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Posted: Jun 21, 2007

Truly green nanotechnology - growing nanomaterials in plants

(Nanowerk Spotlight) A lot of buzz has been created by the term "green nanotechnology". In a broad sense, this term includes a wide range of possible applications, from nanotechnology-enabled, environmentally friendly manufacturing processes that reduce waste products (ultimately leading to atomically precise molecular manufacturing with zero waste); the use of nanomaterials as catalysts for greater efficiency in current manufacturing processes by minimizing or eliminating the use of toxic materials (green chemistry principles); the use of nanomaterials and nanodevices to reduce pollution (e.g. water and air filters); and the use of nanomaterials for more efficient alternative energy production (e.g. solar and fuel cells). Unfortunately, there is a flip side to these benefits. As scientists experiment with the development of new chemical or physical methods to produce nanomaterials, the concern for a negative impact on the environment is also heightened: some of the chemical procedures involved in the synthesis of nanomaterials use toxic solvents, could potentially generate hazardous byproducts, and often involve high energy consumption (not to mention the unsolved issue of the potential toxicity of certain nanomaterials). This is leading to a growing awareness of the need to develop clean, nontoxic and environmentally friendly procedures for synthesis and assembly of nanoparticles. Scientists are now exploring the use of biological organisms to literally grow nanomaterials.
In nature, numerous inorganic materials are synthesized by living organisms. These bioinorganic materials can be extremely complex both in structure and function, and also exhibit exquisite hierarchical ordering from the nanometer to macroscopic length scales which has not even remotely been achieved in laboratory-based syntheses. Inorganic materials in the form of hard tissues are an integral part of most multicellular biological systems. Hard tissues (e.g. bone or nacre) are generally biocomposites containing structural biomacromolecules and some 60 different kinds of minerals that perform a variety of vital structural, mechanical, and physiological functions (quoted from "Microbial Nanoparticle Production").
The possibility of using such microorganisms and plants in the deliberate synthesis of nanomaterials is a recent phenomenon.
The first report of live plants synthesizing nanoparticles appeared in 2002 when it was shown that gold nanoparticles, ranging in size from 2 to 20 nm, could form inside alfalfa seedlings ("Formation and Growth of Au Nanoparticles inside Live Alfalfa Plants"). Subsequently it was shown that alfalfa also could form silver nanoparticles when exposed to a silver rich solid medium ("Alfalfa Sprouts: A Natural Source for the Synthesis of Silver Nanoparticles").
There are also cases of synthesizing metallic nanoparticles from dead biomass. A recent example is the fabrication of silver nanoparticles ranging from 55 to 80 nm in size, and triangular or spherical shaped gold nanoparticles, from the sundried biomass of Cinnamomum camphora leaf (commonly know as camphor tree) with aqueous silver or gold precursors at ambient temperature ("Biosynthesis of silver and gold nanoparticles by novel sundried Cinnamomum camphora leaf").
What all these reports show is that a simple synthesis of nanoparticles not only from microbes but also from dead or live plants is feasible. However, this is a largely unexplored area of research.
Scientists are already working on finding out what exactly the biochemical processes are that lead to nanoparticle formation in plants; if and how nanoparticle shape modulation and size control can be managed; what range of chemical composition of nanoparticles can be biosynthesized (so far it is mostly confined to metals, but the synthesis of oxides, nitrides and carbides etc. is also of great interest); and whether plant extracts can be used in nanobiosynthesis.
The big question of course is whether these methods could be used for large scale fabrication of nanomaterials. At this point there just is not enough knowledge to answer the question if biosynthesis techniques will ever be able to compete with existing physical and chemical synthesis processes. But once scientists understand the processes involved in biosynthesis, and with some genetic engineering, who knows, maybe one day we will see nanomaterial production in microbe farms and truly green nanotechnology vegetable factories.
By . Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology (RSC Nanoscience & Nanotechnology) and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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