Self-assembly Of Au Nanoparticles Research Articles

High-Power Impulse Magnetron Sputter Deposition of Ag on Self-Assembled Au Nanoparticle Arrays at Low-Temperature Dewetting Conditions.

Plasmons have facilitated diverse analytical applications due to the boosting signal detectability by hot spots. In practical applications, it is crucial to fabricate straightforward, large-scale, and reproducible plasmonic substrates. Dewetting treatment, via applying direct thermal annealing of metal films, has been used as a straightforward method in the fabrication of such plasmonic nanostructures. However, tailoring the evolution of the dewetting process of metal films poses considerable experimental complexities, mainly due to nanoscale structure formation. Here, we use grazing-incidence small- and wide-angle X-ray scattering for the in situ investigation of the high-power impulse magnetron sputter deposition of Ag on self-assembled Au nanoparticle arrays at low-temperature dewetting conditions. This approach allows us to examine both the direct formation of binary Au/Ag nanostructure and the consequential impact of the dewetting process on the spatial arrangement of the bimetallic nanoparticles. It is observed that the dewetting at 100 °C is sufficient to favor the establishment of a homogenized structural configuration of bimetallic nanostructures, which is beneficial for localized surface plasmon resonances (LSPRs). The fabricated metal nanostructures show potential application for the surface-enhanced Raman scattering (SERS) detection of rhodamine 6G molecules. As SERS platform, bimetallic nanostructures formed with dewetting conditions turn out to be superior to those without dewetting conditions. The method in this work is envisioned as a facile strategy for the fabrication of plasmonic nanostructures.

Self-Assembly of Strain-Adaptable Surface-Enhanced Raman Scattering Substrate on Polydimethylsiloxane Nanowrinkles.

Flexible surface-enhanced Raman scattering (SERS) substrates adaptable to strains enable effective sampling from irregular surfaces, but the preparation of highly stable and sensitive flexible SERS substrates is still challenging. This paper reports a method to fabricate a high-performance strain-adaptable SERS substrate by self-assembly of Au nanoparticles (AuNPs) on polydimethylsiloxane (PDMS) nanowrinkles. Nanowrinkles are created on prestrained PDMS slabs by plasma-induced oxidation followed by the release of the prestrain, and self-assembled AuNPs are transferred onto the nanowrinkles to construct the high-performance SERS substrate. The results show that the nanowrinkled structure can improve the surface roughness and enhance the SERS signals by ∼4 times compared to that of the SERS substrate prepared on flat PDMS substrates. The proposed SERS substrate also shows good adaptability to dynamic bending up to ∼|0.4| 1/cm with excellent testing reproducibility. Phenolic pollutants, including aniline and catechol, were quantitatively tested by the SERS substrate. The self-assembled flexible SERS substrate proposed here provides a powerful tool for chemical analysis in the fields of environmental monitoring and food safety inspection.

Interface self-assembly of plasmonic nanolayer for sensitive detection of heavy metals in water using NELIBS

Nowadays, high-stable and ultrasensitive heavy metal detection is of utmost importance in water quality monitoring. Nanoparticle-enhanced laser-induced breakdown spectroscopy (NELIBS) shows high potential in hazardous metal detection, however, encounters unstable and weak signals due to nonuniform distribution of analytes. Herein, we developed an interface self-assembly (ISA) method to create a uniformly distributed gold nanolayer at a liquid-liquid interface for positive heavy metal ions capture and NELIBS analysis. The electrostatically self-assembled Au nanoparticles (NPs)-analytes membrane was prepared at the oil-water interface by injecting ethanol into the mixture of cyclohexane and Au NPs-analytes water solution. Then, the interface self-assembled Au NPs-analytes membrane was transformed onto a laser-processed superhydrophilic Si slide for detection. Three heavy metals (cadmium (Cd), barium (Ba), and chromium (Cr)) were analyzed to evaluate the stability and sensitivity of the ISA method for NELIBS. The results (Cd: RSD ​= ​3.6 ​%, LoD ​= ​0.654 ​mg/L; Ba: RSD ​= ​3.4 ​%, LoD ​= ​0.236 ​mg/L; Cr: RSD ​= ​7.7 ​%, LoD ​= ​1.367 ​mg/L) demonstrated signal enhancement and high-stable and ultrasensitive detection. The actual sample detection (Cd: RE ​= ​7.71 ​%, Ba: RE ​= ​6.78 ​%) illustrated great reliability. The ISA method, creating a uniform distribution of NP-analytes at the interface, has promising prospects in NELIBS.

Realization of ultrastrong coupling between LSPR and Fabry–Pérot mode via self-assembly of Au-NPs on p-NiO/Au film

Abstract The realization of higher coupling strengths between coupled resonant modes enables exploration of compelling phenomena in diverse fields of physics and chemistry. In this study, we focus on the modal coupling between localized surface plasmon resonance (LSPR) of Au nanoparticles (Au-NPs) and Fabry–Pérot mode (p-NiO/Au film). The effects of nanoparticle size, projected surface coverage (PSC), interparticle distance (IPD), and arrangement to the coupling strength between the two modes are theoretically investigated using finite-difference time-domain (FDTD) method. Au-NPs/p-NiO/Au film (ANA) nanostructures with NPs size of 10 nm, 30 nm, and 50 nm are considered. Numerical calculations point to larger size and higher projected surface coverage (also smaller IPD) of NPs as pre-eminent factors in enhancing the strength of modal coupling. ANA nanostructure with NPs size of 30 nm (ANA-30) and 50 nm (ANA-50) are experimentally fabricated via a facile air–liquid interface self-assembly. The fabricated nanodevices exhibit immense Rabi splitting energies of 655 meV (ANA-30) and 770 meV (ANA-50), and thus fulfill the ultrastrong coupling condition with remarkable splitting energy to bare (plasmon) energy ratio of 0.35 (ANA-30) and 0.4 (ANA-50). The physical insights presented in this study, together with the simple and scalable fabrication process, establish a viable approach to realize stronger coupling between LSPR and Fabry–Pérot mode in metal NPs/dielectric/metal film systems. This will be vital to take advantage of the promising performance enhancements of plasmonic-based nanostructures under strongly coupled regimes in areas such as solar to fuel conversion, sensing, opto-electronics, and quantum applications.

Nb2C MXene self-assembled Au nanoparticles simultaneously based on electromagnetic enhancement and charge transfer for surface enhanced Raman scattering

Recent years, two-dimensional transition metal carbonitrides (MXene) have attracted much attention in the field of surface-enhanced Raman scattering (SERS). However, the relatively low enhancement of MXene is a major challenge. Herein, Nb2C-Au NPs nanocomposites were prepared by electrostatic self-assembly method, which have a synergistically conjugated SERS effect. The EM hot spots of Nb2C-Au NPs are significantly enlarged and expanded, while the surface Fermi level is decreased. This synergistic effect could improve the SERS performance of the system. Consequently, for the dye molecules CV and MeB, the detection limits reach 10−10 M and 10−9 M, respectively, while for biomolecule adenine, the detection limit is as low as 5 × 10−8 M. The results also show the good concentration-dependent linearity, uniformity, reproducibility and stability of SERS substrate. Nb2C-Au NPs could be a fast, sensitive and stable SERS platform for label-free and non-destructive detection. This work may expand the application of MXene based materials in the field of SERS.

Templated Out‐of‐Equilibrium Self‐Assembly of Branched Au Nanoshells (Small 12/2023)

Branched Au Nanoshells Out-of-equilibrium self-assembly of metal nanoparticles achieving 3D structures with controlled morphology is still rare. In article number 2206712, Claudia Pigliacelli, Pierangelo Metrangolo, and co-workers adopt a peptide-gold superstructure as template to control the out-of-equilibrium self-assembly of Au nanoparticles, obtaining a transient 3D branched Au-nanoshell stabilized by surfactant. The nanoshell dismantles upon concentration gradient equilibration, leading to disassembly and regression to the template.

Top-Down Fabrication of Luminescent Graphene Quantum Dots Using Self-Assembled Au Nanoparticles.

A new graphene quantum dot (GQD) fabrication method is presented, which employs a lithographic approach based on self-assembled Au nanoparticles formed by solid-state dewetting. The GQDs are formed by the patterned etching of a graphene layer enabled by Au nanoparticles, and their size is controllable through that of the Au nanoparticles. GQDs are fabricated with four different diameters: 12, 14, 16, and 27 nm. The geometrical features and lattice structures of the GQDs are determined using transmission electron microscopy (TEM). Hexagonal lattice fringes in the TEM image and G- and 2D-band Raman scattering evidence the graphitic characteristics of the GQDs. The oxygen content can be controlled by thermal reduction under a hydrogen atmosphere. In GQDs, the absorption peak wavelengths in the ultraviolet range tend to decrease as the size of the GQDs decreases. They also exhibit apparent photoluminescence (PL). The PL peak wavelength is approximately 600 nm and becomes shorter as the size of the GQDs decreases. The blue shift in the optical absorption and PL of the smaller GQDs is attributed to the quantum confinement effect. The proposed GQD fabrication method can provide a way to control the physical and chemical properties of GQDs via their size and oxygen content.

Templated Out-of-Equilibrium Self-Assembly of Branched Au Nanoshells.

Out-of-equilibrium self-assembly of metal nanoparticles (NPs) has been devised using different types of strategies and fuels, but achieving finite 3D structures with a controlled morphology through this assembly mode is still rare. Here, a spherical peptide-gold superstructure (PAuSS) is used as a template to control the out-of-equilibrium self-assembly of Au NPs, obtaining a transient 3D-branched Au-nanoshell (BAuNS) stabilized by sodium dodecyl sulphate (SDS). The BAuNS dismantles upon SDS concentration gradient equilibration over time in the sample solution, leading to NPs disassembly and regression to PAuSS. Notably, BAuNS assembly and disassembly promotes temporary interparticle plasmonic coupling, leading to reversible and tunable changes of their plasmonic properties, a highly desirable behavior in the development of optoelectronic nanodevices.

Properties of SiC and Si3N4 Thin Films Containing Self-Assembled Gold Nanoparticles

The properties of semiconductor materials can be strongly affected by the addition of metallic nanoparticles. Here we investigate the properties of SiC + Au and Si3N4 + Au thin films prepared by magnetron sputtering deposition followed by thermal annealing. The influence of gold addition on the optical and electrical properties is explored. We show the formation of self-assembled Au nanoparticles in SiC and Si3N4, with the size and arrangement properties determined by the deposition and annealing conditions. Both SiC- and Si3N4-based films show an increase in the overall absorption with increasing Au content, and its decrease with increasing annealing temperature. All films show the presence of surface plasmon resonance, whose peaks shift toward larger wavelengths with increasing Au nanoparticle size. The resistivity significantly drops with the Au content increase for both types of matrices, although the resistivity of Si3N4-based films is much higher. The incorporated quantity of Au in the host matrix was chosen in such a way to demonstrate that a huge range of optical and electrical characteristics is achievable. The materials are very interesting for application in opto-electronic devices.

Self-Assembled Au Nanoparticle Monolayers on Silicon in Two- and Three-Dimensions for Surface-Enhanced Raman Scattering Sensing.

Gold nanoparticle/silicon composites are canonical substrates for sensing applications because of their geometry-dependent physicochemical properties and high sensing activity via surface-enhanced Raman spectroscopy (SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized via wet-chemistry on functionalized flat silicon (Si) and vertically aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective approach to prepare such substrates. Herein, we report on the critical parameters that influence nanoparticle coverage, aggregation, and assembly sites in two- and three-dimensions to prepare substrates with homogeneous optical properties and SERS activity. We show that the degree of AuNP aggregation on flat Si depends on the silane used for the Si functionalization, while the AuNP coverage can be adjusted by the incubation time in the AuNP solution, both of which directly affect the substrate properties. In particular, we report the reproducible synthesis of nearly touching AuNP chain monolayers where the AuNPs are separated by nanoscale gaps, likely to be formed due to the capillary forces generated during the drying process. Such substrates, when used for SERS sensing, produce a uniform and large enhancement of the Raman signal due to the high density of hot spots that they provide. We also report the controlled self-assembly of AuNPs on VA-SiNW arrays, which can provide even higher Raman signal enhancement. The directed assembly of the AuNPs in specific regions of the SiNWs with a control over NP density and monolayer morphology (i.e., isolated vs nearly touching NPs) is demonstrated, together with its influence on the resulting SERS activity.

Self-Assembled Ligand-Capped Plasmonic Au Nanoparticle Films in the Kretschmann Configuration for Sensing of Volatile Organic Compounds

Films of close-packed Au nanoparticles are coupled electrodynamically through their collective plasmon resonances. This collective optical response results in enhanced light-matter interactions, which can be exploited in various applications. Here, we demonstrate their application in sensing volatile organic compounds, using methanol as a test-case. Ordered films over several cm2 were obtained by interfacial self-assembly of colloidal Au nanoparticles (~10 nm diameter) through controlled evaporation of the solvent. Even though isolated nanoparticles of this size are inherently non-scattering, when arranged in a close-packed film the plasmonic coupling results in a strong reflectance and absorbance. The in-situ tracking of vapor phase methanol concentration through UV-Vis transmission measurements of the nanoparticle film is first demonstrated. Next, in-situ ellipsometry of the self-assembled films in the Kretschmann (also known as ATR) configuration is shown to yield enhanced sensitivity, especially with phase difference measurements. Our study shows the excellent agreement between theoretical models of the spectral response of self-assembled films with experimental in-situ sensing experiments. At the same time, the theoretical framework provides the basis for the interpretation of the various observed experimental trends. Combining periodic nanoparticle films with ellipsometry in the Kretschmann configuration is a promising strategy towards highly sensitive and selective plasmonic thin-film devices based on colloidal fabrication methods for volatile organic compound (VOC) sensing applications.

Topography Mapping with Scanning Electrochemical Cell Microscopy

High-resolution scanning electrochemical cell microscopy (SECCM), synchronously visualizing the topography and electrochemical activity, could be used to directly correlate the structure and activity of materials nanoscopically. However, its topographical measurement is largely restricted by the size and stability of the meniscus droplet formed at the end of the nanopipette. In this paper, we report a scheme that could reliably gain several tens nanometer resolution (≥65 nm) of SECCM using homemade ∼50 nm inner diameter probes. Furthermore, the topography and hydrogen evolution reaction (HER) activity of ∼45 nm self-assembled Au nanoparticles monolayer were simultaneously recorded successfully. This scheme could make mapping of both topologic and chemical properties of samples in the nanometer regime with SECCM routinely, which potentially can largely expand the field of SECCM applications.

DNA self-assembled Au nanoparticle clusters on silver nanorod arrays for high-sensitive and multiplex detection of cancer-related biomarkers.

To sensitively detect multiple and cross-species disease-related targets from a single biological sample in a quick and reliable manner is of high importance in accurately diagnosing and monitoring diseases. Herein, a surface-enhanced Raman scattering (SERS) sensor based on a functionalized multiple-armed tetrahedral DNA nanostructure (FMTDN) immobilized silver nanorod (AgNR) array substrate and Au nanoparticle (AuNP) SERS tags is constructed to achieve both multiplex detection and enhanced sensitivity using a sandwich strategy. The sensor can achieve single, dual, and triple biomarker detections of three lung cancer-related nucleic acid and protein biomarkers, i.e., miRNA-21, miRNA-486 and carcinoembryonic antigen (CEA) in human serum. The enhanced SERS signals in multiplex detections are due to the DNA self-assembled AuNP clusters on the silver nanorod array during the assay, and the experimentally obtained relative enhancement factor ratios, 150 for AuNP dimers and 840 for AuNP trimers, qualitatively agree with the numerically calculated local electric field enhancements. The proposed FMTDN-functionalized AgNR SERS sensor is capable of multiplex and cross-species detection of nucleic acid and protein biomarkers with improved sensitivity, which has great potential for the screening and clinical diagnosis of cancer in the early stage.

Silver(I)-selective impedimetric and potentiometric detection at 3-hydroxy-N-(4-mercaptophenyl)benzamide functionalized gold surface – Analysis of water and packaged food products

In the current work, a monothiol, 3-hydroxy-N-(4-mercaptophenyl) benzamide (HMB) was synthesized via amide coupling of 4-aminothiophenol with 3-hydroxybenzoic acid followed by its self-assembly on the surface of a gold electrode which was confirmed using electrochemical techniques: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and spectroscopic techniques: AFM and FT-IR. Further, HMB functionalized gold electrode (HMB-Au) was utilised as a platform for impedimetric sensing in the presence of one-electron reversible redox probe, [Fe(CN)6]3-/4- and found to show sensitivity towards Ag+ over a wide linear concentration range from 1 × 10−9 M to 1 × 10−4 M (r2 = 0.99 ± 0.005) at pH = 8 with a detection limit of 3.60 × 10−9 M. Furthermore, HMB self-assembled Au nanoparticles (HMB-AuNPs) were incorporated as modifier to construct a carbon paste electrode (CPE) for potentiometric detection of Ag+. Ag+-CPE exhibited a Nernstian slope of 61.70 ± 0.02 mV per decade in the concentration range of 1.0 × 10−4 M - 1.0 × 10−1 M (r2 = 0.99 ± 0.01) with a detection limit of 4.46 × 10−5 M and a short response time of 15 s. Both HMB based EIS sensor and CPE have shown an acceptable selectivity towards silver(I) ions and successfully applied for estimation of silver(I) content in various samples of water and packaged food products.

Optofluidic modulator based on thermoplasmonically controlled liquid-liquid interface.

We report a thermoplasmonically actuated optical modulator based on a liquid-liquid interface decorated with self-assembled Au nanoparticles (Au NPs). The system comprises heptane (top layer) and water (bottom layer), and an Au NPs array at their interfaces. Focused excitation with the plasmonic wavelength (532 nm) generates a localized temperature rise at the interface (ΔT=3.2±0.7∘C), resulting in a thermocapillary flow. We optimized the temperature gradient and the heptane layer thickness so that the resulting thermocapillary flow leads to the formation of a "self-healing hole" at the irradiating zone, which we exploited as an all-optical modulator. A signal beam (655 nm) positioned through the top layer, parallel to the interface, gives a maximum output when the layer is intact (no-hole situation, ON state) and a minimum output when the hole is present (OFF state). Nearly 100% optical modulation is achieved in a reversible manner, highlighting the potential of responsive and reconfigurable fluid-fluid interfaces for optical applications.

Enhanced electrical conductivity of polymer microspheres by altering assembly sequence of two different shaped conductive fillers

Conductive microspheres are prepared using two different shaped conductive fillers, carbon nanotubes (CNTs) and Au nanoparticles (AuNPs), by a dry particle coating (DPC) process and a self-assembly method. CNTs, which are one-dimensional conductive fillers, facilitate the formation of conducting pathways so that the zero-dimensional AuNP content can be significantly reduced to obtain electrical percolation from the conductive microspheres. Two fabrication methods are investigated: (1) the self-assembly of AuNPs followed by the DPC of the CNTs on polystyrene (PS) microspheres (t-PS/AuNP/CNT) and (2) the DPC of the CNTs followed by the self-assembly of AuNPs (PS/t-CNT/AuNP). In both cases, the electrical percolation of the conductive microspheres is achieved using a significantly lower amount of AuNPs (1 wt%), which is 17 times lower than that of samples prepared using only AuNPs (17 wt%). Interestingly, each sample exhibits distinct conducting behaviors with different amounts of AuNPs owing to the different hybridization structures of AuNPs and CNTs.

PM Level and Large Dynamic Range Glucose Detection Based on a Sandwich Type Plasmonic Fiber Sensor

A sandwich type titled fiber Bragg grating (TFBG) plasmonic sensor based on self-assembled p-mercaptophenylboronic acid (PMBA) monolayer and Au nanoparticles (AuNPs), has been demonstrated and successfully applied for sensitive and stable low-concentration glucose detection. The sensor was immobilized with PMBA monolayer acting as the highly sensitive and stable glucose recognition molecule instead of a traditional unstable enzyme. In the experiments, the high sensitivity for glucose detection is achieved, establishing a competitive limit of detection (LOD) of 295 pM and large dynamic measurement range from 1 nM to 10 mM simultaneously. Moreover, due to the unique two-set diol configurations of glucose, the combination of TFBG-SPR with AuNPs and PMBA exhibit a good selectivity over fructose and galactose, which both possess only one set of diol configurations. Non-invasive saliva glucose detection, as a better alternative to blood, has emerged and attracted many researchers' attention. Therefore, the practicality of this method was successfully validated by detecting glucose in a simulated human saliva sample. We believe that this method represents an appealing solution for flexible, temperature-insensitive, high-sensitivity, and high-stability detection of ultra-low concentration glucose in modern non-invasive point-of-care testing as well as tumor micro-environment (TME) related cancer research.

PH-Driven Reversible Assembly and Disassembly of Colloidal Gold Nanoparticles.

Owing to the localized surface plasmon resonance (LSPR), dynamic manipulation of optical properties through the structure evolution of plasmonic nanoparticles has been intensively studied for practical applications. This paper describes a novel method for direct reversible self-assembly and dis-assembly of Au nanoparticles (AuNPs) in water driven by pH stimuli. Using 3-aminopropyltriethoxysilane (APTES) as the capping ligand and pH-responsive agent, the APTES hydrolyzes rapidly in response to acid and then condenses into silicon. On the contrary, the condensed silicon can be broken down into silicate by base, which subsequently deprotonates the APTES on AuNPs. By controlling condensation and decomposition of APTES, the plasmonic coupling among adjacent AuNPs could be reversible tuned to display the plasmonic color switching. This study provides a facile and distinctive strategy to regulate the reversible self-assembly of AuNPs, and it also offers a new avenue for other plasmonic nanoparticles to adjust plasmonic properties via reversible self-assembly.

Plasmon-enhanced coherent anti-stokes Raman scattering vs plasmon-enhanced stimulated Raman scattering: Comparison of line shape and enhancement factor.

Plasmon-enhanced coherent Raman scattering microscopy has reached single-molecule detection sensitivity. Due to the different driven fields, there are significant differences between a coherent Raman scattering process and its plasmon-enhanced derivative. The commonly accepted line shapes for coherent anti-Stokes Raman scattering and stimulated Raman scattering do not hold for the plasmon-enhanced condition. Here, we present a theoretical model that describes the spectral line shapes in plasmon-enhanced coherent anti-Stokes Raman scattering (PECARS). Experimentally, we measured PECARS and plasmon-enhanced stimulated Raman scattering (PESRS) spectra of 4-mercaptopyridine adsorbed on the self-assembled Au nanoparticle (NP) substrate and aggregated Au NP colloids. The PECARS spectra show a nondispersive line shape, while the PESRS spectra exhibit a dispersive line shape. PECARS shows a higher signal to noise ratio and a larger enhancement factor than PESRS from the same specimen. It is verified that the nonresonant background in PECARS originates from the photoluminescence of nanostructures. The decoupling of background and the vibrational resonance component results in the nondispersive line shape in PECARS. More local electric field enhancements are involved in the PECARS process than in PESRS, which results in a higher enhancement factor in PECARS. The current work provides new insight into the mechanism of plasmon-enhanced coherent Raman scattering and helps to optimize the experimental design for ultrasensitive chemical imaging.

Tunable Raman scattering enhancement due to self-assembled Au nanoparticles layer on porous AAO: The influence of the alumina support

In this investigation, SERS substrates were synthesized based on the growth of Au nanoparticles (AuNPs) layer over porous support of anodized aluminum oxide (AAO). The enhancement factor (EF) of the SERS substrates was determined by the measurement of Rhodamine 6G (R6G) allowed a detection limit of 10−11 (measured over molar concentration). The AAO was grown by a two-stage anodizing method, and subsequently, an Au precursor layer in different thicknesses was sputtered on each alumina support. The coated samples were heat-treated under nitrogen to promote different spatial distributions of AuNPs on the surface. Au-Np layer was achieved with disordered pore-like as demonstrated by low-pressure Nitrogen adsorption and BET analysis (isotherm curve type IV-H3). The sample that exhibited the strongest SERS effect consisted of a 50 nm Au precursor layer on the AAO support. The method reported in this work is highly reproducible, given its simplicity, with a homogeneous size distribution compared to other methods and other substrates such as Silicon.