The term nano originated from the Greek nanos which means ‘dwarf’. It is one billionth of a meter. Therefore, whenever we think about nanoscience or nanotechnology, very small objects come to the mind. Indeed, this branch of science and technology deals with materials having at least one spatial dimension in the size range of 1 to 100 nm. 
The most common working definition of nanoscience is: ‘Nanoscience is the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale. Nanotechnologies are defined thus: ‘Nanotechnologies are the design, characterization, production, and application of structures, devices, and systems by controlling shape and size at the nanometer scale. 
Nanoparticles have unique catalytic, electronic, magnetic, chemical, photoelectrochemical, and optical properties and are important in technology and medicine. Au nanoparticles are highly stable, low reactogenic, and biocompatible. They generally lack specific toxicity, come in a variety of shapes, have unique optical and electronic properties, and can be used in optics, electronics, catalysis, and biomedicine (diagnostics, therapy of cancer and other diseases, and drug and gene delivery). nanoparticles, owing to their unique physical, chemical, and biological properties, are effective in chemical catalysis, optoelectronics, biomedicine, and other fields , nanoparticles have antiviral, antifungal, antibacterial, antitumor, anti-inflammatory, and antioxidant properties, which open extensive possibilities for the use of nanoparticles in biomedicine to treat infections and cancers, as well as to prepare medical devices, advanced-therapy medicinal products, and cosmetics, The physicochemical properties, biological activity, and degree of toxicity of nanoparticles depend on their size and shape and this dependence calls for novel procedures to prepare nanoparticles with required properties and characteristics.
Chemical, physical, and biological methods have been developed to synthesize nanoparticles but chemical and physical methods  using hazardous compounds such as hydrazine, sodium borohydride, and dimethylformamide (DMF) as reducing agents  are involved in the production of toxic byproducts which are hazardous moreover the methods are very expensive. To synthesis stable metal nanoparticles with controlled size and shape, there has been searching for inexpensive, safe, and reliable and “green” approach. The novel methods so-called green/biosynthesis have been recently developed by a variety of plant extract 
Green synthesis of metal nanoparticles using plant extracts can minimize their toxicity, Furthermore, synthesis of metal nanoparticles using plant extracts is very cost effective, and therefore can be used as an economical and valuable alternative to the large-scale production of metal nanoparticles. In addition, the full utilization of plant waste is a sustainable path for development. 
Applications of nanotechnology
- Nanoparticle involved in drug delivery
- Nanofood is a term used to describe foods that use nanotechnology techniques
- Gene delivery it is a technique that plays a vital role that can efficiently introduce a gene of interest in order to express its encoded protein in a suitable host or host cell
- There are a variety of nanoparticle systems currently under investigation to be applied in biomedical with the emphasis on cancer therapeutics.
- In recent years nanoparticles are involved with new applications in areas like information & communication technology, power engineering, industrial engineering, environmental engineering, chemical industry, medicine, in pharmaceuticals and cosmetics.
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