# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
###### Abstract
The cells in the human body mimic the functionalities of the so-called smart materials (counter-intuitively) with diverse innate attributes encompassing self-healing, self-replicating, auto-immunity, and multi-component organization. From an extensive literature review, it has been observed that it is practically not feasible to provide a fixed definition of SA as the terminology is widely used by several scientific disciplines, thereby presenting an infinite flexibility in generalization. However, from a perspective of nano-engineering, which is the topic of the current review, we present a broad definition for SA: 'the regulated (reversibly controllable) and self-governing union of individual components (or building blocks) from a disordered or less ordered state (at one equilibrium) to a higher order or better-ordered state (with an altered equilibrium state), under the influence of external forces [25; 26], so as to realize robust functionalities for a desired application' [9; 24]. The primary requirements to accomplish nanoparticle SA can be listed as follows: (i) NMs that are utilized for obtaining the final self-assembled product, (ii) an adequate understanding of the basic chemical and physical properties of the nanoparticles (NPs) under consideration, (iii) a critical examination of the free energy difference between the reactants (NPs) and the products (nano-assemblies), and (iv) a careful analysis of the interparticle interactions (van der Waals, H-bonding, ionic, electrostatic, magnetic, and molecular) among the NPs used or with that of the template (if incorporated). Importantly, the information that is implicitly coded into the nano-constructs with respect to the shape, size, mass, charge, and intrinsic polarizability (magnetic and electric), established the SA pathway, and realized in the final self-assembled functional units [22; 27; 28]. While the nano-engineered materials display staggering physicochemical properties that are dramatically different from their bulk counterparts, the engineered nano-assemblies present novel optoelectronic characteristics that are distinctly different from their nano-material counterparts [29; 30; 31; 32; 33; 34; 35; 36]. Fundamentally, there are two types of SA, namely, (i) static and (ii) dynamic SA. The static SA is a process in which the nano-constructs and the end products are at a particular local or global equilibrium, devoid of the dissipation of energy. Some of the examples of static SA that occur naturally are the specific folding occurringin globular proteins, the pairing of the bases in a cell, and the formation of a lipid bilayer. Further, the generation of atomic, ionic, and molecular crystals as well as liquid and colloidal crystals have been categorized as the static SA, presenting applications in the fields of optoelectronics, displays, molecular sieves, and the development of newer band gap materials. On the contrary, dynamic SA is a process where the individual nano-constructs assemble into higher-order structures with the concomitant dissipation of energy [36; 37; 38]. A simple example of dynamic SA from the macroscopic standpoint is the association of different forces such as gravity, electromagnetism, and entropy that contribute to the organization of materials in nature around us. Another example that is extensively observed in nature is the indispensable role of SA and associated processes that occur at the cellular level, sustaining the lifeforms on the planet [9; 10; 28]. Furthermore, different types of classifications of NP SA have evolved in the past decade [28; 29; 39; 40; 41]. The general processes involved in the SA formation can be sub-categorized into directed, co-assembly, and hierarchical SA [9; 42].