Jaquay et al. have shown that the excitation of the resonance modes of photonic crystal arrays results in near-field patterns that give rise to optical forces that can drive colloidal nanoparticles to assemble into well-ordered patterns, Figure ). The resonantly-enhanced attractive optical force that drives the nanoparticles towards the photonic crystal interface is stronger than the inter-particle forces. Alternatively, Jin et al. demonstrated that illumination on plasmonic colloidal nanoparticles (Au nanorods, nanostars, and nanospheres) results in photothermal convection that drives the nanoparticles to assemble at the point of illumination. The size of the assembly can be increased by either increasing the intensity of light or the illumination time, Figure (e). Another photothermal route towards reversible self-assembly of nanoparticles is by the integration of a thermo-responsive component. For instance, poly-n-isopropylacrylamide (PNIPAM) -capped Ag-Fe3O4 heterodimers are thermally responsive as the PNIPAM chains above its low critical solution temperature (LCST) of 32 o C collapse due to immiscibility. Thus, the Ag-Fe3O4 heterodimers can be reversibly assembled and disassembled by changing the temperature with illumination that heats up the Ag part photothermally, Figure (f). Figure 6. (a) Schematic of light assisted nanoparticle self-assembly by surface photo-chemistry (b) Self-assembly of COOH surface terminated nanoparticles by light-induced acidity regulation (pH). The acidity is regulated by merocyanine molecules (MCH + ) in the solvent that transform to spiropyran under visible light. Reprinted with permission © 2015, Nature Publishing Group (c) Light induced selforganization of nanoparticles trapped at the air-liquid interface is also possible by cis-trans switching of a photosensitive surfactants (Trans-AzoTAB to cis-AzoTAB) at the interface. Reprinted with permission © 2019 John Wiley & Sons. (d) Excitation of the resonance modes of photonic crystals gives rise to optical forces due to strong near-field gradients that induce nanoparticle clustering. Reprinted with permission © 2013, American Chemical Society. (e) Intense illumination of nanoparticle colloids leads to photothermal convective currents that assemble the nanoparticles at single spot. Reprinted with permission © 2018 John Wiley & Sons. (f) Thermo-responsive PNIPAM capped nanoparticles for assemblies when the temperature reaches the low critical solution temperature (LCST) of 32 o C by photothermal heating, leading to immiscibility of the ligands and hence colloidal instability. The self-assembly process is reversible. Reprinted with permission © 2013 Royal Society of Chemistry.