The develop
ment of gate-like
sy
ste
ms able to perfor
m certain progra
mmed function
s i
s anintere
sting way of taking che
mi
stry to the frontier
s of nano
science. In relation to thi
s field, we report aco
mplete
study of the behavior of a pH-driven and anion-controlled nano-
supra
molecular gate-like en
se
mbleobtained by anchoring
suitable polya
mine
s on the pore outlet
s of
me
soporou
s material
s of the type MCM-41 (
solid
N3-S). The relea
se of an entrapped dye (Ru(bipy)
32+) fro
m the pore void
s into the bulk
solutionallow
s u
s to
study the gating effect. A pH-driven open/clo
se
mechani
sm wa
s ob
served that ari
se
s fro
m thehydrogen-bonding interaction between a
mine
s at neutral pH (open gate) and Coulo
mbic repul
sion
s at acidicpH between clo
sely located polya
mmoniu
ms at the pore opening
s (clo
sed gate). Molecular dyna
mic
ssi
mulation
s u
sing force field
method
s have been carried out to explain the pH-driven open/clo
se
mechani
sm.For thi
s purpo
se, a
me
soporou
s silica
structure wa
s con
structed, taking a
s ba
se the (1
mage
s/entitie
s/one
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mage
s/gifchar
s/beta2.gif" BORDER=0 ALIGN="
middle">-cry
stoballite
structure on which large hexagonal nanopore
s and anchored polya
mine
s were included.Fro
m the
se calculation
s, it wa
s ob
served how co
mpletely unprotonated a
mine
s di
splay poor coverage ofthe pore (fully open gate), wherea
s co
mpletely protonated a
mine
s (
si
mulating a pH 2 or lower) re
sult in aclear reduction of the pore aperture, in agree
ment with the experi
mental re
sult
s. In additional to the pH-driven protocol, opening/clo
sing of the gate-like en
se
mble can al
so be
modulated via an anion-controlled
mechani
sm. Thi
s study wa
s carried out by
monitoring the dye relea
sed fro
m the pore void
s of the
N3-Ssolid at a certain pH in the pre
sence of a range of anion
s with different
structural di
men
sion
s and charge
s,including chloride,
sulfate, pho
sphate, and ATP (
Canion = 1 × 10
-2 mol d
m-3). The choice of a certainanionic gue
st re
sult
s in a different gate-like en
se
mble behavior, ranging fro
m ba
sically no action (chloride)to co
mplete (ATP) or partial pore blockage, depending on the pH (
sulfate and pho
sphate). The re
markableanion-controllable re
spon
se of the gate-like en
se
mble can be explained in ter
ms of anion co
mplex for
mationwith the tethered polya
mine
s. The
se experi
mental
studie
s are al
so in agree
ment with co
mputational
si
mulation
s with fluoride, chloride, iodide, and dihydrogen pho
sphate anion
s. In the
model, larger anion
spu
sh the tethered polya
mine
s toward the pore opening
s more efficiently, and therefore the pore aperturedecrea
se
s. The
studie
s al
so
show that, for anion
s showing a
strong tendency to for
m hydrogen-bondingnetwork
s (e.g., pho
sphate), co
mplete pore blockage wa
s ob
served at acidic pH. Finally,
selectivity pattern
shave been di
scu
ssed in ter
ms of kinetic rate
s of the liberation of the Ru(bipy)
32+ dye fro
m the a
mine-functionalized dye-containing
material
N3-S.