Synthesis of carbon nanotubes. Bionanoparticles preparation using micro organism and plant extracts

7

Synthesis of carbon nanotubes. Bionanoparticles preparation using micro organism and plant extracts

Synthesis of carbon nano-tube

Arc discharge

·         Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes.

·         During this process, the carbon contained in the negative electrode sublimates because of the high discharge temperatures. Because nanotubes were initially discovered using this technique, it has been the most widely-used method of nanotube synthesis.

·         The yield for this method is up to 30 percent by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects.

Laser ablation

·         In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses.

·         The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, it is more expensive than either arc discharge or chemical vapor deposition.

Chemical vapor deposition (CVD)

·         During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination. The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still being studied. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate. Fluidised bed reactor is the most widely used reactor for CNT preparation. Scale-up of the reactor is the major challenge.

·         CVD is a common method for the commercial production of carbon nanotubes. The metal nanoparticles are mixed with a catalyst support such as MgO or Al₂O₃ to increase the surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have proven effective for nanotube growth.

·         If a plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes (i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.

·         Of the various means for nanotube synthesis, CVD shows the most promise for industrial-scale deposition, because of its price/unit ratio, and because CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst.

Super-growth CVD

·         Super-growth CVD (water-assisted chemical vapour deposition) process was developed by Kenji Hata, Sumio Iijima and co-workers at AIST, Japan. In this process, the activity and lifetime of the catalyst are enhanced by addition of water into the CVD reactor. Dense millimeter-tall nanotube "forests", aligned normal to the substrate, were produced. The forests growth rate could be expressed, as  In this equation, β is the initial growth rate and τ_o is the characteristic catalyst lifetime.

·         Their specific surface exceeds 1,000 - 2,200 m²/g, surpassing the value of 400–1,000 m²/g for HiPco samples.

·         The synthesis efficiency is about 100 times higher than for the laser ablation method.

·          The time required to make SWNT forests of the height of 2.5 mm by this method was 10 minutes. Those SWNT forests can be easily separated from the catalyst, yielding clean SWNT material (purity >99.98%) without further purification. For comparison, the as-grown HiPco CNTs contain about 5-35% of metal impurities; it is therefore purified through dispersion and centrifugation that damages the nanotubes. The super-growth process avoids this problem. Patterned highly organized single-walled nanotube structures were successfully fabricated using the super-growth technique.

·         The mass density of super-growth CNTs is about 0.037 g/cm³. It is much lower than that of conventional CNT powders (~1.34 g/cm³), probably because the latter contain metals and amorphous carbon.

·         The super-growth method is basically a variation of CVD. Therefore, it is possible to grow material containing SWNT, DWNTs and MWNTs, and to alter their ratios by tuning the growth conditions. Their ratios change by the thinness of the catalyst. Many MWNTs are included so that the diameter of the tube is wide.

·         The vertically aligned nanotube forests originate from a "zipping effect" when they are immersed in a solvent and dried. The zipping effect is caused by the surface tension of the solvent and the van der Waals forces between the carbon nanotubes. It aligns the nanotubes into a dense material, which can be formed in various shapes, such as sheets and bars, by applying weak compression during the process. Densification increases the Vickers hardness by about 70 times and density is 0.55 g/cm³.

·         The packed carbon nanotubes are more than 1 mm long and have a carbon purity of 99.9% or higher; they also retain the desirable alignment properties of the nanotubes forest

Difference between HiPco CNT and Super growth SWNT

Parameter	HiPco CNT	Super growth SWNT
Purity	5-35%	>99.98%
Purification	purified through dispersion and centrifugation that damages the nanotubes	Purification not needed Nodamage
Patterned highly organized single-walled nanotube structures	Not possible	successfully fabricated
Mass density	1.34 g/cm3	0.037 g/cm3
Impurity	Metals and amorphous carbon	nil

Biological synthesis of nanomaterials

Biological synthesis of metal nanoparticles is environmentally friendly processes that do not use toxic chemicals. Biological processes are classified into three methods: using microorganism such as bacteria and fungi, using enzyme, and using plant or plant extracts.

 NANOPARTICLE SYNTHESIS USING MICROORGANISM

BACTERIA

Pseudomonas stutzeri AG259 isolated from silver mines has been shown to produce silver nanoparticles. The synthesis of magnetic nanoparticles has been reported by using magnetotactic bacteria. Magnetotactic bacteria such as Magnetospirillummagneticum produce two types of particles; some produce magnetic (Fe₃O) nanoparticles in chains and some produce greigite (Fe₃S₄) nanoparticles, while some other produce both types of nanoparticles. Similarly in the presence of exogenous electron donor, sulphate-reducing bacterium Desulfovibriodesulfuricans NCIMB 8307 has been shown to be synthesizing palladium nanoparticles. The common Lactobacillus strains found in buttermilk assisted the growth of microscopic gold, silver, and gold-silver alloy crystals of well-defined morphology. Recently, bacterial cell supernatant of Pseudomonas aeruginosa was used for the reduction of gold ions resulting in extracellular biosynthesis of gold nanoparticles . This would help in understanding the biochemical and molecular mechanism of nanoparticles synthesis. The cell filtrate has helped in achieving better control over size and polydispersity of nanoparticles. Further, this has documented that extracellular synthesis of nanoparticles using cell filtrate could be beneficial over intracellular synthesis.

Morphological control over the shape of gold nanoparticles has been achieved by using Plectonemaboryanum UTEX 485, a filamentouscyanobacterium. When it was reacted with aqueous Au(S₂O₃)₂ ^3- and AuCl₄^- solutions at 25–100°C for up to 1 month and at 200° C for 1 day resulted in the precipitation of cubic gold nanoparticles and octahedral gold platelets, respectively. The mechanisms of gold bioaccumulation by cyanobacteria (Plectonemaboryanum UTEX 485) from gold (III)chloride solutions have documented that interaction of cyanobacteria with aqueous gold(III)-chloride initially promoted the precipitation of nanoparticles of amorphous gold(I)-sulfide at the cell walls, and finally deposited metallic gold in the form of octahedral (III) platelets near cell surfaces and in solutions. Adding further to the mechanism, a sulfate-reducing bacterial enrichment was used to destabilize gold(I)-thiosulfate complex to elemental gold and proposed that this could occur by three possible mechanisms involving iron sulfide, localized reducing conditions, and metabolism.

ACTINOMYCETES

It has been observed that the extremophilicactinomycete, Thermomonospora sp. when exposed to gold ions reduced the metal ions extracellularly, yielding gold nanoparticles with a much improvedpolydispersity. However, in an effort towards elucidating mechanism or conditions favouring the formation of nanoparticles with desired features. The reduction of metal ions and stabilization of the gold nanoparticles were believed to occur by an enzymatic process. . Monodisperse gold nanoparticles synthesis could be due to extreme biological conditions such as alkaline and slightly elevated temperature conditions used for the synthesis of nanoparticles. Based on the hypothesis alkalotolerant Rhodococcus sp. has been used for intracellular synthesis of good quality monodisperse gold nanoparticles. The concentration of nanoparticles were more on the cytoplasmic membrane than on the cell wall. This could be due to reduction of the metal ions by enzymes present in the cell wall and on the cytoplasmic membrane but not in the cytosol. These metal ions were not toxic to the cells, which are producing them, and were continued to multiply even after the biosynthesis of gold nanoparticles.

YEAST

The Candida glabrata and Schizosaccharomycespombe were used for the first time in the biosynthesis of cadmium sulphide (CdS) nanocrystals. These nanocrystals were produced using cadmium salts and are now used in quantum semiconductor crystallites. Further experiments have been conducted to improve the quantity of semiconductor CdS nanocrystals production that was achieved by using Schizosaccharomycespombe cells. When these cells were incubated with 1mM Cd during their mid-log phase of growth, maximal nanocrystals were obtained. This study has suggested that the formation of CdS nanocrystals was dependent on the growth phase of yeast. When Cd was added during stationary phase, its uptake as well as production of CdS nanocrystals was decreased or resulted in no CdS formation. Upon adding Cdduring early exponential phase of yeast growth, CdS nanocrystals were formed but this time it was affecting the cellular metabolism of the yeast and resulted in efflux of Cd from the cells. The possible mechanism of decrease in CdS nanocrystals formation could be like this: upon exposure to Cd as a stress, a series of biochemical reactions were triggered to overcome the toxic effects of this metal. Firstly, an enzyme phytochelatinsynthase was activated to synthesize phytochelatins (PC) that chelated the cytoplasmicCd to form a low molecular weight PC-Cd complex and ultimately transport them across the vacuolar membrane by an ATP-binding cassette-type vacuolar membrane protein (HMT1). In addition to Cd, sulphidecould also be added to this complex in the membrane and that results in formation of high molecular weight PCCdS₂ complex that also allow them to sequestered into vacuole. Conditions have also been standardized for the synthesis of large quantities of silver nanoparticles by using silver-tolerant yeast strain MKY3. The procedure for separation of these silver particles has also been documented that was based on differential thawing of the samples. Recently, yeast strains have been identified for their ability to produce gold nanoparticles, whereby con trolling growth and other cellular activities controlled size and shape of the nanoparticles was achieved.

FUNGI

In addition to good monodispersity, nanoparticles with well defined dimensions can be obtained by using fungi. This has been shown with an experiment where bioreduction of aqueous AuCl^-₄  ions was carried out using the fungus Verticillium sp. that led to the formation of gold nanoparticles with fairly well-defined dimensions and good monodispersity. These results have documented that the trapping of AuCl^-₄  ions on the surface of fungal cells could occur by electrostatic interaction with positively charged groups (such as, lysine residues) in enzymes that are present in the cell wall of the mycelia. Here then gold ions were reduced by enzymes within the cell wall leading to aggregation of metal atoms and formation of gold nanoparticles. However, they could not find the exact mechanism of formation of gold nanoparticles. It can be concluded from their study that compared to bacteria, fungi could be a source for large amount production of nanoparticles. Since fungi are known to secrete much higher amounts of proteins, thus might have significantly higher productivity of nanoparticles in biosynthetic approach. Towards elucidating mechanism of nanoparticles formation, an in vitro approach was followed where species specific NADH dependent reductase, released by the Fusariumoxysporum, were successfully used to carry out the reduction of AuCl₄ions to gold nanoparticles. This has first time opened up a novel fungal/enzyme-based in vitro approach for nanomaterials synthesis. Based on properties of Fusariumoxysporum, it was also used in the formation of extremely stable silver hydrosol. The acidophilic fungus Verticillium sp. has capability of producing gold as well as silver nanoparticles upon their incubation with Ag⁺and AuCl^-₄  ions. However, a novel biological method for the intra- and extra-cellular synthesis of silver nanoparticles using the fungi, Verticillium and Fusariumoxysporum respectively has been documented. This has opened up an exciting possibility wherein the nanoparticles may be entrapped in the biomass in the form of a film or produced in solution, both having interesting commercial potential. The fungus, Aspergillusflavus also resulted in the accumulation of silver nanoparticles on the surface of its cell wall when incubated with silver nitrate solution.

Extracellularly produced nanoparticles were stabilized by the proteins and reducing agents secreted by the fungus. A minimum of four high molecular weight proteins released by the fungal biomass have been found in association with nanoparticles. One of these was strain specific NADH-dependent reductase. However, emission band produced by fluorescence spectra indicate the native form of these proteins present in the solution as well as bound to the surfaces of nanoparticles. Further, the reduction of metal ions and surface binding of the proteins to the nanoparticles did not compromise the tertiary structure of the proteins. Endophytic fungus Collitotrichumsp. growing in the leaves of geranium was used for the synthesis of stable and various shaped gold nanoparticles. Reducing agent in this fungus was also polypeptides/enzymes (Shankar et al. 2003a). Instead of fungi culture, isolated proteins from them have been used successfully in nanoparticles production. Nanocrystallinezirconia was produced at room temperature by the cationic proteins. These proteins were similar in nature to silicatein, secreted by the Fusariumoxysporum, and were capable of hydrolyzing aqueous ZrF₆^2- ions extracellularly.

            Growth conditions play an important role during the production of nanoparticles while using the fungi cultures. When gold ions were incubated with the Trichothecium sp. biomass under stationary conditions led to the formation of extracellular nanoparticles. While under shaking conditions, this was resulted in the formation of intracellular gold nanoparticles. The possible reason for this could be the enzymes and proteins responsible for the synthesis of nanoparticles. These proteins were released into the medium under stationary conditions and did not release under shaking conditions.

Synthesis of magnetic nanoparticles by using Fusariumoxysporum and Verticillium sp. at room temperature. Both fungi secreted proteins which were capable of hydrolyzing iron precursors extracellularly to form iron oxides predominantly in the magnetite (Fe₃O₄)phase. Also a nitrate-dependent reductase and a shuttle quinone from several Fusariumoxysporum strains were found to be involved in extracellular production of silver nanoparticles or silver hydrosol. However, it was not true with all Fusarium sp. Fusariummoniliforme produces reductase enzyme but could not form silver nanoparticles upon their incubation with silver ions. However, by controlling the amount of cofactor NADH, synthesis of quite stable Au–Ag alloy nanoparticles of various compositions have been made possible. This approach can be further employed for producing various other composite nanoparticles. Towards elucidating the mechanism of synthesis of nanoparticles, α-NADPH-dependent nitrate reductase and phytochelatin isolated from Fusariumoxysporum has been used for in vitro silver nanoparticle production.

Minimum time, miniaturization and non-hazardous processes are key parameters for any kind of technology acceptance. In this effort they could get fairly monodispersed silver nanoparticles within 10 min by using Aspergillusfumigatus. This was the first report of such rapid synthesis of nanoparticles using fungus. The production was even faster compared to the physical and chemical processes of nanoparticles synthesis. Hence, this process could be suitable for developing a biological process for mass scale production of nanoparticles. Tetragonal barium titanate (BaTiO) nanoparticles of sub–10 nm dimensions produced by Fusariumoxysporum under ambient conditions have been observed an ecofriendly and economically viable methods for the synthesis of complex oxide nanomaterials of technological interest. The presence of ferroelectric properties in these nanoparticles would revolutionize the electronic industries by making ultra small capacitors and ultrahigh density nonvolatile ferromagnetic memories. Also, the synthesis of highly luminescent CdSe quantum dots at room temperature, reported recently by the fungus, Fusariumoxysporum when incubated with a mixture of CdCl₂ and SeCl₄would be of great importance. Recently, Fusariumoxysporum fungus has also been used for the production of silica and titaniananoparticles from aqueous anionic complexes SiF₆^2- and TiF₆^2- respectively.

Extra-cellular protein-mediated hydrolysis of the anionic complexes results in the facile room temperature synthesis of crystalline titania particles while calcination at 300° C is required for crystallization of silica.

NANOPARTICLE SYNTHESIS USING PLANTS OR PLANT EXTRACTS

Using plants for nanoparticle synthesis can be advantageous over other biological processes because it eliminates the elaborate process of maintaining cell cultures and can also be suitably scaled up for large-scale synthesis of nanoparticles demonstrated the synthesis of gold and silver nanoparticles within live alfalfa plants from solid media.

Figure4  illustrates the scheme of nanoparticle formation using plant extract. Extracellular nanoparticle synthesis using plant leaf extracts other than whole plants would be more economical due to the easier downstream processing. The pioneering works of nanoparticle synthesis using plant extracts have been carried out. whereas the presence of CI⁺ ions distorted the nanotriangle morphology and induced the formation of aggregated spherical nanoparticles.