Types of Nanomaterials and Their Main Applications
ENMs have found a range of applications in the chemical, textile, cosmetics, electronics, sports, and automotive industries as well as in the fields of medicine, energetics, biotechnology, environmental protection, telecommunications, and transportation [Sahu 2016; Inshakova and Inshakov 2017]. Thanks to nanotechnology, a wide range of unique products are already available, e.g. extremely durable construction materials, compact computers with immense data processing capabilities, quantum computers, thin optical fibers, dichroic glass, high-efficiency microchips, carbon nanotube antennae that collect light in the visible spectrum, super-slippery coatings, walkways and non-woven textiles, air purification systems, specialized biomaterials and biomarkers, as well as drug carriers. The following section is a review of the types of nanomaterials and their applications [EC 2012; EPA 2017; Dreaden et al. 2012; Gajanan and Tijare 2018].
This class of nanomaterials includes mainly nanofibers and carbon nanotubes, fuller- enes, carbon black, and graphene. Carbon nanofibers are almost entirely composed of stretched carbon structures, chemically similar to graphite. Their high mechanical strength is due to a highly organized chemical and geometric structure. These materials are also infusible, chemically resistant, as well as resistant to wear and sudden changes in temperature. Carbon nanofibers have mainly found applications as a dispersive phase material in nanocomposites (carbon-metal, carbon-ceramic, carbon-polymer, and carbon-carbon) or composite materials in which nanostruc- tured materials constitute the dispersive phase. They are also used in light constructions, lithium-ion batteries, fuel cells, vibration damping materials, filtration fabrics, and fuel lines.
Carbon nanotubes (CNTs) are composed of one or more layers of carbon atoms, rolled up in a hexagonal lattice analogous to that in graphite, constituting either long single-wall carbon nanotubes (SWCNTs) or multi-wall carbon nanotubes (MWCNTs). They exhibit high electrical and thermal conductivity and a high strength-to-w'eight ratio. These materials have found applications in, among others, disk drives and automobile fuel lines. They are also used as polymer additives in paints, fuel cells, electrodes, electrolytes, and battery membranes.
Fullerenes are molecules composed of an even number of 60 or more carbon atoms, creating a polycyclic system (as a kind of nanotubes closed at both ends).
Their applications are chiefly in the biomedical field (as contrast agents and in drug delivery systems).
Carbon black is a black powder containing 80-95% amorphous carbon. The majority of these particles are in the 1-100 nm range as well as in the agglomerated state. The material is used mainly for the production of tires, pigments, inks, antistatic fillers, decorative fibers, electrodes, and carbon brushes.
Graphene exhibits both the properties of metals and semiconductors but is not, in fact, classified under any of these categories. It is, however, an excellent conductor of heat and electricity. Graphene is also characterized by low resistivity, high opacity, and high tensile strength. Its current applications include integrated circuits, transistors, transparent conductive electrodes, solar and fuel cells, and a range of sensors and filters, e.g. for the sorption of heavy metal ions from contaminated waters.
Metals and Metal Alloys
The most popular, currently produced metal nanoparticles are gold, silver, iron, copper, and titanium. The most common metal alloys are platinum and palladium. Gold nanoparticles are most often used in medical applications such as diagnostics. They are also used in optics, solar cell technology, lubricants, catalytic converters, sensors, and special coatings.
Nanosilver is mainly used for its antibacterial, antifungal, and antiviral properties (products typically containing nanosilver are used in hospital textiles, dressings, containers for contact lenses, sports clothing, odorless undergarments, cosmetics, etc.). Other commercially available metals often used as nanomaterials are, among others, platinum nanoparticles, platinum and palladium alloys (in electronics and chemical catalysis and copper nanopowder for printed electronics and inks), iron nanoparticles (magnetic recording tapes), and titanium alloys (medical implants and materials used in the automotive, aviation, and space industries).
Inorganic Non-metallic Nanomaterials
The most common commercially available inorganic and non-metallic nanomaterials are synthetic amorphous silica, titanium dioxide, zinc oxide, aluminum oxide, aluminum hydroxide, iron oxides, cerium dioxide, and zirconium dioxide. Various nanoforms of synthetic amorphous silica have found applications in sectors such as textiles, leather, paper, cosmetics, food, electronics, and construction as well as in the production of detergents, paints, and varnishes. They are also used to reinforce elastomers (mainly tires), shoes, and cable shields, for crude oil refining, and as drying agents to protect products during transportation and storage. Titanium dioxide nanoparticles are good UV radiation filters. TiO, nanoparticles have special electrical, photocatalytic, and antimicrobial properties. This material is used in sunscreens, plastic and metal coatings, varnishes for maintenance, solar cells, catalysts, protective coatings, the so-called self-cleaning products (windows, cements, tiles, hospital textiles), deodorants, and air purification systems.
Zinc nano-oxide is a colorless and effective UV filter but in a different spectrum than Ti02. It is used as an active agent in self-cleaning products, sunscreens, varnishes, ceramics, and electronics, as well as in liquid crystal displays and solar cells.
Aluminum oxide nanoparticles increase the coatings’ wear resistance and are used as fillers in polymers and tires as well as in protective glasses and scratch- resistant windows, floors, bar code scanners, precise optical components, catalysts, ceramic filtration membranes, and flame retardants.
Iron oxide nanoparticles have found a number of applications as components of pigments, polishes, catalysts, fuel cells, oxygen sensors, and optoelectronic equipment as well as in water purification systems and the remediation of soil and ground waters.
Cerium (IV) nano-oxide has special optical properties due to which it has found applications in optical, electrooptical, microelectronic, and optoelectronic devices. It is used to polish silicon wafers and glass surfaces as an anticorrosive material and is also used as a catalytic diesel fuel additive.
Zirconium nano-dioxide is used in optical connectors, fuel cells, lithium-ion batteries, catalysts, ceramic membranes, cements, dental fillers, dentures, fluorescent lamps, and polishes. This material is sintered in the powder form of ceramic materials with unique properties, chiefly high fracture toughness.
Nanoclays are nanoparticles of layered mineral silicates, such as kaolinite, bentonite, montmorillonite, hectorite, and halloysite. Nanoclays have found applications as components of tires, paints, inks, greases, polymer nanocomposites, cosmetics, and drug carriers.
Nanopolymers and Dendrimers
There are currently commercially available polymer nanofilms, nanotubes, nanowires, nanorods, and nanoparticles. Polymer nanoparticles are polymer units used in the nanoscale. They are used in drug carriers and as fillers in polymer composite matrices. Nanostructured polymer films are used as coatings, e.g. in biomedical applications. Polymer nanotubes, nanowires, and nanorods have potential applications in sensors and in micromechanical, electronic, magnetic, optic, and optoelectronic devices.
Currently, work is being carried out on the development of conductive textiles, wearables (wearable devices), and “intelligent fibers”, which change their properties depending on the environmental conditions.
Dendrimers are a distinct group characterized by specific polymer structures. They are fractal-like, with a regular, branched structure and a high specific surface area. Dendrimers have found applications in drug concentration controllers, diagnostic tests, liquid diffraction grids, lasers and light-emitting diodes, catalysts, or semi-permeable membranes.
Quantum dots are crystalline semiconductors with dimensions from 2 to 10 nm, and their electrical properties depend on the shape and size of individual crystals.
Quantum dots are particles small enough to have their properties significantly changed after adding a single electron. The fluorescence of quantum dots can be controlled by modifying their size and geometry. Because of their absorption spectrum tuning and a high extinction coefficient, quantum dots are already used, or are planned to be used, in photovoltaic devices, lasers, LED diodes, photodetectors, sensors, and single-photon sources. Due to properties resulting from their nanometric dimensions, they are more stable and precise as markers in medical diagnostics than organic dyes used to date. Plans are underway to use them in drug carriers, for tracking viruses in the body, and in single-electron transistors. They are most commonly obtained from indium phosphide, cadmium selenide, and sulfide.
Nanomaterials can constitute the dispersive phase in composites, particularly in a polymer matrix. The sheer number of possible combinations of materials and their constituents, as well as their weight ratios in the final products, makes it highly complicated to single out particular nanocomposites for consideration in this monograph. They also significantly differ in material properties, making it impossible to list those most important for health safety. However, it is important to take note of nanocomposites as a group of materials. They are mainly used in the production of high-strength sport and construction materials, self-cleaning surfaces, conductive polymers, and bone implants.
Impact of Nanomaterials on Human Health
Effects of Specific Properties of Nanomaterials on Toxicity
Nanomaterials can have toxic effects on the human body depending on their chemical nature and physical properties, the most significant of which are size and shape of the particles, their surface area, the state of aggregation and agglomeration, solubility, surface charge, surface modifications, crystalline structure, etc. The properties of substances with particles in the nanoscale are different from their bulk counterparts in many respects. Nanoparticles have a relatively low mass, an extensive specific surface area (the outer area of a solid substance in relation to the mass of the substance (m2/g)), varying chemical reactivity, a larger capacity for oxidation, a different surface charge, and varying solubility in liquids.
A number of these properties may influence the behavior of the nanoparticles in living organisms. Nanogold is an apt example. In the bulk form, gold is a yellow metal, which does not interact with biological materials; however, nanogold is purple or red, depending on the grain size, and it easily binds to proteins. This is why nanogold can be seen as both an anticancer medicine and as a potentially toxic substance.