# 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  
Light-responsive (photo-responsive) SA gained much attention among all the exogenous stimuli due to its economical, easy, and precise control over the intensity and remote delivery capability. Both monochromatic lights and polychromatic lights have been used as a light source to initiate the SA of NMs [86]. The SA of NPs can be achieved by using a photo-responsive surface functionality [87]. Traditionally, this process can be acquired by functionalizing the surface with a monolayer of photo-switchable molecules and the response can be tuned by the different wavelengths of light depending on the specific photo-sensitivity component on the surface of individual NPs. In general, light-responsive stimuli are a reversible process when the photosensitizers undergo photo-isomerization. The most commonly used photo-isomerization compounds include azobenzene, spiropyran, stilbene, diazonaphthoquinone, dithienylethene, and pheophorbide [88]. Amongst these options, azobenzene is a well-known and most-common photo-switchable component to acquire the SA of NMs upon irradiation with light. Azobenzene undergoes a transition that induces trans to cis transformation where nanomaterial agglomerate is controllable [89], and the respective graphical representation is shown in the Figure 7A. In a classical example, the light-responsive methacrylic-type azobenzene monomer is grafted on the surface of gold NPs. UV-light irradiation at 365 nm for 30 min triggers the azobenzene moieties that induces trans-cis isomerization, which results in the aggregation of gold NPs in toluene. The transformation of trans to cis increases the polarity, and it leads to the formation of insoluble products. Additionally, it can also be useful for the switchable wettability that can be useful in various applications including the development of hydrophobic surfaces that help in self-cleaning technology. (Figure 6B left (i)) The red shift with an increase in the broadness of the plasmonic resonance peak of gold NPs confirms the photo-responsive SA of the nanoparticle. Notably, the interparticle spacing between self-assembled particles was influenced by the chain length of the grafted polymer. The before and after SA was shown in Figure 7B ((ii) top) and Figure 7B ((ii) down), respectively. For example, 13 k, 35 k, and 48 k molar mass polymer chains were grafted on the surface of gold NPs and irradiated under UV light, resulting in an increase in interparticle spacing [90]. Separately, spiropyran-functionalized nanoparticles are another example of a photo-induced SA technique. In this process, spiropyran undergoes light-induced isomerization via a spiropyran closed ring to open ring merocyanine isomer when irradiated under UV light and can be reversible in visible light (Figure 7C) [91]. Here, the spiropyran-functionalized NPs can be used in SERS (surface enhancement Raman spectroscopy) applications. The gaps raised due to the aggregation of nanoparticles during the SA acts a hotspot for SERS [92]. Recently, [PERSON] and co-workers developed a mirror-like conductive surface on a glass slide via a photosensitive formulation that contained a photosensitizer to generate free radicals upon exposure to UV light to induce SA. Authors have developed a self-assembled silver NP on a glass slide without using a stabilizing agent or polymer matrix. A photosensitive formulation that contained a mixture of silver nitrate and 2-hydroxy-2-methylpropiophene was applied like a paste on a glass slide and irradiated under UV light to acquire the mirror-like conductive surface in a liquid-air interface, as shown in the Figure 7D [93]. This process can be advantageous to develop advanced light reflectors, solar cells, and smart windows. Apart from the photo-isomerization and free radical process, the photo-cleavage technique is also one of the techniques used in SA processes. The word photo-cleavage refers the change in the structure of a molecule due to bond breakage when exposed to light.