Current Commentary: Sustainable Nanotechnology.

The following will be published in the December 2014 issue of the journal Science Progress http://www.sciencereviews2000.co.uk/view/journal/science-progress, of which I am an editor - so, this is a (very early) preview!

We welcome proposals and manuscripts from potential authors on most aspects of science and technology, guided by the following description:
"The journal's objective is to provide reviews of a range of current topics, which are both in-depth in their content, and of general appeal, presenting the reader with an overview of contemporary science and technology, and its impacts on humanity." The style of the following article is intended to illustrate this.

Keywords.

sustainable nanotechnology, silver nanoparticles, cellulose, nanocellulose, gold nanoparticles, zeolite, toxicology, C. elegans,

 
1. Can a definition for “sustainable nanotechnology” be agreed upon?

The U.S. National Nanotechnology Initiative1 defines nanotechnology as, “the manipulation of matter with at least one dimension in the range 1—100 nanometres (nm),” where the tendency is for quantum mechanical effects to become increasingly important toward the smaller end of the range. It is critical that the particular materials, and devices made from them, should possess properties that are different from the bulk (micrometric or larger) materials, as a consequence of their small size, which may include enhanced mechanical strength, chemical reactivity, electrical conductivity, magnetism and optical effects. The term sustainable has been used overly and often incorrectly, essentially to mean things that are environmentally benign, but including a degree of “greenwash” in some cases. [Greenwash is a compound word based on "whitewash", and refers to a deceptive form of promotion (spin) that portrays the products, aims or policies of an organisation in an environmentally benign (green) light]. In ecology, sustainable systems are self-sustaining, or self-regenerating (regenerative)2, as occur in nature. We may note that the phrase “sustainable agriculture” has been described2 as an oxymoron, since agriculture is by its very nature unsustainable, relying as it does on inputs of all kinds, e.g. petroleum, natural gas and water, and that it renders the soil vulnerable to erosion with the progressive and global loss of productive land3.

It is even more vexing to find a precise definition of sustainable nanotechnology, since the individual sustainability aspects of all components must be considered. ACS Sustainable Chemistry & Engineering has recently presented its second special issue concerning Sustainable Nanotechnology. Those papers featured in this issue were presented at the 2nd Sustainable Nanotechnology Organization (SNO) Conference held in November 2013. The conference was attended by over 200 delegates working in academia, industry, and government agencies. The editorial of this special issue offers the following definition4: “Sustainable nanotechnology is the research and development of nanomaterials that have economic and societal benefits with little or no negative environmental impacts. The successful application of nanotechnology is contingent upon scientific excellence that provides economic, ethical, and societal benefits.” While from the associate director of the Virginia Tech’s Center for Sustainable Nanotechnology we have5: “Sustainable nanotechnology is the development of science and technology within the 1 – 100 nanometer scale, with considerations to the long-term economic viability and a sensible use of natural resources, while minimizing negative effects to human health and the environment. Potential negative effects may be caused by engineered nanomaterials or by anthropogenic changes in the prevalence of naturally occurring nanomaterials.”

As she further stresses: “When addressing “sustainable nanotechnology”, we must address economic needs, human safety, and environmental conservation. Sustainable nanotechnology demands extra creativity and innovation in an already innovative field. How can we make materials safer to people? How can we make manufacturing less energy intensive? How can we minimize waste? These are a few good driving questions towards sustainable nanotechnology. Actually, these should be driving questions in whatever work you do, whether it is related to nanotechnology or not.” Such definitions allow us to distil the essence that the nanomaterials must have positive economic and societal benefits in their use, while effectively being “harmless”; however, issues over the manufacture of the nanomaterials themselves must also pertain, for example the likely availability of their component elements in the future, and hence how sustainable their long-term supply might prove be6. It should be noted that we are already somewhat removed from the ecological definition of sustainability, and regeneration, since the nanomaterials are required as an external and continual input to whatever systems are being “improved” by their presence. Some mitigation both of this demand, and of consequent environmental impacts, might be achieved through nano-recycling.


(2) Properties of nanoparticles.

At the nanoscale, the fraction of the total atoms in the particle that are at the surface becomes substantial, in contrast to bulk materials, and it is this very high surface area that is responsible for many of the unique properties of nanoparticles. [We may note that 1 kg of particles of 1 mm3 diameter has the same surface area as 1 mg of particles of 1 nm3 diameter]. Additionally, due to the effect of quantum confinement of the electrons, unexpected optical effects may occur: thus, nanoparticles of gold and silicon (respectively yellow and grey in their bulk forms) are reddish in colour (Fig. 1). As a further phenomenon, it was found that a sample of 2.5 nm diameter gold nanoparticles melted at ~300 °C, which is far lower than the normal 1064 °C melting point of gold7. By varying their size, shape, and chemical composition, it is possible to tune the absorption of solar radiation by nanoparticles, which in any case tend to absorb radiation more strongly than do the corresponding bulk materials, with implications for both solar PV and solar thermal applications. Nanoparticles can be created using various different methods, some of which are now outlined8.

Attrition is carried out using a mechanical device such as a ball-mill, to breakdown macro- or micro-sized materials into smaller particles, from which the nanoparticle fraction is isolated. Pyrolysis involves burning a liquid or gaseous precursor that has been forced through an orifice at high pressure, and the oxide nanoparticles are recovered from the solid product, usually by air-classification. [Air classification is a separation technique in which the material stream to be sorted is injected into a chamber which contains a column of rising air. Within the chamber, the effect of air-drag supplies an upward force on the particles which counteracts the force of gravity and lifts the material to be sorted up into the air. Since the effect of air-drag varies according to the size and shape of the particles, the latter are sorted vertically in the moving air column, and are hence separated from one another].

In order to avoid the formation of aggregates and agglomerates, ultrasonic nozzle spray pyrolysis (USP) is employed, which results in single primary particles. Thermal plasmas, which operate at temperatures in the region of 10,000 K, may be used to vapourise small micrometer-size particles from a solid, leading to the formation of nanoparticles by cooling beyond the exit point of the plasma region. RF-induction plasma torches have been used in the production of ceramic nanoparticles such as oxides, carbides, and nitrides of Ti and Si, among other materials. Nanoparticles may also be prepared using methods of radiation chemistry. In this approach, electrons, generated by radiolysis of water molecules in aqueous solutions, reduce metal cations to the corresponding metal atoms which coalesce to form nanoparticles. A surfactant is present, which surrounds the particles as they are formed and regulates their growth, and in high enough concentrations, the surfactant molecules remain in association with the nanoparticles, so preventing them from dissociating or forming clusters with other particles. The shape and size of the particles can be adjusted according to the concentrations of the materials and the dose of gamma-rays, which may be up to 10,000 Gy (1 MRad)9.

The radiolytic formation of free radicals has been studied previously using ESR and related techniques10, including in zeolite nanomaterials11. Sol-gel methods have also been found useful in the preparation of nanoparticles. The sol-gel process is used for the preparation of, typically, metal oxide materials in the fields of materials science and ceramic engineering, starting from an appropriate solution (sol) of chemicals, which acts as the precursor to an integrated network (or gel) of either discrete particles or network polymers. The sol can be deposited onto a substrate to form a film, or it may be cast into a suitable container with the desired shape to produce monolithic ceramics, glasses, fibres, membranes, and aerogels, or for the synthesis of powders (microspheres or nanospheres). The method permits the fine control of the chemical composition of a product, is inexpensive, and is carried out at low temperatures.



(3) Morphology and characterisation of nanoparticles.

The terms nanotubes, nanospheres, nanoreefs, nanoboxes, nanostars and even12 nano-cabbage and (nano) sea-anemone have appeared in the literature, in reference to the apparent similarity between the various nanoparticle morphologies and the shapes of objects that are more commonly encountered by humans (Fig. 2). It is sometimes the presence of templating or directing agents, such as micellar emulsions or anodized alumina pores, that causes the various shapes to form spontaneously; alternatively, they may arise from the innate crystallographic growth patterns of the materials themselves13. As a consequence of their structural isotropy, amorphous particles tend to form spheres. Once formed, it is necessary to characterize the nanoparticles, and a range of techniques are used for this purpose, most commonly: electron microscopy (TEM, SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), x-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), ultraviolet-visible spectroscopy, Rutherford backscattering spectrometry (RBS), dual polarisation interferometry, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR). A more recently developed method has been introduced for the characterisation of nanoparticles, which is termed Tunable Resistive Pulse Sensing (TRPS) which enables the size, concentration and surface charge to be determined simultaneously for a wide variety of nanoparticles14.



(4) Recent research in sustainable nanotechnology.

That this field is one of rapid growth is emphasised by the fact that the ACS Sustainable Chemistry & Engineering journal has recently published its second special issue concerning Sustainable Nanotechnology, from which the following studies are now highlighted. To provide a “completely green and environmentally friendly” catalyst for environmental decontamination, a γ-Fe2O3-pillared montmorrilonite nanocomposite was synthesized, which was characterized using scanning electron microscopy (SEM) methods, transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). An 85% degradation of dichlorophenol (DCP) was obtained in 2.5 hours, in the presence of peroxymonosulfate, with somewhat reduced levels after 3.5 hours, with H2O2 (50%) and peracetic acid (70%) of DCP [15]. Reusable, magnetically separable, magnetite-supported copper (nanocat-Fe-CuO) 20-30 nm nanoparticles were prepared as catalysts for the synthesis of pyrazole derivatives, 4-methoxyaniline and Ullman-type condensation reactions, under mild conditions. The particles were recovered and reused six times without any loss in catalytic activity [16]. Intercellularly synthesised gold nanoparticles were characterised using surface-enhanced Raman spectroscopy (SERS).

Both intracellular and extracellular gold nanoparticles, biosynthesized by the green algae Pseudokirchneriella subcapitata were imaged sing SERS to identify surface-associated biomolecules and aid in the determination of the mechanism for the nanoparticle biosynthesis [17]. Despite the widespread use of ZnO nanoparticles (NP) in various applications, they are actually among the more toxic NPs known. Thus there is the incentive to produce safer ZnO NPs, while preserving the essential optical, electronic, and structural features of these materials. Thus, two ZnO samples of equal dimension (9.26 ± 0.11 nm) were synthesized from the same zinc acetate precursor using a forced hydrolysis process, but with different solvents, which permitted the modification of their surface structures. While the lattice parameters, optical properties, and band gap (3.44 eV) of the two ZnO NP samples were preserved, FTIR spectroscopy showed there were significant differences between them in their surface structures and surface-bound functional groups. Accordingly, the zeta potential, hydrodynamic size, and photocatalytic rate differed considerably. It was found that the ZnO NP sample with the higher zeta potential and greater catalytic activity was more cytotoxic to cancer cells by a factor of 1.5 [18]. In another study, silver and gold NPs were produced using antioxidants from extracts of natural fruits and spices blackberry, blueberry, pomegranate and turmeric). The NPs were characterized using XRD, TEM, high-resolution TEM (HR-TEM), particle size analysis, UV–vis spectroscopy, and thermogravimetric analysis [19].

Despite the fact that iron-based nanoparticles are known to be effective for the degradation of organic dyes, organochlorine compounds, and arsenic contaminants, their characterisation has been ambiguous. In a recent study, iron-based NPs were produced by reduction with green tea extract and were fully characterized by TEM, XRD, and UV–vis spectrometry. According to the XRD and TEM results, the iron formed amorphous nanosized particles, whose size depended on the reaction time. It was shown that iron(II,III) NPs prepared as green tea extract (GT–Fe nanoparticles) had negative ecotoxicological impacts on important aquatic organisms such as cyanobacterium (Synechococcus nidulans), alga (Pseudokirchneriella subcapitata), and invertebrate organisms (Daphnia magna) too20. Cacumen Platycladi (CP) extract was employed in a green bioreductive strategy to form bimetallic Au–Pd/TiO2 catalysts intended for the solvent-free oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) using an ambient pressure of O2. It was found that the uncalcined Au–Pd/TiO2 catalyst prepared at 90 °C from Au–Pd NPs (Au/Pd molar ratio: 2:1) could convert BzOH to BzH in a yield of 74.2%, and with a 95.8% selectivity, and that the catalyst preserved its activity over 7 recycling events21.

TiO2 is an efficient photocatalyst in water treatment applications, and for other purposes22. In order to adjust the band gap and electronic structure of the catalyst, a method has been presented by which an N-doped TiO2 may be obtained, which is hybridized with graphene sheets. The reaction occurs at a relatively low temperature (180 °C), in which NH3 solution is both the N source and the reaction medium. After a reaction time of 14 hours, the product contained 2.4 atom % of N. A dramatic suppression of the photoluminescence (PL) intensity was observed in the N-doped TiO2/graphene composites, as compared with undoped TiO2, demonstrating that the electron–hole pair recombination rate was dramatically reduced in the doped composite. It was further demonstrated that the N-doped TiO2/graphene photocatalyst degraded methylene blue twice as fast as a commercial Degussa P25 catalyst23. Also with water decontamination in mind, this time from Pb2+ cations, rather than organic pollutants, magnetic attapulgite/fly ash/poly(acrylic acid) (ATP/FA/PAA) ternary nanocomposite hydrogels were prepared by inverse suspension polymerization, after the inorganic materials were modified in situ. It was found that the hydrogels exhibited a high adsorption selectivity toward Pb2+ cations, which could be completely desorbed by treatment with aqueous HCl. The hydrogels are cheap to make, have a high mechanical stability and retain their magnetic properties, which suggests that they may prove to be very useful materials for the treatment of Pb2+-contaminated water24.

It was shown that when the surfaces of membrane filters were modified using graphene-based materials, their activity toward Escherichia coli and Bacillus subtilis was improved significantly. It was concluded that part of this improved antimicrobial property was due to the formation of reactive oxygen species (ROS) by the nanomaterials, and that such surface-modification may provide enhanced filters for the treatment of water and wastewaters25. The contamination of waterways by phosphate, mostly from agricultural runoff waters, is an environmental problem since it leads to eutrophication, and the formation of algal blooms and dead zones. In this regard, it has been shown that nanoscale zero-valent iron (NZVI) particles offer promise for the recovery of phosphate from aqueous media. To determine the bioavailability of the phosphate, as sorbed onto the NZVI particles, spinach (Spinacia oleracea) and algae (Selenastrum capricornutum) grown in hydroponic solutions, were used. It was found that the concentration of algae increased by 6.7 times when the only source of phosphate was spent NZVI, in comparison with algae grown in standard all-nutrient media, which contains phosphate. An elevated iron content was measured in the roots, leaves, and stems of the spinach that had been treated with spent NZVI, respectively, as compared to the controls, which might indicate a role for iron in enhancing the overall plant growth26.

Cellulose has been converted to n-hexane using an Ir-ReOx/SiO2 (Re/Ir = 2) catalyst combined with HZSM-5 as a co-catalyst in a biphasic reaction system (n-dodecane + H2O), from which the yield of n-hexane reached 83% from ball-milled cellulose and 78% from microcrystalline cellulose. The yield remained high (71%) even when a high concentration of water was present (cellulose to water: 1:1). By recalcination of the catalyst, the yields were maintained over three cycles of the catalyst. The mechanism involves the hydrolysis of cellulose to glucose via water-soluble oligosaccharides, hydrogenation of glucose to sorbitol, and successive hydrogenolysis of sorbitol to n-hexane. The Ir-ReOx/SiO2 catalyst is active for one hydrogenation and hydrogenolysis step, while HZSM-5 enhanced both the hydrolysis of cellulose and the C–O bond hydrogenolysis activity of the Ir-ReOx/SiO2 catalyst27. 5-hydroxymethylfurfural (5-HMF) is a useful renewable biofuel and biochemical, which, it has been shown, may be produced in a “one pot” procedure by the hydrolysis of microcrystalline cellulose over a zeolite described as “Bimodal-HZ-5”, which was obtained by post-synthesis modification of H-ZSM-5 with desilication. The conversion of cellulose was 67%, with a 5-HMF yield of 46%. The catalyst was found to be reusable for four cycles, with no reduction of its activity28.

Lignin may be recovered from the Kraft process, as a component of wood-pulp. The name “Kraft” comes from the German word for “strong” in regard to the superior paper quality that was obtained from it. In a recent study, a water-soluble lignin-based polyoxyethylene ether (KL-PEG) was synthesized from kraft lignin (KL) and poly(ethylene glycol) (PEG). PEGs with various polyoxyethylene ether lengths were functionalized preferentially with epichlorohydrin using BF3-Et2O as the Lewis acid catalyst and were then grafted onto KL by blocking the phenolic hydroxyl groups. It was demonstrated that the PEG content in the KL-PEG copolymer could be controlled by varying the mass ratio of PEG to KL, and the molecular weight was controlled according to the molar ratio of epichlorohydrin to PEG. It was proposed that this novel amphiphilic KL-PEG copolymer, with its renewable lignin backbone and branched PEG groups might be used as a dispersant for a variety of materials, including agricultural suspension concentrates29.

The lithium ion battery has a high profile as an energy storage device for the potential sustainable transport of power. By means of pulse electrodeposition, dense Sn films were deposited on a Cu substrate, followed by various post-treatments at 200 °C, of which electroplating a Cu-film-coating and heat treatment in different atmospheres are representative. When the films were assembled in an Sn-based anode with a Cu coating, post-heating in argon for 12 hours, a surface was formed with Cu6Sn5/Sn as the primary phase. The latter exhibited the greatest first cycle charge/discharge capacity and largest irreversible capacity loss (IRC); in contrast, the least IRC was found for a Sn-based anode sintered in air for 48 h was surface modified by SnO. The first cycle capacity and IRC of the anode were both enhanced by defects in the Cu6Sn5/Sn phase, while the decrease in the first cycle IRC of the anode is aided by the presence of SnO30. Well-defined carbon black/polypyrrole (CB/PPy) composite hollow nanospheres have been evaluated for their potential application as electrode materials for supercapacitors. These materials were prepared via in situ chemical oxidative interfacial polymerization of pyrrole, in the presence of sulphonic acid-modified carbon black (CB-SO3H) NPs, using an inert solvent (toluene) as the soft template, in which sodium dodecyl benzenesulphonate (SDBS) was used as both the surfactant for the emulsion and the dopant for both the produced polypyrrole (PPy) and the CB-SO3H NPs. A maximum electrical conductivity of 0.045 S/cm and a specific capacitance of 29 F/g was achieved, and even after 1000 cycles, the latter parameter was reduced only by 5%, indicating an excellent cycling performance for these nanospheres31.

A series of dye-sensitized solar cells was prepared in which different 9,10-diaryl-substituted anthracene groups acted as a π-bridge with a 2,6-linkage mode. Where present, tert-butylphenyl and hexyloxyphenyl groups in the 9 and 10 positions of the anthracene unit occupied practically perpendicular orientations in respect to the conjugated ring-plane, so helping to discourage possible π–π stacking and reducing the extent of charge recombination. When an optimum arrangement of substitutents was present at the anthracene ring, a Jsc (short circuit current) of 13.42 mA cm–2, Voc (open circuit voltage) of 722 mV, and FF (fill factor) of 0.66, corresponding to an overall conversion efficiency of 6.42%, were obtained32.

A bulk hetero-junction organic polymer solar cell based on poly(3-hexylthiophene) (P3HT) and PC70BM, containing a near-infrared absorbing dye, bis[4-(2,6-di-tert-butyl)vinyl-pyrylium] squaraine (TBU-SQ), was presented, in which the dye raised the power conversion efficiency (PCE) to 4.55% from 3.47% for the binary blend alone. This improvement was attributed to the light-harvesting efficiency in the near-infrared region of the solar spectrum and the increased exciton dissociation into free charge carriers in the ternary blended film. After the film had been thermally annealed, the PCE was found to increase to 5.15%, which corresponded with a red shift and broadening of the film’s absorption profile as a result of the thermal teratement33.

Carbon nanotubes (CNTs) are of interest as potential materials with which to enhance the performance of the anodes and cathodes in lithium-ion batteries, although there are some concerns that CNTs may be toxic34. A material flow analysis (MFA) with a stock dynamics and logistic model has been employed to predict the timescale for the transition from conventional Li-ion batteries in portable computers to CNT Li-ion batteries, and the according waste generation of CNTs in obsolete laptop batteries. State-specific recycling rates for electronic waste were projected, from which to estimate the quantities of CNTs in laptop batteries that will arise to be recycled, incinerated, or placed in landfill. It is concluded that as the various markets for CNT-enabled electronics begin to expand, collection and recycling facilities in the U.S., may need to inaugurate new processes or controls to reduce the potential for the emissions of and exposures to CNTs34. A survey has been made of both conventional and emerging techniques that are available for characterizing engineered nanoparticles in complex matrices35, which included microscopy (TEM, SEM, HRTEM, DLS, SNOM), chromatography (HDC, FFF), mass spectroscopy (ICP-MS, SEC-ICP/MS, MALDI, FFF-ICP-MS), sp-ICP-MS, and electrochemical techniques. The design of a portable nanoparticle analyzer based on tangential flow filtration and electrochemical detection (EC-TFF) was presented in the form of a case study, and its application for the characterization of engineered nanosilver in actual environmental samples. A 98.5% removal efficiency was obtained for Ag-NPs with varying particle sizes35.

Uncertainties are confronted by nanotechnology companies on a number of levels which concern regulation of the manufacture and use of NPs, their demand, and advances in technology. It is accordingly difficult to make predictions as to how the capacity of engineered nanomaterials or nanoenabled products is likely to expand in the future, and the influence of this on associated revenue income. However, Monte Carlo simulations may prove useful in arriving at optimal decisions regarding sustainable capacity expansion, and to aid decision makers in setting sustainable manufacturing goals by reducing any unnecessary capacity expansion and the degree of occupational exposure to NPs36.



(5) Health and environmental concerns.

In view of their high surface to volume ratio, with an associated high reactivity, NPs may present dangers both to the human organism and to the environment34. While it is known that NPs can pass through cell membranes in organisms, how they interact precisely with biological systems is relatively unknown and potentially complex: as in a recent investigation which found varying degrees of cytotoxity for ZnO NPs toward human immune cells37. It is likely that safety data that were obtained during clinical studies of prior medicines that did not contain NPs will not be entirely adequate from which to assess more recent nano-formulations, which may require their own specific safety-evaluation. Nanomaterials are widely used in various sunscreen and cosmetic formulations, but any likely risks to human health are mostly unknown at present38. Combustion processes39 in general are of concern, in that they may generate respirable NPs, and as of 2013 the Environmental Protection Agency has written the following in regard to its investigation of the safety of the following categories of NPs, in anticipation of a market based upon them that is expected to be worth $1 trillion by 2015(40):

· Carbon Nanotubes: carbon materials have a wide range of uses, ranging from composites for use in vehicles and sports equipment to integrated circuits for electronic components. The interactions between nanomaterials such as carbon nanotubes and natural organic matter strongly influence both their aggregation and deposition, which strongly affects their transport, transformation, and exposure in aquatic environments. In past research, carbon nanotubes exhibited some toxicological impacts that will be evaluated in various environmental settings in current EPA chemical safety research. EPA research will provide data, models, test methods, and best practices to discover the acute health effects of carbon nanotubes and identify methods to predict them.

· Cerium oxide: nanoscale cerium oxide is used in electronics, biomedical supplies, energy, and fuel additives. Many applications of engineered cerium oxide nanoparticles naturally disperse themselves into the environment, which increases the risk of exposure. There is ongoing exposure to new diesel emissions using fuel additives containing CeO2 nanoparticles, and the environmental and public health impacts of this new technology are unknown. EPA’s chemical safety research is assessing the environmental, ecological, and health implications of nanotechnology-enabled diesel fuel additives.

· Titanium dioxide: nano titanium dioxide is currently used in many products. Depending on the type of particle, it may be found in sunscreens, cosmetics, and paints and coatings. It is also being investigated for use in removing contaminants from drinking water.

· Nano Silver: nano silver is being incorporated into textiles and other materials to eliminate bacteria and odour from clothing, food packaging, and other items where antimicrobial properties are desirable. In collaboration with the U.S. Consumer Product Safety Commission, EPA is studying certain products to see whether they transfer nano-size silver particles in real-world scenarios. EPA is researching this topic to better understand how much nano-silver children come in contact with in their environments.

· Iron: while nano-scale iron is being investigated for many uses, including “smart fluids” for uses such as optics polishing and as a better-absorbed iron nutrient supplement, one of its more prominent current uses is to remove contamination from groundwater. This use, supported by EPA research, is being piloted at a number of sites across the U.S.40.

There is a very readable account of how nanomaterials get into the environment available41, and another which considers some of the prospects for nano-recycling42. Clearly, NPs can have an undesirable impact on the environment, since one study demonstrated that earthworms exposed to gold nanoparticles in soil produced 90% fewer offspring42. Since earthworms have been described as the “tractors” of the soil, in their contribution to the soil food web2, any die-back in the population of soil-organisms could have serious consequences for the world food supply3. Similarly, it has been shown that Ag-NPs dramatically reduce the reproduction potential of the soil nematode Caenorhabditis elegans43 (Fig. 3). It is thought that oxidative stress may play an important role in the toxicity of Ag-NPs toward C.elegans. It is reported that Ag-NPs are to be used in Nigeria to treat patients who are infected with the ebola virus, although the judiciousness of this approach is under question44.


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Captions to figures.

Fig. 1. Silicon nanopowder. http://en.wikipedia.org/wiki/Nanoparticle#mediaviewer/File:Nano_Si_640x480.jpg

Fig. 2. Nanostars of vanadium(IV) oxide. http://en.wikipedia.org/wiki/Nanoparticle#mediaviewer/File:Nanostars-it1302.jpg

Fig. 3. The soil nematode Caenorhabditis elegans http://pl.wikipedia.org/wiki/Caenorhabditis_elegans#mediaviewer/Plik:Enlarged_c_elegans.jpg