A. V. Zaitseva

Percolation Transitions in Composites Formed by the Evaporation of Droplets of Silver Nanoparticle Dispersions

Conducting properties are studied for deposits that result from evaporating droplets of dispersions of silver nanoparticles with average diameters of 6.2 and 11.9 nm. The deposits represent two ring-shaped structures with diameters of several dozen and several hundred micrometers, which are formed along the perimeter of an evaporating droplet. It is shown that only the external ring-shaped deposit is an electric conductor. The dependence of its conductivity on the content of metal precursor (AgNO3) in a solution used to synthesize nanoparticles appears to be similar to a percolation transition. Mechanisms of charge transfer in the examined ring-shaped deposit are discussed assuming that it represents a metal-polymer composite.

Evaporation of droplets of silver nanoparticle dispersions on metal surfaces

The formation of ring-shaped deposits during the evaporation of droplets of silver nanoparticle dispersions on aluminum, copper, and nickel substrates has been studied, and the geometric characteristics and conductivity of the deposits have been determined. The formation process of the deposits on the above substrates has been shown to exhibit some peculiarities to compare with that on hydrophilic glass substrates. These peculiarities lead to substantial differences in the geometric parameters and structure of the deposits formed on substrates of different natures. Therewith, the qualitative regularities of variations in the geometric parameters and conductivity of the deposits with the size (numerical concentration) of silver nanoparticles remain preserved.

Metal nanoparticles on polymer surfaces: 6. Probing of non-glassy polystyrene surface layer

Peculiarities of the state of the surface layer of the amorphous glassy polymer polystyrene are studied with a specially developed experimental approach. The essence of the method consists in the observation via atomic force microscope for the depth and rate of embedding of gold nanoparticles in a polymer after their preliminary adsorption on the polymer surface from hydrosol. It is shown that the polymer glass-transition temperature near the boundary with air is substantially lowered relative to its bulk value. “Equilibrium” thickness of the non-glassy (“melted”) surface layer is determined through analysis of the data on the kinetics of nanoparticle embedding, and it is revealed that the layer thickness increases with temperature, reaching, near the “bulk” glass-transition temperature, the magnitude that is close to the diameter of the macromolecular coil. The results obtained are analyzed with allowance for published data, and the semi-empirical formula describing variations in the thickness of the non-glassy surface layer as a function of temperature in the interval between the “surface” and “bulk” glass-transition temperatures of a polymer is proposed.

A new method for loading mesoporous silica nanoparticles with drugs: Sol–gel synthesis using drug micelles as a template

It has been shown that mesoporous nanocontainers from SiO2 may be obtained by the sol–gel synthesis using drug (Miramistin) micelles as a template. The nanocontainers resulting from the combination of the stages of their synthesis and loading are characterized by a very high content of the drug (no less than 0.9 g per 1 g of SiO2). The kinetics of Miramistin desorption from the mesoporous particles into an aqueous medium has been studied under static and quasi-dynamic conditions. The desorption has been shown to rather strongly depend on pH. Possible mechanisms of the desorption process have been discussed.

The Formation of Nanocomposites as a Result of Evaporating Droplets of Dispersions of Silver Nanoparticles in Low-Volatile and Binary Dispersion Media

The regularities of the formation of nanocomposites in the form of ring-shaped deposits during the evaporation of dispersions of cube-shaped silver nanoparticles with an average size of 70 nm on hydrophilic (glass) and hydrophobic (copper) substrates have been studied. The nanoparticles have been synthesized by the polyol method in an ethylene glycol solution. In addition, droplets of dispersions of the same nanoparticles in an ethylene glycol/ethanol mixture and water have been experimentally studied. Geometric parameters and conductivity of the formed ring-shaped deposits have been determined.

Nucleation and Growth of Gold Nanoparticles on Adsorption Layers and in Ultrathin Films of Poly(2-vinylpyridine)

Peculiarities of the nucleation and growth of gold nanoparticles on adsorption layers and in ultrathin films of poly(2-vinylpyridine) (PVP) in the chemical reduction of sorbed Au(III) ions and the consequent thermal treatment of systems are studied by X-ray photoelectron spectroscopy and optical spectroscopy. It is shown that nitrogens of PVP pyridine groups coordinate gold atoms. It is revealed that, even at relatively short contact between PVP film saturated with chloroauric acid and the solution of strong reductant NaBH4, Au(III) ions are reduced to metal. As a result, quasi-metal gold particles are formed. At the same time, when exposing a PVP-Au system to the solution of weak reductant NH2OH, the process of reduction proceeds in several stages. First, Au(III) ions are reduced to Au(I) followed by the reduction to Au(0) as a result of disproportionation reaction. It is demonstrated for the first time that, upon using NH2OH, the rate of reduction, as well as the structure of prepared PVP-Au nanocomposite films depends to substantial extent on solution pH. Prolonged annealing of ultrathin nanocomposites at 150 °C, i.e., above the glass transition temperature of polymer matrix, leads to an increase in the sizes of metal particles and the formation of systems characterized by intense absorption within the 500–600-nm range due to the localized plasmon resonance of gold nanoparticles.

New two-dimensional polymer-metal composites based on highly ordered ensembles of nanoparticles. Design and optical properties

A new method has been proposed for designing composite materials that represent highly ordered two-dimensional metal nanoparticles ensembles having variable geometric parameters and being embedded into the surface layer of a polymer matrix to a predetermined depth. The method is based on diblock copolymer micellar lithography and the effect of decreased glass-transition temperature of a polymer surface as compared with its bulk value. The possibility of independent variations in the depth of embedding of metal nanoparticles (by system annealing) and their size (by seeded growth) has resulted in the systematic study of the influence of the temperature and time of annealing on the kinetics of nanoparticle embedding into a polymer. For the first time, the plasmon-resonant properties of hexagonally ordered ensembles of gold nanoparticles located at a polymer-air interface have been experimentally studied. It has been established that the embedding of an ensemble of nanoparticles into a polymer is accompanied by a bathochromic shift of the maximum of its localized surface plasmon resonance due to a growth in the effective dielectric permittivity of the environment of the nanoparticles. An empirical equation has been proposed, which satisfactorily describes these experimental data.

Macroscopic deformation of a substrate as a new method for controlling geometry and plasmon-resonant properties of highly ordered planar ensembles of metal nanoparticles

A new approach to creating highly ordered two-dimensional ensembles of nanoparticles with variable geometric parameters is proposed. It combines diblock copolymer micellar lithography and controlled deformation of a polymer substrate. The key feature of the approach is the formation of a monolayer of hexagonally packed metal precursor-containing micelles of an amphiphilic diblock copolymer on the surface of an isotropically stretched polymer plate. The average distance between micelle centers is 140 nm. Subsequent thermal treatment (or isotropic stretching) of the sample results in the shrinkage (or elongation) of the substrate, which enables one to vary the distance between micelle centers in a range of 80–200 nm while retaining hexagonal packing of the micelles in the monolayer. At the final stage, ensembles of hexagonally ordered gold nanoparticles are obtained by exposing the micellar films to air plasma. It is demonstrated that gold nanoparticles in these ensembles can be enlarged by seeded growth. The systematic study of the plasmon-resonant properties of the resulting ensembles shows that the gradual increase in the distance between 35-nm gold particles from 80 to 200 nm leads to an unexpected nonmonotonic shift of the maximum of localized surface plasmon resonance, which is, from our point of view, caused by the high degree of organization of nanoparticles on the substrate.

Diblock Copolymer Micellar Lithography: 1. Intermicellar Interactions and Pathways for Control of Monomicellar Film Structure

The formation of reverse micelles of amphiphilic diblock copolymers of styrene and 2-vinylpyridine in selective (for one of the blocks) solvent (toluene) is studied by dynamic light scattering and atomic force and transmission electron microscopies, as well as by absorption spectroscopy and X-ray photoelectron spectroscopy techniques. It is revealed that the behavior of micelles of block copolymers with different ratios of block lengths and absolute molecular masses in solution is fundamentally different depending on the amount of added metal salt. The possibility of controlled variations in the characteristic sizes of two-dimensional ordered ensembles of micelles on the surface of silicon wafers is demonstrated. It is shown that, in some cases, the distance between the centers of micelles in ensemble depends on the concentration of copolymer solution and the amount of metal salt preliminarily added to the solution.

Effect of temperature on ring-shaped-deposit formated at evaporation of droplets of silver-nanoparticle dispersions

The effect of temperature is studied on the geometric parameters and conductivity of ring-shaped deposits formed at evaporation of droplets of dispersions of silver nanoparticles on hydrophilic (glass) and hydrophobic (copper) substrates. It has been shown that increasing temperature leads to substantial changes in the deposit profile. Therewith, the effects of temperature on droplet evaporation on glass and copper substrates are different. It has been found that the lateral conductivity of a ring-shaped deposit formed on a glass substrate increases stepwise similarly to a percolation transition at a droplet-evaporation temperature of 58°C. It has been suggested that the reason for the temperature effect is related to a change in the ratio between the rates of physicochemical processes occurring at different stages of droplet evaporation.