Biomedical Engineering Reference
In-Depth Information
Here, we find water, as the most important matter in the world, can be designed
to become an excellent medium for the transmission of signals. Water molecules
confined within nanoscale channels exhibit structures and dynamics that are very
different from bulk [
6
,
15
,
17
,
22
,
32
,
34
,
43
,
52
,
99
-
103
]. Particularly, in nanochan-
nels with suitable radii, water molecules are confined to form one-dimensional
chains linked by hydrogen bonding with the “concerted” water dipole orientations
[
21
,
22
,
43
,
104
], which provide an excellent example for the transmission of signals
due to water dipole interactions. Note that the stable water molecular chain is a
prerequisite for signal transmission as a medium.
Fortunately, the reorientation of this water dipole chain has its characteristic time
estimated to be in the range of 2-3 ns for CNTs with a length of 1.34 nm [
21
]. More
remarkably, the water molecule chains in a nanotube can remain dipole-ordered up
to macroscopic lengths of 0.1 mm, with durations up to 0.1 s [
104
].
As an example, Fig.
1.43
shows the “concerted” orientations of water molecules
(water dipoles are ordered cooperatively inside the CNT) [
21
,
22
,
40
,
43
,
59
,
104
]. If
we can “tune” the orientation of one water molecule using a single charge at one end,
we might be able to control the orientations of other water molecules in the same
channel or other connected channels. In other words, we have a molecular level
“signal transmission,” i.e., converting a charge signal to water dipole orientation
signal (at one end) and then transmitting the water dipole orientation signal to other
ends, which might be converted back to a charge signal again. Furthermore, if we
use Y-shaped nanochannels, i.e., three nanochannels connected with each other to
form a Y-junction, we can even achieve a “signal multiplication,” which means that a
water dipole orientation signal can be multiplied into many water dipole orientation
signals [
59
]. It is also observed that the signal transmission and multiplication, via
water molecules confined in nanoscale channels, can be effectively shielded from
thermal fluctuations.
We have applied MD simulations, which are widely used in nanoscale- and
molecular-scale simulations for both physical and biological systems [
22
,
59
,
105
-
111
], to investigate this interesting phenomenon of water-mediated signal
transmission. NVT ensemble simulations have been carried out at a constant
temperature of 300 K using a Berendsen thermostat [
105
] and in constant volumes
(L
x
L
y
L
z
D
6:01 nm
6:01 nm
11:00 nm in SWNT systems with 12,922 water
molecules using the molecular modeling package Gromacs 4.0.5 [
106
,
108
], and
more settings are referred to Ref. [
59
]). A typical (6, 6) uncapped armchair SWNT
with a width of 0.81 nm and a length of 5.13 nm is adopted as shown in Fig.
1.43
.
This device is constrained at the center of the simulation boxes by using the position
restraints (the whole tube length is partitioned evenly into three segments with an
interval 1.71 nm, and thus the four rings of carbon atoms are constrained) solvated
with water molecules with constant density. We have adopted the particle-mesh
Ewald method [
112
] to model long-range electrostatic interactions, and we have
applied periodic boundary conditions in all directions. A time step of 2 fs is used,
and data are collected every 0.5 ps. In all of our simulation results, the TIP3P water
model is applied and the carbon atoms are modeled as uncharged Lennard-Jones
particles with a cross section of
CC
D
0:34nm and
CO
D
0:3275 nm, and a
