SILVER NANOPARTICLES

Silver nanoparticles are nanoparticles of silver of between 1 nm and 100 nm in size.
Silver (Ag) Nanoparticles, nanodots or nanopowder are spherical or nanoflake high surface area metal particles with properties and uses that include inhibiting transmission of HIV and other viruses.
Nano Silver Particles are also available in Ultra high purity and high purity, coated, oleic oil-coated, dispersed, and polymer-dispersed forms.

CAS Number: 7440-22-4
EC Number: 231-131-3
Molecular Formula: Ag
Molecular Weight: 107.87

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While frequently described as being 'silver' some are composed of a large percentage of silver oxide due to their large ratio of surface to bulk silver atoms.
Numerous shapes of nanoparticles can be constructed depending on the application at hand.

Commonly used silver nanoparticles are spherical, but diamond, octagonal, and thin sheets are also common.
Their extremely large surface area permits the coordination of a vast number of ligands.
The properties of silver nanoparticles applicable to human treatments are under investigation in laboratory and animal studies, assessing potential efficacy, biosafety, and biodistribution.

Silver nanoparticles are nanoparticles of silver of between 1 nm and 100 nm in size.
Nanoscale Silver Particles are available in the size range of 10-200 nm, with specific surface area (SSA) in the 30-60 m2/g range and also available as flakes with an average particle size of 2-10 micron range with a specific surface area of approximately 40-80 m2/g.

Nano Silver Particles are also available in Ultra high purity and high purity, coated, oleic oil-coated, dispersed, and polymer-dispersed forms.
Nanofluids are generally defined as suspended nanoparticles in solution either using surfactant or surface charge technology.
Other nanostructures include nanorods, nanowhiskers, nanohorns, nanopyramids and other nanocomposites.

Surface functionalized nanoparticles allow for the particles to be preferentially adsorbed at the surface interface using chemically bound polymers.
Silver (Ag) Nanoparticles, nanodots or nanopowder are spherical or nanoflake high surface area metal particles with properties and uses that include inhibiting transmission of HIV and other viruses.

Silver nanoparticles are nanoparticles of silver of between 1 nm and 100 nm in size.
Silver (Ag) Nanoparticles, nanodots or nanopowder are spherical or nanoflake high surface area metal particles with properties and uses that include inhibiting transmission of HIV and other viruses.

Applications of Silver Nanoparticles:
Silver nanoparticles are one of the most commonly utilized nanomaterials due to their anti-microbial properties, high electrical conductivity, and optical properties.

Medical Applications:
Silver nanopaticles are widely incorporated into wound dressings, and are used as an antiseptic and disinfectant in medical applications and in consumer goods.
Silver nanoparticles have a high surface area per unit mass and release a continuous level of silver ions into their environment.

The silver ions are bioactive and have broad spectrum antimicrobial properties against a wide range of bacteria.
By controlling the size, shape, surface and agglomeration state of the Silver Nanoparticles, specific silver ion release profiles can be developed for a given application.

Household Applications:
There are instances in which silver nanoparticles and colloidal silver are used in consumer goods.
Samsung for example claimed that the use of silver nanoparticles in washing machines would help to sterilize clothes and water during the washing and rinsing functions, and allow clothes to be cleaned without the need for hot water.

The Silver Nanoparticles in these appliances are synthesized using electrolysis.
Through electrolysis, silver is extracted from metal plates and then turned into silver nanoparticles by a reduction agent.
This method avoids the drying, cleaning, and re-dispersion processes, which are generally required with alternative colloidal synthesis methods.

Importantly, the electrolysis strategy also decreases the production cost of Silver Nanoparticles, making these washing machines more affordable to manufacture.

Uses of Silver Nanoparticles:
Silver nanoparticles (Ag NPs) are used in various consumer products including cosmetics, textiles, and health-care products owing to their strong antimicrobial activity.
Silver nanoparticles (AgNPs) are widely used in medicine, physics, material sciences, and chemistry.

Silver nanoparticles have attract increasing attention for the wide range of applications in biomedicine.
Silver nanoparticles, generally smaller than 100 nm and contain 20–15,000 silver atoms, have distinct physical, chemical and biological properties compared to their bulk parent materials.

The optical, thermal, and catalytic properties of silver nanoparticles are strongly influenced by their size and shape.
Additionally, owning to their broad-spectrum antimicrobial ability, silver nanoparticles have also become the most widely used sterilizing nanomaterials in consuming and medical products, for instance, textiles, food storage bags, refrigerator surfaces, and personal care products.

Catalysis uses:
Using silver nanoparticles for catalysis has been gaining attention in recent years.
Although the most common applications are for medicinal or antibacterial purposes, silver nanoparticles have been demonstrated to show catalytic redox properties for dyes, benzene, and carbon monoxide.
Other untested compounds may use silver nanoparticles for catalysis, but the field is not fully explored.

Supported on silica spheres – reduction of dyes uses of Silver Nanoparticles:
Silver nanoparticles have been synthesized on a support of inert silica spheres.
The support plays virtually no role in the catalytic ability and serves as a method of preventing coalescence of the silver nanoparticles in colloidal solution.

Thus, the silver nanoparticles were stabilized and it was possible to demonstrate the ability of them to serve as an electron relay for the reduction of dyes by sodium borohydride.
Without the silver nanoparticle catalyst, virtually no reaction occurs between sodium borohydride and the various dyes: methylene blue, eosin, and rose bengal.

Mesoporous aerogel – selective oxidation of benzene uses:
Silver nanoparticles supported on aerogel are advantageous due to the higher number of active sites.
The highest selectivity for oxidation of benzene to phenol was observed at low weight percent of silver in the aerogel matrix (1% Ag).

This better selectivity is believed to be a result of the higher monodispersity within the aerogel matrix of the 1% Ag sample.
Each weight percent solution formed different sized particles with a different width of size range.

Silver alloy – synergistic oxidation of carbon monoxide uses of Silver Nanoparticles:
Au-Ag alloy nanoparticles have been shown to have a synergistic effect on the oxidation of carbon monoxide (CO).
On Silver nanoparticles own, each pure-metal nanoparticle shows very poor catalytic activity for CO oxidation; together, the catalytic properties are greatly enhanced.

Silver nanoparticles is proposed that the gold acts as a strong binding agent for the oxygen atom and the silver serves as a strong oxidizing catalyst, although the exact mechanism is still not completely understood.
When synthesized in an Au/Ag ratio from 3:1 to 10:1, the alloyed nanoparticles showed complete conversion when 1% CO was fed in air at ambient temperature.

The size of the alloyed particles did not play a big role in the catalytic ability.
Silver nanoparticles is well known that gold nanoparticles only show catalytic properties for CO when they are ~3 nm in size, but alloyed particles up to 30 nm demonstrated excellent catalytic activity – catalytic activity better than that of gold nanoparticles on active support such as TiO2, Fe2O3, etc.

Light-enhanced uses:
Plasmonic effects have been studied quite extensively.
Until recently, there have not been studies investigating the oxidative catalytic enhancement of a nanostructure via excitation of Silver nanoparticles surface plasmon resonance.

The defining feature for enhancing the oxidative catalytic ability has been identified as the ability to convert a beam of light into the form of energetic electrons that can be transferred to adsorbed molecules.
The implication of such a feature is that photochemical reactions can be driven by low-intensity continuous light coupled with thermal energy.

The coupling of low-intensity continuous light and thermal energy has been performed with silver nanocubes.
The important feature of silver nanostructures that are enabling for photocatalysis is their nature to create resonant surface plasmons from light in the visible range.

The addition of light enhancement enabled the particles to perform to the same degree as particles that were heated up to 40 K greater.
This is a profound finding when noting that a reduction in temperature of 25 K can increase the catalyst lifetime by nearly tenfold, when comparing the photothermal and thermal process.

Sensors uses:
Peptide capped silver nanoparticle for colorimetric sensing has been mostly studied in past years, which focus on the nature of the peptide and silver interaction and the effect of the peptide on the formation of the silver nanoparticles.
Besides, the efficiency of silver nanoparticles based fluorescent sensors can be very high and overcome the detection limits.

Optical probes uses:
Silver nanoparticles are widely used as probes for surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF).
Compared to other noble metal nanoparticles, silver nanoparticles exhibits more advantages for probe, such as higher extinction coefficients, sharper extinction bands, and high field enhancements.

Antibacterial agents uses:
Silver nanoparticles are most widely used sterilizing nanomaterial in consuming and medical products, for instance, textiles, food storage bags, refrigerator surfaces, and personal care products.
Silver nanoparticles has been proved that the antibacterial effect of silver nanoparticles is due to the sustained release of free silver ions from the nanoparticles.

Catalyst uses:
Silver nanoparticles have been demonstrated to present catalytic redox properties for biological agents such as dyes, as well as chemical agents such as benzene.

The chemical environment of the nanoparticle plays an important role in their catalytic properties.
In addition, Silver Nanoparticles is important to know that complicated catalysis takes place by adsorption of the reactant species to the catalytic substrate.

When polymers, complex ligands, or surfactants are used as the stabilizer or to prevent coalescence of the nanoparticles, the catalytic ability is usually decreased due to reduced adsorption ability.
In general, silver nanoparticles are mostly used with titanium dioxide as the catalyst for chemical reactions.

Conductive Composites uses:
Incorporation of silver particles into plastics, composites, and adhesives increases the electrical conductivity of the material.
Silver pastes and epoxies are widely utilized in the electronics industries.

Silver Nanoparticles based inks are used to print flexible electronics and have the advantage that the melting point of the small silver nanoparticles in the ink is reduced by hundreds of degrees compared to bulk silver.
When scintered, these silver nanoparticle based inks have excellent conductivity.

Plasmonics Uses:
Silver nanoparticles have unique optical properties because they support surface plasmons.
At specific wavelengths of light the surface plasmons are driven into resonance and strongly absorb or scatter incident light.
This effect is so strong that Silver nanoparticles allows for individual Silver Nanoparticles as small as 20 nm in diameter to be imaged using a conventional dark field microscope.

This strong coupling of metal nanostructures with light is the basis for the new field of plasmonics.
Applications of plasmonic silver nanoparticles include biomedical labels, sensors, and detectors.
Silver Nanoparticles is also the basis for analysis techniques such as Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Fluorescent Spectroscopy.

Photovoltaics Uses:
There is increasing interest in utilizing the large scattering and absorption cross sections of plasmonic silver nanoparticles for solar applications.
Since the Silver Nanoparticles act as efficient optical antennas, very high efficiencies can be obtained when the nanoparticles are incorporated into collectors.

Properties of Silver Nanoparticles:

Optical Properties:
When silver nanoparticles are exposed to a specific wavelength of light, the oscillating electromagnetic field of the light induces a collective coherent oscillation of the free electrons, which causes a charge separation with respect to the ionic lattice, forming a dipole oscillation along the direction of the electric field of the light.

The amplitude of the oscillation reaches maximum at a specific frequency, called surface plasmon resonance (SPR).
The absorption and scattering properties of silver nanoparticles can be changed by controlling the particle size, shape and refractive index near the particle surface.

For example, smaller nanoparticles mostly absorb light and have peaks near 400 nm, while larger nanoparticles exhibit increased scattering and have peaks that broaden and shift towards longer wavelengths.
Besides, the optical properties of silver nanoparticles can also change when particles aggregate and the conduction electrons near each particle surface become delocalized.

*Antibacterial Effects of Silver Nanoparticles:
The antibacterial effects of silver nanoparticles have been used to control bacterial growth in a variety of applications, including dental work, surgery applications, wounds and burns treatment, and biomedical devices.
Silver Nanoparticles is well known that silver ions and silverbased compounds are highly toxic to microorganisms.

Introduction of silver nanoparticles into bacterial cells can induce a high degree of structural and morphological changes, which can lead to cell death.
Scientists have demonstrated that the antibacterial effect of silver nanoparticles is mostly due to the sustained release of free silver ions from the nanoparticles, which serve as a vehicle for silver ions.

Products And Functionalization Of Silver Nanoparticles:
Synthetic protocols for silver nanoparticle production can be modified to produce silver nanoparticles with non-spherical geometries and also to functionalize nanoparticles with different materials, such as silica.
Creating silver nanoparticles of different shapes and surface coatings allows for greater control over their size-specific properties.

Anisotropic structures:
Silver nanoparticles can be synthesized in a variety of non-spherical (anisotropic) shapes.
Because silver, like other noble metals, exhibits a size and shape dependent optical effect known as localized surface plasmon resonance (LSPR) at the nanoscale, the ability to synthesize Silver Nanoparticles in different shapes vastly increases the ability to tune their optical behavior.

For example, the wavelength at which LSPR occurs for a Silver Nanoparticles of one morphology (e.g. a sphere) will be different if that sphere is changed into a different shape.
This shape dependence allows a silver nanoparticle to experience optical enhancement at a range of different wavelengths, even by keeping the size relatively constant, just by changing Silver nanoparticles shape.

This aspect can be exploited in synthesis to promote change in shape of Silver Nanoparticles through light interaction.
The applications of this shape-exploited expansion of optical behavior range from developing more sensitive biosensors to increasing the longevity of textiles.

Triangular nanoprisms:
Triangular-shaped Silver Nanoparticles are a canonical type of anisotropic morphology studied for both gold and silver.

Though many different techniques for silver nanoprism synthesis exist, several methods employ a seed-mediated approach, which involves first synthesizing small (3-5 nm diameter) silver nanoparticles that offer a template for shape-directed growth into triangular nanostructures.

The silver seeds are synthesized by mixing silver nitrate and sodium citrate in aqueous solution and then rapidly adding sodium borohydride.
Additional silver nitrate is added to the seed solution at low temperature, and the prisms are grown by slowly reducing the excess silver nitrate using ascorbic acid.

With the seed-mediated approach to silver nanoprism synthesis, selectivity of one shape over another can in part be controlled by the capping ligand.
Using essentially the same procedure above but changing citrate to poly (vinyl pyrrolidone) (PVP) yields cube and rod-shaped nanostructures instead of triangular nanoprisms.

In addition to the seed mediated technique, silver nanoprisms can also be synthesized using a photo-mediated approach, in which preexisting spherical silver nanoparticles are transformed into triangular nanoprisms simply by exposing the reaction mixture to high intensities of light.

Nanocubes:
Silver nanocubes can be synthesized using ethylene glycol as a reducing agent and PVP as a capping agent, in a polyol synthesis reaction (vide supra).
A typical synthesis using these reagents involves adding fresh silver nitrate and PVP to a solution of ethylene glycol heated at 140 °C.

This procedure can actually be modified to produce another anisotropic silver nanostructure, nanowires, by just allowing the silver nitrate solution to age before using Silver nanoparticles in the synthesis.
By allowing the silver nitrate solution to age, the initial nanostructure formed during the synthesis is slightly different than that obtained with fresh silver nitrate, which influences the growth process, and therefore, the morphology of the final product.

Coating with silica:
In this method, polyvinylpyrrolidone (PVP) is dissolved in water by sonication and mixed with silver colloid particles.
Active stirring ensures the PVP has adsorbed to the Silver Nanoparticles surface.
Centrifuging separates the PVP coated Silver Nanoparticles which are then transferred to a solution of ethanol to be centrifuged further and placed in a solution of ammonia, ethanol and Si(OEt4) (TES).

Stirring for twelve hours results in the silica shell being formed consisting of a surrounding layer of silicon oxide with an ether linkage available to add functionality.
Varying the amount of TES allows for different thicknesses of shells formed.
This technique is popular due to the ability to add a variety of functionality to the exposed silica surface.

Synthesis Methods of Silver Nanoparticles:

Wet chemistry:
The most common methods for nanoparticle synthesis fall under the category of wet chemistry, or the nucleation of particles within a solution.
This nucleation occurs when a silver ion complex, usually AgNO3 or AgClO4, is reduced to colloidal Ag in the presence of a reducing agent. When the concentration increases enough, dissolved metallic silver ions bind together to form a stable surface.

The surface is energetically unfavorable when the cluster is small, because the energy gained by decreasing the concentration of dissolved particles is not as high as the energy lost from creating a new surface.
When the cluster reaches a certain size, known as the critical radius, Silver nanoparticles becomes energetically favorable, and thus stable enough to continue to grow.

This nucleus then remains in the system and grows as more silver atoms diffuse through the solution and attach to the surface.
When the dissolved concentration of atomic silver decreases enough, Silver nanoparticles is no longer possible for enough atoms to bind together to form a stable nucleus.
At this nucleation threshold, new nanoparticles stop being formed, and the remaining dissolved silver is absorbed by diffusion into the growing nanoparticles in the solution.

As the particles grow, other molecules in the solution diffuse and attach to the surface.
This process stabilizes the surface energy of the particle and blocks new silver ions from reaching the surface.

The attachment of these capping/stabilizing agents slows and eventually stops the growth of the particle.
The most common capping ligands are trisodium citrate and polyvinylpyrrolidone (PVP), but many others are also used in varying conditions to synthesize particles with particular sizes, shapes, and surface properties.

There are many different wet synthesis methods, including the use of reducing sugars, citrate reduction, reduction via sodium borohydride, the silver mirror reaction, the polyol process, seed-mediated growth, and light-mediated growth.
Each of these methods, or a combination of methods, will offer differing degrees of control over the size distribution as well as distributions of geometric arrangements of the nanoparticle.

A new, very promising wet-chemical technique was found by Elsupikhe et al. (2015).
They have developed a green ultrasonically-assisted synthesis.

Under ultrasound treatment, silver nanoparticles (AgNP) are synthesized with κ-carrageenan as a natural stabilizer.
The reaction is performed at ambient temperature and produces silver nanoparticles with fcc crystal structure without impurities.
The concentration of κ-carrageenan is used to influence particle size distribution of the AgNPs

Monosaccharide reduction:
There are many ways silver nanoparticles can be synthesized; one method is through monosaccharides.
This includes glucose, fructose, maltose, maltodextrin, etc., but not sucrose.
Silver nanoparticles is also a simple method to reduce silver ions back to silver nanoparticles as it usually involves a one-step process.

There have been methods that indicated that these reducing sugars are essential to the formation of silver nanoparticles.
Many studies indicated that this method of green synthesis, specifically using Cacumen platycladi extract, enabled the reduction of silver.
Additionally, the size of Silver Nanoparticles could be controlled depending on the concentration of the extract.

The studies indicate that the higher concentrations correlated to an increased number of nanoparticles.
Smaller Silver Nanoparticles were formed at high pH levels due to the concentration of the monosaccharides.
Another method of silver nanoparticles synthesis includes the use of reducing sugars with alkali starch and silver nitrate.

The reducing sugars have free aldehyde and ketone groups, which enable them to be oxidized into gluconate.
The monosaccharide must have a free ketone group because in order to act as a reducing agent Silver nanoparticles first undergoes tautomerization.
In addition, if the aldehydes are bound, Silver Nanoparticles will be stuck in cyclic form and cannot act as a reducing agent.

For example, glucose has an aldehyde functional group that is able to reduce silver cations to silver atoms and is then oxidized to gluconic acid.
The reaction for the sugars to be oxidized occurs in aqueous solutions. The capping agent is also not present when heated.

Citrate reduction:
An early, and very common, method for synthesizing silver nanoparticles is citrate reduction.
This method was first recorded by M. C. Lea, who successfully produced a citrate-stabilized silver colloid in 1889.
Citrate reduction involves the reduction of a silver source particle, usually AgNO3 or AgClO4, to colloidal silver using trisodium citrate, Na3C6H5O7.

The synthesis is usually performed at an elevated temperature (~100 °C) to maximize the monodispersity (uniformity in both size and shape) of the particle.
In this method, the citrate ion traditionally acts as both the reducing agent and the capping ligand, making Silver nanoparticles a useful process for AgNP production due to Silver nanoparticles relative ease and short reaction time.
However, the silver particles formed may exhibit broad size distributions and form several different particle geometries simultaneously.
The addition of stronger reducing agents to the reaction is often used to synthesize particles of a more uniform size and shape.

Reduction via sodium borohydride:
The synthesis of silver nanoparticles by sodium borohydride (NaBH4) reduction occurs by the following reaction:
Ag+ + BH4− + 3 H2O → Ag0 +B(OH)3 +3.5 H2
The reduced metal atoms will form nanoparticle nuclei.

Overall, this process is similar to the above reduction method using citrate.
The benefit of using sodium borohydride is increased monodispersity of the final particle population.
The reason for the increased monodispersity when using NaBH4 is that Silver nanoparticles is a stronger reducing agent than citrate.

The impact of reducing agent strength can be seen by inspecting a LaMer diagram which describes the nucleation and growth of nanoparticles.
When silver nitrate (AgNO3) is reduced by a weak reducing agent like citrate, the reduction rate is lower which means that new nuclei are forming and old nuclei are growing concurrently.

This is the reason that the citrate reaction has low monodispersity.
Because NaBH4 is a much stronger reducing agent, the concentration of silver nitrate is reduced rapidly which shortens the time during which new nuclei form and grow concurrently yielding a monodispersed population of silver nanoparticles.

Particles formed by reduction must have their surfaces stabilized to prevent undesirable particle agglomeration (when multiple particles bond together), growth, or coarsening.
The driving force for these phenomena is the minimization of surface energy (nanoparticles have a large surface to volume ratio).

This tendency to reduce surface energy in the system can be counteracted by adding species which will adsorb to the surface of the nanoparticles and lowers the activity of the particle surface thus preventing particle agglomeration according to the DLVO theory and preventing growth by occupying attachment sites for metal atoms.

Chemical species that adsorb to the surface of nanoparticles are called ligands.
Some of these surface stabilizing species are: NaBH4 in large amounts, poly(vinyl pyrrolidone) (PVP), sodium dodecyl sulfate (SDS), and/or dodecanethiol.

Once the particles have been formed in solution they must be separated and collected.
There are several general methods to remove nanoparticles from solution, including evaporating the solvent phase or the addition of chemicals to the solution that lower the solubility of the nanoparticles in the solution.
Both methods force the precipitation of the nanoparticles.

Polyol process:
The polyol process is a particularly useful method because Silver nanoparticles yields a high degree of control over both the size and geometry of the resulting nanoparticles.
In general, the polyol synthesis begins with the heating of a polyol compound such as ethylene glycol, 1,5-pentanediol, or 1,2-propylene glycol7.
An Ag+ species and a capping agent are added (although the polyol itself is also often the capping agent).

The Ag+ species is then reduced by the polyol to colloidal nanoparticles.
The polyol process is highly sensitive to reaction conditions such as temperature, chemical environment, and concentration of substrates.

Therefore, by changing these variables, various sizes and geometries can be selected for such as quasi-spheres, pyramids, spheres, and wires.
Further study has examined the mechanism for this process as well as resulting geometries under various reaction conditions in greater detail

Seed-mediated growth:
Seed-mediated growth is a synthetic method in which small, stable nuclei are grown in a separate chemical environment to a desired size and shape.
Seed-mediated methods consist of two different stages: nucleation and growth.

Variation of certain factors in the synthesis (e.g. ligand, nucleation time, reducing agent, etc.), can control the final size and shape of Silver
Nanoparticles, making seed-mediated growth a popular synthetic approach to controlling morphology of nanoparticles.

The nucleation stage of seed-mediated growth consists of the reduction of metal ions in a precursor to metal atoms.
In order to control the size distribution of the seeds, the period of nucleation should be made short for monodispersity.

The LaMer model illustrates this concept.
Seeds typically consist small Silver Nanoparticles, stabilized by a ligand.
Ligands are small, usually organic molecules that bind to the surface of particles, preventing seeds from further growth.

Ligands are necessary as they increase the energy barrier of coagulation, preventing agglomeration.
The balance between attractive and repulsive forces within colloidal solutions can be modeled by DLVO theory.

Ligand binding affinity, and selectivity can be used to control shape and growth.
For seed synthesis, a ligand with medium to low binding affinity should be chosen as to allow for exchange during growth phase.

The growth of nanoseeds involves placing the seeds into a growth solution.
The growth solution requires a low concentration of a metal precursor, ligands that will readily exchange with preexisting seed ligands, and a weak or very low concentration of reducing agent.

The reducing agent must not be strong enough to reduce metal precursor in the growth solution in the absence of seeds.
Otherwise, the growth solution will form new nucleation sites instead of growing on preexisting ones (seeds).
Growth is the result of the competition between surface energy (which increases unfavorably with growth) and bulk energy (which decreases favorably with growth).

The balance between the energetics of growth and dissolution is the reason for uniform growth only on preexisting seeds (and no new nucleation).
Growth occurs by the addition of metal atoms from the growth solution to the seeds, and ligand exchange between the growth ligands (which have a higher bonding affinity) and the seed ligands.

Range and direction of growth can be controlled by nanoseed, concentration of metal precursor, ligand, and reaction conditions (heat, pressure, etc.).
Controlling stoichiometric conditions of growth solution controls ultimate size of particle.
For example, a low concentration of metal seeds to metal precursor in the growth solution will produce larger particles.

Capping agent has been shown to control direction of growth and thereby shape.
Ligands can have varying affinities for binding across a particle.

Differential binding within a particle can result in dissimilar growth across particle.
This produces anisotropic particles with nonspherical shapes including prisms, cubes, and rods.

Light-mediated growth:
Light-mediated syntheses have also been explored where light can promote formation of various Silver Nanoparticles morphologies.

Silver mirror reaction:
The silver mirror reaction involves the conversion of silver nitrate to Ag(NH3)OH.
Ag(NH3)OH is subsequently reduced into colloidal silver using an aldehyde containing molecule such as a sugar.

The silver mirror reaction is as follows:
2(Ag(NH3)2)+ + RCHO + 2OH− → RCOOH + 2Ag + 4NH3

The size and shape of the Silver Nanoparticles produced are difficult to control and often have wide distributions.
However, this method is often used to apply thin coatings of silver particles onto surfaces and further study into producing more uniformly sized Silver Nanoparticles is being done.

Ion implantation:
Ion implantation has been used to create silver nanoparticles embedded in glass, polyurethane, silicone, polyethylene, and poly(methyl methacrylate).
Particles are embedded in the substrate by means of bombardment at high accelerating voltages.

At a fixed current density of the ion beam up to a certain value, the size of the embedded silver nanoparticles has been found to be monodisperse within the population, after which only an increase in the ion concentration is observed.

A further increase in the ion beam dose has been found to reduce both the Silver Nanoparticles size and density in the target substrate, whereas an ion beam operating at a high accelerating voltage with a gradually increasing current density has been found to result in a gradual increase in the Silver Nanoparticles size.

There are a few competing mechanisms which may result in the decrease in Silver Nanoparticles size; destruction of NPs upon collision, sputtering of the sample surface, particle fusion upon heating and dissociation.

The formation of embedded Silver Nanoparticles is complex, and all of the controlling parameters and factors have not yet been investigated.
Computer simulation is still difficult as Silver nanoparticles involves processes of diffusion and clustering, however it can be broken down into a few different sub-processes such as implantation, diffusion, and growth.

Upon implantation, silver ions will reach different depths within the substrate which approaches a Gaussian distribution with the mean centered at X depth.
High temperature conditions during the initial stages of implantation will increase the impurity diffusion in the substrate and as a result limit the impinging ion saturation, which is required for Silver Nanoparticles nucleation.

Both the implant temperature and ion beam current density are crucial to control in order to obtain a monodisperse Silver Nanoparticles size and depth distribution.
A low current density may be used to counter the thermal agitation from the ion beam and a buildup of surface charge.

After implantation on the surface, the beam currents may be raised as the surface conductivity will increase.
The rate at which impurities diffuse drops quickly after the formation of the Silver Nanoparticles, which act as a mobile ion trap.
This suggests that the beginning of the implantation process is critical for control of the spacing and depth of the resulting Silver Nanoparticles, as well as control of the substrate temperature and ion beam density.

The presence and nature of these particles can be analyzed using numerous spectroscopy and microscopy instruments.
Silver Nanoparticles synthesized in the substrate exhibit surface plasmon resonances as evidenced by characteristic absorption bands; these features undergo spectral shifts depending on the Silver Nanoparticles size and surface asperities, however the optical properties also strongly depend on the substrate material of the composite.

Biological synthesis:
The biological synthesis of Silver Nanoparticles has provided a means for improved techniques compared to the traditional methods that call for the use of harmful reducing agents like sodium borohydride.
Many of these methods could improve their environmental footprint by replacing these relatively strong reducing agents.

The commonly used biological methods are using plant or fruit extracts, fungi, and even animal parts like insect wing extract.
The problems with the chemical production of silver Silver Nanoparticles is usually involves high cost and the longevity of the particles is short lived due to aggregation.

The harshness of standard chemical methods has sparked the use of using biological organisms to reduce silver ions in solution into colloidal Silver Nanoparticles.

In addition, precise control over shape and size is vital during Silver Nanoparticles synthesis since the NPs therapeutic properties are intimately dependent on such factors.
Hence, the primary focus of research in biogenic synthesis is in developing methods that consistently reproduce NPs with precise properties.

Fungi and bacteria:
Bacterial and fungal synthesis of Silver Nanoparticles is practical because bacteria and fungi are easy to handle and can be modified genetically with ease.
This provides a means to develop biomolecules that can synthesize AgNPs of varying shapes and sizes in high yield, which is at the forefront of current challenges in Silver Nanoparticles synthesis.

Fungal strains such as Verticillium and bacterial strains such as Klebsiella pneumoniae can be used in the synthesis of silver Silver Nanoparticles.
When the fungus/bacteria is added to solution, protein biomass is released into the solution.
Electron donating residues such as tryptophan and tyrosine reduce silver ions in solution contributed by silver nitrate.

These methods have been found to effectively create stable monodisperse Silver Nanoparticles without the use of harmful reducing agents.
A method has been found of reducing silver ions by the introduction of the fungus Fusarium oxysporum.
The Silver Nanoparticles formed in this method have a size range between 5 and 15 nm and consist of silver hydrosol.

The reduction of the Silver Nanoparticles is thought to come from an enzymatic process and Silver Nanoparticles produced are extremely stable due to interactions with proteins that are excreted by the fungi.

Bacterium found in silver mines, Pseudomonas stutzeri AG259, were able to construct silver particles in the shapes of triangles and hexagons.
The size of these Silver Nanoparticles had a large range in size and some of them reached sizes larger than the usual nanoscale with a size of 200 nm.
The Silver Nanoparticles were found in the organic matrix of the bacteria.

Lactic acid producing bacteria have been used to produce silver nanoparticles.
The bacteria Lactobacillus spp., Pediococcus pentosaceus, Enteroccus faeciumI, and Lactococcus garvieae have been found to be able to reduce silver ions into silver nanoparticles.

The production of the Silver Nanoparticles takes place in the cell from the interactions between the silver ions and the organic compounds of the cell.
Silver nanoparticles was found that the bacterium Lactobacillus fermentum created the smallest silver nanoparticles with an average size of 11.2 nm.

Silver nanoparticles was also found that this bacterium produced the Silver Nanoparticles with the smallest size distribution and the Silver Nanoparticles were found mostly on the outside of the cells.
Silver nanoparticles was also found that there was an increase in the pH increased the rate of which the Silver Nanoparticles were produced and the amount of particles produced.

Plants:
The reduction of silver ions into silver nanoparticles has also been achieved using geranium leaves.
Silver Nanoparticles has been found that adding geranium leaf extract to silver nitrate solutions causes their silver ions to be quickly reduced and that the nanoparticles produced are particularly stable.

The silver nanoparticles produced in solution had a size range between 16 and 40 nm.
In another study different plant leaf extracts were used to reduce silver ions.
Silver Nanoparticles was found that out of Camellia sinensis (green tea), pine, persimmon, ginko, magnolia, and platanus that the magnolia leaf extract was the best at creating silver nanoparticles.

This method created particles with a disperse size range of 15 to 500 nm, but Silver Nanoparticles was also found that the particle size could be controlled by varying the reaction temperature.
The speed at which the ions were reduced by the magnolia leaf extract was comparable to those of using chemicals to reduce.

The use of plants, microbes, and fungi in the production of silver nanoparticles is leading the way to more environmentally sound production of silver nanoparticles.
A green method is available for synthesizing silver nanoparticles using Amaranthus gangeticus Linn leaf extract.

Biological Research of Silver Nanoparticles:
Researchers have explored the use of silver nanoparticles as carriers for delivering various payloads such as small drug molecules or large biomolecules to specific targets.
Once the AgNP has had sufficient time to reach Silver nanoparticles target, release of the payload could potentially be triggered by an internal or external stimulus.
The targeting and accumulation of Silver Nanoparticles may provide high payload concentrations at specific target sites and could minimize side effects.

Chemotherapy:
The introduction of nanotechnology into medicine is expected to advance diagnostic cancer imaging and the standards for therapeutic drug design.
Nanotechnology may uncover insight about the structure, function and organizational level of the biosystem at the nanoscale.
Silver nanoparticles can undergo coating techniques that offer a uniform functionalized surface to which substrates can be added.

When the Silver Nanoparticles is coated, for example, in silica the surface exists as silicic acid.
Substrates can thus be added through stable ether and ester linkages that are not degraded immediately by natural metabolic enzymes.
Recent chemotherapeutic applications have designed anti cancer drugs with a photo cleavable linker, such as an ortho-nitrobenzyl bridge, attaching Silver nanoparticles to the substrate on the Silver Nanoparticles surface.

The low toxicity Silver Nanoparticles complex can remain viable under metabolic attack for the time necessary to be distributed throughout the bodies systems.
If a cancerous tumor is being targeted for treatment, ultraviolet light can be introduced over the tumor region.
The electromagnetic energy of the light causes the photo responsive linker to break between the drug and the Silver Nanoparticles substrate.

The drug is now cleaved and released in an unaltered active form to act on the cancerous tumor cells.
Advantages anticipated for this method is that the drug is transported without highly toxic compounds, the drug is released without harmful radiation or relying on a specific chemical reaction to occur and the drug can be selectively released at a target tissue.

A second approach is to attach a chemotherapeutic drug directly to the functionalized surface of the silver nanoparticle combined with a nucelophilic species to undergo a displacement reaction.
For example, once the Silver Nanoparticles drug complex enters or is in the vicinity of the target tissue or cells, a glutathione monoester can be administered to the site.

The nucleophilic ester oxygen will attach to the functionalized surface of the Silver Nanoparticles through a new ester linkage while the drug is released to Silver nanoparticles surroundings.
The drug is now active and can exert Silver nanoparticles biological function on the cells immediate to its surroundings limiting non-desirable interactions with other tissues.

Multiple drug resistance:
A major cause for the ineffectiveness of current chemotherapy treatments is multiple drug resistance which can arise from several mechanisms.
Nanoparticles can provide a means to overcome MDR.
In general, when using a targeting agent to deliver nanocarriers to cancer cells, Silver nanoparticles is imperative that the agent binds with high selectivity to molecules that are uniquely expressed on the cell surface.

Hence NPs can be designed with proteins that specifically detect drug resistant cells with overexpressed transporter proteins on their surface.
A pitfall of the commonly used nano-drug delivery systems is that free drugs that are released from the nanocarriers into the cytosol get exposed to the MDR transporters once again, and are exported.

To solve this, 8 nm nanocrystalline silver particles were modified by the addition of trans-activating transcriptional activator (TAT), derived from the HIV-1 virus, which acts as a cell-penetrating peptide (CPP).
Generally, AgNP effectiveness is limited due to the lack of efficient cellular uptake; however, CPP-modification has become one of the most efficient methods for improving intracellular delivery of nanoparticles.

Once ingested, the export of the AgNP is prevented based on a size exclusion.
The concept is simple: the nanoparticles are too large to be effluxed by the MDR transporters, because the efflux function is strictly subjected to the size of Silver nanoparticles substrates, which is generally limited to a range of 300-2000 Da.
Thereby the nanoparticulates remain insusceptible to the efflux, providing a means to accumulate in high concentrations

Antimicrobial:
Introduction of silver into bacterial cells induces a high degree of structural and morphological changes, which can lead to cell death.
As the silver nanoparticles come in contact with the bacteria, they adhere to the cell wall and cell membrane.

Once bound, some of the silver passes through to the inside, and interacts with phosphate-containing compounds like DNA and RNA, while another portion adheres to the sulfur-containing proteins on the membrane.
The silver-sulfur interactions at the membrane cause the cell wall to undergo structural changes, like the formation of pits and pores.

Through these pores, cellular components are released into the extracellular fluid, simply due to the osmotic difference. Within the cell, the integration of silver creates a low molecular weight region where the DNA then condenses.
Having DNA in a condensed state inhibits the cell's replication proteins contact with the DNA.

Thus the introduction of silver nanoparticles inhibits replication and is sufficient to cause the death of the cell.
Further increasing their effect, when silver comes in contact with fluids, Silver nanoparticles tends to ionize which increases the Silver Nanoparticles' bactericidal activity.

This has been correlated to the suppression of enzymes and inhibited expression of proteins that relate to the cell's ability to produce ATP.
Although Silver nanoparticles varies for every type of cell proposed, as their cell membrane composition varies greatly, Silver nanoparticles has been seen that in general, silver nanoparticles with an average size of 10 nm or less show electronic effects that greatly increase their bactericidal activity.
This could also be partly due to the fact that as particle size decreases, reactivity increases due to the surface area to volume ratio increasing.

Silver nanoparticles have been shown to have synergistic antibacterial activity with commonly used antibiotics such as; penicillin G, ampicillin, erythromycin, clindamycin, and vancomycin against E. coli and S. aureus.
Furthermore, synergistic antibacterial activity has been reported between silver nanoparticles and hydrogen peroxide causing this combination to exert significantly enhanced bactericidal effect against both Gram negative and Gram positive bacteria.

This antibacterial synergy between silver nanoparticles and hydrogen peroxide can be possibly attributed to a Fenton-like reaction that generates highly reactive oxygen species such as hydroxyl radicals.
Silver nanoparticles can prevent bacteria from growing on or adhering to the surface.

This can be especially useful in surgical settings where all surfaces in contact with the patient must be sterile.
Silver nanoparticles can be incorporated on many types of surfaces including metals, plastic, and glass.
In medical equipment, Silver nanoparticles has been shown that silver nano particles lower the bacterial count on devices used compared to old techniques.

However, the problem arises when the procedure is over and a new one must be done.
In the process of washing the instruments a large portion of the silver nano particles become less effective due to the loss of silver ions.
They are more commonly used in skin grafts for burn victims as the silver nano particles embedded with the graft provide better antimicrobial activity and result in significantly less scarring of the victim.

These new applications are direct decedents of older practices that used silver nitrate to treat conditions such as skin ulcers.
Now, silver nanoparticles are used in bandages and patches to help heal certain burns and wounds.
An alternative approach is to use AgNP to sterilise biological dressings (for example, tilapia fish skin) for burn and wound management.

They also show promising application as water treatment method to form clean potable water.
This doesn't sound like much, but water contains numerous diseases and some parts of the world do not have the luxury of clean water, or any at all.
Silver nanoparticles wasn't new to use silver for removing microbes, but this experiment used the carbonate in water to make microbes even more vulnerable to silver.

First the scientists of the experiment use the nanopaticles to remove certain pesticides from the water, ones that prove fatal to people if ingested.
Several other tests have shown that the silver nanoparticles were capable of removing certain ions in water as well, like iron, lead, and arsenic.

But that is not the only reason why the silver nanoparticles are so appealing, they do not require any external force (no electricity of hydrolics) for the reaction to occur.
Conversely, post-consumer silver nanoparticles in waste water may adversely impact biological agents used in waste water treatment

Metrology of Silver Nanoparticles:
A number of reference materials are available for silver nanoparticles.
NIST RM 8017 contains 75 nm silver nanoparticles embedded in a cake of the polymer polyvinylpyrrolidone to stabilize them against oxidation for a long shelf life.

They have reference values for mean particle size using dynamic light scattering, ultra-small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy; and size distribution reference values for the latter two methods.
The BAM-N001 certified reference material contains silver nanoparticles with a specified size distribution with a number-weighted median size of 12.6 nm measured by small-angle X-ray scattering and transmission electron microscopy.

Handling And Storage of Silver Nanoparticles:

Precautions for safe handling:

Hygiene measures:
Immediately change contaminated clothing.
Apply preventive skin protection.
Wash hands and face after working with substance.

Stability And Reactivity of Silver Nanoparticles:

Chemical stability:
Silver nanoparticles is chemically stable under standard ambient conditions (room temperature).

Possibility of hazardous reactions:
No data available

First Aid Measures of Silver Nanoparticles:

If inhaled:

After inhalation:
Fresh air.

In case of skin contact:
Take off immediately all contaminated clothing.
Rinse skin with water/ shower.
Consult a physician.

In case of eye contact:

After eye contact:
Rinse out with plenty of water.
Call in ophthalmologist.
Remove contact lenses.

If swallowed:

After swallowing:
Immediately make victim drink water (two glasses at most).
Consult a physician.

Indication of any immediate medical attention and special treatment needed:
No data available

Fire Fighting Measures of Silver Nanoparticles:

Suitable extinguishing media:
Water
Foam
Carbon dioxide (CO2)
Dry powder

Unsuitable extinguishing media:
For this substance/mixture no limitations of extinguishing agents are given.

Further information:
Prevent fire extinguishing water from contaminating surface water or the ground water system.

Accidental Release Measures of Silver Nanoparticles:

Environmental precautions:
Do not let product enter drains.

Methods and materials for containment and cleaning up:
Cover drains.
Collect, bind, and pump off spills.

Observe possible material restrictions.
Take up dry.

Dispose of properly.
Clean up affected area

Exposure Controls/Personal Protection of Silver Nanoparticles:

Personal protective equipment:

Eye/face protection:
Use equipment for eye protection
Safety glasses

Skin protection:
Handle with gloves.
Wash and dry hands.

Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0,11 mm
Break through time: 480 min

Splash contact:
Material: Nitrile rubber
Minimum layer thickness: 0,11 mm
Break through time: 480 min

Body Protection:
protective clothing

Respiratory protection:
Recommended Filter type: Filter type P2

Control of environmental exposure:
Do not let product enter drains.

Identifiers of Silver Nanoparticles:
CAS Number: 7440-22-4
EC Number: 231-131-3
MDL Number: MFCD00003397
Linear Formula: Ag

CAS Number: 7440-22-4
EC Number: 231-131-3
Molecular Formula: Ag
Molecular Weight: 107.87

Properties of Silver Nanoparticles:
Molecular Weight: 107.87
Appearance: Powder
Melting Point: 961.78 °C
Boiling Point: 2162 °C
Density: N/A
Bulk Density: 0.312 g/cm3
True Density: ~10.5 g/cm3
Size Range: 80-100 nm
Average Particle Size: Specific Surface Area: 5.37 m2/g
Morphology: spherical
Solubility in H2O: N/A
Crystal Phase / Structure: cubic
Poisson's Ratio: 0.37
Thermal Expansion: (25 °C) 18.9 µm·m-1·K-1
Vickers Hardness: 251 MPa
Young's Modulus: 83 GPa
Linear Formula: Ag
MDL Number: MFCD00003397
EC No.: 231-131-3
Beilstein/Reaxys No.: N/A
Pubchem CID: N/A
IUPAC Name: N/A
SMILES: [Ag]
InchI Identifier: InChI=1S/Ag
InchI Key: BQCADISMDOOEFD-UHFFFAOYSA-N