Detailed Review on Synthesis of Silver Nanoparticles

Silver nanoparticles are nanoparticles of silver having size range between 1 and 100 nm. Silver is one such metal which has been used for various biomedical applications. They penetrate the human body via different organs like liver, kidney, lungs, spleen and brain. AgNPs are extremely small, but their large surface area allows them to dissolve quickly. They exhibit several beneficial features, including chemical stability, strong conductivity, localized surface plasmon resonance, which make them effective against viruses. since they can act on multiple targets, the likelihood of causing drug resistance reduced. Nanotechnology, which focuses on the creation and manipulation of structures at the scale of 1–100 nm, has become a rapidly expanding area of modern research. Nanoparticles (NPs) are particularly valuable due to their broad range of applications in medicine, cosmetics, food, environmental monitoring, optics, catalysis, and electronics. Among the various branches of this field, nanobiotechnology has gained prominence, combining nanoscience with biological approaches to design functional nanomaterials with controlled size, shape, and dispersion. This interdisciplinary area involves contributions from chemistry, physics, biology, engineering, and medicine, and has enabled the development of innovative materials for healthcare and technology. Recently, increasing focus has been placed on green synthesis methods, which utilize non-toxic, environmentally friendly reagents and solvents. Such approaches offer sustainable alternatives to conventional chemical routes, reducing environmental risks while ensuring effective and reproducible nanoparticle production. These advancements highlight the potential of nanotechnology for both scientific innovation and industrial application.

Structure Of Silver Nanoparticles

Silver nanoparticles are defined as silver particles with a diameter ranging from 1 to 100 nanometers. While they are most commonly spherical, they can also be synthesized in various other forms, including diamond and octagonal shapes, as well as thin sheets. A key characteristic of silver quantum dots is their tunable bandgap, which can be adjusted by altering the dot's size. Furthermore, these nanoparticles exhibit Surface Plasmon Resonance (SPR), a property that makes them a highly desirable dopant material for use in photovoltaic cells to enhance their efficiency. Silver is a noble metal whose nanoparticles possess distinct and adjustable plasmonic characteristics. The frequency of their surface plasmon resonance (SPR) can be precisely controlled by modifying the particles' size and shape, as this determines how they interact with their surrounding environment. While nanoparticles made from pure, uncoated silver already demonstrate excellent physical and chemical properties, their functionality can be further tailored for specific applications. This is often achieved by coating them with a different metal, such as gold, to form a core-shell structure.

Figure 1 – Structure of Silver Nano Particles

The physical and chemical characteristics of nanoparticles are used to describe their properties

1. Physical Characteristics

• Optical properties
• Mechanical properties
• Magnetic and electrical properties

2. Chemical Characteristics

• Oxidation
• Reduction
• Stability
• Sensitivity
• The biological effects and toxicity of silver nanoparticles (AgNPs) are primarily governed by their unique physicochemical properties. Key characteristics that determine their cytotoxicity include
• Surface Chemistry: The composition and reactivity of the nanoparticle's surface.
• Size and Size Distribution: Smaller particles typically have higher reactivity and toxicity due to a larger surface area-to-volume ratio.
• Shape and Morphology: Particles can be spherical, rod-shaped, triangular, octagonal, or flower-like, each shape influencing its interaction with biological systems differently.
• Particle Composition and Purity.
• Surface Coating/Capping: Organic or inorganic molecules used to stabilize the nanoparticles.
• Agglomeration State: The tendency to form clusters, which affects their bioavailability.
• Dissolution Rate: The speed at which AgNPs release silver ions (Ag?), a key mechanism of their toxicity.
• Reactivity in Solution.
• Efficiency of Ion Release.
• The Type of Reducing Agent used in their synthesis.
• These properties are crucial for understanding how AgNPs will behave in biological environments and are fundamental to assessing their safety and efficacy.

Synthesis Of Silver Nanoparticles

There are two primary strategies   for producing silver nanoparticles: the top -down and the bottom -up approaches. These approaches can be carried out using physical, chemical, or biological methods. Generally physical techniques follow the top – down route, while chemical and biological methods rely on bottom -up synthesis. In the top- down method, a bulk material is taken as the starting point and broken-down step by step into smaller particles until nanoparticles are obtained. On the other hand, bottom -up method builds nanoparticle by assembling atoms and molecules. This approach is generally less toxic, more cost effective and more efficient.

Figure 2 – Synthesis of Silver Nano Particles

Physical approaches such as evaporation–condensation and laser ablation are among the most widely applied methods for producing silver nanoparticles. One advantage of these techniques is the absence of solvent contamination, which often occurs in chemical synthesis, thereby ensuring higher purity of the nanoparticles. Additionally, these methods allow greater uniformity in particle distribution. However, certain limitations exist. For example, synthesis using a tube furnace under atmospheric pressure requires a large amount of space, consumes significant energy, and generates high operating temperatures, which increases environmental concerns. Furthermore, achieving stable thermal conditions demands extended preheating times. A smaller ceramic heater has been proposed as an alternative, which can reduce energy requirements while maintaining adequate evaporation of silver to form nanoparticles. Rapid cooling of the vapor also promotes the formation of nanosized particles. Overall, physical methods are effective but less energy-efficient compared to chemical approaches, limiting their broader application. Among the physical techniques, evaporation–condensation and laser ablation are the most widely used methods. Their major advantages over chemical synthesis are the absence of solvent contamination in the resulting thin films and the uniform distribution of nanoparticles. However, producing silver nanoparticles with a tube furnace at atmospheric pressure has certain drawbacks. Tube furnaces require a large setup space, consume high amounts of energy, increase the surrounding temperature, and take considerable time to reach thermal stability. In addition, they typically demand power consumption of several kilowatts and need a long preheating period before achieving stable operating conditions. Studies have shown that silver nanoparticles can also be generated using a small ceramic heater with a localized heating zone. This approach allows efficient evaporation of the source material, and the vapor condenses quickly because the steep temperature gradient near the heater surface promotes rapid cooling. Compared to a tube furnace, this method is more efficient and facilitates the formation of smaller nanoparticles.

2.  Chemical Methods

Chemical reduction is one of the most commonly employed strategies for preparing silver nanoparticles, as it enables large-scale production using relatively simple procedures. This approach involves reducing silver salts with organic or inorganic agents in the presence of stabilizers that prevent particle agglomeration. A wide range of reducing agents—including sodium borohydride, hydrazine, ascorbic acid, and glucose—have been reported. Stabilizers such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), or natural polymers are often added to control particle size, shape, and dispersion. Despite its popularity, chemical synthesis faces challenges such as particle instability, aggregation, and the need for post-synthesis purification. Moreover, many reducing agents are toxic and raise environmental concerns. To address these issues, researchers are increasingly exploring “green” chemical methods that utilize eco-friendly solvents and biological stabilizers while maintaining efficiency and reproducibility. Such modifications aim to overcome the drawbacks of conventional chemical routes and improve their suitability for biomedical and industrial purpose.  The synthesis of silver nanoparticles (AgNPs) is most commonly achieved through chemical reduction using either organic or inorganic reducing agents. Typically, compounds like sodium citrate, ascorbate, sodium borohydride (NaBH?), elemental hydrogen, polyol methods, Tollens’ reagent, N, N-dimethylformamide (DMF), and poly (ethylene glycol)-based copolymers are employed to reduce silver ions (Ag?) in aqueous or non-aqueous systems. These agents convert Ag? into metallic silver (Ag?), which subsequently aggregates into small oligomeric clusters. Over time, these clusters evolve into metallic colloidal silver nanoparticles. To ensure stable dispersions during nanoparticle synthesis, protective agents are necessary. They safeguard the nanoparticles by preventing aggregation and enabling them to bind or adsorb onto surfaces. Surfactants containing functional groups such as thiols, amines, acids, and alcohols play a key role in particle stabilization. They promote controlled particle growth, reduce sedimentation, and preserve the surface properties Surfactants containing functional groups such as thiols, amines, acids, and alcohols play a key role in particle stabilization. They promote controlled particle growth, reduce sedimentation, and preserve the surface properties of nanoparticles. Additionally, polymers like poly (vinyl alcohol), poly(vinylpyrrolidone), and poly (ethylene glycol) are often incorporated as stabilizing agents during synthesis.

Various eco-friendly synthesis techniques such as the use of polyoxometalates, pollens, polysaccharides, and irradiation methods are employed for producing silver nanoparticles. Green synthesis refers to the application of biologically compatible materials like plants, bacteria, and fungi for nanoparticle preparation. These methods offer significant advantages over conventional chemical synthesis, as they avoid many of its limitations. One widely used approach involves plant extracts, which serve as both reducing and capping agents in the formation of silver nanoparticles. This method is environmentally safe and non-toxic compared to traditional chemical reduction techniques. The bioactive molecules present in plant extracts—such as amino acids, vitamins enzymes, and proteins—help in reducing silver ions (Ag?) and stabilize the nanoparticles, ensuring uniform size and shape. A wide range of plants, including moringa, bitter apple, fenugreek, neem, and others, have been utilized for this purpose.

Other Methods

1. Microwave Assisted Synthesis

Microwave assisted synthesis is a promising technique for producing silver nano particle. Compared to traditional oil bath heating, microwave heating consistently yields nanostructures with smaller sizes, narrower size distributions, and higher crystallization degrees. This method offers shorter reaction times, lower energy consumption, and better product yields, reducing particle agglomeration. Beyond eliminating the need for an oil bath, microwave assisted synthesis with environmentally friendly reaction media can significantly reduce chemical waste and reaction times in various organic syntheses and chemical transformations. It has been reported that silver nanoparticles can be synthesized using microwave assistance with carboxy methyl cellulose sodium as both a reducing and stabilizing agent. The size of the nanoparticles depends on the concentrations of carboxy methyl cellulose and silver nitrate. The resulting nanoparticles were uniform, stable, and showed specific characteristics.

2. Laser Irradiation

Laser irradiation can be used to synthesize silver nanoparticles in an aqueous solution containing silver salt and surfactant. This method produces nanoparticles with a well-defined shape and size distribution. In a photo sensitization method using benzophenone, laser irradiation at low powers created silver nanoparticles of about 20 nm, while higher powers resulted in nanoparticles of about 5 nm. Both lasers and mercury lamps can serve as light sources for producing silver nanoparticles.  

3. Electrochemical Synthetic Method

The electrochemical synthetic method is used to synthesize silver nanoparticles. By adjusting electrolysis parameters and changing the composition of electrolytic solutions we can control particle size and improve the homogeneity of silver nanoparticles

4. Photoinduced Reduction

Silver nanoparticles can be produced through several photoinduced or photocatalytic reduction techniques. This photochemical approach is a clean method known for its high precision, ease of use, and adaptability. A key advantage is the ability to create nanoparticles in diverse environments such as cells, emulsions, polymer films, surfactant micelles, and glasses. In one application , polyelectrolyte capsules served as microreactors for the photoinduced synthesis of  silver nanoparticles with an average size of 8 nm .The same photoinduced  process has also been used to transform spherical silver nanoparticles into triangular nanocrystals with controlled edge lengths between 30 and 120 nm , this morphological change was managed using a dual beam illumination technique with citrate acting as stabilizers .

5. UV Initiated Photoreduction 

UV-initiated photoreduction is a straightforward and efficient technique for synthesizing silver nanoparticles (NPs). This process employs compounds like citrate, polyvinylpyrrolidone, poly (acrylic acid), and collagen to facilitate the reaction. For example, one study used laponite clay suspensions as a stabilizer to prevent aggregation during the UV photoreduction of silver nitrate. The characteristics of the resulting nanoparticles were found to depend on the duration of UV exposure. After three hours of irradiation, the NPs had a bimodal size distribution and were relatively large. Continued UV exposure broke these particles down into smaller sizes with a single, stable distribution mode. In another application of this technique, silver nanostructures—including nanospheres, nanowires, and dendrites—were produced at room temperature using poly (vinyl alcohol) as a protective stabilizing agent. The final morphology of the nanostructures was significantly influenced by the concentrations of both the poly (vinyl alcohol) and the silver nitrate.

6. Microemulsion Techniques

 Microemulsion methods offer a way to create uniform silver nanoparticles (NPs) with controllable sizes. This process uses a two-phase (aqueous-organic) system where the reactants—a metal precursor and a reducing agent—are initially separated into two immiscible liquids. A quaternary alkyl-ammonium salt acts as a mediator, controlling the transport and interaction rate between these phases at their interface. Metal clusters form at this liquid interface. They are stabilized by coating with surfactant molecules present in the non-polar organic medium and are then shuttled into the organic phase by the same mediator. A significant drawback of this technique is the requirement for large quantities of harmful organic solvents and surfactants. These must later be separated and removed from the final nanoparticle product, complicating the process. Additionally, a separate green chemistry approach has been documented. This method enables the one-step synthesis and stabilization of silver nanostructures in water at room temperature.

7. Laser Ablation Synthesis

Silver nanoparticles (NPs) can be produced through laser ablation, a process that involves using a laser to vaporize material from a solid silver target immersed in a solution. The properties of the resulting nanoparticles are influenced by several key parameters:

• The wavelength of the laser used.
•  The duration of its pulses (e.g., femtosecond, picosecond, or nanosecond).
•  The laser's fluence (energy density).
•  The total ablation times.
• The composition of the liquid medium, which may or may not contain surfactants.

A significant benefit of laser ablation over chemical synthesis methods is that it does not require chemical reagents. This allows for the creation of pure, contaminant-free silver colloids ideal for sensitive applications. For instance, one study generated silver nano spheroids (20-50 nm) in water using an 800 nm femtosecond laser. When compared to ablation with nanosecond pulses, the femtosecond method resulted in a lower production efficiency and generally yielded smaller colloidal particles.

Types Of Silver Nanoparticles

Nanoparticles (NPs) are categorized into three primary groups: inorganic, carbon-based, and organic nanoparticles.

Inorganic Based NPs

This classification encompasses nanoparticles that do not contain carbon atoms. This group primarily includes nano-sized particles composed of metals or metal oxides.

1. Metal NPs

This category includes nanoparticles made from metals such as cadmium (Cd), aluminum (Al), copper (Cu), cobalt (Co), gold (Au), iron (Fe), silver (Ag), zinc (Zn), and lead (Pb). Their distinctive properties—which arise from their size and inherent characteristics—include a high surface area, specific pore size, surface charge density, and varied forms like cylindrical and spherical shapes. They also exhibit diverse colors and can have either amorphous or crystalline structures. Furthermore, external environmental conditions like air, heat, sunlight, and moisture can influence these properties.

2. Metal Oxide NPs

Since certain metals readily form oxides, their oxide nanoparticles are synthesized to improve and enhance their material characteristics. Due to the propensity of certain metals to form oxides, their oxide nanoparticles are synthesized to achieve enhanced material properties. For instance, iron nanoparticles readily oxidize in the presence of oxygen (O?) at room temperature, forming iron oxide (Fe?O?), which exhibits greater reactivity than its metallic counterpart. Commonly synthesized metal oxide nanoparticles—including titanium oxide (TiO?), silicon oxide (SiO?), zinc oxide (ZnO), magnetite (Fe?O?), iron oxide (Fe?O?), aluminum oxide (Al?O?), and cerium oxide (CeO?)—are known for their improved efficiency, reactivity, and overall performance. Literature indicates that these metal oxide NPs often possess superior properties compared to pure metal nanoparticles.

Organic-Based NPs

Organic-based nanoparticles, also known as nano capsules, are generally non-toxic and environmentally friendly. This category includes ferritin, liposomes, micelles, and dendrimers—all of which are composed of polymeric or organic materials. These nanoparticles demonstrate high responsiveness to external stimuli such as light and heat. Owing to their distinct and advantageous characteristics, they are increasingly preferred by researchers as a promising alternative in various applications. These distinctive properties make organic NPs a superior and preferred choice for applications like drug delivery. Their stability, high drug-loading capacity, and ability to specifically adsorb or encapsulate therapeutic compounds position them as highly efficient carriers for active pharmaceutical ingredients. Their effectiveness is further defined by their unique surface morphology, size, shape, and chemical composition. In targeted drug delivery systems, organic NPs are specifically designed to transport and release active agents at precise sites of action within the body.

Carbon-Based NPs

Nanoparticles that are composed entirely of a carbon framework are classified as carbon-based nanoparticles. This category includes several distinct types such as graphene, fullerenes, carbon nanofibers, carbon nanotubes, black carbon, and activated carbon.

1. Fullerenes

Fullerenes are spherical carbon nanoparticles formed by carbon atoms connected via sp² hybridization. Their structure can consist of a single layer (monolayer) or multiple layers (poly-layered). Monolayer fullerenes, composed of roughly 28 to 1500 carbon atoms, can reach diameters up to 8.3 nm, while poly-layered versions typically range from 4 to 36 nm in diameter.

2. Graphene

Graphene is a two-dimensional, planar allotrope of carbon where the atoms are arranged in a hexagonal lattice. A single layer of this material has an extremely small thickness of approximately 1 nm.

3. Carbon Nanotubes

Carbon nanotubes are produced by rolling a graphene nano-foil into a hollow, cylindrical tube. Single-layer (monolayer) nanotubes have a very small diameter, typically less than0.7 nm. Multi-layer nanotubes can vary significantly in length, from micrometers to several millimeters, and their ends may be either sealed or open.

4. Nanofibers of Carbon

Carbon nanofibers are also fabricated from graphene nano-foils. However, unlike the uniform cylinders of nanotubes, they are coiled into shapes such as cups or cones.

5. Black Carbon

Black carbon nanoparticles are amorphous, spherical particles with diameters between 20 and 70 nm. They exhibit strong particle interactions and tend to combine into larger aggregates, forming agglomerates around 500 nm in size.

Figure 3 - showing Different types of silver nanoparticles

Analyzing the physicochemical properties of nanoparticles is essential for understanding their behavior, distribution within biological systems, safety, and effectiveness. A variety of analytical techniques are employed to characterize silver nanoparticles (AgNPs) and evaluate their functional performance. Key methods include:

• UV-vis spectroscopy
• X-ray diffractometry (XRD)
• Fourier transform infrared spectroscopy (FTIR)
• X-ray photoelectron spectroscopy (XPS)
• Dynamic light scattering (DLS)
• Scanning electron microscopy (SEM)
• Transmission electron microscopy (TEM)
•  Atomic force microscopy (AFM)

The following sections detail the fundamental principles of these primary techniques.

1. UV-Visible Spectroscopy

This technique is a fundamental and widely used tool for the initial characterization of nanoparticles, particularly for monitoring their synthesis and stability. AgNPs possess distinct optical properties, notably a surface plasmon resonance (SPR) absorption band. This band arises from the collective oscillation of free electrons in the nanoparticles when they interact with specific wavelengths of light. UV-vis spectroscopy is favored because it is rapid, straightforward, sensitive, and can be used on colloidal suspensions without the need for calibration.

2. X-Ray Diffraction (XRD)

XRD is a versatile analytical method used to determine the crystalline structure of materials at the atomic level. It is applied for phase identification, quantitative analysis, and measuring properties like crystallinity and particle size. The technique operates on Bragg's law, analyzing the diffraction patterns produced when X-rays interact with a crystalline sample. It is non-destructive and can be used to characterize a wide array of organic, inorganic, and nanoscale materials.

3. Dynamic Light Scattering (DLS)

DLS is a primary technique for determining the size distribution of nanoparticles in a solution or suspension, typically in the 2–500 nm range. It works by measuring the fluctuations in the intensity of laser light scattered by particles undergoing Brownian motion. This data is used to calculate the hydrodynamic diameter of the particles, providing crucial information about their size in a liquid medium.

4. Fourier Transform Infrared (FTIR) Spectroscopy.

FTIR spectroscopy is used to identify molecular fingerprints and functional groups present on the surface of nanoparticles. It is highly accurate and can detect very small absorbance changes. This makes it invaluable for confirming the involvement of biomolecules in green synthesis processes and for studying functional molecules attached to nanomaterials like silver, carbon nanotubes, and graphene.

5. Scanning Electron Microscopy (SEM).

SEM is a high-resolution imaging technique that uses a focused beam of electrons to provide detailed information on the surface morphology, size, shape, and distribution of nanoparticles. When combined with Energy-Dispersive X-ray spectroscopy (EDX), SEM can also provide the elemental composition of the sample, allowing researchers to correlate structure with chemistry.

Applications Of Silver Nanoparticles

Silver has been utilized for its antibacterial properties for over 5,000 years. As nanoparticles (AgNPs), silver is particularly valued because it combines potent antibacterial effects with low toxicity to human cells. This antibacterial activity is defined as the ability to kill or inhibit the growth of bacteria while leaving surrounding cells unharmed. Beyond this, AgNPs have a wide range of applications, including antiviral, antifungal, and anti-inflammatory uses, as well as in cosmetics, water treatment, healthcare, biosensing, drug delivery, imaging, and textiles.

Figure 4 – Application Of Silver Nano particles