Gold 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 physical and chemical methods. Although these two methods have been well researched and the process technology is relatively mature at home and abroad, they suffer from the disadvantages of high production costs and easy environmental pollution. With the proven ability of some microorganisms to reduce metal ions, it was realized that the reduction of silver ions by microorganisms could be used to produce nanoscale monolithic silver. And the unique advantages of this new method will likely lead to its development as a different means of preparing nanosilver than before.
Preparation Method of Gold Nanoparticles
In recent years, due to the unique physical and chemical properties, good biocompatibility and biosafety of gold nanoparticles, more and more researchers have been attracted to carry out extensive research and development on them. The properties of different forms of gold nanoparticles vary greatly, and a variety of methods have been used to synthesize gold nanoparticles of different sizes and forms in a controlled manner, such as gold nanospheres, gold nanorods and gold nanocages. The following are some of the main methods for the synthesis of gold nanoparticles.
Sodium Citrate Reduction Method
The sodium citrate reduction method also known as the Turkevich method, was proposed by Turkevich in 1951 and is still one of the common methods used to synthesize gold nanoparticles. Only three raw materials are required for this method: chloroauric acid, sodium citrate and ultrapure water. In an aqueous solution under high-temperature conditions, the sodium citrate acts as a reducing agent and also as a stabilizer, allowing the preparation of gold nanoparticles of different particle sizes. This method is mostly used to prepare spherical gold nanoparticles with a particle size below 100 nm, but it is not possible to prepare gold nanoparticles that are too small.
Crystal Seed Growth Method
The seed growth method allows people to control the synthesis of the shape, size, composition and structure of gold nanoparticles. Murphy's group at the University of South Carolina proposed in 2001 that rod-shaped gold nanoparticles could be synthesized by using smaller gold colloidal particles as seeds. The method is divided into two steps: nucleation and growth. Firstly, tiny gold nanoparticles are prepared by chemical reduction method as seeds, and then the seeds are placed in the growth solution with different proportions of reducing agents, surface stabilizers, etc., so that the free Au3+ in the growth solution is continuously reduced to zero-valent Au atoms and directional deposited on the seed crystal, finally forming gold nanoparticles of various sizes and shapes. The different proportions of the growth solution and the additional ratio of the seed crystals are the keys to controlling the size and shape of the gold nanoparticles.
Electrochemical Method
This method was proposed earlier by Wang et al. to prepare gold nanoparticles. First, the gold plate is used as the anode, and the anode electrode is sacrificed to generate gold ions under energization, and the platinum plate is used as the cathode to reduce the gold ions, and a mixed solution of surface active agents cetyltrimethylammonium bromide (CTAB) and dodecyl bromide chloride is used as the electrolyte, and electrolysis is carried out under the conditions of ultrasonic and controlled current stability. This method is convenient to control the size of gold nanoparticles by changing deposition time, voltage, current and other conditions, and the obtained gold nanoparticles have a uniform particle size, but the energy consumption is large and the production cost is high.
Two-Phase Method
In 1994, Brust et al. synthesized gold nanoparticles through a two-phase method. Compared with other methods, this method has greater potential in the synthesis of smaller particle sizes. In the mixed liquid of aqueous phase and organic phase, surfactants are used to transfer free Au3+ ions in the aqueous phase to the organic phase, and then Au3+ is reduced to Au atoms by reducing agent and thiol stabilizer and protected by thiol coating. The gold nanoparticles prepared by this method have good stability and are easy to be modified.
In addition to the above-mentioned preparation methods of gold nanoparticles, there are template methods, photochemical methods, self-assembly methods, biological reduction methods, and some special physical methods, all of which can prepare a variety of gold nanoparticles.
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