Traditional and green approaches to the synthesis of copper-containing nanoparticles using chemical reduction

. The problem of obtaining copper-containing nanoparticles by chemical reduction has been studied. A comparison is made of traditional and “green” approaches to synthesis. The experimental data obtained during the synthesis of copper nanoparticles in the presence of a polymer stabilizer (polyvinylpyrrolidone) and a reducing agent (tert-butylamine-borane), as well as literature data on the synthesis of nanoparticles using plant and conditionally “green” substances (glucose, extracts of Ocimum sanctum and Celastrus paniculatus leaves, Sarcostemma acidum stem). The advantages and disadvantages of these approaches are identified and discussed. It is noted that the industrial applicability of “green” methods is limited due to the need to create completely new production of reagents for synthesis. In addition, the application of green synthesis is difficult due to scalability and standardization issues. Environmental pollution associated with the production of plant extracts also requires additional assessment. It has been established that traditional synthesis methods remain relevant for modern industry, and “green” methods have prospects for use in the distant future.


Introduction
The production of nanomaterials (nanoparticles, nanocomposites, etc.) with specified characteristics has been an urgent problem in nano chemistry for several decades.Due to the unique properties caused by dimensional effects, such materials find their use in various branches of modern industry.The automotive and road industries are no exception.Nanostructures are used both in a wide range of structural materials in the production of automobiles and in the creation of new road construction materials.In addition, the attention of researchers is drawn to studying the possibility of using nanofluids in engine cooling systems, as well as the inclusion of nanoparticles in multicomponent fuels.
In recent years, there has been a need to search for technologies for producing nanomaterials that are effective not only from an economic point of view, but also from an environmental point of view.Green approaches to the formation of nanostructures are constantly being researched and developed.
One of the promising traditional technologies for producing metal nanoparticles is the chemical reduction of metal ions in aqueous solutions of polymers using various reducing agents (hydrazine, tert-butylamine-borane, etc.) [1,2].
Green chemical reduction methods use green reagents as well as materials with low environmental risks.For example, glucose and plant extracts can be used as reducing agents and stabilizers [3][4][5][6].
This work compares synthesis products based on traditional and "green" methods for producing nanoparticles.

Materials and methods
The nanoparticles were obtained by chemical reduction of a copper salt in a polymer solution with tert-butylamine-borane.Polyvinylpyrrolidone (PVP) was used as a polymer protector.The water for solutions was pre-purified by double-distillation.
The synthesis product (copper-containing sol) was analyzed using transmission electron microscopy (TEM) and electron spectroscopy.
To analyze TEM images, the ImageJ program was used [7,8].Data on nanoparticles synthesized using green methods are obtained from literature sources.The preparation of nanoparticles using glucose as a reducing agent [3], extracts of holy basil leaves (Ocimum sanctum) [4], the stem of Sarcostemma acidum [5], and leaves of black oil plant (Celastrus paniculatus) [6] were considered.
A comparison of the synthesis products, taking into account the synthesis conditions, was carried out based on such characteristics of nanoparticles as size, shape, and stability.

Optical properties
In all the cases considered, different characteristic patterns were obtained by electron spectroscopy.Thus, during experimental synthesis, nanoparticles were obtained that exhibit a peak characteristic of the phenomenon of localized surface plasmon resonance (LSPR) in the absorption band with a wavelength of 570 nm, which corresponds to metallic copper [9].When using glucose as a reducing agent, the peak was observed at a wavelength of 475 nm, the extract of holy basil leaves -247 nm (before adding PVP -342 nm), the extract of Sarcostemma acidum stem -282 nm, the extract of Celastrus paniculatus leaves-269 nm.
From the electron spectroscopy data, it follows that during synthesis using traditional and "green" approaches, copper-containing nanoparticles of different compositions and sizes were obtained.It is believed that Cu nanoparticles are characterized by a peak due to the LSPR at a wavelength of about 570 nm [9], for CuO -about 390 nm [10], for Cu2O -about 475 nm [11].In this case, the LSPR phenomenon, as well as the exact values of the wavelength depend on the size and shape of the particles and can differ greatly from the presented values [12].Therefore, to accurately identify the composition of nanoparticles, it is necessary to use methods that provide more unambiguous results.

Particles morphology and size
By analyzing images obtained by electron microscopy, the size and shape of the nanoparticles were determined.Using all "green" methods, as in the case of experimental synthesis (Fig. 1), spherical nanoparticles were obtained.It should be noted that the particle size differed significantly depending on the synthesis method.When particles were obtained experimentally, the particle size was in a narrow range (5-7 nm), and the average diameter was 5.6 nm.A similar narrow range of size distribution was observed for particles obtained with the simultaneous use of PVP and Sarcostemma acidum stem extract (27-32 nm).When glucose was used as a reducing agent, the particle size was in a wide range from 150 to 200 nm.The widest range of particle sizes was observed when using holy basil leaf extract (5-200 nm).Nanoparticles with a medium size distribution range were obtained using Sarcostemma acidum stem extract (32-82 nm) and an extract of the leaves of Celastrus paniculatus (2-10 nm).
In general, it is known that polymer stabilizers can provide a narrow particle size distribution, limiting their growth and preventing aggregation.However, the effect of plant extracts on this function requires additional research into the mechanisms of action of nanoparticles on the growth.

Nanoparticles stability
The problem of sustainability and stability of the resulting nanoparticles is an important parameter characterizing the possibility of their storage and use.In experimental synthesis (as in the case of using glucose as a reducing agent), long periods of stability to oxidation and aggregation (up to 2 months) were achieved.However, this property was not considered by the authors when studying the use of plant extracts.This may be due to the low stability of synthesis products using plant extracts.

Methods perspective
The experimental results and literature data (Table 1) showed that the production of coppercontaining nanoparticles is possible using both traditional and "green" methods.
However, compared to traditional methods, "green" technologies based on plant extracts give less predictable synthesis results due to difficulties in determining the mechanisms of reduction and stabilization by plant extracts.These difficulties are associated with the multicomponent composition of plant extracts.It should also be noted that due to the lack of industrially produced plant extracts, the process of nanoparticle synthesis is difficult to scale up and standardize.
In addition, the source material has a significant influence on the production of plant extracts.The quality and characteristics of the source material depend on geography, climate, soil, and environmental situation.Extracts from plants collected in different locations may have significant differences in composition and characteristics.
The environmental impact of pollution associated with extract production also needs to be assessed.Such pollution includes two independent stages: agricultural (pollution caused by plant cultivation) and industrial (directly related to the production of extracts).

Conclusion
The possibilities of traditional and "green" methods for producing nanoparticles by chemical reduction were investigated.
It has been established that the production of nanoparticles is possible using both types of methods.However, green methods based on the use of plant extracts are less stable.With green synthesis, it is more difficult to control the characteristics of the resulting nanoparticles.In addition, the industrial applicability of such "green" methods is associated with a number of problems (scalability, standardization, additional capacity for obtaining and analyzing plant extracts).
Green methods may have prospects for application in the distant future.However, for application "here and now", traditional methods and conditionally "green" methods based on the use of industrially produced reagents remain relevant.
To introduce green methods into industry, future research must first form a fundamental basis for stable green synthesis, as well as compare the environmental risks of green and traditional methods.

Fig. 1 .
Fig. 1.TEM micrograph of copper-containing nanoparticles obtained by the traditional method.

Table 1 .
Comparative table of traditional and "green" methods for the synthesis of copper-containing nanoparticles.