Molecular gates spin photons from chemicals
The human eye uses millions of nanoscale photoreceptors that output chemical signals when they are stimulated by photons. By reversing that process, a team of researchers in Northern Ireland and Japan has engineered a tiny molecular transistor that emits photons when supplied with the right chemicals.
The researchers showed just how small logic gates can be made when using individual molecules—in this case, 3nm in radius. And besides demonstrating the ability to operate nanoscale molecular logic gates (a capability that has been shown elsewhere in solutions inside test tubes), the team showed how such gates could be embedded in an organic thin film. Arrays of logic gates could be assembled on such membranes, heralding a comprehensive architecture for future molecular-sized computers, the team said.
"We introduced molecular logic in 1993 and, since then, have slowly expanded what molecules can do in the computation field. The small size of molecules vis-a-vis semiconductors has been an important advantage to exploit," said A. Prasanna de Silva, the Queen's University of Belfast professor who headed the team. Other members included University of Tokyo professors Seiichi Uchiyama and Kaoru Iwai as well as post-doctoral researcher Gareth McClean.
The current demonstration was for an AND gate, but the group has previously demonstrated NOT, OR and NOR gates. The principle used is dubbed fluorescent photoinduced electron transfer (PET). Molecules designed for PET variously accept either protons or ions as input. When their inputs are satisfied, they switch on a fluorophene molecule, which then produces light.
For the AND gate, the group used a sodium ion and a hydrogen ion as inputs. The gate only switched on when both inputs were present—the classical AND logic function.
A supermolecule was assembled into a logic gate by joining a fluorophene molecule and an input receptor molecule with a spacer molecule that disabled the fluorophene's normal fluorescence capability. When the two input receptors received their corresponding chemical inputs, PET was initiated in a process similar to what's seen during green-plant photosynthesis. By turning on the fluorophene molecule, the molecular AND gate switched on the previously suppressed fluorescence.
"We have shown for the first time that membranes can provide a fine host for synthetic computing molecules," said de Silva. "We are casting the various molecular logic operations we have achieved into membrane media."
An anthracene molecule was used for the experimental AND gate. It was linked by a short methylene spacer to a trialkylamino and benzo-15-crown-5, which had two receptors—one for the hydrogen ion and one for the sodium ion. The membrane architecture allowed the receptors to stick out of one side of the thin film, with the output fluorophene molecule sticking out on the other side, thus isolating the output photons from the liquid chemical inputs.
The membrane used was a thin film of tetramethylammonium dodecyl sulfate. "The molecular logic gate straddles the membrane, and strong fluorescence only occurs when both receptors catch the correct ions," said de Silva.
According to de Silva, the two-input AND logic gate is only one of 30 molecular computational operations that have been demonstrated by his and other research groups worldwide. Now, he believes that all of these logical operations should be amenable to the PET technique his group has developed.
For the future, de Silva plans to demonstrate complex logical operations at the molecular level and identify other biological processes that are amenable to molecular-logic operations.
- R. Colin Johnson
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