Metallic core-shell nanoparticles for conductive coatings and printing

The review is focused on bimetallic nanoparticles composed of a core formed by low-cost metal having high electrical conductivity, such as Cu and Ni, and a protective shell composed of stable to oxidation noble metal such as Ag or Au. We present the chemical and physical approaches for synthesis of such particles, as well as the combination of the two, the stability to oxidation of core-shell nanoparticles at various conditions, and the formulation of conductive compositions and their application in conductive coatings and printed electronics.

Effect of Oxalic Acid Treatment on Conductive Coatings Formed by Ni@Ag Core-Shell Nanoparticles

Low-cost metallic nanoink based on nickel-silver core-shell nanoparticles (Ni@Ag NPs) was used for the formation of conductive metallic coatings with low sintering temperature, which can be successfully applied for replacement of currently used silver-based nanoinks in printed electronics. The effect of oxalic acid (OA) on the sintering temperature and conductivity of coatings formed by Ni@Ag NPs was evaluated. It was found that the addition of OA to the ink formulation and post-printing treatment of deposited films with this acid provided a noticeable decrease in the sintering temperature required for obtaining conductive patterns that is especially important for utilizing the polymeric substrates. The obtained resistivity of metallic coatings after sintering at temperature as low as 100 °C was found to be 30 µΩ·cm, only ~4 times higher compared to the resistivity of bulk Ni that is promising for future application of such materials for fabrication of low-cost flexible printed patterns.

3D Printing Materials for Soft Robotics

Soft robotics is a growing field of research, focusing on constructing motor-less robots from highly compliant materials, some are similar to those found in living organisms. Soft robotics has a high potential for applications in various fields such as soft grippers, actuators, and biomedical devices. 3D printing of soft robotics presents a novel and promising approach to form objects with complex structures, directly from a digital design. Here, recent developments in the field of materials for 3D printing of soft robotics are summarized, including high-performance flexible and stretchable materials, hydrogels, self-healing materials, and shape memory polymers, as well as fabrication of all-printed robots (multi-material printing, embedded electronics, untethered and autonomous robotics). The current challenges in the fabrication of 3D printed soft robotics, including the materials available and printing abilities, are presented and the recent activities addressing these challenges are also surveyed.

Polydispersity vs. Monodispersity. How the Properties of Ni-Ag Core-Shell Nanoparticles Affect the Conductivity of Ink Coatings

The effect of polydispersity of nickel-silver core-shell nanoparticles (Ni-Ag NPs) on the conductivity of ink coatings was studied. Ni-Ag NPs of various average diameters (100, 220, and 420 nm) were synthesized and utilized for the preparation of conductive inks composed of monodisperse NPs and their polydisperse mixtures. The shell thickness of synthesized Ni-Ag NPs was found to be in the range of 10–20 nm and to provide stability of a core metal to oxidation for at least 6 months. The conductivity of metallic films formed by inks with monodisperse Ni-Ag NPs was compared with those formed by polydisperse inks. In all cases, the optimal conditions for the formation of conductive patterns (weight ratio of monodisperse NPs for polydisperse composition, the concentration of the wetting agent, sintering temperature, and duration) were determined. It was found that metallic films formed by polydisperse ink containing 100, 220, and 420 nm Ni-Ag NPs with a mass ratio of 1:1.5:0.5, respectively, are characterized by the lowest resistivity, 10.9 µΩ·cm, after their thermal post-coating sintering at 300 °C for 30 min that is only 1.6 higher than that of bulk nickel.

Conductive nanomaterials for 2D and 3D printed flexible electronics

This review describes recent developments in the field of conductive nanomaterials and their application in 2D and 3D printed flexible electronics, with particular emphasis on inks based on metal nanoparticles and nanowires, carbon nanotubes, and graphene sheets. We present the basic properties of these nanomaterials, their stabilization in dispersions, formulation of conductive inks and formation of conductive patterns on flexible substrates (polymers, paper, textile) by using various printing technologies and post-printing processes. Applications of conductive nanomaterials for fabrication of various 2D and 3D electronic devices are also briefly discussed.

Additive Manufacturing of Transparent Silica Glass from Solutions

A sol, aqueous solution-based ink is presented for fabrication of 3D transparent silica glass objects with complex geometries, by a simple 3D printing process conducted at room temperature. The ink combines a hybrid ceramic precursor that can undergo both the photopolymerization reaction and a sol-gel process, both in the solution form, without any particles. The printing is conducted by localized photopolymerization with the use of a low-cost 3D printer. Following printing, upon aging and densifying, the resulting objects convert from a gel to a xerogel and then to a fused silica. The printed objects, which are composed of fused silica, are transparent and have tunable density and refractive indices.

Evaporation of Nanosuspensions on Substrates with Different Hydrophobicity

Liquid drop evaporation on surfaces is present in many industrial and medical applications, e.g., printed electronics, spraying of pesticides, DNA mapping, etc. Despite this strong interest, a theoretical description of the dynamic of the evaporation of complex liquid mixtures and nanosuspensions is still lacking. Indeed, one of the aspects that have not been included in the current theoretical descriptions is the competition between the kinetics of evaporation and the adsorption of surfactants and/or particles at the liquid/vapor and liquid/solid interfaces. Materials formed by an electrically isolating solid on which a patterned conducting layer was formed by the deposits left after drop evaporation have been considered as very promising for building electrical circuits on flexible plastic substrates. In this work, we have done an exhaustive study of the evaporation of nanosuspensions of latex and hydrophobized silver nanoparticles on four substrates of different hydrophobicity. The advancing and receding contact angles as well as the time dependence of the volume of the droplets have been measured over a broad range of particle concentrations. Also, mixtures of silver particles and a surfactant, commonly used in industrial printing, have been examined. Furthermore, the adsorption kinetics at both the air/liquid and solid/liquid interfaces have been measured. Whereas the latex particles do not adsorb at the solid/liquid and only slightly reduce the surface tension, the silver particles strongly adsorb at both interfaces. The experimental results of the evaporation process were compared with the predictions of the theory of Semenov et al. (Evaporation of Sessile Water Droplets: Universal Behavior in the Presence of Contact Angle Hysteresis. Colloids Surf. Physicochem. Eng. Asp. 2011, 391 (1-3), 135-144) and showed surprisingly good agreement despite that the theory was developed for pure liquids. The morphology of the deposits left by the droplets after total evaporation was studied by scanning electronic microscopy, and the effects of the substrate, the particle nature, and their concentrations on these patterns are discussed.

Plasma-Induced Decomposition of Copper Complex Ink for the Formation of Highly Conductive Copper Tracks on Heat-Sensitive Substrates

The use of Cu-formate-2-amino-2-methyl-1-propanol ink and low-pressure plasma for the formation of highly conductive patterns on heat sensitive plastic substrates was studied. It was found that plasma results in decomposition of copper complex to form metallic copper without heating at high temperatures. Ink composition and plasma parameters (predrying conditions, plasma treatment duration, gas type, and flow rate) were optimized to obtain uniform conductive metallic films. The morphology and electrical characteristics of these films were evaluated. Exposing the printed copper metallo-organic decomposition (MOD) ink to 160 W plasma for 8 min yielded resistivity as low as 7.3 ± 0.2 μΩ cm, which corresponds to 23% bulk copper conductivity. These results demonstrate the applicability of MOD inks and plasma treatment to obtain highly conductive printed patterns on low-cost plastic substrates and 3D printed polymers.

Merging of metal nanoparticles driven by selective wettability of silver nanostructures

The welding and sintering of nanomaterials is relevant, for example, to form electrical contacts between metallic particles in printed electronic devices. Usually the welding of nanoparticles is achieved at high temperatures. Here we find that merging of two different metals, silver and gold nanoparticles, occurs on contact at room temperature. The merging process was investigated by experimental and molecular dynamics simulations. We discovered that the merging of these particles is driven by selective wettability of silver nanoparticles, independent of their size and shape (spheres or rods); silver behaves as a soft matter, whereas gold as a hard surface being wetted and retaining its original morphology. During that process, the silver atoms move towards the surface of the Au nanoparticles and wrap the Au nanoparticles in a pulling up-like process, leading to the wetting of Au nanoparticles.

Conductive nanomaterials for printed electronics

This is a review on recent developments in the field of conductive nanomaterials and their application in printed electronics, with particular emphasis on inkjet printing of ink formulations based on metal nanoparticles, carbon nanotubes, and graphene sheets. The review describes the basic properties of conductive nanomaterials suitable for printed electronics (metal nanoparticles, carbon nanotubes, and graphene), their stabilization in dispersions, formulations of conductive inks, and obtaining conductive patterns by using various sintering methods. Applications of conductive nanomaterials for electronic devices (transparent electrodes, metallization of solar cells, RFID antennas, TFTs, and light emitting devices) are also briefly reviewed.

Transparent conductors composed of nanomaterials

This is a review on recent developments in the field of transparent conductive coatings (TCCs) for ITO replacement. The review describes the basic properties of conductive nanomaterials suitable for fabrication of such TCCs (metallic nanoparticles and nanowires, carbon nanotubes and graphene sheets), various methods of patterning the metal nanoparticles with formation of conductive transparent metallic grids, honeycomb structures and 2D arrays of interconnected rings as well as fabrication of TCCs based on graphene and carbon nanotubes. Applications of TCCs in electronic and optoelectronic devices, such as solar cells, electroluminescent and electrochromic devices, touch screens and displays, and transparent EMI shielders, are discussed.

Nanostructured electrochromic films by inkjet printing on large area and flexible transparent silver electrodes

Printed electrochromic flexible films were obtained by combining transparent silver grid electrodes formed by self-assembly and inkjet printed WO3 nanoparticles. Concentrated dispersions of WO3 nanoparticles were inkjet printed on transparent plastic silver grid electrodes with a high transparency of 83% in the spectral range of 400-800 nm, and a low sheet resistance in the range of 1-5 Ω sq(-1). These electrodes were used for electrochromic applications for the first time. The resultant patterned nanostructured electrochromic films maintained their coloring and bleaching performance after bending of the flexible films.

Study of expression of TLR2, TLR4 and transckription factor NF-kB structures of galt of rats in the conditions of the chronic social stress and modulation of structure of intestinal microflora

The present study was conducted to investigate of the influence of chronic social stress (CSS) and modulation of the composition of intestinal microflora on the distribution of TLR2+-, TLR4+- and Nf-kB+-cells in the GALT of ileum of the rats. Researchers have been conducted on 84 rats (female) of Wistar line, which were divided on 7 experimental groups: control rats (group 1); rats, which were modeled CSS1 by means of three weeks social isolation and prolong psychoemotional influence (group2); rats, which having CSS 2 modeling by means of keeping animals in over populated cages with every day change of grouping (group 3); rats with CSS1 and CSS2, which were made the modeling of intestinal microflora by means of administrations of aminoglycosed antibiotic kanamycin (group 4 and 5, accordingly); rats with CSS1 and CSS2, which were made the modeling of intestinal microflora by means of everyday administrations of lactobacterine (groups 6 and 7, accordingly). Structure of population of TLR2+-, TLR4+- and Nf-kB+-cells has been studied by the analysis of serial histological sections using the method of direct and indirect immunofluorescense with monoclonal antibodies to TLR2, TLR4 and Nf-kB. CSS development is accompanied with increase in total lymphocytes expressing TLR2 and 4 type GALT rats with the most pronounced in LFV (TLR2+-lymphocytes) and PP LFs (TLR4+-cells) led to an increase in the number of Nf-kB+- cells: in LFV a 1.8-2 fold (p<0.05) in PP at the sub - 52-91% (p<0.05) in PP LFs - for 89-92% (p<0.05), and it is also influenced on the density of TLR2, TLR4, and the concentration of Nf-kB in immunopositive cells. AB and PB injections were accompanied by a decrease in the number of studied cells, so in the separate zone GALT is increased to their number, changing the density of immune system receptors.

Plasma and microwave flash sintering of a tailored silver nanoparticle ink, yielding 60% bulk conductivity on cost-effective polymer foils

A combination of plasma and microwave flash sintering is used to sinter an inkjet-printed and tailored silver nanoparticle formulation. By using two sintering techniques sequentially, the obtained conductivity is 60%, while keeping the processing temperature well below the glass transition temperature (T(g)) of the used polymer substrate. This approach leads to highly conductive features on cost-effective polymer substrates in relatively short times, which are compatible with roll-to-roll (R2R) production. An electroluminescence device is prepared as an example.

Conductive inks with a "built-in" mechanism that enables sintering at room temperature

At present there is no metallic ink that enables formation of conductive patterns at room temperature by a single printing step. Printing conductive features by metallic nanoparticle-based inks must be followed by sintering while heating to elevated temperatures, thus preventing their utilization on most plastic substrates used in plastic electronics. In this report we present a new silver nanoparticle-based conductive ink, having a built-in sintering mechanism, which is triggered during drying of the printed pattern. The nanoparticles that are stabilized by a polymer undergo self-sintering spontaneously, due to the presence of a destabilizing agent, which comes into action only during drying of the printed pattern. The destabilizing agent, which contains Cl(-) ions, causes detachment of the anchoring groups of the stabilizer from the nanoparticles' surface and thus enables their coalescence and sintering. It was found that the new metallic ink leads to very high conductivities, by a single printing step: up to 41% of the conductivity of bulk silver was achieved, the highest reported conductivity of a printed pattern that is obtained from nanoparticles at room temperature.

Triggering the sintering of silver nanoparticles at room temperature

A new approach to achieve coalescence and sintering of metallic nanoparticles at room temperature is presented. It was discovered that silver nanoparticles behave as soft particles when they come into contact with oppositely charged polyelectrolytes and undergo a spontaneous coalescence process, even without heating. Utilizing this finding in printing conductive patterns, which are composed of silver nanoparticles, enables achieving high conductivities even at room temperature. Due to the sintering of nanoparticles at room temperature, the formation of conductive patterns on plastic substrates and even on paper is made possible. The resulting high conductivity, 20% of that for bulk silver, enabled fabrication of various devices as demonstrated by inkjet printing of a plastic electroluminescent device.

Coalescence of silver nanoparticles at room temperature: unusual crystal structure transformation and dendrite formation induced by self-assembly

It was found that during the evaporation of water from a droplet of a silver nanoparticles dispersion a self-assembly process leads to the coalescence of the nanoparticles at room temperature and eventually results in a 3D, micrometer-sized dendrite. Direct in situ HR-TEM observation of coalescence events of individual nanoparticles revealed that during this process a transformation of the nanoparticles' crystal structure takes place, from the common fcc silver structure to the unusual hcp structure. It was found that even-though a majority of the nanoparticles in the dispersion have the fcc structure the obtained dendrites are characterized by the hcp structure, reflecting the crystal structure transformation due to the coalescence process.

Polymer-surfactant interactions: binding mechanism of sodium dodecyl sulfate to poly(diallyldimethylammonium chloride)

The binding mechanism of poly(diallyldimethylammonium chloride), PDAC, and sodium dodecyl sulfate, SDS, has been comprehensively studied by combining binding isotherms data with microcalorimetry, zeta potential, and conductivity measurements, as well as ab initio quantum mechanical calculations. The obtained results demonstrate that surfactant-polymer interaction is governed by both electrostatic and hydrophobic interactions, and is cooperative in the presence of salt. This binding results in the formation of nanoparticles, which are positively or negatively charged depending on the molar ratio of surfactant to PDAC monomeric units. From microcalorimetry data it was concluded that the exothermic character of the interaction diminishes with the increase in the surfactant/polymer ratio as well as with an increase in electrolyte concentration.

Ring stain effect at room temperature in silver nanoparticles yields high electrical conductivity

We demonstrate that metallic rings formed spontaneously at room temperature via evaporation of aqueous drops containing silver nanoparticles (20-30 nm in diameter) exhibit high electrical conductivity (up to 15% of that for bulk silver). The mechanism underlying this self-assembly phenomena is the "ring stain effect", where self-pinning is combined with capillary flow to form a ring consisting of close-packed metallic nanoparticles along the perimeter of a drying droplet. Our macroscopic and microscopic (applying conductive atomic force microscopy) transport measurements show that the conductivity of the ring, which has a metallic brightness, is orders of magnitude larger than that of corresponding aggregates developed without the ring formation, where high conductivity is known to appear only after annealing at high temperature.

Inorganic polysulfides' quantitation by methyl iodide derivatization: dimethylpolysulfide formation potential

Inorganic polysulfides are important intermediates in the formation of dimethylpolysulfides and possibly other volatile sulfur compounds of environmental significance. Currently, direct determination of these ions in the concentration range of natural systems is practically impossible, particularly under oxic conditions. Polysulfide quantification by derivatization with methyl iodide or d6-methyl iodide is emerging as a valuable alternative method for studies of polysulfide formation in natural systems. This manuscript presents detailed studies aimed at the evaluation of this method. We determined the conversion of the inorganic polysulfides to dimethylpolysulfides by methylation with methyl iodide. Close to 100 per cent of the molar concentration of polysulfide salts were converted to organic polysulfides for very low concentrations of dissolved polysulfide solutions, but only a small recovery was obtained for high concentrations of polysulfide precursors or when the solubility limit was exceeded. The recovery of polysulfides based on the calculated dissolved polysulfide concentration exceeds 1,000 per cent for very low dissolved concentrations of polysulfides. This unexpected dependence is attributed to continuous inorganic polysulfide formation from hydrogen sulfide and sulfur precipitate concurrent with, and in fact driven by, the methylation process.

Formation of carbonyl sulfide by the reaction of carbon monoxide and inorganic polysulfides

OCS formation by the reaction of inorganic polysulfides with carbon monoxide, which are both abundant in natural aquatic systems, was studied as a model abiotic route for OCS formation in the dark. The net OCS accumulation rate was a function of a bimolecular formation reaction and simultaneous OCS hydrolysis kinetics. The reaction of polysulfides with CO in the dark was found to be first order with respect to CO concentration and first order with respect to the molar sum of the polysulfide species generated by the disproportionation of the dissolved polysulfide precursors. The pH dependence of the OCS production rate was controlled by the pH-dependent disproportionation of polysulfide precursors. Lower temperatures, intermediate redox potentials, and moderately basic pH conditions increase the steady-state concentration of OCS. The speciation of polysulfides in aqueous solutions is still disputed. Some authors claim that hexasulfide is one of the dominant species while others believe that pentasulfide is the largest sulfide species in aqueous systems. Despite the disagreement between different models for speciation of polysulfides, the proposed rate law agreed very well with the thermodynamic data based on four and on five polysulfide species, with only minor differences in the preexponential kinetic coefficients.

Water-soaked evidence: detectability of explosive traces after immersion in water

Various factors governing the detectability of explosive traces after being soaked in water were studied. The variables are: the type of the surface (surfaces liable to be found in aircraft were chosen), the type of explosive, the type of water (tap or seawater), and movement of the immersed surface in the water. The maximal immersion times (tmax) after which explosive detection was possible were evaluated. This datum was found to depend on the type of explosive (one of the important factors is solubility in water), the surface material and the environmental conditions (tap or seawater movement). Detection of PETN on high-density polyethylene, linoleum, glass and aluminum, by the chemical Explosive Testing Kit (ETK), was possible even after a month of soaking in seawater. In addition, it has been found that movement of bulk water around the samples with deposited explosives considerably decreases tmax values. It is, therefore, recommended to retrieve samples for explosive analysis as soon as possible and in areas where the currents of water is minimal.

Native and Hydrophobically Modified Human Immunoglobulin G at the Air/Water Interface

The adsorption of human immunoglobulin G (IgG) at the air/water interface was monitored both by the in situ radiotracer technique using [(14)C] labeled IgG and by surface tension measurements. The results reveal that adsorption of IgG from single protein systems displays bimodality due to molecular rearrangements at the interface. Above the threshold value of 1.5x10(-2) mg/ml solution concentration, adsorbed IgG molecules reoriented from the side-on to the end-on configuration. The existence of a lag time which did not appear in Gamma=f(t) curves, was observed in Pi=f(t) relationships at low protein concentrations and was due to the limits of the surface pressure technique to detect protein adsorption. The adsorption of native IgG was also carried out in the presence of a hydrophobized IgG obtained by grafting capryloyl residues to its lysine groups by reaction with N-hydroxysuccinimide ester of caprylic acid, which yielded 19 covalently bound alkyl chains to the IgG molecule (19C(8)-IgG). This modified IgG exhibited enhanced adsorption at the air/water interface, as manifested by its increased adsorption efficiency relative to the native protein. Sequential and competitive adsorption experiments from binary mixtures of native IgG and 19C(8)-IgG clearly demonstrate that the displacement of the native protein from the air/water interface strongly depended on the manner of how 19C(8)-IgG and native IgG competed with each other. When the two proteins competed simultaneously, 19C(8)-IgG predominantly occupied the available area but when native IgG was adsorbed first, for 2 h, the sequentially adsorbed 19C(8)-IgG was incapable of substantially displacing it from the interface. Copyright 2001 Academic Press.

Chemically Modified Glucose Oxidase with Enhanced Hydrophobicity: Adsorption at Polystyrene, Silica, and Silica Coated by Lipid Monolayers

Covalent modification of glucose oxidase from Aspergillus niger by the palmitic acid ester of N-hydroxysuccinimide at a molar ratio ester:protein of 56:1 results in the formation of the enzyme derivative with 11 attached palmitic chains. Surface hydrophobicity measurements by a fluorescent probe, 8-anilino-1-naphthalenesulfonate, indicate a drastic increase in the hydrophobicity index of glucose oxidase after such a modification. The modified glucose oxidase displays a much higher adsorption affinity for hydrophilic (silica) as well as for hydrophobic (silica coated by phosphatidyl choline and cholesterol monolayers and polystyrene latex beads) surfaces, and forms more compact surface layers compared to the native glucose oxidase. Such a difference results from a spontaneous formation of micelle-like aggregates (clusters) of the hydrophobized enzyme molecules (average size 500 nm), which come into contact with a surface. A possible structure of the glucose oxidase surface layers and the nature of the forces determining the adsorption of the enzyme on various adsorbents are discussed. Copyright 1999 Academic Press.

Chemically Modified Human Immunoglobulin G: Hydrophobicity and Surface Activity at Air/Solution Interface

Covalent modification of human IgG by fatty acid esters (C8 and C16) of N-hydroxysuccinimide was carried out. Surface hydrophobicity measurements, using the fluorescent probe 8-anilino-1-naphthalenesulfonate, indicate an increase in the surface protein hydrophobicity with an increase in the number and in the length of the attached alkyl chains. The modified IgGs decrease surface tension at the air/solution interface more effectively than the native protein. The values of the molecular cross-sectional areas (DeltaA) estimated from the kinetic data are in the range of 100-300 Å2 and reflect the size of protein segments at the interface during the adsorption process. About 40-50% increase in the DeltaA was observed upon attachment of the C8 groups to the native IgG. The lengthening of the bound alkyl chain from C8 to C16 results in a further increase in this value. The influence of the overall IgG hydrophobicity and the length of the attached alkyl chain on the dimensions of the mobile protein segment at the surface are discussed. Copyright 1999 Academic Press.

Penetration of Glucose Oxidase and of the Hydrophobically Modified Enzyme into Phospholipid and Cholesterol Monolayers

The penetrant ability of the native glucose oxidase, GOx, and of the hydrophobically modified enzyme GO(mod) realized by grafting to its lysine residues alkyl C16 chains, into phosphatidylcholine dibehenoyl (DBPC), phosphatidylcholine dipalmitoyl (DPPC), phosphatidyl-ethanolamine dipalmitoyl (DPPE), phosphatidyl-serine dipalmitoyl (DPPS), and cholesterol (CHOL) monolayers was assessed by surface pressure measurements at constant area by enzyme injection to the aqueous phase beneath spread monolayers. As revealed by the magnitude of surface pressure increments (DeltaPi), both the quantities and the rates of penetration of the enzymes into these monolayers were lipid chemical nature and enzyme concentration dependent. When compared with GOx, GO(mod) displayed an enhanced penetrant ability into all the studied monolayers that resulted in rapidly attained DeltaPi plateau values, characteristic of stable systems. The influence of lipid hydrocarbon chain length and of the polar headgroup charge on the efficiency and effectiveness of GOx and GO(mod) penetration into these monolayers is discussed. Copyright 1999 Academic Press.

Adsorption of Hydrophobized Glucose Oxidase at Solution/Air Interface

The modification of glucose oxidase by palmitic acid ester of N -hydroxysuccinimide leads to the formation of a new hydrophobized enzyme with five covalently bound C16 groups. Such a modification was shown not to alter noticeably the native structure of the enzyme. The modified glucose oxidase displays enhanced surface activity at the water/air interface in comparison with the native enzyme. The maximum reduction of surface tension at all concentrations studied was higher for the modified glucose oxidase than for the native one. The modified enzyme also displayed a much steeper rise of the surface potential with time and a much more rapid attainment of the saturation plateau than the unmodified enzyme.