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Current Commentary Sustainable Nanotechnology

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 journals 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.


 
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 ?-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 organi

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