In the last 150 years, humanity has advanced significantly in terms of technology. Not only the technology has become more efficient in terms of performance, but there is a significant difference between the dimensions of modern technology and its historical counterparts. Since the invention of the microprocessor, modern technology has moved us forward to build smaller and faster products. Engineers and scientists all over the world are pushing technology’s physical boundaries to make it more portable, efficient, and accessible. Richard Feynman, a physicist, proposed in the 1950s that by reducing materials to their simplest form, more may be learned about them. This idea laid the foundations for nanomaterials– the concept of manipulating matter at an atomic or molecular level. The word “nano” derives from the Latin word “nanus,” indicating a person of very low height, i.e. a dwarf. The International System (SI) of units considers nano as a prefix to indicate {10}^{−9} parts of a unit, e.g., a billion of a meter, a billion of a liter, a billion of a kilogram, etc. Nanomaterials are conventionally defined as materials having at least one dimension between 1 and 100 nm. Recent advances in fields like microscopy have provided scientists with new tools to better understand and exploit phenomena that occur naturally when the matter is organized at the nanoscale. These phenomena are based on “quantum effects” and other simple physical effects like increased surface area. Furthermore, because the majority of biological processes take place at the nanoscale, scientists can use models and templates to imagine and build new processes that will benefit their work in medicine, imaging, computing, printing, chemical catalysis, materials synthesis, and a variety of other fields.
Types of Nanomaterials
The umbrella field of nanosciences and nanotechnology consists of a wide range of nanomaterials with various physical, chemical, mechanical, optical, magnetic, and biological properties and different internal and external structures. Although various organizations have proposed several frameworks for the categorization of nanomaterials, a complete internationally agreed terminology is yet to be formed. There are two basic classification systems for different types of nanomaterials.
Classification based on Structural Dimensionality
Zero Dimensional Nanomaterials
Zero-dimensional (0-D) structures include materials with all dimensions at nanoscales of 1 to 100 nm. Most of these materials are spherical in shape; however, cubes and polygonal shapes with nano-dimensions are also found under this class.
One Dimensional Nanomaterials
One-dimensional (1-D) structures are materials with two dimensions at the nanoscale and the other dimension is beyond the nanoscale (>100 nm), meaning that one dimension is outside the nanoscale.
Two Dimensional Nanomaterials
Two-dimensional (2-D) structures are materials with one dimension at the nanoscale, and two of the dimensions are not confined to the nanoscale. 2-D nanomaterials exhibit platelike shapes and can be
Three Dimensional Nanomaterials
Three-dimensional (3-D) structures are materials having three arbitrary dimensions beyond the nanoscale (>100 nm). However, these materials possess a nanocrystalline structure or involve the presence of peculiarities at the nanoscale. They can be composed of multiple arrangements of nanosize crystals, most typically in different orientations.
Classification based on Chemical Composition
Organic Nanomaterials
As the name suggests, organic nanomaterials are the class of carbon-based nanomaterials whose covalent interactions make them compatible for biomedical purposes. In recent years, a significant increase in the studies focused on the uses of nanomaterials with the organic structure for regeneration of bone, cartilage, skin, or dental tissues. There is numerous evidence for several advantages of using natural or synthetic organic nanostructures in a wide variety of dental fields, from implantology, endodontics, and periodontics, to regenerative dentistry and wound healing. Biomedicine stands to profit from the use of organic nanocarriers. Some of the advantages of the nanostructures include higher colloidal stability, improved dispersibility, and improved surface reactivity. The most prominent characteristic of organic nanomaterials continues to be their ability to control the delivery of drugs such as small molecule drugs, proteins, and DNA; however, there are several other potential applications of organic nanomaterials, such as polymers for coatings, nanoscale optoelectronics, and other technical applications.
Inorganic Nanomaterials
Inorganic nanomaterials are the class of nanomaterials primarily composed of metal-based nanomaterials, metal-oxide-based nanomaterials, ceramics, a few non-metals-based nanomaterials, and other nanostructured materials whose central core is composed of inorganic materials that define their fluorescent, magnetic, electrical, and optical properties. Numerous studies have shown that inorganic nanomaterials including gold nanoparticles, nonporous and mesoporous silica nanoparticles, magnetic nanoparticles, and quantum dots have shown great potential in bioimaging, targeted drug delivery, cancer therapies, and other technological sectors, such as biosensing, chemical sensing, electronics, and optical applications.
Hybrid Nanomaterials
Hybrid nanomaterials are defined as unique chemical conjugates of organic and/or inorganic materials, i.e., these are mixtures of two or more inorganic components, two or more organic components, or at least one of both types of components. The resulting material is not a simple mixture of its components but a synergistic material with properties and performance to develop applications with unique properties determined by the interface of the components at the molecular or supramolecular level. Its functionality is associated with the improvement of physicochemical properties. For the electrochemical or biochemical properties through the optimization mainly of magnetic, electronic, optical, and thermal properties or a combination of them.
Examples
Carbon nanotubes (CNTs)
Carbon Nanotubes (CNTs) are cylindrical tubes made of rolled sheets of carbon allotrope graphene with a diameter of nanoscale order. They can be single-walled (SWCNT) with a diameter of less than 1 nanometer (nm) or multi-walled (MWCNT), consisting of several concentrically interlinked nanotubes, with diameters reaching more than 100 nm; however, their length can reach several micrometers or even millimeters. CNTs are considered one of the strongest materials known to man. Carbon nanotubes are prominently known for their remarkable electrical conductivity, thermal conductivity, and exceptional tensile strength. The properties of nanotubes have caused researchers and companies to consider using them in several fields. For example, because carbon nanotubes have the highest strength-to-weight ratio of any known material, researchers at NASA are combining carbon nanotubes with other materials into composites that can be used to build lightweight spacecraft. Different production methods for carbon nanotubes (CNTs) include functionalization, filling, doping, and chemical modification. Parameters such as structure, surface area, surface charge, size distribution, surface chemistry, and agglomeration state, and purity of the samples have a considerable impact on the reactivity of carbon nanotubes.
Nanocomposites
Nanocomposites are hybrid nanomaterials that are produced by mixing polymers with inorganic solids (clays to oxides) at the nanometric scale. Composites are engineered or naturally occurring solid materials which result when two or more different constituent materials, each having their own significant characteristic (physical or chemical properties), are combined to create a new substance with superior properties than original materials in a specific finished structure. In mechanical terms, nanocomposites differ from conventional composite materials due to the exceptionally high surface-to-volume ratio of the reinforcing phase and its exceptionally high aspect ratio. It also changes how the nanoparticles bond with the bulk material. The result is that the composite can be improved many times concerning the application. Some nanocomposite materials are 1000 times tougher than bulk component materials.
Nanocomposites are suitable materials to meet several emerging demands arising from scientific and technological advances. They offer improved performance over monolithic and micro-composite counterparts. Several applications already exist, while many potentials are possible for these materials, which may open new prospects for the future. Given their unique properties, nanocomposites have been utilized in many applications including food, biomedical, electroanalysis, energy storage, wastewater treatment, automotive, etc.
Nanofibers
Nanofibers are defined as fibers having a diameter within the nanoscale. In the fabric industry, this classification is often extended to include fibers as large as 1,000 nm diameter, which are referred to as microfibers. Generally, fibers can be prepared by melt processing, interfacial polymerization, electrospinning, antisolvent-induced polymer precipitation, and electrostatic spinning, whereas carbon nanofibers are graphitized fibers produced by catalytic synthesis. The diameters of nanofibers depend on the type of polymer used and the method of production. All polymer nanofibers are unique for their large surface area-to-volume ratio, high porosity, appreciable mechanical strength, and flexibility in functionalization compared to their microfiber counterparts. Nanoscale fibers are significantly utilized as several doping agents for important applications such as energy storage, fuel cells, aerospace, diodes, capacitors, transistors, drug delivery systems, battery separators, sensors, and information technology. Another very critical application of nanofibers is their use in tissue engineering to provide scaffolds for biodegradable polymers.
Nanowires
Silicon Nanowires
Nanowires are defined as any nanostructured material in the form of a wire with a diameter ranging from 10-100nm. The most prominent feature of nanowires is their length-to-width ratio being greater than 1000. Due to this massive difference in the length to diameter ratio, nanowires are often referred to as quasi-1-dimensional materials and allows the required space for quantum confinement effects, which is why these are also called “quantum wires.” This, together with the atomically sharp junctions achievable by the epitaxial growth techniques, makes nanowires an ideal playground for band-engineering, enabling countless applications of nanowires across electronics, optoelectronics, and quantum technologies at large. Nanowires can be made from a wide variety of materials, including silicon, germanium, carbon, and various conductive metals, such as gold and copper. It is important to note that nanowires are different from nanotubes since nanotubes are hollow, whereas nanowires are solid nanostructures.
Dendrimers
Dendrimers are nano-sized, radially symmetric molecules with well-defined, homogeneous, and monodisperse structures. They are typically defined by three components: a central core, an interior dendritic structure (the branches), and an exterior surface with functional surface groups. The varied combination of these components yields products of different shapes and sizes with shielded interior cores, which are ideal candidates for applications in both biological and materials sciences. The properties of dendrimers are usually defined by the functional group attached to the molecular surface as it affects the solubility and chelation ability of the dendrimer; however, the cores also provide unique properties to the cavity size, absorption capacity, and capture-release characteristics. Dendrimers are ideal candidates for biomedical applications because of their characteristics, including hyper branching, well-defined globular structures, excellent structural uniformity, multivalency, variable chemical composition, and high biological compatibility. Dendrimers are prominently emerging as a drug delivery mechanism in the field of biomedical research. Other potential applications of dendrimers include gene transfection, a catalyst for nanostructures, rheology modification, etc.
Quantum Dots (QDs)
A schematic of a QD and its atomic like energy levels
Quantum dots are nanocrystals of a semiconducting material with diameters in the range of 2-10 nanometers (10-50 atoms). They display unique electronic properties, intermediate between those of bulk semiconductors and discrete molecules, resulting from unusually high surface-to-volume ratios for these particles. When the quantum dots are illuminated by UV light, an electron in the quantum dot can be excited to a state of higher energy, resulting in the transition of an electron from the valence band to the conductance band. The excited electron can drop back into the valence band, releasing its energy by the emission of light. When semiconductor particles are made small enough for quantum effects to occur, it affects the allowed energies at which electrons and holes can exist in the particles. In other words, the semiconducting properties of materials can be tuned by controlling their behavior at the nanoscale. Potential applications of quantum dots include single-electron transistors, solar cells, LEDs, lasers, single-photon sources, quantum computing, cell biology research, microscopy, and medical imaging. Their small size allows for some QDs to be suspended in solution, which may lead to use in inkjet printing and spin-coating. These processing techniques result in less expensive and less time-consuming methods of semiconductor fabrication.