pH clock-operated mechanized nanoparticles

Article abstract of DOI:10.1021/ja9010157

Mechanized nanoparticles (MNPs) consisting of supramolecular machines attached to the surface of mesoporous silica nanoparticles are designed to release encapsulated guest molecules controllably under pH activation. The molecular machines are comprised of

Full text of DOI:10.1021/ja9010157

Published on Web 08/25/2009
pH Clock-Operated Mechanized Nanoparticles
Sarah Angelos,† Niveen M. Khashab,‡ Ying-Wei Yang,†,§ Ali Trabolsi,‡ Hussam A. Khatib,‡
J. Fraser Stoddart,*,‡ and Jeffrey I. Zink*,†
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095, and
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208
Received February 9, 2009; E-mail: stoddart@northwestern.edu; zink@chem.ucla.edu
Mesoporous silica nanoparticles, mechanized with nanovalves1
and other artificial molecular machines,2 are promising vehicles
for drug delivery because they are biologically compatible and are
capable of storing and releasing molecules on command. Previously,
mechanized nanoparticles (MNPs) designed to release their contents
in response to an externally controlled light source were used to
deliver therapeutic molecules to human cancer cells.3 However,
for some applications, self-opening drug carrier vehicles that
respond autonomously to a particular biostimulus are preferred. In
the work reported here, we present tunable pH-operable MNPs in
which biologically relevant pH changes are used to trigger the
release of guest molecules.
One of the design challenges inherent in preparing MNPs is
selecting suitable mechanical components such that the system
operates under the desired conditions. Central to the design of these
systems is the knowledge that mesoporous silica nanoparticles are
noncytotoxic and undergo cellular uptake into acidic lysosomes
through endocytosis.4 We conceived that a MNP system designed
to remain closed at neutral pH levels (i.e., the bloodstream) but
open under mildly acidic conditions (i.e., in lysosomes) could
release its contents autonomously upon cell uptake.
The supramolecular machines described here that fulfill these
requirements are based5 on the pumpkin-shaped macrocycle,
cucurbit[6]uril (CB[6]), which forms6 pH-dependent complexes with
aminoalkanes when they are protonated. We recently reported1a
a
type of MNP based on the interaction of CB[6] with a bisammonium
pseudorotaxane that operates under basic conditions, but it is
unsuitable for biological applications because of the high pH (∼10)
that is required for activation. The unique biologically relevant
supramolecular machines described here consist7 of bistable CB[6]/
trisammonium pseudorotaxanes that are attached to the surface of
mesoporous silica nanoparticles and operate to encapsulate Pro-
pidium Iodide (PI) guest molecules at neutral pH and then release
the contents under mildly acidic conditions. This design (Figure 1)
incorporates the novel feature that the pH required for activation
of this MNP system can be selectively “dialed-in.”
The operation of the MNPs presented here relies on three
prominent design features: (1) the difference in basicity of the
anilinium nitrogen atom in comparison with the other two nitrogen
atoms, (2) the difference in the length of the oligomethylene spacers,
and (3) the identity of functional group R. The anilinium nitrogen
atom is approximately 106-fold less basic than the alkyl nitrogen
atoms, resulting in it being unprotonated at neutral pH. By this
design, at neutral pH, the CB[6] ring resides on the tetramethyl-
enediammonium recognition unit so that both portals of the
macrocycle are able to engage in ion-dipole binding interactions,
Figure 1. Design and pH-dependent operation of MNP-1 (R ) H) and
MNP-2 (R ) OMe). When the pH is lowered such that the anilinium
nitrogen atom is protonated, the CB[6] ring shuttles to the distal hexa-
methylenediammonium station, and PI is released (a). At neutral pH,
the CB[6] ring sits on the tetramethylenediammonium recognition
unit, blocking the nanopores (b). When the pH is raised, all of the
nitrogen atoms on the stalk are deprotonated resulting in dethreading of
the CB[6] ring (c). The system is designed such that the pH-response
can be fine-tuned by changing R to modulate the pKa of the anilinium
nitrogen atom.
† University of California, Los Angeles.
‡ Northwestern University.
§ Current Address: Department of Chemistry, University of California, Irvine,
California 92697.
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C O M M U N I C A T I O N S
thereby blocking the pore orifices and encapsulating the guest
molecules. The difference in the oligomethylene spacer length is
an important aspect of the design because the stability constant for
to allow for complexation between the CB[6] rings and the
trisammonium stalks. The final mechanized nanoparticles are highly
dispersible in water with an average diameter of 134 nm as
measured by dynamic light scattering (Supporting Information).
Testing and characterization of the MNP operation were per-
formed in solution using luminescence spectroscopy. A sample of
dye-loaded MNPs (∼10 mg) was loaded into the corner of a cuvette,
and H2O (∼12 mL) was added carefully. A probe beam was
directed into the H2O, and the emission intensity of the dissolved
dye was collected as a function of time to generate a release
profile. At ∼300 s, the pH was adjusted to a specific level
through the addition of either 0.01 M HCl or 0.01 M NaOH.
The percentage of release was calculated using absorbance
spectroscopy to enable quantitative comparison of the release
efficiency (Supporting Information).
The release profiles (Figure 3) demonstrate that MNPs are able
to contain guest molecules at neutral pH but release them in
response to pH changes. When different acidic pH values are used
to activate MNP-1 (Figure 3a), it is apparent that the rate of release
for the MNPs depends directly on the pH that is used for activation,
with the highest release rate occurring at the most acidic pH. When
the pH is adjusted to 3.4, 100% release is obtained after 1800 s.
However, when the pH is adjusted to 4.4, only 71% release occurs
in the same amount of time. The response is even slower when pH
5.0 is used: only 35% of the maximum release is observed after
1800 s. Virtually no release is observed over the course of the
experiment when the pH is adjusted to 6.5. The observed pH-
+
the complexation of NH3+(CH2)6NH3 with CB[6] is an order of
magnitude greater than that for NH3+(CH2)4NH3+.7 The six-carbon
spacer is a better match with the inner dimensions of CB[6] and
allows for optimal ion-dipole interactions between both occuli of
the CB[6] ring and an ammonium center. Therefore, once the pH
is lowered such that the anilinium nitrogen atom becomes proto-
nated, the CB[6] ring shuttles to the distal hexamethylenediammo-
nium recognition unit, such that the pore orifice is unblocked and
the encapsulated contents are released (Figure 2). Finally, by tuning
the pKa of the anilinium nitrogen through chemical modification at
the para postion R of the terminal phenyl group, the pH at which
the MNP is opened can be adjusted. The pKa of the nitrogen atom
in aniline is approximately 1 pKa unit less basic than that of the
nitrogen atom in p-anisidine.8 In following this trend, we have
prepared two different MNP systems, MNP-1 (R ) H) and MNP-2
(R ) OMe), which are tuned to respond at different pH levels.
Because the identity of the substituent R affects the pKa of the
anilinium nitrogen atom, it serves to modulate the acidic pH that
is required for activation. Alternatively, but of less importance to
biological applications, activation of these MNPs can also occur
through addition of base: if the pH is raised such that each of the
ammonium groups is deprotonated, all binding interactions are
disrupted and the nanopore orifices are unblocked as a result of
complete dethreading of the CB[6] rings.
Figure 2. MNPs operate by keeping guest molecules contained at neutral
pH, but release contents when the pH is lowered. Trisammonium stalks
with two different recognition units are spread all over the surface of
mesoporous silica nanoparticles. At neutral pH, the CB[6] ring encircles
the tetramethylenediammonium station, and the MNP is in its closed
configuration. When the pH is lowered, the CB[6] ring shuttles to the distal
hexamethylenediammonium recognition unit such that the pore orifices are
unblocked and the contents are released.
Mesoporous (pore diameter ∼2 nm) silica nanoparticles9 (∼130
nm diameter) were prepared10 through a base-catalyzed sol-gel
process involving the use of cetyltrimethylammonium bromide
(CTAB) surfactant and a tetraethylorthosilicate (TEOS) silica
precursor. The assembly of the dye-loaded MNPs was then
completed in a stepwise process from the nanoparticle surface
outward (Supporting Information). Briefly, beginning with as-
synthesized nanoparticles, acid extraction was performed to remove
the CTAB template and generate empty, accessible nanopores. Next,
amine modification of the silica nanoparticle surface was ac-
complished through treatment with aminopropyltriethoxysilane
(APTES). To the amine-modified nanoparticles, the trisammonium
stalks were attached to the nanoparticle surfaces. Dye loading was
accomplished by soaking the trisammonium-terminated nanopar-
ticles in a solution of propidium iodide (PI) to allow the dye
molecules to diffuse into the empty nanopores. Finally, the
pseudorotaxanes were formedsand the synthesis of the MNP
system was completedsby adding CB[6] to the dye-loading mixture
Figure 3. MNPs are able to contain PI molecules at neutral pH levels
but release them under pH control. (a) The rate of release of PI from
MNP-1 depends on the amount of acid that is added. (b) The pH level
required to fully activate MNP-2 is less acidic than that for MNP-1. (c)
Adjustment of the pH to 10 causes MNP-1 to release PI molecules as a
result of complete deprotonation of the trisammonium stalks and
dethreading of the CB[6] rings.
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dependent release rate is consistent with the mechanism of operation
for the MNP system: release of guest molecules depends on the
rate of protonation of the anilinium nitrogen atom, which occurs
more rapidly at more acidic pH levels.
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Acknowledgment. This work was supported by the US DOD
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Supporting Information Available: Synthetic procedures and
spectral characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
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