Biomedical Engineering Reference
In-Depth Information
orientations with a roughly equal probability in the BTs. This behavior can be partly
explained from the interactions of water molecules at the Y-junction. When the
water orientations are downward in the MT, the O atom of the uppermost water
molecule in the MT attracts two H atoms from the two bottommost water molecules
in the BTs; on the other hand, when the water orientations are upward in the MT,
one of the H atoms of the uppermost water molecule in the MT attracts both O atoms
in the bottommost water molecules in the BTs. The interaction in the former case is
much stronger than interaction in the latter case. Consequently, the average value of
P
(
t
) in the branch tubes is 0.5 for a concerted upward water orientation in the MT
(corresponding to a positive monitored charge).
In Y-shaped nanochannels, water reorientation also holds its time delay in
response to a switch in the charge signal [
59
]. In our simulations, the time delay
associated with the BTs is found to be slow, 40 ns on average with a maximal
duration of 150 ns, to respond to the downward
!
upward signal switch of the
water orientations in the MT, but much faster, 4 ns only, to respond to the
upward
!
downward switch of the water orientations in the MT. Clearly, the former
has longer delay time than the latter. This is because that the downward water
orientations in the MT (where one O atom controls two H atoms at the Y-junction)
are much more stable than the case for the upward water orientations in the MT
(where one H atom controls two O atoms at the Y-junction). It takes more time to
shift from a more stable state to a less stable one. Compared to the time delay for
switches in a single nanochannel (0.07-3.2 ns), the response times in the Y-channels
are also generally longer, particularly for the downward
!
upward switch.
We emphasize that the stability of water chain confined in nanochannels is a
guarantee to achieve water-mediated signal transmission, conversion, and multipli-
cation. The phenomenon that we have observed is robust. First, we note that the
signal multiplication capability is not very sensitive to the angles of the Y-shaped
tube (the angles between SWNTs, currently at 120
ı
, 120
ı
, 120
ı
). We have tried T-
shaped tubes (with an angle of 90
ı
,90
ı
, 180
ı
) and other slightly different angles,
and the results are more or less the same in terms of water orientation propagations.
Next, the orientation of the monitored-water and, consequently, the orientations
of the all water molecules in the nanotubes can be controlled by a single charge with
a quantity of one electron. This is a remarkable capability from the viewpoint of the
signal transmission and conversion. Thus, what is the minimal charge needed for
this capability? Numerical analysis shows that the magnitude of this signal charge
needs to be larger than 0.6
e
—our data using a charge from 0.6
e
to 1.0
e
indicate that
the water orientations can be well controlled with such a single charge. From the
previous discussion [
22
], it is clear that the orientation of the monitored-water can be
“fixated” only when the electrostatic interaction energy between the external charge
and the monitored-water is comparable to the interaction energy of the monitored-
water with either side of its neighboring water molecules. Since the imposed charge
is placed at the center of a second carbon ring of the nanotube, which has a distance
of about 4 A from the centerline of the nanotube (close to the distance between two
water neighboring molecules), the charge required to control the orientation of the
