«By Yusuf Nur A thesis submitted to The University of Birmingham for the Degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental ...»
Zinc oxide: In recent years, nanoscale zinc oxides have received considerable attention due to their novel properties which are applicable in many fields including ultraviolet laser devices (Makino et al., 2002), piezoelectric nanogenerators (Wang and Song, 2006), chemical sensors (Müller and Weißenrieder, 1994), solar cells (Beek et al., 2004), antibacterial agents (Jones et al., 2008) and biomedical labels (Kuo et al., 2009). Zinc oxide is a wide-bandgap Semiconductor that displays luminescent properties in the near ultraviolet and visible region (Huang et al., 2001) and it can be doped with magnetic elements Co, Fe and Ni to store magnetic data. In the literature, several synthesis methods, such as thermal decomposition, chemical vapour deposition, laser ablation, spray pyrolysis, sol–gel method, hydrothermal synthesis, molecular beam epitaxy, etc. have been reported for the preparation of ZnO nanoparticles and films (Tarasenko et al., 2010). Surface modifications using appropriate organic compounds have also remarkably widened the existing applications or opened new ways of applying zinc oxide NPs (Guo et al., 2000).
Quantum dots (QD) are very special group of semiconductors due to their small sizes ranging from 2 -10 nm. The number of atoms inside them may vary from just few atoms to hundreds or thousands depending on their desired final size. They are defined as particles with physical dimensions smaller than the exciton Bohr radius (Chan et al., 2002) which is the distance between exited electron and the hole formed in the ground electronic state due to the excitation of the electron. QDs can be made from alloys of most semiconductor metals (e.g., CdS, CdSe, CdTe, ZnS, PbS) (Alivisatos, 1996, Bailey and Nie, 2003). The small size affects their electronic properties in a way that they form a special group of semiconductors which conducts electricity very fast. A quantum dot made of few atoms displays a big electronic band gap where the excitation of the electrons requires lots of energy and concurrently more energy in the form of fluorescence is released when electrons return to their rest state. This phenomenon gives the QDs optical properties which are different from the bulk material properties in the sense that their emission fluorescence wavelength can be altered by changing the size of the quantum dots. In principle, the smaller the quantum dots the more their fluorescence wavelength is shifted toward blue. QDs have been received as a new technology and, due to their unique electrical and optical properties, have many applications in a variety of fields including: electroluminescent displays like light emitting diodes (LED), solid-state lighting, solar cells, biotechnology and medicine (Jamieson et al., 2007).
126.96.36.199.4 Metals Nanoscale metals have manifested a number of size – dependent interesting properties which makes them different from both bulk and atomic scale materials of the same metal. They have
decorative dyes in ornaments and artworks (Daniel and Astruc, 2003). Their more recent applications exploit many aspects of their unique nanoscale optical, electrical and chemical properties and range from catalysts (Jana et al., 1998), medical diagnostics (Mirkin et al., 1996), ant-bacterial uses to water purifications (Pradeep and Anshup, 2009). Among the variety of ways used to synthesize metal NPs, the following methods are widely applied to tune their size and shape into a specific purpose: electrochemical reduction (Lee et al., 2011, Hirsch et al., 2005), photochemical reduction (Eustis et al., 2005), vapour deposition (Pandey et al., 2011), and chemical reduction using a reducing agent (Brust et al., 1994, Turkevich, 1951, Frens, 1973).
188.8.131.52.4.1 Silver nanoparticles.
Among the different types of metal, nanoparticles silver is by far the most studied in the literature due to its wide and rapidly growing application in a number of scientific areas and in consumer products (Tolaymat et al., 2010) and its known toxic effects on the environment and human health (Khaydarov et al., 2011). Its areas of application vary from textile engineering, catalysts (Lewis, 1993), medicine (Salata, 2004),, water treatment (Solov’ev et al., 2007) and disinfecting to electronics (Lee and Jeong, 2005) and biotechnology (Niemeyer, 2001). Well designed silver nanoparticles of various shapes and sizes can be achieved through a variety of different synthesis approaches using numerous capping agents depending on their applications. Of the above mentioned synthesis approaches the following ones are the most applied methods: photochemical methods, laser ablation (Lee et al., 2001), microwave processing (Soto et al., 2005), thermal decomposition of silver axalate (Navaladian et al., 2006), reduction of both inorganic and organic agents and electron radiation (Bönnemann and Richards, 2001).
Elemental gold has many unique properties which have attracted and fascinated mankind since its discovery. Being very unreactive, gold does not turnish in the atmosphere and so keeps its attractive colour forever(Hutchings et al., 2008). That is one of the main reasons why gold has been used in shaping jewelleries. It has been used for many colourful, decorative, ceremonial and religious artifacts and has been a metal with a high monetary value. Colourful aqueous solutions of gold colloids date back to Roman times and were known to medieval alchemists as aurum potabile (Mellor, 1923). A Roman cup, called the Lycurgus cup, used nanosized (ca 50 nm) gold and silver alloys, with some Cu clusters to create different colours depending on whether it was illuminated from the front or the back.
The cause of this effect was not known to those who exploited it. Michael Faraday was the first to recognise that the colour was due to the minute size of the gold particles (Faraday, 1857). On February 5, 1857, Michael Faraday delivered the Bakerian Lecture of the Royal Society in London entitled “Experimental Relations of Gold (and other metals) to Light”. In his speech, he mentioned that known phenomena (the nature of the ruby glass) appeared to indicate that a mere variation in the size of its particles gave rise to a variety of resultant colours. Nearly a century later, electron microscope investigations on Faraday’s rubycoloured gold colloids have revealed that Faraday’s fluid preparations contain particles of gold of average diameter (6 ± 2 nm) (Turkevich, 1951).
Although some scientists see the Faraday’s experiment as a landmark in the history of nanoscience and nanotechnology (Peter and John Meurig, 2007) the chemical inertness of gold as a bulk metal appeared to provide very little opportunities to open up new and exciting chemistries (Hutchings et al., 2008). The new field of nanotechnology made it possible to discover the unique properties of matter when subdivided to the nanoscale. Gold at
many disciplines of science including: material scientists, catalysts, biologists, surface and synthetic chemists and theoreticians in great number. Today, in the 21st century, gold chemistry is based on solid ground regarding the preparation and characterisation of a wide variety of fundamental compounds with gold atoms and gold clusters as core units (Murray, 2000, Peter, 2000, Gagotsi, 2006). The fact that gold NPs have been studied in many different scientific fields has led not only to a deep understanding of many of the physico-chemical features that determine the characteristic behaviour of these nanoscale gold nanoaprticles but also to invent, test and validate reliable novel procedures for the preparation, synthesis and characterisation of gold nanoparticles of basically any desired size and shape.
The bottom up process described in section 184.108.40.206 is by far more common and effective (Sardar et al., 2009) and has become a popular method in current nano-science and nanoengineering. It has a number of potentially very attractive advantages. These include experimental simplicity down to the atomic size scale, the possibility of three-dimensional assembly and the potential for inexpensive mass fabrication (Brust and Kiely, 2002). The simplest and most common bottom up method employed for the production of the gold nanoparticles of different sizes is the reduction of Au(III) salt (usually HAuCl4) by sodium citrate in water. In this method, pioneered by Turkevich and co-workers in 1951(Turkevich,
1951) and later refined by Frens in the 1970s (Frens, 1973) and more recently further developed by Kumar (Kumar et al., 2006). It is generally accepted that the AuCl4- ions are first reduced to atomic gold (Au), the concentration of which rises quickly to the supersaturation level. Collision of the Au atoms leads to a sudden burst of nuclei formation which marks the start of the nucleation step. It is the attachment and coalescence of those nuclei which results in the growth and formation of desired nanoparticles (Pong et al., 2007). Figure
nanoparticles. It shows that the reduction and nucleation are fast (200 ms) while growth step is the rate determining step since it is much slower than the antecedent nucleation step. Many times, difficulty in controlling the nucleation and growth steps, which are intermediate stages of particle formation process may result in a broad particles size distribution (Bellloni, 1996).
In the presence of various reactive polymers in the reaction medium, that is, polymers having various functional groups, the growing metallic particles are stabilized by the adsorption of the polymer chains onto the surface of the growing metal fragments, thus lowering their surface energy and creating a barrier to further aggregation (King et al., 2003).
Figure 2-5: Schematic illustration for the deduced process of gold nanaoparticles formation. Reduction and nucleation are faster processes than coalescence of nuclei (Polte et al., 2010). Reprinted with the permission from copyright 2010 American chemical society.
One important factor for understanding the behaviour of the natural particles in the environment and the bioavailability of heavy metals loaded on them is their interaction with microorganisms associated with biomass population. The nanoparticles could possibly be immobilised, absorbed, reacted or retarded by biomass in the environment. Since one of the main objectives of this project is to study the effect gold nanoparticles on planktonic bacteria the following section 2.2 will give a general introduction of the overall fate and behaviour of
2.2 Fate and behaviour of the manufactured NPs in the environment The abundant use of NPs and their final disposal into the environment have raised lots of concerns within the scientific community regarding their fate and behaviour in the natural environment and their possible impacts on the living organisms. Overall, the need for understanding the role of nanoparticles in the environment is important to perceive the extent to which we are changing the planet with the application of nanotechnology and to choose to address minimizing those changes (Wigginton et al., 2007).
Some important questions that need to be raised when addressing nanoscience in order for it to be fully understood are: will manufactured nanoparticles from industry and other sources enter the atmosphere, soils, sediments or water(Mueller and Nowack, 2008)? If so, how persistent will they be, and in what concentrations will they occur (Gottschalk et al., 2009) and what form will they take? It is quite clear that large scale use of NPs (Woodrow_Wilson_centre, 2007) suggests that their presence in the environment will increase (Navarro et al., 2008, Barnes et al., 2010). Another important point of consideration is that once nanoparticles originated from different sources get their way in the environment, their properties will not remain the same due to their interactions with water, soil, air or with living organisms such as plants, algae fungus and bacteria.
The key processes that govern nanoparticles behaviour in the aquatic environment are aggregation and dissolution, driven by size and surface properties of the materials which are mainly dependent on environmental factors such as temperature, ionic strength, pH, particle
, composition and shape ( see Figure 2-6 below) (Dunphy Guzman et al., 2006, Filella and Buffle, 1993, Lecoanet et al., 2004).
Figure 2-6: Schematic represantation of the key factors and processes that govern the behaviuor of the nanoparticles in the natural enviroement.(Misra et al., 2012). Reprinted with permission from copyright 2012 Elsevier.
Surface properties determine the stability and mobility of NPs in aquatic and terrestrial systems and their interactions with the living organisms(Navarro et al., 2008). That is why manufactured metallic NPs are for example coated with inorganic or organic agents such as citrate, PVP, polyethelene glycol (PEG), or surfactants such as sodium dodecyl sulfate to stabilize them in the suspensions (Mafuné et al., 2000). It is clear now that those coating agents will influence, if not determine, the surface chemistry of these NPs and have impact on their behaviour in the environment. The effect of pH on the metallic NPs was clarified by the silver nanoparticles which transformed in size or state (soluble silver and silver ions) within minutes in low pH aqueous nitric acid with similar acidity to the natural environment (Elzey and Grassian, 2010). The ionic strength and pH ranges typical of most soils and surface waters present conditions under which clusters or aggregates of TiO2 with diameters
monodisperse nano-TiO2 (French et al., 2009).