Sub-10 ohm resistance gold films prepared by removal of ligands from thiol-stabilized 6 nm gold nanoparticles

Gold nanoparticles aggregation on graphene using Reactive force field: A molecular dynamic study

We examine the aggregation behavior of AuNPs of different sizes on graphene as function of temperature using molecular dynamic simulations with Reax Force Field. In addition, the consequences of such aggregation on the morphology of AuNPs and the charge transfer behavior of AuNP-Graphene hybrid structure are analyzed. The aggregation of AuNPs on graphene is confirmed from the center of mass distance calculation. The simulation results indicate that the size of AuNPs and temperature significantly affect the aggregation behavior of AuNPs on graphene. The strain calculation showed that shape of AuNPs changes due to the aggregation and the smaller size AuNPs on graphene exhibit more shape changes than larger AuNPs at all the temperatures studies in this work. The charge transfer calculation reveals that, the magnitude of charge transfer is higher for larger AuNPs-graphene composite when compared with smaller AuNPs-graphene composite. The charge transfer trend and the trends seen in the number of Au atoms directly in touch with graphene are identical. Hence, our results conclude that, quantity of Au atoms directly in contact with graphene during aggregation is primarily facilitates charge transfer between AuNPs and graphene. Our results on the size dependent strain and charge transfer characteristics of AuNPs will aid in the development of AuNPs-graphene composites for sensor applications.

Morphological transformation of gold nanoparticles on graphene oxide: effects of capping ligands and surface interactions

This article presents the influence of capping ligand and surface interaction types on the coarsening or reshaping behavior of surface-immobilized gold nanoparticles with different core size and shape. The morphological transformation of gold nanoparticles and nanorods on graphene oxide upon heating at temperatures ranging from 50 to 200 °C was investigated. The aggregation and coarsening behaviors of spherical nanoparticles on graphene oxide were slightly affected by the core size of nanoparticles (~ 1, 3, and 10 nm). The comparison of two different surface ligands revealed that glutathione ligands provide much better protection than cetyltrimethylammonium bromide ligands against the morphological transformation of nanoparticles. In addition, the evaluation of surface binding interactions indicated that the attachment of nanoparticles and nanorods onto graphene oxide with additional thiol functional groups could improve the immobilization of particles and therefore decelerate coarsening and reshaping of nanoparticle and nanorods.

Graphene oxide-promoted reshaping and coarsening of gold nanorods and nanoparticles

This paper describes thermally induced reshaping and coarsening behaviors of gold nanorods and nanoparticles immobilized on the surface of graphene oxide. Cetyltrimethylammonium bromide-stabilized gold nanorods with an aspect ratio of ∼3.5 (54:15 nm) and glutathione-capped gold nanoparticles with an average core size of ∼3 nm were synthesized and self-assembled onto the surface of graphene oxide. The hybrid materials were then heated at different temperatures ranging from 50 to 300 °C. The effects of heat treatments were monitored using UV-vis spectroscopy and transmission electron microscopy (TEM). These results were directly compared with those of heat-treated free-standing gold nanorods and nanoparticles without graphene oxide to understand the heat-induced morphological changes of the nanohybrids. The obtained results showed that the gold nanorods would undergo a complete reshaping to spherical particles at the temperature of 50 °C when they are assembled on graphene oxide. In comparison, the complete reshaping of free-standing gold nanorods to spherical particles would ultimately require a heating of the samples at 200 °C. In addition, the spherical gold nanoparticles immobilized on graphene oxide would undergo a rapid coarsening at the temperature of 100-150 °C, which was lower than the temperature (150-200 °C) required for visible coarsening of free-standing gold nanoparticles. The results indicated that graphene oxide facilitates the reshaping and coarsening of gold nanorods and nanoparticles, respectively, during the heat treatments. The stripping and spillover of stabilizing ligands promoted by graphene oxide are proposed to be the main mechanism for the enhancements in the heat-induced transformations of nanohybrids.

Highly sensitive hydrogen sulfide (H₂S) gas sensors from viral-templated nanocrystalline gold nanowires

A facile, site-specific viral-templated assembly method was used to fabricate sensitive hydrogen sulfide (H2S) gas sensors at room temperature. A gold-binding M13 bacteriophage served to organize gold nanoparticles into linear arrays which were used as seeds for subsequent nanowire formation through electroless deposition. Nanowire widths and densities within the sensors were modified by electroless deposition time and phage concentration, respectively, to tune device resistance. Chemiresistive H2S gas sensors with superior room temperature sensing performance were produced with sensitivity of 654%/ppm(v), theoretical lowest detection limit of 2 ppb(v), and 70% recovery within 9 min for 0.025 ppm(v). The role of the viral template and associated gold-binding peptide was elucidated by removing organics using a short O₂ plasma treatment followed by an ethanol dip. The template and gold-binding peptide were crucial to electrical and sensor performance. Without surface organics, the resistance fell by several orders of magnitude, the sensitivity dropped by more than a factor of 100 to 6%/ppm(v), the lower limit of detection increased, and no recovery was detected with dry air flow. Viral templates provide a novel, alternative fabrication route for highly sensitive, nanostructured H2S gas sensors.

Heat-induced coarsening of layer-by-layer assembled mixed Au and Pd nanoparticles

This article shows the coarsening behavior of nanoparticle multilayers during heat treatments which produce larger metallic nanostructures with varying shapes and sizes on glass slides. Nanoparticle multilayer films are initially constructed via the layer-by-layer self-assembly of small and monodispersed gold and/or palladium nanoparticles with different compositions (gold only, palladium only, or both gold and palladium) and assembly orders (compounding layers of gold layers over palladium layers or vice versa). Upon heating the slides at 600°C, the surface nanoparticles undergo coalescence becoming larger nanostructured metallic films. UV-Vis results show a clear reliance of the layering sequence on the optical properties of these metal films, which demonstrates an importance of the outmost (top) layers in each nanoparticle multilayer films. Topographic surface features show that the heat treatments of nanoparticle multilayer films result in the nucleation of nanoparticles and the formation of metallic cluster structures. The results confirm that different composition and layering sequence of nanoparticle multilayer films clearly affect the coalescence behavior of nanoparticles during heat treatments.

Organ weaving: woven threads and sheets as a step towards a new strategy for artificial organ development

The concept of "organ weaving" is presented, a fabrication technique that can be an attractive option for the development of artificial tissues and organs. "Living threads" are created by immersing threads that are soaked in a CaCl(2) solution into a sodium-alginate-loaded cell suspension bath, encapsulating the cells and creating a bio-friendly, easily manageable starting material for building up larger scaffold structures. Such living threads have the advantage of being a particularly mild culturing medium for mammalian cells, protecting the cells during subsequent processing steps from dehydration and other rapid changes in the chemistry of the surrounding environment. Connecting different types of threads into 3D objects gives unique opportunities to address tissue engineering challenges.

Stability and Morphology of Gold Nanoisland Arrays Generated from Layer-by-Layer Assembled Nanoparticle Multilayer Films: Effects of Heating Temperature and Particle Size

This article reports the effects of heating temperature and composition of nanoparticle multilayer films on the morphology, stability, and optical property of gold nanoisland films prepared by nanoparticle self-assembly/heating method. First, nanoparticle-polymer multilayer films are prepared by the layer-by-layer assembly. Nanoparticle multilayer films are then heated at temperature ranging from 500 °C to 625 °C in air to induce an evaporation of organic matters from the films. During the heating process, the nanoparticles on the solid surface undergo coalescence, resulting in the formation of nanostructured gold island arrays. Characterization of nanoisland films using atomic force microscopy and UV-vis spectroscopy suggests that the morphology and stability of gold island films change when different heating temperatures are applied. Stable gold nanoisland thin film arrays can only be obtained after heat treatments at or above 575 °C. In addition, the results show that the use of nanoparticles with different sizes produces nanoisland films with different morphologies. Multilayer films containing smaller gold nanoparticles tend to produce more monodisperse and smaller island nanostructures. Other variables such as capping ligands around nanoparticles and molecular weight of polymer linkers are found to have only minimal effects on the structure of island films. The adsorption of streptavidin on the biotin-functionalized nanoisland films is studied for examining the biosensing capability of nanoisland arrays.