can also be amorphous or crystalline, poly or monocrystalline, single‐ or multi‐element, which also exist in different forms. 2D materials such as nanosheets, nanofilms, and nanoribbons have shown promising applications in the fields of optoelectronics, sensors, and biomedicine (Weaver et al., 2014).
1.3.1.4 Three‐Dimensional NMs
Herein, the materials have all the three dimensions in macroscale but are comprised of uniformly distributed nanometer‐sized grains. Hence, the movement of the electrons can be free across all the three dimensions without any confinement (Jeevanandam et al., 2018). 3D NMs also called bulk NMs are widely used in catalysis, electrodes, and magnetic materials. Nano balls, nano coils, and nanoflowers are typical 3D NMs that have high surface area and can provide maximum adsorption sites for all the molecules in a small‐area framework (Shen et al., 2008).
1.3.2 Classification Based on Chemical Compositions
Similar to dimension, the composition of NMs also plays a vital role in deciding their activities and application. On the basis of composition, NMs are classified into four subcategories, namely: (i) carbon‐based NMs, (ii) organic NMs, (iii) inorganic NMs, and (iv) composite NMs.
1.3.2.1 Carbon‐Based NMs
The NMs with carbon atoms as their backbone are called carbon‐based NMs. They can exist in different forms such as 0D (fullerenes), 1D (carbon nanotubes), 2D (graphene sheets), and 3D (diamond crystal and graphite). General methods to prepare these NM include chemical vapor deposition, arc discharge, and laser ablation. Carbon‐based NMs exist in different forms with multiple shapes such as hollow spheres, nanotubes, and ellipsoids (Jeevanandam et al., 2018). Fullerenes are carbon materials with spherical morphology where the carbon atoms are held by sp2 hybridization. A unique advantage of the fullerenes is their high symmetric property (Astefanei, Núñez, & Galceran, 2015). In general, fullerenes contains 28–980 carbon atoms where the diameter of single layer is up to 8.2 nm and for multilayered fullerenes it is about 4–36 nm (Ealias & Saravanakumar, 2017). Carbon nanotubes are 1D carbon NMs where carbon atoms are wound up to form hollow cylinders, which can also be described as an extension of fullerenes or buckyball. Carbon nanotubes can be single‐walled, double‐ or multi‐walled with thickness varying from 0.7 nm for single‐walled to 100 nm for multi‐walled CNTs. The length of CNTs generally varies from few micrometers to several millimeters (Ealias & Saravanakumar, 2017). CNTs have been exploited in various fields owing to their versatile properties such as elasticity, strength, rigidity, field emission, and electrical conductivity (Saeed & Khan, 2014, 2016). Graphene is one of the 2D carbon‐based materials formed by sp2 hybridized carbon atoms. It is a hexagonal network of carbon atoms with honeycomb atomic structure that is confined to a two‐dimensional planar surface. Graphene elucidates commendable physical, chemical, optical, and mechanical properties owing to their unique honeycomb atomic structure. These unique properties altogether make them remarkable materials that are extensively applied in the fields of electronics, optics, storage, thermal applications, photovoltaics, and composite materials (Goenka, Sant, & Sant, 2014; Pumera, 2010).
1.3.2.2 Organic‐Based NMs
The NMs formed from proteins, lipids, carbohydrates, and other organic substances are termed as organic‐based NMs, which are generally 10 nm to 1 μm in size. Commonly exploited organic NMs are dendrimers, liposomes, micelles, and polymeric NPs. The superior advantage of these systems over the other NM systems is because of their biodegradable and nontoxic nature. Some of these materials like micelles, liposomes, and polymeric NPs have hollow core also called as nanocapsules, which are being exploited for loading and delivery of drug molecules (Biswas et al., 2013; Tiwari, Behari, & Sen, 2008). Apart from that, these materials are sensitive to heat and light, which can be used as platform for responsive and targeted drug delivery system. Similarly, the surface of dendrimers has many chain ends that can also be engineered for specific chemical functions and targeted delivery. Owing to the aforementioned properties coupled with their structural stability, structural integrity, and controlled release profile, organic NMs have emerged as a promising drug delivery system (Wei et al., 2015).
1.3.2.3 Inorganic‐Based NMs
The NMs that are based on metal, metal oxide, and ceramic are called inorganic NMs.
1.3.2.3.1 Metal‐Based NMs
Nanometer‐sized particles that are synthesized from the metal either by constructive or destructive routes are metal‐based NMs. Most of the metals can be synthesized in form of NMs (Salavati‐Niasari, Davar, & Mir, 2008); however, the most extensively studied metal‐based NMs include cadmium, aluminum, silver (Kim et al., 2007), iron, gold (Sun & Xia, 2002), copper (Ramyadevi et al., 2012; Ruparelia et al., 2008), and lead‐based NMs. The size of these materials varies from 10 to 100 nm with high surface area‐to‐volume ratio, unique surface charge, and pore size. Further, they can be either amorphous or crystalline, which can exist in different sizes and shapes such as spheres, and cylinders.
1.3.2.3.2 Metal Oxide‐Based NMs
Metal‐based NMs are sensitive to environmental factors such as heat, sunlight, moisture, and air. In order to overcome the demerits of metal NMs, metal oxide‐based NMs were synthesized. One of the most common examples of metal oxide NPs are iron oxide NPs, which are synthesized from the oxidation of iron particles at room temperature. The metal oxide NPs are preferred over metal NPs due to their increased reactivity and efficiency (Tai et al., 2007). Routinely employed metal oxides include: magnetite (Sun & Zeng, 2002), iron oxide, aluminum oxide (Mukherjee et al., 2011), silicon dioxide, titanium dioxide, zinc oxide (Sharma, Jandaik, Kumar, Chitkara, & Sandhu, 2016), cerium oxide, and copper oxide (Ren et al., 2009).
1.3.2.3.3 Composite‐Based NMs
In general, composite materials are described as materials with two or more different materials combined to blend the properties of all the constituent materials. In the same way, the composite‐based NM is a multiphase material with at least one of the dimensions in nanoscale, which is obtained by combining one NM with other or blending one NM with bulk or larger material to form a NM (Vollath, 2013). Bones and the eggshells are the best examples of naturally occurring composite NMs. These NMs generally possess highly improved physical, chemical, mechanical, and biological properties in comparison to their constituent materials. Nanocomposite materials can be of different combinations such as metal/metal, metal/ceramic, carbon/carbon, and ceramic/ceramic (Jeevanandam et al., 2018).
1.3.3 Classification Based on Origin
Based on their origin, NMs are classified as natural and synthetic NMs. The NMs that are produced by biological species or anthropogenic activities in nature without human intervention are called natural NMs. The NMs formed in nature are present throughout earth's atmosphere, hydrosphere, and lithosphere. This may include the NMs present in whole troposphere, oceans, sea, rivers, lake, groundwater, rocks, lava, soils, even microorganism, and higher organisms (Hochella, Spencer, & Jones, 2015; Sharma et al., 2015). Synthetic NMs are the NMs that are synthesized through physical, chemical, biological, or hybrid methods besides the materials that are produced from engine exhaust, smoke, and mechanical grinding (Wagner et al., 2014). Even though synthetic NMs are more advantageous as aforementioned, the major problem is predicting the fate and behavior of the materials in the environment. Currently there are a lot of strategies to perform the risk assessment of the synthetic NP in various environmental conditions. Still extrapolating the behavior of synthetic NMs from existing knowledge is a major challenge.
1.4 Application of NMs
NMs have found broad applications in various fields