Asymmetric Nanoparticles Research Articles

Plasmon-drag-assisted terahertz generation in a graphene layer incorporating an asymmetric plasmon nanostructure

This Rapid Communication presents a structure and full theoretical analysis to exploit the photon drag effect for THz signal generation in a graphene layer integrated with a plasmonic structure. The plasmonic structure is composed of a periodic array of asymmetric nanoparticles patterned over a graphene layer. The nanoparticles are designed to accomplish two goals: field localization due to the plasmonic resonance and manipulating the phase of the near field to effectively drag the quasiparticles in graphene. Combining the asymmetry with the plasmon resonances of nanoparticles, we show that an enhancement as large as three orders of magnitude is attainable in the power of the generated THz wave. This level of unprecedented enhancement mostly stems from the phase manipulation of the near field caused by asymmetric nanoparticles. Using the achieved enhancement, it is demonstrated that an ultra-wideband THz signal carrying the power of $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{W}$ can be generated using a commercially available femtosecond pulsed laser.

Localized Surface Plasmon-Enhanced Deep-UV Light-Emitting Diodes with Al/Al2O3 Asymmetrical Nanoparticles

In this work, light-extraction enhancement induced by localized surface plasmons (LSPs) on asymmetrical design of metallic nanoparticles (NPs) for AlGaN deep-ultraviolet (UV) light-emitting diodes (LEDs) is investigated numerically. The systems under study consist of Al NPs with varying diameter separated by a nano-gap, and the symmetrical dimer NPs with different geometrical parameters are studied for reference. We have demonstrated that tunable plasmonic NPs and spectral response can be controlled by varying the size of the nanoparticles and nano-gaps. It is found that the enhancement of the electric field and the peak position are subject to geometrical characteristics of the NP components in different manners for symmetrical and asymmetrical systems, respectively. High enhancement ratio for UV wavelength can be obtained by tuning parameters of asymmetrical system, which is potential for improving the performance of deep-UV LEDs.

The Enhanced Catalytic Activities of Asymmetric Au-Ni Nanoparticle Decorated Halloysite-Based Nanocomposite for the Degradation of Organic Dyes.

Janus particles (JPs) are unique among the nano-/microobjects because they provide asymmetry and can thus impart drastically different chemical or physical properties. In this work, we have fabricated the magnetic halloysite nanotube (HNT)-based HNTs@Fe3O4 nanocomposite (NCs) and then anchored the Janus Au-Ni or isotropic Au nanoparticles (NPs) to the surface of external wall of sulfydryl modified magnetic nanotubes. The characterization by physical methods authenticates the successful fabrication of two different magnetic HNTs@Fe3O4@Au and HNTs@Fe3O4@Au-Ni NCs. The catalytic activity and recyclability of the two NCs have been evaluated considering the degradation of Congo red (CR) and 4-nitrophenol (4-NP) using sodium borohydride as a model reaction. The results reveal that the symmetric Au NPs participated NCs display low activity in the degradation of the above organic dyes. However, a detailed kinetic study demonstrates that the employ of bimetallic Janus Au-Ni NPs in the NCs indicates enhanced catalytic activity, owing to the structurally specific nature. Furthermore, the magnetic functional NCs reported here can be used as recyclable catalyst which can be recovered simply by magnet.

Impact of the Nanoscale Gap Morphology on the Plasmon Coupling in Asymmetric Nanoparticle Dimer Antennas.

Coupling of plasmon resonances in metallic gap antennas is of interest for a wide range of applications due to the highly localized strong electric fields supported by these structures, and their high sensitivity to alterations of their structure, geometry, and environment. Morphological alterations of asymmetric nanoparticle dimer antennas with (sub)-nanometer size gaps are assigned to changes of their optical response in correlative dark-field spectroscopy and high-resolution transmission electron microscopy (HR-TEM) investigations. This multimodal approach to investigate individual dimer structures clearly demonstrates that the coupling of the plasmon modes, in addition to well-known parameters such as the particle geometry and the gap size, is also affected by the relative alignment of both nanoparticles. The investigations corroborate that the alignment of the gap forming facets, and with that the gap area, is crucial for their scattering properties. The impact of a flat versus a rounded gap structure on the optical properties of equivalent dimers becomes stronger with decreasing gap size. These results hint at a higher confinement of the electric field in the gap and possibly a different onset of quantum transport effects for flat and rounded gap antennas in corresponding structures for very narrow gaps.

Asymmetric Au-core Pd-shell nanoparticles supported on reduced graphene oxide for enhanced electrocatalytic activity

Asymmetric Au-core Pd-shell nanoparticles supported on rGO simply prepared by a GRR-like technique exhibit superior ORR performance compared to commercial Pt/C.

Challenges in nanoelectrochemical and nanomechanical studies of individual anisotropic gold nanoparticles.

The characterization of nanoparticles and the correlation of physical properties such as size and shape to their (electro)chemical properties is an emerging field, which may facilitate future optimization and tuning of devices involving nanoparticles. This requires the investigation of individual particles rather than obtaining averaged information on large ensembles. Here, we present atomic force - scanning electrochemical microscopy (AFM-SECM) measurements of soft conductive PDMS substrates modified with gold nanostars (i.e., multibranched Au nanoparticles) in peak force tapping mode, which next to the electrochemical characterization provides information on the adhesion, deformation properties, and Young's modulus of the sample. AFM-SECM probes with integrated nanodisc electrodes (radii < 50 nm) have been used for these measurements. Most studies attempting to map individual nanoparticles have to date been performed at spherical nanoparticles, rather than highly active asymmetric gold nanoparticles. Consequently, this study discusses challenges during the nanocharacterization of individual anisotropic gold nanostars.

Free-Standing Bilayered Nanoparticle Superlattice Nanosheets with Asymmetric Ionic Transport Behaviors.

Natural cell membranes can directionally and selectively regulate the ion transport, which is critical for the functioning of living cells. Here, we report on the fabrication of an artificial membrane based on an asymmetric nanoparticle superlattice bilayered nanosheet, which exhibits similar ion transport characteristics. The superlattice nanosheets were fabricated via a drying-mediated self-assembly of polystyrene-capped gold nanoparticles at the liquid-air interface. By adopting a layer-by-layer assembly process, an asymmetric nanomembrane could be obtained consisting of two nanosheets with different nanoparticle size. The resulting nanomembranes exhibit an asymmetric ion transport behavior, and diode-like current-voltage curves were observed. The asymmetric ion transport is attributed to the cone-like nanochannels formed within the membranes, upon which a simulation map was established to illustrate the relationship between the channel structure and the ionic selectivity, in consistency with our experimental results. Our superlattice nanosheet-based design presents a promising strategy for the fabrication of next-generation smart nanomembranes for rationally and selectively regulating the ion transport even at a large ion flux, with potential applications in a wide range of fields, including biosensor devices, energy conversion, biophotonics, and bioelectronics.

Uniform Concave Polystyrene-Carbon Core-Shell Nanospheres by a Swelling Induced Buckling Process.

We have developed a facile procedure that can create asymmetrical building blocks by uniformly deforming nanospheres into C(∞v) symmetry at low cost and high quality. Concave polystyrene@carbon (PS@C) core-shell nanospheres were produced by a very simple microwave-assisted alcohol thermal treatment of spherical PS@C nanoparticles. The dimensions and ratio of the concave part can be precisely controlled by temperature and solvents. The concavity is created by varying the alcohol-thermal treatment to tune the swelling properties that lead to the mechanical deformation of the PS@C core-shell structure. The driving force is attributed to the significant volume increase that occurs upon polystyrene core swelling with the incorporation of solvent. We propose a mechanism adapted from published models for the depression of soft capsules. An extrapolation from this model predicts that the rigid shell is used to generate a cavity in the unbuckled shell, which is experimentally confirmed. This swelling and deformation route is flexible and should be applicable to other polymeric nanoparticles to produce asymmetrical nanoparticles.

Symmetry breaking polymerization: one-pot synthesis of plasmonic hybrid Janus nanoparticles.

Asymmetric hybrid nanoparticles have many important applications in catalysis, nanomotion, sensing, and diagnosis, however ways to generate the asymmetric hybrid nanoparticles are quite limited and inefficient. Most current methods rely on interfacial adhesion and modification of already formed particles. In this article we report a one-pot, facile and scalable synthesis of anisotropic Au-polymer hybrid nanoparticles via interfacial oxidative dispersion polymerization. The interfacial nucleation and polymerization lead to spontaneous symmetry breaking and formation of the Janus particles. The reaction is initiated by monomer radicals generated by the strong oxidant HAuCl4, which is itself later reduced by the electron-rich monomers to self-nucleate and form Au nanoparticles (NPs). The competition between divinylbenzene adsorption and the PVP capping agent results in effective partial surface wetting, forming asymmetric Au-PDVB hybrid nanoparticles, by confining growth of each material to its own phase. Such spontaneous symmetry breaking, important in morphogenesis, with control over the subsequent growth processes should lead to significant advances in the synthesis of asymmetric nanostructures.

Synthesis and characterization of polystyrene coated iron oxide nanoparticles and asymmetric assemblies by phase inversion

ABSTRACTFilms with a gradient concentration of magnetic iron oxide nanoparticles are reported, based on a phase inversion membrane process. Nanoparticles with ∼13 nm diameter were prepared by coprecipitation in aqueous solution and stabilized by oleic acid. They were further functionalized by ATRP leading to grafted polystyrene brush. The final nanoparticles of 33 nm diameter were characterized by TGA, FTIR spectroscopy, GPC, transmission electron microscopy, and dynanmic light scattering. Asymmetric porous nanoparticle assemblies were then prepared by solution casting and immersion in water. The nanocomposite film production with functionalized nanoparticles is fast and technically scalable. The morphologies of films were characterized by scanning electron microscopy and atomic force microscopy, demonstrating the presence of sponge‐like structures and finger‐like cavities when 50 and 13 wt % casting solutions were, respectively, used. The magnetic properties were evaluated using vibrating sample magnetometer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41368.

Ultrasensitive electrochemical immunosensor for carbohydrate antigen 72-4 based on dual signal amplification strategy of nanoporous gold and polyaniline–Au asymmetric multicomponent nanoparticles

A sandwich electrochemical immunosensor is described for carbohydrate antigen 72-4 (CA72-4) based on a dual amplification strategy with nanoporous gold (NPG) film as the sensor platform and polyaniline–Au asymmetric multicomponent nanoparticles (PANi–Au AMNPs) as labels. In this study, the second anti-CA72-4 antibody (Ab2) adsorbed onto the Au of the PANi–Au AMNPs, which could be simply synthesized by interfacial reaction and have many characteristics of polyaniline and Au nanoparticle, such as well-controlled size, high conductivity, biocompatibility and catalysis. NPG film was used as electrode substrate material to fix a large number of antibodies, due to its unique properties: good biocompatibility, high conductivity, large surface area, and stability. The synergetic of NPG film and PANi–Au AMNPs could increase signal response, and significantly improve sensitivity of the immunosensor. The proposed immunosensor exhibited a wide linear range from 2 to 200U/mL, with a detection limit of 0.10U/mL CA72-4, good reproducibility, selectivity and stability. This new type of labels for immunosensors may provide many potential applications in the detection of carbohydrate antigen in immunoassays.

One-pot preparation of novel asymmetric structure nanoparticles and its application in catalysis

One-pot method has been applied to the synthesis of new Janus structure nanorods, in which Au nanoparticles are located at one end of mesoporous silica rods. Janus nanorods are synthesized directly by the chloroauric acid solution, without prior preparation of gold or silica nanoparticles. This method is easy and cost-effective for high yield preparation of Janus nanoparticles catalyst. Meanwhile, new Janus nanoparticles have high catalytic activity. Therefore, these new nanoparticles will have broad prospects in catalysis, drug delivery, self-propelled nanodevices, etc.

Facile synthesis of asymmetric Ag-organosilica hybrid nanoparticles with tunable morphologies and optical properties.

This study reports a simple and novel wet-chemical method for synthesizing asymmetric Ag-organosilica hybrid nanoparticles with finely-tuned morphologies and optical properties.

A minor lipid component of soy lecithin causes growth of triangular prismatic gold nanoparticles

Reduction of bromoauric acid by ascorbic acid in the presence of crude soybean lecithin produced a mixture of spherical and triangular prismatic nanoparticles while higher purity phosphatidylcholine (PC) produced only spherical nanoparticles. The triangular prismatic nanoparticles made with lecithin had an average edge length of 90 nm and a localized surface plasmon resonance between 700 and 1050 nm, depending on the synthetic conditions used. Although crude soy lecithin is composed of 75% PC and phosphatidylethanolamine, additional lipids and other small molecules are present. Preparatory gel electrophoresis was used to separate the different nanoparticle shapes and phosphatidic acid (PA) was identified as bound to the triangular prismatic nanoparticles by mass spectrometry of the gel-purified nanoprisms. Furthermore, PA proved essential to the asymmetric growth. When bromoauric acid was reduced by ascorbic acid using a mixture of pure PA and pure PC, triangular prismatic gold nanoparticles also resulted, confirming the role of PA and providing a second route to these near infrared active nanoparticles. Nanoparticles prepared in soy lecithin or PA–PC mixtures exhibited extended stability with no aggregation after months of storage, in contrast to nanoparticles prepared in pure PC. These nanoparticles were prepared without the use of the alkyl ammonium salts that have limited in vivo applications with other asymmetric gold nanoparticles. This is a versatile synthetic method providing stable, shape-controlled gold nanoparticles using a mild reducing agent and soy lecithin as an environmentally benign ligand source.

Popcorn-like morphology of asymmetric nanoparticles co-adjusted by hydrophilicity and crosslinking degree of the seeds

This paper presents a facile synthesis method for poly(methyl methacrylate-co-acetoacetoxyethyl methacrylate)–polystyrene popcorn-like particles by soap-free seeded emulsion polymerization. In this study, popcorn-like, raspberry-like and reverse core–shell structures of the P(MMA-co-AAEM)–PS particles could be obtained in a reliable manner. And it was found that the hydrophilicity and crosslinking degree of the seed particles, which were respectively controlled by the addition amount of comonomer AAEM and crosslinker ethyleneglycol dimethacrylate in the P(MMA-co-AAEM) seed particles, co-adjusted the popcorn-like morphology of P(MMA-co-AAEM)–PS particles. In order to make sense of the formation mechanism of the morphology, a thermodynamic model utilizing an equilibrium among the elastic-retractile force of the cross-linked polymer network, the mixing force of monomer and polymer and the interfacial tension force between the particle and water, was used to explain the formation of anisotropic particles.

Plasmonic off-axis unidirectional beaming of quantum-well luminescence

Plasmonic off-axis unidirectional beaming of luminescence is demonstrated using nitride semiconductor quantum wells. The underlying mechanism involves the near-field excitation of surface plasmon polaritons on an ultrathin metal film, which are then diffractively scattered by an adjacent periodic array of asymmetric metallic nanoparticles. By tailoring the nanoparticles shape, we show that forward scattering can be suppressed in favor of backward diffraction (or vice versa), thereby enabling unidirectional beaming at geometrically tunable oblique angles. These nanostructures can be used to control the output light directionality of arbitrary planar luminescent devices, with a spatial resolution that would be unattainable with bulk optics.

Asymmetric Plasmonic Aptasensor for Sensitive Detection of Bisphenol A

A sensitive plasmonic chirality-based aptasensor for the detection of bisphenol A (BPA) was developed. Asymmetric plasmonic nanoparticle dimers were produced by the hybridization of a BPA aptamer and its complementary sequence modified nanoparticles. Under different concentrations of BPA, the intensity of the chiral signal was varied. A low limit of detection of 0.008 ng/mL was obtained in the range 0.02-5 ng/mL.

A Sensitive DNAzyme-Based Chiral Sensor for Lead Detection.

A DNAzyme-based sensor for the determination and quantification of lead ions (Pb2+) has been established, which combines the recognition and catalysis of DNAzyme with the optical properties of nanomaterials. Circular dichroism (CD) signals were obtained by a DNAzyme-based assembly of asymmetric silver nanoparticle (AgNPs) dimers. A good linear relationship between CD signals and Pb2+ concentration was obtained ranging from 0.05 ng∙mL−1 to 10 ng∙mL−1 with a limit of detection (LOD) of 0.02 ng∙mL−1. The specificity of this sensor in lead ion detection was excellent, and a satisfactory recovery was obtained in the analysis of tap water samples. The proposed technique possesses both high sensitivity and good specificity, giving it great potential for the analysis of Pb2+ in water.

High-Frequency Mechanical Stirring Initiates Anisotropic Growth of Seeds Requisite for Synthesis of Asymmetric Metallic Nanoparticles like Silver Nanorods

High-speed stirring at elevated temperatures is shown to be effective in the symmetry-breaking process needed for the growth of the hard-to-synthesize silver nanorods from the polyol reduction of silver ions. This process competes with the facile formation of more symmetrical, spherical and cubic, nanoparticles. Once the seed is formed, further growth proceeds predominantly along the long axis, with a consequent increase of the particles' aspect ratio (that of the nanorod). When stirring is stopped shortly after seed formation, nanorods with a broad distribution of aspect ratios are obtained, while when the high-frequency stirring continues the distribution narrows significantly. The width of the nanorods can only be increased if the initial concentration of Ag(+) ions increases. Reducing the stirring speeds during seed formation lowers the yield of nanorods. Molecular dynamics simulations reveal that the formation of a nanometer-scale thin boundary region between a solid facet of the nanoparticle and the liquid around it, and the accommodation processes of metal (Ag) atoms transported through this boundary region from the liquid to the solid growth interface, are frustrated by sufficiently fast shear flow caused by high-frequency stirring. This arrests growth on seed facets parallel to the flow, leading, together with the preferential binding of the capping polymer to the (100) facet, to the observed growth in the (110) direction, resulting in silver nanorods capped at the ends by (111) facets and exposing (100) facets on the side walls.

Synthesis of Janus Nanoparticles via a Combination of the Reversible Click Reaction and “Grafting to” Strategies

A critical challenge in nanoparticle functionalization has been the preparation of polymer-grafted asymmetric (Janus) nanoparticles (diameter < 100 nm). We describe a robust and cyclic method involving a reversible click reaction and "grafting to" strategies to synthesize such nanoparticles. Mechanochemistry was used in a protection-deprotection process to separate nanoparticles cleanly that were anchored to larger particles, and the recovered azide-functionalized larger particles could be recycled as face-blocking moieties. With this combination of strategies, we prepared 15 nm silica nanoparticles that were partially functionalized with poly(methyl methacrylate). Additionally, the unique self-assembly behaviors of the resultant Janus nanoparticles were investigated in different solvents at different concentrations.