Surfactant Layers on Gold Nanorods

ConspectusGold nanorods (Au NRs) are an exceptionally promising tool in nanotechnology due to three key factors: (i) their strong interaction with electromagnetic radiation, stemming from their plasmonic nature, (ii) the ease with which the resonance frequency of their longitudinal plasmon mode can be tuned from the visible to the near-infrared region of the electromagnetic spectrum based on their aspect ratio, and (iii) their simple and cost-effective preparation through seed-mediated chemical growth. In this synthetic method, surfactants play a critical role in controlling the size, shape, and colloidal stability of Au NRs. For example, surfactants can stabilize specific crystallographic facets during the formation of Au NRs, leading to the formation of NRs with specific morphologies.The process of surfactant adsorption onto the NR surface may result in various assemblies of surfactant molecules, such as spherical micelles, elongated micelles, or bilayers. Again, the assembly mode is critical toward determining the further availability of the Au NR surface to the surrounding medium. Despite its importance and a great deal of research effort, the interaction between Au NPs and surfactants remains insufficiently understood, because the assembly process is influenced by numerous factors, including the chemical nature of the surfactant, the surface morphology of Au NPs, and solution parameters. Therefore, gaining a more comprehensive understanding of these interactions is essential to unlock the full potential of the seed-mediated growth method and the applications of plasmonic NPs. A plethora of characterization techniques have been applied to reach such an understanding, but many open questions remain.In this Account, we review the current knowledge on the interactions between surfactants and Au NRs. We briefly introduce the state-of-the-art methods for synthesizing Au NRs and highlight the crucial role of cationic surfactants during this process. The self-assembly and organization of surfactants on the Au NR surface is then discussed to better understand their role in seed-mediated growth. Subsequently, we provide examples and elucidate how chemical additives can be used to modulate micellar assemblies, in turn allowing for a finer control over the growth of Au NRs, including chiral NRs. Next, we review the main experimental characterization and computational modeling techniques that have been applied to shed light on the arrangement of surfactants on Au NRs and summarize the advantages and disadvantages for each technique. The Account ends with a "Conclusions and Outlook" section, outlining promising future research directions and developments that we consider are still required, mostly related to the application of electron microscopy in liquid and in 3D. Finally, we remark on the potential of exploiting machine learning techniques to predict synthetic routes for NPs with predefined structures and properties.

Using adsorption kinetics to assemble vertically aligned nanorods at liquid interfaces for metamaterial applications

Vertically aligned monolayers of metallic nanorods have a wide range of applications as metamaterials or in surface enhanced Raman spectroscopy. However the fabrication of such structures using current top-down methods or through assembly on solid substrates is either difficult to scale up or have limited possibilities for further modification after assembly. The aim of this paper is to use the adsorption kinetics of cylindrical nanorods at a liquid interface as a novel route for assembling vertically aligned nanorod arrays that overcomes these problems. Specifically, we model the adsorption kinetics of the particle using Langevin dynamics coupled to a finite element model, accurately capturing the deformation of the liquid meniscus and particle friction coefficients during adsorption. We find that the final orientation of the cylindrical nanorod is determined by their initial attack angle when they contact the liquid interface, and that the range of attack angles leading to the end-on state is maximised when nanorods approach the liquid interface from the bulk phase that is more energetically favorable. In the absence of an external field, only a fraction of adsorbing nanorods end up in the end-on state (≲40% even for nanorods approaching from the energetically favourable phase). However, by pre-aligning the metallic nanorods with experimentally achievable electric fields, this fraction can be effectively increased to 100%. Using nanophotonic calculations, we also demonstrate that the resultant vertically aligned structures can be used as epsilon-near-zero and hyperbolic metamaterials. Our kinetic assembly method is applicable to nanorods with a range of diameters, aspect ratios and materials and therefore represents a versatile, low-cost and powerful platform for fabricating vertically aligned nanorods for metamaterial applications.

Morphological and Orientational Controls of Self-Assembly of Gold Nanorods Directed by Evaporative Microflows

Evaporative self-assembly of noble metal nanoparticles into ordered structures holds great promise for fabricating optical and plasmonic devices by virtue of its low cost, high efficiency, and ease of operation. However, poor control of Marangoni flows is one of the challenges accounting for realizing a well-defined assembly. Herein, based on the theoretical analysis of the influence of evaporative intensity on the assembly, two simple but reliable flow-field-confinement platforms are designed to control the evaporative microflows and to work concurrently with depletion forces to enable the regulated self-assembly of gold nanorods. Orientationally ordered assemblies are realized by the designed strong unidirectional microflow in a capillary, and a device-scale assembly of monolayer membrane is obtained by the created weak convection in homemade glass cells. Morphologically diversified superstructure assemblies, such as spherulite-like, boundary-twisted, chiral spiral assemblies, and merging membranes with a π-twisted domain wall, are obtained due to the spontaneous symmetry breaking or in the presence of defects, such as surface steps and screw dislocations. Optical anisotropy and polarization-dependent behaviors of these assemblies are further revealed, implying the potential applications in plasmonic coupling devices and optoelectronic components. An understanding of the entropy-driven assembly behaviors and control of evaporative microflows to guide the self-assembly of gold nanorods provides insights into the general bottom-up approach that is helpful for constructing complex yet robust nanosuperstructures.

Synthesis of Uniform Gold Nanorods with Large Width to Realize Ultralow SERS Detection

In this work, we successfully demonstrate high-yield synthesis of high-quality gold nanorods (Au NRs) with width ranging from 6.5 nm to 175 nm by introducing heptanol molecules as secondary templating agents during cetyltrimethylammonium bromide-templated, seeded growth method. The results show that an appropriate concentration of heptanol molecules not only alter the micellization behavior of CTAB in water, but also help silver ions impact the symmetry-breaking efficiency of additional Au-NP seeds in addition to enhancing the utilization of gold precursors. Moreover, the generality and versatility of the present strategy for synthesis of Au NRs with flexible controlled dimensions are further demonstrated by successful synthesis of Au NRs with the assistance of other fatty alcohols with properly long alkyl chains. Furthermore, when arrays of vertically aligned Au NRs with large width (AVA-Au_120×90 NRs) are used as SERS substrates, they can achieve the ultralow limit of detection for crystal violet (10^-16  M) with good reliability and reproducibility, and the rapid detection and identification of residual harmful substances.

Oriented Gold Nanorod Arrays: Self-Assembly and Optoelectronic Applications

Self-assembly of anisotropic plasmonic nanomaterials into ordered superstructures has become popular in nanoscience because of their unique anisotropic optical and electronic properties. Gold nanorods (GNRs) are a well-defined functional building block for fabrication of these superstructures. They possess important anisotropic plasmonic characteristics that result from strong local electric field and are responsive to visible and near-IR light. There are recent examples of assembling the GNRs into ordered arrays or superstructures through processes such as solvent evaporation and interfacial assembly. In this Minireview, recent progress in the development of the self-assembled GNR arrays is described, with focus on the formation of oriented GNR arrays on substrates. Key driving forces are discussed, and different strategies and self-assembly processes of forming oriented GNR arrays are presented. The applications of the oriented GNR arrays in optoelectronic devices are also overviewed, especially surface enhanced Raman scattering (SERS).

Self-assembled vertically aligned Au nanorod arrays for surface-enhanced Raman scattering (SERS) detection of Cannabinol

Self-assembled multi-layered vertically aligned gold nanorod (AuNR) arrays have been fabricated by a simple preparation process that requires a balance between the particle concentration and the ionic strength of the solvent. An experimentally determined critical AuNR concentration of 2.0nM and 50mM NaCl produces well-ordered vertically aligned hexagonally close-packed AuNR arrays. We demonstrate surface treatment via UV Ozone cleaning of such samples to allow introduction of analyte molecules (benzenethiol and cannabinol) for effective surface enhanced Raman scattering detection. This is the first demonstration of the SERS analysis of cannabinol. This approach demonstrates a cost-effective, high-yield and simple fabrication route to SERS sensors with application in the screening for the cannabinoids.

Self-assembly of segmented anisotropic particles: tuning compositional anisotropy to form vertical or horizontal arrays

Columnar arrays of anisotropic nano- and microparticles, in which the long axes of the particles are oriented perpendicular to the substrate, are of interest for photovoltaics and other applications. Array assembly typically requires applied electric or magnetic fields and/or controlled drying, which are challenging over large areas. Here, we describe a scalable approach to self-assemble multicomponent nanowires into columnar arrays. Self-assembly of partially etched nanowires (PENs) occurred spontaneously during sedimentation from suspension, without drying or applied fields. PENs, which have segments that are either gold or "empty" (solvent-filled) surrounded by a silica shell, were produced from striped metal nanowires by first coating with silica and then removing sacrificial segments by acid etching. Electrostatic repulsion between the particles was necessary for array assembly; however, details of PEN surface chemistry were relatively unimportant. The aspect ratio and relative center of mass (COM) of the PENs were important for determining whether the PEN long axes were vertically or horizontally aligned with respect to the underlying substrate. Arrays with predominantly vertically aligned particles were achieved for PENs with a large offset in COM relative to the geometric center, while other types of PENs formed horizontal arrays. Assemblies were formed over >10 cm(2) areas, with over 60% of particles standing. We assessed array uniformity and reproducibility by imaging many positions within each sample and performing multiple assemblies of differently segmented PENs. This work demonstrates the versatility of gravity-driven PEN array assembly and provides a framework for designing other anisotropic particle systems that self-assemble into columnar arrays.

Gold nanorods dispersed in homopolymer films: optical properties controlled by self-assembly and percolation of nanorods

In this paper, polymer nanocomposite films containing gold nanorods (AuNRs) and poly(2-vinyl pyridine) (P2VP) have been investigated for their structure-optical property relationship. Using transmission electron microscopy (TEM), the assembly of AuNRs (7.9 nm × 28.4 nm) grafted with a P2VP brush in P2VP films is examined as a function of the AuNR volume fraction Ø(AuNRs) and film thickness h. For h ∼ 40 nm, AuNRs are confined to align parallel to the film and uniformly dispersed at low Ø(AuNRs). Upon increasing Ø(AuNRs), nanorods form discrete aggregates containing mainly side-by-side arrays due to depletion-attraction forces. For Ø(AuNRs) = 2.7%, AuNRs assemble into a 2D network where the discrete aggregates are connected by end-to-end linked nanorods. As Ø(AuNRs) further increases, the polymer-rich regions of the network fill in with nanorods and rod overlap is observed. Monte Carlo simulations capture the experimentally observed morphologies. The effect of film thickness is investigated at Ø(AuNRs) = 2.7%, where thicker films (40 and 70 nm) show a dense array of percolated nanorods and thinner films (20 nm) exhibit mainly isolated nanorods. Using Rutherford backscattering spectrometry (RBS), the AuNRs are observed to segregate near the substrate during spin-casting. Optically, the longitudinal surface plasmon resonance (LSPR) peaks are correlated with the local orientation of the AuNRs, where side-by-side and end-to-end alignments induce blue and red shifts, respectively. The LSPR undergoes a red shift up to 51 nm as Ø(AuNRs) increases from 1.6 to 2.7%. These studies indicate that the optical properties of polymer nanocomposite films containing gold nanorods can be fine-tuned by changing Ø(AuNRs) and h. These results are broadly applicable and provide guidelines for dispersing other functional nanoparticles, such as quantum dots and carbon nanotubes.

Vertical arrays of anisotropic particles by gravity-driven self-assembly

Anisotropic particles assemble to spontaneously form columnar arrays. Hybrid nanotube/nanowire particles (silica nanotubes partially filled with metallic cores) deposit with their denser metallic ends towards the surface, orienting them vertically. Up to 84% are observed to be standing over a 0.64 cm(2) area within 15 min. Standing percentage is found to be dependent on particle surface concentration.

Assembly of colloidal semiconductor nanorods in solution by depletion attraction

Arranging anisotropic nanoparticles into ordered assemblies remains a challenging quest requiring innovative and ingenuous approaches. The variety of interactions present in colloidal solutions of nonspherical inorganic nanocrystals can be exploited for this purpose. By tuning depletion attraction forces between hydrophobic colloidal nanorods of semiconductors, dispersed in an organic solvent, these could be assembled into 2D monolayers of close-packed hexagonally ordered arrays directly in solution. Once formed, these layers could be fished onto a substrate, and sheets of vertically standing rods were fabricated, with no additional external bias applied. Alternatively, the assemblies could be isolated and redispersed in polar solvents, yielding suspensions of micrometer-sized sheets which could be chemically treated directly in solution. Depletion attraction forces were also effective in the shape-selective separation of nanorods from binary mixtures of rods and spheres. The reported procedures have the potential to enable powerful and cost-effective fabrication approaches to materials and devices based on self-organized anisotropic nanoparticles.