"I am concerned about the serious effects of artificial chemicals on the environment and living bodies," Masahiro Fujiwara told nanotechweb.org. "One approach to the problem is the use of those chemicals only 'when they are required, where they are required' and in the required quantity. I wish to create the material for this technology."

Fujiwara and colleagues used the hexagonal mesoporous silica MCM-41, which they modified with molecules containing coumarin groups. The team attached these molecules to the outlets of the pores, rather than to their inside walls, by using MCM-41 still filled with the template molecules that directed its pore formation. As a result, the coumarin molecules formed a "molecular door" at the pore outlet.

Shining ultraviolet light with a wavelength longer than 310 nm onto the silica caused the coumarin-bearing molecules to dimerize (i.e. join together) so that they blocked the exit from the pore (see figure). Applying light with a wavelength of around 250 nm, on the other hand, split up the dimers and opened the "hinged double door".

"The significant technical factor is the preparation method to attach the coumarin substituent selectively near the outlet of the pore," explained Fujiwara. "If the substituent is not attached densely near the outlet of the pore, the controlled release is not effective."

Now the researchers, who reported their results in Nature, plan to use stimuli other than light to open or close the doors, for example pH, temperature, ions, molecules, enzymes or viruses.