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interface (Saptarshi et al., 2013). Quartz crystal balance is used to measure changes in mass on the surface of oscillating quartz caused by the NM–protein interaction. In a study, adsorption of proteins myoglobin, bovine serum albumin, and cytochrome over the surface of gold NPs was studied using quartz crystal balance (Kaufman et al., 2007). Confocal Raman spectroscopy and confocal spectroscopy can be employed to study and visualize NM–protein interaction and intake of NMs into cells by fluorescent labeling of NPs. In recent times, a combination of these techniques has been strategically employed to study the different aspects of NM–protein/biomolecule interaction. NMs can be synthesized through different routes such as chemical, physical, and green methods. Changes in the synthesis methods, concentration of reactants, and conditions can definitely modulate the morphological parameters (size and shape) of NMs. Taking this into account, the selection of synthesis route also plays an important role in governing NMs' morphological features and their functions. Keeping the aforementioned perspectives in mind, this chapter describes the effects of size and shape of NMs on their biological activity.
1.2 Synthesis of Nanomaterials
NMs can be synthesized by a number of methods that are grouped into two different categories: (i) bottom‐up and (ii) top‐down method. Schematics of typical methods for synthesis of NMs are given in Figure 1.1 as described by Ealias and Saravanakumar (2017).
Figure 1.1 Schematics of typical methods for synthesis of NMs.
1 Bottom‐up method: It is a constructive method wherein the atoms build up clusters that in turn form the NMs. This category includes methods such as sol–gel, spinning, chemical vapor deposition, pyrolysis, and biosynthesis.
2 Top‐down method: On the contrary, the top‐down method is a destructive method where the bulk materials are reduced into nanoscale materials. It includes methods such as mechanical milling, nanolithography, laser ablation, sputtering, and thermal decomposition. The typical method of synthesis for various NMs is given in Table 1.1.
1.3 Classification of NMs
In nature, NMs are built by nanoscale or submicron‐sized blocks that exhibit size‐dependent effects. Over the last two decades, a number of NMs or nanostructured materials have been developed and a lot of new developments are underway. The abundant increase in the number of NMs has set forth the need for the classification of these materials. An understanding of the classification would give insight into the interaction of NMs with various surfaces and resultant functionality of the NMs. The first classification of NMs was given by Gleiter (2000) in which the materials were classified on the basis of crystalline forms and chemical compositions. The scheme subdivided the materials into three classes where each class has four kinds of materials (Gleiter, 2000). However, the list of Gleiter was not considered complete as it failed to take account of 0D and 1D NMs such as fullerenes and nanotubes into the classification.
Table 1.1 Various approaches for NM synthesis (Ealias & Saravanakumar, 2017).
| S. No | Category | Method | NMs |
|---|---|---|---|
| 1 | Bottom‐up | Sol–gel | Metal and metal oxide and carbon NMs |
| Spinning | Organic polymers | ||
| Chemical vapor deposition | Carbon and metal NMs | ||
| Pyrolysis | Metal oxide and carbon NMs | ||
| Biosynthesis | Metal and organic polymer NMs | ||
| 2 | Top‐down | Mechanical milling | Metal, metal oxide, and polymeric NMs |
| Nanolithography | Metal NMs | ||
| Laser ablation | Carbon and metal oxide NMs | ||
| Sputtering | Metal NMs | ||
| Thermal decomposition | Metal oxide and carbon NMs |
1.3.1 Classification Based on Dimensions
Later, Pokropivny and Skorokhod (2007) proposed a new scheme of NM classification where the dimensionality (shape and size or form) of the NMs was considered as a primary criterion. In general, nanostructures are structures with at least one dimension d equal to or less than 100 nm, which is considered as d*. The value d* is always dictated by physical phenomena such as path length of phonons and electrons, diffusional length, length of de Broglie wave, penetration length, and correction length. According to the scheme, NMs were classified into four major categories: 0D, 1D, 2D, and 3D (Pokropivny & Skorokhod, 2007).
1.3.1.1 Zero‐Dimensional NMs
Zero‐dimensional NMs are defined as materials where all the three dimensions are confined to the nanoscale (1–100 nm). The same definition could also be stated on basis of the movement of electrons along the dimensions of the NMs. Zero‐dimensional materials are materials where the electrons are merely entrapped in a dimensionless space without any possible movement (Jeevanandam et al., 2018). The best examples for 0D NMs are NPs and quantum dots. Over the past decades, 0D materials have gained a lot of interest where a number of methods have been designed to fabricate 0D NMs with precise dimensions. 0D NMs can be crystalline or amorphous in nature. They may be mono‐ or polycrystalline, single‐ or multi‐element, and may exist in various forms (shapes and sizes). These materials have found application in a number of fields such as solar cells (Lee et al., 2009), light‐emitting diodes (Stouwdam & Janssen, 2008), single‐electron transistors (Mokerov et al., 2001), lasers, therapeutics and diagnosis (Azzazy, Mansour, & Kazmierczak, 2007).
1.3.1.2 One‐Dimensional NMs
One‐dimensional NM are the materials where one of the dimensions is in macroscale with other two dimensions confined to the nanoscale (<100 nm) (Xia et al., 2003). Herein, the electrons can move across one axis freely whereas they entrapped in other two dimensions of the NMs (Jeevanandam et al., 2018). These 1D NMs are ideal choice for studying the dimension‐dependent activity of the materials. Similar to 0D NMs they also can be amorphous or crystalline, mono‐ or polycrystalline, ceramic, polymeric or composite materials of different shapes and sizes. 1D materials such as nanotubes, nanowires, and nanofibers have attracted a lot of interest in the development of hierarchal nanostructures such as nanofilms, nanosheets, and nanoribbons with profound applications in the field of optoelectronics and nanoelectronics (Cui et al., 2001; Kong et al., 2000).
1.3.1.3 Two‐Dimensional NMs
Materials with one of the dimensions in the nanoscale (≤100 nm) and the other two dimensions in macroscale are called 2D materials. Here the electrons are confined in one direction whereas they can move across in other two