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, it 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, it 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. It 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 the nanoparticle could be controlled depending on the concentration of the extract. The studies indicate that the higher concentrations correlated to an increased number of nanoparticles. The
monosaccharide must have a free ketone group because in order to act as a
reducing agent it first undergoes
tautomerization. In addition, if the aldehydes are bound, it 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, :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 it 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,
sodium dodecyl sulfate (SDS), 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 Both methods force the
precipitation of the nanoparticles.
Polyol process The
polyol process is a particularly useful method because it 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.
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.), Seeds typically consist small 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 nanoparticle 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 nanoparticles produced are difficult to control and often have wide distributions. 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 nanoparticle 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 nanoparticle size. There are a few competing mechanisms which may result in the decrease in nanoparticle size; destruction of NPs upon
collision,
sputtering of the sample surface, particle fusion upon heating and dissociation. Both the implant temperature and ion beam current density are crucial to control in order to obtain a monodisperse nanoparticle 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 problems with the chemical production of 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 nanoparticles. In addition, precise control over shape and size is vital during nanoparticle 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 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 nanoparticle synthesis. Fungal strains such as
Verticillium and bacterial strains such as
Klebsiella pneumoniae can be used in the synthesis of silver nanoparticles. When the fungus/bacteria is added to solution,
protein biomass is released into the solution.
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 nanoparticles takes place in the cell from the interactions between the silver ions and the organic compounds of the cell. It was found that the bacterium
Lactobacillus fermentum created the smallest silver nanoparticles with an average size of 11.2 nm. It was also found that this bacterium produced the nanoparticles with the smallest size distribution and the nanoparticles were found mostly on the outside of the cells. It was also found that there was an increase in the
pH increased the rate of which the 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. It 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. 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.
Products and functionalization At small sizes silver nanoparticles typically contain twins, either
Icosahedral or
decagedral. 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 Ag nanoparticles in different shapes vastly increases the ability to tune their optical behavior. For example, the wavelength at which LSPR occurs for a nanoparticle 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 its shape. This aspect can be exploited in synthesis to promote change in shape of nanoparticles through light interaction.
Triangular nanoprisms Triangular-shaped 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. 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. This procedure can actually be modified to produce another anisotropic silver nanostructure, nanowires, by just allowing the silver nitrate solution to age before using it 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 nanoparticle surface.
Centrifuging separates the PVP coated 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. ==Metrology==