Design, Fabrication, and Applications of DNA Nanomachines - DNA Nanotechnology: From Structure to Function - page 249

of the nanopores, and K C -simulated organization of G-quadruplexes is used to
control the opening and closing of the pores, thus switching on and off the ion
permeability [ 103 ].
Besides the pore surface, a DNA machine with switchable motion was immo-
bilized on the surface of a quantum dot by the group of Willner (Fig. 11.18 c)
[ 104 ]. The motional elements include the anti-adenosine monophosphate (AMP)
sequence-specific aptamer and a labeled fluorophore. Sequential addition of AMP
and its hydrolysis enzyme (adenosine deaminase) results in the reversible transloca-
tion of the moving strand along the track, leading to the change of FRET efficiency
between the fluorophore and the quantum dot surface.

11.5 Perspective

The unique, predictable, sequence-dependent structure formation and stimuli-
responsive features of nucleic acids enable DNA to be highly promising for
molecular machines. The complexity of the nanomachines has been enhanced
from single-molecular motor to autonomous molecular spider, and the applications
have been expanded from sensing to autonomous creation of new compounds
and even the programmed assembly of nanoobjects, such as aptamer-gated DNA
nanorobot, Au NP helix on folded DNA origami, and cargo assembly on robot-
like DNA machine, etc. However, DNA machines are still in the early stages, and
several issues remain challenging for future development in this field. The first
is the productivity scale of DNA strands, which limits the mechanical devices
especially with enhanced complexity, for example, DNA origami-based devices,
in the laboratory-scale construction and operation. The second is that the current
DNA devices are still very primitive, including the fueling system, the automation,
the cooperativity between DNA machinery components, and the reliability and
the reusability of the machinery devices. The third originates from the chemical
structure of the DNA backbones, which leads to the incompatibility of DNA with
a lot of species and environment. An example is the difficulty in operations in
the organic solution. The fourth is the inefficient incorporation of DNA machines
onto the solid surface, together with the difficulty in the resolution of imaging the
motion on the surface. Although researchers have succeeded in the immobilization
of single-molecule nanomotor onto the surface to regulate the surface functions, the
difficulties in complex nanomachines are inevitable, regarding the characterization
and the operation.
It is impossible to predict the future of the field of DNA nanomachines.
But the enhancement of complexity of mechanical devices and focusing on
the machinery functions will be some of the directions, for example, more
diverse environments, smarter system with fast and synchronous functions of
sensing/responding/analyzing/memory/feedback/instructing, or self-generating
DNA machines. The exciting progress of the devices, witnessed in the past decade,