Au Nanostructures Research Articles

A comprehensive study of the nonlinear optical response exhibited by ion-implanted silica plates with Au and Pt nanostructures

Third-order nonlinear optical properties exhibited by Platinum nanoclusters and Gold nanoparticles integrated in a two-stage configuration are reported. Comparative nanostructures were nucleated in a monolithic complex platform form based on a single matrix by a dual ion-implantation process and a reduction in the ablation threshold from 2.2 J/cm2 to 1.2 J/cm2 was obtained. This modification in the photothermal response exhibited by the nanoparticles seems to derive from a fractional transfer of energy. Femtosecond z-scan measurements at 800 nm revealed a strong two-photon absorption in the dual-implanted sample together to an ultrafast electronic nonlinearity responsible for a self-defocusing effect. These findings highlight the potential for tailoring quantum systems by integration of hybrid photonic circuits using multi-implantation processes.

Study on SERS activity of Au-Ag bimetallic nanostructures synthesized using different reducing agents

Abstract The Au-Ag bimetallic nanostructures were synthesized independently using three different reducing agents [ascorbic acid (C6H8O6), sodium borohydride (NaBH4) and tri-sodium citrate dihydrate (C6H9Na3O9)] to understand their effect on the morphology of the nanostructures. The nanostructures obtained using the reducing agents mentioned in above order, were found in sprouted potato, cubical, and prolate spheroid (PS) shapes. All the synthesized nanostructures were characterized using energy-dispersive X-ray spectroscopy and extinction spectroscopy. The surface enhanced Raman scattering (SERS) activity of all these nanostructures was studied using Raman probe solutions with micro and nanomolar concentrations. Relatively, the PS shaped nanostructures were found to be more stable and better for SERS studies. Therefore, these nanostructures were further characterized by Transmission electron microscopy and X-ray photoelectron spectroscopy. The usefulness of PS shaped Au-Ag nanostructures was also demonstrated by recording the spectra of both methylene blue (MB) and 4-Aminothiophenol (4-ATP) solutions with femtomolar concentration. The experimental enhancement factor (EEF) was found to be around 1.1 × 1011 in case of MB solution with femtomolar concentration. The possible origin for the EEF is explained using quasi-static theory and Finite Element Method (FEM) based simulations.

Enhancing osteogenesis of adipose-derived mesenchymal stem cells using gold nanostructure/peptide-nanopatterned graphene oxide

Graphene derivatives are highly promising materials for use in stem-cell-based regenerative therapies, particularly for bone regeneration. Herein, we report a graphene oxide (GO)-based hybrid platform (GOHP) that is highly effective for guiding the osteogenesis of human adipose-derived mesenchymal stem cells (hAMSCs). A GO-coated indium tin oxide (ITO) substrate was electrochemically modified with Au nanostructures (GNSs), following which a cysteine-modified quadruple-branched arginine-glycine-aspartic acid was self-assembled on the ITO-GO-GNS hybrid via Au-S bonds. The synthesized GOHP, with the highest density of GNSs (deposition time of 120 s), exhibited the highest osteogenic differentiation efficiency based on the osteogenic marker expression level, osteocalcin expression, and osteoblastic mineralisation. Remarkably, although GO is known to be less efficient than the high-quality pure graphene synthesised via chemical vapour deposition (CVD), the fabricated GOHP exhibited an efficiency similar to that of CVD-grown graphene in guiding the osteogenesis of hAMSCs. The total RNA sequencing results revealed that CVD graphene and GOHP induced the osteogenesis of hAMSCs by upregulating the transcription factors related to direct osteogenesis, Wnt activation, and extracellular matrix deposition. Considering that GO is easy to produce, cost-effective, and biocompatible, the developed GOHP is highly promising for treating various diseases/disorders, including osteoporosis, rickets, and osteogenesis imperfecta.

Morphological Control in Gold Nanostructures Synthesis Process

Noble metal nanoparticles (NPs) have unique optical, electronic and catalytic properties. Dome-like gold (Au) NPs having a size as small as [Formula: see text][Formula: see text]nm and a high density of [Formula: see text][Formula: see text]cm[Formula: see text] were prepared by very high frequency (VHF) hydrogen (H[Formula: see text] plasma-assisted radio frequency magnetron sputtering (RFMS) technique. The influence of RF power and sputtering time on the morphology and structure of islands-like Au nanostructures (NSs) were determined. Besides, optimum sputtering sample (OSS) was treated by VHF H2 plasma technique to fabricate dense Au NPs with great uniformity. Prepared samples were studied by atomic force microscope (AFM), energy dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscope (HR-TEM). AFM images of the sputtering samples show a good density of islands-like Au NSs ([Formula: see text][Formula: see text]cm[Formula: see text] with relatively homogeneous distribution. VHF H2 plasma treatment dependent alterations in the surface morphology of the OSS (shape, size, distribution, number density and average surface roughness) were clearly manifested in the AFM images. EDX spectra disclosed prominent Au peaks highly sensitive to the H2 plasma treatment. Our novel fabrication method may contribute to the development of Au NPs based nanodevices.

Plasmon-Driven Chemistry on Mono- and Bimetallic Nanostructures.

ConspectusHot carriers are highly energetic species that can perform a large spectrum of chemical reactions. They are generated on the surfaces of nanostructures via direct interband, phonon-assisted intraband, and geometry-assisted decay of localized surface plasmon resonances (LSPRs), which are coherent oscillations of conductive electrons. LSPRs can be induced on the surface of noble metal (Ag or Au) nanostructures by illuminating the surfaces with electromagnetic irradiation. These noble metals can be coupled with catalytic metals, such as Pt, Pd, and Ru, to develop bimetallic nanostructures with unique catalytic activities. The plasmon-driven catalysis on bimetallic nanostructures is light-driven, which essentially enables green chemistry in organic synthesis. During the past decade, surface-enhanced Raman spectroscopy (SERS) has been actively utilized to study the mechanisms of plasmon-driven reactions on mono- and bimetallic nanostructures. SERS has provided a wealth of knowledge about the mechanisms of numerous plasmon-driven redox, coupling, and scissoring reactions. However, the nanoscale catalytic properties of both mono- and bimetallic nanostructures as well as the underlying physical cause of their catalytic reactivity and selectivity remained unclear for decades.In this Account, we focus on the most recent findings reported by our and other research groups that shed light on the nanoscale properties of mono- and bimetallic nanostructures. This information was revealed by tip-enhanced Raman spectroscopy (TERS), a modern analytical technique that has single-molecule sensitivity and subnanometer spatial resolution. TERS findings have shown that plasmonic reactivity and the selectivity of bimetallic nanostructures are governed by the nature of the catalytic metal and the strength of the rectified electric field on their surfaces. TERS has also revealed that the catalytic properties of bimetallic nanostructures directly depend on the interplay between the catalytic and plasmonic metals. We anticipate that these findings will be used to tailor synthetic approaches that are used to fabricate novel nanostructures with desired catalytic properties. The experimental and theoretical results discussed in this Account will facilitate a better understanding of TERS and explain artifacts that could be encountered upon TERS imaging of a large variety of samples. Consequently, plasmon-driven chemistry should be considered as an essential part of near-field microscopy.

Stretchable and Transparent Electrochemical Sensor Based on Nanostructured Au on Carbon Nanotube Networks for Real-Time Analysis of H2O2 Release from Cells.

Various electrochemical biosensors have been developed for direct and real-time recording of biomolecules released from living cells. However, since these traditional electrodes are commonly rigid and nonflexible, in situ monitoring of biochemical signals while cell deformation occurs remains a great challenge. Herein, we report a facile approach for the development of a stretchable and transparent electrochemical cell-sensing platform based on Au nanostructures (nano-Au) and carbon nanotube (CNT) films embedded in PDMS (nano-Au/CNTs/PDMS). The sandwich-like nanostructured network of nano-Au/CNTs endows the sensor with excellent mechanical stability and electrochemical performance. The obtained nano-Au/CNTs/PDMS electrode displays desired performance for H2O2 detection with a wide linear range (20 nM-25.8 μM) and low detection limit (8 nM). Owing to good biocompatibility and flexibility, HeLa and human umbilical vein endothelial cells can be directly cultured on the electrode and real-time monitoring of H2O2 release from cells under their stretched state was realized. The proposed strategy demonstrated in this work provides an effective way for design of stretchable sensors and more opportunities for sensing biomolecules from mechanically sensitive cells.

Polarized SERS Controlled by Anisotropic Growth on Ordered Curvature Substrate.

Colloidal lithography is an efficient and low-cost method to prepare an ordered nanostructure array with new shapes and properties. In this study, square-shaped and cone-shaped Au nanostructures were obtained by 70° angle deposition onto polystyrene bead array with the diameter of 500 nm when a space of 120 nm is created between the neighbor beads by plasma etching. The gaps between the units decrease when the Au deposition time increases, which leads to the polarized enhanced local field, in agreement with the surface-enhanced Raman scattering spectra (SERS) observations and finite-difference time-domain (FDTD) simulations. When the Au deposition time increased to 5 min, 5 nm gaps form between the neighbor units, which gave an enhancement factor of 5 × 109. The SERS chip was decorated for the detection of the liver cancer cell marker Alpha-fetoprotein (AFP) with the detection limit down to 5 pg/mL.

Investigating the Effect of Ag and Au Nanostructures with Spherical and Rod Shapes on the Emission Wavelength of OLED

Noble metals, especially Ag and Au nanostructures, have unique and adjustable optical attributes in terms of surface plasmon resonance. In this research, the effect of Ag and Au nanoparticles with spherical and rod shapes on the light extraction efficiency and the FWHM of OLED structures was investigated using the finite difference time domain (FDTD) method. The simulation results displayed that by changing the shape and size of Ag and Au nanostructures, the emission wavelength can be adjusted, and the FWHM can be reduced. The presence of Ag and Au nanoparticles in the OLEDs showed a blue and red shift of the emission wavelength, respectively. Also, the Ag and Au nanorods caused a significant reduction in the FWHM and a shift to the longer wavelengths in the structures. The structures containing Ag nanorods showed the narrowest FWHM and longer emission wavelength than the other structures.

Yolk-Shelled Gold@Cuprous Oxide Nanostructures with Hot Carriers Boosting Photocatalytic Performance.

Plasmonic Au nanoparticles (NPs) have been commonly used to enhance the photocatalytic activity of Cu2O. Till now, core-shell Au NP@Cu2O composites have been reported in previous studies. Yet, these Au@Cu2O composites only exhibit visible light response. Other special Au nanostructures, such as Au nanorods (NRs) or Au nanobipyramids (NBPs), which possess near-infrared light absorption, were rarely used to endow the near-infrared light response for Cu2O. In this work, for the first time, we used Au NPs, Au NRs, and Au NBPs and employed a handy and universal method to synthesize a series of yolk-shelled Au@Cu2O composites. The results showed that the yolk-shelled Au@Cu2O composites had much higher photocatalytic activity than their solid-shelled ones and pure Cu2O. More importantly, yolk-shelled Au NR@Cu2O and Au NBP@Cu2O composites indeed presented excellent near-infrared light-driven photocatalytic activity, which were impossible for Au NP@Cu2O and pure Cu2O. This outstanding performance for yolk-shelled Au NR@Cu2O and Au NBP@Cu2O could be attributed to the transfer of abundant hot electrons from Au NRs or Au NBPs to Cu2O, and the timely utilization of hot holes on Au through the rich pore channels on their yolk-shelled structure. Furthermore, yolk-shelled Au@Cu2O also showed better stability than pure Cu2O, owing to the migration of the oxidizing holes from Cu2O to Au driven by the built-in electric field. This work may give a guide to fabricate controllable and effective photocatalysts based on plasmonic metals and semiconductors with full solar light-driven photocatalytic activities in the future.

Detection and quantification of toxicants in food and water using Ag–Au core-shell fractal SERS nanostructures and multivariate analysis

Here we demonstrate a facile, two step formation of silver core - gold shell (Ag–Au) nanostructures using microelectrodes and assess their performance as surface-enhanced Raman scattering (SERS) substrates to detect and quantify toxicants. Ag nanostructures, serving as the scaffolds for the bimetallic structures, were grown first by using electrochemical deposition on the edges of microelectrodes functionalized with the alkanethiol, 11-mercaptoundecanoic acid. Subsequently, different concentrations of HAuCl4 were used to perform a galvanic reaction on the surfaces of the Ag nanostructures with aqueous droplets being placed on the microelectrodes for 10 min before the substrate was rinsed and dried. Lower HAuCl4 concentrations were found to better preserve the fractal morphology of the formed Ag–Au nanostructures, while higher concentrations resulted in Ag–Au fragments. The SERS enhancement factor for the Ag–Au nanostructures was estimated to have a max value of 6.51 x 105. Combining a data reduction technique with a linear classifier, both identification and quantification were demonstrated with 100% success. The toxicants thiram, thiabendazole, malachite green and biphenyl-4-thiol were all detected and identified at 1 ppm. Lastly, as a proof of concept, the Ag–Au nanostructures were transferred to a PDMS film resulting in a flexible SERS substrate capable of direct detection of thiram on an apple peel without any additional sample pre-treatment.

Time-dependent eco-friendly method to produce Au nanometric assemblies with fluorescent properties

A time-dependent eco-friendly synthesis of Au assemblies was carried out using Taraxacum officinale plant extract in the present study. Au nanomaterials have potential applications in imaging therapy cancer diagnostics due to their biocompatibility and fluorescence response when irradiated with Ultraviolet light. Also, synthesis methods whose chemistry minimizes toxic residues are of interest to scientists, mainly when intended to use Au nanomaterials for biomedical treatments. Furthermore, the obtained products' fluorescence intensities were also measured to determine if they could be helpful biosensors. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV–Vis spectroscopy, X-ray diffraction (XRD), Infrared spectroscopy (FTIR), Atomic absorption spectrophotometry (AAS), and Zeta potential (ZP) analyzed the obtained products. SEM shows Au self-assemblies with dendritic morphology and an average size of 158 nm. TEM images also confirm dendrites with flat-flower morphology with a thickness of around 10 nm. UV–Vis spectrum displays surface plasmon resonance (SPR) bands typical of Au nanostructures. XRD results further confirm the Au fcc nanostructure of dendrites. FTIR spectra show that the Au ions reduction compounds were sugars, proteins, and compounds with benzene rings. Also, AAS indicates 89% of conversion efficiency from Au ions to assemblies. Fluorescence analyses show that the Au nanostructures' intensities in 280 u.a. could be significant for widespread applications as fluorescent biosensors in therapy and imaging. Finally, the ZP value for Au assemblies was –26.6 mV indicating good colloidal stability at the end of the reaction.

A Flexible Sensor Based on 3D Gold@Carbonaceous Nanohybrid with Defect Sites of Conductivity for the Wearable Sensing at Low Stress

The detection of subtle strain on the human body by flexible sensors suffers from the inferior sensitivity. In general, poor sensitivity derives from the intact structural integrity of building blocks of electric material. Inspired by the rolling friction in daily life, a three-dimensional (3D) nanohybrid of gold@carbonaceous nanoballs (Au@CBs) is developed. Compared with the pure Au nanostructure, the CBs increase the defect sites of conductivity and deformation sensitivity between Au nanostructures. The Flexible Stress Sensor (FSS) based on the 3D nanohybrid of Au@CBs is fabricated, and due to the enough displacement variation of 3D nanohybrid of Au@CBs, the change of resistance could be triggered by the tiny stress. The as-prepared sensor has high sensitivity at wide working range ([Formula: see text][Formula: see text]KPa[Formula: see text], 0.1–10[Formula: see text]KPa and [Formula: see text][Formula: see text]KPa[Formula: see text], 10–100[Formula: see text]KPa) and good cyclic stability (5000 cycles). Finally, the FSS based on the 3D nanohybrid of Au@CBs is employed for the detection of subtle vibration and movement of the human body, such as pulse and joint bending, which exerts great potential as wearable electronics.

Tailoring the Optical Properties of Sputter-Deposited Gold Nanostructures on Nanostructured Titanium Dioxide Templates Based on In Situ Grazing-Incidence Small-Angle X-ray Scattering Determined Growth Laws.

Gold/titanium dioxide (Au/TiO2) nanohybrid materials have been widely applied in various fields because of their outstanding optical and photocatalytic performance. By state-of-the-art polymer templating, it is possible to make uniform nanostructured TiO2 layers with potentially large-scale processing methods. We use customized polymer templating to achieve TiO2 nanostructures with different morphologies. Au/TiO2 hybrid thin films are fabricated by sputter deposition. An in-depth understanding of the Au morphology on the TiO2 templates is achieved with in situ grazing-incidence small-angle X-ray scattering (GISAXS) during the sputter deposition. The resulting Au nanostructure is largely influenced by the TiO2 template morphology. Based on the detailed understanding of the Au growth process, characteristic distances can be selected to achieve tailored Au nanostructures at different Au loadings. For selected sputter-deposited Au/TiO2 hybrid thin films, the optical response with a tailored localized surface plasmon resonance is demonstrated.

Modulating electron density of vacancy site by single Au atom for effective CO2 photoreduction

The surface electron density significantly affects the photocatalytic efficiency, especially the photocatalytic CO2 reduction reaction, which involves multi-electron participation in the conversion process. Herein, we propose a conceptually different mechanism for surface electron density modulation based on the model of Au anchored CdS. We firstly manipulate the direction of electron transfer by regulating the vacancy types of CdS. When electrons accumulate on vacancies instead of single Au atoms, the adsorption types of CO2 change from physical adsorption to chemical adsorption. More importantly, the surface electron density is manipulated by controlling the size of Au nanostructures. When Au nanoclusters downsize to single Au atoms, the strong hybridization of Au 5d and S 2p orbits accelerates the photo-electrons transfer onto the surface, resulting in more electrons available for CO2 reduction. As a result, the product generation rate of AuSA/Cd1−xS manifests a remarkable at least 113-fold enhancement compared with pristine Cd1−xS.

Dynamic metallization of spherical DNA via conformational transition into gold nanostructures with controlled sizes and shapes

Despite the reversible condensation properties of DNA, DNA metallization during controlled conformational transitions has been rarely investigated. We perform dynamic metallization of spherically condensed DNA nanoparticles (DNA NPs) via a globule-to-coil transition. A positively charged new Au3+ reagent is prepared via ligand-exchange of conventional complex Au3+ ions, which was used to synthesize spherically condensed DNA NPs simply based on the fundamental electrostatic and coordinative interactions between DNA and Au3+ions. Interestingly, the size of the Au3+-condensed DNA NPs (Au3+-DNA NPs) and the type of reducing agents lead to the formation of different Au nanostructures with unprecedented morphologies (cracked NPs, bowl-shaped NPs, and small NPs), owing to the controlled conformational changes in the Au3+-DNA NPs during metallization. The condensed DNA NPs play significant roles for Au nanostructures as (1) the dynamic template for the synthesis, (2) the reservoir and supply of Au3+ for the growth, and (3) the surface stabilizer. The synthesized Au nanostructures are remarkably stable against high ionic strength and exhibit catalytic activities and excellent SERS properties. This is the first study on the morphological control and concomitant dynamic metallization of spherically condensed DNA, proposing new synthetic routes for bioinorganic nanomaterials.

First-principles based analysis of thermal transport in metallic nanostructures: Size effect and Wiedemann-Franz law

Metallic nanostructures (the nanofilms and nanowires) are widely used in electronic devices, and their thermal transport properties are crucial for heat dissipation. However, there are still gaps in understanding thermal transport in metallic nanostructures, especially regarding the size effect and validity of the Wiedemann-Franz law. In this work, we perform mode-by-mode first-principles calculations combining the Boltzmann transport equation to understand thermal transport in metallic nanostructures. We take the gold (Au) and tungsten (W) nanostructures as prototypes. It is found that when the size of nanostructures is on the order of several tens of nanometers, the electronic/phonon thermal conductivity is smaller than the bulk value and decreases with size. The phonon contribution increases in nanostructures for those metals with small bulk phonon thermal conductivity (like Au), while the phonon contribution may increase or be suppressed in nanostructures for those metals with large bulk phonon thermal conductivity (like W). By assuming that the grain boundary does not induce inelastic electron-phonon scattering, the Wiedemann-Franz law works well in both Au and W nanostructures if the Lorentz ratio is estimated using electronic thermal conductivity. The Wiedemann-Franz law also works well in Au nanostructures when the Lorentz ratio is estimated by total thermal conductivity.

Facile fabrication of Au nanoworms covered polyethylene terephthalate (PET) film: Towards flexible SERS substrates

Simple, reproducible, large-area fabrication of flexible highly-enhanced polymer-based SERS substrate is well required. In this paper, a facile strategy to obtain worm-like Au nanostructures on polyethylene terephthalate (PET) film is proposed, based on Ar plasma etching and Au evaporation. A systematic study on SERS performance versus Ar plasma etching duration and Au deposition thickness is performed for the first time. The optimal flexible Au nanoworms covered PET substrate shows high SERS EF (~1.2 × 108) and trace level detection sensitivity (10-9 M R6G), which helps to push SERS technique forward to fulfill various real-life tasks.

Preparation, Functionalization, Modification, and Applications of Nanostructured Gold: A Critical Review

Gold nanoparticles (Au NPs) play a significant role in science and technology because of their unique size, shape, properties and broad range of potential applications. This review focuses on the various approaches employed for the synthesis, modification and functionalization of nanostructured Au. The potential catalytic applications and their enhancement upon modification of Au nanostructures have also been discussed in detail. The present analysis also offers brief summaries of the major Au nanomaterials synthetic procedures, such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, sonochemical deposition, electrochemical deposition, microwave and laser pyrolysis. Among the various strategies used for improving the catalytic performance of nanostructured Au, the modification and functionalization of nanostructured Au produced better results. Therefore, various synthesis, modification and functionalization methods employed for better catalytic outcomes of nanostructured Au have been summarized in this review.

High optical enhancement in Au/Ag alloys and porous Au using Surface-Enhanced Raman spectroscopy technique

We report high optical enhancement in Ag/Au alloys and porous gold nanostructures using Surface Enhanced Raman Spectroscopy (SERS) technique. Scanning electron microscopy investigation shows the formation of Ag/Au alloys particles during irradiation of Ag–Au bilayer deposited on FTO (SnO2:F) substrate by laser fluency equal to 0.5 J/cm2 or 1.0 J/cm2 with 12 ns laser pulse duration. The dealloying process of these Au–Ag alloy particles leads to the formation of Au nanoporous particles. The obtained nanostructures were studied with SERS and revealed a promising enhancement factor in porous Au nanostructure and tunability of localized surface plasmon resonance. The highly dense strong hot spots and large specific area in porous structure of gold nanostructures is the origin of the highly enhancement factor observed experimentally and theoretically. A very good agreement between simulation and experimental results was found confirming the potential of Au/Ag alloys and particularly porous gold nanostructure in SERS application.

Enantioselective SERS sensing of pseudoephedrine in blood plasma biomatrix by hierarchical mesoporous Au films coated with a homochiral MOF

Here, we present a new family of hierarchical porous hybrid materials as an innovative tool for ultrasensitive and selective sensing of enantiomeric drugs in complex biosamples via chiral surface-enhanced Raman spectroscopy (SERS). Hierarchical porous hybrid films were prepared by the combination of mesoporous plasmonic Au films and microporous homochiral metal-organic frameworks (HMOFs). The proposed hierarchical porous substrates enable extremely low limit of detection values (10−12 M) for pseudoephedrine in undiluted blood plasma due to dual enhancement mechanisms (physical enhancement by the mesoporous Au nanostructures and chemical enhancement by HMOF), chemical recognition by HMOF, and a discriminant function for bio-samples containing large biomolecules, such as blood components. We demonstrate the effect of each component (mesoporous Au and microporous AlaZnCl (HMOF)) on the analytical performance for sensing. The growth of AlaZnCl leads to an increase in the SERS signal (by around 17 times), while the use of mesoporous Au leads to an increase in the signal (by up to 40%). In the presence of a complex biomatrix (blood serum or plasma), the hybrid hierarchical porous substrate provides control over the transport of the molecules inside the pores and prevents blood protein infiltration, provoking competition with existing plasmonic materials at the limit of detection and enantioselectivity in the presence of a multicomponent biomatrix.