Natural Nanoparticles: A Particular Matter Inspired by Nature

Abstract

During the last couple of decades, the rapidly advancing field of nanotechnology has produced a wide palette of nanomaterials, most of which are considered as "synthetic" and, among the wider public, are often met with a certain suspicion. Despite the technological sophistication behind many of these materials, "nano" does not always equate with "artificial". Indeed, nature itself is an excellent nanotechnologist. It provides us with a range of fine particles, from inorganic ash, soot, sulfur and mineral particles found in the air or in wells, to sulfur and selenium nanoparticles produced by many bacteria and yeasts. These nanomaterials are entirely natural, and, not surprisingly, there is a growing interest in the development of natural nanoproducts, for instance in the emerging fields of phyto- and phyco-nanotechnology. This review will highlight some of the most recent-and sometimes unexpected-advances in this exciting and diverse field of research and development. Naturally occurring nanomaterials, artificially produced nanomaterials of natural products as well as naturally occurring or produced nanomaterials of natural products all show their own, particular chemical and physical properties, biological activities and promise for applications, especially in the fields of medicine, nutrition, cosmetics and agriculture. In the future, such natural nanoparticles will not only stimulate research and add a greener outlook to a traditionally high-tech field, they will also provide solutions-pardon-suspensions for a range of problems. Here, we may anticipate specific biogenic factories, valuable new materials based on waste, the effective removal of contaminants as part of nano-bioremediation, and the conversion of poorly soluble substances and materials to biologically available forms for practical uses.

Keywords: bioreduction; homogenization; microbes; nanoparticles; redox; selenium; silver; sulfur.

Figure 1

Nature itself is a skilled nanotechnologist. Microscopic and nanoscopic particles are formed, for instance, by combustion and are found: (a) near open fire; (b) as result of volcanic activity; (c) in form of precipitates; and (d) as bioreductively formed deposits of elements in certain bacteria. Photos provided by Marc Schäfer and Muhmmad Jawad Nasim.

Figure 2

Examples of natural and biological materials which contain nanoscopic particles. (a) Naturally occurring nanoparticles of inorganic, elemental sulfur, for instance, are found at mineral wells rich in hydrogen sulfide, such as the Elisenbrunnen in Aachen. (b) In contrast, mechanically produced nanomaterials of natural products have been evaluated for medical and agricultural applications. (c) Eventually, there are also naturally produced nanomaterials of natural, biological products, such as nanoscopic particles of elemental selenium coated with microbial proteins which are formed by bioreductive or oxidative metabolism in bacteria and fungi.

Figure 3

(a) The mineral wells in and near the town of Aachen in Germany are rich in sulfur, primarily in form of hydrogen sulfide (H₂S). Solid deposits of inorganic matter can therefore be found, for instance, at the Marktbrunnen in Burtscheid (image kindly provided by Roman Leontiev). (b) A microscopic investigation at 10,000-fold magnification reveals numerous microscopic and sub-microscopic particles and irregular agglomerates in this kind of water which (c) according to Energy Dispersion X-ray spectroscopy (EDX) consist of primarily of calcium salts and elemental sulfur [42].

Figure 4

Schematic illustration of the biogenic factory which is able to turn biological substances, extracts, plants, algae and even waste biomass into amazing new products and nanomaterials. A particular interest resides on the added value resulting from the use and “up-cycling” of by-products and waste, such as de-oiled herbs, spent grains and coffee grounds, as these materials initially are not only food-grade but otherwise would go to waste and hence impact negatively on the environment. Photo taken at Hassel (Saar) and kindly provided by Elizabeth Jacob.

Figure 5

The emerging field of phyto-nanotechnology employs isolated biological components and substances to form, modify or coat nanoparticles. These particles often exhibit interesting properties, such as pronounced biological activity, and may therefore be employed in medicine or agriculture. Phyto-nanotechnology also offers new and innovative uses for plant materials and biomass, which otherwise may have been wasted. Here, the field of phyco-nanotechnology, which is centered around algae, for many biological, manufacturing, ecological and economical reasons today represents a particularly interesting area of research and development.

Figure 6

Properties of nanoparticles: (a) increased rate of dissolution; (b) enhanced saturation solubility; (c) decreased diffusional distance; (d) higher concentration gradient; and (e) improved adhesiveness.

Figure 7

A schematic overview of theutilization of nanosizing techniques for turning waste into value.

Figure 8

Natural nanotechnology with its various inorganic and biological aspects provides a wide range of opportunities and applications, not only in medicine and cosmetics, but also in less obvious areas such as agriculture and waste removal.