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).     

Long-range alignment of gold nanorods in electrospun polymer nano/microfibers

In this study, a scalable fabrication technique for controlling and maintaining the nanoscale orientation of gold nanorods (GNRs) with long-range macroscale order has been achieved through electrospinning. The volume fraction of GNRs with an average aspect ratio of 3.1 is varied from 0.006 to 0.045 in aqueous poly(ethylene oxide) solutions to generate electrospun fibers possessing different GNR concentrations and measuring 40-3000 nm in diameter. The GNRs within these fibers exhibit excellent alignment with their longitudinal axis parallel to the fiber axis n. According to microscopy analysis, the average deviant angle between the GNR axis and n increases modestly from 3.8 to 13.3° as the fiber diameter increases. Complementary electron diffraction measurements confirm preferred orientation of the {100} GNR planes. Optical absorbance spectroscopy measurements reveal that the longitudinal surface plasmon resonance bands of the aligned GNRs depend on the polarization angle and that maximum extinction occurs when the polarization is parallel to n.     

PEGylated gold nanorod separation based on aspect ratio: characterization by asymmetric-flow field flow fractionation with UV-Vis detection

The development of highly efficient asymmetric-flow field flow fractionation (A4F) methodology for biocompatible PEGylated gold nanorods (GNR) without the need for surfactants in the mobile phase is presented. We report on the potential of A4F for rapid separation by evaluating the efficiency of functionalized surface coverage in terms of fractionation, retention time (t _R ) shifts, and population analysis. By optimizing the fractionation conditions, we observed that the mechanism of separation for PEGylated GNRs by A4F is the same as that for CTAB stabilized GNRs (i.e., according to their AR) which confirms that the elution mechanism is not dependent on the surface charge of the analytes and/or the membrane. In addition, we demonstrated that A4F can distinguish different surface coverage populations of PEGylated GNRs. The data established that a change in M_w of the functional group and/or surface orientation can be detected and fractionated by A4F. The findings in this study provide the foundation for a complete separation and physicochemical analysis of GNRs and their surface coatings, which can provide accurate and reproducible characterization critical to advancing biomedical research.

Bottom‐Up On‐Surface Synthesis of Two‐Dimensional Graphene Nanoribbon Networks and Their Thermoelectric Properties

Graphene, the one‐atom‐thick two‐dimensional (2D) carbon material, has attracted tremendous interest in both academia and industry due to its outstanding electrical, mechanical, and thermal properties. For electronic applications, the challenging task is to make it as a semiconductor. The bottom‐up synthesis of semiconducting one‐dimensional (1D) nanometer‐wide graphene strips, namely, graphene nanoribbons (GNRs), has attracted much attention owing to its promising electronic, optical, and magnetic properties. In this regard, we report the fabrication of cove‐type 2D GNR networks (GNNs) via the interconnection of 1D self‐assembled GNRs on the surface of Au(111). The cove‐type 2D GNRs networks (GNNs) were fabricated from the GNR, 5‐CGNR‐1‐1 , synthesized using the precursor of DBSP . Annealing of high‐density self‐assembled GNRs on the surface of Au(111) through two‐zone chemical vapour deposition (2Z CVD) successfully generated a 2D interconnected structure with high yield via the fusion and ladder coupling reactions of GNR chains. In order to validate the later fusion reaction, we have also synthesized the GNR, 7‐AGNR‐1‐1 , using the precursor of DBBA . The GNNs, which consist of hybridized metallic‐like and semiconducting GNRs, are a new class of carbon‐based materials. Further, we applied this material for thermoelectric (TE) applications and found a very low cross‐plane thermal conductivity of 0.11 Wm^?1 K^?1, which is one of the lowest value among the carbon‐based materials as well as inorganic semiconductors, while maintaining the cross‐plane electrical conductivity of 188 S m^?1.     

Supramolecular Nanostructures of Structurally Defined Graphene Nanoribbons in the Aqueous Phase

Structurally well‐defined graphene nanoribbons (GNRs) have attracted great interest because of their unique optical, electronic, and magnetic properties. However, strong π–π interactions within GNRs result in poor liquid‐phase dispersibility, which impedes further investigation of these materials in numerous research areas, including supramolecular self‐assembly. Structurally defined GNRs were synthesized by a bottom‐up strategy, involving grafting of hydrophilic poly(ethylene oxide) (PEO) chains of different lengths (GNR‐PEO). PEO grafting of 42–51 % percent produces GNR‐PEO materials with excellent dispersibility in water with high GNR concentrations of up to 0.5 mg mL^?1. The “rod–coil” brush‐like architecture of GNR‐PEO resulted in 1D hierarchical self‐assembly behavior in the aqueous phase, leading to the formation of ultralong nanobelts, or spring‐like helices, with tunable mean diameters and pitches. In aqueous dispersions the superstructures absorbed in the near‐infrared range, which enabled highly efficient conversion of photon energy into thermal energy.     

Synthesis of Polybenzoquinolines as Precursors for Nitrogen‐Doped Graphene Nanoribbons

Graphene nanoribbons (GNRs) represent promising materials for the next generation of nanoscale electronics. However, despite substantial progress towards the bottom‐up synthesis of chemically and structurally well‐defined all‐carbon GNRs, strategies for the preparation of their nitrogen‐doped analogs remain at a nascent stage. This scarce literature precedent is surprising given the established use of substitutional doping for tuning the properties of electronic materials. Herein, we report the synthesis of a previously unknown class of polybenzoquinoline‐based materials, which have potential as GNR precursors. Our scalable and facile approach employs few synthetic steps, inexpensive commercial starting materials, and straightforward reaction conditions. Moreover, due to the importance of quinoline derivatives for a variety of applications, the reported findings may hold implications across a diverse range of chemical and physical disciplines.     

Atomically Precise Nanocluster Assemblies Encapsulating Plasmonic Gold Nanorods

The self‐assembled structures of atomically precise, ligand‐protected noble metal nanoclusters leading to encapsulation of plasmonic gold nanorods (GNRs) is presented. Unlike highly sophisticated DNA nanotechnology, this strategically simple hydrogen bonding‐directed self‐assembly of nanoclusters leads to octahedral nanocrystals encapsulating GNRs. Specifically, the p‐mercaptobenzoic acid (pMBA)‐protected atomically precise silver nanocluster, Na₄[Ag₄₄(pMBA)₃₀], and pMBA‐functionalized GNRs were used. High‐resolution transmission and scanning transmission electron tomographic reconstructions suggest that the geometry of the GNR surface is responsible for directing the assembly of silver nanoclusters via H‐bonding, leading to octahedral symmetry. The use of water‐dispersible gold nanoclusters, Au_≈250(pMBA)_n and Au₁₀₂(pMBA)₄₄, also formed layered shells encapsulating GNRs. Such cluster assemblies on colloidal particles are a new category of precision hybrids with diverse possibilities.     

Imaging translational and rotational diffusion of single anisotropic nanoparticles with planar illumination microscopy

Here we demonstrated a simple yet powerful method, planar illumination microscopy, to directly track the rotational and translational diffusion dynamics of individual anisotropic nanoparticles in solution and living cells. By illuminating gold nanorods (GNRs) with two orthogonal sheets of light and resolving the polarized scattering signal with a birefringent crystal, we readily achieved three-dimensional angular resolving capability for single GNRs in noisy surroundings. The rotational dynamics of individual GNRs dispersed in glycerol/water mixtures with different chemical modification were tracked, and the measured rotational diffusion coefficient was well fitted to a previously reported theoretical model (Torre, J. G. d. l.; Martinez, M. C. L. Macromolecules 1987, 20, 661-666; Tirado, M. M.; Torre, J. G. d. l. J. Chem. Phys. 1980, 73, 1986-1993). In addition, the translational and rotational movements of individual GNRs transported by kinesin motor protein on microtubules inside living cells were directly imaged. Compared to its motion in free solution, a GNR attached to motor-protein did not rotate significantly while moving forward. Our method can be further generalized to allow determination of three-dimensional orientation of single dipoles using many different illumination modes.     

Efficient Bottom‐Up Preparation of Graphene Nanoribbons by Mild Suzuki–Miyaura Polymerization of Simple Triaryl Monomers

Herein an efficient bottom‐up solution‐phase synthesis of N=9 armchair graphene nanoribbons (GNRs) is described. Catalyzed by Pd(PtBu₃)₂, Suzuki–Miyaura polymerization of a simple and readily available triaryl monomer provides a novel GNR precursor with a high molecular weight and excellent solubility. Upon cyclodehydrogenation, the resulting GNR exhibits semiconducting properties with an approximately 1.1 eV band gap (LUMO: ?3.52 eV; HOMO: ?4.66 eV) as characterized by UV/Vis‐NIR spectroscopy and cyclic voltammetry.     

Heterostructures through Divergent Edge Reconstruction in Nitrogen‐Doped Segmented Graphene Nanoribbons

Atomically precise engineering of defined segments within individual graphene nanoribbons (GNRs) represents a key enabling technology for the development of advanced functional device architectures. Here, the bottom‐up synthesis of chevron GNRs decorated with reactive functional groups derived from 9‐methyl‐9H‐carbazole is reported. Scanning tunneling and non‐contact atomic force microscopy reveal that a thermal activation of GNRs induces the rearrangement of the electron‐rich carbazole into an electron‐deficient phenanthridine. The selective chemical edge‐reconstruction of carbazole‐substituted chevron GNRs represents a practical strategy for the controlled fabrication of spatially defined GNR heterostructures from a single molecular precursor.     

PAA-derived gold nanorods for cellular targeting and photothermal therapy

A single-step LbL procedure to functionalize CTAB-capped GNRs via electrostatic self-assembly is reported. This approach allows for consistent biomolecule/GNR coupling using standard carboxyl-amine conjugation chemistry. The focus is on cancer-targeting biomolecule/GNR conjugates and selective photothermal destruction of cancer cells by GNR-mediated hyperthermia and NIR light. GNRs were conjugated to a single-chain antibody selective for colorectal carcinoma cells and used as probes to demonstrate photothermal therapy. Selective targeting and GNR uptake in antigen-expressing SW 1222 cells were observed using fluorescence microscopy. Selective photothermal therapy is demonstrated using SW 1222 cells, where >62% cell death was observed after cells are treated with targeted A33scFv-GNRs.     

On‐Surface Synthesis and Characterization of Triply Fused Porphyrin–Graphene Nanoribbon Hybrids

On‐surface synthesis offers a versatile approach to prepare novel carbon‐based nanostructures that cannot be obtained by conventional solution chemistry. Graphene nanoribbons (GNRs) have potential for a variety of applications. A key issue for their application in molecular electronics is in the fine‐tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non‐benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) is a highly appealing strategy. Herein we present the selective on‐surface synthesis of a Por–GNR hybrid, which consists of two Pors connected by a short GNR segment. The atomically precise structure of the Por–GNR hybrid has been characterized by bond‐resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc‐AFM). The electronic properties have been investigated by scanning tunneling spectroscopy (STS), in combination with DFT calculations, which reveals a low electronic gap of 0.4 eV.     

Recent Progress and Challenges in Graphene Nanoribbon Synthesis

Graphene, the thinnest two‐dimensional material in nature, has abundant distinctive properties, such as ultrahigh carrier mobility, superior thermal conductivity, very high surface‐to‐volume ratio, anomalous quantum Hall effect, and so on. Laterally confined, thin, and long strips of graphene, namely, graphene nanoribbons (GNRs), can open the bandgap in the semimetal and give it the potential to replace silicon in future electronics. Great efforts are devoted to achieving high‐quality GNRs with narrow widths and smooth edges. This minireview reports the latest progress in experimental and theoretical studies on GNR synthesis. Different methods of GNR synthesis—unzipping of carbon nanotubes (CNTs), cutting of graphene, and the direct synthesis of GNRs—are discussed, and their advantages and disadvantages are compared in detail. Current challenges and the prospects in this rapidly developing field are also addressed.     

Luminescent Gold Nanorods To Enhance the Near-Infrared Emission of a Photosensitizer for Targeted Cancer Imaging and Dual Therapy: Experimental and Theoretical Approach

Strong plasmon absorption in the near-infrared (NIR) region renders gold nanorods (GNRs) amenable for biomedical applications, particularly for photothermal therapy. However, these nanostructures have not been explored for their imaging potential because of their weak emission profile. In this study, the weak fluorescence emission of GNRs is tuned to match that of the absorption of a photosensitizer (PS) molecule, and energy transfer from the GNR to PS enhances the emission profile of the GNR–PS combination. GNR complexes generally quench the fluorescence emission of nearby chromophores. However, herein, the complex retains or rather enhances the fluorescence through competition in energy transfer. Excitation-dependent energy transfer has been explained experimentally and theoretically by using DFT calculations, the CIE chromaticity diagram, and power spectrum. The final GNR–PS complex modified for tumor specificity serves as an excellent organ-specific theranostic probe for bioimaging and dual therapy both in vitro and in vivo. Principal component analysis designates photodynamic therapy a better candidate than that of photothermal therapy for long-term efficacy in vivo.     

Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection

The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehensively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge involved in the fractionation of positively charged nanoparticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical parameter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios.     

Enhanced Photochromism of Diarylethene Induced by Excitation of Localized Surface Plasmon Resonance on Regular Arrays of Gold Nanoparticles

Localized surface plasmon resonance (LSPR) excitation on the photochromic reaction of a diarylethene derivative (DE) was studied by surface enhanced Raman scattering (SERS). UV and visible light irradiations transform reversibly DE between open-form (OF) and closed-form (CF) isomers, respectively. A mixture of PMMA and DE (either OF or CF isomer) was spin-coated onto gold nanorods (GNRs) arrays, designed by electron beam lithography, with two localized surface plasmon resonances (LSPR) at distinct wavelengths, due to their anisotropy. The photochromic reaction rates from CF to OF isomers, under LSPR excitation, were monitored from SERS spectral changes under different polarizations, on the same GNR substrate to compare the effect of LSPR field strength. It appears that the photoisomerization rate was faster when LSPR was excited with the polarization parallel to the GNR long axis. The present results highlight a potential genuine mechanism, from near field LSPR excitation, involved in the photochromic enhancement of diarylethene photochromes.     

A Supramolecular‐Based Dual‐Wavelength Phototherapeutic Agent with Broad‐Spectrum Antimicrobial Activity Against Drug‐Resistant Bacteria

With the ever‐increasing threat posed by the multi‐drug resistance of bacteria, the development of non‐antibiotic agents for the broad‐spectrum eradication of clinically prevalent superbugs remains a global challenge. Here, we demonstrate the simple supramolecular self‐assembly of structurally defined graphene nanoribbons (GNRs) with a cationic porphyrin (Pp4N) to afford unique one‐dimensional wire‐like GNR superstructures coated with Pp4N nanoparticles. This Pp4N/GNR nanocomposite displays excellent dual‐modal properties with significant reactive‐oxygen‐species (ROS) production (in photodynamic therapy) and temperature elevation (in photothermal therapy) upon light irradiation at 660 and 808 nm, respectively. This combined approach proved synergistic, providing an impressive antimicrobial effect that led to the complete annihilation of a wide spectrum of Gram‐positive, Gram‐negative, and drug‐resistant bacteria both in vitro and in vivo. The study also unveils the promise of GNRs as a new platform to develop dual‐modal antimicrobial agents that are able to overcome antibiotic resistance.     

Wrinkle engineering: a new approach to massive graphene nanoribbon arrays

Wrinkles are often formed on CVD-graphene in an uncontrollable way. By designing the surface morphology of growth substrate together with a suitable transfer technique, we are able to engineer the dimension, density, and orientation of wrinkles on transferred CVD-graphene. Such kind of wrinkle engineering is employed to fabricate highly aligned graphene nanoribbon (GNR) arrays by self-masked plasma-etching. Strictly consistent with the designed wrinkles, the density of GNR arrays varied from ～0.5 to 5 GNRs/μm, and over 88% GNRs are less than 10 nm in width. Electrical transport measurements of these GNR-based FETs exhibit an on/off ratio of ～30, suggesting an opened bandgap. Our wrinkle engineering approach allows very easily for a massive production of GNR arrays with bandgap-required widths, which opens a practical pathway for large-scale integrated graphene devices.     

Study of Electronic,Optical Absorption and Emission in Pure and Metal‐Decorated Graphene Nanoribbons (C29H14‐X; X=Ni,Fe, Ti,Co+, Al+, Cu+): First Principles Calculations

Density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations were performed with the basis sets 6‐31G for DFT and 6‐31G(d), 6‐31+G(d,p) for TDDFT on pure graphene nanoribbon (GNR) C₃₀H₁₄ and metal‐decorated C₂₉H₁₄‐X (MGNRs; X=Ni, Fe, Ti, Co⁺, Al⁺, and Cu⁺). The metal/carbon ratio (X:C 3.45 %) and the doping site were fixed. Electronic properties, that is, the dipole moment, binding energy, and HOMO–LUMO gaps, were calculated. The absorption and emission properties in the visible range (λ=400–720 nm) were determined. Optical gaps, absorption and emission wavelengths, oscillator strengths, and dominant transitions were calculated. Pure graphene was found to be the most stable form. However, of the MGNRs, C₂₉H₁₄?Co⁺ and C₂₉H₁₄?Al⁺ were found to be the most and least stable, respectively. All GNRs were found to have semiconducting nature. The optical absorption of pure graphene undergoes a shift on metal doping. Emission from the pure graphene followed Kasha′s rule, unlike the metal‐doped GNRs.     

Detection of nanoscale η‐MgZn2 phase dissolution from an Al‐Zn‐Mg‐Cu alloy by electrochemical microtransients

In this article we demonstrate how the dissolution of nanosize intermetallic particles present in Al alloys can be detected using microelectrochemical techniques. The local electrochemical properties of a high‐strength Al‐Zn‐Mg‐Cu alloy, which contains nanoscale η‐MgZn₂ phases, were investigated with a microcapillary cell. At the open‐circuit potential (OCP) with the sample surface in the range of 1000 µm², potential fluctuations (microtransients) can be observed. Under polarization of such small areas, current transients are detected in the passive range of the alloy. An estimation of the size of dissolution events from the charge passed during the current transient leads to the conclusion that the current transients could stem from the dissolution of η‐phase particles, with diameters in the order of 100 nm. This size scale corresponds well with the size of the grain‐boundary MgZn₂. Transmission electron microscopy (TEM) and high‐resolution SEM images provide further insights for possible mechanisms leading either to potential or to current microtransients. Copyright © 2008 John Wiley & Sons, Ltd.