these studies, functional molecules and nanoparticles can be selectively placed at specific positions of DNA origami in a programmed fashion. In addition, DNA origami can be integrated with top‐down nanotechnology. These methods could be applied for creating nanoscale devices with novel functionality when the functional origami is precisely integrated on the fabricated surface.
1.11 Dynamic DNA Origami Structures Responsive to External Stimuli
1.11.1 DNA Origami Structures Responsive to External Stimuli
DNA origami technology enables the creation of robust 3D structures, which can perform various mechanical movements [98, 99]. A dynamic DNA origami that changes 3D structure in response to external stimuli such as salt concentration and temperature was developed. In the design, shape fitting and π–π stacking interactions between the base pairs were utilized. As shown in (Figure 1.13a), the DNA origami consists of two shape‐complementary rods that can rotate around the pivot, and the opened X‐shaped structure closes into a closed rod shape depending on the salt concentration [100]. By detecting open and close states by fluorescence, the structure operated more than 1000 open/close cycles without breaking by heating and cooling. The performance and robustness as a mechanical molecular device were excellent. Furthermore, a movable grid assembly at the micrometer scale was constructed by further assembling the dynamic DNA origami. In combination with these mobile parts, as a demonstration, a nanoscale “human‐shaped robot” was also constructed, that can open and close the arms by controlling the salt concentration (Figure 1.13b). Using DNA origami in this way, it is possible to create designable self‐assembled materials that can be driven in response to various external stimuli.
1.11.2 Stimuli‐Responsive DNA Origami Plasmonic Structures
A DNA origami nanodevice that controls the plasmonic interaction between two gold nanorods (AuNRs) was constructed by controlling the rotation of the structure. A structure with two plates (80 nm × 16 nm × 8 nm) was created that could rotate at the central connection (Figure 1.13c) [102]. The direction of rotation could be controlled by selective DNA strand displacement. The conformations of the structures were characterized by CD due to the chirality generated from DNA and the plasmonic interaction between the two AuNRs. Depending on the sequence of the toehold‐containing DNA strands, the structure can form a relaxed or locked state and rotate to the right or left in a programmed fashion. Stepwise reactions can be directly monitored by detecting changes in the CD spectra.
1.11.3 Photo‐Controlled DNA Origami Plasmonic Structures
Photoswitches were incorporated into the DNA origami plates to control the locked and relaxed states of the AuNR‐bound plates by photoreaction (Figure 1.13d) [101]. Photoresponsive DNA strands containing a photoisomerizable azobenzene derivative were incorporated into the sides of the two plates. Dissociation and binding of the photo‐responsive DNA strands by UV and visible light irradiation induced the open (relaxed state) and closed (locked) states, respectively. Two AuNRs (40 nm × 10 nm) were attached to each plate via hybridization of DNA strands introduced to the AuNRs and the plates. CD bands were observed at around 740 nm in the spectrum when the two plates were locked by hybridization of photoresponsive DNA strands. When the two plates were relaxed by dissociation of the photoresponsive DNA strand, the CD signals cancelled out and became extremely weak because the angle between the two arms was not fixed. Because of the reversibility of the cis–trans photoisomerization of azobenzene, two conformations (relaxed and locked state) could be repeatedly formed by alternating UV and visible irradiation, and conformational changes could be read spectroscopically in real time. The results show that these dynamic DNA origami devices can be used for molecular memory by reading out the reversible conformational change induced by photoirradiation.
Figure 1.13 A dynamic DNA origami structure that changes the structure in response to external stimuli. (a) It consists of two units and can rotate at the central pivot. The shape of the side surfaces to be joined fits each other and close. The structure can open and close reversibly in response to temperature. (b) A molecular robot that opens and closes arms in response to salt concentration.
Source: Gerling et al. [100]/with permission of American Association for the Advancement of Science.
(c) A rotatable DNA origami structure. Two plates can rotate on the central axis. A locked state and a relaxed state are formed by addition and removal of specific DNA strands. Left‐handed and right‐handed form can be controlled by a DNA strand exchange reaction. Due to the plasmonic interaction between AuNRs and chirality, locked and relaxed state are detected by CD spectra.
Source: Kuzyk et al. [101]/Springer Nature/CC BY 4.0.
(d) A plasmonic structure that combines AuNRs that open and close in response to photoirradiation. Repeated ON/OFF of the CD signals is monitored by alternative visible and UV irradiation.
Source: Kuzyk et al. [101]/Springer Nature/CC BY 4.0.
1.12 Conjugation of DNA Origami to Lipid
1.12.1 DNA Origami Channel with Gating
Robust 3D DNA origami can be used for the construction of artificial channels. Channels in the cell membrane transfer specific molecules and ions with a controlled gate system. DNA origami transmembrane channels were created and incorporated into the lipid bilayer [103]. This architecture consists of a robust cylinder‐like structure with a hollow interior and a short tubular pore that penetrates the lipid membrane (Figure 1.14a). To tightly attach to the lipid bilayer surface, multiple cholesterol molecules were incorporated at the bottom of the main body of the channel structure (Figure 1.14b). Using single‐channel electrophysiological measurements, the response of the origami channel showed properties similar to those of the natural ion channels in terms of conductance and channel gating. Using this channel, different lengths of single‐stranded DNAs with a stopper such as a stem and G‐quadruplex were discriminated by electrophysiological measurements. These results show that the origami channels can be further used to identify target molecules by detecting single‐molecule translocation.
1.12.2 DNA Origami Templated Synthesis of Liposomes
DNA nanostructures provide various scaffolds to place size‐controlled materials. Artificial liposomes are a useful tool for studying membrane structures and for applications such as drug delivery. However, it is difficult to control the size of liposomes in a customized fashion. Lin and coworkers created a method for producing sub‐100 nm‐sized liposomes using a DNA origami template (Figure 1.14c,d) [104]. Ring‐shaped DNA origami structures with different diameters were designed and prepared. Lipid molecules were placed via hybridization of the DNA–lipid conjugate onto the inner surface of the DNA ring. Then, the ring with handles was mixed with
