Molecular Logic Gate Operates in Nanospace

MOLECULAR COMPUTATION

Chemists in Northern Ireland and Japan have designed a fluorescent molecule that carries out a logical computation in the nanospace of a membrane (J. Am. Chem. Soc. 2005, 127, 8920). The system operates as a two-input AND logic gate, in which two conditions must both be satisfied to produce an output.

The paper shows the true miniaturization capability of computation using molecules by achieving a logic operation inside a nanospace with a 3-nm radius, says A. Prasanna de Silva, a chemistry professor at Queens University, Belfast, who carried out the work with University of Tokyo assistant professor of chemistry Seiichi Uchiyama and coworkers. This is the highest level miniaturization, by far, of a computational device. To the best of our knowledge, the smallest known silicon devices have features of 65 nm. Our paper also shows, for the first time, that membranes provide a fine host for synthetic computing molecules.

The computation molecule has an anthracene fluorophore linked by short methylene spacers to trialkylamino and benzo-15-crown-5 moieties that are receptors for H⁺ and Na⁺ ions, respectively. The two ions serve as the inputs for the AND logic operation. Both have to be present for an output–fluorescence–to occur. The trialkylamino receptor is connected to a long alkyl chain that anchors the molecular logic gate to the membrane and away from bulk water.

The membrane is a detergent micelle made from tetramethylammonium dodecyl sulfate. The nanospace straddles the membrane, de Silva explains. It starts from the center of the micelle and extends beyond the sulfate groups to include a good part of the diffuse counterion cloud of tetramethylammonium cations and arriving H⁺ and Na⁺ ions.

The two-input AND logic operation that we demonstrate is only one of around 30 molecular computational operations available in the literature that should now be demonstrable in the same way, he adds.

David A. Leigh, professor of organic chemistry at the University of Edinburgh, in Scotland, remarks that the paper is a lovely piece of work. Two of the inherent problems with molecular logic gates are, how can you compartmentalize them, and how small can these compartments be? he says. The paper shows that if you incorporate the artificial logic system into micelles, the compartments can be very small indeed. The work is notable for the elegant simplicity that often characterizes output from these two groups. I expect the idea to be widely adopted.