 65% is achieved by simply varying the reagents ratio in the incubation mixture.Because of the high anisotropy of the flat nanoparticles, the AuNTPs-containing hydrosols show intense, well-defined absorptions in the NIR spectral region, which make them excellent candidates as biosensors also for in vivo assays with biological tissues and fluids.When subjected to UV light, both triangular and nearly spherical PCD_ABr-coated gold nanoparticles exhibit fluorescent properties and Raman features typical of disordered, sp2-based carbon nanostructures. The photogeneration of graphite-like moieties within the diacetylene shell instead of expected polydiacetylene skeleton is most likely due to poor alignment of the monomer chains on the gold surfaces and seems to be favored on isotropic nanoparticles.

Long Lifetime and Coupling of Acoustic Vibrations of Gold Nanoplates on Unsupported Thin Films.

Radiation losses due to energy transfer to acoustic waves in the surroundings lead to short vibrational lifetimes of plasmonic nanoresonators and severely limits their applications in quantum technologies and mass sensing. Here we demonstrate a general method for improving the acoustic vibration lifetimes of Au nanoplates (flat structures with edge lengths of several microns and thicknesses of 10-40 nm) by using unsupported thin films instead of thick substrates, which blocks the outgoing acoustic waves. We experimentally confirm the vibrational quality factor improvement of Au nanoplates on Si3N4 and Al2O3/Si3N4 thin films with thicknesses less than 100 nm. Through this substrate phonon engineering, we successfully observe strong vibrational coupling between two Au nanoplates. Furthermore, the increased vibrational lifetimes allow us to study the vibrations of the nanoplates relative to the substrate and, for stacked nanoplates, relative to each other. The frequencies of these relative vibrational modes are well described by a simple coupled mass model, which provides the effective spring constants between interfaces separated by surface bonded molecules. We also demonstrate that the continuous thin films can be used for small plasmonic nanostructures (dimensions of several hundred nm), with similar increases in the vibrational lifetimes.

Using a Graphene-Polyelectrolyte Complex Reducing Agent To Promote Cracking in Single-Crystalline Gold Nanoplates.

It is a challenge to produce single-crystalline gold nanoparticles having regular size definition designed for controlled light absorbance and internal structural inhomogeneities to enhance electro-magnetic fields. Here, we report a synthetic strategy to generate large single-crystalline triangular or hexagonal gold nanoplates with multiple cracks within the plates using a graphene-polyelectrolyte complex as both a surface adsorbent and bulk reducing agent. Large-scale gold nanoplates can be synthesized within 48 h. First-principles calculations indicate that the nanoplates have a kinetically limited morphology resulting from prior growth of {111} facets confined by the graphene-polyelectrolyte multilayer. The nanocracks result from the inability of the bulk reducing agent to enter narrow defect spaces during growth that remained permanently. The nanoplates had extraordinary physical-chemical detection sensitivity when used for surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). The limit of rhodamine 6G (Rh6G) SERS detection is as low as 5 × 10-13 M. The gold nanoplates also showed a remarkable light-to-heat conversion efficiency (68.5%). The approach described may be applicable to other metals so that tunable nanostructures can be generated by the graphene-polyelectrolyte multilayer strategy.

Supersensitive Detection of the Norovirus Immunoplasmon by 3D Total Internal Reflection Scattering Defocus Microscopy with Wavelength-Dependent Transmission Grating.

Norovirus (NoV) is a major foodborne pathogen, and even low levels of virus can cause infection and gastroenteritis. We developed a supersensitive NoV sensor that detects NoV group-I capsid protein (NoVP) via three-dimensional (3D) total internal reflection scattering defocus microscopy (TIRSDM) with wavelength-dependent transmission grating (TG). The combination of evanescent wave scattering and TG significantly enhanced the detection sensitivity and selectivity of NoVP in first-order spectral images (n = +1) by minimizing spectroscopic interference and background noise. In particular, wavelength-dependent 3D defocused TG imaging (3D TG-TIRSDM) separated silver nanotag and gold nanoplate signals on a NoVP immunoplasmon chip along the x, y, and z coordinates simultaneously. Additionally, the use of wavelength-dependent TG increased the spectral resolution by 5-fold along the xy-axis and 1.4-fold along the z-axis compared to conventional 3D TIRSDM at the subdiffraction limit. The NoVP sensor exhibited a lower limit of detection of 820 yM, which is 29 000 times better than the previous potentiometer method, and a wide dynamic detection range of 820 yM to 92.45 pM (R = 0.9801). This new method could be applied to detect various pathogenic viruses during the initial stage of infection.

Catalytic Activity of Anisotropic Gold Nanoplates towards Oxygen Reduction

AbstractGlassy carbon electrode (GCE) coated with anisotropic gold nanoplates (aAuNPs) was used for the study of oxygen reduction reaction (ORR) in 0.5 M sulfuric acid instead of bulk gold electrodes. The electrode cleaning/activation procedure lead to the removal of any charged and uncharged residues on the gold nanoplates, leaving the nanostructured surface highly active towards oxygen reduction. The advantages: much lower overpotential and larger current densities of oxygen reduction are ascribed to the unique nanostructures present on the carbon electrode surface‐the gold nanoplates. They are rich in edges providing a large population of Au (100) sites with unsaturated coordination exposed to the solution, and catalytically active. Measurements performed using a rotating disc electrode, modified with the gold nanoplates, confirmed that ORR proceeds via two separate steps: oxygen is reduced to hydrogen peroxide, and the peroxide is further reduced in a two‐electron reduction to water.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects.

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

Single-particle Correlation Study: Polarization-dependent Differential Interference Contrast Imaging of Two-dimensional Gold Nanoplates

Questions surrounding the optical properties of two-dimensional (2D) triangular single gold nanoplates (AuNPs) remain largely unanswered. Herein, a scanning-electron microscopy-correlated single-particle study was conducted to identify polarization-dependent optical properties of AuNPs under dark-field (DF) and differential interference contrast (DIC) microscopy. AuNPs with an aspect ratio of ∼3 showed a single broad DF scattering spectrum without separation of the two dipole and quadrupole resonance modes present in 2D AuNPs. Polarization-sensitive interference properties of the individual AuNPs were revealed through periodic changes in the intensities and types of DIC images obtained. A dipole resonance mode was found to mainly contribute to the polarization-sensitive interference properties of AuNPs. Furthermore, DIC polarization anisotropy allowed us to track the real-time orientation of a dipole resonance mode of a AuNP rotating on a live cell membrane.