Toxicity of nanoparticles
Abstract:
This chapter summarizes the most important applications of nanotechnology in the construction industry together with aspects of nanoparticles toxicity. Nanotechnology is currently applied in the construction industry predominantly in cement, coatings, paints and insulating materials. Nanoparticles incorporated into existing construction materials confer them novel and extraordinary properties, such as increased strength, self-sensing, self-cleaning, antimicrobial, or pollution remediation capabilities. On the other hand, nanoparticles from construction materials that are released into the environment can be extremely detrimental to health. Nanoparticles can enter the human body via inhalation, ingestion, or skin contact. The range of pathologies related to exposure to nanoparticles encompasses respiratory, cardiovascular, lymphatic, autoimmune, neurodegenerative diseases, and a variety of cancers that can manifest immediately following exposure or many years later.
15.1 Introduction to nanoparticle and nanomaterial toxicity
15.1.1 Definition and general information
A nanoparticle is defined as a particle with length in two or three dimensions larger than 1 nm and smaller than about 1000 nm (Buzea et al., 2007), although some authors prefer a narrower range, from 1 nm up to 100 nm (Borm et al., 2006; Shvedova et al., 2010; Dobrovolskaia and McNeil, 2007). Nanoparticles are a growing subclass of the larger classification of nanomaterials, which includes all materials with a structural component smaller than 1000 nm in at least one dimension. In addition to nanoparticles, the class of nanomaterials includes complex nanostructures attached to a substrate, such as computer chips or thin films with a porous structure. While nanomaterials that are fixed on a substrate are benign as long as they do not detach and become airborne, many types of free nanoparticles can be associated with negative health effects.
From the physico-chemical point of view, there are two main subtypes of nanoparticles: soluble and insoluble. Soluble or biodegradable nanoparticles, depending on their composition and as a result of biological and/or photo-chemical decomposition, may convert into molecular by-products. These resulting molecular components may be easier to clear from the body, sometimes making them less harmful than their nanoparticle progenitor (Park et al., 2009). The insoluble, or biopersistent, subtype comprises nanoparticles made of metals and/or their compounds, such as titanium dioxide (TiO2) or quantum dots. They can also be non-metals such as carbon-based nanoparticles including fullerenes C60, carbon nanotubes, carbon soot, and silicon-based nanoparticles, among others.
Natural and anthropogenic nanoparticles can be toxic to life forms due to their noxious interactions with intracellular organelles, proteins or genes. A schematic depicting the pathways of exposure with nanoparticles and their adverse effects in humans is shown in Fig. 15.1. Three types of research give insight on nanoparticle toxicity: epidemiological studies directed to determine whether there is an association between environmental or occupational exposure to nanoparticles and human disease; in vitro experiments, utilizing either human or animal cell lines, designed to elucidate the molecular determinants of potentially toxic nanoparticles; and in vivo studies performed on animals that are usually the correlate for initial in vitro observations.
15.1.2 Sources of nanoparticles on Earth and their toxicity
One might be inclined to associate the terms nanoparticle and nanomaterial with nanotechnology alone. However, the sources of nanoparticles on Earth are varied, encompassing natural as well as man-made. The nanoparticle pollution resulting from both anthropogenic and natural sources on Earth is considerable in very populated parts of the globe, and can be readily visualized from satellite images.
Natural nanoparticles
The main natural causes of nanoparticle generation are incomplete combustion during volcanic eruptions, physical and chemical weathering of rocks, incomplete combustion from forest fires, water evaporation, and extraterrestrial dust. Natural nanoparticles such as dust, smoke, and ash, are ubiquitous on Earth. Dust particles with interplanetary origin accrete on Earth at a rate of about 40 kilotons per year (Johnson, 2001). Nanoparticles from large volcanic eruptions can affect regions up to hundreds of kilometers from the volcano by means of volcanic aerosols. Nanoparticles from dust storms are not confined only to the desert regions on Earth, but have been shown to travel across and among continents, with global impacts (Husar et al., 2001; McKendry et al., 2001). Natural nanoparticles can create a wide range of undesired health effects, ranging from respiratory conditions related to incomplete combustion and dust nanoparticles, swelling of the limbs due to the blockage of lymphatic vessels by volcanic soil particles, and diseases involving calcifications associated with nanoparticle production by nanobacteria, such as renal stone formation (Ciftcioglu et al., 2005; Wood and Shoskes, 2006).
Anthropogenic nanoparticles
These are substantial in the most populated regions of the world (Murr and Garza, 2009). Sources of anthropogenic nanoparticles include incomplete combustion due to vehicle and automobile exhaust, burning of wood, coal, combustion of petroleum and their derivatives for cooking, heating, and power generation, waste incineration, and smoking. Occupational activities, such as welding, mining, or building demolition (Buzea et al., 2007; Lowers and Meeker, 2005), are also an important source of nanoparticle exposure for workers and their immediate environment. Furthermore, commercialized products containing engineered nanoparticles are rapidly becoming an important source of nanoparticle exposure. Among anthropogenic sources, arguably the most important source of nanoparticle pollution is incomplete combustion due to vehicle and automobile exhaust. Vehicle pollution comprises not only greenhouse gases, such as carbon dioxide, but a large array of nanoparticles that compose more than 90% of the number of diesel generated particles (Westerdahl et al., 2005; Sioutas et al., 2005). A high concentration of such nanoparticles has been found in the air hundreds of meters away from major roads and highways (Sioutas et al., 2005). Studies have found an association between exposure to these particles and increased cardiopulmonary mortality (Vermylen et al., 2005). Multiple epidemiological studies link vehicle particulate pollution to a wide range of adverse health effects, ranging from respiratory to cardiovascular disease (Garshick et al., 1988; Vermylen et al., 2005; Hoek et al., 2002; Knox, 2005; Bigert et al., 2003). Few studies have associated the exposure to engine exhaust with specific childhood cancers (Knox, 2005). Researchers have also reported higher incidences of heart attack and lung cancer in professional drivers (Bigert et al., 2003; Garshick et al., 1988). In conclusion, there is a growing body of epidemiological evidence that correlates high levels of outdoor microparticle/nanoparticle pollution to an increased incidence and mortality from lung and cardiovascular disease.
Engineered nanoparticles
Engineering nanoparticles are a class of nanomaterials that are likely to become a significant source of nanoparticle pollution if not properly handled and recycled. Some engineered nanoparticles are already commercialized in a variety of everyday use products such as cosmetics, sunscreens, toothpaste, food additives, stain-resistant clothing, sporting goods, tires and in construction materials (Buzea et al., 2007). There are currently more than 300 products on the market containing nanomaterials (Maynard et al., 2006). The health effects of the long-term use of these products are basically unknown. Despite the fact that many nanoparticles may be potentially toxic, an increasing number of consumer products containing engineered nanoparticles are commercially available long before their safety is assessed (Kulthong et al., 2010; Hagens et al., 2007).
Recent concern has been raised on the widespread use of titanium dioxide nanoparticles in a variety of products as a colorant and UV-ray blocking agent. Due to its tinctorial effects (white pigment), it is currently used as a colorant for food, medications, toothpaste, paint, and plastic; as an active UV blocking agent it is used in sunscreens and cosmetic creams (European Commission, 2007). Titanium dioxide has proved to be safe when introduced to similar products in bulk form. However, studies to determine its safety at the nanoscale are lacking. The long term effects of incorporating titanium dioxide, if any, remain to be discovered.
In vitro studies indicate that silver nanoparticles are even more toxic to human and animal cells than asbestos (Soto et al., 2005). Inhaled silver nanoparticles migrate via the circulatory system and may reach several target organs. Silver nanoparticles have been found in the blood of patients with blood dyscrasias (Gatti et al., 2004), and in the intestine of patients with colon cancer (Gatti, 2004). Whether silver nanoparticles are directly involved in the pathogenesis of these diseases has not been properly addressed. Furthermore, there are reports indicating that silver nanoparticles have been discovered lodged in the liver, kidneys and heart of otherwise healthy patients (Takenaka et al., 2001). Why and how silver nanoparticles have reached these organs was not explained or understood. Silver nanoparticles are now used due to their antiseptic properties in fabrics, socks, washing machines, vacuum cleaners, and food storage containers, and as burn wound dressings.
15.2 Morphology, classification, and properties of nanomaterials
‘Nano’ can be considered a different state of aggregation of matter in addition to the solid, liquid, gas, and plasma states. This is because nanoparticles of a specific material exhibit quite different physical (optical, electromagnetic), chemical (catalytic), and mechanical properties from the same material in bulk form. The two main factors that make nanoparticles behave differently from bulk form are surface effects and quantum effects. As a result, nanomaterials may have different chemical reactivity, mechanical, optical, electrical and magnetic properties compared to their bulk counterparts.
Surface effects manifest as smooth scaling of physical properties due to an increased fraction of the atoms at the particle surface compared to the interior. These effects include increased chemical reactivity and reduced melting point of nanoparticles compared to larger particles or bulk material (Buzea et al., 2007). Nanoparticles have a very large relative surface area (and a large particle number per unit mass) compared to microparticles or larger particles (Buzea et al., 2007). Hence, nanoparticles have a much larger surface area available for chemical reactions compared to microparticles. For instance, if we consider the same mass of nanoparticles (with diameter of 60 nm) and compare them with their microparticle counterpart (with diameter of 60 microns), nanoparticles will exhibit 1000-fold enhanced reactivity. This is explained by the fact that the atoms at the surface of a nanoparticle have fewer neighbors than the atoms in bulk. Consequently, the binding energy per atom decreases with particle size (Roduner, 2006). This translates to a reduced melting point as a function of particle diameter, according to the Gibbs–Thomson equation where Tbulk is the melting point of the bulk material, d is the diameter of the nanoparticle, and c is the material constant (Roduner, 2006). For example, the melting temperature of 3 nm diameter gold nanoparticles is more than 300 K lower than the melting temperature of bulk gold, 1336 K.
We can classify nanoparticles based on their dimensionality, morphology, composition, uniformity, and agglomeration (Fig. 15.2). Among the nanostructures presented in Fig. 15.2, those that can easily be released into the environment are nanoparticles that may pose a serious health risk. One must emphasize that various coatings, due to wear and finite lifetime, could release nanoparticles that may become airborne and be inhaled or get into dermal contact. In contrast, if the nanoparticles are firmly attached to substrates or larger objects and will not be released into the atmosphere, they are considered not to pose a health risk. Figure 15.3 shows transmission electron microscope (TEM) and scanning electron microscope (SEM) images of nanoparticles with various morphologies and compositions.
15.2 Classification of nanostructured materials from the point of view of dimensionality, morphology, composition, uniformity and agglomeration state (Buzea et al., 2007).
15.3 TEM images of (a) Fe3O4 nanoparticles; (b), (c), (d) gold nanoparticles with various morphologies; (e) SiO2 nanoparticles; (f) TiO2 nanoparticles embedded in paint. (reprinted from Zhu et al., 2011, copyright 2011, with permission from Elsevier), (image by Byeongdu Lee, courtesy of Argonne National Laboratory), (reprinted from Sanchez and Sobolev, 2010, copyright 2010, with permission from Elsevier), (reprinted from Caballero et al., 2010, copyright 2010, with permission from Elsevier)
15.2.1 Carbon nanotubes
A very important class of newly engineered nanomaterials are carbon nanotubes (CNTs). Similarly to asbestos fibers, CNTs have a long aspect ratio. CNTs can be visualized as graphite sheets (hexagonal patterns of carbon atoms) rolled up in a tube structure (Makar and Beaudoin, 2003) that may have a single wall (single-walled CNT; SWCNTs) or multiple walls (multi-walled CNTs; MWCNTs). CNTs can be modeled to exhibit a variety of shapes and sizes. Their diameter can vary between 0.4 and 100 nm, while their length ranges between several nanometers up to centimeters (Dai, 2002). Some MWCNTs have exposed edge planes along the surface, constituting potential sites for chemical and physical interactions (Sanchez and Sobolev, 2010). CNTs can be synthesized with different types of chirality (the orientation of the hexagonal patterns with respect to the tube axis) (Ma et al., 2010). Interestingly, two tubes with the same diameter may have different structures and hence different properties.
CNTs present unique electronic, chemical, mechanical, and thermal properties (Ajayan, 1999; Salvetat et al., 1999). They exhibit a low density, ranging between 1.3 and 2 g/cm2, high flexibility and high aspect ratio – 1000 (Sahoo et al., 2010; Wernik and Meguid, 2010). Their electronic behavior ranges from metallic to semiconductor, depending on their chirality (Makar and Beaudoin, 2003) and oxygen doping (Collins et al., 2000). CNTs seem to be the strongest materials, having Young’s elasticity modulus of approximately 1 TPa (Salvetat et al., 1999) and a tensile strength of 50–200 GPa (Ma et al., 2010). Thus, not surprisingly, CNTs are very flexible, and can be bent to form circles or create knots (Lourie et al., 1998). Moreover CNTs can be scrolled enabling yarn weaving, sewing or braiding. CNTs have been used as scaffolding to fabricate yarns of superconductors and TiO2 used for photocatalysis (Lima et al., 2011). CNTs suspended in liquids exhibit a remarkably high thermal conductivity (Choi et al., 2001), approaching the theoretical limit for carbon materials (Berber et al., 2000). The thermal conductivity of SCNTs and MWCNTs can attain 6000 W/(mK) and 2000 W/(mK), respectively. Thus, they possess a negligible coefficient of thermal expansion (Ma et al., 2010). Due to their anisotropic structure, it is believed that their thermal conductivity is also anisotropic, being much higher along than across the tube (Makar and Beaudoin, 2003).
The superior mechanical properties of CNTs make them of great interest as structural materials; however, due to their similar morphology to asbestos (Fig. 15.4 (a), (b)) and the lack of rules and regulations regarding their handling, they might soon become a public health issue as well (Donaldson et al., 2010). Despite the institution of strict regulations in many countries, asbestos continues to have severe consequences on the health of workers and consumers for many years to come. After decades of debate, it is now well accepted that mesothelioma, a cancer of the lining of the lungs, may develop after either environmental or occupational exposure to asbestos (Nishikawa et al., 2008). Although some countries have opted to ban the use of asbestos, unfortunately a few others have not established any regulatory measures. These trends are illustrated in Fig. 15.4 (c), showing the mortality trends for mesothelioma in relation to change in asbestos use.
15.4 SEM images of (a) asbestos fibers and (b) multi-wall carbon nanotubes; (c) mortality trends for mesothelioma in males in relation to change in asbestos use. (courtesy of United States Geological Survey (USGS) at Denver, CO, http://usgsprobe.cr.usgs.gov/picts2.html), (reprinted with permission from Ajayan, 1999, copyright 1999 American Chemical Society), (reprinted from Environmental Health Perspectives, Nishikawa et al., 2008)
15.3 Types of building materials incorporating nanomaterials
To date, the majority of nanotechnology applications in construction on the market have focused on developing coatings, paints and composites that exploit the photocatalytic properties of TiO2 nanoparticles. TiO2’s photo-catalytic properties may be regulated by either UV-light or light of the visible spectrum. Depending on the association, TiO2 composites may acquire self-cleaning, microbicidal, anti-fog or hydrophilic properties; remarkably, they may also gain anti-pollutant effects. Multiple composite materials have already been generated by admixing various nanomaterials and conventional building materials such as concrete, glass, ceramics, polymers or metals. It is envisioned that nanomaterials will not only improve the properties of the conventional materials, but they may also be designed as self-sensing devices to monitor structural integrity. Besides composites, nanoparticle coatings have been used to confer fireproof, scratch resistance, antireflective, or antimicrobial properties to already edified structures. A schematic of nanotechnology application in the construction industry is shown in Fig. 15.5.
15.5 Schematics of nanotechnology application in construction materials and their resulting properties.
15.3.1 Concrete–nanoparticles composites
Nanocomposites incorporate nanoparticles into a matrix such as polymers, metals, or ceramics. The incorporation of nanoparticles can improve drastically the properties of the nanocomposite compared to that of the matrix alone, including mechanical strength, toughness and electrical or thermal conductivity. Although greater nanoparticle concentrations may be used, the usual range reported varies from 0.5% to 5.0% (v/v).
Concrete is probably the most important construction material nowadays, and ordinary Portland cement is the most common form of cement binder used in concrete. Portland cement is formed by grinding amorphous masses of various oxides, such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite, together with gypsum into powder (Makar and Beaudoin, 2003). After mixing it with water, the oxides suffer hydration reactions rendering them into solid cement binders. The cement grains have usual dimensions between 5 and 30 microns; however, grains with smaller dimensions are also present (Makar and Beaudoin, 2003). A typical microscale image of Portland cement is shown in Fig. 15.6(a). Cement nanocomposites are expected to have improved mechanical properties. It is thought that nanospheres would interrupt cracking, while nanofibers would act as reinforcing systems. In general, nanoparticles would have a high surface area to volume ratio, resulting in increased chemical reactivity (Fig. 15.6).
15.6 SEM images of (a) Portland cement; (b) fly ash; (c) carbon nanotubes and (d) carbon nanotubes–fly ash cement composite; (e) particle size and specific surface area related to concrete materials. (reprinted from Chaipanich et al., 2010, copyright 2010, with permission from Elsevier), (reprinted from Sanchez and Sobolev, 2010, copyright 2010, with permission from Elsevier)
Various types of nanoparticles have been added to concrete and the resulting composites have shown improved properties compared to simple concrete, such as strength and reduced permeability to water. Most of the research in nanocomposite cement has been done by incorporating nanoparticles of SiO2 (Bjornstrom et al., 2004; Tao, 2005; Jo et al., 2007; Hui et al., 2004; Li et al., 2006b, 2007; Ye et al., 2007; Lin et al., 2008; Potapov et al., 2011; Li, 2004) and T1O2 (Li et al., 2006b, 2007) into concrete.
The addition of SiO2 nanoparticles to concrete improves concrete workability and structural strength (Sobolev and Gutierrez, 2005a, 2005b; Li et al., 2004), accelerates the hydration reaction (Bjornstrom et al., 2004; Lin et al., 2008), increases the compressive and flexural strength of mortar (Li et al., 2004; Sanchez and Sobolev, 2010) and increases resistance to water penetration (Tao, 2005).
The addition of TiO2 nanoparticles to concrete provides the mix with outstanding self-cleaning and anti-pollutant capabilities. The concrete–TiO2 composite will induce the photocatalytic degradation of NOx, CO, VOC, chlorophenols and aldehydes, common pollutants present in vehicle and industrial emissions (Ruot et al., 2009; Chen and Poon, 2009). More detailed description of the photocatalytic mechanism is provided in Section 15.3.4. Self-cleaning and de-polluting concrete is already commercially available and has been used in the construction of buildings and roads in Italy and Japan (see Fig. 15.9 on page 447). In addition to photocatalytic benefits, TiO2 nanoparticles accelerate the early-stage hydration of cement, and enhance the compressive and flexural strength as well as the abrasion resistance (Sanchez and Sobolev, 2010; Li et al., 2006b, 2007).
15.9 Photographs of self-cleaning exterior building materials: (a) the church ‘Dives in Misericordia’ in Rome built with self-cleaning concrete that incorporates TiO2; (b) and (c) self-cleaning test for an outdoor PVC tent material with the left half coated with TiO2 (manufacturer Taiyo Kogyo) situated in the Photocatalyst Museum, KAST; (d) self-cleaning tiles on MM Towers in Yokohama; (e) self-cleaning glass on Matsushita Denso building (Nippon Sheet Glass); (f) self-cleaning sound-proof wall; (g) self-cleaning tiles and glass in eco-life-type housing; (h) self-cleaning roof on a train station in Motosumiyoshi. (reprinted from Pacheco-Torgal and Jalali, 2011, copyright 2011, with permission from Elsevier), (courtesy of Toto), (courtesy of Sekisui), (courtesy of PanaHome) (courtesy of Taiyo Kogyo) (b–h reprinted from Fujishima et al., 2008, copyright 2008, with permission from Elsevier)
Similarly, the addition of Fe2O3 nanoparticles confers concrete self-sensing capabilities and enhanced compressive and flexural strength (Hui et al., 2004; Li et al., 2004), while incorporating Al2O3 leads to a remarkable increase of the modulus of elasticity (Li et al., 2006c).
Due to their unique mechanical and electrical properties and a high aspect ratio, CNTs are probably the most promising nanomaterial to be used as cement composite. CNTs are likely to enhance cement’s mechanical properties and make it more resistant to crack propagation. Remarkably, CNT-cement composites exhibit electromagnetic field shielding and self-sensing properties that may have numerous applications in the near future (Sanchez and Sobolev, 2010).
Successful incorporation of CNTs into cement requires proper dispersion of the CNT particles and suitable bonding to the cement (Makar and Beaudoin, 2003; Ma et al., 2010). Van der Waals forces make CNTs adhere to one another, making even dispersion difficult. This impediment can be overcome by using specific surfactants, solvents (Forney and Poler, 2011), acid treatment of the nanotubes (Park et al., 2011), sonication (Ma et al., 2010), or surface functionalization (Ma et al., 2010; Sahoo et al., 2010), among other methods. CNT-cement matrix bonding is currently under investigation and is proving to be a great challenge.
Several authors have reported improved properties of CNT-cement composites, including increased compressive and flexural strength, enhanced fracture resistance properties, decreased porosity, increase in the hydration rate, and the development of strong bonds between CNTs and cement (Chaipanich et al., 2010; Konsta-Gdoutos et al., 2010; Li et al., 2005; Makar and Chan, 2009; Luo et al., 2011). In addition to giving increased strength, CNT networks embedded in a cement matrix act as in situ sensors for wireless detection of damage in concrete structures (Saafi, 2009).
15.3.2 Glass, ceramic, metallic and polymer nanocomposites
CNT composites also include polymers, metals and alloys, and glass. Glass CNT composites are promising materials due to their increased mechanical properties while retaining light transmittance. CNTs have been used for reinforcing polymers, such as epoxy, polyurethane, and phenol–formaldehyde resins, polyethylene, polypropylene, polystyrene, and nylon (Fig. 15.7) (Ma et al., 2010; Sahoo et al., 2010). As before, the mechanical properties of CNT composites depend on the dispersion state of the CNTs and their bonding to the matrix, in addition to the properties of the matrix material. It has been demonstrated that the technique employed for CNT dispersion can influence the mechanical properties of CNT–polymer nanocomposite (Ma et al., 2010). In addition, the random orientation and alignment of CNTs can also alter significantly the composite properties (Ma et al., 2010).
15.7 (a) SEM image of a fracture surface in a SWCNT–epoxy resin composite; (b) TEM image of a broken surface of a MWCNT–polymer composite. (reprinted with permission from Ajayan, 1999, copyright 1999 American Chemical Society)
While polymer–CNT composites have achieved some improved mechanical properties, research is ongoing to advance their performance. For example, polystyrene nanocomposites reinforced with 1.0 wt% CNTs with a high aspect ratio exhibited more than 35% and 25% increases in elastic modulus and tensile strength, respectively (Qian et al., 2000). CNTs can also confer self-sensing capabilities to the polymer matrix (Zhang et al., 2006).
Attempts to fabricate CNT–Al metal composites (Kuzumaki et al., 1998; Choi and Bae, 2011) and CNT–ceramic composites (Ma et al., 1998) have also been made. They encountered similar problems as with the concrete–CNT and polymer–CNT composites and additional difficulties due to the higher temperatures required for the matrix materials.
Enhanced scratch resistance can be attained by the addition of silica nanoparticles to polycarbonate. The resulting nanocomposite had a remarkably enhanced scratch resistance and significantly improved hardness compared to simple polycarbonate (Luyt et al., 2011).
The incorporation of copper nanoparticles at the steel-grain boundaries in steel offers the resulting composite materials a higher corrosion resistance and weldability (Agrawal and Agrawal, 2011).
15.3.3 Nanocoatings with scratch resistance, antireflection, anticorrosive, and fireproof properties
Coatings and paints with nanoparticles are probably the most successfully commercialized application to date. This is probably for several reasons: nanoparticles have a greater surface area interaction with the underlying substrate, penetrate deeper than common coatings, have an improved coverage and are transparent to visible light (van Broekhuizen and van Broekhuizen, 2009). Nanocoatings with different particles can confer the substrates various properties, such as water-repellence properties, scratch resistance, UV protection, antireflection, anticorrosive, fireproof, or self-cleaning abilities. These coatings can be applied to almost any kind of surfaces, from plastic and glass, to metals. Paints incorporating nanoparticles are mostly used for their photocatalytic, antibacterial, and self-cleaning properties.
Anticorrosive properties
Nanozinc coatings can have anticorrosive properties (Pereyra et al., 2010).
Fireproof properties
Nanosilica-based coatings have fireproof properties, and magnesium hydroxide nanoparticles together with micro-trihydrated alumina act as flame-retardant pigments (Pereyra et al., 2010). Silica nanolayers sandwiched between two glass sheets confer fireproof properties (Mann, 2006). In addition to photocatalytic abilities, TiO2 coatings can confer flame-retardant capabilities, hindering the ignition or growth of fire (Moafi et al., 2011).
Scratch resistance properties
Scratch resistance properties can be conferred to coatings by the addition of SiO2 or Al2O3 nanoparticles (van Broekhuizen and van Broekhuizen, 2009).
Antireflective and adjustable light transmittance properties
Nanocoatings for glass are also very interesting for indoor climate control by blocking the infrared and visible light. The coating of glass with nanosilica results in antireflective properties, contributing to energy conservation (Troitskii et al., 2010). Nanocoatings can use thermochromic, photochromic, or electrochromic technologies to reversibly change their absorption of infrared light by reacting to temperature, light intensity, or applied voltage (van Broekhuizen and van Broekhuizen, 2009). For example, tungsten oxide and Prussian blue are materials that exhibit electro-chromism–the property of reversibly changing color when placed in an electric field. They are used in smart windows with adjustable visible light transmittance between 75% and 8%, and solar transmittance between 56% and 6% (Kraft and Rottmann, 2009).
15.3.4 Photocatalytic coatings and composites
Photocatalytic nanomaterials like TiO2, ZnO, and other semiconductor materials are very promising for antiseptic properties and pollution remediation. TiO2 is a semiconductor with a band gap of 3.2 eV and 3.02 eV in anatase and rutile forms, respectively (Markowska-Szczupak et al., 2011). Upon excitation by UV light, an electron–hole pair is generated on its surface (Geng et al., 2008). These highly unstable states are very reactive and lead to the conversion of water and oxygen molecules into reactive oxygen species, such as hydroxyl radicals, superoxide ion, and hydrogen peroxide, which chemically react with microbes and pollutant molecules, and degrade them (Linsebigler et al., 1995). The photocatalytic reactions kill microorganisms and ultimately oxidize them to water and carbon dioxide (Hochmannova and Vytrasova, 2010). The hydroxyl radicals have a short lifetime and are produced only on the surface of TiO2 molecules in contact with water, while superoxide ions are long lived (Markowska-Szczupak et al., 2011). Hydrogen peroxide is highly reactive and can penetrate cell membranes. Upon light irradiation and in the presence of water, photocatalytic coatings are able to kill a wide range of bacteria, fungi, algae, viruses, and prions (Markowska-Szczupak et al., 2011; Caballero et al., 2010). TiO2 nanoparticles are very effective in killing a wide range of bacteria, both Gram-negative, such as Escherichia coli (Rincon and Pulgarin, 2004; Liou et al., 2011), Salmonella Typhimurium, and Vibrio cholerae (Berney et al., 2006), and Gram-positive bacteria, such as Staphylococcus aureus (Cheng et al., 2009). TiO2 is also very effective in suppression of antibiotic-resistant bacteria, such as methicillin-resistant Staphylococcus aureus, and bacteria highly resistant to UV light, such as Enterobacter cloacae (Dunnill et al., 2011; Page et al., 2009; Markowska-Szczupak et al., 2011). Under UV irradiation, TiO2 photocatalysts were shown to deactivate most viruses, including herpes simplex virus, hepatitis-B, poliovirus, rotavirus and influenza virus (Markowska-Szczupak et al., 2011).
There are several mechanisms by which photocatalytic nanoparticles are assumed to kill bacteria. First, light-irradiated nanoparticles in contact with the bacterial membrane generate reactive oxygen species that damage the membrane in less than 20 minutes (Liou et al., 2011). Second, nanoparticles smaller than 20 nm penetrate the bacteria and give rise to photocatalytic processes inside bacteria, damaging intracellular components and producing the oxidation or reduction of intracellular Coenzyme A that causes loss of bacterial respiratory activity and cell death (Maness et al., 1999). Most of the studies show that the bactericidal action occurs within hours or earlier, depending on the TiO2 concentration and the amount of light exposure.
Most of the studies of photocatalytic activity of TiO2 have focused on antibacterial activity under UV light; however, the use of this process would be limited only to environments with enough UV light. This impediment can be overcome by doping TiO2 with metals such as Fe, Ag, Ni, Pt, Au, Ag, Cu, Rh and Pd, oxides such as: ZnO, WO3, SiO2 and CrO3, or non-metals such as N, C and S, that can shift their photocatalytic properties in visible light (Markowska-Szczupak et al., 2011; Zhang et al., 2003, 2007; He et al., 2008; Anpo, 2000; Fujishima et al., 2008). For example, the addition of Fe to TiO2 powders in latex paint leads to a redshift into visible light of the absorption threshold of TiO2, making possible the use of photocatalytic paints indoors under visible light irradiation (He et al., 2008). The photo-catalytic sterilization of Escherichia coli with Fe-doped TiO2 paints exceeded 99% in less than 4 h under visible light irradiation.
The photocatalytic reactions occur not only with organic species, such as bacteria and viruses, but with inorganic compounds, such as ammonia, NOx, SOx, NH3, CO gases and VOC (Steyn, 2009; Geng et al., 2008; Maggos et al.,2007, 2008; Chen et al., 2011; Auvinen and Wirtanen, 2008). International pilot projects have tested and demonstrated the benefits of using these photocatalytic composites and coatings in air pollution remediation: Photocatalytic Innovative Coverings Application for Depollution Assessment (PICADA) 2006, and CAMDEN 2007. Experimental data demonstrate that TiO2-treated mortar lowers significantly the levels of NOx gases compared to untreated mortar, between 36% and 82% (Maggos et al., 2008). The coating of several substrates (glass, gypsum, and polymer) with TiO2 based paints demonstrates that VOC remediation via photocatalysis is not influenced by the substrate (Auvinen and Wirtanen, 2008).
Anti-fog capabilities
When moist air cools down on surfaces of mirrors or glass they form water droplets or fog that impairs visual clarity. TiO2–SiO2 based surface coatings can be hydrophilic under UV illumination (Watanabe et al., 1999; Wang et al., 1997). This means that the water spreads evenly across the surface, substantially improving the visual clarity (Fig. 15.8). Automobile side-view mirrors as well as adhesive plastic films using this technology are manufactured in Japan.
15.8 Antifog glass: (a) hydrophobic surface before UV irradiation; (b) highly hydrophilic surface after UV irradiation; (c) hydrophobic TiO2-coated glass with water vapor – the fog or small water droplets impedes the view of the text placed behind the glass; (d) antifog surface after UV irradiation. High hydrophilicity prevents the formation of water droplets, making the text clearly visible. (reprinted by permission from Macmillan Publishers Ltd: Nature (Wang et al., 1997), copyright 1997)
Self-cleaning capabilities
Because the TiO2 embedded in the surface can decompose organic contaminants under ultraviolet or visible light, the surface can clean itself. Moreover, the self-cleaning effect of TiO2 can be enhanced in the presence of water flow, such as natural rainfall (Wang et al., 1998). This enhancing phenomenon is due to the super-hydrophilic property of the TiO2 surface; more exactly, the water penetrates the space between the contaminant and the super-hydrophilic TiO2 surface (Fujishima et al., 2008). Even if the light exposure is not sufficient to decompose the surface contaminants by photocatalysis, the surface will remain clean when flowing water is present. Exterior construction materials that are exposed to plenty of sunlight and rainfall would benefit the most from this technology. Materials with self-cleaning capabilities have been commercially available since the late 1990s, mostly in Japan and lately in Italy. They are very attractive because they can be used to mitigate urban air pollution and confer self-cleaning capabilities to structures. Self-cleaning materials do not require the use of harsh chemicals or solvents, giving a clear environmental benefit (Fig. 15.9(b), (c)). They can be found as photocatalytic concrete and other coating materials for architectural walls (Fig. 15.9(a)), self-cleaning tiles (Figure 15.9(d), (g), (h)), photocatalytic paints, and filters. An architectural structure that uses self-cleaning concrete was completed in 2003 in Rome, Italy. The Jubilee Church (or Dives in Misericordia) in Rome uses Portland cement with photocatalytic additives with the intention of keeping the building clean. The photocatalytic materials are widely used in Japan; several thousand buildings are covered with self-cleaning tiles; Nagoya International Airport uses self-cleaning glass; there are self-cleaning sunshades in parks, bus and train stations; and eco-life style type houses have been marketed since 2003 (Fujishima et al., 2008). Photocatalytic capabilities are also beneficial to air-cleaning filters, that can kill and decompose bacteria and other organic substances instead of accumulating them (Grinshpun et al., 2007).
Photo-road technology
Photocatalysis of TiO2 is able to remove NOx from automobile exhaust, which is becoming a serious source of pollution in urban areas (Ichiura et al., 2003). When colloidal TiO2 solution is mixed with cement, the nitric acid formed during the oxidation of nitrogen oxides reacts with the cement, forming calcium nitrate, a compound that is easily washed off by rainwater. This ‘photo-road technology’ has been tested in Japan for more than 10 years (Fujishima et al., 2008). For example, 300 m2 on a highway in Tokyo was estimated to remove 50–60 mg of NOx per day, corresponding to the amount exhausted by 1000 automobiles. This technology is currently being tested by an Italian cement company in Rome and Paris as well.
Antibacterial applications
Due to their antimicrobial properties, paints or tiles with photocatalytic abilities can be used on the floors and walls of hospitals, operating rooms, childcare centers and food processing centers where sterile conditions are very important (Fujishima et al., 2008). Antibacterial tiles on the walls lead to decreased bacterial counts not only on the walls but in the surrounding air. Antibacterial tiles are already used in Japan in hospitals, hotels and restaurants, among others. Using surfaces with TiO2 as antibacterial agent has the advantage of passive operation, without the need for electrical power or chemical agents, the already available light and oxygen being sufficient. In addition, it will not result in environmental pollution from using harsh chemicals.
Anticorrosion properties
It was found that TiO2 coatings have a protective anticorrosive effect on steel (Ohko et al., 2001). After UV irradiation, the TiO2 coating injected electrons into the steel, this process giving a protective effect against corrosion. The photo-generated holes also play a role in decomposing organic contaminants and provide a self-cleaning function. Later it was observed that TiO2 coupled with WO3 maintained an anticorrosion effect even in the dark for a period of time, due to the energy storage capabilities of WO3 (Tatsuma et al., 2002; Fujishima et al., 2008).
The issues regarding photocatalytic compounds include concerns on the generation of more than just water and carbon dioxide, but pollutants harmful to human health; the lifetime of photocatalytic ability, and last but not least, the possibility of airborne nanoparticulates that can be inhaled and cause significant toxicity.
15.3.5 Antimicrobial coatings
Several types of nanoparticles (Ag, Cu, ZnO, and TiO2) exhibit high toxicity to a broad spectrum of pathogens and can be used as antimicrobial agents. Silver nanoparticles can be incorporated into paints (Lansdown, 2002) and fabrics (Li et al., 2011) and confer them antimicrobial properties.
Silver-based compounds are able to reduce the growth of several kinds of bacteria, including Escherichia coli, Staphylococcus aureus, Leuconostoc mesenteroides, Bacillus subtilis, Klebsiella mobilis, and Pseudomonas aeru-ginosa (Marambio-Jones and Hoek, 2010). In addition, silver nanoparticles have a biocidal activity against other organisms, such as fungi and algae (Marambio-Jones and Hoek, 2010). They were found to have antiviral activity as well, inhibiting the replication of Hepatitis B virus (Lu et al., 2008) and respiratory Syncytial virus (Sun et al., 2008) and HIV-1 virus (Mastro et al., 2010). Ag, Cu, and Ag–Cu nanoparticles strongly inhibit the replication of the HIV-1 virus, not only preventing it from attaching to host cells but by denaturing the resulting proteins of the target organisms by binding to reactive groups and inactivating them (Mastro et al., 2010). An excellent review on the biocidal activity of silver can be found in Marambio-Jones and Hoek (2010).
Silver-based antimicrobial agents are important due to the low toxicity of Ag ions to human cells, having a long-lasting biocidal activity and low volatility, and being thermally stable (Williams et al., 1989). However, studies of Ag nanoparticle toxicity revealed that silver nanoparticle aggregates are more toxic than asbestos (Soto et al., 2005, 2007). Hence, much attention must be paid to the careful incorporation of silver nanoparticles into paints and active surfaces in order to avoid their release into the environment and their uptake by humans.
15.3.6 Insulating materials
Nanomaterials used in insulations are aerogels, made of nanofoam with nano-bubbles or nano-holes. Aerogels are probably the most promising thermal insulation materials for building applications, with a potentially large impact on energy consumption and greenhouse gas emission due to heating (Fig. 15.10). First discovered in the 1930s (Kistler, 1931), aerogels are slowly being developed for commercial applications (Baetens et al., 2011). Aerogels, also known as ‘solid smoke’, are composed of a very small amount (0.2%) of silica (or other material) and 99.8% air (Pacheco-Torgal and Jalali, 2011). With the lowest thermal conductivity of any solid, down to 13 mW/mK, and a high transmittance in the solar spectrum, aerogels are of particular interest in the construction sector for the construction of future high-insulation windows (Baetens et al., 2011).
15.3.7 Nanosensors and actuators
As a result of fatigue and environmental effects, concrete structures suffer damage in the form of cracks. Continuous health monitoring of concrete structures is needed to make decisions regarding their maintenance and repair. The development of nanotechnology opens the avenue for their use as smart advanced sensing materials that can be used for in situ monitoring of the health condition of structures. A conductive network of carbon fibers embedded in concrete will change its resistivity as a function of strain (piezoresistivity) and will act as in situ sensors for the wireless detection of damage in concrete structures. Experimental results indicate that wireless measurement of the resistance of CNTs to change makes possible the early detection of structural damage, such as cracks (Saafi, 2009). In addition to CNTs (Saafi, 2009; Han et al., 2010b), other conductive particles have been researched as cement nanocomposite components with piezoelectric properties, such as Ni (Han et al., 2009, 2010a) and carbon black (Li et al., 2006a; Xiao et al., 2011).
15.3.8 Possible future applications
Among the many applications of CNTs in construction, there are three main areas where CNTs are very promising: their incorporation in existing construction materials (composites), CNT-based cables, and heat transfer systems (Makar and Beaudoin, 2003). The application of CNTs in ropes and cables is currently restricted due to the limited lengths of CNTs. With advances in nanotechnology, if longer CNTs can be fabricated they could be woven into ropes and cables and used for bridges, elevators, etc. CNT aligned composites could be applied in the heating of buildings, taking advantage of the differences in thermal conductivity across and along the tubes, in a new system for heated floors.
15.4 The uptake of nanoparticles and their toxicity
Nanoparticles are ubiquitous on Earth and beyond. The Earth and the Solar System are believed to have formed by the contraction of clouds of molecules and nanoparticles, their condensation into larger particle seeds, and finally the accretion of these seeds. Life on Earth has been exposed to nano-particles since its beginnings. Several billion years of life on Earth and 7 million years of human evolution have led to specific evolutionary adaptations of humans and other life forms to nanoparticle and microparticle intruders. For instance, in humans, any inhaled particle encounters just to cite a few defense mechanisms: (1) at the cellular level the mucociliary escalator, performed by specialized cilia of the respiratory epithelium moving particles away from the lungs towards the upper respiratory tract; (2) at the molecular level, the labeling of non-self nanoparticles with immunological molecules – opsonins (e.g. antibodies or complement molecules) – existing in the lung-lining fluid; labeling facilitates the recognition of nanoparticles by phagocytes – cells specialized in the ingestion and sequestration of foreign intruders by the process of phagocytosis; and (3) phagocytosis by phagocytes and other cells with phagocytic abilities, occurring both in the lungs and throughout the body. Under normal circumstances, the success of these defense mechanisms will depend on the amount, size, and composition of the inhaled nanoparticles. When phagocytosis is impaired, the nanoparticles accumulate and generate oxidative stress and cellular damage. Oxidative stress plays a significant role in the etiopathogenesis of various diseases, such as cancer, cardiovascular, and neurodegenerative diseases. Tissue damage provoked by oxidative stress at the cellular level may evoke acute and chronic inflammation. Inappropriate immune stress is a well-recognized player in complex cardiovascular and respiratory illnesses, such as asthma and chronic obstructive pulmonary disease (COPD).
Currently, our belief is that the main molecular mechanism by which oxidant or transition metals in nanoparticle form elicit their deleterious effects is by shifting the redox balance towards oxidation (Buzea et al., 2007; Unfried et al., 2007). Due to their small size, nanoparticles are able to enter cells by phagocytosis and other mechanisms, such as passive uptake or adhesive interactions mediated by Van der Waals forces, steric interactions, and electrostatic charges (Peters et al., 2006; Geiser et al., 2005). Their uptake can thus occur even in the absence of specific cell surface receptors, and associated subcellular structures. Depending on their intracellular localization, nanoparticles can produce oxidative stress or damage DNA. It is also accepted that nanoparticles may modulate the immune response (Zolnik et al., 2010). Studies have reported that some nanoparticles can enhance the expression of specific viral receptors and lead to excessive inflammation, while others can decrease the expression of certain viral and bacterial receptors, leading to lower resistance to some microorganisms (Lucarelli et al., 2004).
Just how toxic each type of nanoparticle is remains a controversial subject (Schierow, 2008). We do know that the toxicity of nanoparticles depends on several factors, amongst which are chemical composition, crystalline structure, size, and aggregation. Since at nano level the basic physicochemical properties of materials can change with small variations in size, it is virtually impossible to extrapolate their adverse effects from their bulk properties. Very often, a material that is considered nontoxic in bulk form might be extremely toxic at the nanoscale (Buzea et al., 2007). Indeed, much research is needed in order to assess the toxicity spectrum of each material at the nanoscale.
Chemical composition
Nanoparticle composition is important in their chemical interaction with cells. Depending on their composition, nanoparticles may have different cellular uptake mechanisms and intracellular localization and may or may not induce oxidative stress. Studies comparing the toxic effects of various nanoparticles to those of particles with known toxic effects, such as asbestos, indicate that silver nanoparticle aggregates are more toxic than asbestos (Soto et al., 2005, 2007); carbon nanotubes are extremely toxic, having more detrimental effects to the lungs than carbon black or silica nanoparticles. Contrastingly, titanium oxide, iron oxide, and zirconium oxide are less toxic than asbestos (Soto et al., 2005). These effects are summarized in Table 15.1. Once inhaled, some metallic dusts are known to cause respiratory diseases such as asthma, pulmonary fibrosis, or lung cancer (Nemery, 1990). Some metallic dusts are found in the brain matter of patients affected by neurological diseases such as Alzheimer’s and Parkinson’s disease (Wang et al., 2009; Antonini et al., 2006; Weiss, 2006). Furthermore, the exposure to certain dusts containing silica or asbestos is linked to autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus (Noonan et al., 2006).
Table 15.1
Relative cytotoxicity index (RCI) of nanoparticles with different composition on murine macrophage cells (after Soto et al.n 2005)
*SWCNT = single-walled carbon nanotubes; MWCNT = multi-walled carbon nanotubes.
Crystalline structure
The toxicity of nanoparticles varies greatly with the crystalline structures for particles with the same composition. While nanoparticles with specific crystalline structures are benign, other allotropes can prove very detrimental to health. For instance, titanium dioxide allotropes – rutile and anatase – have different levels of toxicity (Soto et al., 2005). Rutile nanoparticles induce oxidative DNA damage in the absence of light, while anatase nanoparticles of the same size (200 nm) do not (Gurr et al., 2005). Other studies demonstrated that 10–20 nm nanoparticles of TiO2 stimulated reactive oxygen species production even in the absence of UV light (Long et al., 2006). When inhaled, TiO2 dust has been classified by the International Agency for Research on Cancer (IARC) as a substance possibly carcinogenic to humans (Markowska-Szczupak et al., 2011).
Size
Size is a very important factor in determining nanoparticle toxicity. In general, smaller nanoparticles of the same composition are able to pass in an easier manner through physiological barriers and reach organs than their larger counterparts. In addition, nanoparticles with sizes smaller than 100 nm are not effectively phagocytized compared to larger nanoparticles, and are therefore able to evade this defense mechanism. Experiments on animal models suggest that smaller nanoparticles produce higher inflammatory reactions in rat lungs than larger nanoparticles (Oberdorster et al., 1994). The larger surface area of a given volume of small nanoparticles causes more oxidation and DNA damage than an equal volume of larger particles (Buzea et al., 2007).
Shape
As previously mentioned, shape is also an important determinant of the uptake and cytotoxicity of nanoparticles. The internalization rate of a particle will change depending on the aspect ratio if the composition and size are kept constant (Huang et al., 2010). CNT pulmonary exposure results in the biopersistence of CNTs within the lungs for up to several months (Deng et al., 2007; Elgrabli et al., 2008). The higher the aspect ratio, the higher the toxicity of a material tends to be, as shown in Table 15.1.
Aggregation
Experimental studies indicate that a higher concentration of nanoparticles resulting in aggregates is not as toxic as smaller concentrations of the same nanoparticles that fail to coalesce. Nanoparticles in small concentrations have a higher probability of being distributed to the circulatory system and organs, while a high concentration of nanoparticles leads to the formation of larger aggregates that can be phagocytized more efficiently (Takenaka et al., 2001). However, a very large concentration of nanoparticles is likely to produce lung injury, due to the higher rate of accumulation than clearance. Depending on nanoparticle type, the clearance can occur within months or years, while some nanoparticles might never be eliminated. Longer residence times of nanoparticles within the lungs, circulatory system, and organs result in greater occurrence of negative health effects; this is particularly relevant for nanoparticles with mutagenic potential.
Surface chemistry
Nanoparticle surface chemistry is a major factor affecting the response of biological systems to nanoparticle exposure, and largely determines particle distribution within the body (Araujo et al., 1999) as well as their compatibility with the immune system (Dobrovolskaia and McNeil, 2007). The toxicity profile of a material in nanoparticle form can be drastically affected by its surface functionalization, which can render toxic nanoparticles nontoxic and vice-versa.
15.5 Diseases associated with nanoparticle exposure
The dimensions of nanoparticles are similar to those of viruses and some small bacteria, and analogous to these microorganisms, they have the ability to affect cellular processes and cause disease. Their minute size allows them to translocate to organs (Sonavane et al., 2008) and undergo cell uptake (Buzea et al., 2007). An example of cellular internalization is shown in Fig. 15.11. The potential effects of nanoparticle exposure on cellular processes are diverse, ranging from uncontrolled cell proliferation, as occurs in cancer, to premature cell death, as seen in neurodegenerative diseases. Many nano-particles have a significant genotoxic effect (Landsiedel et al., 2009).
15.11 Uptake of Fe2O3 nanoparticles by human aortic endothelial cells. (reprinted from Zhu et al., 2011, copyright 2011, with permission from Elsevier)
Responses to nanoparticle exposure vary greatly between individuals, as both epigenetic and genetic factors as well as pre-existing diseases contribute to the severity of the ensuing health effects. Pre-existing diseases, such as asthma or diabetes, may facilitate the uptake of nanoparticles and increase their translocation to organs (Buzea et al., 2007). The adverse health effects associated with nanoparticle exposure can take anywhere from several hours up to many years to manifest clinically.
Exposure pathways together with the most severe adverse health effects associated with nanoparticle exposure are summarized in Fig. 15.1. The range of pathologies associated with nanoparticle exposure encompasses respiratory, cardiovascular, lymphatic, autoimmune, gastro-intestinal, and nervous system diseases. Interestingly, specific nanoparticles seem to be linked to some diseases with unknown etiology, such as autoimmune (Noonan et al., 2006), Crohn’s (Gatti, 2004; Lomer et al., 2002), Alzheimer’s and Parkinson’s disease (Weiss, 2006; Kawahara, 2005; Miu and Benga, 2006; Quintana et al., 2006). Scanning electron microscopy has identified micro- and nanoparticle debris in organ tissue and circulating blood of patients with worn orthopedic implants (Gatti and Rivasi, 2002), blood disease (Gatti et al., 2004), colon cancer, Crohn’s disease, ulcerative colitis (Gatti, 2004), and idiopathic diseases (Gatti and Rivasi, 2002). Autopsy results revealed that coal workers have an increased amount of particles in the liver and spleen compared to non-coal workers (Donaldson et al., 2005). Animal studies with inhaled stainless steel welding fumes indicate that manganese accumulates in the blood and liver (Donaldson et al., 2005). Experiments in rats have shown that following inhalation exposure with 4–10 nm silver nanoparticles, within 30 min the nanoparticles enter the circulatory system, and after one day can be found in the liver, kidney, and heart (Takenaka et al., 2001).
Experimental studies have demonstrated that inhaled nanoparticles with diameter smaller than 30 nm can reach the brain via the olfactory nerves or blood–brain barrier (Borm et al., 2006). Multiple studies suggest that the accumulation in the brain of a high concentration of metals (such as copper, aluminum, zinc, manganese, and iron), together with oxidative stress, may initiate and promote the development of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease (Buzea et al., 2007; Weiss, 2006; Kawahara, 2005; Miu and Benga,2006; Quintana et al., 2006). Epidemiological reports indicate possible association between manganese dust inhalation and the earlier than expected occurrence of neurological diseases in miners (Weiss, 2006) and welders (Antonini et al., 2006). Currently, it is unknown if the high concentration of metals in the brain of patients with neurodegenerative diseases is due to the migration to the brain of nanoparticles themselves, or their soluble components followed by aggregation. Nonetheless, studies on animals and autopsy reports indicate that exposure to particulate pollution produces chronic brain inflammation and pathological findings similar to those of the early stages of Alzheimer’s disease (Peters et al., 2006). The number of deaths caused by Alzheimer’s disease shows a linear increase in recent years, from less than 1.9% in 1999 to 3% in 2006 of the total number of deaths per year in the United States (Anderson, 2001, 2002; Anderson and Smith, 2003, 2005; Heron and Smith, 2007; Heron, 2007; Kung et al., 2008).
Nanoparticles that reach the lungs and are not removed via mucociliary clearance or phagocytosis will accumulate and/or pass into the circulatory and lymphatic systems. Inhalation of nanoparticles is associated with respiratory diseases such as asthma, pulmonary fibrosis, emphysema, mesothelioma, and lung cancer (Buzea et al., 2007). A large body of epidemiological research supports the role of particulate pollution in causing these adverse effects. In general, exposure to pollution nanoparticles with various compositions, aggregations, and sizes, is associated with pulmonary and cardiovascular diseases (Buzea et al., 2007). Elevated levels of particulate pollution are linked to higher numbers of hospital admissions for respiratory illnesses, such as pneumonia, bronchitis, and lung cancer (Iwai et al., 2005), and to increased mortality due to either cardiopulmonary failure or lung cancer (Pope et al., 2002). Indeed, hospital admissions for cardiovascular illness increase on days with higher levels of particle pollution (Schwartz and Morris, 1995). There is significant correlation between inhalation of nano- particulate matter and various cancers (lung, breast, endometrium, and ovary) (Iwai et al., 2005; Knox, 2005).
Due to their small size, inhaled nanoparticles can cross physiological barriers and enter the circulatory system. Experimental evidence suggests that inhaled metallic nanoparticles smaller than 30 nm pass easily into the circulatory system (Geiser et al., 2005; Takenaka et al., 2001; Donaldson et al., 2005; Oberdorster et al., 2005). Within the circulatory system nanoparticles can interact with blood and endothelial cells, potentially leading to circulatory diseases such as arteriosclerosis and thrombus formation. Within 30 minutes of exposure, intra-tracheally instilled 30 nm gold nanoparticles have been found within pulmonary platelets in rats (Oberdorster et al., 2005), indicating a possible connection between nanoparticles and the experimental formation of thrombus. It is interesting to note that nanoparticles of various compositions such as gold, silver, cobalt, titanium, tungsten, nickel, zinc, barium, iron, chromium, silicon, glass, talc, and stainless steel have been collected from explanted vena cava filters of patients deemed to be at risk of developing thrombo-emboli (Fig. 15.12(e), (f)) (Gatti et al., 2004). Furthermore, the same study reported that nanoparticles were present within microthrombi trapped at the vertex of the implanted vena cava filters. Human aortic endothelial cells have been shown to release inflammatory mediators upon the uptake of Y2O3 or ZnO nanoparticles (Gojova et al., 2007). A large body of literature supports the notion that nanoparticles with their pro-inflammatory properties may lead to the induction of heart arrhythmia and cardiac failure (Vermylen et al., 2005).
15.12 Nano- and microparticles found in colon cancer tissue from human biopsies: (a) ceramics (filosilicates); (b) stainless steel; (c) silver and (d) silicate; and debris entrapped inside tissue formed around a filter inserted in the vena cava for 156 days: (e) debris of Fe-Cr-Ni; (f) debris of Pb-Cu. (reprinted from Gatti, 2004, copyright 2004, with permission from Elsevier), (with kind permission from Springer Science + Business Media: Gatti et al., 2004)
The size of nanoparticles entering the circulatory system and organs is partly determined by the junction sizes of the endothelial cells lining the vascular system. These junctions range between 2 and 100 nm in size, depending on the organ or tissue (Hussain et al., 2001; Schwab and Pang, 2000). However, capillary permeability considerably increases in the presence of inflammation and in subjects with pre-existing respiratory and circulatory diseases, allowing more and larger nanoparticles to enter into circulation. Inhaled nanoparticles have been shown to reach organs, translocating from lungs to blood (Geiser et al., 2005; Takenaka et al., 2001; Donaldson et al., 2005; Oberdorster et al., 2005), liver, kidneys (Takenaka et al., 2001; Gatti and Rivasi, 2002; Ballestri et al., 2001), spleen (Jani et al., 1990), brain (Peters et al., 2006; Takenaka et al., 2001), and heart (Takenaka et al., 2001). Ex vivo experiments using a human placenta perfusion model have shown that polystyrene particles with diameter smaller than 240 nm are able to cross the fetus–placental barrier, establishing the potential risk of nanoparticle exposure in utero (Wick et al., 2010). Experiments on mice demonstrate that intravenously injected 70 nm silica and 35 nm titanium dioxide nanoparticles, respectively, are associated with pregnancy complications in pregnant mice (Yamashita et al., 2011). These nanoparticles cross the placental barrier, being found in the fetal liver and fetal brain (Yamashita et al., 2011).
In addition to inhalation, nanoparticles can also be ingested. Common sources of exogenous nanoparticles in the gastro-intestinal tract are food additives and colorants (such as titanium dioxide), pharmaceuticals and cosmetics (toothpaste and lipstick), dental prosthesis wear, and inhaled nanoparticles (Buzea et al., 2007). Contaminants may also be an important source of ingested nanoparticles (Gatti et al., 2009). Experiments on animals performed by Jani et al. showed that oral uptake of polystyrene microspheres results in systemic absorption and size-dependent localization to organs, including the liver, spleen, blood, and bone marrow (Jani et al., 1990). The length of stay of particles can also become a determinant factor on their toxicity. Indeed, nanoparticles have been found consistently in the gastro-intestinal tract, blood, and organs of patients with Crohn’s disease, ulcerative colitis, and colon cancer (see Fig. 15.12(a-d)) (Gatti, 2004). It is remarkable that the nanoparticles found in diseased subjects have various chemical compositions (carbon, ceramics, silicon, stainless steel, silver and zirconium, among others) and are not toxic in bulk form (Gatti, 2004). The sources of nanoparticles in the gut of patients with Crohn’s disease are both anthropogenic and natural, and include titanium dioxide from food additives, aluminosilicates from natural clay, and environmental silicates (Powell et al., 1996). Taking into account the possible adverse health effects that nanoparticles may elicit in the gastro-intestinal tract and their lack of nutritional value, the commercialization of dietary nanoparticles should be strictly regulated.
Skin constitutes a third portal of entry for nanoparticles into organisms. Though a controversial subject, experimental evidence indicates that specific nanoparticles are able to penetrate skin, with their cytotoxicity dependent on their composition. Skin uptake of soil nanoparticles has been shown to result in the impairment of the lymphatic system, due to inadequate fluid drainage in podonconiosis (Blundell et al., 1989). In vitro experiments have evidenced the uptake of multi-wall carbon nanotubes by human epidermal keratinocytes (Monteiro-Riviere et al., 2005). Both in vitro and in vivo experiments suggest that sun-illuminated titanium dioxide nanoparticles from sunscreens catalyze DNA damage, and produce inflammation in human endothelial cells (Serpone et al., 2001; Dunford et al., 1997). Additionally, silver nanoparticles, used in pharmaceuticals as antibacterial wound dressings, are not only bactericidal but may also be toxic to kerati-nocytes and fibroblasts (Poon and Burd, 2004).
The relationship between nanoparticle exposure and the immune response is not well understood. Exposure to silica and asbestos dusts is linked to autoimmune diseases with unknown etiology, such as rheumatoid arthritis and systemic lupus erythematosus (Noonan et al., 2006; Pfau et al., 2005). Recent research indicates that, depending on the material, size, and physico-chemical properties, various nanoparticles can either suppress or stimulate the immune response (Dobrovolskaia and McNeil, 2007; Zolnik et al., 2010), and inhibit (Ryan et al., 2007) or exacerbate allergic reactions (Dobrovolskaia and McNeil, 2007; Dwivedi et al., 2009). For instance, fullerenes have antiallergic properties, inhibiting allergic responses in vitro and anaphylaxis in vivo (Ryan et al., 2007). Carboxyfullerenes were found to enhance the ability of neutrophils to destroy specific Streptococcus pyogenes (Dobrovolskaia and McNeil, 2007). Functionalized polylactic-glycolic acid nanoparticles reduce inflammation in arthritis animal models (Dobrovolskaia and McNeil, 2007). One must note that while some nano-particles could be used to suppress the immune response in the treatment of inflammatory diseases, some undesirable immunosuppression could be detrimental in managing cancer and infections.
The composition, size, and other physico-chemical properties of nanoparticles, determine their ability to penetrate cells, affect organelles, and influence basic cellular processes, such as the cell cycle and cell death. Experimental evidence shows that nanoparticles are internalized by a variety of cell types, ranging from alveolar macrophages (Takenaka et al., 2001; Hoet et al., 2004), platelets (Nemmar et al., 2002), red blood cells (Peters et al., 2006; Rothen-Rutishauser et al., 2006), and endothelial cells (Gojova et al., 2007), to nerve cells (Hutter et al., 2010). Nanoparticles have been found at different locations within cells, such as the cytoplasm, mitochondria (Xia et al., 2006), and nucleus (Porter et al., 2006). The cytotoxicity varies with a multitude of factors, including the cell type, nanoparticle composition, size, surface chemistry, and aggregation. In general, the cytotoxic effect emerges with increasing dose and length of exposure, in a manner which varies with nanoparticle composition. Associated increases in cell death are related to cell internalization and subsequent generation of reactive oxygen species. There is not clear evidence whether nanoparticles can interact with surface or intracellular receptors. We consider that it is reasonable to speculate that some nanoparticles may interact with pattern recognition receptors (PRRs) such as the toll-like receptor (TLR) family of genes at the cell surface, or once internalized they may also interact with the NOD-like receptor (NLR) family of genes triggering cytotoxic pathways (Martinon et al., 2009). It is also plausible that danger-associated molecular patterns (DAMP) released from injured cells inflammasomes, as observed with microparticles of asbestos and silica, may amplify and perpetuate the inflammatory response.
Beneficial effect of some nanoparticles
It is important to note that while most nanoparticles may be potentially toxic, there are significant exceptions (Buzea et al., 2007; Connor et al., 2005; Goodman et al., 2004). Indeed, some nanoparticles appear to have antioxidant properties, and hence are beneficial to human health (Buzea et al., 2007; Bosi et al., 2003; Schubert et al., 2006). Examples include functionalized fullerenes (Bosi et al., 2003) and nanoparticles of non-stoichiometric rare earth oxides (CeO2 and Y2O3) (Schubert et al., 2006), which are neuroprotective and exhibit consistently anti-apoptotic activity in various cell types due to their antioxidant properties.
Surface functionalization has been used to render toxic nanoparticles nontoxic, making them valuable tools for use at the molecular level. This technique is currently used to fabricate contrast agents in molecular imaging and designing artificial molecular receptors (Asiyanbola and Soboyejo, 2008). Further possible applications of this technique include targeting specific malignant cells or microbes. Nanoscale drug delivery systems targeting cancer are used to deliver chemotherapy drugs with lower systemic toxicity, improved selectivity and specificity (Malam et al., 2009). The toxicity of some nanoparticles to specific microorganisms makes them important antiseptic nanotools (Lara et al., 2010). Silver, titanium dioxide, zinc oxide, and magnesium oxide nanoparticles all exhibit antibacterial activity, while fullerenes and silver nanoparticles have antiviral activity. In addition, sea-salt aerosols resulting from water evaporation are probably among the few natural nanoparticles with beneficial health effects, helping restore the mucociliary clearance in patients with respiratory diseases (Ballard et al., 2006).
15.6 Detection of occupational nanoparticles and remedial action
Airborne nanoparticles may pose a health risk to workers as well as to consumers of products containing nanoparticles that may be released into the environment. The National Institute for Occupational Safety and Health (NIOSH) developed a ‘Nanoparticle Emission Assessment Technique’ (NEAT) that uses a combination of measurement techniques and instruments able to assess the inhalation exposure to nanoparticles (Methner et al., 2010a, 2010b). They used NEAT to assess potential inhalation exposure of nanoparticles in facilities producing and handling nanomaterials, such as CNTs, fullerenes, metal oxides, and quantum dots (Methner et al., 2010a). It was found that occupational exposure to CNTs occurs during their handling and transportation. Precautions should be taken during the weighing, mixing, collection, manual transfer, cleaning operations, drying, spraying, chopping, and sonication of CNTs (Methner et al., 2010a; Stella, 2011).
Unfortunately, currently there are no laws or regulations to impose occupational exposure limits to most nanoparticles (Hallock et al., 2009). This is mostly due to the swift commercialization of products containing nanoparticles and a slower pace of research into their toxicity mechanisms. In order to avoid possible inhalation of nanoparticles, much care should be taken during all the fabrication stages involving nanoparticle composites and coatings, and the wearing of protective equipment and filters should be mandatory (Fig. 15.13). In addition, Material Safety Data Sheets (MSDSs) should be developed and provided for the most hazardous nanoparticles (van Broekhuizen and van Broekhuizen, 2009).
15.13 Technician cleaning plastic sheets with ink containing SWCNTs. (courtesy Mary Schubauer-Berigan and Matt Dahm, NIOSH)
Until toxicology information becomes available for all types of nanoparticles, workers using nanoparticles should prevent inhalation, dermal and ingestion exposure (Hallock et al., 2009). Studies show that airborne nano-particles can stay suspended in air for days or weeks and if inhaled about 35% will be deposited into the lungs (Maynard and Kuempel, 2005). Best practices in preventing nanoparticle exposure are summarized below (Hallock et al., 2009):
• Inhalation exposure prevention: handling of nanoparticles in a fume hood; the use of HEPA filters; transport in sealed containers.
• Dermal exposure prevention: use preferably disposable gloves or double gloves, laboratory coats and eye protection.
• Cleaning procedures: wet-wipe surfaces after use; do not sweep or use compressed air for cleaning; use HEPA vacuum cleaners for spills.
• Disposal: dispose of nanomaterials and nanomaterial-contaminated laboratory materials as hazardous waste labeled as nanoparticles.
• Toxicity information: obtain current toxicity information on nanomaterials because MSDSs most often report health effects of micron-sized materials, while nanoparticles are more toxic.
15.7 Sources of further information and advice
Several excellent reviews on the application of nanotechnology in constructions are Fujishima et al. (2008), van Broekhuizen and van Broekhuizen, Raki et al. (2010), Allen et al. (2005), Hochmannova and Vytrasova, Lee et al. (2010), Mann (2006), Bystrzejewska-Piotrowska et al. (2009), Makar and Beaudoin (2003), Sanchez and Sobolev (2010), Steyn (2009), and Pacheco-Torgal and Jalali (2011). Some excellent reviews regarding nanoparticle toxicity are Borm et al. (2004, 2006), Buzea et al. (2007), Bystrzejewska-Piotrowska et al. (2009), Crosera et al. (2009), Dobrovolskaia and McNeil (2007), Donaldson et al. (2005), Fubini et al. (2010), Hoet et al. (2004), Oberdorster et al. (2005), Sharma (2010), Shvedova et al. (2010), Simko and Mattsson (2010), Sioutas et al. (2005), Stone et al. (2007), Vermylen et al. (2005), Warheit et al. (2008), Xia et al. (2009), and Peralta-Videa et al. (2011).
15.8 Conclusion and future trends
We are just discovering the wonderful properties of materials at the nanoscale, and bottom-up approaches will allow the fabrication of novel nanostructures and nanotechnologies that will likely be implemented in all the areas of our life. There is no doubt that nanotechnology will revolutionize the construction industry, with lighter and stronger materials, and smart sensing structures, with self-cleaning antimicrobial surfaces, that improve energy conservation and remediate pollution in the cities of the future. However, without rules and regulations for the handling and disposal of nanomaterials they might have an adverse impact on our society. The pace at which nanomaterials find their way to new applications does not parallel the pace of research that should be directed towards ensuring their safety before they reach the hands of consumers. Current research indicates that many nanoparticles are toxic to living organisms. Their toxicity depends on various physico-chemical characteristics whose relative importance is still unknown. Nanoparticles are associated with a wide range of diseases that can manifest themselves immediately following exposure or up to many years later. While today the main focus of criticism is on engineered nanoparticles due to their toxicity and potential environmental impact, we must emphasize that particulate pollution from vehicle exhaust and from incomplete combustion of various fuels currently constitutes the majority of anthropogenic nanopollutants associated with adverse health effects. Because it is impossible to extrapolate the toxicity of a nanomaterial from the properties of the bulk material, a case-by-case approach is required in order to identify hazardous nanomaterials. Thus, sustained research programs are needed to determine the toxicity spectrum of newly developed materials in nanoparticle form. Each type of nanoparticle has to be treated as a unique material, and its toxicity investigated prior to its use in consumer products or as medicine. It is not only the responsibility of the industry, but ours as society, to ensure that the safety of all materials is assessed prior to their commercialization and the appearance of deleterious effects.
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