Introduction to Nanoscience and Nanotechnology. Chris Binns
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Название: Introduction to Nanoscience and Nanotechnology
Автор: Chris Binns
Издательство: John Wiley & Sons Limited
Жанр: Отраслевые издания
isbn: 9781119172253
isbn:
The uptake of nanoparticles in the intestinal tract is complicated and there are other routes into the circulation as well as through the enterocytes. It has been shown that translocation depends on particle size, coating and surface charge [6]. The first barrier to cross is a mucous layer, which is excreted by specialist cells known as goblet cells that are interspersed with the enterocytes in the villi walls and is designed to act as a filter. It was shown more than 20 years ago that 14 nm latex particles could cross this barrier in 2 minutes while it took 415 nm particles 30 minutes and 1000 nm particles were unable to cross at all [11]. The enterocytes act as a further barrier and again, the ability to be absorbed and pass into the interior of the villi depends on a number of factors including size. As well as blood vessels, the villi are permeated with lymph vessels and nanoparticles that make it through the wall can enter the lymph system, where they are likely to trigger an immune response or the blood circulation where the ones that are not removed (<10%) can be deposited in different organs.
Figure 2.7 Villi and microvilli of the small intestine. The surface of the central tube (lumen) of the small intestine is made of finger‐like projections (villi), which are composed of enterocytes that themselves have hairlike projections (microvilli). The internal surface presents an area of some 200 m2 to the passing fluid containing nutrients.
Source: BallenaBlanca. https://commons.wikimedia.org/wiki/File:Villi_%26_microvilli_of_small_intestine.svg. Licensed under CC BY‐SA 4.0 (https://creativecommons.org/licenses/by‐sa/4.0/deed.en).
2.2.3 Nanoparticles and the Skin
Human skin, with a typical surface area of 1.5 m2, is an impenetrable barrier to the environment preventing the entry of any foreign matter. The structure, shown in Figure 2.8, is divided into three layers called the epidermis, the dermis and the subcutaneous layer with the surface layer of the epidermis composed of a layer of dead cells about 10 μm thick known as the stratum corneum. The dermis is a layer of living cells permeated with blood vessels near its base containing the oil and sweat glands and the hair follicles. The epidermis is also composed of living cells apart from the stratum corneum and contains the specialist melanin producing cells (melanocytes) that determine skin color.
Zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles in the size range 20–30 nm are widely used in cosmetic products such as sunscreens and there has been some concern that penetration to the bottom of the dermis could allow such particles to enter the blood circulation. To date, however there is no evidence that this can occur, indeed, studies of 18 nm ZnO nanoparticles [12] show that they do not penetrate the stratum corneum. Although particles can enter the hair follicles at the hair root, this part of the channel is also covered with a dead layer and prevents the particles reaching live layers. There has been some interest in transdermal applications of drugs, which is possible using microemulsions [13] though this is not relevant to nanoparticles.
Figure 2.8 Structure of skin. Human skin consists of three basic layers labeled the epidermis the dermis and the subcutaneous layer. The dermis is composed of living cells and contains, hair follicles, oil, and sweat glands. The epidermis is also composed of living cells apart from the top 10 μm, which is a layer of dead cells known as the stratum corneum.
2.2.4 Air Quality Specifications
There are identifiable harmful effects on health from exposure to nanoparticles, especially cardiovascular problems associated with inhaled airborne particles and various guidelines for limits of acceptable particulate densities have been published, for example, in the European Directive on air quality [14]. Current air policies on dust levels only distinguish particle sizes in a broad‐brush manner and focus on all particles smaller than 10 μm (the PM10 fraction) and those smaller than 2.5 μm (the PM2.5 fraction). It is clear from the previous discussion that in the future there will need to be further limits set at PM0.1 and PM0.05 (particles smaller than 50 nm).
2.3 Nanoparticles and Clouds
The presence of aerosol in the atmosphere has a significant influence on climate, its most important role being in the formation of clouds. Pure water vapor in the atmosphere is invisible but when it condenses into microscopic water droplets, suspended as an aerosol, over a region of sky a cloud is born. The process of cloud formation and how they evolve and precipitate is a complex process but an important fundamental consideration relevant to this book is that without a preexisting aerosol of particles, clouds would not form except in extremes of high supersaturation. In a purely gaseous atmosphere, even one saturated with water vapor, it is not possible for water droplets to start growing, unless there are some initial “seed” particles that water can condense onto. These seeds are referred to as cloud condensation nuclei (CCNs). The reason why pure water vapor will not form droplets is described briefly in Advanced Reading Box 2.2 but in a nutshell, although water molecules do stick together, at normal temperatures, and vapor pressures, they do not stay together long enough for a third and fourth molecule to join them and start a droplet growing. However, a water droplet above a critical size that somehow appeared would be stable and in a humid atmosphere would grow. Without CCNs there is no way to achieve the initial water droplet above the critical size. The presence of preexisting CCNs changes that and water molecules can easily condense onto them and grow to a normal cloud droplet size. These fall sufficiently slowly under gravity to be considered as suspended (see Advanced Reading Box 2.1).
Advanced Reading Box 2.2 Condensation of Water Droplets in a Humid Atmosphere
The vapor pressure above a flat liquid surface within a closed container is [15]:
where ns is the atomic density near the surface, Ef СКАЧАТЬ