Krasnoyarsk scientists developed a simple and effective method that makes it possible to produce solutions with unusually high concentrations of metals. Owing to this method, the researchers were able to obtain a solution with a record amount of silver nanoparticles - 1800 grams per liter. This approach will allow creating inks for three-dimensional printing, and also opens up new prospects for the development of materials and technologies. The results of the study were published in a series of articles, with the recent one in the Colloid Journal.

Due to their unique properties, organosols of silver nanoparticles are widely used in optical and semiconductor devices, for the production of electrical and thermally conductive films, as well as in catalysts and antibacterial materials. However, most existing methods for producing organosols of nanoparticles have their limitations. They can reduce the efficiency and profitability of production, require disposal and recycling of large volumes of waste solutions, and the final concentration of silver nanoparticles rarely exceeds one hundred grams per liter.

Scientists from the Krasnoyarsk Science Center SB RAS were the first in the world to create a solution with ultra-high content of silver nanoparticles. To do this, they developed a simple and high-throughput method for producing silver organosols with metal concentrations of up to 1800 grams per liter. The method involves preparing a hydrosol, an aqueous phase of silver nanoparticles, and then transferring it into an organosol, an organic phase. This approach makes it possible to increase the concentration of nanoparticles in solution.

“The main difference between organosols and hydrosols is the type of medium: organosols have an organic medium, while hydrosols have an aqueous medium. The advantage of organosols over hydrosols is that organosols contain organic solvents such as alcohols and ethers, which provide better stability and compatibility with various materials and surfaces. This makes organosols more effective,” explains Sergei Vorobyov, Candidate of Chemical Sciences, senior researcher at the Institute of Chemistry and Chemical Technology SB RAS.

To transfer the nanoparticles into the organic phase, scientists add deionized water and orthoxylene, a petroleum product that is a combination of organic substances, to the purified silver hydrosol. Being low-volatile, moderately toxic and relatively cheap, this substance turned out to be the most effective in the extraction process. After stirring, the resulting mixture is separated into a dark yellow top organic layer and a colorless aqueous layer with a small amount of black sediment at the bottom.

“In our work we used the phase transfer method. This method involves the initial preparation of hydrosols of silver nanoparticles followed by the transfer of the particles into the organic phase. We synthesized metallic silver nanoparticle hydrosols using a previously proposed modified citrate–sulfate method. This transfer to the organic phase increases the hydrophobicity of nanoparticles. At the same time, their size and shape hardly change. The resulting silver organosols are stable, that is, less prone to clumping, enlargement, dissolution and oxidation, and can be concentrated by the partial removal of the solvent to metal content of 1800 grams per liter,” says Sergei Vorobyov about the development of concentrated solutions of nanoparticles.

The specialists emphasize that in addition to unprecedented concentration, the developed organosols are highly stable for more than seven months. They also withstand repeated cycles of drying and returning to ash form. The technique proposed by the Krasnoyarsk scientists will make it possible to develop inks for three-dimensional printing, antimicrobial agents and nanofluids, and it will also open up new opportunities for the creation of materials and technologies.

“Our solutions are much more concentrated, cheaper, safer to obtain, and also very productive compared to analogues. The concentrate, based on silver particles, significantly surpasses both foreign and domestic developments in many respects. Other developments have a concentration of no more than 500 grams per liter and require a labor-intensive production procedure. Our technology is unique and allows us to use silver in the form of nitrate, which is reduced under certain conditions. And the product can be used in printing and inkjet printers to create conductive paths without the need for annealing. This opens the way to the production of printed and applied low-cost electronics on flexible substrates, such as printed circuit boards and thin silver films,” shared Sergei Vorobiev.

Researchers tried to obtain metal films from new silver organosols. For this purpose, organosols were applied to a dry and grease-free glass surface. After the evaporation of orthoxylene, a film with a characteristic silvery metallic shine effect formed on it. The resulting films had a thickness of 1 micrometer and high electrical conductivity which increased more than 6 times after heat treatment of the film. The scientists also discovered that by increasing the amount of the solution applied, thicker films could be obtained, up to 20 micrometers thick.

Krasnoyarsk scientists suggest that almost any material in a nano-sized state can be concentrated in ash solutions using the proposed method. For example, the specialists also synthesized concentrated hydrosols of magnetite nanoparticles containing 1350 grams per liter and copper oxide with a metal concentration of about one kilogram per liter. Such developments can find their application in biomedicine for targeted delivery of drugs: the drug is “attached” to a magnetic nanoparticle, which is directed into a cancer cell using an external magnetic field. Hydrosols can also be used to enrich different types of ores, in extraction processes such as gold ores, and to create metamaterials.

The information was prepared with the support of a grant from the Russian Ministry of Education and Science within the framework of the federal project “Popularization of Science and Technology.”