Nanotechnology: Green Synthesis of Gold Nanoparticles
|image credits: Walterdenkens, wikimedia|
Author: Sanjana Singh
Nanoparticles are really tiny particles with the dimension ranging from only 1nm to 100nm, where any nano-scale technology used to manipulate these particles for its application in real world is known as Nanotechnology . These particles have special and enhanced physical and chemical properties as compared to their bulk materials due to their large reactive and exposed surface area along with quantum-mechanical effect as a result of specific electronic structures. These particles have been widely used in many fields such as electronics, photochemical, biomedicine, chemistry, ware fare and environmental remediation . Nanoparticles in addition have a promising future for fuel production from raw petroleum materials through higher chemical action and it’s uses in many bio-imaging and therapeutic functions .
Metal nanoparticles specifically gold nanoparticles have abundant use in the field of nanotechnology and nanomedicine because they have small size, high surface-area-to-volume ratio, high surface energy allowing immobilization of broad range of biomolecule and they also have many optical properties which are mainly concerned with localized plasmon resonance (PR) . These particles have been widely used in various biomedical applications and drug delivery systems due to their inert nature, stability, high dispersity, non-cytotoxicity, biocidal and antioxidant properties .
The interest in gold nanoparticle is largely due to the relative ease of their synthesis, with good control of their sizes and shapes, their optical characteristics and their good biocompatibility .
Gold nanoparticles are in various size’s ranging from 10 nm-200 nm exhibiting shape and size dependent properties, yellow in bulk but often red in nano-scale. Because there is an electron cloud around gold nanoparticle which can be excited by white light so the energy absorbed determines the color. When light hits the cloud its start oscillating so, there's some polarization and the negative charge dips down. There's positive charge on metal core which act as a restoring force where electron bounce back and there's resonance situation called plasmon resonance. This, plasmon resonance (and thus color) is markedly influenced by shape and interaction of particles with each other.
Plasmon resonance in gold nanoparticle
Another effect that influence color of gold nanoparticle is proximity that is if gold nanoparticles come close to each other by aggregation (or controlled interaction by design) leads to color change.
Different shapes of gold nanoparticles are spherical, sub-octahedral, octahedral, decahedral, icosahedral multiple twined, multiple twined, irregular shape, tetrahedral, nanotriangles, nano-prisms, hexagonal platelets and nanorods.
Different shapes of gold nano-particles
The rod-shaped gold nanoparticle has a definite shape which effect color, when white light hits the rod outer electronic layer and we get both longitudinal and transverse plasmon resonance, and hence rod appear blue or brown, depending on the aspect ratio. This anisotropic geometry enables them to get tunable absorption in both visible and near infrared (NIR) regions and make them suitable for potential applications in the fields of biosensing, gene delivery and photo thermal therapy . The main applications of gold nanoparticles are lateral flow, optical sensing, light scattering applications, drug delivery and cancer therapy.
Covalent attachment of molecules to gold, taking thiolated molecules like alkanes-thiol which have hydrophobic tails have a reasonably high affinity for gold surfaces so thiol groups lie on the surface and tail align to give a hydrophobic organic surface and so instead of metal surface we switch to an organic surface where tail of the herb determines surface properties.
Here, passive binding of antibodies involves systematic pH and salt titrations to optimize binding but there is also significant risk of unwanted aggregation. Coating nanoparticles using functional groups like carboxyl or amine are extremely useful for bioconjugation. InnovaCoat technology, coat entire gold particle with a protective coat which provides incredible colloidal stability, number of surface chemistries which allow molecules to get attached to gold and irreversible covalent attachment of analytes and antibodies.
Effect on InnovaCoat on plasmon resonance.
GREEN SYNTHESIS OF GOLD NANOPARTICLES
Various methods have been developed for the synthesis of gold nanoparticles including chemical, physical, and biological methods.
Green method is a biological method, an alternative to chemical and physical method providing a environmentally friendly way of synthesizing nanoparticles. Moreover, Green chemistry synthesis routes are non-toxic and inexpensive and help to synthesize metallic nanoparticles with various shapes, sizes, contents and physicochemical properties .
Green synthesis or green materials are types of microorganisms, enzymes, plants, or plant extracts, bacteria, actinobacteria, yeasts, molds, algae that can be used as reducing and stabilizing agents in synthesis of metal nanoparticles .
Specific plants contain specific chemical compounds which can act as active substances in the process of reduction and stabilization of nanoparticles. These compounds are alternative environmentally friendly materials in nanoparticle production due to their function to reduce the use of hazardous chemicals, including wastes. Biomolecules in plant extracts that can reduce metal ions into nanoparticles include proteins, enzymes, amine, polysaccharides, alkaloids, flavonoids, terpenoids, and phenolic acids [10,11].
Plants, which have immense potential for detoxification, reduction and accumulation of metals, are promising, fast and economical in synthesising metallic nano-particles. Synthesis process; is initiated by addition of extracts obtained from plant parts such as leaves, roots and fruits into the aqueous solution of metal ions, with the materials present in the plant extract, like sugar, flavonoid, protein, enzyme, polymer and organic acid, acting as a reducing agent.
Several studies have used the extracts of J. sambac leaf , Rosa rugosa leaf , Magnolia kobus and Diospyros kaki leaves , Ocimum sanctum leaf , Aerva lanata leaf , Coriandrum sativum leaf , Phyllanthus , and henna leaf , Morinda citrifolia roots , Pelargonium graveolens (PeG) leaves and its endophytic fungus , Polyscias scutellaria  leaves as reducing agents in AuNPs synthesis by green method. Antioxidants present in these plant extracts carry out an active role in the synthesis when the corresponding plant extracts are used.
Various bacterial and fungal strains like Bacillus cereus (PTCC), Fusarium oxysporum (PTCC), Pseudomonas aeruginosa and Rhodopseudomonas capsulate, etc. also have the ability to produce NPs. Because of rapid development, affordable culturing costs and easy control and manipulation of growth environment, bacteria are clear targets in the production of nanoparticles . The types of reduction differ depending on the nature of the active components that are responsible for the bio-reduction process. That is, if microbial enzymes are responsible for the bio-reduction of the induced toxic ions, then the reaction is an enzymatic one, and if the active sites of the microbial products (i.e., various types of polysaccharides or polypeptides) are responsible for the bio-reduction, then the reaction is non-enzymatic . Moreover, depending on whether the corresponding NPs are present within or outside the microbial cells, the NP production type can be classified as intracellular or extracellular .
Of all the aquatic organisms, algae (eukaryotic aquatic photosites) provide a good source of biomolecules. Since algae contain pigments, proteins, carbohydrates, fats, nucleic acids and secondary metabolites like alkaloids, some aromatic compounds, macrolides, peptides and terpenes, they act as reducing agents to produce nanoparticles from metal salts without producing any toxic by-product. Once the algal biomolecules are identified, the nanoparticles of desired shape or size may be fabricated and also, they act as capping and stabilizing agents for the fabricated nanoparticles. Their size of the nanoparticles is controlled by various factors such as temperature, incubation time, pH and concentration of the solution .
UV-V is spectrophotometry, particle size analyzer (PSA), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy-selected area electron diffraction (TEM-SAED), and X-ray diffraction (XRD) were used to characterize AuNPs on the basis of their shape and size .The FTIR measurements are carried out in attempt to obtain information about the nature of organic protection layer that surrounds the gold nanoparticles. This information is extremely valuable for the design of functionalization procedures if one wants to use the particles for drug delivery. Crystalline nature of the nanoparticles within the FCC structure are confirmed by the peaks in the XRD pattern corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, vivid circular spots in the selected area electron diffraction (SAED) and clear lattice fringes in the high-resolution TEM image .
Recently, a variety of microorganisms and plant extracts have been used to efficiently synthesize metal nanoparticles by green synthesis and characterized with using UV, FTIR, XRD, SEM and HRTEM instruments. Thus, the synthesis of nanoparticles by green synthesis is the most convenient, easy, environmentally friendly way as it minimizes the side effects of chemical and physical methods by preventing the use of toxic chemicals and formation of harmful/dangerous by-products and can be synthesized any need for high pressures, energy environment and temperature.
Green materials hold a huge potential as ecofriendly and cost-effective tools, avoiding toxic, harsh chemicals and the high energy demand required for physiochemical fabrication. In the above review we have discussed the use of green materials in synthesis of gold nanoparticles where proteins, polysaccharides, amines, amino acids, alcohols, pigments, carboxylic acids carbohydrates and sugars have been shown to act as reducing agents. These gold particles have thus revolutionized the field of nanotechnology with its widespread applications in targeted drug delivery, imaging, diagnosis, anti-microorganism, green photocatalyst, adsorbent, detector and therapeutics due to their very tiny size, high expansivity, stability, non-cytotoxicity and tunable optical, physical and chemical properties.
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