Israeli organic chemists have created a calculator the size of a single molecule.

Electronic calculators may have decreased in size dramatically over the past 30 years, but a team of Israeli organic chemists has now shrunk the calculator to the size of a single molecule.

Their calculator is based on the fluorescent molecule fluorescein, which can exist in one of four different ionisation states (cation, neutral, monoanion and dianion). Fluorescein can be switched between these different states by simply changing the pH of the surrounding solution, which can be done by adding an acid (HCl) or a base (NaOH). It will absorb light at different wavelengths depending on its ionisation state.

These special properties allow fluorescein to act like a suite of logic gates, with the acid and base as inputs and light absorption as the output. In previous research, the team from the Weizmann Institute of Science, Rehovot, led by Abraham Shanzer, succeeded in using fluorescein to add and subtract two binary digits. 

To develop a proper molecular calculator, however, Shanzer and his colleagues needed to find a way for fluorescein to add and subtract three binary digits. This would allow it to deal with digits carried over from previous calculations (adding and subtracting binary numbers often produces carry over digits, as with long strings of decimal numbers).

The chemists discovered that, because they’re using chemical inputs, only one three-digit addition (1+1+1) and one three-digit subtraction (0-1-1) posed novel challenges for their fluorescein calculator. They worked out that both calculations could be conducted by adding several base inputs one after another to generate the dianion state. 

This meant, however, that they needed to generate two different outputs from the same ionisation state. They did this by monitoring both the absorbance and fluorescence of the dianion state for the addition, but only monitoring the absorbance for the subtraction.

’This is very clever work,’ says organic chemist AP de Silva from Queen’s University, Belfast, who is a pioneer in the field of molecular computing.

Jon Evans