Reversible assembly and disassembly of gold nanorods induced by EDTA and its application in SERS tuning

Gold on the horizon: unveiling the chemistry, applications and future prospects of 2D monolayers of gold nanoparticles (Au-NPs)

Noble 2D monolayers of gold nanoparticles (Au-NPs) have garnered significant attention due to their unique physicochemical properties, which are instrumental in various technological applications. This review delves into the intricate physical chemistry underlying the formation of Au-NP monolayers, highlighting key interactions such as electrostatic forces, van der Waals attractions, and ligand-mediated stabilization. The discussion extends to the size- and shape-dependent assembly processes of these NP monolayers, elucidating how nanoparticle dimensions and morphologies influence monolayer formation and stability. Moreover, the review explores the diverse interfaces-solid, liquid, and air-where Au-NP monolayers are employed, each presenting distinct advantages and challenges. In the realm of applications, Au-NP monolayers have shown remarkable promises. In memory devices, their ability to facilitate high-density data storage through enhanced electron transport mechanisms is examined. Biosensing applications benefit from the monolayers' exceptional sensitivity and specificity, which are crucial for detecting biomolecular interactions. Furthermore, the role of Au-NP monolayers in electrocatalysis is explored, with a focus on their catalytic efficiency and stability in various electrochemical reactions. Despite their potential, the deployment of Au-NP monolayers faces several challenges. The review addresses current limitations such as scalability, reproducibility, and long-term stability, proposing potential strategies to overcome these hurdles. Future prospects are also discussed, including the development of multifunctional monolayers and integration with other nanomaterials to enhance performance across different applications. In conclusion, while significant strides have been made in understanding and utilizing 2D Au-NP monolayers, ongoing research is imperative to fully exploit their capabilities. Addressing existing challenges through innovative approaches will pave the way for their widespread adoption in advanced technological applications.

'When is a hotspot a good nanospot' - review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s-100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS' sensitivity, and in need of investigation via scanning probe techniques.

Fluorogenic response from DNA templated micrometer range self-assembled gold nanorod

Plasmonic gold nanorod (AuNR) on a macromolecular matrix exhibits an end-to-end (ETE) long-range self-assembly (AuNR)n with n > 100. In the case of small molecules as a template, the pre-synthesized macromolecular matrix is missing and this brings a synthetic challenge in directed long-range assembly of AuNR. Self-assembly with thiol-modified small DNA and AuNR shows a much short-range ETE assembly with n < 25 via a simple evaporation technique on a solid surface. In this study, the introduction of two short amine modified probe DNAs (∼2.5 nm) and one 22-mer complementary single strand (ss)-DNA template (∼7 nm) show the long-range ETE self-assembly of (AuNR)n with n > 130. In the solution state, the zigzag arrangement within the assembled structure controls the typical change in the absorption behavior for (AuNR)n ETE assembly. The formation of this long-range ETE self-assembly in a solution state was verified from the combined effect of fluorescence resonance energy transfer (FRET) and hotspot-induced fluorescence enhancement. The probe DNAs and templated DNA concentration on fluorescence enhancement have been varied to monitor the effect of (AuNR)n with n = ∼5-130 in ETE self-assembly. Primarily quenched FRET acceptor in the presence of AuNR decisively exhibits remarkable fluorogenic response in ETE self-assembly with maximum n value. Although the FRET efficiencies among the fluorophores are comparable, the fluorogenic boost in ETE AuNR is due to the increased number of intrinsic navigated hotspots in these assemblies.

Combining Acoustic Bioprinting with AI-Assisted Raman Spectroscopy for High-Throughput Identification of Bacteria in Blood

Identifying pathogens in complex samples such as blood, urine, and wastewater is critical to detect infection and inform optimal treatment. Surface-enhanced Raman spectroscopy (SERS) and machine learning (ML) can distinguish among multiple pathogen species, but processing complex fluid samples to sensitively and specifically detect pathogens remains an outstanding challenge. Here, we develop an acoustic bioprinter to digitize samples into millions of droplets, each containing just a few cells, which are identified with SERS and ML. We demonstrate rapid printing of 2 pL droplets from solutions containing S. epidermidis, E. coli, and blood; when they are mixed with gold nanorods (GNRs), SERS enhancements of up to 1500× are achieved.We then train a ML model and achieve ≥99% classification accuracy from cellularly pure samples and ≥87% accuracy from cellularly mixed samples. We also obtain ≥90% accuracy from droplets with pathogen:blood cell ratios <1. Our combined bioprinting and SERS platform could accelerate rapid, sensitive pathogen detection in clinical, environmental, and industrial settings.

Reversible assembly of nanoparticles: theory, strategies and computational simulations

The significant advances in synthesis and functionalization have enabled the preparation of high-quality nanoparticles that have found a plethora of successful applications. The unique physicochemical properties of nanoparticles can be manipulated through the control of size, shape, composition, and surface chemistry, but their technological application possibilities can be further expanded by exploiting the properties that emerge from their assembly. The ability to control the assembly of nanoparticles not only is required for many real technological applications, but allows the combination of the intrinsic properties of nanoparticles and opens the way to the exploitation of their complex interplay, giving access to collective properties. Significant advances and knowledge gained over the past few decades on nanoparticle assembly have made it possible to implement a growing number of strategies for reversible assembly of nanoparticles. In addition to being of interest for basic studies, such advances further broaden the range of applications and the possibility of developing innovative devices using nanoparticles. This review focuses on the reversible assembly of nanoparticles and includes the theoretical aspects related to the concept of reversibility, an up-to-date assessment of the experimental approaches applied to this field and the advanced computational schemes that offer key insights into the assembly mechanisms. We aim to provide readers with a comprehensive guide to address the challenges in assembling reversible nanoparticles and promote their applications.

Inhalation of Silver Silicate Nanoparticles Leads to Transient and Differential Microglial Activation in the Rodent Olfactory Bulb

Engineered silver nanoparticles (AgNPs), including silver silicate nanoparticles (Ag-SiO₂ NPs), are used in a wide variety of medical and consumer applications. Inhaled AgNPs have been found to translocate to the olfactory bulb (OB) after inhalation and intranasal instillation. However, the biological effects of Ag-SiO₂ NPs and their potential nose-to-brain transport have not been evaluated. The present study assessed whether inhaled Ag-SiO₂ NPs can elicit microglial activation in the OB. Adult Sprague-Dawley rats inhaled aerosolized Ag-SiO₂ NPs at a concentration of 1 mg/ml for 6 hours. On day 0, 1, 7, and 21 post-exposure, rats were necropsied and OB were harvested. Immunohistochemistry on OB tissues were performed with anti-ionized calcium-binding adapter molecule 1 and heme oxygenase-1 as markers of microglial activation and oxidative stress, respectively. Aerosol characterization indicated Ag-SiO₂ NPs were sufficiently aerosolized with moderate agglomeration and high-efficiency deposition in the nasal cavity and olfactory epithelium. Findings suggested that acute inhalation of Ag-SiO₂ NPs elicited transient and differential microglial activation in the OB without significant microglial recruitment or oxidative stress. The delayed and differential pattern of microglial activation in the OB implied that inhaled Ag-SiO₂ may have translocated to the central nervous system via intra-neuronal pathways.

Laying down of gold nanorods monolayers on solid surfaces for surface enhanced Raman spectroscopy applications

Laying-down gold nanorods (GNRs) of a monolayer immobilized on a solid substrate was realized with a hybrid method, a combination of three elemental technologies: surface modification, electrophoresis, and solvent evaporation. The self-assembly of CTAB-protected GNRs in the solution was induced by 0.05 mM of EDTA. The assembled GNRs were deposited in a laying-down form on the solid surface during the hybrid method. The final coverage was over 71% on the substrate with an area larger than 0.6 cm². The spacing between the sides of the GNRs was fixed to be 4.6 ± 0.9 nm by the thermal annealing-promoted crystalline packing of the bilayer of CTAB salt-bridged with EDTA. The obtained laying-down GNRs of a monolayer on the gold substrate show a small shift of the transverse LSPR around 550-570 nm (with a width of around 100 nm) and a large red shift of the longitudinal LSPR to be 900-1050 nm (with a width of 500 nm), because of the strong electromagnetic coupling between the GNRs and gold substrate. Therefore it can be used in a wide range of wavelengths for surface enhanced Raman spectroscopy (SERS) applications. The film has a high enhancement factor with 10⁵ for R6G.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Synthesis of Asymmetric One-Dimensional Pd on Au Bimetallic Nanostructures

Nanostructures composed of a gold nanorod (AuNR) core and a Pd/Pt shell are of great interest due to their potential application as plasmon resonance-enhanced catalysts. However, the synthesis of well-defined one-dimensional bimetallic nanostructures with precise control over shell thickness and length remains a challenge. In this study, we report a detailed and systematic study on the chemical synthesis of a uniform Pd shell on single crystalline and pentahedrally twinned (PHT) AuNRs of various lengths. AuNRs were used as a template, and the slow and controlled reduction of Pd(II) ions on preformed AuNRs was carried out for the formation of rectangular-shaped Au@Pd bimetallic nanorods. The Pd shell thickness around the AuNRs was controlled by the supply of Pd(II) ions in the growth solution. We were able to grow a ∼20 nm uniform Pd shell around the AuNR, keeping the rod-like morphology intact without local nucleation to form irregular shapes and randomly overgrown nanostructures. The formation of bimetallic nanorods was also extended beyond typical single crystalline nanorods to PHT high aspect ratio gold nanorods and nanowires, using them as templates. To our surprise, unusually curved asymmetric nanorods were formed when the Pd deposition was carried out on AuNRs longer than ∼800 nm which could be possibly due to a Pd and Au lattice mismatch at the interface and higher flexibility of the nanorods when they exceeded certain lengths.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Configuration-Modulated Hot Electron Dynamics of Gold Nanorod Assemblies

Comprehension and modulation of hot electron dynamics at an ultrafast time scale are crucial for exploring the hot electron-assisted energy transfer processes. Here, we report the hot electron dynamics of dispersed gold nanorods and their controlled assemblies measured by time-resolved pump-probe spectroscopy. Both assembly configurations are shown to accelerate the hot electron decay in comparison with dispersed nanorods. The hot electron dynamics exhibit different variations with aspect ratio in transverse and longitudinal polarizations. The hot electron lifetime and the spectral signature of the induced absorption modification are found to be highly sensitive to photon energy as well as assembly configuration and aspect ratios, showing different contributions of plasmon coupling and electron-surface scattering. This work not only improves the understanding of the underlying mechanisms of hot electron dynamics but also paves the way to optimize performance characteristics of hot carrier-assisted photocatalysis, photovoltaics, and all-optical high-rate photonic processing applications.

Self-assembly and wetting properties of gold nanorod-CTAB molecules on HOPG

The formation of self-assembled superstructures of cetyltrimethylammonium bromide (CTAB) after drying on a nonwetting highly ordered pyrolytic graphite (HOPG) surface have been investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Although SEM did not reveal coverage of CTAB layers, AFM showed not only CTAB assembly, but also the dynamics of the process on the surface. The self-assembled layers of CTAB molecules on the HOPG terraces prior to nanorod deposition were shown to change the wettability of the surface, and as a result, gold nanorod deposition takes place on nonwetting HOPG terraces.

Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures

This manuscript describes a reversible wet chemical process for the tip-selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip-selective dissolution was achieved by oxidation of AuNRs with a Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on the AuNR surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. A decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in a blue shift in the longitudinal plasmon band (LPB) position, and red shifts in the LPB position were achieved by increasing the AR by 1D growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when the desired AR and/or LPB position is achieved. In addition, the tip-selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long AuNWs.

Appearance of SERS activity in single silver nanoparticles by laser-induced reshaping

We report simultaneous plasmonic scattering and Raman spectroscopic observations of single citrate capped silver nanoparticles (AgNPs) which exhibit surface enhanced Raman scattering (SERS) upon meeting specific conditions induced by laser (532 nm) exposure. We show that nanoparticles which are not initially SERS active become SERS active by laser-induced reshaping/reorientation. A set-up developed for these observations enabled in situ high speed time-lapse characterization using plasmonic and Raman spectroscopies in conjunction with dark-field microscopy (DFM). Changes in the AgNPs were confirmed by monitoring plasmonic scattering spectra and DFM images. Time-lapse observations have shown that laser-induced changes in the plasmonic properties of AgNPs resulted in the appearance of SERS. Spectral matching between plasmon resonance and downward molecular vibronic transitions for molecules adsorbed on the surface of plasmonic nanomaterials is attributed to the nanoparticle SERS. We have further shown that the release of silver ions by silver nanoparticles can be the probable reason for their plasmonic changes. Gold nanoparticles inert to such mild (850 μW, 532 nm) laser-induced changes do not exhibit the appearance of SERS.

Two-Step Assembly of Thermoresponsive Gold Nanorods Coated with a Single Kind of Ligand

Gold nanorods (GNRs) coated with a single kind of ligand show thermoreponsive two-step assembly to provide a hierarchical structure. The GNRs (33 nm in length × 14 nm in diameter) coated with a hexa(ethylene glycol) (HEG) derivative form side-by-side assemblies at 30 °C (T_A1 ) as a steady state through dehydration. By further heating to over 40 °C (T_A2 ), larger assemblies, which are composed of the side-by-side assembled units, are formed as hierarchical structures. The dehydration temperature of the HEG derivative varies depending on the free volume of the HEG unit, which corresponds to the curvature of the GNRs. Upon heating, dehydration first occurs from the ligands on the side portions with a lower curvature, and then from the ligands on the edge portions with a higher curvature. The different sized GNRs (33 × 8 and 54 × 15 nm) also show two-step assembly. Both the T_A1 and T_A2 are dependent on the diameter of the GNRs, but independent of their length. This result supports that the dehydration is dependent on the free volume, which corresponds to the curvature. Anisotropic assembly focusing on differences in curvature provides new guidelines for the fabrication of hierarchical structures.

Dual function of EDTA with silver nanoparticles for root canal treatment-A novel modification

The chelating and antimicrobial capacity of a novel modification of 17% EDTA with silver nanoparticles (AgNPs) (EDTA-AgNPs) was evaluated in-vitro for root canal treatment (RCT). The EDTA-AgNPs solution was characterized by UV-Vis spectroscopy, ζ-potential and high-resolution transmission electron microscopy (HRTEM). Antimicrobial capacity was evaluated against Candida albicans and Staphylococcus aureus in planktonic and biofilm cells by broth macrodilution (24 h) and XTT assays, (1, 10 and 30 min) respectively. The chelating capacity of EDTA-AgNPs was assessed indirectly (smear layer removal) and directly (demineralizing effect) in bovine dentin at two silver concentrations, 16 and 512 μg/ml at 1 and 10 minutes of exposure time. Smear layer removal was evaluated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The demineralizing effect was determined by atomic absorption spectroscopy (AAS), microhardness test (MH) and X-ray diffractometer (XRD). Synthesized AgNPs were quasi-spherical in shape with an average size of 13.09 ± 8.05 nm. 17% EDTA-AgNPs was effective to inhibit C. albicans and S. aureus in planktonic and biofilm cultures. The smear layer removal and demineralizing effect were similar between 17% EDTA-AgNPs and 17% EDTA treatments. The 17% EDTA-AgNPs solution proved to be an effective antimicrobial agent, and has a similar chelating capacity to 17% EDTA alone. These in-vitro studies strongly suggest that EDTA-AgNPs could be used for effective smear layer removal, having an antimicrobial effect at the same time during RCT.

Plasmon-assisted site-selective growth of Ag nanotriangles and Ag-Cu2O hybrids

We report a plasmon-assisted growth of metal and semiconductor onto the tips of Ag nanotriangles (AgNTs) under light irradiation. The site-selective growth of Ag onto AgNTs are firstly demonstrated on the copper grids and amine-coated glass slides. As the irradiation time increases, microscopic images indicate that AgNTs gradually touch with each other and finally “weld” tip-to-tip together into the branched chains. Meanwhile, the redshift of plasmon band is observed in the extinction spectra, which agrees well the growth at the tips of AgNTs and the decrease of the gaps between the adjacent nanotriangles. We also synthesize AgNT-Cu₂O nanocomposites by using a photochemical method and find that the Cu₂O nanoparticles preferably grow on the tips of AgNTs. The site-selective growth of Ag and Cu₂O is interpreted by the local field concentration at the tips of AgNTs induced by surface plasmon resonance under light excitation.

Hierarchical Self-Assembly of Cu7Te5 Nanorods into Superstructures with Enhanced SERS Performance

This paper reports a strategy to get self-assembly of Cu₇Te₅ nanorods into hierarchical superstructures: the side-by-side self-assembly of nanorods into microscale one-dimensional (1D) nanowires (primary structure), the side-by-side alignments of the 1D nanowires into two-dimensional (2D) nanowire bundles (secondary structure), and the further rolling up of the 2D bundles into three-dimensional (3D) microtubes (tertiary structure). It was found that the oleylamine (OLA)/n-dodecanethiol (DDT) mixture as a binary capping agent was key to produce Cu₇Te₅ nanorods in the quantum size regime with high monodispersity, and this was a prerequisite for their hierarchical self-assembly based on elaborate control of the solvent evaporation process. The obtained Cu₇Te₅ microtube superstructures were used as SERS substrate and showed much stronger SERS enhancement than the as-prepared Cu₇Te₅ nanorods before assembly. This was probably ascribed to the remarkably enhanced local electromagnetic field arising from the plasmon coupling of Cu₇Te₅ nanorods in the well-assembled superstructures.

Fabrication of coupled twin-shaped hollow hemispherical calcium molybdate via a facile ultrasound-assisted approach

Coupled twin-shaped hollow hemispherical calcium molybdate (CTHH-CaMoO₄) microstructures were successfully synthesized via a facile ultrasound-assisted template-free route in the presence of absolute ethanol and ethylenediaminetetraacetic acid (EDTA).

Nano-functionalized filamentous fungus hyphae with fast reversible macroscopic assembly & disassembly features

Hyphae help polyaniline nanoparticles to assemble & disassemble macroscopically.

Rational design of Au nanorods assemblies for highly sensitive and selective SERS detection of prostate specific antigen

A simple SERS immunosensor based on AuNRs assembly was developed for rapid detection of specific antigen in early diagnostics.

Gold nanorod biochip functionalization by antibody thiolation

Conjugation of biomolecules on gold nanorod (GNR) surfaces is the basis for successful applications in biosensing, imaging, and drug delivery. Current functionalization methods are often problematic, involving multi-step nanoparticle modification to replace surfactant bilayer, delicate nanoparticle protection during surfactant exchange, and material loss due to inevitable aggregation. Instead of intensive surface modification of GNRs, we describe herein a facile method to functionalize gold nanorod surfaces via covalent Au-S bonds by thiolating receptors. The resulting GNR-bioconjugates showed superior dispersion and stability in buffer for months without morphology change and aggregation. ELISA tests confirmed the high biofunctionality of the thiolated anti-IgG moieties immobilized on the GNR surfaces. Furthermore, this simple method facilitated a straightforward functionalization of GNR assembly on glass substrate to construct a specific biochip, which can detect human IgG targets in a label-free fashion with high sensitivity and specificity. Compared to electropolymeric coating to functionalize the GNR, our method exhibited a five-fold enhancement in the spectral sensitivity to refractive index change caused by the target binding. This universal GNR bioconjugation method can be extended to bind different proteins and antibodies for development of biosensors or drug delivery.

Au nanorod quartets and Raman signal enhancement: towards the design of plasmonic platforms

Quartets of Au nanorods were designed by combining the methodologies of lateral and longitudinal assemblies. A high electric field prevailing at the quartet junctions results in large enhancement in the Raman signals of molecules. FDTD simulations showed that the displacement of the lateral dimers in quartets expands the scope of hot spot distribution.

Replacement of CTAB with peptidic ligands at the surface of gold nanorods and their self-assembling properties

Herein, we describe the self-assembling of gold nanorods (GNRs) induced during the ligand exchange at their surface. An exchange reaction between tricysteine PEGylated peptidic ligands and cetyltrimethylammonium bromide (CTAB)-protected gold nanorods is conducted. We demonstrated that the terminal group charge (positively or negatively charged) and the hydrophobicity of the peptidic ligands (bearing or not an undecanoyl chain) strongly affects the self-organization of the GNRs occurring in solution. Adjusting the amount of short PEGylated peptides causes a self-organization of the gold nanorods in solution, resulting in a red- or blue-shift of the plasmon bands. The decrease of their surface charge and the self-assembling in solution were first shown by zetametry, by Dynamic Light Scattering and UV-spectroscopy. Thanks to Small Angle X-ray Scattering experiments and Transmission Electron Microscopy images, the self-organization of the nanorods in solution was clearly demonstrated and correlated to the spectroscopic change in absorbance. Conversely, in the case of longer PEGylated peptidic ligands including an undecanoyl chain, the GNRs are particularly stable against aggregation for several days after purification. By controlled drying on a substrate, we showed their ability to self-organize into well-defined ordered structures making them very attractive as building blocks to design optical materials.

Polyamine ligand-mediated self-assembly of gold and silver nanoparticles into chainlike structures in aqueous solution: towards new nanostructured chemosensors

Polyamine ligands are very versatile compounds due to their water solubility and flexibility. In the present work, we have exploited the binding ability of a polyamine molecular linker (L (2-)) bearing different functional groups, which favors the self-assembling of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) into 1D nanochains in aqueous solution. The chainlike assemblies of AuNPs and AgNPs were structurally stable for a long period of time, during which their characteristic optical properties remained unchanged. The mechanism of AuNPs and AgNPs chain assembly associated with the induction of electric dipole-dipole interactions arising from the partial ligand exchange of surface-adsorbed citrate ions by (L (2-)) was investigated. UV/Vis spectrophotometry and transmission electron microscopy (TEM) were used to determine timedependent structural changes associated with formation of the 1D nanoparticle structures. Finally, the sensing of Hg(2+) in aqueous solution using AgNPs@(L)(2-) and AuNPs@(L)(2-) assemblies was also carried out in aqueous solution.

Magnetic tuning of plasmonic excitation of gold nanorods

By using gold nanorods as an example, we report the dynamic and reversible tuning of the plasmonic property of anisotropically shaped colloidal metal nanostructures by controlling their orientation using external magnetic fields. The magnetic orientational control enables instant and selective excitation of the plasmon modes of AuNRs through the manipulation of the field direction relative to the directions of incidence and polarization of light.

Solution-dispersible Au nanocube dimers with greatly enhanced two-photon luminescence and SERS

We report the synthesis of 43-nm diameter Au nanocube dimers by using Ag(+) ions as competitive ligands to freeze L-cysteine-induced assembly process of the nanocubes to a desirable stage. Ascribed to the resonant interparticle coupling with an newly arising plasmon band at 710 nm and local field enhancement, the two-photon luminescence intensity of the Au nanocube dimers in solution was over 20 times stronger than that of the monomers in the wavelength range 555-620 nm. Furthermore, by coupling Raman tags onto the nanocube surface, a solution-based surface-enhanced Raman scattering (SERS) of the nanocube dimers had an enhancement factor of over 10 times compared to the isolated nanocubes. To sum up, with high stability in solution and attractive optical properties, the Au nanocube dimers have potential applications in in vivo bio-imaging and solution-based SERS.

Replacement of cetyltrimethylammoniumbromide bilayer on gold nanorod by alkanethiol crosslinker for enhanced plasmon resonance sensitivity

Surface modification of gold nanorods (GNRs) is often problematic due to tightly packed cetyltrimethylammoniumbromide (CTAB) bilayer. Herein, we performed a double phase transfer ligand exchange to achieve displacement of CTAB on nanorods. During the removal, 11-mercaptoundecanoic acid (MUDA) crosslinker is simultaneously assembled on nanorod surfaces to prevent aggregation. The resulting MUDA-GNRs retain the shape and position of plasmon peaks similar to CTAB-capped GNRs. The introduction of carboxyl groups allows covalent conjugation of biological receptors in a facile fashion to construct a robust, label-free biosensor based on localized surface plasmon resonance (LSPR) transduction of biomolecular interaction. More importantly, smaller MUDA layer on the GNRs reduces the distance of target binding to the plasmonic nanostructure interface, leading to a significant enhancement in LSPR assay sensitivity and specificity. Compared to modification using conventional electropolymer adsorption, MUDA-coated gold nanosensor exhibits five times lower detection limit for cardiac troponin I assay with a high selectivity.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.