The develop
ment of gate-like syste
ms able to perfor
m certain progra
mmed functions is aninteresting way of taking che
mistry to the frontiers of nanoscience. In relation to this field, we report aco
mplete study of the behavior of a pH-driven and anion-controlled nano-supra
molecular gate-like ense
mbleobtained by anchoring suitable polya
mines on the pore outlets of
mesoporous
materials of the type MCM-41 (solid
N3-S). The release of an entrapped dye (Ru(bipy)
32+) fro
m the pore voids into the bulk solutionallows us to study the gating effect. A pH-driven open/close
mechanis
m was observed that arises fro
m thehydrogen-bonding interaction between a
mines at neutral pH (open gate) and Coulo
mbic repulsions at acidicpH between closely located polya
mmoniu
ms at the pore openings (closed gate). Molecular dyna
micssi
mulations using force field
methods have been carried out to explain the pH-driven open/close
mechanis
m.For this purpose, a
mesoporous silica structure was constructed, taking as base the (1
mages/entities/one
macr.gif">1) plane of the
mages/gifchars/beta2.gif" BORDER=0 ALIGN="
middle">-crystoballite structure on which large hexagonal nanopores and anchored polya
mines were included.Fro
m these calculations, it was observed how co
mpletely unprotonated a
mines display poor coverage ofthe pore (fully open gate), whereas co
mpletely protonated a
mines (si
mulating a pH 2 or lower) result in aclear reduction of the pore aperture, in agree
ment with the experi
mental results. In additional to the pH-driven protocol, opening/closing of the gate-like ense
mble can also be
modulated via an anion-controlled
mechanis
m. This study was carried out by
monitoring the dye released fro
m the pore voids of the
N3-Ssolid at a certain pH in the presence of a range of anions with different structural di
mensions and charges,including chloride, sulfate, phosphate, and ATP (
Canion = 1 × 10
-2 mol d
m-3). The choice of a certainanionic guest results in a different gate-like ense
mble behavior, ranging fro
m basically no action (chloride)to co
mplete (ATP) or partial pore blockage, depending on the pH (sulfate and phosphate). The re
markableanion-controllable response of the gate-like ense
mble can be explained in ter
ms of anion co
mplex for
mationwith the tethered polya
mines. These experi
mental studies are also in agree
ment with co
mputationalsi
mulations with fluoride, chloride, iodide, and dihydrogen phosphate anions. In the
model, larger anionspush the tethered polya
mines toward the pore openings
more efficiently, and therefore the pore aperturedecreases. The studies also show that, for anions showing a strong tendency to for
m hydrogen-bondingnetworks (e.g., phosphate), co
mplete pore blockage was observed at acidic pH. Finally, selectivity patternshave been discussed in ter
ms of kinetic rates of the liberation of the Ru(bipy)
32+ dye fro
m the a
mine-functionalized dye-containing
material
N3-S.