Properties and applications of colloidal nonspherical noble metal nanoparticles

Silver nanoparticles prepared by gamma irradiation across metal–organic framework templates

In this study, we demonstrate for the first time the successful fabrication of well-dispersed ultrafine silver nanoparticles inside metal–organic frameworks through a single step gamma irradiation at room temperature.

Tuning the structural asymmetries of three-dimensional gold nanorod assemblies

A series of 3D AuNR dimers and trimers were fabricated under the guidance of DNA origami.

Efficient surface enhanced Raman scattering on confeito-like gold nanoparticle-adsorbed self-assembled monolayers

Confeito-like gold nanoparticles adsorbed on the self-assembled monolayer hierarchy of dendrimer/polystyrene spheres can strongly enhance Raman scattering, being superior to those on the self-assembled monolayer of 3-aminopropyltriethoxysilane.

Hierarchical nanospheres based on Pd nanoparticles dispersed on carbon coated magnetite cores with a mesoporous ceria shell: a highly integrated multifunctional catalyst

The novel multifunctional Fe₃O₄@C–Pd@CeO₂ nanospheres show excellent catalytic performance in the Suzuki–Miyaura cross-coupling reaction and the reduction of 4-nitrophenol.

Au@Pd core–shell nanoparticles-decorated reduced graphene oxide: a highly sensitive and selective platform for electrochemical detection of hydrazine

An Au_core–Pd_shell-decorated reduced graphene oxide nanocomposite is successfully employed for the electrochemical detection of low-level hydrazine in an aqueous solution.

Three-dimensional plasmonic hydrogel architecture: facile synthesis and its macroscale effective space

A facile strategy was developed to fabricate a 3D hydrogel decorated with Ag nanoparticles as a SERS substrate. The macro effective depth in this 3D network was confirmed. The substrate produced satisfactory results in the analysis of trace environmental molecules.

Mediator-free total cholesterol estimation using a bi-enzyme functionalized nanostructured gold electrode

A one-step electrochemical route for the synthesis, functionalization and deposition of Au nanostructures and for the bi-enzyme functionalization of a Au electrode has been proposed.

Etching of unmodified Au@Ag nanorods: a tunable colorimetric visualization for the rapid and high selective detection of Hg2+

A simple and cost-effective colorimetric approach based on unmodified Au@Ag nanorods (Au@Ag NRs) was developed for Hg²⁺ detection.

Island-type growth of Au–Pt heterodimers: direct visualization of misfit dislocations and strain-relief mechanisms

Structural and analytical characterization related to the formation mechanism of Au–Pt heterodimers from polyhedral Pt nanocrystals is reported.

Electrochemical detection of azidothymidine on modified probes based on chitosan stabilised silver nanoparticles hybrid material

Silver nanoparticle stabilized by chitosan is synthesized for modification of sensing probe for AZT estimation in human plasma.

Metal/Semiconductor hybrid nanostructures for plasmon-enhanced applications

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

Plasmon mediated shape and size selective synthesis of icosahedral silver nanoparticles via oxidative etching and their 1-D transformation to pentagonal pins

A size- and shape-selective synthesis of pentagonally twinned silver icosahedral nanoparticles (AgIhNPs), one of the five platonic solid morphologies, has been developed by integrating three key factors: nuclei templating by copper, photochemical development using violet LED light and chemical oxidative etching. The presence of copper is essential for AgIhNP shape selection via the promotion of icosahedral nuclei in precursor NPs. Violet light (401-410 nm) is crucial to promote plasmonic selection of near-spherical AgIhNPs. Oxidative etching with hydrogen peroxide and photochemical reduction with citrate establishes a red-ox equilibrium for the photochemical selection of AgIhNPs. The addition of chloride ions improves size- and shape-selectivity. Finally, the demonstration of 1-D growth of AgIhNPs to pentagonal pins initiated at AgIhNP pentagonal-twinned defects highlights a universal role of twinned defects for the formation of anisotropic nanoparticles.

Impact of core dielectric properties on the localized surface plasmonic spectra of gold-coated magnetic core-shell nanoparticles

Gold-coated iron oxide core-shell nanoparticles (IO-Au NPs) are of interest for use in numerous biomedical applications because of their unique combined magnetic-plasmonic properties. Although the effects of the core-dielectric constant on the localized surface plasmon resonance (LSPR) peak position of Au-shell particles have been previously investigated, the impact that light-absorbing core materials with complex dielectric functions have on the LSPR peak is not well established. In this study, we use extended Mie theory for multilayer particles to examine the individual effects of the real and imaginary components of core refractive indices on Au-shell NP plasmonic peaks. We find that the imaginary component dampens the intensity of the cavity plasmon and results in a decrease of surface plasmon coupling. For core materials with large imaginary refractive indices, the coupled mode LSPR peak disappears, and only the anticoupled mode remains. Our findings show that the addition of a nonabsorbing polymer layer to the core surface decreases the dampening of the cavity plasmon and increases LSPR spectral intensity. Additionally, we address apparent discrepancies in the literature regarding the effects of Au-shell thickness on LSPR peak shifts.

Mesoporous gold and palladium nanoleaves from liquid-liquid interface: enhanced catalytic activity of the palladium analogue toward hydrazine-assisted room-temperature 4-nitrophenol reduction

The importance of an interfacial reaction to obtain mesoporous leafy nanostructures of gold and palladium has been reported. A new synthetic strategy involving 1,4-dihydropyridine ester (DHPE) as a potential reducing agent performs exceptionally well for the desired morphologies of both the noble metals at room temperature. The DHPE in turn transforms into its oxidized aromatic form. The as-synthesized gold leaves exhibit high surface-enhanced Raman scattering activity with rhodamine 6G (R6G) due to their hyperbranched structure. It is worthwhile that as-synthesized porous architectures of palladium support the room-temperature hydrogenation of 4-nitrophenol (4-NP) by hydrazine hydrate (N2H4·H2O), reported for the first time. Furthermore, MPL exhibits exceptionally good catalytic activity toward electrooxidation of formic acid. Therefore, an aromaticity driven synthetic technique achieves a rationale to design leafy nanostructures of noble metals from the liquid-liquid interface for multifaceted applications.

LSPR-dependent SERS performance of silver nanoplates with highly stable and broad tunable LSPRs prepared through an improved seed-mediated strategy

The application of the silver plates as a proper substrate for surface enhanced Raman spectroscopy (SERS) was performed to give deep insight on LSPR-dependent SERS performance. Firstly, an improved seed-mediated method is developed to synthesize silver nanoplates (NP) with broad-tuning localized surface plasmon resonance (LSPR) and high stability. The LSPR peaks could be tuned in the range from 485 to ∼1200 nm by controlling the experimental parameters. With the treatment of sodium dodecyl sulfate (SDS), silver NPs exhibit high stability for SERS tests. The LSPR-dependent SERS study was performed by taking four typical silver NPs with LSPR peaks at 485 nm, 614 nm, 906 nm and 1130 nm as substrates. Also, two probe molecules, 4-amino-thiophenol (4-ATP) and rhodamine-6G (R-6G), were used, and both the 458 nm and 633 nm lasers were selected as excitation for the LSPR-dependent SERS study. Our results indicated that the SERS performance is largely dependent on the LSPR of the silver NP substrate at a given excitation wavelength. Specifically, the Raman signals were greatly enhanced when the laser excitation line matched (close to the LSPR band) the peak position of LSPR band. When at the excitation of 633 nm, two orders of magnitude stronger SERS signals would be observed for the Ag-614 substrate than that of the Ag-485 and Ag-1130 substrates with their LSPR peak positions far away from 633 nm. The same result can also be observed when the laser excitation at 458 nm was selected for the Ag-485 substrate. Our study gives a deep insight into LSPR-dependent SERS performance. It also gives a method for giving large SERS enhancement just by selecting a proper excitation wavelength matched to the LSPR of the substrate.

Quantitative understanding of the optical properties of a single, complex-shaped gold nanoparticle from experiment and theory

We report on a combined study of Rayleigh and Raman scattering spectroscopy, 3D electron tomography, and discrete dipole approximation (DDA) calculations of a single, complex-shaped gold nanoparticle (NP). Using the exact reconstructed 3D morphology of the NP as input for the DDA calculations, the experimental results can be reproduced with unprecedented precision and detail. We find that not only the exact NP morphology but also the surroundings including the points of contact with the substrate are of crucial importance for a correct prediction of the NP optical properties. The achieved accuracy of the calculations allows determining how many of the adsorbed molecules have a major contribution to the Raman signal, a fact that has important implications for analyzing experiments and designing sensing applications.

Mesoporous nano/micro noble metal particles: synthesis and applications

The morphology, size and composition often govern the physical and chemical properties of noble metal units with a size in the nano or micro scale. Thus, the controlled growth of noble metal crystals would help to tailor their unique properties and this would be followed by their practical application. Mesoporous nano/micro noble metal units are types of nanostructured material that have fascinating properties that can generate great potential for various applications. This review presents a general view on the growth mechanisms of porous noble metal units and is focused on recent progresses in their synthetic approaches. Then, their potential applications in the field of drug delivery, cell imaging and SERS substrates, as well as fuel cell catalysts are overviewed.

Photo-Thermal Conversion and Stability of Gold and Silver Nanostructures

Gold and silver nanostructures (such as Au nanorods and Ag nanoplates) exhibit strong and tunable surface plasmon resonance in the near-infrared region (NIR). Under a certain NIR laser irradiation, noble metal nanostructrues achieve a high photo-thermal effect, which would be useful in the therapy. In this work, Au nanorods with longitude surface plasmon resonance (SPRL) shifting in the region of 650 ~1100 nm were synthesized by a seed method. Ag nanoplates and nanocubes with SPR located in the region of 650~850 nm were produced by a hydrothermal method. Through adjusting laser power and irradiating time, the photo-thermal conversions of these nanostructures were studied under NIR laser irradiation. Under low power laser (808 nm, <1W) irradiation, the shape of the Au nanorods are stable and the temperature of colloid increase from room temperature to ~57°C. However, Au nanorods undergo deformation from rod to spherical particle under irradiation of high power (808 nm laser; 6W; 1064nm laser, 7W), resulting in the disappearance of SPRL. Morphology evolutions and photo-thermal conversion of Ag nanostructures were also studied. Ag nanostructures have a lower photo-thermal conversion compared with that of Au nanorods colloid. Snipping and dendrite can be observed for Ag nanoplates after irradiating, while Ag nanocubes have no obvious shape change.

Shedding light on the growth of gold nanoshells

Nanostructured particles containing noble metals can have highly tunable localized surface plasmon resonances and are therefore of particular interest for numerous applications. Nanoshells comprising a dielectric core and gold or silver shell are a widely researched systems because of the strong dependence of their optical properties on the ratio of core diameter to shell thickness. Although seeded-growth procedures have been developed to produce these particles, the many reported studies show significant variation in the nanoshell morphologies and hence optical properties. In order to establish processes that reproducibly synthesize nanoshells with high optical quality, it is necessary to develop techniques that monitor changes at the core particle surface during shell growth. For that purpose, we have carried out in situ nonlinear second-harmonic scattering (SHS) and linear vis-NIR extinction spectroscopy simultaneously during the seeded growth of gold nanoshells on silica core particles. Our SHS measurements show a striking variation in the nonlinear optical properties of the growing gold nanoshells. In comparison with linear optical measurements and with scanning electron microscopy (SEM) images made of gold nanoshells produced with varying shell completenesses, the SHS signal was observed to reach a peak intensity at a stage prior to shell closure. We attribute this high sensitivity of the SHS signal to the incomplete nanoshell surface morphology to the generation and subsequent degeneration of regions of electric field enhancement at gaps between isolated gold islands, which grow and coalesce. This conclusion is corroborated by finite-difference time-domain simulations of incomplete nanoshells. We suggest that the in situ analytical approach demonstrated here offers significant promise for future activities regarding the in-process optimization of the morphology and optical properties of metal nanoshells and other nanostructured plasmonic particles.

Controllable preparation of core-shell Au-Ag nanoshuttles with improved refractive index sensitivity and SERS activity

Recent studies have conclusively shown that the plasmonic performance of Au nanostructures can be enhanced by incorporating Ag. Here, we developed a simple and robust approach for preparing core-shell Au-Ag nanoshuttles (NSs) using single-crystal Au nanorods (NRs) as cores. Upon tailoring the temperature of the reaction system containing alkaline glycine buffer (pH 8.5), exceptionally monodisperse Au-Ag NSs with sharp tips and tunable shell thickness could be routinely achieved in high yield through an epitaxial growth process. In particular, high-resolution transmission electron microscopy and nitric acid corrosive experiments revealed that the shells of these NSs consisted of a homogeneous Au-Ag alloy, rather than pure Ag or Au as previously reported. It was found that glycine played an important role in determining the final metal contents of the shell by regulating the reduction kinetics. In addition, the obatined Au-Ag NSs with sharp tips were shown to have significantly improved refractive index sensitivity and surface-enhanced Raman scattering activity relative to the original Au NRs, making these materials promising for biomedical applications, such as biosensing and biolabeling.

Luminescent metal nanoclusters: controlled synthesis and functional applications

Luminescent metal nanoclusters that consist of only several, to tens of, metal atoms and which possess sizes comparable to the Fermi wavelength of electrons have recently attracted significant attention. This new class of luminescent materials not only provides the missing link between atomic and nanoparticle behaviors in metals but also they present abundant novel information for the development of new applicable material systems to meet urgent needs in many areas (such as ultrasensitive sensors for heavy metals, bioimaging, as well as information technology) mainly because of their attractive characteristics, including ultra-small size, good dispersibility, excellent biocompatibility and photostability. In this review, we summarize recent advances in the controlled synthesis and application of luminescent metal nanoclusters, with a particular emphasis on Pt, Mo, Bi and alloy clusters. We also speculate on their future and discuss potential developments for their use in sensors, bioimaging and energy harvesting and conversion.

Ultrathin gold nanoribbons synthesized within the interior cavity of a self-assembled peptide nanoarchitecture

There is increasing interest in gold nanocrystals due to their unique physical, chemical, and biocompatible properties. In order to develop a template-assisted method for the fabrication of gold nanocrystals, we demonstrate here the de novo design and synthesis of a β-sheet-forming nonapeptide (RU006: Ac-AIAKAXKIA-NH2, X = L-2-naphthylalanine) which undergoes self-assembly to form disk-like nanoarchitectures approximately 100 nm wide and 2.5 nm high. These self-assemblies tend to form a network of higher-order assemblies in ultrapure water. Using RU006 as a template molecule, we fabricated ultrathin gold nanoribbons 50-100 nm wide, 2.5 nm high, and micrometers long without external reductants. Furthermore, in order to determine the mechanism of ultrathin gold nanoribbon formation, we synthesized four different RU006 analogues. On the basis of the results obtained using RU006 and these analogues, we propose the following mechanism for the self-assembly of RU006. First, RU006 forms a network by the cooperative association of disk-like assemblies in the presence of AuCl4(-) ions that are encapsulated and concentrated within the interior cavity of the network architectures. This is followed by electron transfer from the naphthalene rings to Au(III), resulting in slow growth to form ultrathin gold nanoribbons along the template network architectures under ambient conditions. The resulting ribbons retain the dimensions of the cavity of the template architecture. Our approach will allow the construction of diverse template architectural morphologies and will find applications in the construction of a variety of metallic nanoarchitectures.

Reliable plasmonic substrates for bioanalytical SERS applications easily prepared by convective assembly of gold nanocolloids

The ability to easily prepare Surface Enhanced Raman Scattering (SERS) substrates by the assembly of chemically synthesized gold nanocolloids is of great interest for the advancement of SERS-based optical detection and identification of molecular species of biological or chemical interest, pollutants or warfare agents. In this work we employ three very simple strategies, which can be implemented in any laboratory without the need for specialized equipment, to prepare assemblies of citrate-stabilized spherical gold colloids: (i) drop-coating, which induces the assembly of colloids in so-called coffee rings; (ii) a simplified variant of convective self-assembly (CSA), based on water evaporation in a constrained geometry, which yields highly uniform strips of nanoparticles (NP); (iii) assembly onto chemically functionalized glass surfaces which yields randomly assembled colloids and colloidal clusters. The SERS properties of the resulting colloidal assemblies are comparatively evaluated under multiple excitation lines with p-aminothiophenol (pATP) as a model Raman scatterer. The NP strips obtained by CSA prove to be SERS-active both in the visible and NIR and possess a highly uniform SERS response as demonstrated by spectra at individually selected sites and by confocal SERS mapping. Further it is shown that these NP strips are effective for the detection of cytosine, a DNA component, and for multi-analyte SERS detection. These results, showing how an efficient SERS substrate can be obtained by a very simple assembly method from easy-to-synthesize colloidal gold NP, can have an impact on the development of analytical SERS applications.

Enhanced tumor accumulation of sub-2 nm gold nanoclusters for cancer radiation therapy

A new type of metabolizable and efficient radiosensitizers for cancer radiotherapy is presented by combining ultrasmall Au nanoclusters (NCs, <2 nm) with biocompatible coating ligands (glutathione, GSH). The new nanoconstruct (GSH-coated Au25 NCs) inherits attractive features of both the Au core (strong radiosensitizing effect) and GSH shell (good biocompatibility). It can preferentially accumulate in tumor via the improved EPR effect, which leads to strong enhancement for cancer radiotherapy. After the treatment, the small-sized GSH-Au25 NCs can be efficiently cleared by the kidney, minimizing any potential side effects due to the accumulation of Au25 NCs in the body.

Glucose detection using SERS with multi-branched gold nanostructures in aqueous medium

Gold nanoparticles (AuNPs), multi-branched gold nanoparticles (MBGNs), and silica-coated MBGNs (MBGNs-silica) were studied for rhodamine B (RB) and α-glucose Raman detection at low concentration in water.

SERS properties of different sized and shaped gold nanoparticles biosynthesized under different environmental conditions by Neurospora crassa extract

Surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces. It is known that metal nanoparticles, especially gold and silver nanoparticles, exhibit great SERS properties, which make them very attractive for the development of biosensors and biocatalysts. On the other hand, the development of ecofriendly methods for the synthesis of metallic nanostructures has become the focus of research in several countries, and many microorganisms and plants have already been used to biosynthesize metallic nanostructures. However, the majority of these are pathogenic to plants or humans. Here, we report gold nanoparticles with good SERS properties, biosynthesized by Neurospora crassa extract under different environmental conditions, increasing Raman signals up to 40 times using methylene blue as a target molecule. Incubation of tetrachloroauric acid solution with the fungal extract at 60°C and a pH value of a) 3, b) 5.5, and c) 10 resulted in the formation of gold nanoparticles of a) different shapes like triangles, hexagons, pentagons etc. in a broad size range of about 10-200 nm, b) mostly quasi-spheres with some different shapes in a main size range of 6-23 nm, and c) only quasi-spheres of 3-12 nm. Analyses included TEM, HRTEM, and EDS in order to corroborate the shape and the elemental character of the gold nanoparticles, respectively. The results presented here show that these ‘green’ synthesized gold nanoparticles might have potential applicability in the field of biological sensing.

One-pot synthesis of trimetallic Au@PdPt core-shell nanoparticles with high catalytic performance

The development of an efficient synthesis method to produce multimetallic nanoparticles (NPs) with a desirable structure is strongly required to clarify the structure-composition-property relationship of NPs and to investigate their possible applications. However, the controlled synthesis of NPs consisting of multiple (n ≥ 3) noble metal components has been relatively unexplored in comparison to bimetallic NPs. In the present work, we have demonstrated a facile one-pot aqueous approach for the controlled synthesis of trimetallic Au@PdPt core-shell NPs with a well-defined octahedral Au core and a highly crystalline dendritic Pd-Pt alloy shell (Auoct@PdPt NPs). The simultaneous reduction of multiple metal precursors with dual reducing agents, namely, ascorbic acid and hydrazine, gave a fine control over the nucleation and growth kinetics of NPs, resulting in the formation of novel Auoct@PdPt NPs. The prepared NPs showed excellent catalytic performance for methanol electrooxidation, which can be attributed to their optimized binding strength toward adsorbate molecules due to the improved charge transfer between core and shell of the NPs. The present strategy can offer a convenient and valuable way to fabricate multicomponent nanostructures with desired structures and functions.

Two-color laser printing of individual gold nanorods

We report on the deposition of individual gold nanorods from an optical trap using two different laser wavelengths. Laser light, not being resonant to the plasmon resonances of the nanorods, is used for stable trapping and in situ alignment of individual nanorods. Laser light, being resonant to the transversal mode of the nanorods, is used for depositing nanorods at desired locations. The power and polarization dependence of the process is investigated and discussed in terms of force balances between gradient and scattering forces, plasmonic heating, and rotational diffusion of the nanorods. This two-color approach enables faster printing than its one-color equivalent and provides control over the angular orientation (±16°) and location of the deposited nanorods at the single-nanorod level.