Photothermal manipulation of the fringing field in gold nanorod dimers towards the apoptosis of cancerous cells

The possibility of coherent manipulation of optical and thermal energies in noble metal nanostructures has given birth to an enduring research arena coined by thermoplasmonics. Upon interaction with electromagnetic radiation, the energy of the produced hot electrons in metallic nanostructures is converted into heat and is transferred to the medium as a consequence of numerous relaxation processes. Gold nanorods have, often, been adopted as the classical anisotropic nanostructures owing to excellent shape-selective plasmonic tunability in the vis-NIR region. When a pair of metallic nanostructures are sufficiently close to each other to imbue electromagnetic interaction, there occurs evolution of collective plasmon modes, substantial enhancement of near field and strong squeezing of electromagnetic energy at the interparticle spatial region of the dimeric nanostructures. Recent advances in the 'tips and tricks' guide to assembling, even, anisotropic nanostructures in colloidal dispersions have offered the opportunity to interplay with the phenomenological plasmonic and thermal characteristics. The photothermal attributes emerging due to electromagnetic coupling of fringing fields have been explored considering parallel and perpendicular configurations of gold nanorod dimers as the prototypical systems from theoretical and experimental perspectives and their biomedical consequences have been realised in a mice model towards the photothermal apoptosis of cancerous cells.

Au nanorod assembly for sensitive SERS detection of airway inflammatory factors in sputum

In this paper, we demonstrate a surface-enhanced Raman spectroscopy (SERS) biosensor based on the self-assembly of gold nanorods (AuNRs) for the specific detection of airway inflammatory factors in diluted sputum. The AuNR surface was modified with an antibody that was able to specifically recognize an airway inflammatory factor, interleukin-5 (IL-5), so that a end-to-end self-assembly system could be obtained, resulting in an order of magnitude amplification of the Raman signal and greatly improved sensitivity. Meanwhile, the outer layer of the biosensor was coated with silicon dioxide, which improved the stability of the system and facilitated its future applications. When the detected concentration was in the range of 0.1-50 pg/mL, the SERS signal generated by the sensor showed a good linear relationship with the IL-5 concentration. Moreover, it had satisfactory performance in diluted sputum and clinical subjects with asthma, which could achieve sensitive detection of the airway inflammatory factor IL-5. Overall, the developed biosensor based on the SERS effect exhibited the advantages of rapid and sensitive detecting performance, which is suitable for monitoring airway inflammatory factors in sputum.

Composite ligand shells on gold nanoprisms - an ensemble and single particle study

The attachment of thiolated molecules onto gold surfaces is one of the most extensively used and robust ligand exchange approaches to exploit the nanooptical features of nanoscale and nanostructured plasmonic materials. In this work, the impact of thiol adsorption on the optical properties of wet-chemically synthesized gold nanoprisms is studied both at the ensemble and single particle level to investigate the build-up of more complex ligand layers. Two prototypical ligands with different lengths have been investigated ((16-mercaptohexadecyl)trimethylammonium bromide - MTAB and thiolated polyethylene glycol - mPEG-SH). From ensemble experiments it is found that composite ligand layers are obtained by the sequential addition of the two thiols, and an island-like surface accumulation of the molecules can be anticipated. The single particle experiment derived chemical interface damping and resonance energy changes further support this and show additionally that when the two thiols are used simultaneously, a higher density, intermixed layer is formed. Hence, when working with more than a single type of ligand during surface modification, sequential adsorption is preferred for the combination of accessible essential surface functionalities, whereas for high overall loading the simultaneous use of the different ligand types is favourable.

Self-assembly of Au@AgNR along M13 framework: A SERS nanocarrier for bacterial detection and killing

Self-assembled functional nanomaterials with electromagnetic hot spots are crucial and highly desirable in surface-enhanced Raman scattering (SERS). Due to its versatile biological scaffold, the M13 phage has been employed to produce novel nano-building blocks and devices. In this study, we propose a novel M13 phage-based SERS nanocarrier, that utilizes the pVIII capsid in M13 to conjugate Au@Ag core-shell nanorod (Au@AgNR) with linker carboxy-PEG-thiol (M13-Au@AgNR) and the pIII capsid to specifically target Escherichia coli (E. coli). The M13-Au@AgNR@DTTC (3,3'- diethylthiocarbocyanine iodide) SERS probe was used to detect E. coli in a concentration range of 6 to 6 × 105 cfu/mL, achieving a limit of detection (LOD) of 0.5 cfu/mL. The proposed SERS platform was also tested in real samples, showing good recoveries (92%-114.3%) and a relative standard deviation (RSD) of 1.2%-4.7%. Furthermore, the system demonstrated high antibacterial efficiency against E. coli, approximately 90%, as measured by the standard plate-count method. The investigation provides an effective strategy for in vitro bacteria detection and inactivation.

Angle-resolved plasmonic photocapacitance of gold nanorod dimers

The assembly of nanostructures with plausible statistical orientations has provided the opportunity to correlate physical observables to develop a diverse range of niche applications. The dimeric configurations of gold nanorods have been chosen as atypical model systems to correlate optoelectronic with mechanical properties at a number of combinations of angular orientations. Metals are considered as conductors in electronics and reflectors in optics - therefore, metallic particles at the nanoscale exhibit unique optoelectronic characteristics that enable the design of materials to meet the demand of the modern world. Gold nanorods have often been adopted as prototypical anisotropic nanostructures owing to their excellent shape-selective plasmonic tunability in the vis-NIR region. When a pair of metallic nanostructures is sufficiently close to exhibit electromagnetic interaction, the evolution of collective plasmon modes, substantial enhancement of the near-field and strong squeezing of the electromagnetic energy at the interparticle spatial region of the dimeric nanostructures occur. The localised surface plasmon resonance energies of the nanostructured dimers strongly depend on the geometry as well as the relative configurations of the neighbouring particle pairs. Recent advances in the 'tips and tricks' guide have even made it possible to assemble anisotropic nanostructures in a colloidal dispersion. The optoelectronic characteristics of gold nanorod homodimers at different mutual orientations with statistical variation of the angle between 0 and 90° at particular interparticle distances have been elucidated from both theoretical and experimental perspectives. It has been observed that the optoelectronic properties are governed by mechanical aspects of the nanorods at different angular orientations of the dimers. Therefore, we have approached the design of an optoelectronic landscape through the correlation of the plasmonics and photocapacitance through the optical torque of gold nanorod dimers.

Coiled-coil peptide-based assembly of a plasmonic core-satellite polymer-metal nanocomposite as an efficient photothermal agent for drug delivery applications

Polymer-metal nanocomposites have widespread applications in biomedical fields such as imaging, catalysis, and drug delivery. These particles are characterized by combined organic and inorganic properties. Specifically, photothermal nanocomposites incorporating polymeric and plasmonic nanoparticles (NPs) have been designed for both triggered drug release and as imaging agents. However, the usual design of nanocomposites confers characteristic issues, among which are the decrease of optical properties and resulting low photothermal efficiency, as well as interactions with loaded drugs. Herein, we report the design of a core-satellite polymer-metal nanocomposite assembled by coiled-coil peptides and its superior photothermal efficiency compared to electrostatic-driven nanocomposites which is the standard design. We also found that the orientation of gold nanorods on the surface of polymeric NPs is of importance in the final photothermal efficiency and could be exploited for various applications. Our findings provide an alternative to current wrapping and electrostatic assembly of nanocomposites with the help of coiled-coil peptides and an improvement of the control over core-satellite assemblies with plasmonic NPs. It paves the way to highly versatile assemblies due to the nature of coiled-coil peptides to be easily modified and sensitive to pH or temperature.

Janus bimetallic nanorod clusters-poly(aniline) nanocomposites with temperature-responsiveness for Raman scattering-based biosensing

Herein, Janus bimetallic nanorod clusters-poly(aniline) nanocomposites (JRCPCs) with gold nanorod clusters (GNRCs) in side-by-side (SBS) or end-to-end (ETE) configuration are synthesized, and applied to surface-enhanced Raman scattering (SERS)-based biosensing of carcinoembryonic antigen (CEA). Taking advantage of their geometrical and chemical anisotropy, GNRCs in both SBS and ETE configurations are prepared by addition of negatively charged citrate anions and poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAAc-b-PNIPAM), respectively, to electrostatically interact with cationic cetyltrimethylammonium bromide surfactant on the side of the gold nanorods (GNRs). Subsequently, the JRCPCs are prepared by unidirectional growth of polyaniline and additional growth of Ag onto these GNRCs. JRCPCs with GNRCs in either the SBS or the ETE configuration show strong enhancement of electromagnetic field at both GNR aggregates and GNRC core-Ag shell gaps of bimetallic nanorod cluster components. In particular, because temperature-responsive PAAc-b-PNIPAM of JRCPCs is embedded at GNR junctions, interparticle gaps generated in GNRCs in ETE configuration are controlled via temperature-triggered hydration-dehydration of the PAAc-b-PNIPAM chains such that optical properties are largely changed. With distinct surface functionalities from JRCPCs, SERS-based quantitative analysis of CEA is achieved using JRCPCs as SERS nanoprobes. This work presents the great potential of advanced Janus nanocomposites for SERS-based biosensing applications.

Fabrication and SERS properties of complex and organized nanoparticle plasmonic clusters stable in solution

SERS activity can be increased by the formation of hot spots at the interparticle junctions of plasmonic nanoparticles in very close proximity, dramatically improving the enhancement factors in comparison with isolated nanoparticles. Controlling the number and geometrical architecture of hot spots, while endowing the clusters with colloidal stability, results in feasible optical sensors, able to provide quantitative SERS responses. Here, we review the approaches proposed to date to produce colloidal stable clusters, focusing on the control of the coordination number of nanoparticle assemblies and interparticle gaps. Clusters of spherical nanoparticles of the same size and rods of the same size are described to subsequently outline core-satellite constructs of nanoparticles of different sizes. Besides, purification processes for nanoparticle clusters are revised to provide efficient production in high yields.

Carboxymethyl Cellulose-Coated Tacrolimus Nonspherical Microcrystals for Improved Therapeutic Efficacy of Dry Eye

Dry eye (DE) is a highly prevalent ocular surface disease which affects the quality of life and results in low working efficiency. Frequent instillation is required due to low bioavailability of conventional eye drops. The aim of this study is to develop a novel formulation of tacrolimus (TAC), routinely prescribed for DE, by combination of the microcrystal technology and layer-by-layer assembly. First, nonspherical tacrolimus microcrystals (TAC MCs) are synthesized by antisolvent-induced precipitation. These TAC MCs are modified by alternate deposition of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) subsequently to obtain CMC-coated TAC MCs (TAC-(PAH/CMC)₃ ). The resultant formulations are evaluated in vivo in a mouse DE model induced by an intelligently controlled environmental system. Compared with commercially available TAC eye drops and the TAC MCs counterpart, TAC-(PAH/CMC)₃ exhibits superior therapeutic performance with reduced drug instillation frequency, which is attributed to the nonspherical geometry of MCs, the lubricant, mucoadhesive effect of CMC, and the anti-inflammatory function of TAC. Therefore, TAC-(PAH/CMC)₃ represents a better option for the management of DE.

Femtosecond laser fabrication of silver nanostructures on glass for surface enhanced Raman spectroscopy

We report on an optimized fabrication protocol for obtaining silver nanoparticles on fused silica substrates via laser photoreduction of a silver salt solution. We find that multiple scans of the laser over the surface leads to a more uniform coverage of densely packed silver nanoparticles of approximately 50 nm diameter on the fused silica surface. Our substrates yield Raman enhancement factors of the order of 10¹¹ of the signal detected from crystal violet. We use a theoretical model based on scanning electron microscope (SEM) images of our substrates to explain our experimental results. We also demonstrate how our technique can be extended to embedding silver nanoparticles in buried microfluidic channels in glass. The in situ laser inscription of silver nanoparticles on a laser machined, sub-surface, microfluidic channel wall within bulk glass paves the way for developing 3D, monolithic, fused silica surface enhance Raman spectroscopy (SERS) microfluidic sensing devices.

Polymer induced one-step interfacial self-assembly method for the fabrication of flexible, robust and free-standing SERS substrates for rapid on-site detection of pesticide residues

We have demonstrated a one-step approach for the fabrication of flexible, robust, reproducible and free-standing SERS substrates (AuNPs/polyvinyl chloride (PVC) film) through a polymer induced interfacial self-assembly method. In this method, the polymer (PVC) plays dual roles, that is, inducing the interfacial self-assembly of nanoparticles and fixing the assembled nanostructure in the PVC matrix. As the assembled nanoparticles are orderly half-embedded in the PVC film, the AuNPs/PVC film exhibits outstanding reproducibility and stability. In addition, the film could be easily regenerated by rinsing with NaBH4 solution. As a proof of concept, the film was directly wrapped on an apple surface for in situ detection of pesticide residues, and a detection limit of 10 ng cm-2 thiram was achieved. Furthermore, rapid on-site and in situ detection of multi-pesticide residues has been proved to be feasible with the aid of a portable Raman spectrometer. Due to its simple preparation, good reliability, outstanding stability and reusability, the AuNPs/PVC film has great potential in practical applications.

Interactions and Attachment Pathways between Functionalized Gold Nanorods

Nanoparticle (NP) self-assembly has been recognized as an important technological process for forming ordered nanostructures. However, the detailed dynamics of the assembly processes remain poorly understood. Using in situ liquid cell transmission electron microscopy, we describe the assembly modes of gold (Au) nanorods (NRs) in solution mediated by hydrogen bonding between NR-bound cysteamine linker molecules. Our observations reveal that by tuning the linker concentration, two different NR assembly modes can be achieved. These assembly modes proceed via the (1) end-to-end and (2) side-to-side attachment of NRs at low and high linker concentrations in solution, respectively. In addition, our time-resolved observations reveal that the side-to-side NR assemblies can occur through two different pathways: (i) prealigned attachment, where two Au NRs prealign to be parallel prior to assembly, and (ii) postattachment alignment, where two Au NRs first undergo end-to-end attachment and pivot around the attachment point to form the side-to-side assembly. We attributed the observed assembly modes to the distribution of linkers on the NR surfaces and the electrostatic interactions between the NRs. The intermediate steps in the assembly reported here reveal how the shape and surface functionalities of NPs drive their self-assembly, which is important for the rational design of hierarchical nanostructures.

Sulfate-Mediated End-to-End Assembly of Gold Nanorods

There is interest in the controlled aggregation of gold nanorods (GNRs) for the production of extended nanoassemblies. Prior studies have relied upon chemical modification of the GNR surface to achieve a desired final aggregate structure. Herein we illustrate that control of electrolyte composition can facilitate end-to-end assembly of cetyltrimethylammonium-bromide-coated (CTAB) GNRs. By adjusting either the sulfate anion concentration or the exposure time it is possible to connect GNRs in chain-like assemblies. In contrast, end-to-end assembly was not observed in control experiments using monovalent chloride salts. We attribute the end-to-end assembly to the localized association of sulfate with exposed quaternary ammonium head groups of CTAB at the nanorod tip. To quantify the assembly kinetics, visible-near-infrared extinction spectra were collected over a predetermined time period, and the colloidal behavior of the GNR suspensions was interpreted using plasmon band analysis. Transmission electron microscopy and atomic force microscopy results support the conclusions reached via plasmon band analysis, and the colloidal behavior is consistent with Derjaguin-Landau-Verwey-Overbeek theory.

Anisotropic noble metal nanoparticles: Synthesis, surface functionalization and applications in biosensing, bioimaging, drug delivery and theranostics

Anisotropic nanoparticles have fascinated scientists and engineering communities for over a century because of their unique physical and chemical properties. In recent years, continuous advances in design and fabrication of anisotropic nanoparticles have opened new avenues for application in various areas of biology, chemistry and physics. Anisotropic nanoparticles have the plasmon absorption in the visible as well as near-infrared (NIR) region, which enables them to be used for crucial applications such as biological imaging, medical diagnostics and therapy ("theranostics"). Here, we describe the progress in anisotropic nanoparticles achieved since the millennium in the area of preparation including various shapes and modification of the particle surface, and in areas of application by providing examples of applications in biosensing, bio-imaging, drug delivery and theranostics. Furthermore, we also explain various mechanisms involved in cellular uptake of anisotropic nanoparticles, and conclude with our opinion on various obstacles that limit their applications in biomedical field.

STATEMENT OF SIGNIFICANCE

Anisotropy at the molecular level has always fascinated scientists and engineering communities for over a century, however, the research on novel methods through which shape and size of nanoparticles can be precisely controlled has opened new avenues for anisotropic nanoparticles in various areas of biology, chemistry and physics. In this manuscript, we describe progress achieved since the millennium in the areas of preparation of various shapes of anisotropic nanoparticles, investigate various methods involved in modifying the surface of these NPs, and provide examples of applications in biosensing and bio-imaging, drug delivery and theranostics. We also present mechanisms involved in cellular uptake of nanoparticles, describe different methods of preparation of anisotropic nanoparticles including biomimetic and photochemical synthesis, and conclude with our opinion on various obstacles that limit their applications in biomedical field.

Assembling patchy nanorods with spheres: limitations imposed by colloidal interactions

For gold nanorods the intrinsic shape-anisotropy offers the prospect of anisotropic assembly, provided that their region-selective surface modification can be realized. Here we developed nanorods with a patchy surface chemistry, featuring positively charged molecules in the tip region and polymer molecules at the sides by careful control of molecule concentrations during ligand exchange. When these patchy nanorods are assembled with small negatively charged spherical particles, electric double layer interaction can direct the assembly of two nanospheres at the opposite ends of the nanorods. The PEG chains promote the selectivity of the procedure. As the size of the nanospheres increases, they start to shift towards the side of the nanorod due to increased van der Waals interaction. When the relative size of the nanospheres is even larger, only a single nanosphere is assembled, but instead of the tip region, they are attached to the side of the nanorods. The apparent cross-over of the region-selectivity can be interpreted in terms of colloidal interactions, i.e. the second spherical particle is excluded due to nanosphere-nanosphere electric double layer repulsion, while the large vdW attraction results in a side positioning of the single adsorbed spherical particle. The results underline the importance of absolute values of the different interaction strengths and length scales in the programmed assembly of patchy nanoscale building blocks.

Composite SERS-based satellites navigated by optical tweezers for single cell analysis

Herein, we have designed composite SERS-active micro-satellites, which exhibit a dual role: (i) effective probes for determining cellular composition and (ii) optically movable and easily detectable markers. The satellites were synthesized by the layer-by-layer assisted decoration of silica microparticles with metal (gold or silver) nanoparticles and astralen in order to ensure satellite SERS-based microenvironment probing and satellite recognition, respectively. A combination of optical tweezers and Raman spectroscopy can be used to navigate the satellites to a certain cellular compartment and probe the intracellular composition following cellular uptake. In the future, this developed approach may serve as a tool for single cell analysis with nanometer precision due to the multilayer surface design, focusing on both extracellular and intracellular studies.

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.

The conquest of middle-earth: combining top-down and bottom-up nanofabrication for constructing nanoparticle based devices

The development of top-down nanofabrication techniques has opened many possibilities for the design and realization of complex devices based on single molecule phenomena such as e.g. single molecule electronic devices. These impressive achievements have been complemented by the fundamental understanding of self-assembly phenomena, leading to bottom-up strategies to obtain hybrid nanomaterials that can be used as building blocks for more complex structures. In this feature article we highlight some relevant published work as well as present new experimental results, illustrating the versatility of self-assembly methods combined with top-down fabrication techniques for solving relevant challenges in modern nanotechnology. We present recent developments on the use of hierarchical self-assembly methods to bridge the gap between sub-nanometer and micrometer length scales. By the use of non-covalent self-assembly methods, we show that we are able to control the positioning of nanoparticles on surfaces, and to address the deterministic assembly of nano-devices with potential applications in plasmonic sensing and single-molecule electronics experiments.

Competitive reaction pathway for site-selective conjugation of Raman dyes to hotspots on gold nanorods for greatly enhanced SERS performance

Common methods to prepare SERS (surface-enhanced Raman scattering) probes rely on random conjugation of Raman dyes onto metal nanostructures, but most of the Raman dyes are not located at Raman-intense electromagnetic hotspots thus not contributing to SERS enhancement substantially. Herein, a competitive reaction between transverse gold overgrowth and dye conjugation is described to achieve site selective conjugation of Raman dyes to the hotspots (ends) on gold nanorods (GNRs). The preferential overgrowth on the nanorod side surface creates a barrier to prevent the Raman dyes from binding to the side surface except the ends of the GNRs, where the highest SERS enhancement factors are expected. The SERS enhancement observed from this special structure is dozens of times larger than that from conjugates synthesized by conventional methods. This simple and powerful strategy to prepare SERS probes can be extended to different anisotropic metal nanostructures with electromagnetic hotspots and has immense potential in in-depth SERS-based biological imaging and single-molecule detection.

Wet chemical synthesis of soluble gold nanogaps

A central challenge in molecular electronics is to create electrode pairs separated by only a few nanometers that can accommodate a single molecule of interest to be optically or electrically characterized while residing in the gap. Current techniques for nanogap fabrication are largely based on top-down approaches and often rely on subsequent deposition of molecules into the nanogap. In such an approach, the molecule may bridge the gap differently with each experiment due to variations at the metal-molecule interface. Conversely, chemists can readily synthesize gold nanorods (AuNRs) in aqueous solution. Through controlled end-to-end assembly of the AuNRs into dimers or chains, facilitated via target molecules, they can be used as electrical contacts. In this way, the preparation of AuNR-molecule-AuNR junctions by wet chemical methods may afford a large number of identical devices with little variation in the interface between molecule and electrode (AuNR). In this Account, we highlight recent progress in using chemically synthesized AuNRs as building blocks for molecular electronic applications. We outline the general synthesis and properties of AuNRs and describe the aqueous growth of dimeric AuNR structures from an insulating molecule linked to AuNR precursors (gold seeds). Conjugated, electronically active molecules are typically not soluble under the conditions required for the bottom-up growth of AuNRs. Therefore, we present a strategy that utilizes host-guest chemistry in order to make such π-systems compatible with the AuNR growth procedure. In order to electrically characterize the AuNR-molecule-AuNR constructs, we must transfer them onto a substrate and contact external electrodes. We discuss the implications of using electron-beam lithography for making this contact. In addition, we introduce a novel fabrication approach in which we can grow AuNR nanogap electrodes in situ on prepatterned substrates, thus circumventing post-processing steps that potentially damage the nanogap environment. Due to the inherent optical properties of AuNRs, electromagnetic field enhancement in the nanogaps lets us spectroscopically characterize the molecules via surface-enhanced Raman scattering. We discuss the incorporation of oligopeptides functionalized with acetylene units having uniquely identifiable vibrational modes. This acetylene moiety allows chemical reactions to be performed in the gaps via click chemistry, and the oligopeptide linking platform opens for integration of larger biological components.

Understanding the microscopic origin of gold nanoparticle anisotropic growth from molecular dynamics simulations

We use molecular dynamics simulations in order to understand the microscopic origin of the asymmetric growth mechanism in gold nanorods. We provide the first atomistic model of different surfaces on gold nanoparticles in a growing electrolyte solution, and we describe the interaction of the metal with the surfactants, namely, cetyltrimethylammonium bromide (CTAB) and the ions. An innovative aspect is the inclusion of the role of the surfactants, which are explicitly modeled. We find that on all the investigated surfaces, namely, (111), (110), and (100), CTAB forms a layer of distorted cylindrical micelles where channels among micelles provide direct ion access to the surface. In particular, we show how AuCl2(-) ions, which are found in the growth solution, can freely diffuse from the bulk solution to the gold surface. We also find that the (111) surface exhibits a higher CTAB packing density and a higher electrostatic potential. Both elements would favor the growth of gold nanoparticles along the (111) direction. These findings are in agreement with the growth mechanisms proposed by the experimental groups of Murphy and Mulvaney.

End-to-end alignment of nanorods in thin films

A simple approach to obtain end-to-end assemblies of nanorods over macroscopic distances in thin films is described. Nanorods with aspect ratio of 8-12 can be aligned parallel to the surface in an end-to-end fashion by imposing geometric confinement via block copolymer-based supramolecular assemblies. Successful control over the orientation and location of nanorods requires a balance of particle-particle interactions and entropy associated with geometric confinement from the supramolecular framework, as well as consideration of the kinetics of assembly.

Gold nanorods and their plasmonic properties

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

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.

Effect of metal-liquid interface composition on the adsorption of a cyanine dye onto gold nanoparticles

Synthesis of asymmetric nanoparticles, such as gold nanorods, with tunable optical properties providing metal structures with improved SERS performance is playing a critical role in expanding the use of SERS to imaging and sensing applications. However, the synthetic methods usually require surfactants or polymers as shape-directing agents. These chemicals normally remain firmly bound to the metal after the synthesis, preventing the direct adsorption of a large number of potential analytes and often hampering the chemical functionalization of the surface unless extended, and critical for the nanoparticle stability, postremoval steps were performed. For this reason, it is of great importance for the full exploitation of these nanostructures to gain a deeper insight into the dependence of the analyte-metal interaction to the metal-liquid interface composition. In this article, we investigated in detail the role played by each component of the gold nanorod (GNR) interface in the adsorption of indocyanine green (ICG) as a probe molecule. Citrate-reduced gold nanospheres were used as a model substrate since the negative citrate anions adsorbed onto the metal surface can be easily displaced by those chemicals usually involved in the GNR synthesis, allowing the GNR-like interface composition to be progressively rebuilt and modified at will on the citrate-capped nanoparticles. The obtained results provide a meticulous description of the role played by each individual component of the metal-liquid interface on the ICG interaction with the metal, illustrating how apparently minor experimental changes can dramatically modify the affinity and optical properties of the ICG probe adsorbed onto the nanoparticle.

Rational design of oriented assembly of gold nanospheres with nanorods by biotin-streptavidin connectors

Through the different functionalities on Au nanosphere (AuNSs) and Au nanorod (AuNRs) surfaces, we successfully control AuNSs attachment onto either the end or side surface of anisotropic AuNRs via bio-recognition, and then consciously construct side-by-side or end-to-end assembly nanostructures. This study provides a feasible approach to organize nanoparticles with different morphologies into controllable assembly geometries, which can potentially benefit the construction of future nanodevices.

Application of well-defined indium tin oxide nanorods as Raman active platforms

We determine the surface enhanced Raman (SER) capability of indium tin oxide nanorods (ITO NRs) whose physical, chemical, and optical properties are precisely and uniformly controlled during synthesis. We demonstrate that the Raman intensities observed from varying concentrations of the pure and mixed molecules of rhodamine 6G and 4',6-diamidino-2-phenylindole are much larger on ITO NRs relative to those measured on commercially available ITO-coated glass or Si. Our efforts signify the first attempt to assess the SER capability of precisely controlled metal oxide NRs and will be highly beneficial to many basic and applied Raman applications requiring exceptional detection sensitivity.

Self-assembly of gold nanorods into symmetric superlattices directed by OH-terminated hexa(ethylene glycol) alkanethiol

The self-assembly of anisotropic gold nanorods (GNRs) into ordered phases remains a challenge. Herein, we demonstrated the fabrication of symmetric circular- or semicircular-like self-assembled superlattices composed of multilayers of standing GNRs by fine-tuning the repulsive interactions among GNRs. The repulsive force is tailored from electrostatic interaction to steric force by replacing the surface coating of cetyltrimethylammonium bromide (CTAB) (ζ potential of 20-50 mV) with an OH-terminated hexa(ethylene glycol) alkanethiol (here termed as EG(6)OH, ζ potential of -10 mV). The assembly mechanism is discussed via theoretical analyses of the major interactions, and an effective balance between the repulsive steric and attractive depletion interactions is the main driving force for the self-assembly. The real-time observations of solution assembly (UV-vis-NIR absorption spectroscopy) supports the mechanism that we suggested. The superlattices obtained here not only enrich the categories of the self-assembled structures but more importantly deepen the insight of the self-assembly process and pave the way for various potential applications.