Dual Aperture Control on pH- and Anion-Driven Supramolecular Nanoscopic Hybrid Gate-like Ensembles

Abstract

The development of gate-like systems able to perform certain programmed functions is an interesting way of taking chemistry to the frontiers of nanoscience. In relation to this field, we report a complete study of the behavior of a pH-driven and anion-controlled nano-supramolecular gate-like ensemble obtained by anchoring suitable polyamines on the pore outlets of mesoporous materials of the type MCM-41 (solid N3-S). The release of an entrapped dye (Ru(bipy)3(2+)) from the pore voids into the bulk solution allows us to study the gating effect. A pH-driven open/close mechanism was observed that arises from the hydrogen-bonding interaction between amines at neutral pH (open gate) and Coulombic repulsions at acidic pH between closely located polyammoniums at the pore openings (closed gate). Molecular dynamics simulations using force field methods have been carried out to explain the pH-driven open/close mechanism. For this purpose, a mesoporous silica structure was constructed, taking as base the (111) plane of the beta-crystoballite structure on which large hexagonal nanopores and anchored polyamines were included. From these calculations, it was observed how completely unprotonated amines display poor coverage of the pore (fully open gate), whereas completely protonated amines (simulating a pH 2 or lower) result in a clear reduction of the pore aperture, in agreement with the experimental results. In additional to the pH-driven protocol, opening/closing of the gate-like ensemble can also be modulated via an anion-controlled mechanism. This study was carried out by monitoring the dye released from the pore voids of the N3-S solid at a certain pH in the presence of a range of anions with different structural dimensions and charges, including chloride, sulfate, phosphate, and ATP (C(anion) = 1 x 10(-2) mol dm(-3)). The choice of a certain anionic guest results in a different gate-like ensemble behavior, ranging from basically no action (chloride) to complete (ATP) or partial pore blockage, depending on the pH (sulfate and phosphate). The remarkable anion-controllable response of the gate-like ensemble can be explained in terms of anion complex formation with the tethered polyamines. These experimental studies are also in agreement with computational simulations with fluoride, chloride, iodide, and dihydrogen phosphate anions. In the model, larger anions push the tethered polyamines toward the pore openings more efficiently, and therefore the pore aperture decreases. The studies also show that, for anions showing a strong tendency to form hydrogen-bonding networks (e.g., phosphate), complete pore blockage was observed at acidic pH. Finally, selectivity patterns have been discussed in terms of kinetic rates of the liberation of the Ru(bipy)3(2+) dye from the amine-functionalized dye-containing material N3-S.

Mesoporous Silica Nanoparticles: Their Projection in Nanomedicine

“…In the recent years, an intensive work has been performed in the functionalization of the pore outlet of these materials with organic and bioorganic moieties as gating devices, able to control the release of the trapped molecules in response to different stimuli such as chemicals [121][122][123][124][125], pH [126][127][128], temperature [129], redox reactions [130][131][132], enzymes [133], light [134][135][136][137][138][139][140], and antibodies [141] ( Figure 8). …”

Enrichment of stirred yogurts with folic acid encapsulated in pH-responsive mesoporous silica particles: Bioaccessibility modulation and physico-chemical characterization

“…However it is well known that large anions are able to interact with the protonated polyamines forming strong complexes which increased pore blockage and enhanced the inhibition of the cargo release (Casasús et al, 2008). Having this in mind a release assay from S1 was also performed in water solution adjusted at pH 4 with lactic acid.…”

Mesoporous silica nanoparticles equipped with surface nanovalves for pH-controlled liberation of doxorubicin

“…This is caused by combination of unique properties of silica (high drug loading capacity, tunable pore morphology, easy functionalization, biocompatibility) with the ability of immobilized nanostructures to respond to external influences. Construction of pH-sensitive molecular [9][10][11][12][13] or supramolecular [14][15][16][17][18][19][20][21][22] pore-blocking structures on the surface of silica nanoparticles with hexagonally arranged mesoporous channels causes a great interest of scientists as a promising approach to the creation of transport systems for pH-controlled drug release. Their action is based on the difference between the acidity of the tissues in which an inflammatory process takes place and the physiological pH of healthy cells, in particular, the acidity of tumour cells is substantially lower than that of normal ones.…”