# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 3 Stimuli Dependent Techniques  
### Chemical Stimuli-Based Self-Assembly of Nanomaterials  
#### 3.1.1 Solvent-Induced Self-Assembly  
The colloidal stability of NPs is immensely influenced by the characteristics of their ligand passivation dynamics, which in turn is affected by the nature of the solvent. Some solvents may cause the NPs to be colloidally stable, while others may cause them to aggregate (controlled or uncontrolled) in the solution. In general, the addition of an immiscible solvent (also known as a bad solvent) to a colloidal NP solution can trigger SA of the NPs, as the immiscible solvent causes the surface ligand molecules to switch between stabilized and destabilized states. This process reduces the steric repulsion between ligands, leading to NP aggregation. At large, the controlled aggregation may lead to self-assembled nano-architectures [55; 56]. Although solvent-driven assembly is generated through non-covalent interactions, it can be easily disrupted by removing the stimuli (immiscible solvent) or by introducing competing stimuli (miscible solvent). Thus, the solvent-induced SA approach relies on the interplay between immiscible and miscible solvents to achieve the controlled aggregation of NPs. In a classical example, the use of a solvent-assistant SA route can improve the quantum yield of Cu nanoclusters (NCs). Initially, 1.8 nm-sized triphenylphosphine-capped Cu NCs were synthesized in DMSO (dimethyl sulfoxide) solvent, to which an immiscible solvent (typically glycerol, a bad solvent) was introduced to induce the SA of Cu NCs, as shown in Figure 2A (top (i)). Upon introducing glycerol to the stable phosphine Cu NCs-DMSO solution, Cu NCs started aggregating in the DMSO-Glycerol system. As the glycerol concentration increased in the DMSO-Glycerol system, Cu NCs aggregated and precipitated in highly ordered structures with branched morphologies, due to the stacking of ligands and non-covalent interactions with adjacent Cu NCs in an end-to-end manner (Figure 2A (down left (ii))) [57]. Additionally, a higher amount of glycerol resulted in a change in the final morphology, with an increase in the size of Cu NCs, as well as a strong yellow-green emission, which is depicted in Figure 2A (down right (iii)). The manipulation of NPs that have a heterogeneous chemical nature and different facet reactivities could lead to another strategy for the solution-SA. Here, non-spherical NPs are mostly involved in assembled nanoarchitectures. In particular, side-by-side and end-to-end assemblies of NRs were obtained by triggering the interactions among distinct stabilizing agents upon the addition of stimuli [58; 59]. Generally, they are in the form of anti-solvents [60]. For example, the SA of gold NRs stabilized with CTAB (cetyl trimethyl ammonium bromide) on the long facet and PS (polystyrene) in the short facet manifest two distinct assemblies depending on the solvent used, as shown in Figure 2B. In a water-tetrahydrofuran (THF) system, the presence of THF acted as an anti-solvent for CTAB that reduced the steric repulsions on longer facets of NRs. As a result, it triggered the side-by-side interactions and Individual Nanorods self-assembled to bundle shaped structures, as shown in Figure 2B (left (i)). On the other hand, in the dimethylformamide-water system, where water was treated as an anti-solvent for PS, end-to-end interactions of gold nanorods were observed. As shown in Figure 2B (right (ii)), the SA in the form of a long chain interconnected with the PS-PS short facets was present [61]. Additionally, the assembly of NPs obtained at liquid-liquid interfaces offer an efficient pathway to build up multi-dimensional arrays. In particular, the SA of amphiphilic NPs at a liquid-liquid interface is a well-known approach [62]. In a typical example, amphiphilic Au NPs were synthesized and dispersed in chloroform, where they were stabilized by a block co-polymer (polystyrene-b-poly(ethylene oxide)) ligand.