Free-standing Nanoparticle Research Articles

Non-enzymatic glucose detection using free-standing gold nanoparticle loaded carrageenan hydrogel electrodes

The current manuscript reports the development of free-standing nanomaterial-modified hydrogels that can be explored for sensing applications. Most of the nanomaterial-hydrogel systems developed till now rely on the modification of an electrode as the supporting substrate on which the electrochemical reactions can take place. These substrates can be pencil electrodes and or screen-printed electrodes. This widely limits the use of hydrogels which can be used as scaffolds and can be explored for the fabrication of free-standing electrodes. Since the functionality of only hydrogel is quite limited the current manuscript reports the loading of gold nanoparticles inside the hydrogel matrix which makes it functionally active. The successful synthesis of the modified hydrogel was confirmed with Scanning Electron Microscopy (SEM) and elemental analysis. The rheological data confirmed the positive effect of gold loading on the hydrogel storage modulus. As a proof of concept, the work reports the non-enzymatic electrochemical detection of glucose using gold nanoparticle loaded carrageenan hydrogel. The sensor showed a linear response towards varying glucose concentrations (0 to 2.5 mM). The hydrogel systems can be incorporated into wearables and implantable sensors and can be extended for the detection of other biomarkers in bodily fluids.

Molecular simulation of molecular and surface properties of random copolymer nanoparticle

Random copolymer nanoparticles were studied by Monte Carlo simulation of coarse-grained models on the high coordination lattice. Molecular and surface properties of free-standing nanoparticle were characterized at fixed 50% co-monomer content as a function of the interaction strength between co-monomer units. Intramolecular interaction for all copolymer chains were treated by the Rotational Isomeric State (RIS) model of polyethylene (PE). To simulate the copolymer model, the non-bonded interactions for comonomer units were described by different parameter sets of the Lennard-Jones energy. When the co-monomer interaction was stronger, the bulk densities of nanoparticles were increased and the interfacial widths were narrower. Near the surface, end beads of copolymer chains were segregated and the bond orientation was preferably perpendicular to the surface. Molecular shape and molecular size of copolymer chains were slightly changed as a function of co-monomer interaction strength and most of which occurs close to the surface. For chain orientation, the largest principal axis tended to orient parallelly to the surface, and then changed toward randomly isotropic orientation for weaker co-monomer interaction. The intrachain energies were decreased at the surface region implying more trans conformation while the non-bonded energies were increased due to better monomer packing at the surface region. The change of system energetics was more apparent as a function of the co-monomer interaction.

UV cross-linked smart microgel membranes as free-standing diffusion barriers and nanoparticle bearing catalytic films.

In this study we use poly(N-isopropylacrylamide) (PNIPAM) based copolymer microgels to create free-standing, transferable, thermoresponsive membranes. The microgels are synthesized by copolymerization of NIPAM with 2-hydroxy-4-(methacryloyloxy)–benzophenone (HMABP) and spin-coated on Si wafers. After subsequent cross-linking by UV-irradiation, the formed layers easily detach from the supporting material. We obtain free standing microgel membranes with lateral extensions of several millimetres and an average layer thickness of a few hundred nanometres. They can be transferred to other substrates. As one example for potential applications we investigate the temperature dependent ion transport through the membranes via resistance measurements revealing a sharp reversible increase in resistance when the lower critical solution temperature of the copolymer microgels is reached. In addition, prior to cross-linking, the microgels can be decorated with silver nanoparticles and cross-linked afterwards. Such free-standing nanoparticle hybrid membranes are then used as catalytic systems for the reduction of 4-nitrophenol, which is monitored by UV/Vis spectroscopy.

Molecular simulation study of the curling behavior of the finite free-standing kaolinte layer

Since kaolinite nanolayers potentially have many important applications, it is crucial to determine the factors that govern their curling behavior. The curling of a single-layer, free-standing kaolinite nanoparticle consisting of nearly 1 million atoms is studied with classical molecular dynamics simulation. Two up-to-date force fields are employed to describe the atomic interactions in the clay. The influence of force field details and of the use of different treatments of interactions (long range correction, potential cut-off radius) on the curling direction is systematically examined. That includes a practically infinite potential cut-off, which means that all atomic interactions are considered explicitly. For every inspected case, the structure is characterized by the overall shape of the particle, the axis of roll-up and particular bond lengths and angles. Both of the two possible curling directions are detected in the present simulations, which contradicts the crystallographic assumption that the constituent tetrahedral sheet can exclusively be on the concave side of curled kaolinite nanolayers.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

Free-Standing 2D Janus Gold Nanoparticles Monolayer Film with Tunable Bifacial Morphologies via the Asymmetric Growth at Air-Liquid Interface.

Large scaled two-dimensional free-standing monolayer films of gold nanoparticles show distinctive optical, electrical, and chem-physical propertie making them a new class of advanced plasmonic materials differing from bulk materials and individual nanoparticles in solution. The conventional 2D gold nanoparticle films usually possess symmetric structures and identical properties of gold nanoparticles on both sides. Herein, we developed an easy and efficient approach to construct a new type of free-standing 2D gold nanoparticle monolayer film with asymmetric gold nanoparticle structures and functions, called a 2D Janus gold nanoparticle film. The remarkable feature of our method is the subsequent asymmetric growth on one side of the interfacial self-assembled gold nanoparticle monolayer film at the air-liquid interface. It is very easy to control the morphology of the Janus film by simply and precisely adjusting the size and shape of the gold nanoparticles on the top side, and selectively tuning the structure and composition on the bottom side of the film by growing gold nanoparticles or other noble metals such as Ag, Pt, and Pd. Unlike the conventionally prepared Janus films at solid substrate that require long-time etching and transfer procedures, other features of our method include the short time in which the interfacial self-assembly and the subsequent asymmetric growth are completed as well as the easily transferable property of the Janus film onto different substrates, such as quartz glass sheets, silicon wafers, and PDMS. The obtained Janus gold nanoparticle film shows asymmetric wettabilities, optical properties, and surface-enhanced Raman scattering (SERS) effects, which is promising for a range of potential applications in optical devices, sensors, and asymmetric catalysis.

Assessing the Surface Oxidation State of Free-Standing Gold Nanoparticles Produced by Laser Ablation.

The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose we performed the experiment on a controlled free-standing nanoparticle beam produced by combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68% of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au nanoparticles.

Self-assembly and characterization of 2D plasmene nanosheets

Freestanding plasmonic nanoparticle (NP) superlattice sheets are novel 2D nanomaterials with tailorable properties that enable their use for broad applications in sensing, anticounterfeit measures, ionic gating, nanophotonics and flat lenses. We recently developed a robust, yet general, two-step drying-mediated approach to produce freestanding monolayer, plasmonic NP superlattice sheets, which are typically held together by holey grids with minimal solid support. Within these superlattices, NP building blocks are closely packed and have strong plasmonic coupling interactions; hence, we termed such freestanding materials 'plasmene nanosheets'. Using the desired NP building blocks as starting material, we describe the detailed fabrication protocol, including NP surface functionalization by thiolated polystyrene and the self-assembly of NPs at the air-water interface. We also discuss various characterization approaches for checking the quality and optical properties of the as-obtained plasmene nanosheets: optical microscopy, spectrophotometry, transmission/scanning electron microscopy (TEM/SEM) and atomic force microscopy (AFM). With regard to different constituent building blocks, the key experimental parameters, including NP concentration and volume, are summarized to guide the successful fabrication of specific types of plasmene nanosheets. This protocol, from initial NP synthesis to the final fabrication and characterization, takes ~33.5 h.

Ultraviolet laser-induced damage of freestanding silica nanoparticle films

Silica nanoparticle antireflection films are essential for the development of high-power laser systems because of their high laser damage threshold in ultraviolet (e.g., 351 nm or 355 nm). However, most laser damage reports focus on the films standing on substrates, and the intrinsic laser damage behaviors of the silica nanoparticle films are poorly understood due to the influence of the substrates. Here, we investigate the ultraviolet laser damage behaviors of the freestanding silica nanoparticle films. We find that silica nanoparitcle films demonstrate higher laser damage threshold than the fused silica substrates under the irradiation of 355-nm laser. For the films on the fused silica substrates, the laser damage is caused by the substrates. In the case of freestanding films, laser-damage resistance is strongly dependent on the film thickness and the amount of organic groups in the films. The laser damage threshold increases from 103 to 159 J/cm2 @ 355 nm with decreasing the film thickness from 322 to 31 nm. In addition, removal of organic groups from the films results in higher laser damage threshold. Our findings provide insight into the laser damage behavior of the silica nanoparticle films and foreshow a possible way to improve the laser-damage resistance.

Binder-free freestanding flexible Si nanoparticle–multi-walled carbon nanotube composite paper anodes for high energy Li-ion batteries

Abstract

Free-Standing Metal Oxide Nanoparticle Superlattices Constructed with Engineered Protein Containers Show in Crystallo Catalytic Activity.

The construction of defined nanostructured catalysts is challenging. In previous work, we established a strategy to assemble binary nanoparticle superlattices with oppositely charged protein containers as building blocks. Here, we show that these free-standing nanoparticle superlattices are catalytically active. The metal oxide nanoparticles inside the protein scaffold are accessible for a range of substrates and show oxidase-like and peroxidase-like activity. The stable superlattices can be reused for several reaction cycles. In contrast to bulk nanoparticle-based catalysts, which are prone to aggregation and difficult to characterize, nanoparticle superlattices based on engineered protein containers provide an innovative synthetic route to structurally defined heterogeneous catalysts with control over nanoparticle size and composition.

Tunable Mechanical Properties of Nanoparticle Monolayer Membranes via Ligand Phase Control and Defect Distribution.

In this study, the effects of ligand phase, morphology, and temperature on the elastic modulus of free-standing alkanethiol-capped gold nanoparticle membranes are reported. Langmuir films of 2.5 nm gold nanoparticles capped with tetradecanethiol were prepared at temperatures above and below the phase transition temperature (Tm) of the ligand shell and transferred to holey carbon grids (containing 1.2 μm holes) to form free-standing membranes. Force–indentation measurements are used to measure the elastic modulus of the membranes using an atomic force microscope in the temperature range 10–40 °C. These films are compared with membranes of dodecanethiol-capped gold nanoparticles, which do not undergo a ligand order–disorder transition in the temperature range investigated. The ligand phase effect is observed in the tetradecanethiol-capped gold nanoparticle films, where an abrupt change in the elastic modulus is seen near Tm. The temperature (relative to Tm) during the fabrication of the films is determined to play an important role in tuning the mechanical strength of these films in this temperature range by both changing the nature of the interparticle interactions and by affecting microscale film morphology.

Free-standing nanoparticle superlattice sheets: From design to applications(a)(a)Contribution to the Focus Issue Self-assemblies of Inorganic and Organic Nanomaterials edited by Marie-Paule Pileni.

Here I summarize my perspective in free-standing nanoparticle superlattice sheets with regards to their fabrication, properties, and applications.

Self-Organized Freestanding One-Dimensional Au Nanoparticle Arrays.

One-dimensional Au nanoparticle arrays encapsulated within freestanding SiO2 nanowires are fabricated by thermal oxidation of Au-coated Si nanowires with controlled diameter and surface modulation. The nanoparticle diameter is determined by the Si nanowire diameter and Au film thickness, while the interparticle spacing is independently controlled by the Si nanowire modulation. The optical absorption of randomly oriented Au nanoparticle arrays exhibits a strong plasmonic response at 550 nm. Scanning transmission electron microscopy (STEM)-electron energy loss spectrum (EELS) of nanoparticle arrays confirmed the same plasmonic response and demonstrated uniform optical properties of the Au nanoparticles. The plasmonic response in the STEM-EELS maps is primarily confined around the vicinity of the nanoparticles. On the other hand, examination of the same nanowires by energy-filtered transmission electron microscopy also revealed significant enhancement in the plasmonic excitation in the regions in between the nanoparticles. This versatile route to synthesize one-dimensional Au nanoparticle arrays with independently tailorable nanoparticle diameter and interparticle spacing opens up opportunities to exploit enhanced design flexibility and cost-effectiveness for future plasmonic devices.

Light welding nanoparticles: from metal colloids to free-standing conductive metallic nanoparticle film

Bottom-up assembly of nanostructured thin films could offer an alternative low-cost approach to electronic thin films. However, such solution-processed thin films are often plagued by excessive inter-particle resistance and only exhibit limited current delivering capability. Here, we report a novel approach to fabricate highly conductive free-standing metallic thin film, accomplished by combining interfacial self-assembly of nanoparticles (NPs) and a light welding process. We found that light from a xenon lamp can weld adjacent Ag and Au NPs assembled at the water-air interface, forming a highly interconnected, free-standing metallic thin film structure with excellent electrical transport properties. With such a unique structure, the resultant thin metallic films show not only high flexibility and robustness, but also high conductivity comparable to bulk metallic thin films. Our studies offer a low-cost, room-temperature, and solution-processable approach to highly conductive metallic films. It can significantly impact solution-processable electronic and optoelectronic devices.

Freestanding Ultrathin Nanoparticle Membranes Assembled at Transient Liquid–Liquid Interfaces

A synthetic route is presented for the realization of ultrathin freestanding nanoparticle membranes that are built of gold nanoparticles protected with trimethoxysilane‐bearing ligands. The mechanism relies on interfacial assembly in an oil–water mixture. Upon shaking, nanoparticles are transported to the liquid–liquid interface of the oil droplets and form a network through the formation of Si–O–Si bridges. Reticulation of the nanoparticles during the dynamic process of droplet coalescence allows the formation of ultrathin membranes of only a few nanoparticle layers in thickness and square centimeters in dimension. The membranes can be manipulated, such as locally perforated, without causing their collapse. Furthermore they can be transferred onto solid or holey substrates. The synthetic route is compatible with a coassembly of dopants. As an example, membranes were doped with single walled carbon nanotubes, which resulted in a sizable increase of their electric conductivity.

Investigation of Heterogeneous Catalysts by an Electrochemical Method: Ceria and Titania-Supported Iridium Nanoparticles for Ethylene Oxidation

Electrochemical promotion of catalysis (EPOC) is a well-established phenomenon in heterogeneous catalysis for enhancing catalytic activity through the application of a small electrical stimulus between the catalyst-working and counter electrode deposited on a solid electrolyte (e.g., yttria-stabilized zirconia (YSZ))1. This electrical stimulus causes the backspillover of ionic species, in this case O2-, from the solid electrolyte to the catalyst surface. A resulting change in catalytic activity is due to the modification of the electronic properties of the catalyst. The extent to which these properties can be modified depends on several factors, such as temperature, type of catalyst material and solid electrolyte, catalyst morphology, e.g., particle size and dispersion, ionic conductivity, and gas phase composition 2,3. EPOC is an interfacial phenomenon where two-phase boundaries, i.e., catalyst/gas and catalyst/solid electrolyte, as well as the three-phase boundary (tpb), i.e., electrolyte/catalyst/gas phase play a key role in the reaction. Recent EPOC studies have shown catalytic enhancement of highly dispersed, nano-structured catalysts through the use of mixed ionic electronic conducting (MIEC) materials, i.e., CeO2 4 and TiO2 5, which are in contact with the YSZ solid-electrolyte. This approach allows for considerable reduction of the mass of metal catalyst required, while maintaining the electrical conductivity of the working electrode needed to complete the electrochemical cell 6,7. Both CeO2 and TiO2 are considered reducible supports. More specifically, CeO2, due to its non-stoichiometry, has the ability to undergo conversion between Ce4+ and Ce3+ quite easily 7. TiO2 has also been known to strongly influence the performance of the supported metal catalysts due to this reducibility 8. These properties make the use of ceria- and titania-containing catalysts of interest for many catalytic applications. This study investigates the complete oxidation of ethylene over low particle size (1.1 nm) ruthenium and (1.0 nm) iridium nanoparticles supported on CeO2 and TiO2 under open (o.c.) and closed circuit conditions. Furthermore, detailed electrochemical characterization of the Ir- and Ru- based catalysts was carried out using a steady-state polarization technique. The results obtained were correlated with their o.c. catalytic performance. The discussion of this study includes the effect of temperature and applied potential on the catalytic activity of the supported and freestanding Ir and Ru catalysts. Ru and Ir nanoparticles, synthesized using a polyol reduction method, were supported on CeO2 and TiO2 resulting in a 1 wt% catalyst loading (RuNPs/CeO2, IrNPs/CeO2, RuNPs/TiO2 and IrNPs/TiO2). The supported and free-standing nanoparticle catalysts were deposited on one side of a YSZ solid electrolyte disk in order to apply polarization 4. Gold counter and reference electrodes were applied on the opposite side of YSZ disk 9. Figure 1 shows representative polarization curves of the Ir/CeO2 catalyst at various temperatures. Positive current density (i) is due to electrochemical oxidation of ethylene and oxygen evolution at the tpb, whereas anodic current density is due to O2 electro-reduction. As can be seen, the rate of both positive and negative current densities (i) increases with temperature, indicating an increase in the reaction rates at the tpb as a result of an increase in electrolyte conductivity. i was also found to increase from 1.26, 3.5 to 29.5 µA.cm-2 for 350, 375 and 400 oC, respectively. As a result, this indicates that as temperature is increased, there is more available O2- at the tpb. A combination of open circuit catalytic and electrochemical measurements was used to evaluate and understand the catalytic performance of highly-dispersed Ru- and Ir-based, free-standing, and CeO2 and TiO2-supported catalysts with regards to the effect of O2- ions from the support. The presence of CeO2 and TiO2 was shown to play a significant role in enhancing the catalytic activity of Ru and Ir nanoparticles. Acknowledgment The financial support from Natural Science and Engineering Research Council (NSERC) is acknowledged.

Free-Standing Monolayered Metallic Nanoparticle Networks as Building Blocks for Plasmonic Nanoelectronic Junctions.

The effective coupling of optical surface plasmons (SPs) and electron transport in a plasmonic-electronic device is one of the fundamental issues in nanoelectronics and the emerging field of plasmonics, and offer promise in providing a solution to next generation nanocircuits in which all coupling is in the near field. Attempts toward this end, however, are limited because of the integration challenge to compatible nanodevices. To date, direct electrical detection of SP-electron coupling from metallic nanostructures alone are not reported, and thus it remains a great experimental challenge. In this paper, we succeed in preparing a new suspended-film-type nanoelectronic junction, in which free-standing 2D fractal nanoparticle networks act as plasmonically active nanocomponents. Direct electrical detection of optical collective SPs was evidenced by photocurrent response of the junction upon illumination. Room-temperature I-V characteristics, differing from nonlinear to Ohmic behaviors, are found to be sensitive to the nanometer-scale morphology changes of the nanomembranes. The finding and approach may enable the development of advanced plasmonic nanocircuits and new nanoelectronics, nanophotonics, and (solar) energy applications.

Electrostatically driven drumhead resonators based on freestanding membranes of cross-linked gold nanoparticles.

Freestanding, nanometer-thin membranes of alkanedithiol cross-linked gold nanoparticles represent elastic, mechanically robust and electrically conductive materials, which are interesting for the fabrication of novel nano- and microelectromechanical devices. In this work we present the first electrostatically driven drumhead resonators based on such nanoparticle membranes. These circular membranes have a thickness of 33 to 52 nm, a diameter of either 50 μm or 100 μm, and are equally spaced from their back electrode by ∼10 μm. Using an interferometric nanovibration analyzer various vibrational modes with resonance amplitudes of up to several 100 nm could be detected when the membranes are excited by applying AC voltages (<30 V) with drive frequencies of up to 2 MHz. Further, spatial amplitude distributions of different vibrational modes could be imaged. The devices showed fundamental resonance frequencies in the high kHz range and quality factors Q up to ∼2000. Finally, vibrational spectra and observed mode patterns could be well interpreted using the theory for a clamped circular membrane with negligible bending stiffness. Our findings mark an important step towards the integration of freestanding gold nanoparticle composite membranes into electromechanical devices with various applications, such as novel types of pressure or mass sensors.

Fabrication of freestanding nanoparticle membranes over wells.

Freestanding nanoparticle membranes over circular wells are prepared by utilizing surface engineering. The crucial step of this method is the hydrophobic treatment of the substrate surface, which causes the water droplet to be suspended over wells during drying. Consequently, the nanoparticle monolayer self-assembled at the surface of the water droplet would drape itself over wells instead of being dragged into wells and ruptured into patches after the evaporation of water. This scenario was confirmed by the results of control experiments with changes in the hydrophobicity of the surface and the depth of wells. Moreover, the NaCl crystallization experiment provides additional evidence for the dynamic process of drying. Freestanding nanoparticle membranes with different nanoparticle core sizes and different lengths of ligands have been successfully prepared using the same route. The Young's modulus of one typical kind of prepared freestanding nanoparticle membrane was measured with force microscopy.