Biomedical Engineering Reference
In-Depth Information
on the precise operation of an instrument, as well as the selective hybridization
of DNA. Functional units coupled to DNA oligomers are picked up from a depot
area by a complementary DNA strand bound to an AFM tip. These units are
transferred and deposited on a target area to create desired patterns, assembled
from different functional units. Each of the cut-and-paste steps was demonstrated by
single-molecule force spectroscopy and single-molecule fluorescence microscopy.
11.4.5
Responsive Surface
The concept of signal-triggered functional DNA machines constructed by pro-
grammed nucleic acid nanostructures have been further extended by fabricating
DNA machines on surfaces. The purpose of these efforts is to design nucleic acid-
modified surfaces where the macroscopic surface properties and functions can be
controlled by the nanoscale DNA machine driven by external triggers.
Other than in solution, i-motif-based nanomachines present high reversibility
even on solid surface and show interesting alterations in surface properties. The
set of nanomachine that utilizes oscillatory variant of the Landolt reaction shows
high efficiency and reversibility when the DNA strands are immobilized on a gold
surface to form a 2D array [
98
]. Liu and Zhou et al. actuated reversibly an array of
gold surface-immobilized DNA nanomotor by cycling the solution between slightly
acidic and basic pH, resulting in the conformation change between an i-motif and
duplex structure [
99
]. One end further away from the surface of the DNA is labeled
with a fluorophore. Through the mechanical work of the DNA, the fluorophore is
lifted up and brought down toward the gold surface, and the motion is transduced
into an optical “on-off” nanoswitch of the surface patterns.
The integration between an array of microfabricated silicon cantilevers and
an ensemble of i-motif motors has firstly facilitated the generation of repulsive
surface forces induced by protons, causing the microscale mechanical motion [
100
].
Changes on arrays of i-motif sequences fueled by protons have been employed to
alter the wettability of the surface between superhydrophilic and superhydrophobic
states [
101
](Fig.
11.18
a). The i-motif sequence-contained single-stranded DNA is
immobilized on the gold surface through Au-S bond on one end of strand, while the
other end of DNA is functionalized with a hydrophobic Bodipy-type fluorophore.
At low pH, DNA motors fold into the closed states and the hydrophilic phosphate
backbones of the strands are exposed on the surface, leading to a hydrophilic
surface. When the pH is neutralized, the i-motif structures unfold into single strands
and the hydrophobic groups migrated to the top of the surface, increasing the
hydrophobicity of the surface. However, the surface gives a metastable hydrophobic
state, observed by a dynamic spreading process of the water droplet, due to the
relatively loose arrangement of flexible single-stranded DNA, resulting in entropy-
driven molecular rearrangement of DNA motors at interface. The addition of
complementary DNA strands form the closed packed duplex DNA structures on
the surface and the spreading process is prevented. In this case, the surface is in a
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