pH- and competitor-driven nanovalves of cucurbit[7]uril pseudorotaxanes based on mesoporous silica supports for controlled release

The controlled release of the supramolecular nanovalves of cucurbit[7]uril (CB[7]) pseudorotaxanes based on mesoporous silica MCM-41 driven by dual stimuli were investigated in detail. The encirclement of CB[7] onto the protonated 1,4-butanediamine stalks anchored on the MCM-41 surfaces led to the tight closing of nanopores. The closed pores could be activated by the methods of deprotonation and competitive binding in aqueous media, so that the entrapped model molecules calcein were released from mesopores into bulk solutions. The nanovalve closing and opening were easily detected from the change in fluorescence intensity of calcein and even directly visualized from the change in color of dispersion solutions. The increase of pH resulted in the deprotonation of the initially protonated 1,4-butanediamine units and the dissociation of the supramolecular complexes. The amount of released calcein increased with increasing pH. The competitors cetyltrimethylammonium bromide (CTAB) and 1,6-hexanediamine could activate the nanovalves at near neutral pH through the shift of the supramolecular complex equilibrium. The increases in binding affinity and concentration of the competitors resulted in the increases of release rate and efficiency. Both of the methods could facilitate the nanovalves to realize controlled release on demand. The supramolecular nanovalves driven by a combination of deprotonation and competitive binding under various conditions will have many potential applications in different fields.