Silver nanoparticles have a wide range of applications in many fields of biochemistry due to their unique physicochemical properties, and their preparation methods have been mainly based on physical and chemical methods. Although the research on these two methods is relatively sufficient, they suffer from the disadvantages of high production costs and easy environmental pollution. As some microorganisms were confirmed to have the ability to reduce metal ions, it was realized that the reduction of silver ions by microorganisms could be used to prepare silver nanoparticles. The unique advantages of this new method will likely lead to its development as a different means of silver nanoparticle preparation.
Preparation Method of Silver Nanoparticles
According to different principles, the preparation methods of silver nanoparticles can be divided into three categories: physical methods, chemical methods and biological reduction methods.
Physical Method
The evaporation condensation method and ion sputtering method commonly used in the preparation of metal nanoparticles have been used for the preparation of silver nanoparticles for a long time. These two methods are not easy to introduce impurities, and the average particle size of the obtained silver particles is also small. Mechanical grinding is also a relatively simple and commonly used method, which can obtain silver particle powder with an average particle size of about 20 nm. In addition, silver nanoparticles can also be prepared by laser sputtering.
In general, various physical methods for preparing metal nanoparticles are applicable to the preparation of silver nanoparticles. The principle of the physical method is simple, and the obtained product has fewer impurities and high quality, but its disadvantages are high requirements for equipment and high production costs.
Chemical Method
Chemical method is the most commonly used preparation method of silver nanoparticles, which is to reduce Ag+ through chemical reaction to form nano-scale particles. According to whether the obtained product is supported on other carriers, the preparation method can be divided into two types: supported and non-supported silver nanoparticles.
Preparation of supported nano-silver: The silver nanoparticles produced are dispersed in a solid phase carrier to reduce agglomeration of the generated silver particles and maintain the product particle size by using the carrier on the silver particles. It is mainly used in processes such as catalyst preparation where a carrier needs to be introduced. According to different methods of reducing silver ions, the preparation of supported silver nanoparticles can be divided into pyrolysis method, electroless plating method and activated carbon fiber reduction method.
Preparation of non-supported nano-silver: according to the principle of reducing silver ions and preventing the aggregation of silver particles, the preparation methods of non-supported nano-silver can be divided into chemical reduction method, ray irradiation method, microemulsion method, supercritical fluid method, electrochemical method, etc.
| Chemical Method |
Detail Methods |
Description |
| Preparation of supported nano-silver |
Pyrolysis method |
If the carrier impregnated with silver salt solution is treated at high temperature to decompose the silver salt, the movement of Ag+ ions, Ag0 atoms and Ag metal particles is restricted in the micropores of the carrier, and the resulting silver element It is loaded on the carrier in the form of nano-sized particles. |
| Electroless plating method |
Deposit silver elemental particles generated by chemical reactions on a certain carrier. For example, if formaldehyde-silver ammonia solution is used to conduct chemical silver plating on 10-20 nm Al2O3 powder under the action of ultrasonic waves, Ag-Al2O3 composite powder with a particle size of 50-60nm can be obtained, and it has good uniformity. |
| Activated carbon fiber reduction method |
Activated carbon fiber not only has abundant micropores and huge specific surface area, but also contains a large number of organic functional groups on its surface, so it is easy to react with metal ions under certain conditions. If the activated carbon fiber impregnated with the solution containing silver ions is vacuum-dried, the activated carbon fiber loaded with metallic silver particles can be obtained, and the particle size of the loaded silver particles is tens of nanometers. |
| Preparation of non-supported nano-silver |
Chemical reduction method |
One of the most commonly used methods for the preparation of silver nanoparticles is the reaction of silver salts such as silver nitrate and silver sulfate with appropriate reducing agents such as zinc powder and sodium citrate in the liquid phase so that the Ag+ ions are reduced by the Ag0 atoms and grown as singlet particles. The impurity content in the silver nanoparticles prepared by this method is relatively high. Moreover, due to the large surface interaction energy between them, the generated silver particles are easy to agglomerate, so the particle size of the silver powder produced by the chemical reduction method is generally larger and the distribution is very wide. Adding a dispersant can reduce the agglomeration of the generated silver particles and reduce the particle size, but increases the reaction by-products and increases the production cost. |
| Electrochemical method |
Nano-silver is prepared directly by electrolysis, and a coordination stabilizer needs to be added during the electrolysis process to prevent the agglomeration of the elemental particles. |
| Microemulsion method |
The size of the product particles produced within the microemulsions is controlled because the micelles formed by the surfactants on the microemulsion micelles restrict the exchange of material between the micelle particles, which have the property of maintaining and stabilising their original size. This property of microemulsions can be used to prepare silver nanoparticles. |
Microbial Method
There are two different mechanisms for the reduction of metal ions by microorganisms: the enzymatic mechanism of microorganisms and the non-enzymatic reduction mechanism.
Microbial enzyme catalysis mechanism: The enzymes produced by microorganisms, such as hydrogenase in the periplasm, play a catalytic role, and as electron transporters, transfer the electrons of reducing substances such as hydrogen and formate to metal ions, so that they are reduced. The sites for the catalytic reduction of biological enzymes can be in the periplasm, the extracellular surface and outside the cell. The enzymes involved in the catalytic reduction of metal ions vary from microorganism to microorganism.
Non-enzymatic reduction mechanism: Some functional groups on the surface of microbial cells can undergo redox reactions with metal ions in solution. The reaction process is mainly through the physical and chemical effects of organic functional groups on the cell surface, independent of the biological activity of microorganisms. After metal ions are reduced to zero-valent atoms, they are easy to agglomerate to form small particles with large surface energy. The strong interaction between the cell surface and such small particles prevents their migration and thus reduces aggregation. At present, the bacteria that can reduce silver ions in a non-enzymatic way reported in the literature mainly include D01 and A09.
Compared with physical and chemical methods, the preparation of silver nanoparticles by microbial reduction method has many advantages. The source of microbial raw materials is wide, cheap and easy to obtain, the conditions of the biological reduction reaction are mild, the chemical reagents added in the reaction process and the toxic by-products produced are less, and the product nanoparticles are not easy to agglomerate when attached to the bacteria. The main difficulty is to find new strains with strong enzymatic or non-enzymatic reduction of silver ions, and at the same time, it is necessary to solve the problem of the purity of silver nanoparticles products caused by the introduction of microbial cells.
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