# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 3 Stimuli Dependent Techniques
### Physical Stimuli-Based Self-Assembly of Nanomaterials
#### 3.2.1 Light-Induced Self-Assembly
The most commonly used molecules are o-nitrobenzyl, pyrene, thymine, and coumarin; those undergo bond cleavage due to their interaction with UV light [88]. Amongst these options, coumarin was the most widely used photo-cleavage group. When compared to other photosensitive molecules, coumarin undergoes a cycloaddition reaction when ex posed to a shorter wavelength. For example, coumarine-functionalized gold nanoparticles were self-assembled in a THF solution. When coumarine-gold NPs were irradiated under a 365 nm wavelength, the SA of gold nanoparticles in the THF medium was triggered (Figure 7E (top i(a-d)). Interestingly, the disassembly process was acquired by the exposure to even short wavelength light. The respective graphical images and high-resolution transmission electron microscopy (HRTEM) images were shown in the Figure (Figure 7E (down ii(e-h))) [94]. Thus, light-induced SA can be influenced by various parameters such as the solvent, ligand concentration, polymer chain length, light intensity, and irradiation time. The careful control of parameters during the manipulation of SA provides precise control over the size and density of NMs [86]. Figure 7: (**A**) represents the graphical representation of photo-responsive stimuli. The top image is cited and reproduced from the reference [91]. (**B**) (**i**) shows the change in the absorbance spectra of azobenzene-functionalized gold nanoparticles before and after photo-responsive stimuli (**ii**) HRTEM  
images of gold nanoparticles at different polymer chain lengths. This image was cited and reproduced from the reference [90]. (**C**) graphical representation of spiropyme transformation to merocyanine and its reversibility under UV-Vis light. This image is cited and reproduced from the reference [91]. (**D**) development of mirror-like conductive surface on glass slide via light-induced assembly at liquid-air interface. This image was cited and reproduced from the reference [93]. (**E**) (i(a-d)) HRTEM images of coumarin-functionalized gold nanoparticle assembly and (**E**) (ii(e-h)) disassembly under light exposure. (**E**) (ii) represents the graphical representation of coumarin photo-response. In the advancement of light-induced SA, optical tweezers are a versatile method to precisely control or manipulate the arrangement of NPs into ordered structures. Manipulation of micro- or nano-sized particles via optical forces gained notable attention among researchers in many fields of science. Generally, optical tweezers are a non-contact manipulation technique where a highly focused laser beam exerts a gradient force on an object, and it is capable of trapping and manipulating nano-sized particles [10]. In detail, when a laser beam focuses through the objective lens, the highest light intensity at a focal point experiences a gradient force that pulls particles towards itself. Once trapped, NPs moved and positioned in close proximity to other NPs through the optical tweezers technique facilitated electrostatic and van der Waals interactions between the NPs, resulting in the assembly of NPs into specific patterns. Therefore, SA patterns can be adjusted, or multiple traps can be acquired by moving the focal point to the specific site. On the other hand, the scattering force arises due to the reflection of light and pushes particles in the light propagation direction. Thus, the balance between scattering force and gradient force determines the stability of self-assembled nanostructures [95]. Generally, the manipulation and SA of metallic particles are highly challenging due to their high reflective nature that tends to push the particles far from the focal point. The choice of using an infrared laser that is far from the plasmonic resonance region of gold NPs made Svobada and Block trap 5 nm gold metallic NPs for the first time [96]. Non-spherical metallic NPs usually have a multiple plasmonic resonance that responds distinctly to polarized light.