Field of the invention:

The present invention relates to the field of environmental nano-technology. More particularly the present invention relates to a process of preparing bimetallic silver-gold nanoparticles by eco friendly and remunerative utilization of otherwise harmful weeds.

Background of the invention and prior art:

There is great surge of interest in metal nanoparticles due to the realization that the catalytic activity, and the antimicrobial, electronic, optical, magnetic and medical properties of metallic nanoparticles often differ vastly from that of the concerned bulk metal. This large contrast in properties can be attributed to the change in surface area to volume ratio that occurs as a consequence of nano-sizing. As a consequence, synthesis of metallic nanoparticles for applications in catalysis, electronics, pollution control, medicine, etc is an area of intense scientific activity at present. Even more exciting are the possibilities of bimetallic nanoparticles as they are expected to exhibit virtues not possible in their monometallic counterparts.

Prior arts for the synthesis of bimetallic nanoparticles reveal that so far such particles have been synthesized by physical and chemical methods which employ laser-based, solution phase, sonochemical, and photochemical techniques, etc. But these methods often involve hazardous reagents and/or process conditions which leads to generation of pollutants. Moreover the presence of some toxic chemical species adsorbed on the nanoparticle surface may have adverse effects in medical applications. On the other hand the physical methods are highly energy-intensive and expensive.

Hence the need for alternative ways to synthesize bimetallic nanoparticles has arisen due to the above-mentioned demerits of physical and chemical processes. This search for alternate pathways have led to the use of microorganisms and then plant extracts for the synthesis of metal nanoparticles. Bacteria, fungi, and actinomycetes were the first to be tried but the rate of synthesis by microbiological methods was found to be very slow. In contrast extracts of plants (botanical species) have proved more potent bioagents.

So far different authors have used about 12 species of plants to generate bimetallic (Au/Ag) nanoparticles. These species encompass fruits, vegetables, grains, spices, other foodstuff, medicinal plants, etc, for example Azadirachta indica (Shankar et al., 2004), Lemongrass (Rai ef a/., 2007), D/ospyros kaki (Song and Kim, 2008), sorghum, marjoram, parsley (Hoag et al., 2010), Anacardium occidentale (Sheny et al., 2011), Swietenia mahogany (Mondal et al., 2011), and red grape pomace (Varma et al., 2011). The prior art is summarized as follows:

Shankar et al. (2004) have published a research article in Journal of Colloid and Interface Science., 275, 496-502, disclosing the rapid synthesis of Au, Ag and bimetallic Au core-Ag shell nanoparticles using Neem (Azadirachta indica) leaf broth.

An article published by Rai et al. (2007), in Materials Research Bulletin, 42 (7), 1212-1220, reveals the synthesis of triangular Au core-Ag shell nanoparticles using lemongrass extract.

In Korean Journal of Chemical Engineering, 25 (4), 808-811 (2008), Song and Kim, - have published an article on biological synthesis of bimetallic Au/Ag nanoparticles using persimmon (Diospyros kaki) leaf extract.

Mondal et al. (2011) have published an article in Colloids and Surfaces B: Biointerfaces, 82 (2), 497-504, which discloses biogenic synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles using aqueous extract of mahogany leaves.

An article published by Sheny et al. (2011) in Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy, disclose phytosynthesis of Au, Ag and Au-Ag bimetallic nanoparticles using aqueous extract and dried leaf of Anacardium occidentale.

US application number 20100200501 discloses the use of tea, coffee, parsley, sorghum, marjoram, and/or lemon balm in synthesizing bimetallic nanoparticles.

US application number 20110110723 discloses the use of fruit extracts (red wine or red grape pomace) for the production of bimetallic and multimetallic nanoparticles.

The advantage of using plant extracts, as revealed in the above mentioned prior arts, for the synthesis of bimetallic nanoparticles is that the extracts are non-hazardous, safe to handle and possess a broad variability of metabolites that facilitates nanoparticle formation. But all of the prior art has involved species which are highly useful to mankind in a number of other ways. All the plants used so far in the prior arts have well-established uses as food items, source of cosmetics, medicine, etc. Hence their employment in nanoparticles synthesis, when done on a large-scale, will not only clash with their already well-established uses but would also, consequently, entail substantial costs of production.

In contrast the present invention distinguishes itself by its successful development of bimetallic nanoparticle generation process which utilizes invasive plants (weeds) which are not only worthless but also harmful to the environment. Moreover, unlike prior art which relies only on the use of plant leaves, for example Azadirachta indica (Shankar et a/., 2004), Lemongrass (Rai et a/., 2007), Diospyros kaki (Song and Kim, 2008), Anacardium occidentale (Sheny et al., 2011), and Swietenia mahogany (Mondal et a/., 2011), the present invention is based on the use of the whole plant of the terrestrial weeds lantana, ipomoea, mimosa and coral vine. This is a very significant improvement because on one hand it enhances the utility value of each plant manifold (by enabling the use of the whole plant instead of just leaves) and on the other hand it makes the utilization of the invasives so potentially gainful that it may become remunerative to control the invasives through their harvesting and use. Hence the invention has far-reaching beneficial portent for the protection of large tracts of terrestrial ecosystems.

Object of the invention:

The main object of the present invention is to synthesize bimetallic (Au/Ag) nanoparticles by a clean-green, eco-friendly and inexpensive process.

Another object of the present invention is to synthesis bimetallic (Au/Ag) nanoparticles extracellularly using plant extract.

Yet another object of the present invention is to utilize otherwise worthless weeds for the synthesis of bimetallic (Au/Ag) nanoparticles.

Yet another object of the present invention is to utilize at least one of weeds selected from the group comprising of lantana, ipomoea, mimosa, and coral vine for the synthesis of bimetallic (Au/Ag) nanoparticles.

Yet another object of the present invention is to utilize the whole plant extract of the weeds plant in a gainful manner for the generation and stabilization of bimetallic (Au/Ag) nanoparticles

Yet another object of the present invention is to develop a simplified process for the synthesis of bimetallic (Au/Ag) nanoparticles under ambient temperature, pressure without toxic reactants and products.

Yet another object of the present invention is to optimize the process for the synthesis of bimetallic (Au/Ag) nanoparticle.

Yet another object of the present is to develop processes for the synthesis of mono and poly dispersed bimetallic (Au/Ag) nanoparticles under benign, non hazardous, process conditions.

Yet another object of the present invention is to produce bimetallic (Au/Ag) nano particles of desired size and shape by a clean-green, environment-friendly, and inexpensive process.

Further object of the present invention is to utilize the harmful and undesirable weeds as they have to be otherwise controlled by using herbicides, and other control methods which toxify the environment.

Summary of the invention:

The present invention relates to a green eco-friendly and one pot synthetic method for the synthesis of bimetallic (Au/Ag) nanoparticles of desired size and shape. The method comprises of reacting silver and gold metal salt solutions with weed extracts in aqueous medium. The weed extract used in the present invention is prepared from any one of the weed selected from the group comprising of lantana, ipomoea, mimosa, or coral vine. From the method it is possible to prepare bimetallic (alloy-Au/Ag) nanoparticles, with the metals (Au/Ag) substantially interspersed throughout the particles by altering the order of addition of the reagents and time of addition. Mono and poly dispersed " bimetallic (Au/Ag) nanoparticles of desired size and shape can also be prepared by optimizing the reagents and reaction conditions.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a process of preparing bimetallic (Au/Ag) nanoparticles by eco friendly and remunerative utilization of otherwise harmful weeds.

The present invention utilizes terrestrial weeds lantana (Lantana camara), Ipomoea (Ipomoea carnea), mimosa (Mimosa pvdica) and coral vine (Antigonon leptopus) which are freely available in several parts of the world in very huge quantities. These weeds are not merely worthless but are regarded as a nuisance. In fact expenditure equivalent to billions of rupees is routinely incurred across the world in attempts to eradicate these weeds, or control them with herbicides. This is because these weeds play havoc with the economic productivity and ecological integrity of land and water; and cause very seriously adverse impacts on human and livestock health.

The four terrestrial weeds used in this invention, lantana (Lantana camara), Ipomoea (Ipomoea carnea), mimosa (Mimosa pudica) and coral vine {Antigonon leptopus) have not been used for bimetallic nanoparticle synthesis ever before. This is a very significant because on one hand it enhances the utility value of each plant manifold (by enabling the use of the whole plant instead of just leaves) and on the other hand it makes the utilization of the invasives so potentially gainful that it may become remunerative to control the invasives through their harvesting and use in this nanoparticle manufacturing process. Hence the invention has far-reaching beneficial portent for the protection of large tracts of terrestrial ecosystems.

In one of the preferred embodiment the present invention shall disclose a process for biosynthesis of plant-metal based bimetallic nanoparticles by reacting a first metal salt solution, a second metal salt solution and a weed extract in an appropriate stoichiometric proportion in an aqueous medium. The reaction is allowed for a period of time to form the bimetallic nanoparticles. The weed extract acts as a reducing-cum-capping agent of different potencies thereby the two metal ions are reduced, capped and stabilized into bimetallic nanoparticles of desired size. According to the invention the weed extract used is a whole weed extracts of lantana, ipomoea, mimosa or coral vine. As per the invention the metal salts used are chloroauric acid and silver nitrate.

As per the invention the whole weed means to include leaf, stem, root, singly or in combination thereof. According to the invention first and the second metal ion solutions are introduced simultaneously to react with the weed extract to form the bimetallic nanoparticles in which the first and second metals are interspersed throughout the metal nanoparticles where the first metal is gold and second metal is silver or alternatively the first metal is silver and said second metal is gold.

According to the invention the first metal ion solution is introduced first to react with the weed extract and after a delay time period the said second metal ion is introduced to form the bimetallic nanoparticles in which the first metal is silver and second metal is gold .As per the invention the delay time period ranges from 10 minutes to 28th hour and more preferably 10 minutes for lantana, 8 hours for coral vine and 28 hours for mimosa and ipomoea. Delayed addition of second metal did not favor the bimetallic formation where the first metal is gold and second metal is silver.

In another preferred embodiment the present invention utilizes at least one of the weed extracts selected from the group comprising of lantana, ipomoea, mimosa, or coral vine for the synthesis of bimetallic (Au/Ag) nanoparticles.

According to the invention the size of the bimetallic nanoparticles is determined by the concentration and proportion of the first metal salt solution, second metal salt solution and the weed extract. By varying the stoichiometry and the proportion of the reagents varying sizes of bimetallic (Au/Ag) nanoparticles is obtained. This is characterized using UV-Vis spectral patterns, Hr-SEM, and TEM studies.

The size of the bimetallic nanoparticles is also determined by the reducing-cum-capping property of the weed extracts, because a particular proportion (metal salt solutions and weed extract) exhibits varying spectral pattern when studied among the species. The variation is due to the difference in size and shape of the particles synthesized.

Also reaction time is a factor influencing the size of the nanoparticles. The reaction mixtures are continuously monitored under UV-Vis spectral recording. The size of the particles synthesized is determined by their peak wavelength, shift in wavelength and broadness of the peak formed.

According to the invention the process for synthesizing the bimetallic (Au/Ag) nanoparticles is a one pot synthetic route. During the reaction ie., once aqueous metal salt solutions and weed extracts are mixed in appropriate stoichiometric proportions,
nucleation starts without any intermediary products such as AgO , AgCI Au203 or AU2CI3. This is confirmed by the EDAX spectrum, where only clear peaks of Ag and Au found at 2 keV and 3 keV along with least peaks of other elements C, N and O.

According to the invention the optimization for the process of preparing mono and polydispersed bimetallic (Au/Ag) nanoparticles has been carried out. The exact concentration of the reagent for synthesizing the mono or polydispersed bimetallic (Au/Ag) nanoparticles is determined by varying the concentration of the reagents and recording the resultant peak wavelength and absorbance in UV-Vis spectra.

Characterization of the synthesized bimetallic nanoparticles, were determined by SEM, Hr-SEM, and TEM analysis, the average particle size was found to be around 50 nm.

The process of the present invention does not involves any hazardous chemicals and can be easily performed at room temperature and pressure without the need of control of temperature, pressure, rigorous humidity, etc. Further no toxic pollutant is generated - in the process. No special culture-preparation and isolation procedures are required for the process. Hence the process is easily scalable and very energy economical. The shape and size of the synthesized bimetallic nanoparticles can be easily controlled based on the concentrations of the reagents, reducing cum capping property of the weed extract and reaction time. Further the diagnostic tool for controlling the nanoparticle formation and consistency of its shape/size is spectrophotometry which is a relatively inexpensive and standard tool.

Although the invention has been described above with reference to the disclosed embodiments, those skilled in the art will readily appreciate that the specific experiments detailed are only illustrative of the invention. It should be understood that " various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.

We claim:

1. A process for extra-cellular biosynthesis of bimetallic nanoparticles comprising of

reacting a first metal salt solution, a second metal salt solution and a weed extract in an appropriate stoichiometric proportion in an aqueous medium and allowing the reaction for a period of time to form the bimetallic nanoparticles wherein the said weed extract acts as a reducing-cum-stabilizing agent thereby converting the metal salts into bimetallic nanoparticles of desired size.

characterized in the weed extract as whole weed extract selected from a group consisting of lantana, ipomoea, mimosa and coral vine and in the metal salts which are chloroauric acid and silver nitrate.

2. The process as claimed in claim 1 wherein the said first and the second metal ion solutions are introduced simultaneously to react with the weed extract to form the said bimetallic nanoparticles wherein the said first metal is gold and the said second metal is silver.

3. The process as claimed in claim 1 wherein the said first and the second metal ion solutions are introduced simultaneously to react with the weed extract to form the said bimetallic nanoparticles wherein the said first metal is silver and the said second metal is gold.

4. The process as claimed in claim 1 wherein the said first metal ion solution is introduced first to react with the weed extract and after a delay time period the said second metal ion is introduced to form the bimetallic nanoparticles wherein the said first metal is silver and said second metal is gold.

5. The process as claimed in claim 4, wherein the said delay time period ranges from 10 minutes to 28 hour and more preferably 10 minutes for lantana, 8 hours for coral vine and 28 hours for mimosa and ipomoea extract

6. The process as claimed in claim 1, wherein the reaction of metal salt solutions with the weed extract initiates nucleation but without forming any intermediates such as AgO, AgCI. AU2O3 and AU2CI3.

7. The process as claimed in claim 1, wherein the size and the shape of the said bimetallic nanoparticles is determined by the proportion and concentration of metal ions with respect to the weed extracts and further determined by the synthesis time.

8. The process as claimed in claim 1, wherein the said bimetallic nanoparticles so obtained may be mono dispersed or poly dispersed.

9. The process as claimed in claim 1 wherein the whole weed means to include leaf, stem, root, singly or in combination thereof.