The controlled release of drugs by biostimuli is highly desirable under physiological conditions for their potential use in advanced applications. The enzyme-inspired controlled release of cucurbituril nanovalves by using magnetic mesoporous silica nanoparticles (MSNs) in near-neutral aqueous solutions is reported for the first time. The encirclement of cucurbit[7]uril (CB[7]) onto the protonated 1,4-butanediamine stalks tethered to the external surfaces of superparamagnetic Fe₃O₄-embedded mesoporous silica particles leads to tight blocking of the nanopores. The supramolecular nanovalves are activated by the enzymatic decarboxylation products of lysine, cadaverine (in the protonated form), which has a high affinity for CB[7], so that the encapsulated guest molecules, calcein, in the nanopores are released into the bulk solution. The release of calcein can be controlled in small portions on command by alternating changes in enzymatic decarboxylation products and CB[7]. The amino acid derived polyamines have long been associated with cell growth and cancers. The guest molecules released from the delivery system of magnetic MSNs can act not only on sensing probes for levels of decarboxylases and polyamines, but also on efficacious drugs to specific tissues and cells for regulation of polyamine synthesis.

This paper describes the structure and properties of the complex of acridine orange (AO) with cucurbit[7]uril (CB[7]) in aqueous solutions, studied by ¹H NMR spectroscopy, matrix-assisted laser desorption/ionization time-of-flight (MALDI–TOF) mass spectrometry, UV/Vis spectroscopy, and fluorescence spectroscopy with the aid of competitive binding methods. It was found that AO was included in the CB[7] cavity through strong ion–dipole interactions and hydrogen bonds. Theoretical studies based on calculations performed from first principles further confirmed this binding mode. Unusual downfield NMR shifts of the AO proton resonances in the presence of CB[7] were observed, in sharp contrast with cases of inclusion of other organic molecules in the CB[7] cavities, which resulted in upfield shifts of their proton resonances. The downfield shifts of the proton resonances of the AO·CB[7] complex were found to be the net result of small upfield shifts arising from the inclusion of AO in the CB[7] cavity and large downfield shifts resulting from the deaggregation of the AO aggregates, which were readily formed even at low concentrations in aqueous solutions. Further theoretical calculations explained the unusual downfield NMR shifts well. These research results are of importance for understanding of the structures and properties of complexes of related organic molecules with macrocyclic hosts. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture shows the formation of the inclusion complex of acridine orange (AO) with cucurbit[7]uril (CB[7]) and the unusual downfield NMR shifts of the proton resonances. This shift is the net result of large downfield shifts arising from the deaggregation of AO aggregates followed by small upfield shifts resulting from the inclusion of AO into the CB[7] cavity. The background shows the color change of the aqueous AO solution from yellow to green in the absence and presence of CB[7]. Details are discussed in the article by J. Ma, X. Du et al. on p.4931 ff.