Mechanized silica nanoparticles based on reversible bistable [2]pseudorotaxanes as supramolecular nanovalves for multistage pH-controlled release

Article abstract of DOI:10.1039/c4cc01442a

Cucurbit[6]uril-based reversible bistable [2]pseudorotaxanes were designed, synthesized and installed on the surface of mesoporous silica nanoparticles as supramolecular nanovalves. The assembled mechanized silica nanoparticles realize the multistage pH-controlled release of benzotriazole and the potential for reutilization.

Full text of DOI:10.1039/c4cc01442a

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Mechanized silica nanoparticles based on reversible
bistable [2]pseudorotaxanes as supramolecular
nanovalves for multistage pH-controlled release†
Cite this: Chem. Commun., 2014,
50, 5068
Received 25th February 2014,
Accepted 25th March 2014
MingDong Wang,‡ Tao Chen,‡ ChenDi Ding and JiaJun Fu*
DOI: 10.1039/c4cc01442a
www.rsc.org/chemcomm
Cucurbit[6]uril-based reversible bistable [2]pseudorotaxanes were the reciprocating motion of macrocycles on the linear stalks. To our
designed, synthesized and installed on the surface of mesoporous knowledge, investigations of bistable [2]rotaxanes to realize rever-
silica nanoparticles as supramolecular nanovalves. The assembled sible switching function are still rare. Stoddart and Zink et al.
mechanized silica nanoparticles realize the multistage pH-controlled attached the bistable [2]rotaxanes covalently to the surface of
release of benzotriazole and the potential for reutilization.
MCM-41 and finish a cycle for loading and releasing of guest
molecules, in which cyclobis(paraquat-p-phenylene) (CBPQT4+)
Mechanized silica nanoparticles (MSNPs), which are composed of macrocycles shuttled between the tetrathiafulvalene (TTF) stations
mesoporous silica nanoparticles as scaffolds and mechanically and the dioxynaphthalene (DNP) stations under redox control.8
interlocked molecules as supramolecular nanovalves, are capable Lately, they have designed light-triggered reversible nanovalves
of accommodating and releasing cargo molecules on demand and based on the azobenzene a-cyclodextrin system.9
have received great attention due to their potential applications in
Herein, we introduce a novel design for MSNPs based on
the field of stimuli-responsive controlled drug delivery systems and mesoporous silica nanoparticles equipped with the reversible bis-
intelligent anticorrosion coatings.1,2 As the key part of MSNPs, table [2]pseudorotaxanes to control pore access. The molecular
supramolecular nanovalves, usually in the form of [2]pseudorotax- structure of the bistable [2]pseudorotaxane, which contains 1,6-
anes and bistable [2]rotaxanes, undergo the relative mechanical hexanediammonium (HDA) and 1,6-bis(pyridinium)hexane (BPH)
movement between their ring components and threads on the as two recognition sites, as well as cucurbit[6]uril (CB[6]) as macro-
nanoscale, by which MSNPs can encapsulate the cargo molecules cycles are shown in Scheme 1. In this interlocked molecule, CB[6]
within the mesopores under normal conditions and release them in macrocycles can be switched between two distinct recognition sites
response to environmental changes, such as pH, redox potential, according to pH changes. Under neutral or acidic solution, the CB[6]
light, biomolecules, etc.3–6
In the past decade, a multitude of supramolecular nanovalves
based on [2]pseudorotaxanes have been designed and fabricated.
The transition states of nanovalves from "closed" to "open" are
mainly caused by the gradually weak supramolecular interactions
between the macrocycles and the linear stalks anchored on the
surface of mesoporous silica nanoparticles upon external stimuli.
However, after accomplishing release missions, the complete struc-
tures of MSNPs are broken owing to the dethreading of macrocyclic
gatekeepers from the linear stalks.7 As comparison, making use of
bistable [2]rotaxanes as nanovalves not only actualizes multistage
controlled release in a "release–halt–release" way, but also achieves
another important objective for reloading and reusing of MSNPs by
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, China. E-mail: fujiajun668@gmail.com; Fax: +86 025 84315609;
Tel: +86 025 84315609
† Electronic supplementary information (ESI) available: Experimetal details and
analytical data. See DOI: 10.1039/c4cc01442a
Scheme 1 Schematic representation of operation of the MSNPs 1.
‡ These authors contributed equally to this article.
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macrocycles choose to stay on the HDA stations rather than the BPH
stations, preventing cargo molecules escape. When the pH value
turns to alkaline, the ammonium moieties of HDA are deprotonated
and the binding affinity between CB[6] macrocycles and HDA
stations is weakened, which leads to the migration of CB[6] macro-
cycles from the HDA recognition stations to the BPH stations far
away from the entrance of the pores. At this moment, the nano-
valves are opened and the cargo molecules are free to diffuse out. If
the pH of solution is adjusted from alkaline to weak acidic condi-
tions once more, the CB[6] macrocycles originally recognized on the
BPH stations will shuttle back to the HDA stations, which facilitates
the well-designed MSNPs fulfil the multistage pH-controlled release
and therefore, can be recycled and reused.
MCM-41-type mesoporous silica nanoparticles (MSNs) were
synthesized according to the previous literature and used as starting
nanocarriers.10 Transmission electron microscopy, small-angle X-ray
powder diffraction, and N2 adsorption–desorption isotherms
demonstrate that the monodispersed, diameter of about 150 nm
Fig. 2 13C CP-MAS solid-state NMR spectra of MSNs 1 (a), MSNs 2 (b), and
MSNs 3 (c).
nanospheres have hexagonal channels with an average pore dia- MSNs. The 13C CP-MAS solid-state NMR spectra of MSNs 1, MSNs 2
meter of 2.65 nm and a surface area of 1118 m2 gÀ1 (Fig. S2, ESI†). and MSNs 3 are illustrated in Fig. 2 and provide further evidence of
The assembly process of MSNP 1 is depicted in Fig. S1 (ESI†). The functionalization. The NMR spectrum of MSNs 1 shows three
as-prepared MSNs were first functionalized with (3-aminopropyl) resonance signals at 10, 26 and 43 ppm that can be attributed to
trimethoxysilane to obtain MSNs 1, which were then treated with the peaks of carbon a, b, and c of 3-aminopropyl group. In the NMR
1,6-dibromohexane to give MSNs 2. After that, the MSNs 2 were spectrum of MSNs 2, apart from the peaks of carbons a, b and c,
reacted with compound 1 (the detailed synthesis process is provided some new intense signals were observed at 22, 51, 55, 60 ppm,
in the ESI†) to yield MSNs 3. To obtain MSNPs 1, the MSNs 3 were corresponding to the carbons f–g, e–h, i and d on the hexyl chain.
loaded with 1H-benzotriazole (BTA), which is chosen as the probe As for MSNs 3, the additional three resonance signals appearing
molecule and capped with CB[6] macrocycles.
around 128, 146 and 160 ppm can be assigned to the characteristic
Three-step functionalization processes are confirmed through aromatic carbons (k–t, l–s–u and j) of the pyridine groups, indicating
Fourier transform infrared (FTIR) spectroscopy. As shown in Fig. 1, the formation of the second recognition sites, BPH stations,
MSNs only show the various Si–OH and Si–O–Si vibrations, while which is also proved by the 29Si CP-MAS solid-state NMR spectra
MSNs 1 exhibit an absorption band at 2930 cmÀ1, which corre- (Fig. S3, ESI†).
sponds to the C–H stretching vibration. Compared with MSNs 1, a
X-ray photoelectron spectroscopy (XPS) is also used to verify the
new absorption band at 580 cmÀ1, a characteristic stretching band introduction of the intact stalks inside the framework of MSNs. The
of the C–Br bond is shown in the FTIR spectrum of MSNs 2. After apparent differences between MSNs 3 and MSNs (Fig. S4, ESI†) are
coupled with compound 1, the intensity band of C–Br bond is in the presence of N 1s and C 1s signals in the XPS spectrum of
considerably reduced, and the absorption peaks at 1500–1400 cmÀ1 MSNs 3, but not detected in the MSNs. From thermogravimetric
appear due to the pyridine skeleton vibration, indicating the analysis data as shown in Fig. S5 (ESI†), when MSNs are successively
successful attachment of organic stalks on the exterior surfaces of functionalized with (3-aminopropyl) trimethoxysilane, 1,6-dibromo-
hexane and compound 1, the approximate total weight loss is
11.5 wt% (about 0.86 mmol gÀ1 MSNs), 8.5 wt% (about
0.5 mmol gÀ1 MSNs), and 7.4 wt% (about 0.42 mmol gÀ1 MSNs),
respectively. Although the intensity of the diffraction peak, the
specific surface area and the pore volume are inevitably
decreased after each step of functionalization, this does not
greatly influence the adsorption capacity of mesoporous materi-
als (Fig. S6 and S7, ESI†).
After installation of the planned stalks, the MSNPs 1 were
assembled. Three different measurement methods were adopted
to investigate the operation modes of MSNP 1 in detail. First of
all, Fig. 3 shows release profiles by plotting the absorbance
intensity of BTA at 265 nm from the MSNPs 1 in response to
different pH as a function of time. The leakage of BTA at pH 7.0
was negligible (less than 1.2% total amount of the adsorbed
BTA), revealing that the entrance of pores were blocked by CB[6]
Fig. 1 FTIR spectra of pure MSNs (a), MSNs 1 (b), MSNs 2 (c), and MSNs 3 (d). macrocycles. But in the alkaline solution, the UV/Vis absorbance
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no leakage of BTA under pH 4.5. When the releasing process lasted
for 60 min under pH 9.5, the pH of solution was readjusted to 4.5,
simultaneously supplemented by heating solution to 45 1C for 2 h.
After that, the release process was found to be shut off and the
concentration of benzotriazole in solution was very stable through
continuous monitoring, demonstrating that CB[6] macrocycles
shuttled back to the HDA stations and restarted to regulate the
guest molecules diffusing in or out of the MSNPs 1.
Based on the anticipative results of the multistage pH-controlled
experiment, the reloading ability of MSNPs 1 was finally investi-
gated. The approximately complete release of BTA was performed at
pH 10.5 for 12 h. The reloading procedure is similar to the initial
adsorption experiment. The re-collected MSNPs 1 were suspended
in 15 mg mLÀ1 aqueous BTA solution at pH 10.5 for 12 h, which
ensures the encirclement of CB[6] macrocycles on the BPH stations
during the reloading process, and then closed the nanovalves,
Fig. 3 Release profiles of BTA from MSNPs 1 at different pH values.
of BTA increased immediately, manifesting the rapid escape of washed for the second release measurement. As shown in Fig. 4b,
BTA from MSNPs 1. Furthermore, the release rate is obviously the pH-controlled release profile for the second cycle exhibits the
dependent on the alkalinity of solution. The stronger the alkali- similar features as that from the first cycle. However, the reloading
nity is, the higher deprotonation degree of HDA units is, the capacity is less than 20% for the first cycle, which may be attributed
more numbers of CB[6] macrocycles detached from the HDA to the space steric effect of CB[6] macrocycles.
stations there are, and the more rapid the release rate we observe.
In summary, a new type of MSNPs with the reversible bistable
In the condition of near neutral or acidic solution, CB[6] macro- [2]pseudorotaxanes as supramolecular nanovalves has been devel-
cycles are threaded onto the HDA stations due to that the stability oped. The one-dimensional piston movement of CB[6] macrocycles
constant for the host–guest complex between CB[6] and HDA units between the HDA and the BPH stations guarantee the reversible
is more than that between CB[6] and BPH units.11 As solution switching activity of nanovalves. The operational mechanisms make
alkalinity is increased, 1,6-hexanediammonium are gradually in the MSNPs 1 open or close repeatedly at the molecular lever in response
deprotonated form.12 The high binding affinity of complex to the pH stimulus, which will expand their application area.
HDACCB[6] dramatically declines, the CB[6] macrocycles cannot
This research was financially supported by the National
reside on the HDA stations and transfer to the BPH stations where Natural Science Foundation of China (No. 51102135), the
the complex formation of BPHCCB[6] is little affected by pH values. Natural Science Foundation of Jiangsu Province (No.
In order to validate the reversibility of the moving process of CB[6] BK2011711), the 2013-ZiJin-0102 Talent Program, NUST, and
macrocycles, the second step, the multistage pH-controlled release Jiangsu Graduate Innovation Project (No. CKLX13_196).
experiment was conducted. As depicted in Fig. 4a, there was almost
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