Chemistry Reference
In-Depth Information
with different molecular logic elements can be easily distinguished in an ensemble
on the basis of the logic response of their fluorescence upon addition of acid and
alkali solutions. The number of distinct chemically switched tags available can be
scaled up by increasing the number of logic types and including those that respond
to more than one input (AND, OR, XOR, INH, etc.). Luminophores other than the
anthracene signaling unit used in compounds 1-3 could be employed, each having
characteristic excitation/emission spectral features and luminescence lifetime.
This study shows that a large number of molecular logic gates can be designed
such that each displays a unique signature (luminescence output) in response to
chemical (inputs of ions or molecules) or physical (light or heat) stimulation under
determined experimental conditions (excitation/emission wavelengths, input
threshold values, temperature, etc.). This starting set of tags can be increased further
if the targeted objects are marked with mixtures of logic gates in a chosen molar
ratio, and more than two output levels are considered. The final MCID tag address
of a given object can be represented by a sequence of terms, like a car license plate
or an Internet Protocol address, for instance, (
l
max,em
).(logic types and
combinations).(input types).(input thresholds). To give an example, the tag of
bead D in Fig.
6b
can be represented as (368).(422).(PASS 1
l
max,exc
).(
þ
YES, 1:1).
(H
þ
,H
þ
).(none, pH
4.9) [
35
].
Molecular logic can also be useful for the development of nanosystems for
therapeutic applications. Molecular devices able to generate a chemical output by
processing chemical inputs according to programmed logic functions can be viewed
as “secured” or “smart” delivery systems. Such systems could release a drug
molecule only in response to a predetermined set of external inputs, or when the
concentrations of a given number of chemical inputs signaling a specific condition
(e.g
.
, a disease) rise above (or fall below) appropriate threshold values.
An interesting example is a semibiological molecular device capable of
controlling the folding of a protein with AND logic in response to ATP (adenosine
triphosphate) and light stimulations [
36
]. Noticeably, a molecular automaton based
on DNA (deoxyribonucleic acid) and DNA-manipulating enzymes [
37
,
38
] has
been utilized to achieve logical analysis of gene expression and consequent con-
trolled administration of a biologically active molecule. The automaton was
programmed to identify and analyze in vitro messenger RNAs (ribonucleic acid)
of disease-related genes associated with some forms of cancer, and generate a
single-stranded DNA molecule modeled after an anticancer drug [
38
]. This work
is an important step towards the construction of molecular computers operating
in vivo and capable of autonomously diagnosing a disease and effecting a therapy.
¼
6 From Molecules to Molecular Devices and Machines
In the last 30 years, chemists have learnt to assemble molecules [
18
] and now, by
exploiting the molecule-by-molecule “bottom-up” approach [
39
,
40
], they have
virtually unlimited possibilities to design and construct supramolecular species and
enter the field of nanotechnology.
