Introduction to Nanoscience and Nanotechnology. Chris Binns
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1.14 shows a plot of the strain (relative elongation of a sample) vs. stress (load) for various nanostructured aluminum alloys compared to normal (coarse‐grained) aluminum alloy [14]. The plastic limit or yield strength occurs at the point where the slope changes, and it is seen that nanostructured materials have a value that is up to four times higher than the conventional material. This is a dramatic increase in strength but even higher values have been found in other metals, for example, a 10‐fold increase in copper [15]. A problem with nanostructured materials is also revealed by the plot, however, and that is that they fail (break) at relatively low strains. Problems such as this are being addressed by improvements in processing [15].
1.4 The Chemical Properties of Nanoparticles
Another size‐dependent property of nanoparticles is their chemical reactivity. This is demonstrated most dramatically by gold, which in the bulk is the archetypal inert material. This is one of the reasons it is so highly valued since it does not corrode or tarnish and so has a timeless quality. The only acid that is known to attack it is a hellish brew of concentrated nitric and hydrochloric acids mixed to form what has been poetically named aqua regia (royal water). Gold would therefore seem to be useless as a catalyst to speed up chemical reactions, but this is not so for gold nanoparticles. Catalysts provide a surface whose chemical properties reduce the energy required for a specific reaction between two other species to occur and are vital to the chemical industry. Because it is only the surface that is active, catalysts are in the form of nanoparticles to maximize the surface area per gram of material (see Problem 1) and so their chemical properties are different from the bulk form, which is also true for gold. When gold is in the form of nanoparticles with diameters less than about 5 nm it becomes a powerful catalyst, especially for the oxidation of carbon monoxide (CO). The full story is quite complicated because the reactivity of gold nanoparticles appears to depend not only on their size but also on the material on which they are supported.
An assessment of a number of research papers on the effectiveness of gold in catalyzing the above reaction, however [16], has concluded that the dominant effect is that of the gold nanoparticle size, with the nature of the support playing a secondary role. Figure 1.15 shows a compilation of data on the oxidation of carbon monoxide (CO) by gold nanoparticles on various supports as a function of their size and shows the impressive performance of the gold, which is completely inert in macroscopic‐sized pieces. Since catalysis can only occur at the surface layer of atoms, the dominant size effect is the proportion of gold atoms that are at the surface. In fact, the most important atoms for catalysis are those at the corners between different facets. These low co‐ordinated atoms are where the reacting carbon monoxide (CO) molecules preferentially bond during the reaction. The fraction of this type of atom (highlighted in red in Figure 1.15) is proportional to 1/d3, where d is the particle diameter, and the black line in Figure 1.15 is a fit to the data using this law demonstrating that the dominant size effect is indeed the proportion of corner atoms at the surface. The focus here has been on gold because of its extreme demonstration of a size‐dependence, that is, from a completely inert material to a powerful catalyst but as a general rule, the performance of all catalysts depends on the particle size. Because of the importance of catalysts to the chemical industry, the effect is the focus of a significant research activity.
Figure 1.15 Reactivity of gold nanoparticles. Measured activities of gold nanoparticles on various supports (box) for carbon monoxide oxidation as a function of particle size. The black line is a fit using a 1/d3 law and is seen to broadly represent the variation indicating that the dominant size effect is the proportion of gold atoms that are at a corner between facets at the surface (see text). Such atoms are highlighted in red on the nanoparticle shown.
Source: Reproduced with the permission of Elsevier Science from N. Lopez et al. [16].
1.5 Nanoparticles Interacting with Bacteria and Viruses
Chapter 8 deals in detail with nanoparticles interacting with living systems, and the introduction described types of nanoparticles that could be used for treating cancer (see Figure I.5). In this section, just one type of application will be considered where size is important and that is nanoparticles, mostly silver, interacting with bacteria and viruses, which has become an increasingly important area for medical research. It has been known for some time that silver is highly toxic to a wide range of bacteria, and silver‐based compounds have been used extensively in bactericidal applications. This property of silver has caused great interest especially as new antibiotic‐resistant strains have become a serious problem in public health. For example, in 2017, such bacteria killed more than 35 000 people in the United States alone [17] and any method of attacking them, not involving normal antibiotics, is becoming increasingly important.
Before the discovery of antibiotics, silver was used as an antiseptic in the treatment of open wounds and burns. The highly reactive silver ions can bind to the bacterial cell walls inhibiting the cell respiration or they can pass into the cell and denature the bacterial DNA, which inhibits the replications and ultimately leads to cell distortion and death. Silver in the form of chemical compounds applied to wound dressings, although effective, was shown to be less effective than conventional antibiotics [18].
Silver has made a comeback in the form of nanoparticles, however, because of its greatly increased effectiveness, partly because of the high surface/volume fraction so that a large proportion of silver atoms are in direct contact with their environment. In addition, entire nanoparticles are sufficiently small to pass through outer cell membranes and enter cells' inner mechanisms. A study using nanoparticles with a wide size range [19] showed that only silver nanoparticles with sizes in the range 1–10 nm were able to enter cells and disrupt them. Some particles attached to the cell membrane and disturbed its functions, such as respiration. Others penetrated the outer membrane and caused further damage including damage to the cell DNA. Figure 1.16 shows an electron microscope image of some Pseudomonas aeruginosa5 bacteria after they had been exposed to silver nanoparticles, and the high magnification inset reveals some nanoparticles attached to the cell membranes. This strain can be particularly problematic to cystic fibrosis sufferers as it readily colonizes the favorable environment found in their lungs. It protects itself by producing a thick mucus layer, which makes it difficult to treat with antibiotics once it is established. Silver nanoparticles suspended in an aerosol could prove to be an effective treatment. Other metal nanoparticles such as copper, magnesium, titanium, gold, and zinc have also been found to be effective antibacterial agents and one attractive aspect of using inorganic materials such as these is that it is harder for bacterial cells to evolve ways to become resistant.
There has been less research on the antiviral effectiveness of nanoparticles but it has become a more active topic of research following the Covid‐19 pandemic of 2020. Viruses are much smaller than bacteria and are inhabitants of the Nanoworld as illustrated in the introduction (see Figure I.1). The original report of the interaction of silver nanoparticles with viruses came in 2005 from the same team that carried out the work described above on bacteria [20]. They exposed HIV‐1, the virus responsible for AIDS, to silver nanoparticles and discovered that, again, the important size range is 1–10 nm. As shown in Figure 1.17c, nanoparticles