Biomedical Engineering Reference
In-Depth Information
orientations of the monitored-water (see Fig.
1.43
). Moreover, we have found that
the monitored-water determined all the water orientations in the nanotube, upward
or downward in concert [
59
]. Quantitatively, we define the water dipole orientation
in the nanotube as an angle
i
between the
i
th water molecule and the nanotube
axis [
59
]:
D
acos.p
i
*
p
i
j
/;
i
u
=
j
*
p
i
where
is the dipole of
i
th water molecule and
u
is the axis unit vector of the
N
.t/ is computed by
nanotube. The averaged dipole angle
N
X
N
.t/
D
i
.t /=N.t /;
i
where the average runs over all the water molecules inside a nanotube at time
t
,and
N
(
t
) is the number of water molecules within this tube. The averaged dipole angle is
used to characterize the water-mediated signal transmission. The results are shown
in Fig.
1.43
. It is clear that ' falls into very different ranges most of the time for a
positive or negative signal charge: 110
ı
<
N
'<170
ı
for
q
DC
e
and 10
ı
<
N
'<
70
ı
for
q
D
e
, indicating that the water molecules within each nanotube are well
ordered (i.e., in concert). Thus, the charge (
C
e
/-
e
) signal at one end of the nanotube
can be readily distinguished from the dipole orientation (upward/downward) of the
water molecules at the other end of the nanotube.
Next, from the aforementioned systems, the water orientation states are extracted
every 10 ns after the first 30 ns of simulation time. In these states, we switch the
attached charge polarity and adopt them as initial states for new next simulations.
Based on these simulations with switched charge polarity, we discuss the time delay
of the water orientations in response to a switch in the charge signal. Figure
1.44
display the averaged angle
N
.t/ for four such scenarios. The time delay associated
with the branch tubes is 3.2 ns on average, with a minimal duration of 0.04 ns
(Fig.
1.44
(the last one)) and the maximal one 9.2 ns to respond to the
C
e
!
-
e
signal switch (which is slower) and approximately 0.07 ns only to respond to
the -
e
!C
e
switch (which is much faster). This obvious disparity in the response
time results from the interaction of the monitored-water with the signal charge. The
negative charge attracts the two hydrogen atoms of the monitored-water and limits
the mobility of the oxygen atom to a certain extent. Positive charge attracts the
oxygen atom, and the two hydrogen atoms have more freedom to rotate or swing,
which enables an easier switch for the configurations. This point can also be easily
seen from Fig.
1.43
, as we observe more fluctuations in the case with a positive
signal charge. We note that the short (average 0.07-3.2 ns) response time delay
implies that the signal with frequency in gigahertz range can be expected.
In response to a switch in the charge signal, it takes only tens of picoseconds
for the orientation of the entire water chain to flip over inside the nanochannels.
This delay time mainly results from the reorientation time of the monitored-water
