Assembly Of Plasmonic Nanoparticles Research Articles

Hierarchically Structured Plasmonic Nanoparticle Assemblies with Dual-Length Scale Electromagnetic Hot Spots for Enhanced Sensitivity in the Detection of (Bio)Molecular Analytes

High sensitivity in plasmon-enhanced spectroscopies results not only from high electromagnetic (EM) field enhancements at the vicinity of nanostructured metal surfaces but also the ability to lever...

Multiplexed and single-cell detection of microRNA with plasmonic nanoparticle assemblies

We present a label-free, low cost, and miniatured biosensing platform based on the disassembly of core-satellite plasmonic nanoparticle assemblies. The rapid and selective detection of an exemplary nucleic acid biomarker, has-miRNA-210-3p, was achieved via the strand displacement nucleic acid reaction. Target binding leads to dehybridization of the DNA linkers and changes in the scattering properties of nanostructures as monitored by darkfield microscopy. We demonstrate the ability to detect microRNA expunged from single cells and the potential to multiplex discrete assemblies to enable diverse biological applicability. The work may help translate the applicability of microRNA as diagnostic biomarkers, quantitate their abundance in the microenvironment, and facilitate the study of their correlation or causation to other biomolecules at the single-cell level.

Near-Infrared Chiral Plasmonic Microwires through Precision Assembly of Gold Nanorods on Soft Biotemplates

Directing the assembly of plasmonic nanoparticles into chiral superstructures has diverse applications including, chiroptical sensing, nonlinear optics, and biomedicine. Though soft template-mediated assemblies of both spherical and nonspherical gold nanoparticles have made significant progress, most approaches require sophisticated chemical synthesis or advanced methodologies. Besides, reports of structurally precise chiral plasmonic assemblies beyond nanoscale are limited. Here, we propose an efficient yet simple strategy to grow such precision assemblies up to mesoscale, which is beneficial for a broader community. Briefly, cationic gold nanorods (AuNRs) are allowed to systematically assemble along atomically precise, chiral, rodlike tobacco mosaic virus (TMV) particles via electrostatic attraction under ambient condition. This leads to spontaneous formation of helical hybrid microwires with high structural precision, as evidenced by cryogenic transmission electron microscopy and tomography. Resulting composite superstructures show a strong circular dichroism response at the plasmon wavelength of the AuNRs, which is supported by simulations using discrete dipole approximation. Further, chirality of the system is investigated at a single-microwire level using polarized dark-field scattering microscopy. An alternative chiral template, negatively charged colloidal cellulose nanocrystals, also arrange AuNRs into similar chiral microstructures. Thus, our report proposes a generic methodology to obtain chiral plasmonic response at the NIR region using inexpensive templates that will encourage the exploration of a wider range of nanoscale templates for creating hybrid mesostructures with emerging optoelectronic properties.

Ligand-Free Fabrication of Ag Nanoassemblies for Highly Sensitive and Reproducible Surface-Enhanced Raman Scattering Sensing of Antibiotics.

The assemblies of plasmonic nanoparticles (NPs) are the universal methods for enhancing their surface-enhanced Raman scattering (SERS) activities. However, the present methods suffer from the problems of poor reproducibility, complicated fabrication, or the adsorption of ligands on the surface, which limit their practical applications. In this work, by using a facile freeze-thaw method, we are able to fabricate the assemblies of Ag NPs with highly reproducible SERS activity without the use of ligands. Moreover, the Ag NPs can be well kept in a frozen state for a long time with few influences on the reproducibility (relative standard deviation, RSD ca. 7%), while those kept in colloid (4 °C) suffer from gradual surface oxidation and aggregation. Such a simple freeze-thaw method does not require the introduction of any ligands (or linkers) with long-term stability and reproducibility, implying its wide applications in practical SERS sensing.

Chemical processing of interfacially assembled metal nanowires for surface‐enhanced Raman scattering detection of food contaminants

AbstractWe report a new strategy for the fabrication of Ag–Au bimetallic nanowire (NW) arrays by taking advantage of interfacial assembly of plasmonic nanoparticles and galvanic replacement reactions. Galvanic replacement reactions led to the formation of nanospikes on NWs, giving rise to a high density of hotspots across the entire NW array. We employed this NW array as a surface‐enhanced Raman scattering (surface‐enhanced Raman spectroscopy [SERS]) substrate for the detection of two types of food contaminants: thiram (a pesticide) and melamine (an adulterant). Limits of detection (LODs) of 1 and 10 nM were achieved for thiram in juice samples and melamine in milk samples, respectively. Both LODs are lower than the maximum acceptable concentrations imposed by the US government. The results demonstrate tremendous practical potential of Ag–Au NW arrays as surface‐enhanced Raman scattering substrates for the detection of food contaminants.

Structural order in plasmonic superlattices

The assembly of plasmonic nanoparticles into ordered 2D- and 3D-superlattices could pave the way towards new tailored materials for plasmonic sensing, photocatalysis and manipulation of light on the nanoscale. The properties of such materials strongly depend on their geometry, and accordingly straightforward protocols to obtain precise plasmonic superlattices are highly desirable. Here, we synthesize large areas of crystalline mono-, bi- and multilayers of gold nanoparticles >20 nm with a small number of defects. The superlattices can be described as hexagonal crystals with standard deviations of the lattice parameter below 1%. The periodic arrangement within the superlattices leads to new well-defined collective plasmon-polariton modes. The general level of achieved superlattice quality will be of benefit for a broad range of applications, ranging from fundamental studies of light–matter interaction to optical metamaterials and substrates for surface-enhanced spectroscopies.

Rational Design of Fractal Gold Nanosphere Assemblies with Optimized Photothermal Conversion Using a Quantitative Structure Property Relationship (QSPR) Approach

Assemblies of plasmonic nanoparticles have been proposed for various applications, including photothermal therapy, exploiting surface plasmon coupling phenomena. However, the rational design of fractal nanoparticle assembly remains challenging due to the lack of structural characterizations and modelization of real systems. Here we used the quantitative structure property relationship (QSPR) approach, driven by experimental data and statistical analysis, to establish a relationship between structural descriptors of fractal gold nanoparticle (GNP) aggregates and their light-to-heat conversion. A total of 160 assemblies of various size spherical GNPs with different polyelectrolyte chains were synthesized, which differ in their global charge, size, mass fractal dimension, and plasmonic properties. Fifteen independent descriptors of structure and properties were extracted and further analyzed by QSPR. Principal component analysis and multilinear regression reveal that light-to-heat conversion is mainly governed by the structure of the aggregates and not by the characteristics of the building blocks. This highlights the key role of the fractal dimension of the aggregate and of the ratio of GNP/polyelectrolyte mass to optimize photothermal effects. Rational criteria to optimize light-to-heat conversion within nonideal fractal assemblies of GNP were identified, relaxing on the choice of other parameters, such as GNP or aggregate size, that can be adapted to the desired biomedical applications.

Size-Dependent Thermo- and Photoresponsive Plasmonic Properties of Liquid Crystalline Gold Nanoparticles.

Achieving remotely controlled, reversibly reconfigurable assemblies of plasmonic nanoparticles is a prerequisite for the development of future photonic technologies. Here, we obtained a series of gold-nanoparticle-based materials which exhibit long-range order, and which are controlled with light or thermal stimuli. The influence of the metallic core size and organic shell composition on the switchability is considered, with emphasis on achieving light-responsive behavior at room temperature and high yield production of nanoparticles. The latter translates to a wide size distribution of metallic cores but does not prevent their assembly into various, switchable 3D and 2D long-range ordered structures. These results provide clear guidelines as to the impact of size, size distribution, and organic shell composition on self-assembly, thus enhancing the smart design process of multi-responsive nanomaterials in a condensed state, hardly attainable by other self-assembly methods which usually require solvents.

Metabolic mapping with plasmonic nanoparticle assemblies.

A rapid and simple methodology for the biomolecular analysis of single cells and microenvironments via a stick-and-peel plasmonic sensing platform is reported. Substrate-bound assemblies of plasmonic gold nanoparticles linked by reconfigurable oligonucleotides undergo disassembly upon target binding. Changes in the light scattering intensity of thousands of discrete nanoparticle assemblies are extrapolated concomitantly to yield the mapping of local target concentrations. The methodology is completely free of labelling, purification and separation steps. We quantified the intracellular ATP levels for two ovarian cancer cell lines to elucidate the differences and cellular distribution, and demonstrated the potential of the stick-and-peel platform for mapping the microenvironment of a 2D heterogeneous surface. The portable and economical analytical platform may broaden the affordability and applicability of single-cell based analyses and enable new opportunities in clinical care such as on-site molecular pathology.

Ag Ion Soldering: An Emerging Tool for Sub-nanomeric Plasmon Coupling and Beyond.

Self-assembly represents probably the most flexible way to construct metastructured materials and devices from a wealth of colloidal building blocks with synthetically controllable sizes, shapes, and elemental compositions. In principle, surface capping is unavoidable during the synthesis of nanomaterials with well-defined geometry and stability. The ligand layer also endows inorganic building blocks with molecular recognition ability responsible for their assembly into desired structures. In the case of plasmonic nanounits, precise positioning of them in a nanomolecule or an ordered nanoarray provides a chance to shape their electrodynamic behaviors and thereby assists experimental demonstration of modern nanoplasmonics toward practical uses. Despite previous achievements in bottom-up nanofabrication, a big challenge exists toward strong coupling and facile charge transfer between adjacent nanounits in an assembly. This difficulty has impeded a functional development of plasmonic nanoassemblies. The weakened interparticle coupling originates from the electrostatic and steric barriers of ionic/molecular adsorbates to guarantee a good colloidal stability. Such a dilemma is rooted in fundamental colloidal science, which lacks an effective solution. During the past several years, a chemical tool termed Ag ion soldering (AIS) has been developed to overcome the above situation toward functional colloidal nanotechnology. In particular, a dimeric assembly of plasmonic nanoparticles has been taken as an ideal model to study plasmonic coupling and interparticle charge transfer. This Account starts with a demonstration of the chemical mechanism of AIS, followed by a verification of its workability in various self-assembly systems. A further use of AIS to realize postsynthetic coupling of DNA-directed nanoparticle clusters evidences its compatibility with DNA nanotechnology. Benefiting from the sub-nanometer interparticle gap achieved by AIS, a conductive pathway is established between two nanoparticles in an assembly. Accordingly, light-driven charge transfer between the conductively bridged plasmonic units is realized with highly tunable resonance frequencies. These situations have been demonstrated by thermal/photothermal sintering of silica-isolated nanoparticle dimers as well as gap-specific electroless gold/silver deposition. The regioselective silver deposition is then combined with galvanic replacement to obtain catalytically active nanofoci (plasmonic nanogaps). The resulting structures are useful for real time and on-site Raman spectroscopic tracking of chemical reactions in the plasmonic hotspots (nanogaps) as well as for study of plasmon-mediated/field-enhanced catalysis. The Account is concluded by a deeper insight into the chemical mechanism of AIS and its adaption to conformation-rich structures. Finally, AIS-enabled functional pursuits are suggested for self-assembled materials with strongly coupled and easily reshapable physicochemical properties.

Shaping Liquid Crystals with Gold Nanoparticles: Helical Assemblies with Tunable and Hierarchical Structures Via Thin-Film Cooperative Interactions.

The availability of helical assemblies of plasmonic nanoparticles with precisely controlled and tunable structures can play a key role in the future development of chiral plasmonics and metamaterials. Here, a strategy to efficiently yield helical structures based on the cooperative interactions of liquid crystals and gold nanoparticles in thin films is developed. These nanocomposites exhibit exceptional long-range hierarchical order across length scales, which results from the growth mechanism of nanoparticle-coated twisted nanoribbons and their ability to form organized bundles. The helical assembly formation is governed by the presence of rationally functionalized nanoparticles. Importantly, the thickness of the achieved nanocomposites can be reversibly reconfigured owing to the polymorphic nature of the liquid crystal. The versatility of the proposed approach is demonstrated by preparing helices assembled from nanoparticles of different geometries and dimensions (spherical and rod-like). The described strategy may become an enabling technology for structuring nanoparticle assemblies with high precision and fabricating optically active materials.

Nanoline-gap controlled self assembly of plasmonic nanoparticles inside plasmonic nanolines

This paper presents plasmonic substrates fabricated by a gap—controlled, template-assisted self assembly of plasmonic nanoparticles—such as spherical nanoparticles and nanorods—inside one dimensional plasmonic nanoline templates fabricated using Deep-UV lithography. This hybrid fabrication process—which combines the bottom-up process of capillary-force self assembly with the top-down process of Deep-UV lithography—can potentially be employed for large-area fabrication (6 inch or 12 inch wafers) of plasmonic substrates with sub-10 nm gaps. These small gaps lead to a high electromagnetic enhancement which can be highly beneficial if these substrates are employed for sensing based on surface enhanced Raman scattering (SERS). We demonstrate that the templates of plasmonic nanolines act as lithographed traps and direct the capillary-force assembly of metallic nanoparticles. The gaps between the nanolines, along with the shape and dimensions of the nanoparticles, together determine the interparticle distance, packing pattern and the orientation of the assembled nanoparticles inside these nanolines. Moreover, the electromagnetic behavior of these substrates is exhaustively analyzed using Finite Difference Time Domain (FDTD) modeling. Thus, we demonstrate template-directed, capillary-force assembly of plasmonic nanoparticles inside plasmonic nanolines such that the assembly can be controlled by modulating the structural parameters of the template or the assembled nanoparticles, and can be potentially carried out on a large area.

Plasmonic Supercrystals.

For decades, plasmonic nanoparticles have been extensively studied due to their extraordinary properties, related to localized surface plasmon resonances. A milestone in the field has been the development of the so-called seed-mediated growth method, a synthetic route that provided access to an extraordinary diversity of metal nanoparticles with tailored size, geometry and composition. Such a morphological control came along with an exquisite definition of the optical response of plasmonic nanoparticles, thereby increasing their prospects for implementation in various fields. The susceptibility of surface plasmons to respond to small changes in the surrounding medium or to perturb (enhance/quench) optical processes in nearby molecules, has been exploited for a wide range of applications, from biomedicine to energy harvesting. However, the possibilities offered by plasmonic nanoparticles can be expanded even further by their careful assembly into either disordered or ordered structures, in 2D and 3D. The assembly of plasmonic nanoparticles gives rise to coupling/hybridization effects, which are strongly dependent on interparticle spacing and orientation, generating extremely high electric fields (hot spots), confined at interparticle gaps. Thus, the use of plasmonic nanoparticle assemblies as optical sensors have led to improving the limits of detection for a wide variety of (bio)molecules and ions. Importantly, in the case of highly ordered plasmonic arrays, other novel and unique optical effects can be generated. Indeed, new functional materials have been developed via the assembly of nanoparticles into highly ordered architectures, ranging from thin films (2D) to colloidal crystals or supercrystals (3D). The progress in the design and fabrication of 3D supercrystals could pave the way toward next generation plasmonic sensors, photocatalysts, optomagnetic components, metamaterials, etc. In this Account, we summarize selected recent advancements in the field of highly ordered 3D plasmonic superlattices. We first analyze their fascinating optical properties for various systems with increasing degrees of complexity, from an individual metal nanoparticle through particle clusters with low coordination numbers to disordered self-assembled structures and finally to supercrystals. We then describe recent progress in the fabrication of 3D plasmonic supercrystals, focusing on specific strategies but without delving into the forces governing the self-assembly process. In the last section, we provide an overview of the potential applications of plasmonic supercrystals, with a particular emphasis on those related to surface-enhanced Raman scattering (SERS) sensing, followed by a brief highlight of the main conclusions and remaining challenges.

Unidirectional light scattering by electric dipoles induced in plasmonic nanoparticles

We proposed and demonstrated that unidirectional light scattering, usually realized by utilizing induced electric and magnetic dipole moments, can also be achieved via only electric dipole moments induced in an assembly of plasmonic nanoparticles. The condition for such kinds of unidirectional scattering has been derived in the dipole approximation. Calculated with the coupled dipole method, a contrast between the forward and backward scattering has been demonstrated with nearly five orders of magnitude for a dimer of gold nanospheres. It can be attributed to the destructive interference of scattered far field from the induced electric dipole moments with similar magnitudes and a relative phase of about π. Such properties are general for an assembly of electric dipole moments, and a trimer structure has been demonstrated to realize unidirectional light scattering as well as steering. Our results suggest a simple but yet versatile platform for manipulating light scattering.

Phase transition induced by localized surface plasmon resonance of nanoparticle assemblies

A numerical model concerning the phase transition and temperature distribution of matrix media around plasmonic nanoparticle assemblies under illumination of monochromatic light is investigated. The critical intensity of incident light for isolated nanosphere, nanodisk, and nanorod to melting the surrounding ice is obtained. For nanospheres and nanodisk dimers, enhancement of photothermal conversion (PTC) is observed. Besides, the phase transition of matrix media is not only influenced by PTC, but also the configuration of nanoparticle assemblies. In particular, we demonstrate that for nanorod dimers arranged end-to-end and side-to-side, the dissipation of light for two nanorods are the same in every polarization angles. The change of polarization angle only influence the overall light-to-heat conversion of the dimer. However, for L geometry nanorod dimer, the dissipation of light are different for the two nanorods. This present a way to manipulate and precisely control of temperature distribution and phase transition in nanoscale by tuning the light polarization angles.

Reversible Assembly and Dynamic Plasmonic Tuning of Ag Nanoparticles Enabled by Limited Ligand Protection.

Dynamic manipulation of optical properties through the reversible assembly of plasmonic nanoparticles offers great opportunities for practical applications in many fields. The previous success, however, has been limited to Au nanoparticles. Reversible assembly and plasmonic tuning of Ag nanoparticles (AgNPs) have remained a significant challenge due to difficulty in finding an appropriate surface agent that can effectively stabilize the particle surface and control their interactions. Here, we overcome the challenge by developing a limited-ligand-protection (LLP) strategy for introducing poly(acrylic acid) with precisely controlled coverage to the AgNP surface to not only sufficiently stabilize the nanoparticles but also enable effective control over the surface charge and particle interaction through pH variation. The as-synthesized AgNPs can be reversibly assembled and disassembled and accordingly display broadly tunable coupling of plasmonic properties. Compared to the Au-based system, the success in the reversible assembly of AgNPs represents a significant step toward practical applications such as colorimetric pressure sensing because they offer many advantages, including broader spectral tuning range, higher color contrast, a one-pot process, and low materials and production cost. This work also highlights LLP as a new avenue for controlling the interparticle forces, their reversible assembly, and dynamic coupling of physical properties.

Modular Assembly of Plasmonic Nanoparticles Assisted by DNA Origami.

Arraying noble metal nanoparticles with nanoscale features is an important way to develop plasmonic devices with novel optical properties such as plasmonic chiral metamolecules, optical waveguides, and so forth. Along with top-down methods of fabricating plasmonic nanostructures, solution-based self-assembly provides an alternative approach. There are mainly two routes to organizing metal nanoparticles via self-assembly. One is directly linking nanoparticles through linker molecules, and the other is using nanoparticles to decorate a preformed template. We combine these two routes and herein report a strategy for the DNA origami-assisted modular assembly of gold nanoparticles into homogeneous and heterogeneous plasmonic nanostructures. For each module, we designed W-shaped DNA origami with two troughs as two domains. One domain is used to host a gold nanoparticle, and the other domain is designed to capture another gold nanoparticle hosted on a different module. By simply tuning the sequences of capture DNA strands on each module, gold nanoparticles including spherical and rod-shaped gold nanoparticles (denoted as AuNPs and AuNRs) could be well organized in a predefined manner to form versatile plasmonic nanostructures. Since the interparticle distances could be precisely controlled at the nanoscale, we also studied the plasma coupling among the assembled plasmonic nanostructures. This modular assembly strategy represents a simple yet general and effective design principle for DNA-assembled plasmonic nanostructures.

External-Stimuli-Assisted Control over Assemblies of Plasmonic Metals.

Assembly of plasmonic nanoparticles (NPs) in suspensions is a promising approach for the control of optical and sensing properties that depend on the assembled states of plasmonic NPs. This review focuses on the controlling methods to assemble the NP via external stimuli such as pH, temperature, light, magnetic field, and electric field. External stimuli are introduced as powerful tools to assemble the NPs because of various operational factors, such as the intensity, application time, and frequency, which can be employed. In addition to a summary of recent studies on the controlling methods, a future study on the reversible control over assembled states of the plasmonic NPs via external stimuli is proposed.

Unary- or binary-plasmonic nanoparticle-assemblies formed within hollow silica particles with a surfactant-assisted method

In this study, a surfactant-assisted method was employed to successfully prepare plasmonic nanoparticle assemblies encapsulated in hollow silica particles. In this method, silica was precipitated in the presence of plasmonic nanoparticles with single nanometer sizes and an anionic surfactant, sodium oleate (NaOA). The absorption peak of the composite particles was red-shifted compared with that for the well-dispersed suspension of the plasmonic nanoparticles. The extent of red-shift was easily tuned by changing the concentration of nanoparticles incorporated in the hollow silica particles. A binary-nanoparticle assembly, comprising gold and silver nanoparticles, was also prepared. Hence, this method can be extended to the design of novel multifunctional particles based on different nanoparticle assemblies.

Environmentally responsive plasmonic nanoassemblies for biosensing.

Assemblies of plasmonic nanoparticles enable new modalities for biosensing. Engineered superstructures from metal nanoparticles can enhance the plasmon resonances and chiroptical activity of nanoscale dispersions. Such phenomena are keys to the fabrication of highly sensitive, selective and fast-responding detection platforms, making them promising candidates for clinical applications. This tutorial review summarizes and discusses recent advances in this area. The topics covered in the review include the basic strategies adopted for assembly and engineering of plasmonic nanoparticles, optical properties of the assembled nanostructures and their applications to both in vitro and in vivo detection of biological compounds. We also offer our vision of the future prospects of this field of research. Among emerging applications in this area are novel nanosensors and platforms, for food safety, environmental monitoring, health safeguarding, as well as biodefense.