SYNTHESIS OF NANOPARTICLES

The synthesis of NPs adopts two approaches—a top-down approach and a bottom-up approach.

In the top-down approach, bigger materials are broken down into smaller units using many physical, chemical, and thermal techniques. Top manufacturing involves the construction of parts through methods such as cutting, carving, and molding, and due to our limitations in these processes, highly advanced nanodevices are yet to be manufactured. Laser ablation, milling, nanolithography, hydrothermal teclmiques, physical vapor deposition, and electrochemical methods (electroplating) use the top-down approach for nanoscale material manufacturing. In a study, colloidal carbon spherical particles were synthesized by continuous chemical adsorption of polyoxometalates on the carbon interfacial surface. Adsorption made the carbon black to aggregate into relatively smaller spherical particles, with high dispersion capacity and narrow size distribution (Garrigue et al., 2004).

The bottom-up approach results in the generation of nanoparticles with uneven edges and cavities on the surface of NPs. The bottom-up approach results in the synthesis of good NPs with minimum wastage of chemicals. Examples of the bottom-up approach are sedimentation and reduction techniques (Iravani, 2011). The bottom-up method prepares nanomaterials with a veiy small size, whereas the top-down process cannot deal with very tiny objects. The bottom-up approach generally produces structures with fewer defects as compared to the nanostructures produced by the top-down approach. The main driving force behind the bottom-up approach is the reduction in Gibbs free energy. Therefore, the materials produced are close to their equilibrium state.

Nanotechnology in its broadest terms refers to devices with dimensions in the range of 1-100 nm, while nanofabrication involves the manipulation of matter on the nanoscopic length scale to design structures and patterns with desired functions. It occurs by bottom-up synthetic chemistry; self-assembly has provided a powerful way of making materials and organizing them into functional constructs designed for a specific purpose. Self-assembly teaches that matter of all kinds, exemplified by atoms and molecules, colloids, and polymers, can undergo spontaneous organization to a higher level of structural complexity, driven by a map of forces operating over multiple length scales.

The fabrication of NPs is done by shaping, positioning, and organizing at the nanoscale using two approaches, that is, top-down and bottom-up nanofabrication. The former involves the use of ion and electron, photon, and atom beams to sculpt matter from macroscopic to nanoscopic dimensions in a serial process to form functional constructs with purposeful utility, while the latter self-assembles these constructs in a parallel manner from nanoscale building blocks.

The stability and ordering of these NP structures are the most important features to achieve function for long-term applications. In nanotechnology, items are constructed from the bottom-up method using techniques and tools to make complete, high-performance products. Nanostructured materials are assembled by physical assembly and chemical assembly. The physical assembly techniques are based on the assembly of nonfunctional- ized NPs on the surfaces by physical forces such as convective or capillary assembly (Denkov et al., 1993), spin coating (Ozin and Yang, 2001), and sedimentation (Wijnhoven and Vos, 1998). These techniques result in relatively simple close-packed 2D or 3D particle arrays with limited stabilities.

Self-assembly of NPs is an important feature of nanomaterials and refers to the autonomous organization of components into patterns or structures without human intervention. It is a fundamental principle that creates structural organization from the disordered components in a system. The design of a nanodevice lies in the self-assembly of a target structure from the spontaneous organization of building blocks like molecules or nanoscale clusters. The nanomaterials are functionalized to deposit NPs with surface functionalities onto a functionalized substrate. The nanosurface can be modified to cater to the requirements of the applications. The surface of nanotubes can be modified by covalent interactions including oxidation, cycloaddition reactions, esterification, and amidation. The nanostructures are designed by controlling the functional groups on the surface of NPs and tailoring the nanostructures in a predictable manner to generate functional, more complex nanostructured architectures on the surfaces to meet the needs for specific applications, such as molecular electronics and biosensing (Lahav et al., 2000). Various chemical interaction strategies, for example, covalent bonding (Paraschiv et al., 2002), electrostatic forces (Decher, 1997), and host-guest interactions (Crespo-Biel et al., 2005), have been employed to chemically govern the self-assembly of NPs onto surfaces. Cross-linking of the neighboring particles with chemical forces by selective binding can further enhance the stability of NP assemblies (Zirbs et al., 2005). These methods control the spatial distribution of NPs across a large area in more complex patterns when combined with nanopatteming schemes. The ability to attach NPs onto planar surfaces in a well-defined, controllable, and reliable maimer is an important prerequisite for the fabrication of micro- or nanostructured devices suitable for the application in the field of (bio) nanotechnology. Nanofabrication allows control over functional groups on the surface of NPs to tailor the nanostructures in a predictable manner, resulting in the formation of functional, more complex nanostructured architectures on the surfaces to meet the needs for specific applications such as in molecular electronics and biosensing (Lahav et al., 2000).

Metallic nanomaterials have received peculiar interest in diverse fields of applied science such as materials science and biotechnology (Greque de Morais et al., 2014). The extremely small size and a high surface-to-volume ratio of nanomaterials have evolved nanomaterials as sensors (Collins et al., 2000). Research in nanotechnology reflects the improvement in the design and application of materials, devices, and models that exhibit sustainable future. All of these methods employ hazardous chemicals. In the case of NP synthesis, the bottom-up (or self-assembly) procedures involve a homogeneous system wherein catalysts (e.g., reducing agents and enzymes) are producing nanostructures affected by catalyst properties, reaction media, and reaction conditions (e.g., solvents, stabilizers, and temperature). For example, the chemical reduction method is the most commonly employed synthetic route for metallic particle synthesis. Silver NPs are increasingly used in various fields such as medical, food, health care, and industries due to their unique physical and chemical properties (Guiunanthan et al., 2015; Li et al., 2010). The chemical reduction teclmique is based on the reduction of metal salts like silver nitrate in an appropriate medium using reducing agents like citrate or branched polyethyleneimine. However, citrate produces negatively charged silver NPs, while branched polyethyleneimine produces positively charged ones (Mallick et al., 2004). Particularly, intermediates and byproducts are playing a ciucial role in NP synthesis. Indeed, the physicochemical properties and surface, and morphological characteristics of NPs will influence their fate, activity, transport, and toxicity.