Photoisomerization of an individual azobenzene molecule in water: an on-off switch triggered by light at a fixed wavelength.

The alpha-hemolysin (alphaHL) pore was used as a nanoreactor for the direct observation of the reversible photoisomerization of individual tethered azobenzene molecules in an aqueous environment. alphaHL pores, PAZO, were used that had been derivatized within the lumen at a single cysteine residue with 4-((4-(2-chloroethanoamido)phenyl)diazenyl)benzenesulfonate. Trans-cis isomerizations were monitored at the single-molecule level by observing the modulation of the current passing through PAZO by electrical recording in planar bilayers. When PAZO was irradiated at 330 nm, continuous interconversion between the trans and cis states was observed. Either the trans or the cis state was maintained in the dark, depending upon which was present when the light source was shuttered. The cis state of PAZO was surprisingly stable in the dark, and no cis --> trans transitions were seen over a total observation period of more than 8 h. Therefore, based on our findings, it might be possible to make fast digital nanoscale switches operated by light of a fixed wavelength.