This cycle is repeated five times, demonstrating that the entire process (SP to MC to MC–H+ to SP) is repeatable. It is important to note that although there appears to be slight drift, an imaginary line drawn through the semi-filled circles associated with the response measured at 431 nm (Fig. 2B)
has almost unity slope (slope =−0.00532 a.u./cycle).
This indicates reasonably stable response and an effective return to the baseline after each cycle, so the pattern is clearly reproducible.
The resulting film can be photoswitched to the strongly coloured MC form which can bind H+ ions. Exposing the MC–H+ complex to white light causes expulsion of the H+ ions and reversion to the SP form.
This implies that it should be possible to maintain the surface in the passive SP form, and activate to the H+ binding MC form using UV light when required, confirm the presence of the H+ ion using
colour/UV–vis measurements, expel it and subsequently revert to the passive form, again under photonic control.