Use of biological entities for the synthesis of various nanoparticles.
Nanotechnology emerges from the physical, chemical, biological and engineering sciences where novel techniques are being developed to probe and manipulate single atoms and molecules. In nanotechnology, a nanoparticle (10-9m) is defined as a small object that behaves as a whole unit in terms of its transport and properties. The science and engineering of nanosystems is one of the most challenging and fastest growing sectors of nanotechnology.
This review attempts to explain the diversity of the field, starting with the history of nanotechnology, the physics of the nanoparticle, various strategies of synthesis, the various advantages and disadvantages of different methods, the possible mechanistic aspects of nanoparticle formation and finally ends with the possible applications and future perspectives. Though there are a few good reviews dealing with the synthesis and applications of nanoparticles, there appears to be scanty information regarding the possible mechanistic aspects of nanoparticle formation. This review attempts to fill the void.
The review is organized into five sections. In section 2, we discuss about the early history of nanotechnology and the significant contributions made by eminent scientists in this field. In the next section we describe about the unique properties of nanoparticles, their classification and significance of inorganic nanoparticles. The next section discusses about the various methods of synthesis of nanoparticles and the possible mechanistic aspects. The last section highlights the recent advances and possible applications of nanparticles.
2. Early history
The concept of nanotechnology though considered to be a modern science has its history dating to as back as the 9th century. Nanoparticles of gold and silver were used by the artisans of Mesopotamia to generate a glittering effect to pots. The first scientific description of the properties of nanoparticles was provided in 1857 by Michael Faraday in his famous paper “Experimental relations of gold (and other metals) to light” (Faraday, 1857).
In 1959, Richard Feynman gave a talk describing molecular machines built with atomic precision. This was considered the first talk on nanotechnology. This was entitled “There’s plenty of space at the bottom”.
The 1950’s and the 1960’s saw the world turning its focus towards the use of nanoparticles in the field of drug delivery. One of the pioneers in this field was Professor Peter Paul Speiser. His research group at first investigated polyacrylic beads for oral administration, then focused on microcapsules and in the late 1960s developed the first nanoparticles for drug delivery purposes and for vaccines. This was followed by much advancement in developing systems for drug delivery like (for e.g.) the development of systems using nanoparticles for the transport of drugs across the blood brain barrier. In Japan, Sugibayashi et al., (1977) bound 5-fluorouracil to the albumin nanoparticles, and found denaturation temperature dependent differences in drug release as well as in the body distribution in mice after intravenous tail vein injection. An increase in life span was observed after intraperitoneal injection of the nanoparticles into Ehrlich Ascites Carcinoma-bearing mice (Kreuter, 2007).
The nano- revolution conceptually started in the early 1980’s with the first paper on nanotechnology being published in 1981 by K. Eric Drexler of Space Systems Laboratory, Massachuetts Institute of Technology. This was entitled “An approach to the development of general capabilities for molecular manipulation”.
With gradual advancements such as the invention of techniques like TEM, AFM, DLS etc., nanotechnology today has reached a stage where it is considered as the future to all technologies.
3. Unique properties of nanoparticles
A number of physical phenomena become more pronounced as the size of the system decreases. Certain phenomena may not come into play as the system moves from macro to micro level but may be significant at the nano scale. One example is the increase in surface area to volume ratio which alters the mechanical, thermal and catalytic properties of the material. The increase in surface area to volume ratio leads to increasing dominance of the behaviour of atoms on the surface of the particle over that of those in the interior of the particle, thus altering the properties. The electronic and optical properties and the chemical reactivity of small clusters are completely different from the better known property of each component in the bulk or at extended surfaces. Some of the size dependant properties of nanoparticles are quantum confinement in semiconductors, Surface Plasmon Resonance in some metallic nanoparticles and paramagnetism in magnetic nanoparticles.
Surface plasmon resonance refers to the collective oscillations of the conduction electrons in resonance with the light field. The surface plasmon mode arises from the electron confinement in the nanoparticle. The surface plasmon resonance frequency depends not only on the metal, but also on the shape and size of the nanoparticle and the dielectric properties of the surrounding medium (Jain et al., 2007). For example, noble metals, especially gold and silver nanoparticles exhibit unique and tunable optical properties on account of their Surface Plasmon Resonance.
Superparamagnetism is a form of magnetism that is a special characteristic of small ferromagnetic or ferromagnetic nanoparticles. In such superparamagnetic nanoparticles, magnetization can randomly change direction under the influence of temperature. Superparamagnetism occurs when a material is composed of very small particles with a size range of 1- 10nm. In the presence of an external magnetic field, the material behaves in a manner similar to paramagnetism with an exception that the magnetic moment of the entire material tends to align with the external magnetic field.
Quantum confinement occurs when one or more dimensions of the nanoparticle is made very small so that it approaches the size of an exciton in the bulk material called the Bohr exciton radius. The idea behind confinement is to trap electrons and holes within a small area (which may be smaller than 30nm). Quantum confinement is important as it leads to new electronic properties. Scientists at the Washington University have studied the electronic and optical changes in the material when it is 10nm or less and have related it to the property of quantum confinement.
Some of the examples of special properties that nanoparticles exhibit when compared to the bulk are the lack of malleability and ductility of copper nanoparticles lesser than 50nm. Zinc oxide nanoparticles are known to have superior UV blocking properties compared to the bulk.
3.1. Classification of nanoparticles
Nanoparticles can be broadly grouped into two: namely organic and inorganic nanoparticles. Organic nanoparticles may include carbon nanoparticles (fullerenes) while some of the inorganic nanoparticles may include magnetic nanoparticles, noble metal nanoparticles (like gold and silver) and semiconductor nanoparticles (like titanium dioxide and zinc oxide).
There is a growing interest in inorganic nanoparticles as they provide superior material properties with functional versatility. Due to their size features and advantages over available chemical imaging drugs agents and drugs, inorganic nanoparticles have been examined as potential tools for medical imaging as well as for treating diseases. Inorganic nanomaterials have been widely used for cellular delivery due to their versatile features like wide availability, rich functionality, good biocompatibility, capability of targeted drug delivery and controlled release of drugs (Xu et al., 2006). For example mesoporous silica when combined with molecular machines prove to be excellent imaging and drug releasing systems. Gold nanoparticles have been used extensively in imaging, as drug carriers and in thermo therapy of biological targets (Cheon & Horace, 2009). Inorganic nanoparticles (such as metallic and semiconductor nanoparticles) exhibit intrinsic optical properties which may enhance the transparency of polymer- particle composites. For such reasons, inorganic nanoparticles have found special interest in studies devoted to optical properties in composites. For instance, size dependant colour of gold nanoparticles has been used to colour glass for centuries (Caseri, 2009).
4. Strategies used to synthesize nanoparticles
Traditionally nanoparticles were produced only by physical and chemical methods. Some of the commonly used physical and chemical methods are ion sputtering, solvothermal synthesis, reduction and sol gel technique. Basically there are two approaches for nanoparticle synthesis namely the Bottom up approach and the Top down approach.
In the Top down approach, scientists try to formulate nanoparticles using larger ones to direct their assembly. The Bottom up approach is a process that builds towards larger and more complex systems by starting at the molecular level and maintaining precise control of molecular structure.
4.1. Physical and chemical methods of nanoparticle synthesis
Some of the commonly used physical and chemical methods include:
Sol-gel technique, which is a wet chemical technique used for the fabrication of metal oxides from a chemical solution which acts as a precursor for integrated network (gel) of discrete particles or polymers. The precursor sol can be either deposited on the substrate to form a film, cast into a suitable container with desired shape or used to synthesize powders.
Solvothermal synthesis, which is a versatile low temperature route in which polar solvents under pressure and at temperatures above their boiling points are used. Under solvothermal conditions, the solubility of reactants increases significantly, enabling reaction to take place at lower temperature.
Chemical reduction, which is the reduction of an ionic salt in an appropriate medium in the presence of surfactant using reducing agents. Some of the commonly used reducing agents are sodium borohydride, hydrazine hydrate and sodium citrate.
Laser ablation, which is the process of removing material from a solid surface by irradiating with a laser beam. At low laser flux, the material is heated by absorbed laser energy and evaporates or sublimates. At higher flux, the material is converted to plasma. The depth over which laser energy is absorbed and the amount of material removed by single laser pulse depends on the material’s optical properties and the laser wavelength. Carbon nanotubes can be produced by this method.
Inert gas condensation, where different metals are evaporated in separate crucibles inside an ultra high vacuum chamber filled with helium or argon gas at typical pressure of few 100 pascals. As a result of inter atomic collisions with gas atoms in chamber, the evaporated metal atoms lose their kinetic energy and condense in the form of small crystals which accumulate on liquid nitrogen filled cold finger. E.g. gold nanoparticles have been synthesized from gold wires.
4.2. Biosynthesis of nanoparticles
The need for biosynthesis of nanoparticles rose as the physical and chemical processes were costly. So in the search of for cheaper pathways for nanoparticle synthesis, scientists used microorganisms and then plant extracts for synthesis. Nature has devised various processes for the synthesis of nano- and micro- length scaled inorganic materials which have contributed to the development of relatively new and largely unexplored area of research based on the biosynthesis of nanomaterials (Mohanpuria et al., 2007).
Biosynthesis of nanoparticles is a kind of bottom up approach where the main reaction occurring is reduction/oxidation. The microbial enzymes or the plant phytochemicals with anti oxidant or reducing properties are usually responsible for reduction of metal compounds into their respective nanoparticles.
The three main steps in the preparation of nanoparticles that should be evaluated from a green chemistry perspective are the choice of the solvent medium used for the synthesis, the choice of an environmentally benign reducing agent and the choice of a non toxic material for the stabilization of the nanoparticles. Most of the synthetic methods reported to date rely heavily on organic solvents. This is mainly due to the hydrophobicity of the capping agents used (Raveendran et al., 2002). Synthesis using bio-organisms is compatible with the green chemistry principles: the bio-organism is (i) eco-friendly as are (ii) the reducing agent employed and (iii) the capping agent in the reaction (Li et al., 2007). Often chemical synthesis methods lead to the presence of some toxic chemical species adsorbed on the surface that may have adverse effects in medical applications (Parashar et al., 2009). This is not an issue when it comes to biosynthesized nanoparticles as they are eco friendly and biocompatible for pharmaceutical applications.
4.2.1. Use of organisms to synthesize nanoparticles
Biomimetics refers to applying biological principles for materials formation. One of the primary processes in biomimetics involves bioreduction.
Initially bacteria were used to synthesize nanoparticles and this was later succeeded with the use of fungi, actinomycetes and more recently plants.
4.2.2. Use of bacteria to synthesize nanoparticles
The use of microbial cells for the synthesis of nanosized materials has emerged as a novel approach for the synthesis of metal nanoparticles. Although the efforts directed towards the biosynthesis of nanomaterials are recent, the interactions between microorganisms and metals have been well documented and the ability of microorganisms to extract and/or accumulate metals is employed in commercial biotechnological processes such as bioleaching and bioremediation (Gericke & Pinches, 2006). Bacteria are known to produce inorganic materials either intra cellularly or extra cellularly. Microorganisms are considered as a potential biofactory for the synthesis of nanoparticles like gold, silver and cadmium sulphide. Some well known examples of bacteria synthesizing inorganic materials include magnetotactic bacteria (synthesizing magnetic nanoparticles) and S layer bacteria which produce gypsum and calcium carbonate layers (Shankar et al., 2004).
Some microorganisms can survive and grow even at high metal ion concentration due to their resistance to the metal. The mechanisms involve: efflux systems, alteration of solubility and toxicity via reduction or oxidation, biosorption, bioaccumulation, extra cellular complexation or precipitation of metals and lack of specific metal transport systems (Husseiny et al., 2007). For e.g.
Many microorganisms are known to produce nanostructured mineral crystals and metallic nanoparticles with properties similar to chemically synthesized materials, while exercising strict control over size, shape and composition of the particles. Examples include the formation of magnetic nanoparticles by magnetotactic bacteria, the production of silver nanoparticles within the periplasmic space of
Though it is widely believed that the enzymes of the organisms play a major role in the bioreduction process, some studies have indicated it otherwise. Studies indicate that some microorganisms could reduce silver ions where the processes of bioreduction were probably non enzymatic. For e.g. dried cells of
In the case of bacteria, most metal ions are toxic and therefore the reduction of ions or the formation of water insoluble complexes is a defense mechanism developed by the bacteria to overcome such toxicity (Sastry et al., 2003).
4.2.3. Use of actinomycetes to synthesize nanoparticles
Actinomycetes are microorganisms that share important characteristics of fungi and prokaryotes such as bacteria. Even though they are classified as prokaryotes, they were originally designated as ray fungi. Focus on actinomycetes has primarily centred on their exceptional ability to produce secondary metabolites such as antibiotics.
It has been observed that a novel alkalothermophilic actinomycete,
4.2.4. Use of fungi to synthesize nanoparticles
Fungi have been widely used for the biosynthesis of nanoparticles and the mechanistic aspects governing the nanoparticle formation have also been documented for a few of them. In addition to monodispersity, nanoparticles with well defined dimensions can be obtained using fungi. Compared to bacteria, fungi could be used as a source for the production of large amount of nanoparticles. This is due to the fact that fungi secrete more amounts of proteins which directly translate to higher productivity of nanoparticle formation (Mohanpuria et al., 2007).
Yeast, belonging to the class ascomycetes of fungi has shown to have good potential for the synthesis of nanoparticles. Gold nanoparticles have been synthesized intracellularly using the fungi
Extracellular secretion of the microorganisms offers the advantage of obtaining large quantities in a relatively pure state, free from other cellular proteins associated with the organism with relatively simpler downstream processing. Mycelia free spent medium of the fungus,
Instead of fungi culture, isolated proteins from them have also been used successfully in nanoparticles production. Nanocrystalline zirconia was produced at room temperature by cationic proteins while were similar to silicatein secreted by
The use of specific enzymes secreted by fungi in the synthesis of nanoparticles appears promising. Understanding the nature of the biogenic nanoparticle would be equally important. This would lead to the possibility of genetically engineering microorganisms to over express specific reducing molecules and capping agents and thereby control the size and shape of the biogenic nanoparticles (Balaji et al., 2009).
Microbiological methods generate nanoparticles at a much slower rate than that observed when plant extracts are used. This is one of the major drawbacks of biological synthesis of nanoparticles using microorganisms and must be corrected if it must compete with other methods.
4.2.5. Use of plants to synthesize nanoparticles
The advantage of using plants for the synthesis of nanoparticles is that they are easily available, safe to handle and possess a broad variability of metabolites that may aid in reduction.
A number of plants are being currently investigated for their role in the synthesis of nanoparticles. Gold nanoparticles with a size range of 2- 20 nm have been synthesized using the live alfa alfa plants (Torresday et al., 2002). Nanoparticles of silver, nickel, cobalt, zinc and copper have also been synthesized inside the live plants of
Recently much work has been done with regard to plant assisted reduction of metal nanoparticles and the respective role of phytochemicals. The main phytochemicals responsible have been identified as terpenoids, flavones, ketones, aldehydes, amides and carboxylic acids in the light of IR spectroscopic studies. The main water soluble phytochemicals are flavones, organic acids and quinones which are responsible for immediate reduction. The phytochemicals present in
Recently gold nanoparticles have been synthesized using the extracts of
While fungi and bacteria require a comparatively longer incubation time for the reduction of metal ions, water soluble phytochemicals do it in a much lesser time. Therefore compared to bacteria and fungi, plants are better candidates for the synthesis of nanoparticles. Taking use of plant tissue culture techniques and downstream processing procedures, it is possible to synthesize metallic as well as oxide nanoparticles on an industrial scale once issues like the metabolic status of the plant etc. are properly addressed.
4.2.6. Work on the biomimetic synthesis of nanoparticles in India
There has been considerable significant research in India in the field of biomimetic synthesis of nanoparticles. More research has been found to be concentrated in the area of biomimetic synthesis using plants.
It has been observed that a novel alkalothermophilic actinomycete,
Yeast, belonging to the class ascomycetes of fungi has shown to have good potential for the synthesis of nanoparticles.
In addition to the synthesis of silver nanoparticles,
Recently, scientists in India have reported the green synthesis of silver nanoparticles using the leaves of the obnoxious weed,
|Biological entity||Nanoparticles synthesized||Size||Intra/ E xtracellular||Reference|
|Pseudomonas aeruginosa||Au||15-30nm||E xtracellular||Husseiny et al ., (2007)|
|Bacillus subtilis||Ag||5-60nm||E xtracellular||Saiffudin et al ., (2009)|
|Pseudomonas stutzeri||Ag||Upto 200nm||Periplasmic||Joerger et al ., (2000) Klaus et al ., (1999)|
|Coriolus versicolor||Ag||10-75nm||E xtracellular||Sanghi & Verma (2008)|
|Fusarium semitectum||Ag||10-60nm||E xtracellular||Basavaraja et al ., (2007)|
|Fusarium oxysporum||Ag||5-15nm||E xtracellular||Ahmad et al ., (2003)|
|Phaenerochaete chrysosporium||Ag||-||E xtracellular||Vigneshwaran et al ., (2006)|
|Aspergillus flavus||Ag||8.92+/- 1.62nm||I ntracellular||Vigneshwaran et al ., (2007)|
|Azadirachta indica||Ag, Au, Ag/Au bimetallic||50-100nm||E xtracellular||Shankar et al ., (2003), Tripathy et al ., (2009)|
|Pelargoneum graveolens||Ag||16-40nm||E xtracellular||Shankar et al ., (2003)|
|Capsicum annum||Ag||10-40nm||E xtracellular||Li et al ., (2007)|
4.2.7. Some of the mechanistic aspects of nanoparticle formation
Though there are many studies reporting the biosynthesis of various nanoparticles by bacteria, there is very little information available regarding the mechanistic aspects of nanoparticle production.
The mechanisms of gold bioaccumulation by cyanobacteria (
Scientists in Iran have investigated the extracellular biosynthesis of silver nanoparticles by the cells of
Recently cadmium sulfide nanoparticles have been biosynthesized using the photosynthetic bacteria,
Baker’s yeast (
The synthesis of gold and silver nanoparticles has also been reported using black tea leaf extracts. Black tea leaf extracts are known to contain more amounts of flavonones and polyphenols. It was found that the reduction of metal ions was accompanied by oxidation of polyols (Begum et al., 2009). Table 2 gives a summary of some aspects of nanoparticle formation.
|Biological entity||Nanoparticle||Enzyme/ phytochemical||Reference|
|Rhodopseudomonas palustris||CdS||C-S lyase||Bai et al . (2009)|
|Schizosaccharomyces pombe||CdS||Phytochelatin synthase/ phytochelatins||Mohanpuria et al . (2007)|
|Schizosaccharomyces cerevisiae||Sb2O3||Oxidoreuctases/ quinones||Jha et a l ., (2009)|
|Fusarium oxysporum||Ag||NADH dependant reductase||Duran et al . , (2005)|
|Black tea leaf||Ag/Au||Polyphenols/ flavo noids||Begum et al . , (2009)|
|Azadirachta indica||Ag/ Au||Terpenoids||Shankar et al . , (2004), Tripathy et al ., (2009)|
|Jatropha curcas||Ag||Curcain, curacycline A, curacycline B||Bar et al . , (2009)|
The latex of
Li et al (2007) synthesized silver nanoparticles using the
5. Applications of Nanoparticles
Nanotechnology has a wide range of applications in the fields of biology, medicine, optical, electrical, mechanical, optoelectronics etc.
Silver nanoparticles have also been used for a number of applications such as non linear optics, spectrally selective coating for solar energy absorption, biolabelling and antibacterial activities.
Silver nanoparticles have shown promise against gram positive
Current research is going on regarding the use of magnetic nanoparticles in the detoxification of military personnel in case of biochemical warfare. It is hypothesized that by utilizing the magnetic field gradient, toxins can be removed from the body. Enhanced catalytic properties of surfaces of nano ceramics or those of noble metals like platinum and gold are used in the destruction of toxins and other hazardous chemicals (Salata, 2005).
Photocatalytic activity of nanoparticles has been utilized to develop self- cleaning tiles, windows and anti- fogging car mirrors. The high reactivity of Titania nanoparticles either on their own or when illuminated by UV light have been used for bactericidal purposes in filters.
An important opportunity for nanoparticles in the area of computers and electronics is their use in a special polishing process, chemical-mechanical polishing or chemical-mechanical planarization (CMP), which is critical to semiconductor chip fabrication. CMP is used to obtain smooth, flat, and defect-free metal and dielectric layers on silicon wafers. This process utilizes slurry of oxide nanoparticles and relies on mechanical abrasion as well as a chemical reaction between the slurry and the film being polished. CMP is also used in some other applications, such as the polishing of magnetic hard disks.
Nanoscale titanium dioxide and zinc oxide have been used as sunscreens in cosmetics. The primary advantage of using these nanoparticles is that they are well dispersed and transmit visible light, acting as transparent sunblocks. On the other hand, inorganic sunscreens appear white on the skin- a potential drawback.
The interaction of silver nanoparticles with HIV I has been demonstrated in vitro. It was shown that the exposed sulfur binding residues of the glycoprotein knobs were attractive sites for nanoparticle interaction and that the silver nanoparticles had preferential binding to the gp 120 glycoprotein knobs. Due to this interaction, it was found that the silver nanoparticles inhibited the binding of the virus to the host cells in vitro (Elechiguerra et al., 2005).
Magnetic nanoparticles are also used in targeted therapy where a cytotoxic drug is attached to a biocompatible magnetic nanoparticle. When these particles circulate in the bloodstream, external magnetic fields are used to concentrate the complex at a specific target site within the body. Once the complex is concentrated in the target, the drug can be released by enzymatic activity or by changes in pH or temperature and are taken up by the tumour cells (Pankhurst et al., 2003).
Porous nanoparticles have also been used in cancer therapy where the hydrophobic version of a dye molecule is trapped inside the Ormosil nanoparticle. The dye is used to generate atomic oxygen which is taken up more by the cancer cells when compared to the healthy tissue. When the dye is not entrapped, it travels to the eyes and skin making the patient sensitive to light. Entrapment of the dye inside the nanoparticle ensures that the dye does not migrate to other parts and also the oxygen generating ability is not affected.
Alivisatos (2001) reported the presence of inorganic crystals in magnetotactic bacteria. The bacterium was found to have about 20 magnetic crystals with a size range of 35- 120nm diameter. The crystals serve as a miniature compass and align the bacteria with the external magnetic field. This enables the bacterium to navigate with respect to the earth’s magnetic field towards their ideal environment. These bacteria immobilize heavy metals from a surrounding solution and can be separated by applying a low intensity magnetic field. This principle can be extended to develop a process for the removal of heavy metals from waste water.
Bioremediation of radioactive wastes from nuclear power plants and nuclear weapon production, such as uranium has been achieved using nanoparticles. Cells and S layer proteins of
Biominerals have been formulated by using several bacteria such as
Magnetosome particles isolated from magnetotactic bacteria have been used as a carrier for the immobilization of bioactive substances such as enzymes, DNA, RNA and antibodies (Mohanpuria
Gold nanoparticles are widely used in various fields such as photonics, catalysis, electronics and biomedicine due to their unique properties.
Tripathy et al., (2008) reported the antibacterial applications of the silver nanoparticles synthesized using the aqueous extract of neem leaves. The nanoparticles were coated on cotton disks and their bactericidal effect was studied against
Table 3 gives a list of a few companies which have utilized nanoparticles in their products.
|NanoStellar||Nanocomposite catalyst for use in automotive catalytic converters||Reduced cost due to less platinum use|
|AmericanElements||Catalyst composed of MnO2 nanoparticles to remove volatile organic compounds (VOC)||Capable of destroying VOC down to parts per billion level|
|Zpower||Ag-Zn battery using nanoparticles in the silver cathode||Higher power density, low combustibility|
|Samsung||Ag nanoparticles used in household appliances like clothes washer and refrigerator||Kills bacteria and reduces odour|
|Nanotec||Spray-on liquid containing nanoparticles||Repels water and dirt|
|Aspen Aerogel||Fabric enhanced with nanopores||Insulates against heat|
|Nano horizons||Fabric enhanced with silver nanoparticles||Reduces odours|
|InMat||Nanocomposite barrier film||Prevents air loss from tennis balls|
|Bicycle components strengthened with C nanotubes||Strong, light weight components|
Though the applications of nanoparticles are exhaustive, an effort has been made in this review to highlight specific applications
An important challenge in technology is to tailor optical, electric and electronic properties of nanoparticles by controlling their size and shape. Biomimetic synthesis of nanoparticles has opened its doors to a world of nanoparticles with easy preparation protocols, less toxicity and a wide range of applications according to their size and shape. Nanoparticles of desired size and shape have been obtained successfully using living organisms- simple unicellular organisms to highly complex eukaryotes. The field of nano biotechnology is still in its infancy and more research needs to be focused on the mechanistics of nanoparticle formation which may lead to fine tuning of the process ultimately leading to the synthesis of nanoparticles with a strict control over the size and shape parameters.