BIOSYNTHESIS OF GOLD NANOPARTICLES, SCOPE AND APPLICATION: A REVIEW

BIOSYNTHESIS OF GOLD NANOPARTICLES, SCOPE AND APPLICATION: A REVIEW

S. Tikariha, S. Singh, S. Banerjee, A. S. Vidyarthi*

Department of Biotechnology, Birla Institute of Technology, Mesra, Ranchi-835215, Jharkhand, India

ABSTRACT

The synthesis of gold nanoparticles has received considerable attention and has been a focus of research due to their high chemical and thermal stability, fascinating optical, electronic properties, and promising applications such as nanoelectronics, biomedicine, sensing, and catalysis. Different physical and chemical methods for gold nanoparticles synthesis are known but these methods are either expensive or are not eco-friendly due to use of hazardous chemicals, stringent protocol used during the process. These drawbacks necessitate the development of nonhazardous and greener methods for gold nanoparticles synthesis. Therefore, there has been tremendous excitement in the study of gold nanoparticles synthesis by using natural biological system. Microorganisms thus play a very important role in the eco-friendly and green synthesis of metal nanoparticles. The inherent, clean, nontoxic and environment friendly ability of eukaryotic and prokaryotic microorganisms, plants system to form the metal nanoparticles is particularly important in the development of nanobiotechnology. This review contains a brief outlook of the biosynthesis of gold nanoparticles using various biological resources, characterization and their potential application in various fields.

Biosynthesis

INTRODUCTION:The field of nanotechnology is an immensely developing field as a result of its wide-ranging applications in different areas of science and technology. The word, nanoparticle (10^-9m) can be defined in nanotechnology as a small object that acts as a whole unit in terms of its transport and properties. The word “nano” is derived from a Greek word meaning dwarf or extremely small ¹.

Nanotechnology has a wide variety of applications in various fields like optics, electronics, catalysis, bio-medicine, magnetics, mechanics, energy science, etc. Nanobiotechnology is a multidisciplinary field involving research and development of technology in different fields of science like biotechnology, nanotechnology, physics, chemistry, and material science ^1-2. It deals with bio-fabrication of nano-objects or bi-functional macromolecules usable as tools to construct or manipulate nano-objects. Since, microbial cells offer many advantages like wide physiological diversity, small size, genetic manipulability and controlled culturability, they are thus regarded as ideal producers for the synthesis of diversity of nanostructures, materials and instruments for nanosciences ³.

The methods of biosynthesis can employ either microbial cells or plant extract for production of nanoparticles. Biosynthesis of nanoparticles is an exciting recent area to the large repertoire of various methods of nanoparticles synthesis and now, nanoparticles have entered a commercial exploration period. Gold nanoparticles (GNPs) are presently under intensive study for applications in optoelectronic devices, ultrasensitive chemical and biological sensors and as catalysts ³.  Nanoparticles are metal particles and exhibit different shapes like spherical, triangular, rod, etc. Research on synthesis of nanoparticles is the current area of interest due to the unique visible properties (chemical, physical, optical, etc.) of nanoparticles compared with the bulk material ^4-5.

GNPs are some of the most extensively studied material. These can be easily synthesized, exhibit intense surface plasmon resonance and display high chemical as well as thermal stability ⁶. A variety of gold structures including rods, triangles, hexagons, octagons, cubes and nanowires can be synthesized by using different techniques ^7-10. In biomedicine, GNPs are used in several purposes such as leukemia therapy ¹¹, biomolecular immobilization ¹² and biosensor design. The use of GNPs as anti-angiogenesis, anti-malaria and anti-arthritic agents is also reported by ¹³. Because of the increased demand of gold in many industrial applications, there is a growing need for cost effectiveness as well as to implement green chemistry in the development of new nanoparticles ¹⁴.

Advanced synthesis of Metallic Nanoparticles: The nanoparticles can be synthesized using the top-down (physical) approach which deals with methods such as thermal decomposition, diffusion, irradiation, arc discharge, etc., and bottom-up (chemical and biological) approach which involves seeded growth method, polyol synthesis method, electrochemical synthesis, chemical reduction, and biological entities for fabrication of nanoparticles.

In the top-down approach, the bulk materials are gradually broken down to nano-sized materials by machining and etching techniques. In contrast, the atoms or molecules are assembled into molecular structures in the nanometer range in the bottom-up approach, which is commonly applied for chemical and biological synthesis of nanoparticles ¹⁴.  Generally, the methods used for nanoparticles synthesis employing chemical routes involves conditions such as high temperature and high pressure and also incorporates the use of strong and weak chemical reducing agents along with protective agents (sodium borohydride, sodium citrate and alcohols). These agents are mostly toxic, flammable and they cannot be easily released in environmental and also show a low production rate ^15-16. Moreover, these are capital intensive and are inefficient in materials and energy use ^17-18.

Furthermore, the use of toxic chemicals and organic solvents during nanoparticles synthesis and their occurrence on the surface of nanoparticles limit their applications. Such drawbacks necessitate the development of clean, biocompatible, nonhazardous, and eco-friendly methods for GNPs synthesis. Consequently, biological systems have been focused on and exploited for the synthesis of nanoparticles ¹⁹ providing a safer alternative to physical and chemical methods.

The biological method for the synthesis of nanoparticles employs use of biological agents like bacteria, fungi, actinomycetes, yeast, algae and plants ^20-21 thereby providing a wide range of resources for the synthesis of nanoparticles. The rate of reduction of metal ions using biological agents is found to be much faster and also at ambient temperature and pressure conditions. It is well known that microbes such as bacteria ²², yeast ²³, fungi ²⁴ and alga ^25-26 are capable of adsorbing and accumulating metals. The biological agents secrete a large amount of enzymes, which are capable of hydrolyzing metals and thus bring about enzymatic reduction of metals ions ²⁷.

In case of fungi, the enzyme nitrate reductase is found to be responsible for the synthesis of nanoparticles ^28-29. The biomass used for the synthesis of nanoparticles is simpler to handle, gets easily disposed of in the environment and also the downstream processing of the biomass is much easier. Synthesis of nanoparticles can be carried out at ambient temperature and pressure conditions that require lesser amounts of chemical ¹⁷. The synthesizing process is less labor-intensive, low-cost technique, nontoxic and is more of a greener approach.

Thus, considering the above points the biological method employed for the synthesis of nanoparticles proves to be superior compared with the physical and chemical methods of synthesis due to its environment friendly approach and also as a low cost technique ³⁰.

Therefore, based on their enormous biotechnological applications, microorganisms such as bacteria, fungi, and yeast are regarded as possible eco-friendly “nano-factories” for nanoparticles synthesis.

Mechanism of Biosynthesis of Nanoparticles:  Biosynthesis is the phenomena which takes place by means of biological processes or enzymatic reactions. These eco-friendly processes are referred as green and clean technology, and can be used for better synthesis of metal nanoparticles from microbial cells ³¹. Microorganisms can survive and grow in high concentration of metal ion due to their ability to fight against stress ³². The exact mechanism for the synthesis of nanoparticles using biological agents has not been devised yet as different biological agents react differently with metal ions and also there are different biomolecules responsible for the synthesis of nanoparticles. In addition, the mechanism for intra- and extracellular synthesis of nanoparticles is different in various biological agents ³⁰.

According toBeveridge (1997), the mechanisms which are considered for the biosynthesis of nanoparticles includes efflux systems, alteration of solubility and toxicity via reduction or oxidation, bioabsorption, bioaccumulation, extracellular complexation or precipitation of metals, and lack of specific metal transport systems ³³. The cell wall of the microorganisms also plays a major role in the intracellular synthesis of nanoparticles. The cell wall being negatively charged interacts electrostatically with the positively charged metal ions. The enzymes present within the cell wall bioreduce the metal ions to nanoparticles, and finally the smaller sized nanoparticles get diffused of through the cell wall ³⁴.

Mukherjee et al., (2001) reported stepwise mechanism for intracellular synthesis of nanoparticles using Verticillium species. The mechanism of synthesis of nanoparticles was divided into trapping, bioreduction and synthesis. Similar mechanism was also found in fungus for the synthesis of nanoparticles.  Moreover, in the case of bacteria Lactobacillus sp, Nair and Pradeep (2002) observed that during the initial step of synthesis of nanoparticles, nucleation of clusters of metal ions takes place, and hence there is an electrostatic interaction between the bacterial cell and metal clusters which leads to the formation of nanoclusters ³⁵. Lastly, the smaller sized nanoclusters get diffused through the bacterial cell wall. In actinomycetes also, the reduction of metal ions occur on the surface of mycelia along with cytoplasmic membrane leading to the formation of nanoparticles ^36-37.

The mechanism of extracellular synthesis of nanoparticles using microbes is basically found to be nitrate reductase-mediated synthesis. The enzyme nitrate reductase secreted by the fungi helps in the bioreduction of metal ions and synthesis of nanoparticles. A number of researchers supported nitrate reductase for extracellular synthesis of nanoparticles ^{17, 28-29, 38-40}. A similar mechanism was also reported in the case of extracellular synthesis of GNPs using Rhodopseudomonas capsulata ³⁹.

The bacterium R. capsulata is known to secrete cofactor NADH and NADH-dependent enzymes. The bioreduction of gold ions was found to be initiated by the electron transfer from the NADH by NADH-dependent reductase as electron carrier. Next, the gold ions (Au³⁺) obtain electrons and are reduced to elemental gold (Au⁰) and hence result in the formation of GNPs. Nangia et al., (2009) proposed the synthesis of GNPs by bacterium Stenotrophomonas maltophilia and suggested that the biosynthesis of GNPs and their stabilization via charge capping in S. maltophilia involved NADPH-dependent reductase enzyme which converts Au³⁺ to Au⁰ through electron shuttle enzymatic metal reduction process as shown in Fig. 1 ⁴⁰.

FIG. 1: PROPOSED MECHANISM OF GOLD IONS BIOREDUCTION VIA NADPH-DEPENDANT REDUCTASES

General Chemistry of Gold: Gold can occur in one of the six oxidation states, from -1 to +5, which can be related to its relatively high electronegativity. The most common form of gold complexes is in aurous [Au (I)] and auric [Au (III)] oxidation states ⁴¹. The dissolution of gold in aqueous solution is a combination process of oxidation and complexation. Au (I) and Au (III) can form stable complexes in the presence of a complexing ligand, otherwise they can be reduced in solution to metallic gold ⁴². The stability of gold complexes is related not only to the properties of the complexing ligand, but also more specifically to the donor atom of the ligand that is bonded directly to the gold atom.

According to Nicol et al., (1987), the first rule is that the stability of gold complexes tends to decrease when the electronegativity of the donor atom increases. For example, the stability of gold halide complexes in solution follows the order I^-> Br^-> Cl^-> F^-. The second rule is that Au (III) is generally favored over Au (I) with hard ligands and Au (I) over Au (III) with soft ligands. The preferred co-ordination number of Au (I) is 2, tending to form linear complexes, and that of Au (III) is 4, tending to form square planar complexes. The two precursors which are used for the synthesis of GNPs are gold (III)–chloride complex and gold (I) thiosulfate, in that also, gold (III)–chloride complex is widely used as a precursor in most of the GNPs biosynthesis process.

Biosynthesis of Gold Nanoparticles: The use of microbial cells is now emerging as a novel and green approach for the synthesis of metal nanoparticles. Basic steps for metal nanoparticles biosynthesis includes growth of microorganism in culture media, harvesting biomass from medium and finally incubation of biomass with sub-inhibitory concentration of target metal salts. During the different phases of microbial growth, the metal reduction process may take place by intercellular or extracellular bioreductant ingredients ³⁸. The reaction condition can be optimized by changing experimental factors such as pH, incubation time, presence of light source, temperature, the composition of the culture medium, etc. This optimization will improve the chemical composition, shape and size of the particles synthesized ⁴³.

In general, GNPs precipitate intracellularly and/or extracellularly depending on the species as in Fig. 2 and reaction condition. The shape of GNPs precipitated by bacteria, cyanobacteria, algae, fungi, plants includes spherical, oval, irregular, triangular, tetragonal, hexagonal, octahedral, rod, cubicl, icosahedral, coil or wire, plate, and thin foil, with size ranging from 1 nm to several mm as discussed in Fig. 3.

FIG. 2(a): A TEM MICROGRAPH OF A THIN SECTION OF CYANOBACTERIA CELL WITH THE GOLD NANOPARTICLES INSIDE THE CELL, 2(b): A SEM MICROGRAPH OF GOLD NANOPARTICLES ON THE SURFACE OF SULFATE-REDUCING BACTERIA (DESULFOVIBRIO SP). SCALE BARS IN (a) AND (b) ARE 0.5 AND 1.5 mm, RESPECTIVELY ¹⁴

FIG. 3: TEM AND SEM MICROGRAPHS OF SELECTED GOLD NANOPARTICLES FORMED BY CYANOBACTERIAL INTERACTIONS WITH GOLD (III) CHLORIDE AND GOLD (I) THIOSULFATE COMPLEXES. SCALE BARS IN (a), (b), (c), and (d) are 0.5, 2, 1, AND 0.1 mm, RESPECTIVELY ¹⁴

Synthesis of Gold Nanoparticles by Bacterial System: Ahmad et al., (2003a) demonstrated bacterial synthesis of monodispersed GNPs with extremophilic Thermomonospora sp. biomass via reduction of auric chloride ions (AuCl₄^- ) through enzymatic processes ³⁶. Konishi et al., (2004) reported GNPs synthesis using the mesophilic bacterium Shewanella, where H₂ isacting as an electron donor ⁴⁴.  Shiying et al., (2007) showed that the bacterium Rhodopseudomonas capsulata produced spherical GNPs in the range of 10-20 nm, upon incubation of bacterial biomass with aqueous chlorauric acid (HAuCl₄) solution at a pH range of 4.0-7.0 upon 48 h of incubation ⁴⁵. Further, also discussed that solution pH is an important factor in controlling the morphology of biogenic GNPs and location of gold deposition in cells ³⁹. Alkalotolerant Rhodococcus sp. produced more intracellular monodispersed GNPs on the cytoplasmic membrane than on the cell wall due to reduction of the metal ions by enzymes present in the cell wall and on the cytoplasmic membrane, but not in the cytosol ³⁷.

Bacterial cell supernatants of Pseudomonas aeruginosa have been used for reduction of gold ions and for extracellular biosynthesis of GNPs ⁴⁶.  Bacillus subtilis 168 has been reported to reduce water-soluble Au³⁺ ions to Au⁰ and producenanoparticles of octahedral morphology and dimensions of 5-25 nm inside cell walls ²².

Heterotrophic sulfate-reducing bacterialenrichment from a gold mine has been exploited to reduce gold (I)-thiosulfatecomplex Au(S₂O₃)² to elemental gold of 10 nm size in the bacterial cell envelope,releasing H₂S as an end product of metabolism ^{36, 47}. E. coli DH5α-mediated bioreduction of chloroauric acid to Au⁰ resulted in accumulationof nanoparticles, mostly spherical and some triangles and quasi-hexagons, on thecell surface. These cell-bound nanoparticles offer promising applications in electrochemistryof hemoglobin and other proteins ⁴⁸.

Bioreduction of trivalent aurum has also been reported in the photosyntheticbacterium Rhodobacter capsulatus, which has a higher biosorption capacityfor HAuCl₄ per gram dry weight in the logarithmic phase of growth. The carotenoidsand NADPH-dependent enzymes embedded in the plasma membrane and/orsecreted extracellularly have been found to be involved in the biosorption andbioreduction of Au³⁺ to Au⁰ on the plasma membrane and also outside the cell ⁴⁹. Konishi et al., 2004 found intracellular synthesis of gold by microbial reduction of AuCl₄^- ions using the anaerobic bacterium Shewanella ⁴⁴.

The synthesis of stable gold nanocubes by the reduction of aqueous AuCl₄^- by Bacillus licheniformis has been described by Kalishwaralal (2009) ⁵⁰. The size of gold nanocubes (10–100 nm) in aqueous solution has been calculated using UV–Vis spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) measurements.

Synthesis of Gold Nanoparticles by Fungal System: The fungi are one of the good biological agents in the synthesis of metal nanoparticles. Biosynthesis of metal nanoparticles using fungi such as F. oxysporum ^51-53, Colletotrichum sp. ⁵⁴,  Trichothecium sp., Trichoderma asperellum, T. viride, ^55-57, Phaenerochaete chryso sporium ⁵⁸, Fusarium semitectum ⁵⁹,  Aspergillus fumigates ⁶⁰, Coriolus versicolor ⁶¹, Phoma glomerata ⁶², Penicillium brevicompactum ⁶³,  Cladosporium cladosporioides ⁶⁴,  Penicillium fellutanum ⁶⁵ and Volvariella volvacea ⁶⁶has been extensively studied. Indeed, fungi are regarded as more advantageous for GNPs biosynthesis as compared to other microorganisms because;

(1)   fungal mycelial mesh can withstand flow pressure, agitation, and other conditions in bioreactors compared to bacteria,

(2)   they are fastidious to grow and easy to handle, and;

(3)   they produce more extracellular secretions of reductive proteins and can easily undergo downstream processing ¹⁹.

Absar and coworkers (2005) reported extra- and intracellular biosynthesis of GNPs by fungus Trichothecium sp ⁶⁷. It was observed that when the gold ions reacted with the Trichothecium sp. fungal biomass under stationary condition, it resulted in the rapid extracellular formation of GNPs of spherical rod-like and triangular morphology whereas reaction of the biomass under shaking conditions resulted in intracellular growth of the GNPs. The synthesis of GNPs by the reduction of gold ions using Chinese herbal extract Barbated Skullcup has also been reported ⁶⁸. It has been observed that the extremophilic actinomycete, Thermomonospora sp. when exposed to gold ions reduced the metal ions extracellularly, yielding GNPs with a much improved polydispersity ⁶⁹.

Ahmad et al., (2003a) carried out the reduction of AuCl₄^- ions by using an extremophilic Thermomonospora sp. biomass that has resulted in efficient synthesis of monodisperse GNPs ³⁶. The reduction of metal ions and stabilization of the GNPs were believed to occur by an enzymatic process ^37-38.

Synthesis of Gold Nanoparticles by Cyanobacteria: In the cyanobacterial system, the mechanisms of gold reduction by Plectonema boryanum UTEX 485 from gold(III)–chloride solutions have been studied at several gold concentrations (0.8-7.6 mmol/L) and at 25-80^oC, using both fixed time laboratory and real-time synchrotron radiation XAS experiments ^70-71. The X-ray absorption spectroscopy (XAS) results showed that Au (III) was reduced to Au (I) in a very fast reaction (within minutes), and Au (I) was immediately coordinated with sulfur atoms from cyanobacteria forming gold (I)–sulfide for all gold concentrations and temperatures. The reduction of gold (I)–sulfide to elemental gold was found to be slower at 25^oC than at 60 ^oC and 80^oC. The steps of mechanism of gold reduction and precipitation by cyanobacteria are deduced:

Gold (III) – Chloride (AuCl₄^-)                  Gold (I) – Sulfide (Au₂S)                  Gold (Au)

Synthesis of Gold Nanoparticles By Algae: In the algae system, the mechanisms of gold reduction by Chlorella vulgaris biomass from gold (III) chloride solutions have been studied using XAS ⁷². The XAS results showed that Au (III) was partly reduced to Au (I) and Au (I) was coordinated with sulfur atoms from free sulfhydryl residues and also to a light-atom element, probably nitrogen. Kuyucak and Volesky (1989b) showed that elemental gold was mostly precipitated on the cell wall of Sargassum natans biomass and suggested that the carbonyl (C≡O) groups of the cellulosic materials were the main functional group in the gold binding with N-containing groups involved in a lesser degree ⁷³.

Lin et al., (2005) suggested that the hydroxyl group of saccharides, the carboxylate anion of amino-acid residues, from the peptidoglycan layer on the cell wall appeared to be the sites for gold binding ⁷⁴. However, in case of algal biomass, gold uptake was increased after esterification, suggesting that carboxyl groups played a minor role in gold binding ⁷⁵.

Romero-Gonza˜lez et al. (2003) studied the mechanisms of gold biosorption by dealginated seaweed biomass using fourier transform infrared spectroscopy (FT-IR) and XAS ⁷⁶. FT-IR showed the presence of carboxylate groups on the surface of the biomass and XAS showed that the reduction of gold species occurred on the biomass surfaces to form GNPs and was followed by retention of Au (I) at the sulfur containing sites. Therefore, it was found that the steps of mechanism of gold reduction and precipitation by algae are similar to cyanobacteria (as per above reaction) ¹⁴ .

The biosynthesis of GNPs using marine alga Sargassum wightii has also been investigated ⁷⁷. The stable GNPs in size range of 8 nm to 12 nm were obtained by reduction of aqueous AuCl₄^- ions by extract of marine algae and 95 % of the gold recovery occurred after 12 h of reaction.

Synthesis of Gold Nanoparticles by Plant System: One of the important approaches for biosynthesis of nanoparticles is employing the use of plant extract for biosynthesis reaction. In the case of Azadirachta indica leaf extract a competition bioreduction of Au³⁺ and Ag⁺ ions presented simultaneously in solution was observed. A bimetallic Au core-Ag shell nanoparticles synthesis occurred in solution ⁷⁸. Aloe vera leaf extract has been used for gold nanotriangle and spherical silver nanoparticles synthesis ⁷⁹. The kinetics of GNPs formation was monitored by UV-vis absorption spectroscopy and transmission electron microscopy (TEM).

It was found that after about 5 h of addition of Aloe vera extract to 10^-3 M aqueous solution of HAuCl₄ led to the appearance of a red color in solution. An analysis of the percentage of triangles formed in the reaction medium as a function of varying amounts of the Aloe vera extract showed that more spherical particles were formed with increasing in amount of Aloe vera leaf extract. Leaf extracts of two plants Magnolia kobus and Diopyros kaki were investigated for extracellular synthesis of GNPs ⁸⁰. The GNPs were formed by treating an aqueous HAuCl₄ solution by the plant extract. More than 90% recovery of GNPs was observed in a few minute of reaction at a reaction temperature of 90^oC.

With the use of Emblica Officinalis fruit extract as reducing agent, the extracellular synthesis of highly stable Ag and Au nanoparticles has also been achieved ⁸¹. Adding to the list of plants which are showing potential for nanoparticles production for example Cinnamomum camphora leaf extract has been identified very recently for the production of gold as well silver nanoparticles ². There was a marked difference of shape control between gold and silver nanoparticles which was attributed to the comparative advantage of protective biomolecules and reductive biomolecules. In this case, the polyol components and the water soluble heterocyclic components were mainly found to be responsible for the reduction of silver ions or chloroaurate ions and the stabilization of the nanoparticles, respectively. An overview of some of the reported biological agent synthesizing gold nanoparticles is focused in Table 1.

TABLE 1: BIOLOGICAL AGENTS USED FOR GOLD NANOPARTICLES BIOSYNTHESIS

Scope and application of Gold Nanoparticles: Production of inorganic and metal-based nanomaterials has stimulated the development of a new field that links many disciplines of sciences for the quest for different types of nanoparticles with unique properties. Designing and development of novel and affordable techniques for scale-up production of nanomaterials have not only provided an interesting area of study but in future will also address the expanding human requirements including health safety and environmental issues etc.

In industry, the application of nanomaterials is increasing day by day, and they will soon replace the harmful or toxic chemicals conventionally used as antimicrobial agents ⁹⁰. Application of nanoparticles and their nanocomposites also offers a sound and relatively safer alternative ^91-92 and, therefore, open up new opportunities for development of antimicrobials. Gold is a nobel metal and has been used by many ancient cultures (Egypt, India, and China) to treat diseases such as smallpox, skin ulcers, syphilis, and measles ^93-96.

Gold is currently used for medical devices like pacemaker and gold plated stents ^97-98 are used for the management of heart disease, middle ear gold implants, and gold alloys areused in dental restoration ⁹⁸. Organogold compounds are widely used for the treatment of rheumatoid arthritis but side effects such as proteinuria and skin reactions has been observed at high doses  ^{91, 99}.

The properties of GNPs remarkably differ from the bulk gold because of quantum size confinement imposed by nano-size regimen. The electronic, magnetic, and catalytic properties of GNPs depend mainly on their size and shape ¹⁰⁰. For example, spherical GNPs show a strong absorption band in the visible region of electromagnetic field (~520 nm) but is absent for very small particles (≤ 2 nm) as well as in the bulk gold. With a variety of unique properties, when GNPs are manipulated effectively, it can be applied to many different applications across the field of biology and medicine, environment, and technology ¹⁰¹.

Medical Application: GNPs are excellent labels and have been primarily used for labeling and bioimaging applications for biosensors because they can be detected by numerous techniques, such as optic absorption fluorescence and electric conductivity. GNPs are a very attractive contrast agent ^{96, 102}.The GNPs are directed and enriched at the region of interest, where it provides contrast for observation and visualization. With the characteristic of strongly absorption and scattering visible light, the light energy excites the free electrons in the GNPs to a collective oscillation, known as surface plasmon.

The excited electron gas relaxes thermally by transferring the energy to the gold lattice, and finally the light absorption leads to heating of the GNPs. The interaction of GNPs with light can be used for the visualization of particles using optical microscopy, fluorescence microscopy, photothermal, and photoacoustic imaging. In addition, the interaction of GNPs with both electron waves and X-rays can also be used for visualization using transmission electron microscopy ¹⁴. Gold nanoparticles have been used for a long time for delivery of drug molecules into cells ^{96, 102}. The molecules are adsorbed on the surface of GNPs and then are introduced into the cells using gene guns or particle ingestion.

Inside the cells, these molecules will eventually detach themselves from the GNPs ¹⁴.It gives non-toxic routes to drug and gene delivery application. GNPs are capable of delivering large biomolecules (peptides, proteins, or nucleic acids like DNA or RNA) ¹⁰³.

GNPs due to its biocompatibility and strong interaction with softbases like thiols play a major role in the treatment of cancer ¹⁰⁴. Epithelial ovarian cancer a common malignancy of femalegenital tract could be cured with the use of GNPs. Vascular endothelialgrowth factor (VEGF) performs a vital role in the progression of ovariancancer and also tumor growth and GNPs possess the capability toinhibit the progression of ovarian growth and metastasis ^96-97. Also, in case of multiple myeloma(MM), a cancer of plasma cells, GNPs are observed to inhibit the function of VEGF which induces cell proliferation. This inhibition of VEGFfurther leads to upregulation of cell cycle inhibitor proteins like p21 andp27 which inhibit proliferation ^{104, 106}.

Chronic lymphocytic leukemia (CLL), a cancer caused due to the overproduction of lymphocytes, starts in the bone marrow but could spread to other organs also. It was reported that as GNPs possess the ability to inhibit the function of heparin-based growth factor, GNPs alone can inhibit the function of factors secreted by CLL cells and induce apoptosis ^{104, 106-107}.

Rheumatoid arthritis which is considered as an incurable disease, bare GNPs are found to serve as a possible cure. Newly functionalized GNPs (dendrimers) have been designed for not only targeting and killing tumors but also to fight cancer ^108-109. GNPs is engineered not only to identify, target, and kill tumors but also to carry the additional drug to slow down the growth of cancer cells or kill the cancer cells. Dendrimers acts as an arm to the GNPs so that different molecules are attached to the arms.

Once the cancer cells are surrounded by GNPs, lasers or infrared light heats the gold particles and the dendrimers release the various molecules to kill the tumors ¹⁴. GNPs surface plasmon resonance scattering is predicted in the Mie equations and is found to increase as the size of the nanoparticles increases. By conjugating GNPs to anti-EGFR antibody, it gave the ability to distinguish between cancer and non-cancer cells from the strong scattering images of the GNPs conjugated to antibodies that binds only to the cancer, but not to the non-cancer cells ¹¹⁰. This scattering is observed from a simple optical microscope. They obtain a 600% greater binding ratio to the cancerous cells than to non-cancerous cells, enabling detection of cancerous cells by observing the scattered light on a dark field microscope.

Fig. 4 shows the scattering obtained with GNPs nonspecifically adsorbed on the surface (a–c) and GNPs with anti-EGFR (d–f) antibodies specifically bound to the cancerous cells but not to the non-cancerous cells. Because of this difference, the band shape and the surface plasmon absorption maximum are found to be different and therefore it can be used in medical field to differentiate cancerous cells. These results show that GNPs have enormous power as a diagnostic tool.

FIG. 4: LIGHT SCATTERING OF CELL LABELED WITH (a–c) GOLD NANOPARTICLES AND (d–f) anti-EGFR COATED GOLD NANOPARTICLES. THE anti-EGFR COATED GOLD NANOPARTICLES BIND SPECIFICALLY TO THE CANCEROUS CELLS, WHILE ALL OTHER GOLD NANOPARTICLES ARE NON-SPECIFICALLY BOUND. (a & d) NONMALIGNANT EPITHELIAL CELL LINE HaCaT (HUMAN KERATINOCYTES), (b & e) MALIGNANT EPITHELIAL CELL LINES HOC 313 CLONE 8 (HUMAN ORAL SQUAMOUS CELL CARCINOMA) (c & f) MALIGNANT EPITHELIAL CELL LINES HSC 3 (HUMAN ORAL SQUAMOUS CELL CARCINOMA)

GNPs can also be used for active sensor applications to determine the presence of analyte and to provide its concentration ¹⁰². The plasmon resonance frequency is a reliable feature of GNPs that can be used for sensing. The binding of molecules to the particle surface can change the plasmon frequency directly.

On the other hand, the plasmon resonance frequency is changed when the average distance among GNPs is reduced by forming small aggregates. The effect of plasmon coupling can be used for colorimetric detection of the analyte, known as a gold-based sensor. Raman scattering is enhanced if the analyte is close to a gold surface, called as surface-enhanced Raman scattering.

GNPs modified with Raman-active reporter molecules have been used for the detection of DNA ¹¹¹, protein ¹¹², and two-photon excitation ¹¹³. GNPs can also be used for the transfer of electrons in redox reactions ¹¹⁴. The enzyme is conjugated to the surface of the gold particles and is immobilized on the surface of an electrode ¹¹⁵.  An electrode covered with a layer of GNPs has a much higher surface roughness and larger surface area which lead to higher currents. Another application of gold compounds is as an anti-inflammatory agent due to their ability to inhibit expression of NF-kappa B and subsequent inflammatory reactions ^116-118.

One of the major drawbacks of ionic gold is that they easily get inactivated by complexation and precipitation that limits their desired functions in human system. Here zerovalent GNPs can be a valuable alternative replacing the potential of metallic gold ⁵⁰. GNPs, an emerging nanomedicine is renowned for its promising therapeutic possibilities, due to its significant properties such as biocompatibility, high surface reactivity, resistance to oxidation and Plasmon resonance ¹¹⁹. The inhibitory activity of GNPs against VPF/VEGF165 induced proliferation of endothelial cells provides clear evidence over their therapeutic potential in the treatment of diseases like chronic infiammation, pathological neo-vascularization, rheumatoid arthritis, and neoplastic disorders ¹²⁰.

Thus, gold nanoparticles have so many advantages in meadiacal field as they are in nanometer-size systems that can get easily into the bloodstream and around cells. Also, the multi-functional gold nanoparticles have been demonstrated to be highly stable and versatile scaffolds for drug delivery due to their properties like unique size, along with their chemical and physical properties. Their ability to tune the surface of the particle provides access to cell-specific targeting and thus controlled drug release ¹²¹.

Technological application: GNPs have been designed to improve computer memory ¹¹. A three-dimensional computer memory device composed of layers of GNPs has been developed to increase the memory capacity of a single chip. Another development of computer memory using GNPs is an organic nonvolatile bistable memory, which is a mixture of plastic and gold ⁴⁹.

Environmental application: Technologies based on GNPs are currently being developed for the environmental applications for pollution control and water purification ¹¹⁵.  It has been investigated that bimetallic gold–palladium nanoparticles provides an active catalyst which can be used to  degrade trichloroethene (TCE), which is one of the major pollutants in groundwater, into a non-toxic form ¹¹⁶. GNPs incorporated in a water purification device can effectively capture and remove halocarbon-based pesticides from drinking water ¹¹⁵ and can also enhance the oxidation of mercury generated from coal power plants  ¹²².

The use of GNPs as a catalyst has a major role to play in green chemistry ^123-124. Most industrial oxidation processes tend to use chlorine or organic peroxides which generates large amounts of chloride salts and chlorinated organic byproducts. GNPs supported on carbon active molecular oxygen are found to be able convert alkenes to partial oxidation products such as epoxides at atmospheric pressure and at 60^oC-80^oC ¹²⁵.

GNPs have been developed for selective oxidation of the biomass-derived chemicals, furfural and hydroxymethyl furfural, to form methyl esters as well as for oxidation of carbon monoxide (CO) and trimethylamine. These chemicals are used for flavor and fragrance applications, in plastics and industrial solvents ¹²⁶. Gas sensors based on Au nanoparticles have been developed for detecting a number of gases, including CO and nitrogen oxides (NOx) ¹²⁷.

CONCLUSION: Nanoparticles synthesis from biological route serves as an important alternative in the development of clean, nontoxic, economical and environmentally friendly procedures for the synthesis and assembly of GNPs and has tremendous advantages in comparison to conventional methods for nanoparticles synthesis.

In general, many biological agents have the ability to produce GNPs intracellular as well as extracellular environment. The work on the biosynthesis of GNPs is still largely in the discovery phase. Given the anticipated wide application of GNPs for commercial applications, continuing work is recommended to focus on the mechanisms of the biosynthesis of GNPs and the development of GNPs of well-defined size and shape. Changing properties simply by changing the size or shape of the GNPs is attractive and will continue to be employed in new applications in the future.

GNPs have a number of applications from electronics and catalysis to biology, pharmaceutical and medical diagnosis and therapy.However more research needs to be focused on the mechanistics and kinetics of GNPs formation which may lead to fine tuning of the process ultimately leading to the synthesis of GNPs with a strict control over the size, shape and large scale production of GNPs.

REFERENCE:   

1. Rai M, Yadav A, Gade A: Current trends in phytosynthesis of metal nanoparticles. Crit Re     Biotechnol 2008; 28(4): 277–284.
2. Huang J, Chen C, He N, Hong J, Lu Y, Qingbiao L, Shao W, Sun D, Wang XH, Wang Y, Yiang X: Biosynthesis of silver and gold nanoparticles by novel sundried Cinnamomum camphora leaf. Nanotechnol 2007; 18: 105–106.
3. Villaverde A: Nanotechnology, bionanotechnology and microbial cell factories. Microb Cell Fact 2010; 9:53.
4. Rai M, Yadav A, Bridge P, Gade A: Myconanotechnology: a new and emerging science. CABI publication; United Kingdom, 2009b.
5. Sau TK, Rogach AL: Nonspherical noble metal nanoparticles: colloid-chemical synthesis and morphology control, Adv Mater 2010; 22(16):1781–1804.
6. Jennings T, Strouse G: Past, present, and future of gold nanoparticles. Adv Exp Med Biol 2007; 620:34-47.
7. Jana NR, Gearheart L, Murphy CJ: Evidence for Seed-Mediated Nucleation in the Formation of Gold Nanoparticles from Gold Salts. Chem  Mater 2001; 13:1389-1393.
8. Wang L, Chen X, Zhan J, Chai Y, Yang C, Xu L, Zhuang W, Jing B: Synthesis of gold nano- and microplates in hexagonal liquid crystals. J Phys Chem B 2005; 109:3189-3194.
9. Seo D, Park JC, Song H: Polyhedral gold nanocrystals with O h symmetry: from octahedral to cubes. J Am Chem Soc 2006; 128:14863-14870.
10. 10.Pazos-Pérez N, Baranov D,  Irsen S,  Hilgendorff M, Liz-Marzán L M, Giersig M: Magnetic-noble metal nanocomposites with morphology-dependent optical response. Langmuir 2008; 24: 9855- 9860.
11. Mukherjee P, Bhattacharya R, Bone N, Lee YK, Patra CR, Wang S, Lu L, Secreto C, Banerjee PC, Yaszemski MJ, Kay NE,  Mukhopadhyay D:. Potential therapeutic application of gold nanoparticles in B-chronic lymphocytic leukemia (BCLL): enhancing apoptosis, J Nanobiotechnol 2007; 5:1-13.
12. Huang H, Liu Z, Yang X: Application of electrochemical impedance spectroscopy for monitoring allergen–antibody reactions using gold nanoparticles-based biomolecular immobilization method. Anal Biochem 2006; 356:208-214.
13. Kalishwaralal K, Gopalram S, Vaidyanathan R, Deepak V, Pandian SRK, Gurunathan G: Optimization of α-amylase production for the green synthesis of gold nanoparticles. Colloids Surf B Biointeraces 2010; 77:174-180.
14. Lengke MF, Sanpawanitchakit C, Southam G: Biosynthesis of gold nanoparticles: a review. In:  Rai and N. Duran (eds.), Metal Nanoparticles in Microbiology, 1st edn, Springer, New York, 2011; 37-74.
15. Sharma VK, Yngard RA, Lin Y: Silver nanoparticles: green synthesis and their antimicrobial activities. Adv Colloid Interface Sci 2009; 145: 83–96.
16. Bar H, Bhui DK, Sahoo GP, Sarkar P, Pyne S, Misra A: Green synthesis of silver nanoparticles using seed extract of Jatropha curcas. Colloids Surf A Physicochem Eng Asp 2009b; 348:212–216.
17. Ingle A, Gade A, Pierrat S, Sonnichsen C, Rai MK: Mycosynthesis of silver nanoparticles using the fungus Fusarium acuminatum and its activity against some human pathogenic bacteria. Curr Nanosci 2008; 4:141–144.
18. Rao CNR, Kulkarni GU, Thomas J P, Agrawal V V, Gautam UK, Ghosh M: Nanocrystals of metals, semiconductors and oxides: novel synthesis and applications. Curr Sci 2003; 83:1041–1045.
19. Musarrat  J, Dwivedi S, Singh BR, Saquib Q,  Khedhairy AA: Microbes and Microbial  Technology. London, Springer, 2011.
20. Mohanpuria P, Rana N K, Yadav S K. Biosynthesis of nanoparticles: Technological concepts and future applications, J Nanopart Res 2008; 10(3): 507-517.
21. Thakkar KN, Mhatre SS, Parikh RY: Biological synthesis of metallic nanoparticles. Nanomedicine 2010; 6(2):257–262.
22. Beveridge TJ, Murray RGE: Sites of metal deposition in the cell wall of Bacillus subtilis. J Bacteriol 1980; 141:876–887.
23. Huang CP, Juang CP, Morehart K, Allen L: The removal of Cu (II) from dilute aqueous solutions by Saccharomyces cerevisiae.  Water Res 1990; 24:433–439.
24. Frilis N, Myers-Keith P: Biosorption of uranium and lead by Streptomyces longwoodensis. Biotechnol Bioeng 1986; 28:21–28.
25. Sakaguchi T, Tsuji T, Nakajima A, Horikoshi T: Accumulation of cadmium by green microalgae. Eur J Appl Microbiol Biotechnol 1979; 8:207–215.
26. Darnall DW, Green B, Henzel MJ, Hosea M, McPherson R A, Sneddon J, Alexander MD: Selective recovery of gold and other metal ions from an algal biomass. Environ Sci Technol 1986; 20: 206–208.
27. Rai M, Yadav A, Bridge P, Gade A: Myconanotechnology: a new and emerging science. United Kingdom, CABI publication 2009b.
28. Kumar AS, Ansary AA, Ahmad A, Khan MI: Extracellular biosynthesis of CdSe quantum dots by the fungus, Fusarium Oxysporum. J Biomed Nanotechnol 2007a; 3:190–19.
29. 29.Kumar SA, Abyaneh MK, Gosavi SW, Kulkarni SK, Pasricha R, Ahmad A, Khan MI: Nitrate reductase-mediated synthesis of silver nanoparticles from AgNO₃. Biotechnol Lett 2007b; 29: 439–445.
30. Rai M, Durn N: Metal Nanoparticles in Microbiology. London, Springer 2011.
31. Mandal D, Bolander ME, Mukhopadhyay D, Sarkar G, Mukherjee P: The use of microorganisms for the formation of metal nanoparticles and their application. Appl Microbiol Biotechnol 2006; 69:485–492.
32. Moghaddam KM: An Introduction to Microbial Metal Nanoparticles Preparation Method. J young investigator 2010; 19:1-7.
33. Beveridge TJ, Hughes MN, Lee H, Leung KT, Poole RK, Savvaidis I: Metal–microbe interactions: contemporary approaches. Adv Microb Physiol 1997; 38:177–243.
34. Mukherjee P, Ahmad A, Mandal D, Senapati S, Sainkar SR, Khan MI, Ramani R, Parischa R, Kumar PAV, Alam M, Sastry M, Kumar R:  Bioreduction of AuCl₄^- ions by the fungus, Verticillium sp. and surface trapping of the gold nanoparticles formed. Angew Chem Int Ed 2001; 40:3585–3588.
35. Nair B, Pradeep T: Coalescence of nanoclusters and formation of submicron crystallites assisted by Lactobacillus strains. Cryst Growth Design 2002; 2:293–298.
36. Ahmad A, Mukherjee P, Senapati S, Mandal D, Khan MI, Kumar R, Sastry M: Extracellular biosynthesis of silver nanoparticles using the fungus Fusarium oxysporum. Colloids Surf B Biointerface 2003a; 28:313–318.
37. Ahmad A, Senapati S, Khan MI, Kumar R, Ramani R, Shrinivas V, Sastry M: Intracellular synthesis of gold nanoparticles by a novel alkalotolerant actinomycetes, Rhodococcus species, Nanotechnol 2003b; 14:824–828.
38. Duran N, Marcato PD, Alves OL, DeSouza G, Esposito E: Mechanistic aspects of biosynthesis of silver nanoparticles by several Fusarium oxysporum strains. J Nanobiotechnol 2005; 3:1–8.
39. He S, Guo Z, Zhang Y, Zhang S, Wang J, Gu N: Biosynthesis of gold nanoparticles using the bacteria Rhodopseudomonas capsulata. Mater Lett 2007; 61: 3984–3987.
40. 40.Nangia Y, Wangoo N, Goyal N, Shekhawat G, Suri CR:  A novel bacterial isolate Stenotrophomonas maltophilia as living factory for synthesis of gold nanoparticles. Microb Cell Appl Phys Lett 2009; 94(23):1–3.
41. Emery JF, Leddicotte GW: The radiochemistry of gold. National Academy of Sciences -National Research Council. Washington DC, 1962.
42. Nicol MJ, Fleming CA, Paul RL: The chemistry of the extraction of gold. In: Stanley GG (ed) The extractive metallurgy of gold in South Africa. South African Institute of Mining and Metallurgy, 2nd edn.  Johannesburg, South Africa 1987; 831–905.
43. Klaus T, Joerger R, Olsson E, Granqvist CG: Silver-based crystalline nanoparticles, microbially fabricated. The Proceedings of the National Academy of Sciences Online (US) 1999; 96(24):13611–13614.
44. Konishi Y, Ohno K, Saitoh N, Nomura T, Nagamine S: Microbial synthesis of gold nanoparticles by metal reducing bacterium. Trans Mater Res Soc Jpn 2004; 29: 2341–2343.
45. Shiying H, Zhirui G, Zhanga Y, Zhanga S, Wanga J, Ning G: Biosynthesis of gold nanoparticles using the bacteria Rhodopseudomonas capsulate. Mater Lett 2007; 61:3984–3987.
46. Husseiny MI, Abd El-Aziz M, Badr Y, Mahmoud MA: Biosynthesis of gold nanoparticles using Pseudomonas aeruginosa. Spectrochim Acta A 2007; 67:1003–1006.
47. Lengke MF, Fleet ME, Southam G: Bioaccumulation of gold by filamentous cyanobacteria between 25 and 200^oC. Geomicrobiol J 2006b; 23:591–597.
48. Du L, Jiang H, Xiaohua H, Wang E: Biosynthesis of gold nanoparticles assisted by Escherichia coli DH5a and its application on direct electrochemistry of haemoglobin. Electrochem Commun 2007; 9:1165–1170.
49. Feng Y, Yu Y, Wang Y, Lin X: Biosorption and bioreduction of trivalent aurum by photosynthetic bacteria Rhodobacter capsulatus. Curr Microbiol 2007; 55: 402–408.
50. Kalishwaralal  K, Deepak V, Ram Kumar Pandian S, Gurunathan S: Biosynthesis of gold nanocubes from Bacillus lichemiformis. Bioresour Technol 2009; 100: 5356-5358.
51. Senapati S, Mandal D, Ahmad A, Khan MI, Sastry, M, Kumar R: Fungus mediated synthesis of silver nanoparticles: a novel biological approach. Ind J Phys 2004; 78A: 101–105.
52. Bansal  V, Rautaray D, Ahmad A,  Sastry M: Biosynthesis of zirconia nanoparticles using the fungus Fusarium oxysporum. J Mater Chem 2004; 14:3303–3305.
53. Bansal V, Sanyal A, Rautaray D, Ahmad A, Sastry M: Bioleaching of sand by the fungus Fusarium oxysporum as a means of producing extracellular silica nanoparticles. Adv Mater 2005; 17:889–892.
54. Shankar SS, Ahmad A, Pasrichaa R, Sastry M: Bioreduction of chloroaurate ions by geranium leaves and its endophytic fungus yields gold nanoparticles of different shapes. J Mater Chem 2003; 13:1822–1826.
55. Ahmad A, Senapati S, Khan MI, Kumar R, Sastry M: Extra-/intracellular, biosynthesis of gold nanoparticles by an alkalotolerant fungus, Trichothecium. J Biomed Nanotechnol 2005; 1:47–53.
56. Mukherjee P, Roy M, Mandal BP, Dey GK, Mukherjee PK, Ghatak J, Tyagi AK, Kale SP: Green synthesis of highly stabilized nanocrystalline silver particles by a non-pathogenic and agriculturally important fungus T. asperellum. Nanotechnol 2008; 19: 075-103.
57. Fayaz M, Balaji K, Girilal M, Yadav R, Kalaichelvan PT, Venketesan R: Biogenic synthesis of silver nanoparticles and their synergistic effect with antibiotics: a study against gram-positive and gram-negative bacteria.  Nanomed Nanotechnol Biol Med 2010; 6:103–109.
58. Vigneshwaran N, Kathe AA, Varadarajan PV, Nachane RP, Balasubramanya RH: Biomimetics of silver nanoparticles by white rot fungus Phaenerochaete chrysosporium. Colloids Surf B Biointerfaces 2006; 53:55–59.
59. Basavaraja S, Balaji SD, Lagashetty A, Rajasab AH, Venkataraman A: Extracellular biosynthesis of silver nanoparticles using the fungus Fusarium semitectum.  Mat Res Bull 2008; 43:1164–1170.
60. Bhainsa KC, D’Souza SF: Extracellular biosynthesis of silver nanoparticles using the fungus Aspergillus fumigates. Colloids Surf B Biointerfaces 2006; 47:160–164.
61. Sanghi R, Verma P: Biomimetic synthesis and characterisation of protein capped silver nanoparticles. Bioresour Technol 2009; 100:501–504.
62. Birla SS, Tiwari VV, Gade AK, Ingle AP, Yadav AP, Rai MK: Fabrication of silver nanoparticles by Phoma glomerata and its combined effect against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. Lett Appl Microbiol 2009; 48:173–179.
63. Shaligram NS, Bule M, Bhambure R, Singhal RS, Singh SK, Szakacs G, Pandey A: Biosynthesis of silver nanoparticles using aqueous extract from the compactin producing fungal strain. Proc Biochem 2009; 44:939–943.
64. Balaji DS, Basavaraja S, Deshpande R, Mahesh BD, Prabhakar BK, Venkataraman A: Extracellular biosynthesis of functionalized silver nanoparticles by strains of Cladosporium cladosporioides fungus. Colloids Surf B Biointerfaces 2009; 68:88–92.
65. Kathiresan K, Manivanan S, Nabeel MA, Dhivya B: Studies on silver nanoparticles synthesized by a marine fungus, Penicillium fellutanum isolated from coastal mangrove sediment. Colloids Surf B Biointerfaces 2009; 71:133–137.
66. Philip D: Biosynthesis of Au, Ag and Au-Ag nanoparticles using edible mushroom extract. Spectrochim Acta A 2009; 73:374–381.
67. Absar A, Satyajyoti S, Khan MI, Rajiv K, Sastry M: Extra-/intracellular biosynthesis of gold nanoparticles by an alkalotolerant fungus, Trichothecium sp. J Biomed Nanotechnol 2005; 1: 47-53.
68. Wang Y, He X, Wang K, Zhang, X, Tan W: Barbated Skullcup herb extract mediated biosynthesis of gold nanoparticles and its primary application in electrochemistry. Colloids Surfaces B Biointerfaces 2009; 73:75-79.
69. Sastry M, Ahmad A, Khan MI, Kumar R: Biosynthesis of metal nanoparticles using fungi and actinomycete. Curr Sci 2003; 85:162–170.
70. Lengke MF, Ravel B, Fleet ME, Wanger G, Gordon RA, Southam G: Mechanisms of gold bioaccumulation by filamentous cyanobacteria from gold(III)-chloride complex. Environ Sci Technol 2006c; 40(20):6304–6309.
71. Lengke MF, Ravel B, Fleet ME, Wanger G, Gordon RA, Southam G: Precipitation of gold by the reaction of aqueous gold(III) chloride with cyanobacteria at 25–80^oC – studied by x-ray absorption spectroscopy. Can J Chem 2007; 85(10):651–659.
72. Watkins JW II, Elder RC, Greene B, Darnall D: Determination of gold binding in an algal biomass using EXAFS and XANES spectroscopies. Inorg Chem 1987; 26(7):1147–1151.
73. Kuyucak N, Volesky B: The mechanism of gold biosorption. Biorecovery 1989b; 1:219–235.
74. Lin Z, Wu J, Xue R, Yang Y: Spectroscopic characterization of Au³⁺ biosorption by wastebiomass of Saccharomyces cerevisiae. Spectrochim Acta A 2005; 61:761–765.
75. Gardea-Torresdey JL, Becker-Hapak MK, Hosea JM, Darnall DW: Effect of chemical modification of algal carboxyl groups on metal ion binding. Environ Sci Technol 1990; 24(9):1372–1378.
76. Romero-Gonza˜lez ME, Williams CJ, Gardiner PHE, Gurman SJ, Habesh S: Spectroscopic studies of the biosorption of gold(III) by dealginated seaweed waste. Environ Sci Technol 2003; 37 (18):4163–4169.
77. Singaravelu G, Arockiamary JS, Ganesh Kumar V, Govindaraju K: A novel extracellular synthesis of mondisperse gold nanoparticles using marine alga, Sargassum wightii Greville. Colloids Surf B 2007; 57: 97–101.
78. Shankar SS, Rai A, Ahmad A, Sastry M: Rapid synthesis of Au, Ag, and bimetallic Au core–Ag shell nanoparticles using neem (Azadirachta indica) leaf broth. J Colloid Interf Sci 2004; 275:496–502.
79. Chandra PS, Chaudhary M, Pasricha R, Ahmad A, Sastry M: Synthesis of gold nanotriangles and silver nanoparticles using Aloe vera plant extract, Biotechnol Prog 2006; 22:577-583.
80. Song YJ, Jang HK, Kim SB: Biological synthesis of gold nanopaticles using Magnolia kobus and Diopyros kaki leaf extract. Process Biochem 2009; 44:1133-1138.
81. Ankamwar B, Damle C, Absar A, Mural S: Biosynthesis of gold and silver nanoparticles using Emblica Officinalis fruit extract, their phase transfer and transmetallation in an organic solution. J Nanosci Nanotechnol 2005a; 10:1665–1671.
82. Kashefi K, Tor JM, Nevin KP, Lovley DR: Reductive precipitation of gold by dissimilatory Fe(III)-reducing bacteria and archaea. Appl Environ Microbiol 2001; 67(7):3275–3279.
83. Deplanche K, Macaskie LE: Biorecovery of gold by Escherichia coli and Desulfovibrio desulfuricans. Biotechnol Bioeng 2007; 99(5):1055–1064.
84. Gericke M, Pinches A: Microbial production of gold nanoparticles. Gold Bull 2006b; 39:22– 28.
85. Lengke MF, Fleet ME, Southam G: Morphology of gold nanoparticles synthesized by filamentous cyanobacteria from gold(I)-thiosulfate and gold(III)-chloride complexes. Langmuir 2006a; 22(6):2780–2787.
86. Dyer BD, Krumbein WE, Mossman DJ: Accumulation of gold in the sheath of Plectonema terebrans (filamentous marine cyanobacteria). Geomicrobiol J 1994; 12:91–98.
87. Mata YN, Torres E, Bla¯zquez ML, Ballester A, Gonza¯lez F, Mun´oz JA: Gold(III) biosorption and bioreduction with the brown alga Fucus vesiculosus. J Hazard Mater 2009; 166:612–618.
88. Armendariz V, Herrera I, Peralta-Videa JR, Jose-Yacaman M, Troiani H, Santiago P, Gardea-Torresdey JL: Size controlled gold nanoparticles formation by Avena sativa biomass: use of plants in nanobiotechnology. J Nanopart Res 2004; 6:377–382.
89. Ankamwar B, Chaudhary M, Mural S: Gold nanotriangles biologically synthesized using tamarind leaf extract and potential application in vapor sensing. Synth React Inorg Metal-Org Nanometal Chem 2005b; 35:19–26.
90. Mucha H, Hofer D, ABflag S, Swere M: Antimicrobial finishes and modification. Melliand Textil Int 2002; 83:53–56.
91. Chen Q, Shen X,  Gao H: One-step synthesis of silver-poly (4- vinylpyridine) hybrid microgels by -irradiation and surfactant-free emulsion polymerization, the photoluminescence characteristics. Colloids Surf A Physicochem Eng Asp 2006; 275:45–49.
92. Dimitrov DS: Interactions of antibody-conjugated nanoparticles with biological surfaces, Colloids Surf A: Physicochem Eng Asp 2006; 282–283: 8–10.
93. Huaizhi Z, Yuantao N: China’s ancient gold drugs. Gold Bull 2001; 34:24–9.
94. Richards DG, McMillin DL, Mein EA, Nelson CD: Gold and its relationship to neurological/glandular conditions. Int J Neurosci 2002; 112(1):31–53.
95. Gielen M, Tiekink ERT: Metallotherapeutic drugs and metal-based diagnostic agents, The use of metals in medicine.Wiley 2005; Hoboken, New York.
96. Chen PC, Mwakwari SC, Oyelere AK: Gold nanoparticles: from nanomedicine to nanosensing. Nanotechnol Sci Appl 2008; 1: 45–66.
97. Edelman ER, Seifert P, Groothuis A, Morss A, Bornstein D, Rogers C: Gold-coated NIR stents in porcine coronary arteries. Circulation 2001; 103:429–34.
98. Svedman C, Dune´r K, Kehler M, Moller H, Gruvberger B, Bruze M: Lichenoid reactions to gold from dental restorations and exposure to gold through intracoronary implant of a goldplated stent. Clin Res Cardiol 2006; 95:689–91.
99. Sun RW-Y, Ma D-L, Wong EL-M, Che C-M: Some uses of transition metal complexes as anti-cancer and anti-HIV agents. Dalton Trans 2007; 43:4884–4892.
100. Link S, Mohamed MB, El-Sayed MA: Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant. J Phys Chem B 1999; 103:3073–3077.
101. Oldenburg SJ, Averitt RD, Westcott SL, Halas NJ: Nanoengineering of optical resonances. Chem Phys Lett 1998; 288:243–247.
102. Sperling RA, Gil PR, Zhang F, Zanella M, Parak WJ: Biological application of gold nanoparticles. Chem Soc Rev 2008; 37:1896–1908.
103. Ghosh P, Han G, De M, Kim KC, Rotello MV: Gold nanoparticles in delivery applications. Adv Drug Del Rev 2008; 60:1307-1315.
104. Bhattacharya R, Mukherjee P: Biological properties of naked nanoparticles. Adv Drug Deliv Rev 2008; 60:1289–1306.
105. Bamberger ES, Perrett CW: Angiogenesis in epithelian ovarian cancer. Mol Pathol 2002; 55:348–359.
106. Bhattacharya R, Patra CR, Verma R, Griepp PR, Mukherjee P: Gold nanoparticles inhibit the proliferation of multiple myeloma cells. Adv Mater 2007; 19:711–716.
107. Zent CS, Call TG, Hogan WJ, Shanafelt TD, Kay NE: Uptake on risk-stratified management for chronic lymphocytic leukemia. Leuk Lymphoma 2006; 47:1738–1746.
108. Nam J, Won N, Jin H, Chung H, Kim S: pH-induced aggregation of gold nanoparticles for photothermal cancer therapy. J Am Chem Soc 2009; 131:13639–13645.
109. Escosura-Mun´iz ADL, Sa´nchez-Espinel C, Dı´az-Freitas B, Gonza´lez-Ferna´ndez A: Costa MM, Merkoci. A Rapid identification and quantification of tumor cells using an electrocatalytic method based on gold nanoparticles. Anal Chem 2009; 81(24):10268–10274.
110. El-Sayed I, Huang X, El-Sayed M: Surface plasmon resonance scattering and absorption of anti-EGFR antibody conjugated gold nanoparticles in cancer diagnostics: Applications in oral cancer. Nano Letters 2005; 5:829–834.
111. Krug JT, Wang GD, Emory SR, Nie S: Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals. J Am Chem Soc 1999; 121(39):9208–9214.
112. Ni J, Lipert RJ, Dawson GB, Porter MD: Immunoassay readout method using extrinsic Raman labels adsorbed on immonogold colloids. Anal Chem 1999;  71:4903–4908.
113. Kneipp J, Kneipp H, Kneipp K: Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering. Proc Natl Acad Sci USA 2006; 103(46):17149–17153.
114. Willner B, Katz E, Willner I: Electrical contacting of redox proteins by nanotechnological means. Curr Opin Biotechnol 2006; 17:589–596.
115. Xiao Y, Patolsky F, Katz E, Hainfeld JF, Willner I: Plugging into enzymes: nanowiring of redox enzymes by a gold nanoparticles, Science 2003; 299(5614):1877–1881.
116. Norton S :A brief history of potable gold. Mol Interv 2008; 8(3):120-125.
117. 117.Jeon KI, Byun MS, Jue DM: Gold compound auranofin inhibits Ikappa B kinase (IKK) by modifying Cys-179 of IKKbeta subunit. Exp Mol Med 2003; 35:61-66.
118. Kim NH, Lee MY, Park SJ, Choi JS, Oh MK, Kim IS: Auranofin blocks interleukin-6 signalling by inhibiting phosphorylation of JAK1 and STAT3. Immunology 2007; 122:607-614.
119. Guo R, Song Y, Wang G, Murray RW: Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters? J Am Chem Soc 2005; 127: 2752-2757.
120. Mukherjee P, Bhattacharya R, Wang P, Wang L, Basu S, Nagy JA, Atala A, Mukhopadhyay D, Soker S: Antiangiogenic properties of gold nanoparticles, Clin Cancer Res 2005; 11:3530-3534.
121. Kumawat L, Jain A: Biosynthesis of nanoparticles.Int J Pharm Sci R 2011; 2(11): 2781-2785.
122. Pradeep T, Anshup: Noble metal nanoparticles for water purification: A critical review. Thin Solid Films 2009; 517(24):6441–6478.
123. Dahl JA, Maddux BLS, Hutchison JE: Toward greener nanosynthesis. Chem Rev 2007; 107:2228–2269.
124. Herzing AA, Kiely CJ, Carley AF, Landon P, Hutchings GJ: Identification of active gold nanoclusters on iron oxide supports for CO oxidation. Science 2008; 321:1331–1335.
125. Hughes MD, Xu YJ, Jenkins P, McMorn P, Landon P, Enache DI, Carley AF, Attard GA, Hutchings GJ, King F, Stitt EH, Johnston P, Griffin K, Kiely CJ: Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions. Nature 2005; 437:1132–1135.
126. Taarning E, Nielsen IS, Egeblad K, Madsen R, Christensen CH: Chemicals from renewables: aerobic oxidation of furfural and hydroxymethylfurfural over gold catalysts. ChemSusChem 2008; 1:75–78.
127. Thompson TD: Using gold nanoparticles for catalysis. Nano Today 2007; 2, 4:40-43.
     

     ---