"Considering the great current interest in nanodevices and molecular-based machines, it is evident that a controllable nanovalve will be a key component of the nano-toolbox," Ben Feringa of the University of Groningen told nanotechweb.org. "Realizing that the membranes of our cells are full of valves and channels used for transport, it is not too far-fetched to build a hybrid system based on a natural channel protein and introduce a photoswitch to control the valve's opening and closing."

Feringa and colleagues used a channel protein from the Escherichia coli bacterium. Normally, the mechanosensitive channel of large conductance protein opens and closes a 3 nm-diameter pore to relieve the build-up of hydrostatic pressure inside the bacterial cell.

The researchers modified the protein with photosensitive compounds. Illuminating these compounds with light led to a build-up of charge, causing the valve to open. One of the compounds was reversible: illumination with light of a different wavelength caused the removal of the charge and closing of the valve.

To test the valve's response, the team incorporated a non-reversible valve into a liposomal membrane containing a fluorescent dye. Some dye leakage occurred under ambient conditions, but illuminating the valve with light with a wavelength of 366 nm caused the release of 43% of the liposome's content. Under the same test conditions, the reversible valve released a smaller percentage of the liposome's contents, probably because it had a higher hydrophobicity than the one-way switch.

"The construction of capsules that act as a container for a drug, but have valves that can be opened on command to release the drug, is within reach with our present findings," said Feringa. "The details of the mechanism of channel opening/closing under light [irradiation] will be studied, including factors governing the valve function."

The researchers reported their work in Science.