Self-assembly Of Metal Nanoparticles Research Articles

Mechanistic understanding of nanoparticle interactions to achieve highly-ordered arrays through self-assembly for sensitive surface-enhanced Raman scattering detection of trace thiram

Over the last few decades, there is increasing worldwide concern over human health risks associated with extensive use of pesticides in agriculture. Developing excellent SERS substrate materials to achieve highly sensitive detection of pesticide residues in the food is very necessary owing to their serious threat to human health through food chains. Self-assembled metallic nanoparticles have been demonstrated to be excellent SERS substrate materials. Hence, alkanethiols-protected gold nanoparticles have been successfully prepared for forming larger-scale two-dimensional monolayer films. These films can be disassembled into a fluid state and re-assembled back to crystallized structure by controlling surface pressure. Further investigations reveal that their self-assembled structures are mainly dependent on the diameter of gold nanoparticles and ligand length. These results suggest that the size ratio of nanoparticle diameter/ligand length within the range of 4.45–2.35 facilitates the formation of highly ordered 2D arrays. Furthermore, these arrays present excellent Surface-Enhanced Raman Scattering performances in the detection of trace thiram, which can cause environmental toxicity to the soil, water, animals and result in severe damage to human health. Therefore, the current study provides an effective way for preparing monodispersed hydrophobic gold nanoparticles and forming highly ordered 2D close-packed SERS substrate materials via self-assembly to detect pesticide residues in food. We believe that, our research provides not only advanced SERS substrate materials for excellent detection performance of thiram in food, but also novel fundamental understandings of self-assembly, manipulation of nanoparticle interactions, and controllable synthesis.

Self-Assembly of Metal Nanoparticles in Bacterial Cellulose for the Fabrication of Soft Substrate-Supported Catalysts

Self-Assembly of Metal Nanoparticles in Bacterial Cellulose for the Fabrication of Soft Substrate-Supported Catalysts

Space charge governs the kinetics of metal exsolution

Nanostructured composite electrode materials play a major role in the fields of catalysis and electrochemistry. The self-assembly of metallic nanoparticles on oxide supports via metal exsolution relies on the transport of reducible dopants towards the perovskite surface to provide accessible catalytic centres at the solid–gas interface. At surfaces and interfaces, however, strong electrostatic gradients and space charges typically control the properties of oxides. Here we reveal that the nature of the surface–dopant interaction is the main determining factor for the exsolution kinetics of nickel in SrTi0.9Nb0.05Ni0.05O3–δ. The electrostatic interaction of dopants with surface space charge regions forming upon thermal oxidation results in strong surface passivation, which manifests in a retarded exsolution response. We furthermore demonstrate the controllability of the exsolution response via engineering of the perovskite surface chemistry. Our findings indicate that tailoring the electrostatic gradients at the perovskite surface is an essential step to improve exsolution-type materials in catalytic converters.

Multiscale modeling of the dynamical conductivity of self-assembled nanoparticle networks: Numerical simulations vs analytical models

Impedance spectroscopy experiments are able to reveal the fundamental charge transport properties of a wide variety of complex disordered and nano-structured materials provided that appropriate modeling tools are used. In this paper, we present a numerical simulation-based approach to model the dynamical conductivity of networks formed by self-assembled metal nanoparticles. Inter-particle nano-resistance and nano-capacitance are implemented at the nano-scale assuming inter-particle charge transfer and accumulation mechanisms that can be adapted depending on the nature of the nano-particles and the surrounding medium. The actual positions and spatial arrangements of the nanoparticles within the network are taken into consideration, allowing the attributes of percolating conducting routes to be extracted, classified, and compared in terms of path conductance and statistical distribution of path lengths. Our findings are contrasted to those obtained using analytic models, which are commonly used, but rely on strong assumptions about the electric properties of the conducting paths. We address these assumptions and show that in the case of weakly disordered systems, there is a general agreement between numerical simulations and analytic modeling-based approaches. In the case of disordered networks where the nano-particle size and position fluctuations are included, we show that the path length distribution is frequency-dependent and can differ significantly from the lognormal distribution usually assumed in the analytic models. The impedance of individual pathways may be extracted from the numerical simulations; we discovered that the conductance and susceptance of a specific path are frequency-dependent and inversely proportional to the path length only in ordered networks. Strong scattering of conductance values is caused by disorder effects. The developed numerical approach is generic and applies to most nano-devices where charge transport relies on percolation; it allows to bridge the gap between the nano-scale and micro-scale electric characteristics and, thus, permits a deeper understanding of the charge transport properties of nano-structured materials.

Reduced harmful blue light attenuator based on self-assembly of metal nanoparticles

Reduced harmful blue light attenuator based on self-assembly of metal nanoparticles

Active Plasmonics with Responsive, Binary Assemblies of Gold Nanorods and Nanospheres.

Self-assembly of metal nanoparticles has applications in the fabrication of optically active materials. Here, we introduce a facile strategy for the fabrication of films of binary nanoparticle assemblies. Dynamic control over the configuration of gold nanorods and nanospheres is achieved via the melting of bound and unbound fractions of liquid-crystal-like nanoparticle ligands. This approach provides a route for the preparation of hierarchical nanoparticle superstructures with applications in reversibly switchable, visible-range plasmonic technologies.

Effect of Surface Chemical Modification on the Self Assembly of Metal Nanoparticles

We report the self-assembly of a monolayer of gold nanoparticles (Au-NPs) on unmodified glass and on functionalized glass substrates with 3-aminopropyltriethoxysilane under different temperatures. The interaction between Au-NPs and APTES molecules on the glass substrates was affected by the role of APTES molecules on the glass substrates. The stability of the monolayers was carefully studied by performing a series of characterizations such as UV-Vis-NIR spectroscopy, Scanning electron microscopy (SEM). The self-assembled Au-NPs monolayers show high stability and homogenous distribution. The results of this study are important biosensing applications based on surface plasmon resonance or surface-enhanced Raman spectroscopy, where the detected signals can be adequately enhanced.

The Influence of Dielectric Environment on Spectral Shift of Localized Plasmonic Resonance

In paper the investigation results of dielectric environment influences on spectral shift of localized plasmonic resonance in a system of self-assembled metallic nanoparticles was introduced. It was shown, that the value of spectral shift of localized plasmonic resonance has a linear proportion towards the changes of environment refractive index. For investigated plasmonic system of self-assembled Au nanoparticles the coefficient of proportionality was determined.

Enhanced Spatial Light Confinement of All Inorganic Perovskite Photodetectors Based on Hybrid Plasmonic Nanostructures.

3D incident light confinement by radical electromagnetic fields offers a facile and novel way to break through the performance limit of inorganic perovskite CsPbBr3 quantum dots (QDs). Herein, metallic nanoparticles decorated anodic aluminum oxide (AAO) hybrid plasmonic nanostructures with geometric control are first proposed for cyclic light utilization of perovskite photodetectors, enabled by spatially extended light confinement. The drastic multiple interference induced by plasmonic coupling within AAO matrixes are generated as a function of pore sizes, which can effectively collect the transmitted photons back to the surface. In addition, the self-assembled metallic nanoparticles simultaneously concentrate the incident and reflected light beams into the CsPbBr3 QD layers. The light confinement inherently stems from the metallic nanoparticles due to the variation of the near surface electromagnetic fields. As a result, perovskite photodetectors based on Al nanoparticles/AAO hybrid plasmonic nanostructures with a pore size of 220nm exhibit enhanced photoresponse behavior with remarkably increased photocurrent by ≈43× and maintain low dark current under 490nm light illumination at 1V.

Confined volume plasmon spatial distribution by low-loss EELS on self-assemble bismuth nanoparticles

Bismuth nanoparticles have been synthesized via solvothermal reaction with high yield, as a function of reaction parameters: pH, temperature and precursor concentration (sodium hypophosphite). Morphology characterization of the synthesized products by scanning electron microscopy shows nanobelts formation is favored under acidic pH (2 and 4) conditions. Structural characterization by transmission electron microscopy shows that nanobelts are self-assembled rhombohedral metallic bismuth nanoparticles (5.54 ± 1.87 nm (pH4)) and no other morphology was found, therefore a mechanism for self-assembled nanobismuth is proposed. Study and characterization of bulk plasmon blue-shift on bismuth nanoparticles due to quantum confinement effects was observed by mapping the volume plasmon energy as a function of position by electron energy loss spectroscopy. It shows that self-assembled nanobismuth into nanobelts is a practical structure for the evaluation of bismuth optoelectronic properties in the nano-scale regimen, given that no structural fluctuations due to irradiation of high energy electrons coming from the electron microscope were observed during the mapping of volume plasmon energies on individual bismuth nanoparticles. Association of size, shape and volume plasmon energy on nanobismuth will deliver clearer insight into how the characteristic materials can be tailored for practical applications.

Well-ordered self-assembled plasmonic dimers for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) using structures of metal nanoparticles (NPs) is extremely sensitive to the relative orientation of the polarization of the incident light with the interparticle axis. Self-assembly of metal nanoparticles is a well-known approach to fabricate nanostructures. However, controlling the assembly direction of the NPs in this method is difficult. In this Letter, we propose a controlled self-assembly method using vacuum ultraviolet (VUV) light. By illuminating the surface-functionalized NPs with VUV light, the functionality is cancelled out, and therefore, we can select desirable direction of assembly. In the light of this method, well-ordered dimers (WODs) of silver NPs are fabricated. The Raman enhancement factor of WODs is about twice larger than randomly ordered ones.

Plasmonic Properties of Close-Packed Metallic Nanoparticle Mono- and Bilayers

The self-assembly of metallic nanoparticles is a promising route to metasurfaces with unique properties for many optical applications, such as surface-enhanced spectroscopy, light manipulation, and sensing. We present an in-depth theoretical study of the optical properties of mono- and bilayers assembled from gold and silver nanoparticles. With finite-difference time-domain simulations, we predict the occurrence of two plasmon modes, a bright and a dark mode, which exhibit symmetric and antisymmetric dipole configurations between the layers, respectively. The dark mode resonance energy depends sensitively on the size of the particles and the interparticle gaps. Hotspots with a nearfield intensity enhancement of up to 3000 are expected, which, together with the fact that the dark mode is roughly four times narrower than the bright mode, reveals how promising these materials are for spectroscopy purposes.

Hierarchical assembly of silver and gold nanoparticles in two-dimension: Toward fluorescence enhanced detection platforms

Two-dimensional (2D) metal nanoparticle platforms hold great potential in high-throughput point-of-care testing on basis of their excellent fluorescence enhancement effect. Self-assembly of metal nanoparticles (NPs) is versatile but simple for the preparation of 2D platforms; however, the surface charges on NPs would repel each other and induce large gaps on platforms, which hinder the generation of intense hotspots in electromagnetic fields and thus weaken the fluorescence enhancement effect. Here, we presented the hierarchical assembly of large-sized silver (Ag) NPs and small-sized gold (Au) NPs for constructing fluorescence enhanced platforms. The small Au NPs with weak fluorescence were chosen to complementarily fill into the larger gaps of the pre-assembled Ag platforms for increasing the electromagnetic hotspots. The resulting Ag/Au hybrid platforms (1-, 3- and 6-Ag/Au) exhibited growing fluorescence enhancement effects and the maximum enhanced factor reached 3.6-fold. The inserting of small Au NPs enhanced the fluorescence emission of fluorescent dye (cyanine-5) on the Ag platforms, especially on the monolayer 1-Ag platform, which increased by 161.6%. Moreover, the finite-difference time-domain (FDTD) calculations revealed the underlying cause of the improved electromagnetic fields in the Ag/Au hierarchical architecture in comparison to the Ag or Au NP. This work presents the hierarchical assembly is expected to be a powerful tool in the large-scale fabrication of fluorescence enhanced detection platforms.

Synaptic dynamics in complex self-assembled nanoparticle networks.

We report a detailed study of neuromorphic switching behaviour in inherently complex percolating networks of self-assembled metal nanoparticles. We show that variation of the strength and duration of the electric field applied to this network of synapse-like atomic switches allows us to control the switching dynamics. Switching is observed for voltages above a well-defined threshold, with higher voltages leading to increased switching rates. We demonstrate two behavioral archetypes and show how the switching dynamics change as a function of duration and amplitude of the voltage stimulus. We show that the state of each synapse can influence the activity of the other synapses, leading to complex switching dynamics. We further demonstrate the influence of the morphology of the network on the measured device properties, and the constraints imposed by the overall network conductance. The correlated switching dynamics, device stability over long periods, and the simplicity of the device fabrication provide an attractive pathway to practical implementation of on-chip neuromorphic computing.

SERS active self-assembled diphenylalanine micro/nanostructures: A combined experimental and theoretical investigation.

Enhancing Raman signatures of molecules by self-assembled metal nanoparticles, nanolithography patterning, or by designing plasmonic nanostructures is widely used for detection of low abundance biological systems. Self-assembled peptide nanostructures provide a natural template for tethering Au and Ag nanoparticles due to its fractal surface. Here, we show the use of L,L-diphenylalanine micro-nanostructures (FF-MNSs) for the organization of Ag and Au nanoparticles (Nps) and its potential as surface-enhanced Raman scattering (SERS)-active substrates. The FF-MNSs undergo an irreversible phase transition from hexagonally packed (hex) micro-nanotubes to an orthorhombic (ort) structure at ∼150 °C. The metal Nps form chains on hex FF-MNSs as inferred from transmission electron microscopy images and a uniform non-aggregated distribution in the ort phase. The high luminescence from the ort FF-MNS phase precludes SERS measurements with AgNps. The calculated Raman spectra using density-functional theory shows a higher intensity from rhodamine 6G (R6G) molecule in the presence of an Ag atom bound to ort FF compared with hex FF. The SERS spectra obtained from R6G bound to FF-MNSs with AuNps clearly show a higher enhancement for the ort phase compared with hex FF, corroborating our theoretical calculations. Our results indicate that FF-MNSs both in the hex and ort phases can be used as substrates for the SERS analysis with different metal nanoparticles, opening up a novel class of optically active bio-based substrates.

Significantly enhanced sensitivity of surface plasmon resonance sensor with self-assembled metallic nanoparticles

A surface plasmon resonance (SPR) sensor hybridized with self-assembled metallic nanoparticles is proposed and experimentally demonstrated. The measured sensitivity of the proposed SPR sensor is 110.77 deg/RIU , while that of a conventional SPR sensor is 84.75 deg/RIU . The enhanced sensitivity is attributed to the strong localized surface plasmons and the increased surface interaction area by the nanoparticles. Angle variation measurement, which is an easy detection method using bulk optics, is possible with this structure because a supplementary metallic thin film layer on the nanoparticles leads to utilization of the sensitive variation of the strong localized field by the change of the refractive index. Furthermore, the proposed structure can be fabricated with a very simple three-step nonlithographic process.

Self-Assembled Large-Scale Monolayer of Au Nanoparticles at the Air/Water Interface Used as a SERS Substrate.

Self-assembly of metal nanoparticles has attracted considerable attention because of its unique applications in technologies such as plasmonics, surface-enhanced optics, sensors, and catalysts. However, fabrication of ordered nanoparticle structures remains a significant challenge. Thus, developing an efficient approach for the assembly of large-scale Au nanoparticles films for theoretical studies and for various applications is highly desired. In this paper, a facial approach for fabricating a monolayer film of Au nanoparticles was developed successfully. Using the surfactant polyvinylpyrrolidone (PVP), a large-scale monolayer film of well-ordered, uniform-sized Au nanoparticles was fabricated at the air/water interface. The film exhibited a two-dimensional (2D) hexagonal close-packed (HCP) structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform "hot spots" for surface-enhanced Raman scattering (SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The low spot-to-spot and substrate-to-substrate variations of intensity (<10%), the large surface enhancement factor (∼10(6)), and the high stability (∼45 days) make the substrate suitable for SERS measurements. Transfer of the monolayer film onto a glassy carbon electrode produced an Au electrode with clean, well-defined nanostructure suitable for electrochemical SERS measurements. The adsorption process of ionic liquids on the electrode with the monolayer film is similar to that on bulk metal electrodes. The present strategy provides an effective way for self-assembly of Au nanoparticles into well-defined nanostructures that may form optimal reproducible SERS substrates for quantitative analysis. It also provides an electrode with clean, well-defined nanostructure for electrochemical investigations.

Coulomb blockade based field-effect transistors exploiting stripe-shaped channel geometries of self-assembled metal nanoparticles.

Metallic nanoparticles offer possibilities to build basic electric devices with new functionality and improved performance. Due to the small volume and the resulting low self-capacitance, each single nanoparticle exhibits a high charging energy. Thus, a Coulomb-energy gap emerges during transport experiments that can be shifted by electric fields, allowing for charge transport whenever energy levels of neighboring particles match. Hence, the state of the device changes sequentially between conducting and non-conducting instead of just one transition from conducting to pinch-off as in semiconductors. To exploit this behavior for field-effect transistors, it is necessary to use uniform nanoparticles in ordered arrays separated by well-defined tunnel barriers. In this work, CoPt nanoparticles with a narrow size distribution are synthesized by colloidal chemistry. These particles are deposited via the scalable Langmuir-Blodgett technique as ordered, homogeneous monolayers onto Si/SiO2 substrates with pre-patterned gold electrodes. The resulting nanoparticle arrays are limited to stripes of adjustable lengths and widths. In such a defined channel with a limited number of conduction paths the current can be controlled precisely by a gate voltage. Clearly pronounced Coulomb oscillations are observed up to temperatures of 150 K. Using such systems as field-effect transistors yields unprecedented oscillating current modulations with on/off-ratios of around 70%.

Control of Partial Coalescence of Self-Assembled Metal Nano-Particles across Lyotropic Liquid Crystals Templates towards Long Range Meso-Porous Metal Frameworks Design.

The formation of purely metallic meso-porous metal thin films by partial interface coalescence of self-assembled metal nano-particles across aqueous solutions of Pluronics triblock lyotropic liquid crystals is demonstrated for the first time. Small angle X-ray scattering was used to study the influence of the thin film composition and processing conditions on the ordered structures. The structural characteristics of the meso-structures formed demonstrated to primarily rely on the lyotropic liquid crystal properties while the nature of the metal nano-particles used as well as the their diameters were found to affect the ordered structure formation. The impact of the annealing temperature on the nano-particle coalescence and efficiency at removing the templating lyotropic liquid crystals was also analysed. It is demonstrated that the lyotropic liquid crystal is rendered slightly less thermally stable, upon mixing with metal nano-particles and that low annealing temperatures are sufficient to form purely metallic frameworks with average pore size distributions smaller than 500 nm and porosity around 45% with potential application in sensing, catalysis, nanoscale heat exchange, and molecular separation.

One-Pot Self-Assembled, Reduced Graphene Oxide/Palladium Nanoparticle Hybrid Aerogels for Electrocatalytic Applications

Despite the synergistic features arising from three-dimensional (3D) interconnected porous networks and constitutive reduced grpahene oxide (rGO) sheets, the one-pot synthesis and self-assembly of metal nanoparticles (NPs) deposited on the 3D rGO aerogels (GAs) is still challenging. Herein, we demonstrate the in-situ synthesis and direct deposition of 3D macroporous GA/palladium (Pd) NP hybrids for electrocatalytic applications. The discrete Pd NPs with a narrow size distribution of 8 to 10 nm are uniformly distributed onto the pore surface of GAs, while 3D macroporosity being constructed via a one-pot, hydrothermal process. The morphology, chemical composition and structure, and textural property of the 3D GA/Pd NP hybrids are comprehensively characterized. As a consequence of the coupling effect of the hierarchical porosity and the discrete Pd NPs, the 3D GA/Pd NP hybrids exhibit the greater electrocatalytic activity compared to those of 3D GAs and 2D rGO/Pd NP hybrids.