Now, a team of scientists at the Weizmann Institute of Science in Israel has developed a molecule that can function as an ultraminiature keypad lock.

The molecule is composed of two smaller linked units (fluorescent probes) separated by a molecular chain to which iron can bind. One of these probes shines bright fluorescent blue and the other, fluorescent green, but only when keyboard inputs are correct. Rather than the electric pulses of an electronic keypad, however, inputs consist of iron ions, acids, bases, and ultraviolet light.

The group has previously shown that such molecules can serve as logic gates. As opposed to electronic logic gates, in which electrical switches flip on and off, the molecules, with various combinations of chemical and light inputs, can switch between colors and light intensities to perform arithmetic calculations.

The challenge is generating sequences that are distinguishable one from another. The scientists found that by controlling the opening rate of the logic gate within the reaction time frame, they could produce different, identifiable outputs, depending on input order. By adding light energy, which also influences the molecules' glow, they built a molecule-sized device that lights up only when shown the correct chemical password.

Such molecular keypads could one day serve as tiny ID tags to prevent forgery. Other potential applications include smart diagnostic equipment that detects the release of biological molecules or changes in conditions that indicate disease.