# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
###### Abstract
While the directed SA is triggered through an external entity that directs the entire process, the co-assembly is a process where the final nano-assembled constructs are obtained on account of the interactions between two or more NMs of interest that co-assemble in the presence of each other due to the specific physical or chemical interactions between them. Further, the methodology in which the formation of the final desired product occurs via several steps where each step forms the subset of the final self-assembled product comprises the hierarchical SA approach. It is important to note that a different combination of these three categories with the broad classification of static and dynamic SA is possible. For instance, dynamic co-assembly and static hierarchy have been reported in earlier works. Furthermore, depending on the utility of a particular template used in the assembly formation, the processes are categorized as (i) template-independent and (ii) template-dependent. The well-known examples of self-assembled monolayers (SAM) are template-dependent approaches where the SA of NPs is driven on top of the surface/at specific interfaces of interest. The final structural output of the template-dependent SA techniques are highly determined by the liquid-liquid, liquid-air, and liquid-solid interactions at the interfaces of interest/templates [9; 28; 29]. Such interactions are discussed in detail using the Langmuir-Blodgett method, sedimentation- or evaporation-triggered SA, and the adsorption of NPs at desired surfaces. The widely accepted classification of SA is based on the external stimuli, which are categorized as physical stimuli and chemical stimuli [23]. While the effect of solvents, acid/bases, metal ions, gases, macromolecules, and redox reaction conditions constitute the chemical stimuli-driven SA, the magnetic field, electric field, light, and temperature-driven methods comprise the physical stimuli technologies. The intrinsic magnetic nature (dia, para, ferro, superpara, ferri, antiferro), dipole-dipole interactions, NP size, and polarizability as well as the chemical functionalities (photo-active ligands) determine the formation of the nano-assembled product in such scenarios [23; 42]. In some processes, the conventional nanosynthesis routes are hybridized with the SA approaches to obtain functionally active photo-plasmonic materials. For instance, nanosynthesis may be initiated by the input of light radiation and the NPs formed would immediately SA into ordered structures on account of the interactions of ligand molecules that are capping such NPs. In this case, the activation of ligand molecules to SA, the obtained NPs might be triggered by the input light energy. In other words, the NPs would not self-assemble if they were synthesized via different routes and attempted SA in the dark. Hence, through this example, one can understand that although there are different ways in which the SA of NMs can be classified, by and large, these approaches often overlap. In this background, it is important to note that there are myriad methodologies that are adopted by experimentalists to obtain nano-assemblies. In this context, this review presents an overview of available stimuli-dependent and stimuli-independent approaches for accomplishing nano-assemblies for different applications. Further, we present a focused review on a few state-of-the-art examples encompassing stimuli-dependent approaches and discuss the latest advancements in the domain to cater to the broad audience of nanoscience and nanotechnology research. We believe this review would facilitate further research in the direction of SA by providing new insights, and open new doors to investigate analogous methodologies under the umbrella of stimuli-independent and dependent routes.