Anisotropic Nanostructures Research Articles

Shape-dependent activity of anisotropic Ag nanostructures supported on TiO2 for the photoelectrocatalytic water oxidation

Herein, we report shape-dependent photoelectrocatalytic activity of Ag/Ag2O nanostructures supported on pristine or nitrogen-doped TiO2 (NTiO2). Isotropic (spherical) and anisotropic (cubic and triangular) nanocrystals of Ag/Ag2O nanocrystals were fabricated using complexing and surface directing agents. Microscopic images confirmed the formation of nanospheres (∼ 50–60 nm), nanocubes (∼ 50–70 nm), triangular nanoplates (∼ 40–60 nm). The X-ray photoelectron spectroscopy indicated the presence of metallic silver (Ag0) and oxidized silver (Ag+). The as-prepared photoelectrodes were illuminated by Ultraviolet–Visible (λ = 300–600 nm) or visible radiations (λ = > 420 nm), and photoelectrochemical oxidation of water was examined. The findings indicate a significant correlation in the shape of Ag/Ag2O nanostructures, the incident wavelength, and the photoelectrochemical performance. In addition, different performance trend was discerned under Ultraviolet–Visible and visible excitations with respect to the amount of deposited Ag/Ag2O. The variation in activity observed with different shapes of Ag/Ag2O was correlated to enhanced optical property, arising due to the anisotropic structure, and electrical conductivity.

Hydrodynamic assembly of two-dimensional layered double hydroxide nanostructures

Formation mechanisms of two-dimensional nanostructures in wet syntheses are poorly understood. Even more enigmatic is the influence of hydrodynamic forces. Here we use liquid flow cell transmission electron microscopy to show that layered double hydroxide, as a model material, may form via the oriented attachment of hexagonal nanoparticles; under hydrodynamic shear, oriented attachment is accelerated. To hydrodynamically manipulate the kinetics of particle growth and oriented attachment, we develop a microreactor with high and tunable shear rates, enabling control over particle size, crystallinity and aspect ratio. This work offers new insights in the formation of two-dimensional materials, provides a scalable yet precise synthesis method, and proposes new avenues for the rational engineering and scalable production of highly anisotropic nanostructures.

Synthesis of Gold Nanorods Using Poly(vinylpyrrolidone) of Different Molecular Weights as an Additive

AbstractThe use of poly(vinylpyrrolidone) (PVP) of different molecular weights (5‐360 kDa) during the synthesis of gold nanorods (AuNRs) via seedless and seed‐mediated approaches is reported. For both syntheses, the LSPR band can be tuned from 700 to 1050 nm by modifying the molecular weight of PVP in a time dependent addition of polymer. Moreover, the growth of AuNRs in 100 mM cetyltrimethylammonium bromide (CTAB) is accelerated as the molecular weight of PVP increases from 5 to 360 kDa for the seedless approach. In the case of AuNRs synthesized by the seed‐mediated approach, the aspect ratio decreases from 7.41 to 3.72 as the molecular weight of PVP increases to 55 kDa. For both protocols, high molecular weight of PVP reduces the shape yield despite of producing smaller AuNRs. Therefore, the incorporation of small amounts of PVP during the growth stage can be utilized for the preparation of anisotropic nanostructures to tune their optical properties and improve their shape yield.

Anisotropic Gold Nanostructures: Optimization via in Silico Modeling for Hyperthermia

Protein- and peptide-based manufacturing of self-assembled supramolecular functional materials has been a formidable challenge for biomedical applications, being complex in structure and immunogenic in nature. In this context, self-assembly of short amino acid sequences as simplified building blocks to design metal–biomolecule frameworks (MBioFs) is an emerging field of research. Here, we report a facile, bioinspired route of anisotropic nanostructure synthesis using gold binding peptides (10–15mers) secreted by cancer cells. The bioinformatics tool i-TASSER predicts the effect of amino acid sequences on metal binding sites and the secondary structures of the respective peptide sequence. Electron microscopy, X-ray, infrared, and Raman spectroscopy validated the versatile anisotropic gold nanostructures and the metal–bioorganic nature of this biomineralization. We studied the influence of precursor salt, pH, and peptide concentration on the evolution of nanoleaf, nanoflower, nanofiber, and dendrimer-like a...

Novel anisotropic ordered polymeric materials based on metallopolymer precursors as dye sensitized solar cells

Developing molecular self-assembly is an important step to generate ordered nanostructure materials. In this pursuit, a simple template-free method is reported to develop anisotropic nanostructures using metallopolymer precursors. The phenanthroline-based ruthenium complex monomer (PDAR) and its polymers [3-armed PPDAR (PPDAR-3) and 4-armed PPDAR (PPDAR-4)] were synthesized using ATRP method. These materials displayed higher glass transition temperatures (182 °C for PPDAR-4 and 176 °C for PPDAR-3) compared to the linear polymer, PPDAR (144 °C). The materials showed metal-to-ligand charge transfer (MLCT) absorption peak at 440 nm and armed polymers exhibited higher molar absorption coefficient (PPDAR-4: 7.6 × 105 M−1 cm−1 and PPDAR-3: 6.58 × 105 M−1cm−1) compared to the linear polymer (4.6 × 105 M−1cm−1). The materials were self-assembled in the presence of non-polar solvents to form uniform nano-domain micelles. Thin films of these materials were formed and subjected to elevated annealing temperatures (180 °C) and were fully characterized by AFM, SEM, and XRD techniques to understand the mechanism of self-assembly. Furthermore, dye sensitized solar cell (DSSC) devices were fabricated using the materials as additional components of a liquid electrolyte (I3−/I−) to explore the role of these architectures on open circuit voltage (VOC) as well as cell power conversion efficiency (PCE). Overall, this study provides new insights in the area of metallopolymers.

A NanoFlare-Based Strategy for In Situ Tumor Margin Demarcation and Neoadjuvant Gene/Photothermal Therapy.

Accurate tumor margin demarcation in situ remains a paramount challenge. Herein, a NanoFlare (also known as spherical-nucleic-acid technology) based strategy is reported for in situ tumor margin delineation by transforming and amplifying the pathophysiological redox signals of tumor microenvironment. The NanoFlare designed (named AuNS-ASON) is based on gold nanostar (AuNS) coated with a dense shell of disulfide bridge-inserted and cyanine dyes-labeled antisense oligonucleotides (ASON) targeting survivin mRNA. The unique anisotropic ASON-spike nanostructure endows the AuNS-ASON with universal cellular internalization of tumor cells, while the disulfide bridge inserted confers response specificity toward redox activation. In vitro experiments demonstrate that the AuNS-ASON can discriminate tumor cells rapidly with activated fluorescence signals (>100-fold) in 2 h, and further achieve synergistic gene/photothermal tumor cells ablation upon near-infrared laser irradiation. Remarkably, in situ tumor margin delineation with high accuracy and outstanding spatial resolution (<100 µm) in mice bearing different tumors is obtained based on the AuNS-ASON, providing intraoperative guidance for tumor resection. Moreover, the AuNS-ASON can enable efficient neoadjuvant gene/photothermal therapy before surgery to reduce tumor extent and increase resectability. The concept of NanoFlare-based microenvironment signal transformation and amplification could be used as a general strategy to guide the design of activatable nanoprobes for cancer theranostics.

Gold nanoflowers as efficient hosts for SERS based sensing and bio-imaging

Intrinsically rich in hot spots, anisotropic gold nanostructures have been explored as efficient hosts for Surface Enhanced Raman Scattering (SERS). Limited toxicity, shape/size tunability and high stability of gold nanostructures makes them a better choice to be used as SERS tags compared to other plasmonic metals. In this article, gold nanoflowers using ‘green’ L-ascorbic acid have been synthesized and used as SERS tags to detect low concentrations of Rhodamine 6 G dye. It is found that the gold nanoflowers can detect the presence of R6G molecules up to 10−10 M. The nanoflower (AuNf) petals play an important role in Raman signal enhancement allowing signal improvement of the order of 106. This enhancement can be explained in terms of electromagnetic enhancement mechanism of the metal nanomaterials. We have also studied the cytotoxicity of these nanostructures using MTT assay where human lung carcinoma cell line A549 were treated with AuNf. The synthesized gold nanoflowers show high biocompatibility and improved signals when imaged through confocal microscope indicating its applicability as a multimodal in vitro SERS based sensor and imaging tag.

Near-Field Thermal Radiation between Nanostructures of Natural Anisotropic Material

While nanostructures exhibit remarkable optical and thermal properties, including super-Planckian emission beyond the black-body limit, they are usually made of isotropic materials; anisotropic nanostructures are rarely considered, despite their potential for applications. Investigating near-field thermal radiation between gratings of natural anisotropic graphite, the authors find that nanopatterning enhances heat flux dramatically, due to the excitation of anisotropic surface modes not possessed by plain graphite. This deepened understanding could open routes to more efficient thermal management and thermophotovoltaic energy conversion.

Distinct Mechanosensing of Human Neural Stem Cells on Extremely Limited Anisotropic Cellular Contact.

Human neural stem cells (hNSCs) can alter their fate choice in response to the biophysical cues provided during development. In particular, it has been reported that the differentiation of neural stem cells (NSCs) is enhanced by anisotropic contact, which facilitates focal adhesion (FA) formation and cytoskeletal organization. However, a biomolecular mechanism governing how the cells process the biophysical cues from these anisotropic geometries to their fate commitment is still poorly understood due to the limited availability of geometrical diversities (contact width above 50 nm) applicable to cell studies. Here, we firstly demonstrate that the biomolecular mechanism for enhanced neurogenesis on an anisotropic nanostructure is critically dependent on the resolution of a contact feature. We observed a totally different cellular response to anisotropic geometries by first utilizing a high-resolution nanogroove (HRN) structure with an extremely narrow contact width (15 nm). The width scale is sufficiently low to suppress the integrin clustering and enable us to elucidate how the contact area influences the neurogenesis of hNSCs at an aligned state. Both the HRN and control nanogroove (CN) pattern with a contact width of 1 μm induced the spontaneous topographic alignment of hNSCs. However, intriguingly, the focal adhesion (FA) formation and cytoskeletal reorganization were substantially limited on the HRN, although the cells on the CN showed enhanced FA formation compared with flat surfaces. In particular, the hNSCs on the HRN surface exhibited a strikingly lower fraction of nuclear yes-associated protein (YAP) than on the CN surface, which was turned out to be regulated by Rho GTPase in the same way as the cells sense the mechanical properties of the environment. Considering the previously reported role of YAP on neurogenesis, our finding newly substantiates that YAP and Rho GTPase also can be transducers of hNSCs to process topographical alternation to fate decision. Furthermore, this study with the unprecedented high-resolution nanostructure suggests a novel geometry sensing model where the functional crosstalk between YAP signaling and Rho GTPase integrally regulate the fate commitment of the hNSCs.

Review on Electromechanical Coupling Properties of Biomaterials.

Electromechanical coupling properties of biological materials, especially cellulose from plant cell walls and proteins from animals, are of great interest for applications in biocompatible sensors and actuators and ecofriendly energy harvesters. On the basis of their anisotropic nanostructures, cellulose and fibrous proteins such as collagen, silk, keratin, etc. are expected to be piezoelectric; however, this property does not necessarily translate to cellulose- or protein-containing bulk materials. In fact, the values of piezoelectric coefficients reported for cellulose and proteins in the literature vary over several orders of magnitude, which raises the question of whether these are truly intrinsic piezoelectric properties of biological materials or whether they are obscured with other electromechanical coupling processes such as electrostriction, flexoelectricity, electrochemical transport, or electrostatic deformation. This critical question about intrinsic and extrinsic electromechanical coupling mechanisms is reviewed in this article. The origin of piezoelectricity of cellulose and collagen (the most widely studied protein for piezoelectricity) is discussed based on their molecular structures. Key requirements to construct macroscopic piezoelectric biocomposites are addressed in terms of packing orders or arrangements of polar domains in composites. On the basis of this structural argument, truly piezoelectric responses of macroscopic materials fabricated with or containing cellulose and collagen are found to be extremely difficult to observe or quantify; most values reported in the literature as piezoelectric coefficients of such materials appear to originate from other electromechanical coupling mechanisms. Clarifying these mechanisms is important to properly design electromechanical devices using biobased materials.

Synthesis of Platinum Nanocrystals within Iodine Ions Mediated

A liquid-phase reducing method of synthesizing Pt nanocrystals was demonstrated, and dendrite-, cube-, and cuboctahedron-shaped Pt nanocrystals (NCs) with well-defined monomorphic were successfully synthesized through iodine ions mediated with the CTAB agent. When the KI concentration was increased to thirty times of K2PtCl4 at the nucleation stage, the high-quality Pt nanodendrites could be obtained. However, no matter how many KI were added at the growth age, only cube- and cuboctahedron-shaped Pt nanocrystals formed. The results of high-resolution TEM, EDX, and XRD indicated that the size and shape of Pt NCs could be turned by changing the concentration and time of KI. In the nucleation stage, it might be due to that some iodine ions adsorb on the surfaces of Pt NCs, which probably cause the rapid growth process resulting in the formation of Pt nanodendrites. In the growth stage, although high concentrations of I− ions could contribute to the shape control and generate bigger particles of Pt NCs, small Pt particles do not grow into dendrites. The insight into the role of I− ions in synthesis of Pt NCs reported here provided a viewpoint for clearly understanding the formation mechanism of anisotropic platinum nanostructures.

Exceptional Catalytic Activities and Sensing Performance of Palladium Decorated Anisotropic Gold Nanoparticles

AbstractThe present perspective explores the effect of composition and morphology on the catalytic and surface enhanced Raman scattering (SERS) sensing applications of anisotropic bimetallic gold‐palladium (Au−Pd) nanoparticles. The morphology of the nanoparticles was controlled by a ‘soft’ templating approach by using a hexagonal lyotropic liquid crystalline phase. The mesophases comprised of toluene‐swollen surfactant cylinders, doped with the gold (Au) and palladium (Pd) precursors, stabilized by a monolayer of surfactant and co‐surfactant molecules, arranged in a hexagonal lattice in water as continuous medium. Anisotropic nanostructures of Au having very small Pd nanoparticles deposited on their surface were produced when hexagonal mesophases containing Au and Pd precursors were subjected to gamma irradiation. The synthesized nanomaterials were thoroughly characterized using X‐ray diffraction (XRD), transmission electron microscopy (TEM) and X‐ray photoemission spectroscopy (XPS). These nanomaterials showed exceptionally high catalytic activities for the reduction of 4‐nitrophenol with the normalized rate constant of the bimetallic Au−Pd catalyst being 1.5 times higher than the best catalyst that is reported in the literature. These bimetallic Au−Pd nanostructures also demonstrated very good enhancement for Raman signals of methylene blue by demonstrating an enhancement factor of the order of 106 and a detection limit as low as 0.1 nM.

High-performance polarization beam splitter based on anisotropic plasmonic nanostructures

We propose an alternative design of efficient and compact polarization beam splitters for wide-angle and polarization-sensitive incidence of electromagnetic waves at near-infrared frequencies. The designed system is constructed by depositing periodic metallic strips on top and bottom of a continuous metal film. Based on the optimization of structure parameters, the whole system can suppress transmission (~ 0%) for one polarization and enhance transmission for another polarization (~ 95%). This phenomenon of anisotropic transmission can be attributed to different performances of impedance along different polarization directions. This device may find applications in optical components and polarization filter.

Facile fabrication of silver decorated polyarylene ether nitrile composited micro/nanospheres via microemulsion self-assembling

Although soft micro/nanoreactors obtained from amphiphilic block co-polyolefin are intensively used for metallic nanoparticle synthesis, the intricate crosslinking after self-assembling of copolymers is an indispensable step. In this work, an amphiphilic block copolymer made of aromatic macromolecular with rigid backbone structures (i.e. polyarylene ether nitrile, PEN) is explored to formulate a robust micro/nano-reactor for direct silver nanoparticles (Ag NPs) synthesis without any crosslinking step. We show that the polymeric micro/nanoreactors can be fabricated in both microemulsion and reverse microemulsion system. More importantly, Ag NPs with modulated morphology and optical properties can be in-situ synthesized using these PEN micro/nanoreactors, which is due to the coordination between Ag+ and pendant cyano/sulfonate groups of PEN. Interestingly, the anisotropic polygonal plate-like hybrid nanostructures are obtained via the optimization of nucleation and growth kinetics of Ag NPs using PEN nanoreactor in the water-in-oil W/O reverse microemulsion. Based on these results, the present work would open the way for the facile fabrication of soft micro/nanoreactors for plasmonic nanostructures synthesis using amphiphilic block-copolymers with rigid molecular structures.

TiO2-Capped Gold Nanorods for Plasmon-Enhanced Production of Reactive Oxygen Species and Photothermal Delivery of Chemotherapeutic Agents.

Near infrared (NIR)-absorbing noble metal nanostructures are being extensively studied as theranostic agents, in particular for photoacoustic imaging and photothermal therapy. Because of the electric field enhancement at the tips of anisotropic metal nanostructures, positioning photoactive species at these sites can lead to increased energy absorption. Herein, we show the site-specific placement of NIR-active photosensitizers at the ends of gold nanorods (AuNRs) by growing porous TiO2 caps. The surface plasmon resonance of the AuNRs was carefully tuned to overlap with the exciton absorption of indocyanine green (ICG), a NIR photosensitizer with low quantum yields and poor photostability. In conjugating high amounts of ICG to the TiO2 caps, increased amounts of singlet oxygen (1O2) were generated as compared to when ICG was attached to sidewalls of the AuNRs. Because the AuNRs also cause local increases in temperature upon NIR excitation, DNA strands were next attached to the AuNRs sidewalls and loaded with doxorubicin (DOX). We found that the synergistic effect of increased 1O2 and photothermal-induced drug delivery led to significant improvements in tumor cell killing. This work demonstrates that with careful design over hybrid nanostructure synthesis, higher levels of tumor therapy may be achieved.

Ultrathin PdPt bimetallic nanowires with enhanced electrocatalytic performance for hydrogen evolution reaction

We report a general bottom-up synthetic methodology to epitaxially grow 3-nm-thick single-crystalline PdPt bimetallic nanowires using amphiphilic dioctadecyldimethylammonium chloride as the surfactant template. Mechanistic studies show that the existence of Pd element is critical in the synthesis of bimetallic PdPt nanowires with controllable atomic ratios. Due to synergistic effect of anisotropic single-crystalline ultrathin one-dimensional nanostructures and bimetallic elemental compositions, the resulting PdPt nanowires exhibit superior electrocatalytic activity and stability towards the hydrogen evolution reaction (HER). Among them, bimetallic Pd86Pt14 nanowires show the best HER activity with a small overpotential of only 0.8 mV in acidic media at a current density of 10 mA/cm2 (12.8 mV positive than that of commercial Pt nanoparticles), and excellent stability with only 4.6% loss in activity after current-time chronoamperometric responses for 26 h. This surfactant template-directing approach opens a general method for precise control of anisotropic nanostructures in diverse bimetallic nanomaterials for the high-performance electrocatalysts.

Block-copolymer assisted fabrication of anisotropic plasmonic nanostructures

The anisotropic nanostructures of noble metals are of great interest for plasmonic applications, due to the possibility of tuning the localized surface plasmon resonance (LSPR) across the UV–visible–near infrared (UV–vis-NIR) without sacrificing the linewidth, as well as achieving larger local field enhancement. Here, we report a simple and promising fabrication method of anisotropic gold nanostructures film using polystyrene-b-poly(2vinylpyridine) (PS-b-P2VP) block copolymers (BCPs) as a template. In this approach, PS-b-P2VP spherical micelles were first synthesized as a template, followed by selective deposition of an Au precursor inside the P2VP core of the micelles, using an ethanol solution of Au salt. Subsequently, heat treatment of the precursor deposited BCP films followed by the removal of the BCP template produced anisotropic gold nanostructures of various shapes, such as octahedron, decahedron, tetrahedron, triangles, and triangular prism. A temperature and time dependent study during heat treatment shows the formation of clusters at a higher temperature. Furthermore, measurements of the ensemble extinction spectra of the anisotropic Au nanoparticle films showed two broad distinct LSPR peaks; one in the visible range (∼660 nm), and the other in the NIR range (∼875 nm). The electrodynamic simulation showed that octahedron and decahedron nanoparticles are responsible for the LSPR response in the visible; whereas the triangular shapes are responsible for the LSPR response in the NIR. Our work is expected to open up a new direction for the synthesis of anisotropic nanostructures of noble metals, that can be utilized to tune the LSPR response across the UV–vis-NIR range, using a simple BCP template-based method.

Dipyramidal-Au@SiO2 nanostructures: New efficient electromagnetic nanoresonators for Raman spectroscopy analysis of surfaces

Plasmonic structures act as electromagnetic nanoresonators leading to a local enhancement of the intensity of the electric field of the incident radiation, which leads to an increase in the efficiency of Raman scattering for molecules in close proximity to such nanoresonators. Because many biological molecules can change their structure during interaction with metal surfaces, plasmonic nanostructures are sometimes covered with very thin layers of chemically inert oxides, which prevent direct interaction between the sample being analysed and the metal surface. The Raman analysis of surfaces using surface-protected plasmonic nanoparticles is called shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this contribution, the first example of using dipyramidal-Au@SiO2 nanoparticles as nanoresonators for SHINERS is presented. Since such nanostructures contain many sharp edges and apexes (on which the highest field enhancement is usually generated), the Raman enhancement factor induced by the dipyramidal nanoparticles is significantly (about one order of magnitude) larger than the enhancement factor generated by standard semi-spherical nanostructures. Synthesized dipyramidal-Au@SiO2 nanoparticles were tested as nanoresonators in SHINERS measurements of some thiolate monolayers formed on a platinum surface and for detecting the pesticide thiram deposited on a tomato skin. For example, the limit of detection of thiram in the last system was estimated as about 0.9 ng cm−2. The method of synthesis and purification of dipyramidal-Au nanoparticles applied in this work makes it possible to obtain samples of SHINERS nanostructures having very high homogeneity, significantly better than the homogeneity of anisotropic gold nanostructures previously used for SHINERS measurements.

Synthesis of Highly Uniform Nickel Multipods with Tunable Aspect Ratio by Microwave Power Control.

As the importance of anisotropic nanostructures and the role of surfaces continues to rise in applications including catalysis, magneto-optics, and electromagnetic interference shielding, there is a need for efficient and economical synthesis routes for such nanostructures. The article describes the application of cycled microwave power for the rapid synthesis of highly branched pure-phase face-centered cubic crystalline nickel multipod nanostructures with >99% multipod population. By controlling the power delivery to the reaction mixture through cycling, superior control is achieved over the growth kinetics of the metallic nanostructures, allowing formation of multipods consisting of arms with different aspect ratios. The multipod structures are formed under ambient conditions in a simple reaction system composed of nickel acetylacetonate (Ni(acac)2), oleylamine (OAm), and oleic acid (OAc) in a matter of minutes by selective heating at the (111) overgrowth corners on Ni nanoseeds. The selective heating at the corners leads to accelerated autocatalytic growth along the ⟨111⟩ direction through a "lightning rod" effect. The length is proprtional to the length and number of microwave (MW)-on cycles, whereas the core size is controlled by continuous MW power delivery. The roles of heating mode (cycling versus variable power versus convective heating) during synthesis of the materials is explored, allowing a mechanism into how cycled microwave energy may allow fast multipod evolution to be proposed.

Crystal Phase and Architecture Engineering of Lotus-Thalamus-Shaped Pt-Ni Anisotropic Superstructures for Highly Efficient Electrochemical Hydrogen Evolution.

The rational design and synthesis of anisotropic 3D nanostructures with specific composition, morphology, surface structure, and crystal phase is of significant importance for their diverse applications. Here, the synthesis of well-crystalline lotus-thalamus-shaped Pt-Ni anisotropic superstructures (ASs) via a facile one-pot solvothermal method is reported. The Pt-Ni ASs with Pt-rich surface are composed of one Ni-rich "core" with face-centered cubic (fcc) phase, Ni-rich "arms" with hexagonal close-packed phase protruding from the core, and facet-selectively grown Pt-rich "lotus seeds" with fcc phase on the end surfaces of the "arms." Impressively, these unique Pt-Ni ASs exhibit superior electrocatalytic activity and stability toward the hydrogen evolution reaction under alkaline conditions compared to commercial Pt/C and previously reported electrocatalysts. The obtained overpotential is as low as 27.7 mV at current density of 10 mA cm-2 , and the turnover frequency reaches 18.63 H2 s-1 at the overpotential of 50 mV. This work provides a new strategy for the synthesis of highly anisotropic superstructures with a spatial heterogeneity to boost their promising application in catalytic reactions.