Researchers have found a molecular switch such that two compounds that would readily react with each other can be in the same solution, separated by a very thin membrane and kept from reacting with each other until a molecular switch is thrown. They showed the movement of a single chemical bond can compromise a membrane made up of more than 500 chemical bonds. Their system uses light as a switch to create a reversible, on-demand molecular control mechanism. The study was published in Nature Chemistry.

There are many applications that one can imagine developing from these fundamental findings, especially ones that need a controlled release. "But upon exposure to light, the membrane gets compromised to allow the two components to react with each other," he adds. "The interesting thing is that the membrane is not permanently compromised upon exposure to light, but only when the light is on."

Such reversible molecular controls that respond only when there is a source of energy are quite rare in artificial systems, he says. Usually in artificial, human-made systems, "materials are in an equilibrium state, so if you have a particle that responds to pH change and you put it into an environment that triggers a change, it stays changed. You can't put the genie back into the bottle."

By contrast, nature has engineered some "exquisitely responsive systems," the authors point out, where molecular-scale information is transferred across a membrane that can return to its original, resting state. An example Thayumanavan likes to use refers to ATP, a cellular energy molecule that switches on and off on demand, like their new system. "I tell my students that they may have the impression that a professor never stops talking.

But that is demonstrably false if you just track ATP.  ATP is turned on and being used, but when the ATP is not being used, the professor goes to the resting state, i.e. shuts up. The genie does go back in the bottle." Technically speaking, researchers demonstrate that in their system, light induces actuation of a thin bi-layer of molecules made of a hydrophilic-azobenzene-hydrophobic diblock copolymer.

When light is turned on, the azobenzene bond rotates, and this motion sends a frontal wave across about 500 bonds to compromise the membrane barrier. This allows the entire membrane to allow molecules to travel across. When we turn the light off, it closes again and no molecules can get across the membrane."

The researchers show that the out-of-equilibrium actuation is caused by the photochemical trans-cis isomerization of the azo group, a single chemical functionality, in the middle of the interfacial layer.