Chemical reduction is one of the most common ways to prepare silver nanostructures, and the materials prepared include three main categories, silver salts precursor, reducing agents, and stabilizers or capping agents. Since 1982, when Lee and Meisel first proposed ^[1][62], the use of citrate to reduce AgNO₃ has become one of the most popular methods for preparing silver nanoparticles. During the reaction, Citrate usually has the dual role of reducing and stabilizing silver nanoparticles. The citrate ion concentration is decisive for the properties of the generated silver nanoparticles. The complexation between citrate ions and silver colloids can be promoted by increasing the concentration of citrate ions to reduce the growth of silver particles and thus obtain larger clusters. The role played by sodium citrate on the growth of silver nanoparticles was investigated experimentally and it was found that the absorption properties of Ag colloids were independent of the citrate concentration and the maximum absorbance was maintained at 400 nm. However, the reaction time for the reduction of silver ions by citrate at boiling point temperature is important to achieve complete reduction. Fewer seeds formed in the citrate reduction method and slow cluster growth contribute to the formation of larger silver nanocrystals of varying shape and size ^[2][63]. Katarzyna et al. ^[3][64] studied the effect of the combined use of tannic acid and sodium citrate on the preparation of silver nanoparticles. The synthesis using either of these acids alone does not lead to particles of uniform size and shape. The reduction of silver salts using a mixture of sodium citrate and tannic acid at 100 °C resulted in uniform particles of around 30 nm in diameter. The combined use of sodium citrate and tannic acid allows for controlled nucleation, growth, and stabilization processes, resulting in reproducible monodisperse AgNPs. Another factor affecting the synthesis process is the pH value of the reaction environment. Dong et al. ^[4][65] investigated the effect of pH on the prepared particles when using citrate as a stabilizer and reducing agent for the synthesis of silver nanoparticles. At higher pH, the shapes produced were a mixture of spherical and rod-shaped particles, while at lower pH, the shapes produced were mainly triangular and polygonal particles. Therefore, a stepwise reduction method is proposed, in which nucleation takes place in a high pH environment and growth takes place in a low pH environment. The antibacterial activity of silver nanoparticles is related to their size and other properties (oxidation and release properties) ^[5][66]. In ^[6][67], Four parameters (AgNO₃ concentration, sodium citrate (TSC) concentration, NaBH₄ concentration, and the pH of the reaction) were improved to enhance the antibacterial activity of AgNP in the experiment. It can be seen that the selection and proportion of reducing agent, heating time, and pH value play a crucial role in the morphology and size of the resulting particles when using citrate reduction to generate silver particles. In turn, the morphology and size of the particles play a decisive role in the antibacterial activity of the particles. The antimicrobial properties of silver make it a competitive candidate in the field of antimicrobial sensing ^[7][8][68,69]. Citrate-capped AgNPs have been the choice for SERS-based studies for a long time.

Polyols are also widely used in chemical reduction methods. Among them, ethylene glycol is the most chosen one, which acts as a solvent and reducing agent in this reaction process. Under high-temperature conditions, a precursor silver salt is added to the polyol and a stabilizer or capping agent is added to reduce Ag⁺ to the desired nanostructure. Wiley et al. ^[9][70] introduced the use of ethylene glycol as a reducing agent for silver precursors. When silver nitrate is reduced by ethylene glycol, the initially generated particles may be multiplied twinned, singly twinned, or single-crystal seeds. The structure of silver nuclei varies with their size and available thermal energy, and different seeds grow into different nanostructures, therefore, the crystallinity of the seeds needs to be adjusted in the reaction as a way to control the production of specific shapes. Gautam et al. ^[10][71] used a polymer of polyvinyl alcohol (PVA) as a reducing agent to induce the reduction of Ag⁺. PVA can be used as a surface stabilizer to protect the silver particles during the reaction while preventing the appearance of agglomerates and unwanted growth. Liao et al. ^[11][72] synthesized silver nanowires with controllable morphology by an improved dual-alcohol process. Moreover, the effects of reaction temperature, different control agents, and different AgNO₃ solution droplet acceleration on the reaction were further studied. The synthesis can be automated based on the polyol process. Wolf et al. ^[12][73] proposed an automated synthesis method to stably synthesize AgNPs with an average radius of 3 nm and 5 nm.

Compared to physical and chemical methods, green synthesis methods are environmentally friendly, cost-effective, and easy to synthesize NPs on a large scale, and green synthesis does not require high temperatures, high energy, and harmful chemicals ^[13][74]. Using microorganisms as reducing agents avoids the use of harmful chemicals as sealers or stabilizers. Slawson et al. ^[14][75] reported that silver nanoparticles are biocompatible in some silver-resistant bacteria. As a result, bacteria can aggregate silver on their cell walls. Pooley ^[15][76] suggested that silver could be recovered from ores using bacteria. Klaus et al. ^[16][77] proposed the synthesis of silver nanoparticles using Pseudomonas syringae AG259. The selection of suitable bacteria can effectively accelerate the production of silver nanoparticles. Shahverdi et al. ^[17][78] reported the process of using the culture supernatants of Klebsiella pneumoniae, Escherichia coli, and Enterobacter cloacae to reduce silver ions in aqueous silver nitrate solutions. The synthesis process was rapid, with nanoparticles formed within five minutes of contact between silver ions and bacteria. 

Fungi also have the potential to synthesize silver nanoparticles because they can secrete enzymes and proteins for metal salt reduction and because fungi are generally more readily available than bacteria, and the use of fungi to synthesize silver nanoparticles also makes downstream processing for product recovery easier ^[18][80]. The size and shape of the generated material can also be adjusted by controlling the culture parameters. 

However, the use of microbial assistance requires additional processing steps, and microbial isolation requires additional culture maintenance. Therefore, researchers have proposed the idea of using plants and plant extracts to improve the synthesis pathway, and plant-mediated synthesis or plant synthesis involving extracts of different parts of plants (e.g., leaves, seeds, fruits, stems, roots, etc.) as reducing agents has been studied and considered as a promising strategy for nanoparticle synthesis ^[19][83]. Santos et al. ^[20][84] synthesized silver using three different citrus peel extracts, which were mixed with a solution of AgNO₃ at a concentration of 10⁻³ mol L⁻¹ and stirred at 30 °C for 60 min. The colloids prepared contain a high concentration of silver nanoparticles with a preferential spherical symmetry. The biosynthesis of silver nanoparticles using cell-free extracts of Spirulina has resulted in well-dispersed and highly stable spherical silver nanoparticles with an average particle size of 30–50 nm ^[21][85]. The use of biomolecules as templates for the synthesis of nanomaterials provides an excellent strategy for controlling and modulating their properties. 

In the past decade, green routes for AgNP synthesis have become increasingly popular due to the advantages of providing a one-step synthesis of non-toxic, eco-friendly NPs without the need for preservation or additional maintenance of cultures. However, there are some issues that need to be addressed in the green synthesis method. When using plant extracts for synthesis, the results of synthesis vary due to differences in various phytochemicals. Karthik et al. ^[22][87] using Camellia japonica leaf extract for the preparation of silver suggested that pH is an important factor affecting the morphology and size of the resulting particles and that there are differences in the pH values of different plant extracts and even of extracts from different parts of the same plant. AgNP for commercial use requires very strict uniformity of size. Yet size and shape cannot be well controlled by using green methods ^[23][88].

Seed-mediated growth processes are widely used because of the high degree of control over the size, shape, and structure of the generated particles. Seed-mediated methods can be further divided into two types of growth: homogeneous epitaxy and heterogeneous epitaxy ^[24][89].

Homogeneous epitaxial growth refers to the growth of crystalline seed crystals containing the same metal as that deposited on the seed. Lin et al. ^[25][90] prepared silver nanospheres and silver nanotubes using a homogeneous epitaxial seed-mediated method. Hegde et al. ^[26][91] proposed a method for the synthesis of triangular nanoplates using the seed-mediated method and cetyltrimethylammonium bromide (CTAB) as a single capping agent at a very low concentration of 0.4 mM. Wang et al. ^[27][92] developed a facile synthesis method based on seed-mediated growth using glucose as a reducing agent, where secondary nucleation is prevented during seed-mediated growth due to its weak reducing ability, and the size of silver nanoparticles can be continuously adjusted by continuously adding reactants. The rate of reductant addition also affects the growth, if the rate of reductant addition is too fast so that the consumed reagent is less than the added reagent, the concentration per unit volume will increase or even exceed the critical concentration for secondary nucleation and new nucleation will occur. Using this method, silver nanoparticles with diameters ranging from about 20 nm to 120 nm can be obtained.

Seed-mediated growth offers a simple and robust method for engineering the surface of nanocrystals. The facet can be altered by using a suitable capping agent. For example, Zeng et al. ^[28][97] found that nano-octahedra surrounded by {111} facets and nanocubes/nanorods covered by {100} facets could be selectively synthesized using citrate and PVP as capping agents. The size of nanocrystals can be conveniently tuned and tightly controlled in seed-mediated growth by varying the ratio of precursor to seed under suitable thermodynamic or kinetic conditions. For example, Jeon et al. ^[29][98] produced 2 μm cubes with precise cubic geometry by controlling the reaction atmosphere and the use of hydrochloric acid.

Microwave-assisted growth is receiving increasing attention due to its efficient and environmentally friendly nature. Compared to other conventional methods, microwave radiation requires less energy and can produce nucleation sites in solution at a very fast rate, thus significantly increasing the reaction rate ^[30][99].

Anjana et al. ^[31][100] reported a rapid and green microwave-assisted synthesis method using Cinnamomum cinereum (C. cinereum) as a reducing and capping agent to prepare silver NPs in a crystalline and spherical shape with an average size of 19.25 nm. Microwave-assisted synthesis of nanostars was performed in sealed-bottom flasks at very short time intervals (less than 3 min), and the only chemical precursors in the wet chemistry procedure for synthesis were silver nitrate and trisodium citrate, and the plasma substrates synthesized by this method were able to produce strong SERS spectra.

Another synthetic modality that has received attention is the sonochemical mediated synthesis, where the commonly used excitation source for sonochemistry is mainly high-energy ultrasound, usually in the frequency range of 16 KHz–5 MHz, and most reactions are carried out in the liquid phase ^[32][106]. Ultrasound itself cannot act directly on molecules but affects them through the physical effects of the surrounding environment in turn. When silver nanoparticle is prepared by the sonochemical method, the compression process of cavitation bubbles is rapid, and the heat in the cavitation bubbles is too late to be transferred to the liquid medium, so high temperatures are generated instantly in the hot spot region, and the liquid walls around the collapsed bubbles compress the substances contained in the bubbles, creating a good reflective environment for the chemical reaction.

Zhou et al. ^[33][107] reported a simple preparation method involving ultrasonic irradiation and glutathione as a stabilizer for the fabrication of blue luminescent silver nanoclusters. Kumari et al. ^[34][108] reported a simple method for the synthesis of silver/graphene nanocomposites by sonochemical method using sodium citrate as a reducing agent, which reduces silver ions to silver nanoparticles and forms spherical nanoparticles with an average particle size of 20 nm on graphene sheets.

Sonochemical synthesis methods have the unique advantage of controlling the size of the generated particles by controlling the ultrasonic frequency ^[35][112]. In general, sonochemical synthesis produces spherical metal nanoparticles, and thus the sonochemical process had been limited in preparing other metal nanostructures (e.g., nanorods, nanowires, etc.). To obtain other structures, it is usually necessary to add other substances as structure pointers ^[36][113].