Nanomaterials are highly effective, environmentally friendly, and applicable for various applications. Recently, silver nanoparticles (Ag NPs) are increasingly being synthesized due to their physical, chemical, and biomedical properties. Silver nanoparticles can be synthesized using physical, chemical, and biological methods. Ag NPs are widely applied in electronic and sensing applications.
Nanoscience enables researchers to develop new and cost-effective nanomaterials for energy, healthcare, and medical applications. Silver nanoparticles (Ag NPs) are currently increasingly synthesized for their superior physicochemical and electronic properties. Good knowledge of these characteristics allows the development of applications in all sensitive and essential fields in the service of humans and the environment. This review aims to summarize the Ag NPs synthesis methods, properties, applications, and future challenges. Generally, Ag NPs can be synthesized using physical, chemical, and biological routes. Due to the great and increasing demand for metal and metal oxide nanoparticles, researchers have invented a new, environmentally friendly, inexpensive synthetic method that replaces other methods with many defects. Studies of Ag NPs have increased after clear and substantial support from governments to develop nanotechnology. Ag NPs are the most widely due to their various potent properties.
2. Silver Nanoparticles
According to the Royal Society of Chemistry, the first evidence of silver mining dates to 3000 BC in Turkey and Greece. Even the ancients knew how to polish silver, heat silver ore, and blow air on it. Silver does not react with air, but base metals such as lead and copper oxidize and separate from precious metals. Silver, like gold, is formed from the explosion of a star called a supernova. A 2012 study published in Astronomy and Astrophysics found that exploding small stars produce silver and large stars. When Europeans arrived in the New World in 1492, silver was abundant on Earth. Spanish invaders have enthusiastically drawn this wealth by discovering that South America has silver- and ore-rich veins. According to the Silver Institute, 85% of the silver produced worldwide came from Bolivia, Peru, and Mexico between 1500 and 1800.
Among the various types of NPs, Ag NPs have been widely developed to be utilized in various applications due to their outstanding properties. Typical applications of Ag NPs include clothing and textiles, medical devices, food storage, cosmetics, sunscreens, laundry detergents 
, bandages 
, and sensors 
. Some studies have found that Ag NPs have cytotoxicity that can induce ROS formation in cells 
. Therefore, many products such as detergents, toiletries, etc. In addition, their synthetic or in-use personal care, whether industrial or household, produces a release of NPs, which ultimately ends up in the sewer. This untreated wastewater affects aquatic ecosystems and thus microorganisms. Recently, Ag NPs have great concerns regarding aquatic toxicology due to the difficulty of tracking these particles in the environment and accessing their effects on living organisms 
. The fate of NPs in the aquatic environment and their interactions, the interactions between NPs with biological and abiotic components, and their potential for damage are not well understood, and these uncertainties raise concerns about related risks. These molecules impose on humans on health and the environment 
. Based on the Scopus database 
, the publications on Ag NPs increase with time, where it started in 1990 (two reports) and reached 7105 reports in 2020, as shown in Figure 1
Figure 1. Publications distribution with time on Ag NPs.
2.1. History of Silver Nanoparticles
Colloidal Ag NPs are molecules with an average diameter of 20–40 nm and comprise 80% silver atoms and 20% silver ions. They are the best-selling nanoparticles ahead of carbon nanotubes and titanium nanoparticles and are released into the environment. The demand for Ag NPs has increased due to their applicability in multiple fields. Over the years, various synthesis techniques have been developed, and procedures have been improved to prepare small and uniform Ag NPs. Ag NPs were prepared by a chemical reduction technique, where the silver ions are reduced by sodium citrate 
Further investigations into the role of citrate revealed that besides acting as a reducing agent, the citrate anion also has a proven effect on Ag NPs. At this stage, the LaMer nucleation and growth mechanism was used to characterize the nucleation and growth of the nanoparticles. However, studies revealed that LaMer’s intent and growth 
. Mechanism not related to their feedback. Therefore, Fan Hing et al. examined the mechanism of nucleation and growth that applied to its results. They prepared Ag NPs using a chemical reduction technique in which silver perchlorate ions are reduced by sodium borohydride. It was determined in their study that increasing the concentration of sodium borohydride would lead to the instability of Ag NPs.
In the study conducted by Zielińska et al. 
, the stability of Ag NPs was the main focus. Ag NPs were prepared using the chemical reduction method. Zielińska et al. show that the type of precursor affects particle stability. According to their research, stable and spherical Ag NPs were synthesized using silver nitrate as a precursor in the presence of NaBH4
as a reducing agent and stabilizer. These results were compared with Ag NPs prepared using silver nitrate or silver acetate as a precursor. In this case, the Ag NPs were unstable and precipitated shortly after synthesis. Alternatively, their study presented that it is also possible to synthesize stable and uniform spherical Ag NPs via chemical reduction technique using AgNO3
as a precursor, NaBH4
as a reducing agent, and polymer as a stabilizing agent 
In a study conducted by Hsu and Wu 
, crystalline Ag NPs with a particle size in the range of 3–15 nm were prepared by reducing silver ions by formaldehyde using three stabilizers. Reduction by formaldehyde is relatively slow and requires a catalyst or reaction catalyst to speed up the reaction process. Moreover, the synthesis at high pH favors a decrease in efficiency in the presence of poly (N-vinyl-2-pyrrolidone) (PVP), resulting in spherical Ag NPs with an average particle size of 15 nm. Thiosalicylic acid as a stabilizer will produce spherical Ag NPs with a size of 8 nm. Finally, triethylamine (TEA) has the smallest spherical 3 nm Ag NPs. It was observed that without the addition of a stabilizer, the particles would agglomerate, which was inhibited by Ag-N bonding by TEA. Adjusting the stabilizer concentration is critical, as increasing the TEA concentration increased particle agglomeration 
. Sánchez et al. produced Ag NPs in the application of 20 V to silver electrodes. Another observation is that complete cathode coverage limits the formation of NPs and thus favors the particle deposition process. To avoid cathode precipitation and reduction in nanoparticle production 
, two metallic silver plates as an electrode pair were used to synthesize colloidal Ag NPs. Laser wavelength on particle morphology was investigated. They showed that the deposition of metallic Ag NPs onto a silica substrate by laser ablation under vacuum resulted in reduced particle size as the laser wavelength decreased 
2.2. Synthesis of Silver Nanoparticles
2.2.1. Top-Down and Bottom-Up Approaches
As referenced, various kinds of Ag NPs have been utilized in various applications 
. Specifically, Ag NPs of differing sizes and shapes have been used in a broad scope of uses and clinical gear, such as electronic gadgets, coatings, cleansers, cleansers, swathes, etc. 
. Explicit physical and optical properties of Ag NPs are subsequently essential factors in advancing their use in these applications. In such a manner, the accompanying subtleties of the materials are imperative to consider in their combination: surface property, size dissemination, clear morphology, molecule composition, dissolution rate (i.e., reactivity in arrangement and effectiveness of particle delivery), and kinds of diminishing and capping specialists utilized. The blend techniques for metal NPs are partitioned into top-down and bottom-up approaches, as demonstrated in Figure 2
Synthesis routes of Ag NPs by (a
) Top-down and bottom-up methods, (b
) chemical methods, and (c
) green methods, modified with permission from Ref. 
, MDPI, 2019. The electron transfer initiates the bioreduction through nicotinamide adenine dinucleotide (NADH)-dependent reductase as an electron carrier to form NAD+
. The resulting electrons are obtained by Ag+
ions, which are reduced to elemental Ag NPs.
The big picture perspective disincorporates mass materials to produce the required nanostructures, while the base-up strategy gathers single particles and molecules into bigger nanostructures to create nanosized materials 
. The following areas examine different combination strategies in detail, from the union of spherical Ag NPs to shape-controlled Ag colloids, just as size-controlled Ag NPs are synthesized. The segments also expect to present different courses of union and their instruments, elucidating how shape-and size-controlled amalgamation of Ag NPs can be accomplished through the fitting choice of energy source, antecedent synthetic substances, diminishing and covering specialist, just as well fixation and the molar ratio of synthetic compounds.
2.2.2. Physical Methods
The physical technique usually utilized to prepare Ag NPs is the evaporation-condensation method. It is commonly performed using a tube furnace at atmospheric pressure, synthesizing various sizes 
. Several attempts have been made. For instance, Tsuji et al. 
proposed a new method for synthesizing Ag NPs by a laser ablation technique with focused and unfocused laser beam irradiation carried out at 12 and 900 mJ cm−2
intensities, respectively. The radiation wavelengths used were 355, 532, and 1064 nm. This study revealed that the surface plasmon wavelength of Ag NPs irradiated using 355, 532, and 1064 nm is ~400 nm for focused and unfocused beams.
The physical synthesis of Ag NPs incorporates the evaporation-condensation way and the laser ablation method (Figure 2
. Both approaches can synthesize enormous quantities of Ag NPs with high immaculateness without using chemicals that discharge poisonous substances and endanger human health and climate. Notwithstanding, agglomeration is regularly an incredible test due to the absence of capping agents. Moreover, both approaches devour more noteworthy power and generally need a longer production and complex hardware, increasing the total production cost.
Evaporation–condensation and laser ablation are the principal physical attitudes. The shortfall of solvent contamination in the readied thin films and the consistency of NPs conveyance are the upsides of physical synthesis methods compared to the chemical processes. The biological synthesis of Ag NPs utilizing a tube furnace at an atmospheric pressing factor has a few drawbacks. The tube furnace consumes a considerable space, burns through a great measure of energy while raising the ecological temperature around the source material, and requires a ton of time to accomplish warm steadiness. Additionally, a commonplace tube furnace needs power utilization (a few kilowatts) and several minutes of preheating to arrive at a stable operating temperature 
. It was revealed that Ag NPs could be synthesized using a small ceramic heater with a neighborhood heating zone 
. The little ceramic heater was used to evaporate source materials. The evaporated fume can cool quickly because the temperature angle near the heater surface is exceptionally steep in examining a tube furnace.
Furthermore, Ag NPs sizes obtained from the laser ablation with nanosecond and femtosecond laser beats were analyzed. In the arrangement, the wavelength of the laser utilized was 800 nm. Additionally, a Ti: sapphire laser system created femtosecond laser beats used in the examination 
. It was tracked that Ag NPs sizes retrieved from the nanosecond laser beats were more modest than those from femtosecond laser beats were. The average widths of the obtained Ag NPs from nanosecond and femtosecond laser beat schemes were 27 and 41 nm. For technique enhancement, synthesizing Ag NPs by condensation utilizing a small ceramic heater with a nearby warming area of 500 m2
was projected 
. Unadulterated spherical Ag NPs with shifting measurements from 6.2 to 21.5 nm were effectively created. The examination inferred that the geometric mean breadth, the geometric standard deviation, and the absolute number centralization of Ag NPs increased with the surface temperature of the heater. Their approach could dependably synthesize stable Ag NPs, since the heater surface temperature did not fluctuate with time 
. Various Ag NPs prepared by physical methods are listed in Table 1
Table 1. A list of various physically synthesized Ag NPs.
|Catalytic degradation activity
|Small Ceramic Heater
||Inhalation toxicity studies
2.2.3. Chemical Methods
The famous method for Ag NPs synthesis is a reduction by natural and inorganic reducing agents (Figure 3
). By and large, unique reducing agents, for example, sodium citrate, ascorbate, sodium borohydride, essential hydrogen, polyol measure, Tollens reagent, N, N-dimethylformamide (DMF), and poly (ethylene glycol)-block copolymers are utilized for the reduction of silver particles (Ag+
) in aqueous or non-aqueous arrangements. These reducing agents decrease Ag+
and lead to metallic silver (Ag0
), trailed by agglomeration into oligomeric clusters. These clusters ultimately arrange the metallic colloidal silver particles 
. It is critical to utilize defensive agents to stabilize dispersive NPs during metal nanoparticle planning and ensure that the NPs consumed or tied onto nanoparticle surfaces stay away from their agglomeration 
Ag NPs synthesis mechanism from plant extract, modified with permission from Ref. 
, Royal Society of Chemistry, 2019.
Surfactants involving collaborations with molecule surfaces can stabilize molecule growth and shield particles from sedimentation, agglomeration, or losing their surface properties. Stabilizing dispersive NPs during Ag NPs synthesis is critical. The most well-known methodology utilizes stabilizing agents that can be absorbed outside Ag NPs, evading their agglomeration 
. To stabilize and to dodge agglomeration and oxidation of NPs, capping agents can be utilized, for example chitosan, oleylamine gluconic acid, cellulose or polymers such as poly N-vinyl-2-pyrrolidone (PVP), polyethylene glycol (PEG), polymethacrylic acid (PMAA) and polymethylmethacrylate (PMMA) 
. Adjustment using capping agents can be accomplished either through electrostatic or steric shock. For example, electrostatic adjustment is typically achieved through anionic species such as citrate, halides, carboxylates, or polyoxoanions that adsorb or cooperate with Ag NPs to bestow a negative charge on the outside of Ag NPs. Therefore, the surface charge of Ag NPs can be constrained by covering the particles with citrate particles to give a solid negative charge. Contrasted with utilizing citrate particles, fanned polyethyleneimine makes an amine-functionalized surface with a profoundly specific charge. Other capping agents likewise give extra usefulness. Polyethylene glycol (PEG)-coated NPs show excellent stability in exceptionally salty solutions, while lipoic acid-coated particles with carboxyl gatherings can be utilized for bioconjugation.
2.2.4. Green Methods
Green synthesis methodologies dependent on natural reducing agents rely on different reaction parameters such as solvent, temperature, pressing, and pH conditions (acidic, fundamental, or impartial). For the union of metal oxide nanoparticles, plant biodiversity has been extensively viewed as the accessibility of successful phytochemicals in different plant separates, particularly in leaves such as ketones and aldehydes flavones, amides, terpenoids, carboxylic acids, phenols, and ascorbic acids 
. These parts are fit for decreasing metal salts into MNPs 
. The fundamental focus of such nanomaterials has been researched for biomedical diagnostics, antimicrobials, catalysis, atomic detecting, optical imaging, and marking of natural frameworks.
Essential elements for the green synthesis of Ag NPs are silver salts and bioreducing agents 
. In general, bioreducing agents or various components present in cells act as stabilizers or capping agents, reducing these agents’ need for external inclusion 
. Traditional strategies for producing NPs are costly, harmful, and not environmentally friendly. Therefore, to overcome these problems, experts are adopting a green method for synthesizing NPs. Natural resources and their constituents were used in the synthesis of NPs. Generally, plants and their extracts, bacteria, fungi, and biopolymers can prepare Ag NPs via the green technique, as shown in Table 2
. The green composition of plants, plant extracts, bacteria, fungi, and overall biopolymers is discussed in Section 3.2.5
of this review.
Table 2. Various plant extracts, bacteria, fungi, and biopolymers as bioreducing agents for the synthesis of Ag NPs.
of Silver Nitrate
|The Mechanism for the Synthesis
||Alkaloids, Terpenes, Steroids and Saponins, Flavonoids and Tannins, Alcohol, Phenolic Acids.
||Electrostatic interaction between the functional groups of a respective constituent of plant extract and Ag+
||Bacillus Cereus, Bacillus licheniformis, Staphylococcus aureus, Enterobacteriaceae, Pantoeaananatis, Proteus
|In the extra-cellular synthesis of Ag NPs by Bacillus subtilis, the synthesis of Ag NPs was observed in the reaction mixture after 6 h contact time at room temperature.
||Proteins, peptides enzymes, napthoquinones, NADH, NADPH, peptides, nitrogenous biomacromolecules,
||Intracellular and extracellular synthesis of Ag NPs
||Cellulose, Chitosan, polypeptides, alginate, lignin, protein
||Electrostatic interaction between Ag+ ion and polar groups attached to the polymer
2.5. Green Synthesis by Plant Extract
The plant-based synthesis of Ag NPs is largely embraced more in contrast to techniques that utilize microorganisms, since it tends to be developed effectively, is less biocompromising, and excludes the progression of cell culture growth 
. Leaves, natural products, roots, seeds, and stems contain biomolecules such as compounds, alkaloids, polysaccharides, tannins, terpenoids, phenols, and nutrients of extraordinary restorative worth despite their complex structures, are helpful for the environment 
. Plant extract replaces all harmful synthetic compounds such as trisodium citrate and sodium borohydride. The concentrate from plants helps well in the amalgamation of NPs because of the arrangement of Ag NPs settled by the flavonoid and terpenoid parts present in leaf stock, while the decrease of silver particles is assigned by the polyol water-solvent heterocyclic segments of leaf broth 
. The concentrate of plant Salvia spinosa
under in vitro conditions was utilized interestingly to incorporate Ag NPs 
. The preparation of Ag NPs by Alfalfa sprouts was firstly introduced 
These days, the creation of NPs centers around green synthesis from the extract of various plant parts 
. The multipurpose agents of reduction and adjustment of plant extraction for biological synthesis of NPs are used to execute green science 
. Extraction of nontoxic plants for synthesis nanoparticles offer characteristic capping agents. Besides, as far as the cost for nanoparticle synthesis, plant extraction improves the cost viability over the disconnection of microorganisms for the achievability of nanoparticle synthesis 
. As of late, there is a growing interest in synthesizing MNPs by ‘green’ techniques.
For this reason, extracts of various plants have been used for Ag NPs preparation. One of the primary approaches to utilizing plants as a source for the synthesis of MNPs was with Alfalfa sprouts 
, which was the main report on the arrangement of Ag NPs using a living plant system. Hay roots can engross Ag from agar medium and move them into the plant’s shoots in a similar oxidation state. These Ag molecules orchestrated themselves in the shoots to shape NPs by joining themselves and framing bigger courses of action.
In contrast with bacteria and fungi, green synthesis utilizing plants has all the earmarks of being quicker, and the principal examinations exhibit that synthesis methodology can produce Ag NPs quite quickly. For example, Shankar et al. showed that utilizing Geranium
leaf takes around nine hours, arriving at 90% response contrasted with the 24 to 124 h vital for other responses announced before 
. Therefore, the utilization of plant extracts in green synthesis has prodded various examinations and studies up till now. It was shown that the creation of MNPs utilizing plant extracts could be finished in the metal salt arrangement inside the space of minutes at room temperature, contingent upon the idea of the plant extract. After the decision of the plant extract, the primary influencing boundaries are the grouping of the extract, the metal salt, temperature, pH, and contact time 
. The mechanism of preparing Ag NPs from plant extract is shown in Figure 3
. In addition, Table 3
lists the synthesis of Ag NPs using plant extracts with different sizes, shapes, characterization, and applications.
Table 3. A list of synthesis Ag NPs using plant extracts with different sizes, shapes, characterization, and applications.
|16.987 g of AgNO3 in 1 L of distilled water, 10 mL extract, stirred for 10 min. and kept for 24 h at room temperature
||32.1, 22.6, 14.5 nm, Spherical
|Curcuma longa L.
||10 mL of Curcuma longa L. leaf extrac, 90 mL of 1 mM AgNO3.
||15–40 nm, spherical
|Phaseolus vulgaris L.
||Photocatalytic activity, antimicrobial activity
||Extract (1, 2, 3, and 4 mL), AgNO3 (0.01 M, 50 mL), (300 rpm) for 30 min
||10–20 nm, spherical
|Solanum nigrum L.
||1 mL leaf extract, AgNO3 (10−3 M, 50 mL), stirred continuously at room temperature
||10–50 nm, spherical
||Biocide and anticorrosion
|1 mM AgNO3, ratio 1:1 with tangerine peels extract
||39.6–56.1 nm, round
|AgNO3 (1 mM, 90 mL), 10 mL of extract, at 70 °C.
||Antibacterial and antidiatom activity
||AgNO3 (1 mM, 400 mL), 80 mL of extract, darkness for 24 h
||In vitro cytotoxicity and interaction studies with bovine serum albumin
||Leaf extract and an aqueous solution of 1 mM AgNO3 (20:80, V/V).