# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 3 Stimuli Dependent Techniques
### Chemical Stimuli-Based Self-Assembly of Nanomaterials
#### 3.1.4 Metal Ion-Induced Self-Assembly  
Transition metal ion-mediated SA is another appealing approach in synthesizing various geometries of nanostructures. Since they express a wide range of optical, electronic, magnetic, radioactive, and catalytic properties, metal ion-induced stimuli becomes one of the ideal approaches for using them in later applications [77]. Generally, metal ions act as a vertex of polyhedrons with the attached ligand of NPs. Carboxylate groups of organic ligands usually serve as active binding sites for the metal ion interaction. In short, during the SA approach, metal ions and the surface ligand of NPs were spontaneously bound together via coordinate bonding as a mean of a Lewis acid/base interaction [78]. As shown in Figure 5A, where metal ions (M\({}^{2+}\)) interact with two or more active carboxylate groups of surface ligands, NPs are closer as a result of SA. A balanced series of interactions of attractive and repulsive forces ease the disassembly of NPs that could be promoted with the addition of a strong chelator such as EDTA [79]. Recently, hydrogels of Ag NCs supramolecular structures were self-assembled with the help of a Ba\({}^{2+}\) secondary metal ion. The orientation of enantioselective carboxylic groups promotes the assembly of NCs into nanotubes when coordinated with Ba\({}^{2+}\) as shown in Figure 5B (top). Different concentrations of secondary metal ions were investigated during the formation of Ag nanocluster hydrogels where intertwining increases with the increase in the concentration of Ba(NO\({}_{3}\))\({}_{2}\) as shown in Figure 5B (down (i-iv) SEM images) [80]. In another example, the optical properties of Au nanoclusters were improved via the metal ion coordination approach. [PERSON] and co-workers observed the red shift in PL \(\lambda_{\text{max}}\) and increase in PLQY from 0.1% to 2% with the addition of Zn\({}^{2+}\) cations as shown in Figure 5C. This is due to the restriction on rotation and vibration motions of the compact network surface ligands, which resulted in an increased radiative relaxation through a metal-centered triplet state [81]. As shown in Figure 5C (down (i,ii)), TEM images evidence the aggregation of Au nanoclusters with the addition of Zn\({}^{2+}\) cations. However, NPs self-assembled with the addition of cations unfortunately neutralize charges and simultaneously result in the precipitation or uncontrolled growth of NPs; this result was very common. Therefore, controlling the SA of NPs during an ion-responsive stimuli process would be very important. Recently [PERSON] co-workers studied the controlled dynamic SA of NPs with the help of mixed ligands as shown in Figure 5D. In a typical example, with the addition of Fe\({}^{3+}\) cations to 5 nm-sized Au NPs, they underwent SA to 50 nm quasi-spherical superstructures. However, due to the phase separation of the tannic acid and citrate ligand on the surface, Au NPs restricted the additional growth and addition of strong metal chelator EDTA, which resulted in the disassembly of NPs [82]. Furthermore, the assembly of NPs in an ion-induced approach could be highly directional for site-selective strategies. Recently, [PERSON] and co-workers studied the anisotropic SA of Au/TiO\({}_{2}\) NPs through a site-selective approach. In this work, they have studied the effect of surface ligand density on the morphology of Au NRs. A nanodumbell-shaped assembly of NPs was acquired due to the large deposition of TiO\({}_{2}\) at the lower density and higher curved tip of Au NRs [83].