«By Yusuf Nur A thesis submitted to The University of Birmingham for the Degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental ...»
Natural NPs are ubiquitous and they are distributed throughout the atmosphere, oceans, soil systems, terrestrial water systems (groundwater and surface water), and in and/or on most living organisms both at the micro and macro levels and have been present in the environment forever (Theng and Yuan, 2008, Hochella et al., 2008). They have been formed mainly by the effect of naturally occurring physical, chemical and biological processes such as hydrolysis, erosion, weathering, volcanic eruption, sea spray, photochemical reactions, growth nuclei in super-saturated fluids, plants roots on rocks, minerals and microrganisms.
Forest fires, as volcanic eruptions, for instance, can spread ash and smoke over thousands of miles and lead to an increase in particulate matters, including NPs, exceeding ambient air quality standards (Sapkota et al., 2005).
In many environments, biogenetic nanoparticles are formed directly by microorganism to fulfil metabolic requirements (Suzuki et al., 2002, Schüler and Frankel, 1999) or as an indirect result of microbial activity (Glasauer et al., 2002, Hansel et al., 2004, Banfield et al., 2000). Dominant phases of the natural NPs include: Iron oxides/hydroxides, aluminum oxides/hydroxides, clay minerals (hydrated aluminosilicates of K, Mg, Fe etc.) and silica.
Biological activities also assemble following the bottom up process- a wide range of carbon containing NPs including humic substances, building molecules, functional enzymes, coal, and produce nanoorganisms such as bacteria, viruses, cells and their organelles. Though we usually associate air pollution with human activities such as transportation, industry, and charcoal burning, natural events such as dust storms, volcanic eruptions and forest fires can produce such vast quantities of nanoparticulate matter in the atmosphere that profoundly affect air quality worldwide (Buzea et al., 2007). Therefore, the exposure of human and other
which is more based on the ever increasing production of the engineered nanoparticles which are produced to serve for special purpose and may manifest different properties than above mentioned natural NPs.
2.1.2 Accidental nanoparticles Human activities have been releasing different forms of nanoparticulate matter for millennia as by-products of some activities which were essential for their survival such as agriculture, construction, food cooking, mining and mineral processing. However, accidental NPs input to the environment has risen sharply and dramatically since the beginning of the Industrial Revolution in the mid 18th century and after due to manufacturing emissions, nuclear waste generation and the combustion of fossil fuels (Wiesner et al., 2009). Diesel and automobile exhausts are the primary source of atmospheric nano-and microparticles in urban areas (2002). Since their productions are spontaneous, uncontrolled accidental nanoparticles are more likely to be polydisperse /heterogeneous and have irregular shapes. They contain sulfide, sulphate, nitrate, ammonium, organic carbon, elemental carbon and trace metals (Sioutas et al., 2005).
2.1.3 Engineered nanoparticles The modern nanotechnology exploits the novel nanoscale properties of nanoparticles which has lead to the production of a vast amount of engineered NPs. Engineered NPs are deliberately manufactured by human activities to serve for special purposes and they are different from both incidental NPs that are produced as a side product of human activity, for example, from industrial processes or transport and from natural NPs, for example, humic
Engineered NPs can be divided into a number of classes and not as a single homogeneous group. Based on their core materials, these manufactured nanomaterials can be classified into organic and inorganic. Organic NPs can be further defined as fullerenes (C60 and C70 and derivatives) and carbon nanotubes (multi-walled or single walled CNTs), while inorganic NPs can be sub-divided into metal oxides (of iron, zinc, titanium, cerium etc), metals (mainly silver and gold) and quantum dots (such as cadmiumselenides) (Ju-Nam and Lead, 2008). A brief description of the sources, applications and properties of each of these subgroups will be given in the next sections.
22.214.171.124 Production of engineered nanoparticles Many different techniques have been developed and employed to generate metal nanoparticles, including gold nanoparticles. Those techniques for preparing nanoparticles have advanced rapidly over the recent decades and continue to evolve leading to more and improved control over the size and shape of the particles generated. Two fundamentally different approaches towards the controlled generation of nanostructures have evolved irrespective of the field or discipline (Shenhar and Rotello, 2003). The bottom up method (the chemical approach), where the atoms (produced from reduction of ions) are assembled to generate nanostructures, and the opposite approach, the top down method, also known as the physical method, where material is removed from the bulk material through grinding, milling, chemical methods or volatilisation of solid material followed by condensation of the vapour components, leaving only the desired nanostructures.
Both approaches can be implemented in either gas, liquid, supercritical fluids, solid states, or in vacuum. Most of the manufacturers are interested in the ability to control one or more of
approaches is the stabilisation of the particles to avoid aggregation and coalescence (Ju-Nam and Lead, 2008). Stabilization can occur in many different ways but primarily aggregation is prevented by electrostatic repulsion or steric hindrance. Two of the most important issues confronting nanocrystal synthesis are obtaining a purposeful control over the nanocrystal mean size and routinely producing narrow size distributions (Shields et al., 2010).
The synthesis of small, monodisperse nanoparticles is a major challenge in nanotechnology research. Similar particles experience increased driving forces to aggregate to diminish surface energy. So, protective coating or capping is necessary during synthesis to keep them in a finely dispersed state (Sardar et al., 2009). Both methods have inherent advantages. Top down assembly methods are currently superior for the possibility of interconnection and integration, as in electronic circuitry. Bottom-up assembly is very powerful in creating identical structures with atomic precision, such as the supramolecular functional entities in living organisms(2006). They can also be combined to achieve the material of specific physicochemical properties (Cigang and et al., 2006).
126.96.36.199 Examples of engineered nanoparticles Since the invention of the nanotechnology few decades ago, different types of nanosized materials with different application have been produced and commercialised to take the advantage of their novel properties. The following sections will provide examples of the engineered nanoparticles, their synthesis processes and their applications.
188.8.131.52.1 Fullerenes and Carbon nanotubes Fullerenes, also called as Buckminsterfullerene, are spherical cages composed of 60,
is another allotrope of carbon), fullerenes can conduct electricity due to free electrons on its surface and in this respect has similar properties to graphite. Although fullerenes can be found spontaneously in the nature as by-products of combustion reactions, it was first synthesized in 1985 (Kroto et al., 1985). Since then, many processes for the production of fullerenes of different sizes have been developed including arcing of graphite, combustion of hydrocarbons, thermal and non-thermal plasma pyrolysis of coals and hydrocarbons and thermal decomposition of hydrocarbons (Huczko and Byszewski, 1998).
Figure 2-3: Structure of Fullerene C60 molecule. Purple balls represent the places of carbon atoms(Buseck, 2002).
Reprinted with permission from copyright 2002 Elsevier.
Fullerenes have many applications, including lubrication, superconductors, semiconductors, photoconductors, optical limiters and atom encapsulation. Since fullerenes are empty structures with dimensions similar to several biologically active molecules, they can be filled with different substances and find medical applications. These include anti HIV- protease activity, photodynamic DNA cleavage, free radical scavenger, antimicrobial action and use of fullerenes as diagnostic agents (Mehta and Thakral, 2006, 2001).
have elongated shapes with 1- 2 nm in diameter. Using an arc-discharge evaporation method similar to that used for fullerene synthesis, they were first produced in 1991 (Iijima, 1991).
Normally, carbon nanotubes are made of one sheet of graphite folded to form cylindrical single walled carbon nanotubes (SWCNT) (Iijima and Ichihashi, 1993) although multi walled carbon nanotubes (MWCN) (Iijima, 1991) can be formed by folding more than one sheet of graphite.
Figure 2-4: Representation of SWCNT and MWCN at the top and their TEM images at the bottom(Donaldson et al., 2006).
Reprinted with permission from copyright 2006 Oxford University Press.
At present, the three main methods employed for CNT synthesis are arc-discharge, laser ablation, and chemical vapor deposition (CVD) (Trojanowicz, 2006). Carbon nanotubes are light, chemically stable, have high strength, high aspect ratio (long length compared to a small diameter) and remarkable optical properties (Hou et al., 2002, Tersoff and Ruoff, 1994) and, because of these useful properties, they become ideal material for many applications.
Nanotubes have attracted a considerable amount of interest in the past few decades due to their potential applications in large numbers of academic and industrial areas for diverse application possibilities (Cao and Rogers, 2009) ranging from nanoscale circuits to low voltage devices (Philip Wong, 2005), to light-emitting devices (Freitag et al., 2006), thermal
chemical/biological sensors (Kim et al., 2007b), ultra-strong fibers (Baughman et al., 2002), high-power electrochemical capacitors, (Niu et al., 1997) gas storage components (Gadd et al., 1997, Rakhi et al., 2008), magnetic data storage devices and drug delivery systems (Bianco and Prato, 2003). Waste products from the above mentioned various applications and their disposal to the environment will undoubtedly increase the presence of carbon nanotubes with unknown fate and behavior in the natural environment.
184.108.40.206.2 Metal oxides NPs Metal oxides NPs are synthesized in a variety of ways such as laser ablation, ion implantation, chemical vapour deposition (CVD), photolithography,thermal decomposition, sol-gel process or hydrothermal reaction method (wolf, 2004). Nature is able to synthesize a variety of metal oxide nanomaterials as well under ambient conditions; the magnetic navigation device found in magnetotactic bacteria ( MTB ) is one such example (Lang et al., 2007). As in the case of most NPs, metal oxide NPs are stabilized through surface modifications and the main compounds used for modifying them are phosphonates or silanes (Grancharov et al., 2005). Metal oxides form a class of special interest among inorganic nanoparticles and due to their novel optical, electrical and magnetic properties (Shtykova et al., 2007) they have many applications including catalysis, sensors, electronic materials, biomedical diagnostics and environmental remediation (Oskam, 2006, Hoffmann et al., 1995, Kamat, 1993). Among metal oxide NPs, the following are those which have attracted most of the applications and, as a result, their concentrations in the natural environment are expected to be increasing.
Iron oxide: the most common forms of iron oxide NPs are maghemite, γ-Fe2O3 and magnetite, Fe3O4 and, because of their supermagnetic properties, they offer a high potential
resonance imaging (MRI) and hyperthermia (Arbab et al., 2003, Urs and et al., 2011, Widder et al., 1978, Perez, 2007). Various chemical routes have thus been proposed to synthesize ultra fine nanoparticles of Fe2O3, including the hydrothermal reaction method (Guardia et al., 2009), so l-gel process (Lu et al., 2002), chemical co-precipitation (Laurent et al., 2008) etc.
Titanium oxide. Four crystal forms of titanium dioxide are naturally found: rutile, anatase, brookite and TiO2(B) (Wang et al., 2005) and they can be synthesized using the following techniques: co-precipitation, sol-gel synthesis process (Ramaswamy et al., 2008), chemical vapor deposition (Kawai-Nakamura et al., 2008), reverse micelle synthesis (Li et al., 2008), microemulsion synthesis process (Rashidzadeh, 2008) and hydrothermal reaction method (Kim et al., 2006). It is also worth knowing that the transformation behaviour from the amorphous to the anatase or rutile phase is influenced by the synthesis conditions (Madhusudan Reddy et al., 2001). Because of their attractive properties including high refractive index, light absorption/scattering as well as its chemical stability and relatively low-cost production titanium dioxide, NPs are exploited by a variety of fields (Kim et al., 2007a, Baldassari et al., 2005). Application fields of titanium oxide range from pigments (Pratsinis et al., 1996), to cosmetics, catalysts, (Chen and Yang, 1993) and photocatalysts (Rao and Dube, 1996), etc.
Cerium oxide: A number of attractive properties, such as quantum effects, magnetic properties and catalytic capabilities that cerium oxides display at nanoscale make them ideal for a variety of useful applications in both materials science (Lewis, 2000) and in biology (Kataoka et al., 2001).In material science, cerium oxide NPs are widely applied as catalysis in fuel cell technology (Logothetidis et al., 2003), catalytic wet oxidation (Larachi et al., 2002),
biology, magnetic cerium NPs provide contrast in magnetic resonance imaging (Wilhelm et al., 2002, Mornet et al., 2004) and fluorescent quantum dots is an important tool for biomedical diagnostics (Perez et al., 2002) and cell imaging (Cha et al., 2003, Chen and Gerion, 2004). The above mentioned applications led to a steady increase of the production of cerium oxide NPs which relies - depending on the specific application of the product- on numerous different synthesis methods such as sol-gel, thermal decomposition, solvothermal oxidation, micro-emulsion methods, flame spray pyrolysis and microwave-assisted solvothermal process (Ju-Nam and Lead, 2008).