Nanomanipulation and controlled self-assembly of metal nanoparticles and nanocrystals for plasmonics

Revealing the Cluster-Cloud and Its Role in Nanocrystallization

Elucidating the early stages of crystallization from supersaturated solutions is of critical importance, but remains a great challenge. An in situ liquid cell transmission electron microscopy study reveals an intermediate state of condensed atomic clusters during Pd and Au crystallizations, which is named a "cluster-cloud." It is found that nucleation is initiated by the collapse of a cluster-cloud, first forming a nanoparticle. The subsequent particle maturation proceeds via multiple out-and-in relaxations of the cluster-cloud to improve crystallinity: from a poorly crystallized phase, the particle evolves into a well-defined single-crystal phase. Both experimental investigations and atomistic simulations suggest that the cluster-cloud-mediated nanocrystallization involves an order-disorder phase separation and reconstruction, which is energetically favored compared to local rearrangements within the particle. This finding grants new insights into nanocrystallization mechanisms, and provides useful information for the improvement of synthesis pathways of nanocrystals.

Energy-Converting Nanomedicine

Serious side effects to surrounding normal tissues and unsatisfactory therapeutic efficacy hamper the further clinic applications of conventional cancer-therapeutic strategies, such as chemotherapy and surgery. The fast development of nanotechnology provides unprecedented superiorities for cancer therapeutics. Externally activatable therapeutic modalities mediated by nanomaterials, relying on highly effective energy transformation to release therapeutic elements/effects (cytotoxic reactive oxygen species, thermal effect, photoelectric effect, Compton effect, cavitation effect, mechanical effect or chemotherapeutic drug) for cancer therapies, categorized and termed as "energy-converting nanomedicine," have arouse considerable concern due to their noninvasiveness, desirable tissue-penetration depth, and accurate modulation of therapeutic dose. This review summarizes the recent advances in the engineering of intelligent functional nanotherapeutics for energy-converting nanomedicine, including photo-based, radiation-based, ultrasound-based, magnetic field-based, microwave-based, electric field-based, and radiofrequency-based nanomedicines, which are enabled by external stimuli (light, radiation, ultrasound, magnetic field, microwave, electric field, and radiofrequency). Furthermore, biosafety issues of energy-converting nanomedicine related to future clinical translation are also addressed. Finally, the potential challenges and prospects of energy-converting nanomedicine for future clinical translation are discussed.

Controlling the Self-Assembly of Biomolecules into Functional Nanomaterials through Internal Interactions and External Stimulations: A Review

Biomolecular self-assembly provides a facile way to synthesize functional nanomaterials. Due to the unique structure and functions of biomolecules, the created biological nanomaterials via biomolecular self-assembly have a wide range of applications, from materials science to biomedical engineering, tissue engineering, nanotechnology, and analytical science. In this review, we present recent advances in the synthesis of biological nanomaterials by controlling the biomolecular self-assembly from adjusting internal interactions and external stimulations. The self-assembly mechanisms of biomolecules (DNA, protein, peptide, virus, enzyme, metabolites, lipid, cholesterol, and others) related to various internal interactions, including hydrogen bonds, electrostatic interactions, hydrophobic interactions, π⁻π stacking, DNA base pairing, and ligand⁻receptor binding, are discussed by analyzing some recent studies. In addition, some strategies for promoting biomolecular self-assembly via external stimulations, such as adjusting the solution conditions (pH, temperature, ionic strength), adding organics, nanoparticles, or enzymes, and applying external light stimulation to the self-assembly systems, are demonstrated. We hope that this overview will be helpful for readers to understand the self-assembly mechanisms and strategies of biomolecules and to design and develop new biological nanostructures or nanomaterials for desired applications.

Empowering Electroless Plating to Produce Silver Nanoparticle Films for DNA Biosensing Using Localized Surface Plasmon Resonance Spectroscopy

To facilitate the implementation of biosensors based on the localized surface plasmon resonance (LSPR) of metal nanostructures, there is a great need for cost-efficient, flexible, and tunable methods for producing plasmonic coatings. Due to its simplicity and excellent conformity, electroless plating (EP) is well suited for this task. However, it is traditionally optimized to produce continuous metal films, which cannot be employed in LSPR sensors. Here, we outline the development of an EP strategy for depositing island-like silver nanoparticle (NP) films on glass with distinct LSPR bands. The fully wet-chemical process only employs standard chemicals and proceeds within minutes at room temperature. The key step for producing spread-out NP films is an accelerated ripening of the silver seed layer in diluted hydrochloric acid, which reduces the nucleation density during plating. The reaction kinetics and mechanisms are investigated with scanning (transmission) electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy, with the latter enabling a convenient live monitoring of the deposition, allowing its termination at a stage of desired optical properties. The sensing capabilities of chemically deposited NP films as LSPR transducers are exemplified in DNA biosensing. To this end, a sensing interface is prepared using layer-by-layer (LbL) buildup of polyelectrolytes (PE), followed by adsorption and covalent immobilization of ssDNA. The obtained LSPR transducers demonstrate robustness and selectivity in sensing experiments with binding complementary and unrelated DNA strands.

Optical resonance coupling in compositionally different nanocube–nanosphere heterodimers

Plasmonic nanoparticle dimers with interparticle gap distances (d) in the nanometer scale are able to produce huge electromagnetic field enhancements in the gap region, useful for novel optical applications.

Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications such as plasmon mode excitation, hot carrier generation, and construction of ultracompact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate-Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.

Enhanced Plasmonic Particle Trapping Using a Hybrid Structure of Nanoparticles and Nanorods

Plasmon-enhanced particle trapping was demonstrated using a hybrid structure of nanoparticles and nanorods. In order to intensify localized surface plasmon resonance (LSPR), gold nanoparticles (AuNPs) were deposited on zinc oxide nanorods (ZnONRs). The synergistic effect caused by the hybrid structure was identified experimentally. Numerical analysis revealed that the LSPR-induced photophysical processes such as plasmonic heating and near-field enhancement were improved by the existence of ZnONRs. The role of the ZnONR in enhancing the particle-trapping velocity was explored by examining the scattered electric field, Poynting vector, and temperature gradient over the nanostructures calculated from the simulation. It was found that polystyrene microparticles and Escherichia coli cells were successfully trapped by using the ZnONR/AuNP plasmonic structure. A relatively high dielectric constant and nanorod geometry of ZnO enabled the hybrid substrate to enhance trapping performance, compared with a control case fabricated using only gold nanoislands.

Self-Assembly of Polymer-Coated Plasmonic Nanocrystals: From Synthetic Approaches to Practical Applications

Self-assembly of plasmonic nanocrystals (PNCs) and polymers provides access to a variety of functionalized metallic-polymer building blocks and higher-order hybrid plasmonic assemblies, and thus is of considerable fundamental and practical interest. The hybrid assemblies often not only inherit individual characteristics of polymers and PNCs but also exhibit distinct photophysical and catalytic properties compared to that of a single PNC building block. The tailorable plasmonic coupling between PNCs within assemblies enables the precise control over localized surface plasmon resonance, which subsequently affords a series of light-driven or photo-activated applications, such as surface-enhanced Raman scattering detection, photoacoustic imaging, photothermal therapy, and photodynamic therapy. In this review, the synthetic strategies of a library of PNC-polymer hybrid building blocks and corresponding assemblies are summarized along with the mechanisms of polymer-assisted self-assembly of PNCs and the concepts for bridging the intrinsic properties of PNC-polymer assemblies to widespread practical applications.

Chemically Engineered Au-Ag Plasmonic Nanostructures to Realize Large Area and Flexible Metamaterials

We developed a simple and systematic method to fabricate optically tunable and thermally and chemically stable Au-Ag nanocrystal-based plasmonic metamaterials. An Ag nanocrystal-based metamaterial with desirable optical properties was fabricated via nanoimprinting and ligand-exchange process. Its optical properties were controlled by selectively substituting Ag atoms with Au atoms through a spontaneous galvanic replacement reaction. The developed Au-Ag-based metamaterials provide excellent tunable plasmonic properties required for various applications in the visible and near-infrared regions by controlling the Au-Ag composition according to the conditions of the galvanic displacement. Furthermore, their thermal and chemical stabilities significantly improved because of the protective Au thin layer on the surface. Using this developed process, chemically and thermally stable and flexible plasmonic metamaterials were successfully fabricated on a flexible polyester terephthalate substrate.

Controllable synthesis of AgNWs@PDA@AgNPs core-shell nanocobs based on a mussel-inspired polydopamine for highly sensitive SERS detection

In this work, a series of AgNWs@PDA@AgNPs core-shell nanocobs based on a mussel-inspired polydopamine (PDA) were controllably synthesized and achieve highly sensitive SERS detection. Owing to the existence of abundant catechol and amine functional groups, PDA molecules could assemble a functional layer on the surface of silver nanowires (AgNWs) and exhibit exceptional adhesion performance. More importantly, silver nanoparticles (AgNPs) with controlled coverage and size were achieved on the surface of the PDA layer by in situ reduction of silver ions into AgNPs with catechol functional groups, forming AgNWs@PDA@AgNPs core-shell nanocobs. By regulating synergistical effect between the AgNWs and AgNPs, the AgNWs@PDA@AgNPs core-shell nanocobs demonstrated a highly sensitive and stable SERS response to Rhodamine 6G (R6G) molecules, and a low limit of detection down to 10-12 M. Furthermore, the AgNWs@PDA@AgNPs core-shell nanocobs showed an excellent reproducibility and superior stability as a SERS substrate to achieve trace detection. This strategy would have great potential to fabricate multifarious SERS-active substrates that make it possible to detect single molecules and singles cell in chemical and biological fields.

Optical properties of bimetallic compositional heterodimers

Many important applications of nanometer-sized metal objects arise from the light-induced interactions between their component structures. Here, we demonstrate through state-of-the-art quantum mechanical simulations, the optical response of bimetallic heterodimers (two closely adjacent nanoparticles) composed of Al and Na nanoparticles. We calculate the optical response using time-dependent density functional theory. We found that Al-Na bimetallic heterodimers show rich optical features, strongly depending on the size heterogeneity and interparticle gap distances. In particular, we observe remarkable optical field enhancements and creation of new low-energy absorption peaks with respect to the single Al and Na nanoparticles. We believe that our results may influence the design of future nanoparticle-based optical nanoantenna.

Direct manipulation of metallic nanosheets by shear force microscopy

Micro/nanomanipulation is a rapidly growing technology and holds promising applications in various fields, including photonic/electronic devices, chemical/biosensors etc. In this work, we present that shear force microscopy (ShFM) can be exploited to manipulate metallic nanosheets besides imaging. The manipulation is realized via controlling the shear force sensor probe position and shear force magnitude based on our homemade ShFM system under an optical microscopy for in situ observation. The main feature of the ShFM system is usage of a piezoelectric bimorph sensor, which has the ability of self-excitation and detection. Moreover, the shear force magnitude as a function of the spring constant of the sensor and setpoint is obtained, which indicates that operation modes can be switched between imaging and manipulation through designing the spring constant before experiment and changing the setpoint during manipulation process, respectively. We believe that this alternative manipulation technique could be used to assemble other nanostructures with different shapes, sizes and compositions for new properties and wider applications.

Surface-Enhanced Raman Scattering Spectroscopy for Label-Free Analysis of P. aeruginosa Quorum Sensing

Bacterial quorum sensing systems regulate the production of an ample variety of bioactive extracellular compounds that are involved in interspecies microbial interactions and in the interplay between the microbes and their hosts. The development of new approaches for enabling chemical detection of such cellular activities is important in order to gain new insight into their function and biological significance. In recent years, surface-enhanced Raman scattering (SERS) spectroscopy has emerged as an ultrasensitive analytical tool employing rationally designed plasmonic nanostructured substrates. This review highlights recent advances of SERS spectroscopy for label-free detection and imaging of quorum sensing-regulated processes in the human opportunistic pathogen Pseudomonas aeruginosa. We also briefly describe the challenges and limitations of the technique and conclude with a summary of future prospects for the field.

DNA Origami Route for Nanophotonics

The specificity and simplicity of the Watson-Crick base pair interactions make DNA one of the most versatile construction materials for creating nanoscale structures and devices. Among several DNA-based approaches, the DNA origami technique excels in programmable self-assembly of complex, arbitrary shaped structures with dimensions of hundreds of nanometers. Importantly, DNA origami can be used as templates for assembly of functional nanoscale components into three-dimensional structures with high precision and controlled stoichiometry. This is often beyond the reach of other nanofabrication techniques. In this Perspective, we highlight the capability of the DNA origami technique for realization of novel nanophotonic systems. First, we introduce the basic principles of designing and fabrication of DNA origami structures. Subsequently, we review recent advances of the DNA origami applications in nanoplasmonics, single-molecule and super-resolution fluorescent imaging, as well as hybrid photonic systems. We conclude by outlining the future prospects of the DNA origami technique for advanced nanophotonic systems with tailored functionalities.

A density functional study on the reactivity enhancement induced by gold in IrAu nanoalloys

IrAu nanoalloys have been proven to have remarkable reactivity for several reactions. In this work, mixed IrAu nanoalloys of 8, 27, 48 and 64 total atoms were studied in different atomic compositions (Ir m Au n ) using Density Functional Theory (DFT). A notable segregation tendency is observed, where Ir atoms are located in the inner part and Au atoms in the outermost region of the nanostructure. We found that IrAu nanoalloys present a distinctive synergistic effect with respect to reactivity. In addition, the projected density of electronic states (PDOS) energies were analyzed by examining the d-band shift to estimate the reactivity of various IrAu nanoalloys. Furthermore, the adsorption energies for the CO molecule in the domains of the Ir-Au interface were evaluated. In this sense, the addition of Au atoms to Ir clusters increases the reactivity of Ir by generating unoccupied orbitals near the Fermi level as indicated by the PDOS study.

Insight on agglomerates of gold nanoparticles in glass based on surface plasmon resonance spectrum: study by multi-spheres T-matrix method

The formation of a localized surface plasmon resonance (SPR) spectrum of randomly distributed gold nanoparticles in the surface layer of silicate float glass, generated and implanted by UV ArF-excimer laser irradiation of a thin gold layer sputter-coated on the glass surface, was studied by the T-matrix method, which enables particle agglomeration to be taken into account. The experimental technique used is promising for the production of submicron patterns of plasmonic nanoparticles (given by laser masks or gratings) without damage to the glass surface. Analysis of the applicability of the multi-spheres T-matrix (MSTM) method to the studied material was performed through calculations of SPR characteristics for differently arranged and structured gold nanoparticles (gold nanoparticles in solution, particles pairs, and core-shell silver-gold nanoparticles) for which either experimental data or results of the modeling by other methods are available. For the studied gold nanoparticles in glass, it was revealed that the theoretical description of their SPR spectrum requires consideration of the plasmon coupling between particles, which can be done effectively by MSTM calculations. The obtained statistical distributions over particle sizes and over interparticle distances demonstrated the saturation behavior with respect to the number of particles under consideration, which enabled us to determine the effective aggregate of particles, sufficient to form the SPR spectrum. The suggested technique for the fitting of an experimental SPR spectrum of gold nanoparticles in glass by varying the geometrical parameters of the particles aggregate in the recurring calculations of spectrum by MSTM method enabled us to determine statistical characteristics of the aggregate: the average distance between particles, average size, and size distribution of the particles. The fitting strategy of the SPR spectrum presented here can be applied to nanoparticles of any nature and in various substances, and, in principle, can be extended for particles with non-spherical shapes, like ellipsoids, rod-like and other T-matrix-solvable shapes.

Highly Oriented Nanowire Thin Films with Anisotropic Optical Properties Driven by the Simultaneous Influence of Surface Templating and Shear Forces

The functional properties of nanoparticle thin films depend strongly on the arrangement of the nanoparticles within the material. In particular, anisotropic optoelectronic properties can be achieved through the aligned assembly of 1D nanomaterials such as silver nanowires (AgNWs). However, the control of the hierarchical organization of these nanoscale building blocks across multiple length scales and over large areas is still a challenge. Here, we show that the oriented deposition of AgNWs using grazing incidence spraying of the nano-object suspensions on a substrate comprising parallel surface wrinkles readily produces highly oriented monolayer thin films on macroscopic areas (>5 × 5 mm²). The use of textured substrates enhances the degree of ordering as compared to flat ones and increases the area over which AgNWs are oriented. The resulting microscopic linear arrangement of AgNWs evaluated by scanning electron microscopy (SEM) reflects in a pronounced macroscopic optical anisotropy measured by conventional polarized UV-vis-NIR spectroscopy. The enhanced ordering obtained when spraying is done in the same direction as the wrinkles makes this approach more robust against small rotational offsets during preparation. On the contrary, the templating effect of the wrinkle topography can even dominate the shear-driven alignment when spraying is performed perpendicular to the wrinkles: the concomitant but opposing influence of topographic confinement (alignment along the wrinkles) and of spray-induced shear forces (orientation along the spraying direction) lead to films in which the predominant orientation of AgNWs gradually changes from one direction to its perpendicular one over the same substrate in a single processing step. This demonstrates that exploiting the subtle balance between shear forces and substrate-nanowire interactions mediated by wrinkles offers a new way to control the self-assembly of nanoparticles into more complex patterns.

Magnetic lipid nanocapsules (MLNCs): self-assembled lipid-based nanoconstruct for non-invasive theranostic applications

A novel magnetic nanostructures (MNS) stabilized lipid nanoconstruct is reported that shows superior structural stability and theranostic functionality than conventional lipid based nanocarriers.

Symmetry control of nanorod superlattice driven by a governing force

Nanoparticle self-assembly promises scalable fabrication of composite materials with unique properties, but symmetry control of assembled structures remains a challenge. By introducing a governing force in the assembly process, we develop a strategy to control assembly symmetry. As a demonstration, we realize the tetragonal superlattice of octagonal gold nanorods, breaking through the only hexagonal symmetry of the superlattice so far. Surprisingly, such sparse tetragonal superstructure exhibits much higher thermostability than its close-packed hexagonal counterpart. Multiscale modeling reveals that the governing force arises from hierarchical molecular and colloidal interactions. This force dominates the interactions involved in the assembly process and determines the superlattice symmetry, leading to the tetragonal superlattice that becomes energetically favorable over its hexagonal counterpart. This strategy might be instructive for designing assembly of various nanoparticles and may open up a new avenue for realizing diverse assembly structures with pre-engineered properties.

Controlled growth and shape-directed self-assembly of gold nanoarrows

Self-assembly of colloidal nanocrystals into complex superstructures offers notable opportunities to create functional devices and artificial materials with unusual properties. Anisotropic nanoparticles with nonspherical shapes, such as rods, plates, polyhedra, and multipods, enable the formation of a diverse range of ordered superlattices. However, the structural complexity and tunability of nanocrystal superlattices are restricted by the limited geometries of the anisotropic nanoparticles available for supercrystal self-assembly. We show that uniform gold nanoarrows (GNAs) consisting of two pyramidal heads connected by a four-wing shaft are readily synthesized through controlled overgrowth of gold nanorods. The distinct concave geometry endows the GNAs with unique packing and interlocking ability and allows for the shape-directed assembly of sophisticated two-dimensional (2D) and 3D supercrystals with unprecedented architectures. Net-like 2D supercrystals are assembled through the face-to-face contact of the GNAs lying on the pyramidal edges, whereas zipper-like and weave-like 2D supercrystals are constructed by the interlocked GNAs lying on the pyramidal {111} facets. Furthermore, multilayer packing of net-like and weave-like 2D assemblies of GNAs leads to non-close-packed 3D supercrystals with varied packing efficiencies and pore structures. Electromagnetic simulation of the diverse nanoarrow supercrystals exhibits exotic patterns of nanoscale electromagnetic field confinement. This study may open new avenues toward tunable self-assembly of nanoparticle superstructures with increased complexity and unusual functionality and may advance the design of novel plasmonic metamaterials for nanophotonics and reconfigurable architectured materials.

Seed-mediated synthesis and SERS performance of graphene oxide-wrapped Ag nanomushroom

A facile seed-mediated method was developed to modify core-shell Ag nanosphere@PSPAA with another Ag layer for achieving an enhancement of their surface-enhanced Raman scattering (SERS) activity. Interestingly, an Ag bridge in the polymer shell connected the inner and outer Ag layers, resulting in a mushroom-like nanostructure. The outer Ag grew around the polymer shell to form the cap of the nanomushrooms (NMs) with the extension of the reaction time. The epitaxial growth mechanism of this novel nanostructure was investigated by tuning the type of seed from nanosphere to nanocube and nanorod. With the growth of the outer Ag cap, the SERS intensity of these Ag NMs increased significantly together with the red-shifting and broadening of their typical localized surface plasmon resonance band. Such a phenomenon can be attributed to the formation of SERS hotspots between the inner and outer Ag layers. The Ag NMs were then wrapped with a graphene oxide (GO) shell via static interactions. The GO-wrapped Ag NMs exhibited a further better SERS performance in terms of sensitivity, homogeneity and stability compared with non-wrapped ones, indicating that the heterostructure could be potentially useful for SERS-based immunoassay.

Optical reconfiguration and polarization control in semi-continuous gold films close to the percolation threshold

Controlling and confining light by exciting plasmons in resonant metallic nanostructures is an essential aspect of many new emerging optical technologies. Here we explore the possibility of controllably reconfiguring the intrinsic optical properties of semi-continuous gold films, by inducing permanent morphological changes with a femtosecond (fs)-pulsed laser above a critical power. Optical transmission spectroscopy measurements show a correlation between the spectra of the morphologically modified films and the wavelength, polarization, and the intensity of the laser used for alteration. In order to understand the modifications induced by the laser writing, we explore the near-field properties of these films with electron energy-loss spectroscopy (EELS). A comparison between our experimental data and full-wave simulations on the exact film morphologies hints toward a restructuring of the intrinsic plasmonic eigenmodes of the metallic film by photothermal effects. We explain these optical changes with a simple model and demonstrate experimentally that laser writing can be used to controllably modify the optical properties of these semi-continuous films. These metal films offer an easy-to-fabricate and scalable platform for technological applications such as molecular sensing and ultra-dense data storage.

Fabricating chiroptical starfruit-like Au nanoparticles via interface modulation of chiral thiols

The surface/interface matters as the size of materials enters the nanoscale. Control of surface/interface, therefore, plays an important role in creating novel nanostructures with unusual properties and in obtaining devices with high performance. Herein, we demonstrate unique interface regulation in fabricating nanostructures with strong plasmonic circular dichroism (PCD). With chiral cysteine (Cys) as surface-modulating molecules, starfruit-like Au nanoparticles (NPs) with high PCD responses are obtained via Au overgrowth on Au nanorods (AuNRs). Pre-incubation of the AuNRs with Cys is vital in achieving strong and reproducible PCD responses. Instead of contributing to PCD signals, the pre-adsorbed Cys molecules are found to play a major role in manipulating the Au growth mode and thus the formation of hotspots within the shell. Strong PCD signal mainly comes from the entrapped Cys molecules within the hotspots and is enhanced via local field effect. The distinct roles of the same ligands at different surfaces/interfaces are elucidated. Furthermore, our findings contribute to the strategy of utilizing interface modulation to fabricate complex nanostructures with novel properties.

Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics

Molecular plasmonics uses and explores molecule-plasmon interactions on metal nanostructures for spectroscopic, nanophotonic, and nanoelectronic devices. This review focuses on tailored surfaces/assemblies for molecular plasmonics and describes active molecular plasmonic devices in which functional molecules and polymers change their structural, electrical, and/or optical properties in response to external stimuli and that can dynamically tune the plasmonic properties. We also explore an emerging research field combining molecular plasmonics and molecular electronics.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode-electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm. The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. The EFs for the best performing Au NP monolayer are between 10⁶ and 10⁸ and give quantitative signal response when analyte concentration is changed.

Polarization-Dependent Optical Response in Anisotropic Nanoparticle-DNA Superlattices

DNA-programmable assembly has been used to prepare superlattices composed of octahedral and spherical nanoparticles, respectively. These superlattices have the same body-centered cubic lattice symmetry and macroscopic rhombic dodecahedron crystal habit but tunable lattice parameters by virtue of the DNA length, allowing one to study and determine the effect of nanoscale structure and lattice parameter on the light-matter interactions in the superlattices. Backscattering measurements and finite-difference time-domain simulations have been used to characterize these two classes of superlattices. Superlattices composed of octahedral nanoparticles exhibit polarization-dependent backscattering but via a trend that is opposite to that observed in the polarization dependence for analogous superlattices composed of spherical nanoparticles. Electrodynamics simulations show that this polarization dependence is mainly due to the anisotropy of the nanoparticles and is observed only if the octahedral nanoparticles are well-aligned within the superlattices. Both plasmonic and photonic modes are identified in such structures, both of which can be tuned by controlling the size and shape of the nanoparticle building blocks, the lattice parameters, and the overall size of the three-dimensional superlattices (without changing habit).

Dynamic Nanoparticle Assemblies for Biomedical Applications

Designed synthesis and assembly of nanoparticles assisted by their surface ligands can create "smart" materials with programmed responses to external stimuli for biomedical applications. These assemblies can be designed to respond either exogenously (for example, to magnetic field, temperature, ultrasound, light, or electric pulses) or endogenously (to pH, enzymatic activity, or redox gradients) and play an increasingly important role in a diverse range of biomedical applications, such as biosensors, drug delivery, molecular imaging, and novel theranostic systems. In this review, the recent advances and challenges in the development of stimuli-responsive nanoparticle assemblies are summarized; in particular, the application-driven design of surface ligands for stimuli-responsive nanoparticle assemblies that are capable of sensing small changes in the disease microenvironment, which induce the related changes in their physico-chemical properties, is described. Finally, possible future research directions and problems that have to be addressed are briefly discussed.

Core–shell gold nanocubes for point mutation detection based on plasmon-enhanced fluorescence

A core–shell gold nanocube has been prepared for point mutation detection based on the PEF process.

Silver nanoparticles functionalized with a fluorescent cyclic RGD peptide: a versatile integrin targeting platform for cells and bacteria

A green fluorescent RGD peptide–silver nanoparticle platform to target integrin receptors in cells and bacterial studies.