The host-guest chemistry of cucurbit[8]uril (CB[8]) and the layer-by-layer self-assembly technique are combined to obtain a molecular imprinted polyelectrolyte multilayer film for the recognition and binding of a guest molecule. Cucurbit[8]uril as a ready-made binding site is first associated with a polyelectrolyte and then assembled into a polyelectrolyte multilayer film via layer-by-layer deposition. A cationic guest is subsequently included into the nanocontainer due to specific host-guest interactions. The quantitative analysis of both CB[8] and the included guest molecule in dependence of the surface charge of the multilayer film identifies a high nanocontainer density as well as good to excellent binding efficiencies, therefore yielding a promising imprinted nanomaterial with potential applications in filtration or sensor technology. The investigation of the guest molecule uptake kinetics reveals two processes on different time scales, respectively, which are again related to the charge of the multilayer film surface. The combination of the results obtained from both ultraviolet spectroscopy and dissipative quartz crystal microbalance enables us to describe a full picture of several simultaneous processes initiated by the guest molecule.

Pillar[6]arene-containing multilayer films have been fabricated by alternating deposition of a stoichiometric complex consisting of both pillar[6]arene and methyl viologen with a photoreactive polyelectrolyte, diazoresin (DAR). After photoinduced cross-linking of the multilayer films, the guest molecule, methyl viologen, can be removed. Then, multilayer films with artificial binding sites are fabricated. The films show good properties for molecular uptake and release as well as selectivity for molecules with methyl viologen moieties. It is anticipated that this kind of multilayer films have future applications in the fields of enrichment of molecular dyes and purification of methyl viologen-polluted water.

The powerful host-guest chemistry of cucurbit[8]uril (CB[8]) was employed to obtain photoresponsive polyelectrolyte multilayer films for the reversible and photocontrolled binding and release of an organic guest molecule. For this purpose, we designed and synthesized a polyelectrolyte with azobenzene side groups. Then, CB[8] was associated with the azo side group to obtain a supramolecular host-guest complex that was further used as building block in order to prepare photoresponsive and CB[8]-containing polyelectrolyte multilayer films. Ultraviolet spectroscopy and a dissipative quartz crystal microbalance are employed to monitor the formation of the host-guest complex and the layer-by-layer self-assembly of the multilayer films, respectively. We demonstrate that the photoresponsive properties of the azo side groups are maintained before and after host-guest complexation with CB[8] in solution and within the multilayer films, respectively. A guest molecule was then specifically included as second binding partner into the CB[8]-containing multilayer films. Subsequently, the release of the guest was performed by UV light irradiation due to the trans-cis isomerization of the adjacent azo side groups. Re-isomerization of the azo side groups was achieved by VIS light irradiation and enabled the rebinding of the guest into CB[8]. Finally, we demonstrate that the photocontrolled binding and release within CB[8]-containing multilayer films can reliably and reversibly be performed over a period of more than 2 weeks with constant binding efficiency. Therefore, we expect such novel type of photosensitive films to have promising future applications in the field of stimuli-responsive nanomaterials.

Abstract:A water-soluble, anionic pillar [6]arene derivative (WP6) is applied as monomeric building block for the layer-by-layer self-assembly of thin polyelectrolyte multilayer films, and its pH-dependent host-guest properties are employed for the reversible binding and release of a methylviologen guest molecule. The alternating assembly of anionic WP6 and cationic diazo resin (DAR) is monitored in-situ by a dissipative quartz crystal microbalance (QCM-D). In solution, the formation of a stoichiometric inclusion complex of WP6 and cationic methylviologen (MV) as guest molecule is investigated by isothermal titration calorimetry and UV-vis spectroscopy, respectively, and attributed to electrostatic interactions as primary driving force of the host-guest complexation. Exposure of WP6-containing multilayers to MV solution reveals a significant decrease of the resonance frequency, confirming MV binding. Subsequent release is achieved by pH lowering, decreasing the host-guest interactions. The dissociation of the host-guest complex, release of the guest from the film, as well as full reversibility of the binding event are identified by QCM-D. In addition, UV-vis data quantify the surface coverage of the guest molecule in the film after loading and release, respectively. These findings establish the pH-responsiveness of WP6 as a novel external stimulus for the reversible guest molecule recognition in thin films.

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