Gold nanoparticles are being used for an ever-growing number of applications. Gold nanoparticles are used in electronics, healthcare products, as pharmaceuticals to fight cancer, imaging tumors, carrying drugs, and delivering pulses of heat. Gold nanoparticles could function as in vivo sensors, photoactive agents for optical imaging, drug carriers, disinfection and tissue repair, contrast enhancers in computer tomography and X-ray absorbers in cancer therapy, automobile sensors, cell phones, blood sugar monitors and hydrogen gas production, but the process to make these nanoparticles requires dangerous and extremely toxic chemicals and acids as reducing and/or capping agents which have severe environmental impact. Hence green chemistry should be integrated into nanotechnologies at the source especially when nanomaterials are to be used in medical applications and when metal nanoparticles are intended for application to the human body, toxic chemicals should be excluded entirely from the process. Researchers have found methods that could replace nearly all of the toxic chemicals required to make gold nanoparticles by a natural products like cinnamon, soybeans, cumin, gum arabic, pear, black tea, turmeric, curcumin, geranium extract, bacteria and fungi etc.
Cinnamon
Researchers at University of Missouri have found a method that could replace nearly all of the toxic chemicals required to make gold nanoparticles by a natural product namely cinnamon. The researchers mixed gold salts with cinnamon and stirred the mixture in water to synthesize gold nanoparticles. The new process used no electricity and utilized no toxic agents making it is a true 'green' process.
Soybean
University of Missouri research team has used soybeans as a Phytochemical Reservoir for the Production and Stabilization of Biocompatible Gold Nanoparticles.
Researchers found that by submersing gold salts in water and then adding soybeans, gold nanoparticles could be generated. The water pulls a phytochemical out of the soybean that is effective in reducing the gold to nanoparticles. A second phytochemical from the soybean, also pulled out by the water, interacts with the nanoparticles to stabilize them and keep them from fusing with the particles nearby. This process creates nanoparticles that are uniform in size in a 100-percent green process. No toxic waste was generated.
Cumin
Researchers report an unprecedented synthetic route that involves the production of well-defined spherical gold nanoparticles by simple mixing of cumin to an aqueous solution of sodium tetrachloro aurate. Production of gold nanoparticles in this cumin-mediated Green Nanotechnological process is achieved under biologically benign conditions. The gold nanoparticles generated through cumin-mediated process did not aggregate suggesting that the cocktail of phytochemicals including proteins serve as excellent coatings on nanoparticles and thus, provide robust shielding from aggregations. In addition, the phytochemical coatings on nanoparticles have rendered nontoxic features.
Gum arabic
The researchers became interested in gum arabic, a substance taken from species of the acacia tree, because it is already used to stabilize everyday foods such as yogurt, Big Macs and soda. Gum arabic has unique structural features, including a highly branched polysaccharide structure consisting of a complex mixture of potassium, calcium and magnesium salts derived from arabic acid. The scientists found that gum arabic could be used to absorb and assimilate metals and create a "coating" that makes gold nanoparticles stable and nontoxic. Gum arabic can effectively 'lock' gold nanoparticles to produce nontoxic, nanoparticulate constructs that can be used for potential applications in nanomedicine.
Black tea, turmeric, curcumin or cinnamon
In methods of the invention, an aqueous solution containing gold salts is mixed with polyphenols- or flavanoids-rich plant material. In preferred embodiment methods of making, an aqueous solution containing gold salts is provided. The aqueous solution is mixed with black tea, turmeric, curcumin or cinnamon or a similar naturally occurring polyphenols- or flavanoids-rich plant material. The gold salts react to form biocompatible gold nanoparticles that are stabilized with a coating of the polyphenols- or flavanoids-rich plant material. The black tea, turmeric, curcumin or cinnamon or similar naturally occurring polyphenols- or flavanoids-rich plant material can be a powder or can be in its root or bark form.
Pear
Korean researchers have used pear extract to obtain phytochemically-derived reducing agents for the generation and stabilization of gold nanoparticles. Pear extract is a reservoir of phytochemicals including organic acids, amino acids, peptides and proteins. In addition, the presence of saccharides in the extract provides synergistic reducing power for the rapid transformation of chloroaurate ions into gold nanoparticles. Simple mixing of pear extract (50% [v/v]) with HAuCl4 (2 mM) at 90 oC promptly initiated the appearance of a purple-red color, indicating the formation of gold nanoparticles.
Geranium extract
It has been reported that the synthesis of gold nanoparticles using geranium extract required more time to initiate and was complete in 48 h.
Emblica officinalis fruit extract
The extra cellular synthesis of highly stable Ag and Au nanoparticles has also been achieved using Emblica officinalis fruit extract.
Bacteria and fungi
Biosynthesis of silver and gold nanoparticles using bacteria, and fungi,has been reported, and the time required for completion of the reaction ranged from 24 to 120 h. Intracellular synthesis, prolonged synthesis, multiple purification steps and the maintenance of cell cultures are the drawbacks of microbial procedures.
These methods are attractive because biocompatible gold nanoparticles can be rapidly produced and stabilized under safe conditions.