Molecular logic gate
[citation needed] One of the earliest ideas for the use of π-conjugated molecules in molecular computation was proposed by Ari Aviram from IBM in 1988.
Some early works made some progress in this direction, but they could not realize a complete truth table as their protonated ionic forms could not bind to the substrate in every case.
[7][8] De Silva et al. constructed an anthracene-based AND gate made up of tertiary amine and benzo-18-crown-6 units, both of which were known to show photoinduced electron transfer (PET) processes.
The PET is quenched upon coordination with protons[9] and sodium ions,[10] respectively, for the two receptors, and would cause the anthracene unit to fluoresce.
Sodium ions are encapsulated by the crown ether, resulting in a quenching of the PET process and causing the anthracene unit to fluoresce.
These groups, when attached to anthracene, can simultaneously process information concerning the concentration of the acid and oxidizing ability of the solution.
[12] De Silva et al. constructed an OR molecular logic gate using an aza-crown ether receptor and sodium and potassium ions as the inputs.
[13] Parker and Williams constructed a NAND logic gate based on strong emission from a terbium complex of phenanthridine.
Fluorescence of the highly-emissive boradiazaindacene (input “1”) was found to be quenched in the presence of either a zinc salt [Zn(II)] or trifluoroacetic acid (TFA).
Compound A is a push-pull olefin with the top receptor containing four carboxylic acid anion groups (and non-disclosed counter cations) capable of binding to calcium.
The logic gate operates as follows: without any chemical input of Ca2+ or H+, the chromophore shows a maximum absorbance in UV/VIS spectroscopy at 390 nm.
In organic solution the electron-deficient diazapyrenium salt (rod) and the electron-rich 2,3-dioxynaphthalene units of the crown ether (ring) self-assemble by formation of a charge transfer complex.
An enhanced fluorescence signal is observed only in the presence of excess protons, zinc and sodium ions through interactions with their respective amine, phenyldiaminocarboxylate, and crown ether receptors.
The processing mode operates similarly as discussed above – fluorescence is observed due to the prevention of competing PET reactions from the receptors to the excited anthracene fluorophore.
The specific concentration threshold of each input must be reached to achieve a fluorescent output in accordance with combinatorial AND logic.
When a strong base is added, the phenolic hydroxyl group is deprotonated, effecting a PET that renders the molecule non-emissive.
Due to the great difference in emission intensity, this single molecule is capable of carrying out subtraction at a nanoscale level.
[25] Wen and coworkers designed an INH molecular logic gate with Fe3+ and EDTA as the inputs and a fluorescent output for the detection of ferric ions in solutions.
A BODIPY dye attached to a crown ether and two pyridyl groups separated by spacers works according to an AND logic gate.
[35] The concept of DNA computing arose from addressing storage density issues because of the increasing volumes of data information.
[39] They used lanthanide complexes as fluorescent markers, and their luminescent outputs were detected by FRET-based devices at the terminals of DNA strands.
[43] Llopis-Lorente et al. developed a nanorobot that can perform logic operations and process information on glucose and urea.
[24] Dry molecular gates, such as the one demonstrated by Avouris and colleagues, prove to be possible substitutes for semiconductor devices due to their small size, similar infrastructure, and data processing abilities.
The nanotubes are doped differently in adjoining regions creating two complementary field effect transistors, and the bundle operates as a NOT logic gate only when satisfactory conditions are met.
