Chemistry Reference
In-Depth Information
Fig. 33 A 160-kb molecular
electronic memory device
[
236
]. (a) Structural formula
of the molecular switches
used in the device.
(b) Technomorph
representations depicting how
the molecular switches can be
electrochemically toggled
“on” and “off” in the device
into co-conformational states
with different conductivities.
(c) SEM image of the
intersection between top (
red
)
and bottom (
yellow
)
electrodes for the molecular
switch tunnel junctions.
(d) Microscopic image of the
memory device (
blue
area)
with white blood cells
(
green cicles
) and the
electrical contacts (
red
)
a
b
Oxidation
O
O
O
Reduction
OFF
ON
N
N
S
S
c
S
S
N
N
O
O
O
-
4 PF
6
O
O
O
d
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
corresponding to approximately 250 molecules per junction. Actuation of the
bistable switches in these “molecular switch tunnel junctions” validated them as
storage elements; about 25% of the bits had sufficient on/off ratios to be configured
into a device. The resulting 160-kb memory circuit had a density of 10
11
bits/cm
2
,
for a total memory cell size of 0.0011
m
2
, which is approximately the cross-
sectional area of a white blood cell (Fig.
33b
). A memory circuit of these
dimensions is roughly on par with that of a storage density projected [
237
]tobe
available in 2020 if Moore's Law continues to be obeyed.
The question inevitably arises - couldwe buildmolecularmachines that are “smart”
enough to respond to signals within our bodies, releasing therapeutic drugs or diagnos-
tic contrast agents, for example, only at a localized and desired site? Once again, reality
is not so far from the dream. A number of research groups are developing a variety of
“mechanized nanoparticles” [
238
] with these applications in mind. Efforts thus far
have been associated chiefly with the covalent functionalization of supramolecular and
molecular switches on the surfaces of mesoporous silica nanoparticles (MSNPs) [
239
]
that can act as stimulus-responsive gates to the nanopores [
240
,
241
]. Since 2004,
bistable rotaxanes and pseudorotaxanes at the surface of these nanoparticles have been
demonstrated over and over again to selectively release cargo fromMSNPs in response
to changes in pH [
111
,
242
-
246
] light [
247
-
249
], redox stimuli [
115
,
250
,
251
], and
salt concentration [
252
], as well as oscillating magnetic fields [
253
] and the application
of specific enzymes [
254
] or small molecules [
255
]. Fig.
34
provides one example of
how they are operated, depicting the release profile of “snap-top” rotaxane-gated
m
