## 4 DNA Origami Devices Triggered by External Stimuli or Multiple Interactions  
This section deals with DNA origami nanosystems in which plasmonic effects, or dynamic and controllable movement, e.g., switching or rotation, are induced using a wide spectrum of external stimuli ranging from photore regulation to pH and thermally directed assembly to electromagnetic fields. [PERSON] et al. [99] were among the first to show UV-controllable DNA origami structures. They demonstrated the assembly and disassembly of predesigned multi-orientational patterns constructed from rigid DNA hexagons with photoresponsive azobenzene-modified oligonucleotides inserted either into one, two, or three edges. Later on, [PERSON] and [PERSON] [100] demonstrated light-triggered reconfiguration of a hollow spherical DNA nanostructure. The sphere was obtained from two hemispheres linked together via a DNA scaffold, and the sphere was sealed from the equator using nine crossover strands modified with photolabile \(o\)-nitrobenzyl moieties. Irradiation at the specific wavelength of 302 nm resulted in almost quantitative and irreversible cleavage of nitrobenzyl groups, showing tethered hemispheres in TEM images (Figure 3a). A similar light-controlled container was presented by [PERSON] et al. with capture and release properties of gold nanoparticles [101]. [PERSON] et al. [102] designed an elegant, light-driven 3D plasmonic DNA origami nanostructure, which was based on reversible cis-trans photoisomerization of azobenzene units. They used a similar design to that described in Section 2; however, the design now contained azobenzene units that were assembled to form a chiral template with an adjustable angle. Optical control between locked (ca. 50\({}^{\circ}\) angle) and relaxed (ca. 90\({}^{\circ}\) angle) conformational states was obtained using ultraviolet (UV) and visible (Vis) light illumination. The insertion of two AuNRs into the bundles resulted in a tunable plasmonic chiroptical response with large amplitude modulation upon light stimuli. Elaborating on the previous design [102], [PERSON] et al. [103] demonstrated that the plasmonic metamolecules could also be reconfigured via pH changes, which triggered the selective control over the chiral locked or relaxed state (Figure 3b). The DNA \"lock\" was based on a DNA triplex formation through pH-sensitive and sequence-specific parallel Hoogsteen interactions occurring between an ssDNA and a duplex DNA. Tuning of the relative contents of TAT/CGC triplets enabled the programmability and discrimination of chiral quasi-enantiomers over a wide pH range. The programmable pH response was also successfully used in the reconfiguration of hexagonal DNA origami dimer and trimer systems using i-motif and Hoogsteen-type interactions [104]. [PERSON] et al. [105] introduced chiral plasmonic nanostructures by assembling AuNRs into an L-type configuration using two DNA origami triangles as templates. These two templates were specifically connected using a programmable seam that was able to respond to different stimuli. A clear dynamic and reversible plasmonic effect was detected from circular-dichroism spectra when the system was triggered by light (azobenzene-induced G-quadruplex stretching of the seam) or pH changes (folding/unfolding of cytosine-rich i-motifs). The irreversible cleavage of disulfide bonds of the seam via the reduction of glutathione tripeptide and the cleavage of restriction-enzyme-sensitive DNA sequences was also demonstrated. [PERSON] and co-workers [106] created thermoresponsive DNA origami tweezers in which the actuation was based on reversible temperature-induced coil-to-globule transition of the thermoresponsive polymer poly(\(N\)-isopropylacrylamide) (PNIPAM). Above the lower critical solution temperature (LCST) of 32 \({}^{\circ}\)C, PNIPAM became hydrophobic, causing the folding of both rigid arms (Figure 3c), which was detected as an increased fluorescence of the gold nanoparticle (AuNP) and Cy5 located at the tip of the tweezer in equivalent positions. [PERSON] et al.