A Tetraphenylethene Luminogen-Functionalized Gemini Surfactant for Simple and Controllable Fabrication of Hollow Mesoporous Silica Nanorods with Enhanced Fluorescence

Article abstract of DOI:10.1021/acs.inorgchem.8b02252

Nanoparticles that possess unique structures and properties are highly desired in the production of multifunctional materials because of their combinational performance. In this study, a facile and effective fabricating strategy is developed to controllab

Full text of DOI:10.1021/acs.inorgchem.8b02252

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Tetraphenylethene Luminogen-Functionalized Gemini Surfactant
for Simple and Controllable Fabrication of Hollow Mesoporous Silica
Nanorods with Enhanced Fluorescence
Saisai Yan,†,‡ Zhinong Gao,*,†,‡ Yan Xia,†,‡ Xueming Liao,†,‡ Yifan Chen,§ Jia Han,†,‡
Chenchen Pan,†,‡ and Yingfang Zhang†,‡
†College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China
‡Key Laboratory of Biomedical Polymers, Ministry of Education of China, Wuhan, Hubei 430072, P. R. China
§School of Materials Science and Chemistry Engineering, China University of Geosciences, Wuhan, Hubei 430074, P. R. China
S
* Supporting Information
ABSTRACT: Nanoparticles that possess unique structures
and properties are highly desired in the production of
multifunctional materials because of their combinational
performance. In this study, a facile and effective fabricating
strategy is developed to controllably prepare fluorescent
hollow mesoporous silica nanorods via the cetyltrimethylam-
monium bromide (CTAB) and tetraphenylethene (TPE)
luminogen-functionalized gemini surfactant (CTPE−C6−CTPE
)
guided dual-templating approach. Because of its unique
chemical structure, water solubility, surface activity, and
fluorescent properties, the designed CTPE−C6−CTPE will not
only provide an anchored fluorophore for silica nanoparticles but also serve as an intimate partner of CTAB to regulate their
construction in the structure-directing process. By properly tuning the molar ratio of CTAB/CTPE−C6−CTPE, the shape-
controlled aggregation-induced emission hollow mesoporous silica nanoparticles (AIE-MSNs) can be prepared directly,
producing two kinds of silica nanorods (AIE-MSNs-15 and AIE-MSNs-7). In particular, the incorporated bulky TPE
luminogens will not only endow AIE-MSNs-7 with enhanced fluorescence intensity (2.3-fold) after the removal of CTAB but
also bring about high accessible surface area (606.6 m2/g) and larger pore size (3.2 nm) and pore volume (0.634 cm3/g) for
effective loading and sustained release of the hydrophobic anticancer drug camptothecin. CTPE−C6−CTPE enriches the family of
gemini surfactants and provides important insights into the convenient fabrication of advanced fluorescent mesoporous
materials.
delivery and release in targeted cells.11 With advances in
synthetic nanoscience and nanotechnology, hollow MSNs have
1. INTRODUCTION
Multifunctional nanoplatforms, which integrate various func-
been proposed to optimize the structure of SNs and expand
tional components into a single system, have been in the
their application areas.12−14 The well-defined hollow MSNs
spotlight of producing theranostic nanomedicines for early
can provide both capacious interiors and intact porous shells,
diagnosis and therapies of cancer.1 Remarkable achievements
affording high storage/adsorption capacity and accessible
have been made in the design and fabrication of various
mesoporous channels for mass transfer and diffusion, which
nanoplatforms such as polymers, liposomes, dendrimers,
nanogels, and other hybrid nanomaterials.1−3 Among the
are beneficial for sustained-release behavior and synergistic
effects with guest molecules.15−18 The controllable MSNs with
various materials that have been proposed for preparing smart
nanoplatforms, silica nanoparticles (SNs) are among the most
promising candidates for the integration of designated
modalities and functionalities by virtue of their intrinsically
attractive properties including rigid structure, ideal chemical
stability, tunable pore structure, ease of surface functionaliza-
hollow or rattle structure are considered to be ideal drug-
delivery vehicles, offering additional possibilities for the
tailoring of material properties toward a broad range of
applications.19,20
The recently emerged aggregation-induced emission fluor-
tion, and good biocompatibility.4−7 As a well-developed
ogens (AIEgens) have sparked increasing research interest in
biosensing and imaging because of their unique advantages
nanoplatform, mesoporous SNs (MSNs) have numerous
such as low background interference, a high signal-to-noise
advantages that include stable structures, abundant mesopores,
high specific surface areas, and large pore volumes.8−10 The
incorporation of anticancer drugs, targeting molecules, and
stimuli-responsive elements with MSNs enables guided
Received: August 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. (a) Synthetic Route of the TPE-Functionalized Gemini Surfactant CTPE−C6−CTPE (4) and (b) Schematic
Illustration of the Main Procedures in the Fabrication of AIE-MSNs
ratio, and superior photostability with activatable therapeutic
effects.21 Compared with conventional fluorophores, the AIE
effect offers a timely remedy to tackle the challenge of
notorious aggregation-caused quenching (ACQ) and provides
new degrees of freedom for real-time, on-site, and noninvasive
visualization.22−24 With regard to the construction of multi-
functional nanoplatforms and ultimately advanced materials,
great efforts have recently been made to integrate AIEgens
with silica moieties into one nanostructure so as to develop a
new family of functional composite nanomaterials for sensitive
optical sensing and efficient therapy.22,25 Physical blending is a
simple and facile synthetic approach to generating AIEgens-
functionalized SNs (AIE-SNs) by doping AIE molecules within
the silica matrix.5 Wei and co-workers have made the first
attempt to integrate cell imaging with cancer therapy in one
AIE-MSNs system via the one-pot coassembly of cetyltrime-
thylammonium bromide (CTAB), a silica source, and
hydrophobic AIE molecules.4 Undesirably, leakage of the
dopants from the AIE-MSNs may happen because of weak
interactions between the organic and inorganic phases,
resulting in the inevitable decrease of the fluorescence
intensity. Besides, the reported literatures involved only the
preparation of ordinary nonhollow spherical AIE-MSNs or
AIE-SNs because of the lack of effective directing in the sol−
gel process. The chemical-bonded hollow AIE-MSNs have also
been developed to overcome the above-mentioned draw-
backs.25,26 However, their preparation procedures are rather
complex and mainly rely on multistep improvement based on
the existing nanostructure, which is not favorable in large-scale
applications. It is urgent to overcome these limitations and
develop a new straightforward method for the facile and
controllable preparation of hollow AIE-MSNs.
Surfactants play a critical role in controlling the morphology,
pore size, surface area, and dimension when preparing SNs by
a template-directed method.27−29 Among the various surfac-
tants currently available, gemini surfactants, as a new class of
surfactants containing one unique structure of two amphiphilic
moieties connected with a spacer, have always been considered
to be promising stabilizers and structure director agents for the
preparation of various nanomaterials because of their better
interfacial properties, miscellaneous aggregate morphologies,
charged head groups, and lower critical micelle concentrations
(CMCs).30−32 In our previous work, the improved gemini
surfactant C16−TPE−C16 with a tetraphenylethene (TPE)
luminogen-functionalized spacer, together with conventional
CTAB, was successfully employed to prepare hollow AIE-
MSNs.33 Although the obtained hollow AIE-MSNs exhibit less
uniform morphologies, small mesopores of 2.8 nm, and
unfavorable weakening of the fluorescent intensity after the
removal of CTAB, they inspired us to pursue more suitable
gemini surfactants to improve their structures and properties.
Herein, we designed and synthesized a new TPE-function-
alized gemini surfactant (Scheme 1a, denoted as CTPE−C6−
C
TPE) to serve as an intimate partner of CTAB for the simple
and controllable fabrication of hollow AIE-MSNs. The TPE
luminogens connecting at the tails endowed CTPE−C6−CTPE
with unique surface activity, stability, sensitivity, and AIE
effect, bringing about simultaneous photoluminescence in the
dispersed, aggregated, and solid states. With the help of CTPE
C6−CTPE and CTAB, a facile and effective bottom-up
coassembled strategy is developed to controllably prepare
−
B
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
126.24, 113.56, 67.78, 34.15, 32.86, 29.49, 29.47, 29.41, 29.34, 28.80,
28.21, 26.09.
AIE-SNs in H2O, followed by an extraction process to generate
AIE-MSNs with abundant mesoporous and enhanced fluo-
rescence (Scheme 1b). The designed CTPE−C6−CTPE will not
only provide an anchored fluorophore for AIE-SNs and AIE-
MSNs but also cooperate with CTAB to regulate their
construction as structure-directing agents. By proper tuning
of the molar ratio of CTAB/CTPE−C6−CTPE, the shape-
controlled AIE-MSNs with improved structures and fluores-
cent properties can be prepared directly. Additionally, the
delivery of the hydrophobic anticancer drug camptothecin
(CPT) was further performed to evaluate the drug loading and
release behavior of the resultant hollow AIE-MSNs.
(iii) Synthesis of compound 3: Compound 2 (0.30 g, 0.53 mmol)
and K2CO3 (0.073 g, 0.53 mmol) were added to a 50 mL round-
bottom flask with ethanol (20 mL). Then, dimethylamine (0.048 g,
1.06 mmol) was added slowly to this solution. After that, the reaction
mixture was heated with refluxing for 15 h (monitored by TLC) and
cooled to room temperature. The solution was concentrated, the
residue was poured into a NaOH aqueous solution (20 mL, 1 M), and
the product was taken up with ethyl acetate. The organic layer was
collected and dried with anhydrous Na2SO4. The crude product was
purified by column chromatography with ethyl acetate/methanol
[15:1 (v/v)] as the eluent to obtain the target compound (yellow oil).
1
Yield: 90%. H NMR (400 MHz, CDCl3): δ 6.99−7.14 (m, 15 H),
6.89−6.93 (m, 2 H), 6.60−6.64 (m, 2 H), 3.86 (t, 2 H), 3.24 (m, 2
H), 3.18 (s, 6 H), 1.86 (m, 2 H), 1.73 (m, 2 H), 1.30−1.43 (m, 12
H). 13C NMR (400 MHz, CDCl3): δ 157.59, 143.98, 140.51, 139.91,
135.84, 132.47, 131.33, 127.55, 126.17, 113.48, 71.65, 67.71, 58.32,
55.38, 29.37, 29.31, 29.28, 29.26, 26.60, 26.01, 23.89.
2. EXPERIMENTAL SECTION
2.1. Materials and Instruments. Zinc dust, benzophenone, 4-
hydroxybezophenone, 1,10-dibromodecane, 1,6-dibromohexane, di-
methylamine (33%), and camptothecin (CPT) were purchased from
Aladdin. TiCl4, K2CO3, NaOH, CH2Cl2, Na2SO4, ethanol, methanol,
acetone, pyridine, ethyl acetate, petroleum ether, tetrahydrofuran
(THF), hydrochloric acid (HCl), tetraethylorthosilicate (TEOS), and
cetyltrimethylammonium bromide (CTAB) were purchased from
Sinopharm Chemical Reagent Co. All reagents were of analytical
grade and were used without further purification. H2O was purified by
a Milli-Q system (Millipore, Molsheim, France) and used in all of the
experiments.
(iv) Synthesis of CTPE−C6−CTPE (compound 4): Compound 3
(0.90 g, 1.69 mmol) and 1,6-dibromohexane (0.17 g, 0.70 mmol)
were dissolved in ethyl acetate (20 mL). Then, the reaction was
allowed to stir and reflux for 4 days. After the reaction cooled to room
temperature, the resulting mixture was filtered to obtain a white
precipitate (crude product). The collected product was further
washed with ethyl acetate and recrystallized from ethyl acetate/
ethanol at least three times to get the purified product as a white solid.
1
Yield: 68%. H NMR (400 MHz, CDCl3): δ 6.97−7.09 (m, 30 H),
IR spectra were recorded in the range of 4000−400 cm−1 using KBr
6.86−6.91 (m, 4 H), 6.57−6.62 (m, 4 H), 3.83 (t, 4 H), 3.69 (m, 4
H), 3.47 (m, 4 H), 3.36 (s, 12 H), 1.98 (m, 4 H), 1.70 (m, 8 H),
1.27−1.40 (m, 28 H). 13C NMR (400 MHz, CDCl3): δ 157.58,
143.96, 140.50, 139.90, 135.81, 132.46, 131.29, 127.54, 126.17,
113.48, 67.69, 64.63, 64.05, 50.97, 29.37, 29.33, 29.29, 29.27, 29.25,
26.28, 26.02, 24.45, 22.85, 21.65. IR (KBr, cm−1): 3021 (m), 2925
(vs), 2854 (s), 1604 (s), 1508 (vs), 1470 (s), 1393 (m), 1244 (vs),
1176 (s), 1029 (m), 809 (w), 730 (m), 701 (vs). UV−vis: λmax = 319
nm. ESI-MS (CH3OH, m/z): [M − Br]+, calcd 1227.6, found 1227.5;
[M − 2Br]2+/2, calcd 573.9, found 573.9.
pellets on a Thermo Nicolet 5700 spectrophotometer. H and 13C
1
NMR were obtained from a Bruker Ascend-400 MHz spectrometer
using CDCl3 or dimethyl sulfoxide (DMSO)-d6 as a solvent.
Electrospray ionization mass spectrometry (ESI-MS) spectra were
recorded on a LCQ spectrometer (Finnigan, USA). UV−vis spectra
were measured on a Shimadzu UV-2600 spectrophotometer.
Fluorescence spectra were obtained on a RF-5301PC fluorescence
spectrometer (Shimadzu, Japan) with the slit of 3.0 nm. The field-
emission scanning electron microscopy (FESEM) was performed on a
Zeiss Sigma field-emission scanning electron microscope (20 kV).
Transmission electron microscopy (TEM) images were recorded on a
JEM-2100 microscope (200 kV). The surface tension values were
measured following the Wilhelmy plate procedure at 25.0 °C with a
QBZY-2 tensiometer (Fangrui Corp., China). Electrical conductivity
was performed at 25.0 °C with a WTW conductivity meter (inoLab
Cond730, Germany). The nitrogen adsorption−desorption isotherms
were carried out on a Micromeritics ASAP 2020 system (USA) at 77
K. The Brunauer−Emmett−Teller (BET) method was conducted to
calculate the specific surface areas. The Barrett−Joyner−Halenda
model was utilized to calculate the pore-size distributions from the
desorption branches of the isotherms.
2.2. Preparation of the CTPE−C6−CTPE. The TPE-functionalized
gemini surfactant CTPE−C6−CTPE was synthesized according to the
procedures described in Scheme 1a.
(i) The synthesis of compound 1 was performed by the powerful
McMurry reaction as reported previously.23 Yield: 61%. 1H NMR
(400 MHz, DMSO-d6): δ 9.38 (s, 1 H), 7.16−7.04 (m, 9 H), 6.92−
6.98 (m, 6 H), 6.72−6.75 (d, 2 H), 6.49−6.52 (d, 2 H).
(ii) Synthesis of compound 2: Compound 1 (0.0560 g, 0.16 mmol)
and K2CO3 (0.066 g, 0.48 mmol) were mixed in acetone (20 mL).
Then, 1,10-dibromodecane (0.096 g, 0.32 mmol) was added dropwise
after stirring for 30 min. The resulting mixture was refluxed for 8 h
[monitored by thin-layer chromatography (TLC)] and cooled to
room temperature. After filtration of insoluble K2CO3 and vacuum
distillation, the crude product was purified by silica gel chromatog-
raphy with petroleum ether/ethyl acetate [20:1 (v/v)] as the eluent.
2.3. Preparation of the AIE-MSNs, CTAB-MSNs, MSNs-15,
and MSNs-7. AIE-MSNs were prepared by two steps according to
our previous report.33 Briefly, CTAB and CTPE−C6−CTPE with a total
amount of 0.137 mmol were first dissolved in ultrapure H2O (25 mL).
The molar ratio of CTAB and CTPE−C6−CTPE was as follows:
CTAB:CTPE−C6−CTPE = n:1 (n = 40, 35, 30, 25, 20, 15, 10, 7, and 5).
Then, the aqueous solution of NaOH (0.180 mL, 2.00 M) was
introduced into the mixed solution. The temperature of the mixture
was adjusted to 80 °C and kept for 30 min. Subsequently, TEOS
(0.50 mL, 2.24 mmol) was added dropwise under vigorous stirring.
The mixture was allowed to react for 4 h to give a white precipitate.
The obtained product was collected by centrifugation and washed
with H2O and ethanol several times to yield AIE-SNs. Finally, the
synthesized AIE-SNs (0.5 g) were refluxed in a solution of ethanol
(50.0 mL) and HCl (3.0 mL, 37.4%) for 24 h to remove CTAB. The
obtained product was collected by centrifugation, washed with H2O
and ethanol, and dried at 35 °C in a vacuum to yield the AIE-MSNs.
Additionally, the preparation of comparing CTAB-SNs and CTAB-
MSNs was performed with the individual CTAB under the same
procedure as that described above.
The surfactant template in SNs can be completely removed by the
calcination treatment.14 In order to calcine out the encapsulated
CTPE−C6−CTPE component in AIE-SNs, the obtained AIE-MSNs-15
and AIE-MSNs-7 were further subjected to thermal treatment at 600
°C for 8 h in air, and the corresponding products were designated as
MSNs-15 and MSNs-7, respectively.
2.4. In Vitro Drug Loading and Release. To evaluate the drug
loading capacity of AIE-MSNs, CPT was used as a model guest
molecule in the experiment and the process was performed according
to the reported literature.9 Briefly, 40 mg each of AIE-MSNs (AIE-
MSNs-15 or AIE-MSNs-7) and MSNs (MSNs-15 or MSNs-7) were
respectively dispersed in the solution of CPT (5 mL, 5 mg/mL) in
1
The final product was obtained as a yellowish oil. Yield: 76%. H
NMR (400 MHz, CDCl3): δ 6.99−7.14 (m, 15 H), 6.89−6.93 (m, 2
H), 6.59−6.64 (m, 2 H), 3.86 (t, 2 H), 3.41 (t, 2 H), 1.89 (m, 2 H),
1.72 (m, 2 H), 1.30−1.42 (m, 12 H). 13C NMR (400 MHz, CDCl3):
δ 157.70, 144.05, 140.61, 139.98, 135.90, 132.55, 131.45, 127.62,
C
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. (a) Plot of the surface tension (γ) versus log(C) for CTPE−C6−CTPE. (b) Plot of the conductivity (κ) versus the concentration of CTPE
−
C6−CTPE. (c) Fluorescence spectra of CTPE−C6−CTPE in aqueous solution with the concentration increased from 2 to 180 μM (λex = 340 nm). (d)
Plot of the fluorescence intensity of CTPE−C6−CTPE at 475 nm versus the corresponding concentration in part c. (e) Fluorescence photographs of
CTPE−C6−CTPE in aqueous solution at concentrations from 0 to 180 μM with UV irradiation (365 nm) from the top.
phosphate-buffered saline (PBS, pH = 7.4) and stirred for at least 24 h
in darkness, followed by centrifugation and washing extensively with
PBS to achieve the drug-loaded AIE-MSNs-CPT (AIE-MSNs-15-
CPT or AIE-MSNs-7-CPT) and MSNs-CPT (MSNs-15-CPT or
MSNs-7-CPT). The CPT amount in the supernatant and washed
solutions was collected and measured using UV−vis spectroscopy at
368 nm to determine the residual CPT content (RCPT, mg). The CPT
loading capacity (Q) was evaluated with the loaded CPT mass per
gram of AIE-MSNs and later calculated according to the reported
equation as follows: Q (mg/g) = (25 − RCPT)/0.04.
PBS instead of the tested supernatant to get the release curve. A total
of 8 mg of the sample (AIE-MSNs-15-CPT, AIE-MSNs-7-CPT,
MSNs-15-CPT, or MSNs-7-CPT) was dispersed into 3 mL of PBS
release media, and then the releasing process was performed in the
dark at 37 °C. The supernatant was taken out by centrifugation and
replaced by another 3 mL of fresh PBS every 2 h. The UV−vis
absorbance of the obtained supernatant was measured as well at 368
nm to monitor the CPT release.
3. RESULTS AND DISCUSSION
For drug release assay, 40 mg each of the samples (AIE-MSNs-15-
CPT, AIE-MSNs-7-CPT, MSNs-15-CPT, and MSNs-7-CPT) were
respectively immersed in PBS (20 mL, pH = 7.4) and incubated in a
shaker at 37 °C. At certain time intervals, 3 mL of the supernatant
PBS was taken out by centrifugation to test the drug-released
concentration by virtue of a UV−vis absorption technique and then
returned to the original release media. To better understand their
release behavior, the samples were further incubated by constant fresh
3.1. Surface Activity of CTPE−C6−CTPE. The surface
activity of CTPE−C6−CTPE was evaluated through measure-
ment of their surface tension in aqueous solution, which can be
employed to evaluate the CMC. As shown in Figure 1a, the
surface tension (γ) is plotted against the different concen-
trations (log C) of CTPE−C6−CTPE. At low concentration, the
surfactant unimers were preferentially adsorbed at the air−
D
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. (a) Time-dependent evolution of the fluorescence intensity of CTPE−C6−CTPE at different concentrations. (b) Photostability of CTPE
−
C6−CTPE upon continuous UV excitation at different concentrations. I0 is the initial fluorescence intensity at 475 nm, and I is the fluorescence
intensity of the samples after UV irradiation. (c) Plot of the fluorescence intensity of CTPE−C6−CTPE (50 μM, 2.5 mL) at 475 nm versus the
introducing volume of the CTAB solution with different concentrations. (d) Plot of the fluorescence intensity of CTPE−C6−CTPE (50 μM) versus
the concentration of CTAB in the mixture at a fixed volume. (e) Fluorescence spectra of CTPE−C6−CTPE in a H2O/THF mixture (50 μM) with
different THF volume fractions (vol: 0−90%). (f) Plot of the fluorescence intensity of CTPE−C6−CTPE at 475 nm versus the different THF volume
fractions in part e. Inset: Photographs of CTPE−C6−CTPE (50 μM) in a H2O/THF mixture under 365 nm UV irradiation.
H2O interface, thus leading to a decrease of the surface
tension. In general, the initial adsorption of the surfactant at
the air−H2O interface will reach a saturation state as the
concentration continued to increase and then the surface
tension came to a balance state accompanied by the formation
of micelles. However, because of the strong hydrophobicity
and possible π−π stacking of the TPE moieties, CTPE−C6−
CTPE can be continuously adsorbed at the air−H2O interface
after the break point, resulting in a constant decrease of the
surface tension. The CMC of CTPE−C6−CTPE determined
from the inflection point in the surface tension curve is 21 μM,
which is much lower than that of CTAB (880 μM).34 On the
other hand, the electrical conductivity method was also
the ionic micelles have less charge per unit mass than their
unimers.35 The breakpoint at 21 μM was considered to be the
CMC of CTPE−C6−CTPE and matched well with that obtained
from the surface tension measurement.
3.2. Optical Properties of CTPE−C6−CTPE. The optical
properties of CTPE−C6−CTPE were investigated by UV−vis
absorption and fluorescence spectroscopy. Two characteristic
peaks at about 248 and 319 nm were observed in their
absorption spectra (Figure S1a), which were assigned to
absorption of the phenyl groups and the conjugated TPE.36,37
Meanwhile, their intensities increased linearly with the
concentration of CTPE−C6−CTPE in the range from 0 to 150
μM (Figure S1b). It was noted that the maximum absorption
peak position at 319 nm remained unchanged as the
concentration increased, indicating that the conformation of
CTPE−C6−CTPE molecules did not depend on the micelle
aggregates like other reported TPE derivatives.36
performed to determine the CMC value of CTPE−C6−CTPE
.
A plot of the conductivity (κ) versus the different
concentrations (C) of CTPE−C6−CTPE is shown in Figure 1b.
Initially, the solution conductivity increased linearly with a
higher slope at the low concentration. This straight line was
observed with a decreased slope when the concentrations of
CTPE−C6−CTPE were above 21 μM, ascribed to the fact that
Photoluminescence of CTPE−C6−CTPE was determined by
the restriction degree of intramolecular rotation (RIR) of
propeller-shaped phenyl rings, which will block the non-
E
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
radiative pathway and open up the radiative decay channel.25
On the basis of the RIR mechanism, the light-up fluorescence
behavior of CTPE−C6−CTPE can be realized effectively in the
polar aqueous solution because of their excellent water
solubility. The fluorescence emission spectra of different
concentrations of CTPE−C6−CTPE in aqueous solution are
shown in Figure 1c. Although their maximum emission peak
position remained at 475 nm (typical TPE pattern of light) as
the concentration increased, the intensity cannot keep
increasing as the concentration increased. As shown in Figure
1d, the fluorescence intensity at 475 nm was plotted versus the
corresponding concentration of CTPE−C6−CTPE. Three
straight lines with different slopes and two inflection points
at 20 and 54 μM were found, suggesting the changing
aggregation states of CTPE−C6−CTPE in an aqueous solution.
The fluorescence intensity increased linearly with a higher
slope at the low concentration, and then the speed slowed after
the first inflection point. The decrease in the slope may be
ascribed to combined effects including the weakening of RIR,
inner filter effect, and some other uncertain effects.37 The TPE
units may get more freedom in the nonpolar interior of the
micelles than the outside aqueous environment. The inner
filter effect could also happen if the absorbing molecules were
at high concentration, inducing the disproportional increase of
the photoluminescence intensity with the concentration. This
special inflection point at 20 μM was determined as the CMC
of CTPE−C6−CTPE, which matched well with that obtained
from the surface tension and conductivity measurement.
Moreover, the excitation spectra of CTPE−C6−CTPE were also
recorded below and above the CMC (Figure S2). Remarkable
red shifts of the peaks in the excitation wavelength were
observed when the concentration increased above the CMC,
suggesting that the electron transition could be influenced by
the assembled CTPE−C6−CTPE molecules. In contrast, the
absorbance is simply proportional to photoluminescence
species in other reported AIE surfactants.33,37 As the
concentration of CTPE−C6−CTPE further increased above 54
μM, an unusual decrease of the fluorescence intensity was
observed. This phenomenon could be attributed to the strong
hydrophobicity of the TPE moieties in the aggregated micelles,
which can significantly improve their assembled nonpolar
interior and then weaken the RIR effect as the concentration
continuously increased. These results demonstrate that the
TPE luminogens connected at the tails of CTPE−C6−CTPE were
sensitive to the nonpolar environment and their photo-
luminescence can be fully realized in the individual state.
The light-up bright emission of CTPE−C6−CTPE can be
observed directly by the naked eye from their aqueous
solution. Figure 1e shows clear fluorescent photographs of
the CTPE−C6−CTPE aqueous solution with the assistance of a
UV lamp (365 nm). The brightness of the aqueous solution
enhanced significantly as the concentration increased, and a
strong cyan emission can be presented even at the low
concentration. Such visible fluorescence signals in a dispersed
aqueous solution are typical characteristics of AIE surfactants
because of their excellent water solubility accompanied by a
strong RIR effect.36 Additionally, the bright-cyan fluorescence
of CTPE−C6−CTPE can also be clearly observed in their solid
powder and even frozen state because of the AIE effect (Figure
S3), wherein the intramolecular rotation is suppressed by the
adjacent molecules to open up the irradiative transition
different from the conventional ACQ fluorophores and
AIEgens (nonemissive in the dissolved/dispersed state).21,22
To explore the solution stability of CTPE−C6−CTPE, the
time-dependent evolution of their fluorescence intensities at
different concentrations was investigated by comparing them
with their initial values under the same conditions. As shown in
Figure 2a, although their emission decreased significantly in
the first week, the fluorescence intensities of CTPE−C6−CTPE
with a concentration of less than CMC remain unchanged and
the rest begin to recover slowly in the following weeks.
Furthermore, the recovering tendency was closely related with
the concentration and particularly remarkable in the high-
concentration solutions. The initial decline of the fluorescence
intensity could be attributed to the further aggregation of
CTPE−C6−CTPE molecules in the aqueous solution drive by the
strong hydrophobicity of TPE luminogens. The closer
aggregation can provide more available nonpolar interior to
weaken the RIR effect, resulting in the unusual aggregation-
impaired emission (“AIE”). However, the overcrowded
gathering and π−π stacking between the TPE moieties
would further block and weaken the rotation of the phenyl
rings (AIE effect) in return, leading to the recovery of their
fluorescence emission. Therefore, photoluminescence of
CTPE−C6−CTPE experienced an initial decrease and then
turned to recovery as “AIE” changed to AIE. These results
reveal that the CTPE−C6−CTPE aqueous solutions possess
durable fluorescence properties and their photoluminescence
can also be realized effectively in the aggregated state.
The photostability of CTPE−C6−CTPE was also investigated
by measuring their fluorescence intensities at various time
points upon continuous UV irradiation (Figure 2b). Compared
with the conventional luminescent materials, CTPE−C6−CTPE
exhibited a distinctive changing tendency and the fluorescence
intensities at different concentrations changed inconsistently
under the same conditions. For the low-concentration solution,
a certain degree of decline appeared with an increase of the
exposure time because of photobleaching. However, no
significant change was observed in the following concentration,
and the fluorescence intensity of the high-concentration
solution enhanced rather than decreased with an increase of
the exposure time. This unusual phenomenon may be ascribed
to photoisomerization of the TPE units induced by UV
irradiation,24 which can destroy the assembled nonpolar
environment and then strengthen the RIR effect for high
emission.
3.3. Sensitivity of CTPE−C6−CTPE to CTAB and Organic
Solvents. The buffering capacity of CTPE−C6−CTPE can be
investigated by monitoring their fluorescence intensities under
continuous dilution. As shown in Figure 2c, the fluorescence
intensity of CTPE−C6−CTPE experienced a slight increase at
first and then decreased linearly with the continued
introduction of pure H2O. The initial enhancement of the
fluorescence intensity may be ascribed to the destruction of the
assembled nonpolar interior, where the RIR effect that has
been weakened in the aggregated micelles was recovered.
When the solution was constantly diluted by CTAB (1 mM)
instead of H2O, the promotion proceeded more rapidly and
then decreased linearly. When the solution was diluted by a
higher concentration of CTAB (3 mM), the amplification of
the emission was further strengthened, followed by a dramatic
linear decrease in the subsequent changing curve. Such an
interesting phenomenon probably arose from the changing
roles of CTAB on the TPE units in different stages. When
channel. The simultaneous fluorescent emissions of CTPE
−
C6−CTPE in the aqueous solution and solid state are quite
F
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Representative SEM (a and b) and TEM (c and d) images of AIE-SNs-15. TEM images of AIE-SNs-15 with rare rattlelike structures (e
and f).
CTAB was introduced at the beginning, the RIR effect could
further enhanced by twinning the hydrophobic tails of CTAB
with the phenyl rings. Upon continuous dilution with CTAB,
the nonpolar environment constructed by excess CTAB could
offer freedom instead of the RIR effect for the phenyl rings,
leading to distinct effects on the fluorescence intensity.
Simultaneously, the binding effects of twinning CTAB can
also be observed when CTPE−C6−CTPE was mixed with CTAB
at a fixed volume. As shown in Figure 2d, the fluorescence
intensity increased rapidly when CTAB was introduced and
then reached a plateau after the concentration reached the
same level as that of CTPE−C6−CTPE (50 μM). With the CTAB
concentration continuously increased after 150 μM, the
fluorescence intensity began to decrease gradually because of
the weakening RIR effect arising from excess CTAB. These
results confirm the special sensitivity of CTPE−C6−CTPE to the
nonpolar environment and their intimate relationship with
CTAB.
To investigate the sensitivity of CTPE−C6−CTPE to organic
solvents, the fluorescence spectra of CTPE−C6−CTPE in a H2O/
THF mixture with different THF fractions in volume (vol, 0−
90%) were investigated. With increasing THF fractions from
0% to 20% in the THF/H2O mixture, the fluorescence
intensity of CTPE−C6−CTPE (50 μM) dropped sharply to
almost disappear (Figure 2e) and then leveled off as the THF
fractions reached 30% (Figure 2f). This “turn-off” phenomen-
on could also be observed directly by the naked eye, with the
solution changing from bright cyan to colorless under 365 nm
UV irradiation (inset of Figure 2f). To acquire more detailed
information about their sensitivity, CTPE−C6−CTPE was also
dissolved in other organic solvents (acetonitrile, ethanol, and
acetone). Only weak signals were observed in their
fluorescence spectra (Figure S4). Meanwhile, when ethanol
was slowly added to an aqueous solution of CTPE−C6−CTPE
,
the phase interface could be clearly observed under the bright-
cyan fluorescence (Figure S5a). However, this harmonious
state and bright fluorescence disappeared immediately after
shaking, suggesting that photoluminescence of CTPE−C6−CTPE
was extremely sensitive to organic solvents. In contrast, the
phase interface and bright emission could still be found when
they mixed with n-hexane because of their complete
insolubility (Figure S5b).
3.4. Shape-Controlled Preparation of AIE-SNs and
Derived AIE-MSNs. The shape-controlled AIE-SNs could be
facilely prepared in H2O via a dual-surfactant-guided one-pot
method with CTAB and CTPE−C6−CTPE. As shown in Scheme
1b, CTPE−C6−CTPE and CTAB were dispersed in H2O first
with continuous stirring to form composite micelles for the
facile island nucleation and growth of silica. When TEOS was
introduced, the hydrolyzed silicate precursor could be directed
to condense into ordered silica structures around micellar
templates because of a strong electrostatic interaction.27
Subsequently, a layer of AIE-SNs formed on the composite
micelles via the coassembly of CTAB, CTPE−C6−CTPE, and
silicate oligomers, followed by continuous growth and
aggregation until the reaction finished. To further understand
the role of CTPE−C6−CTPE in tuning AIE-SNs and their
corresponding evolution process, SEM images were used to
characterize the products collected under the different molar
ratios of CTAB/CTPE−C6−CTPE. As the molar ratio of CTAB/
G
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Representative SEM (a and b) and TEM (c and d) images of AIE-SNs-7. TEM images of AIE-SNs-7 with rare rattlelike structures (e and
f).
CTPE−C6−CTPE changed from 40:1 to 5:1, the shapes and sizes
of AIE-SNs transformed significantly and the initial irregular
aggregates gradually evolved into uniform morphologies
(Figure S6). Interestingly, the regular AIE-SNs could further
evolve as the molar ratio continuously changed, achieving the
shape-controlled preparation of AIE-SNs. At the final molar
ratio of 5:1, the AIE-SNs tended to cluster into an elongated
rodlike morphology instead of a rounded one (Figure S6i), and
some could even reach a length of 800 nm (Figure S7). A
summary of the experimental data and processing parameters is
given in Table S1, and the products obtained under different
molar ratios were denoted as AIE-SNs-n (n = 40, 35, 30, 25,
20, 15, 10, 7, 5, and 0).
It is noteworthy that two kinds of silica nanorods, AIE-SNs-
15 (Figure S6f) and AIE-SNs-7 (Figure S6h), could be
prepared directly when the molar ratio of CTAB/CTPE−C6−
CTPE was fixed at 15:1 and 7:1, respectively. As shown in
Figure 3a, SEM observation shows that the obtained AIE-SNs-
15 have rodlike morphology with a width of ∼200 nm and a
length of ∼350 nm. A close observation of AIE-SNs-15 reveals
that the nanorods have a relatively coarse surface with tiny
cracks (Figure 3b). Furthermore, their internal cavities and
shells can also be clearly observed from the high-magnification
SEM images of the damaged AIE-SNs-15 obtained under the
intense ultrasound (Figure S8). TEM images of AIE-SNs-15
indicate a well-defined hollow structure with an outer shell of a
thickness of ∼12 nm (Figure 3c,d). Meanwhile, internal
cavities with thin shells are consistent with their external shape.
Besides the normal hollow silica nanorods, some rare yolk−
shell nanorods with rattle structures, which are difficult to
construct by simple methods, appeared in the product (Figures
3e,f and S9). Because of their large void space for cargo storage
and delivery, yolk−shell nanomaterials composed of a core
within a hollow cavity surrounded by a porous outer shell have
been considered to be ideal carriers and nanoreactors.20
At a molar ratio of 7:1, the product evolved into uniform
narrow silica nanorods in which the width got truncated. As
shown in Figure 4a,b, the SEM images of AIE-SNs-7 exhibit
rodlike morphology with a width of ∼75 nm and a length of
∼250 nm. The coarse surface with tiny cracks as well as their
broken internal cavities and shells can also be clearly observed
at a higher magnification (Figure S8). As can be observed from
the TEM images of AIE-SNs-7 (Figure 4c,d), the hollow
structures can still be found in the product and their shell is
much thinner than that of AIE-SNs-15 with a thickness of only
∼5 nm. Compared with the internal structures of AIE-SNs-15,
the AIE-SNs-7 was mainly yolk−shell nanorods with rattle
structures instead of the normal hollow (Figures 4e,f and S9),
indicating that the internal structures of AIE-SNs were also
developed along with the external morphology evolution as the
molar ratio changed from 15:1 to 7:1.
In addition to the great interest raised by the above
products, the observations allow us to partially conceive the
roles of CTPE−C6−CTPE in achieving the tuning of AIE-SNs.
The dynamic combined structure-directing effects proposed in
our previous research arising from the cooperative CTAB and
CTPE−C6−CTPE can be used to understand their structural
evolution process.33 It is the growing effects of CTPE−C6−CTPE
in the structure-directing process as the molar ratio changes
from 40:1 to 5:1 that lead to the continued evolution of the
H
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. Representative SEM (a) and TEM (c and d) images of AIE-MSNs-15. SEM (b) and TEM (e and f) images of AIE-MSNs-7.
construction. When the amount of CTPE−C6−CTPE is
increased, the effective volume (v) of the hydrophobic group
of CTAB/CTPE−C6−CTPE increases because of the presence of
bulky TPE units, which also increases the average packing
parameter P of the CTAB/CTPE−C6−CTPE cosurfactant (P =
v/a0lc, where a0 is the polar head surface area and lc is the
hydrophobic chain length), resulting in the transformation of
the assembled phase.14 To further understand the structure-
directing effect of CTPE−C6−CTPE in the original system mixed
with CTAB, the compared CTAB-SNs were prepared under
the direction of the individual CTAB (Figure S10). Meanwhile,
AIE-SNs-40, AIE-SNs-30, AIE-SNs-15, and AIE-SNs-5 were
also reprepared under the same conditions without the
addition of CTPE−C6−CTPE (Figure S11). Only regular
nanospheres could be found in their product, and the
nanospheres gradually aggregated into a chaplet shape as the
CTAB concentration decreased (Figure S11D). Additionally, it
is impossible to obtain a hollow or multifunctional structure
with limited CTAB alone according to the literature.4,19,33
These results show the indispensable role of CTPE−C6−CTPE in
the structure-directing process. On the other hand, CTAB, as a
convenient cationic surfactant for building mesoporous silica,
was essential for the whole mixed system and acted as the main
body in guiding the evolution process of AIE-SNs. Only
coralloid AIE-SNs-0 could be obtained under the direction of
pure CTPE−C6−CTPE (Figure S10). All of these results revealed
the presence of the combined structure-directing effects of
CTAB/CTPE−C6−CTPE in the preparation of the tuning of
AIE-SNs. On the basis of the proposed mechanism, other
interesting shapes of AIE-SNs could also be found beyond our
research by adjusting the molar ratio properly.
The interaction between the cationic surfactant head groups
and anionic silica frame could be broken during the subsequent
HCl/ethanol extraction, and then the templating agent was
removed from the mesoporous silica.10 After the coassembly of
CTAB, CTPE−C6−CTPE, and silica, the AIE-SNs can transform
into AIE-MSNs smoothly by removing the CTAB template.
Compared with the simple straight-chain structure of CTAB, it
was difficult to remove CTPE−C6−CTPE from the silica frame
because of its complex structure. In particular, the spacer in the
gemini surfactant can greatly prevent them from escaping from
the rigid structures (Si−O−Si) provided by the silica matrix
(Scheme 1b). Therefore, a large amount of CTPE−C6−CTPE
was persistently encapsulated in the silica frame after the
removal of CTAB, bringing about both abundant mesoporous
and successive fluorescent properties for AIE-MSNs.
SEM and TEM images of AIE-MSNs-15 and AIE-MSNs-7
were investigated to have a better insight of the resultant AIE-
MSNs. As shown in Figure 5a,b, AIE-MSNs-15 and AIE-
MSNs-7 preserve their original morphologies and display no
external differences with AIE-SNs-15 and AIE-SNs-7. The
same result can also be found in the SEM images of CTAB-
MSNs, AIE-MSNs-30, and AIE-MSNs-5 (Figure S12),
revealing the excellent structural stability of AIE-MSNs.
According to the TEM images of AIE-MSNs-15 (Figures 5c
and S13), it can be seen that silica nanorods have already
possessed highly ordered mesoporous channels, wherein the
shells and internal hollow cavities can only be ambiguously
observed because of the presence of numerous pores.
Compared with AIE-SNs-15, the shell thickness of AIE-
MSNs-15 showed no obvious change in size and was estimated
to be ∼12 nm (Figure 5d). Similarly, the indistinct shells and
I
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. (a) Fluorescence spectra for the different kinds of AIE-SNs and CTAB-SNs (0.25 mg/mL). (b) Fluorescence spectra for the different
kinds of AIE-MSNs (0.25 mg/mL). (c) Comparison of the fluorescence intensities for different kinds of AIE-SNs and AIE-MSNs in parts a and b at
475 nm. (d) Fluorescent photographs of CTAB-SNs, AIE-SNs-15, AIE-SNs-7, AIE-MSNs-15, and AIE-MSNs-7 in aqueous dispersion under 365
nm UV irradiation. (e) IR spectra and photographs (taken under room light and 365 nm UV irradiation) of the solid powder of CTAB-SNs, AIE-
SNs, CTAB-MSNs, and AIE-MSNs.
internal hollow structures of AIE-MSNs-7 can also be observed
in their TEM images (Figures 5e and S13). As shown in Figure
5f, AIE-MSNs-7 exhibited the same shell thickness (∼5 nm) as
AIE-SNs-7. Compared with the mesoporous silica in AIE-
MSNs-15, although AIE-MSNs-7 possessed more noticeable
pore structures, their arrangement was inhomogeneous and
disordered (insets of Figure 5d,f). Additionally, the original
yolk−shell nanorods with rattle structure in AIE-MSNs-15 and
AIE-MSNs-7 also became indistinct because of the presence of
numerous pores on the shells, suggesting that the CTAB
template in AIE-SNs has been effectively removed by the
extraction treatment.
3.5. Characterization of AIE-SNs and AIE-MSNs. The
fluorescent properties of AIE-SNs and AIE-MSNs were both
endowed by the embedded CTPE−C6−CTPE. When the
surfactant-directed coassembly was finished, the fluorescence
spectra of the residual supernatant solution were investigated
after AIE-SNs were removed (Figure S14). Their weak signals
suggested that CTPE−C6−CTPE had been encapsulated into
SNs successfully with CTAB. As shown in Figure 6a, the
fluorescence intensities for different kinds of AIE-SNs (0.25
mg/mL) were evaluated at the same time. Compared to
CTAB-SNs, their fluorescence intensities enhanced, with the
types changing from AIE-SNs-40 to AIE-SNs-5. Particularly,
the emission (475 nm, Figure S14b) and absorption (319 nm,
Figure S15a) wavelengths of AIE-SNs occurred at the same
time as the individual CTPE−C6−CTPE in aqueous solution
without a red/blue shift (Figures 1c and S1a), implying that
the TPE units in AIE-SNs were free from the restriction of the
silica matrix. On the contrary, an obvious shift will be observed
if the AIEgens were restricted in the rigid microenvironment
provided by the silica matrix.5,33
The fluorescence spectra for different kinds of AIE-MSNs
with the same concentration (0.25 mg/mL) are shown in
Figure 6b. Compared with the original AIE-SNs, AIE-MSNs
demonstrated enhanced fluorescence intensities instead of the
J
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. Nitrogen adsorption−desorption isotherms (a) and the corresponding pore size distribution curves (b) of AIE-SNs-15, AIE-MSNs-15,
AIE-SNs-7, and AIE-MSNs-7.
traditional weakening caused by the leaking out of fluorescent
molecules in the extraction process. As shown in Figure 6c, the
remarkable enhancement was recorded with the types of SNs
(AIE-SNs-n and AIE-MSNs-n, where n = 40, 35, 30, 25, 20, 15,
10, 7, and 5). Regarding AIE-MSNs-15 and AIE-MSNs-7, their
fluorescence intensities were increased up to 2.2- and 2.3-fold,
respectively, after the extraction treatment. The improved
photoluminescence could also be readily observed by the
naked eye in their aqueous dispersion with the assistance of a
UV lamp (Figure 6d). The results revealed that the removal of
CTAB could bring about not only plenty of mesoporous silica
but also outstanding fluorescence emission for the AIE-MSNs.
One leading cause for their improved performance is the
disappearance of the nonpolar environment constructed by
CTAB, which is especially beneficial for photoluminescence of
the remaining individual CTPE−C6−CTPE because of their
special sensitivity. Additionally, the presence of a large amount
successfully by the extraction process.33 With regard to the
incorporated CTPE−C6−CTPE, the characteristic signal located
at 1508 cm−1 assigned to the stretching vibration of TPE
(Figure S3) could be observed in both AIE-SNs and AIE-
MSNs, revealing the constant presence of CTPE−C6−CTPE. The
results are consistent with the high porosity and successive
fluorescence observed in AIE-MSNs. On the other hand, the
bright-cyan fluorescence could also be observed in the solid
powder of AIE-SNs and AIE-MSNs because of the AIE effect.
As shown in Figure 6e, the solid powders of CTAB-SNs, AIE-
SNs, and AIE-MSNs are white in appearance under normal
room lighting, but strong cyan light can be emitted from AIE-
SNs and AIE-MSNs under 365 nm UV irradiation.
Nitrogen adsorption−desorption isotherms of AIE-SNs-15,
AIE-MSNs-15, AIE-SNs-7, and AIE-MSNs-7 were recorded to
investigate the change of the surface area and porosity before
and after removal of the CTAB template (Table S2). As shown
in Figure 7a, all of the samples exhibited the combination of
type II and IV isotherms with a H4-type hysteresis loop, which
was nearly parallel over an appreciable range of relative
pressure according to the IUPAC classification.38 Moreover, a
sharply increased nitrogen adsorption plot was observed in the
high relative pressure range of 0.8−1.0, which is mainly
attributed to the internal voids formed by the randomly
distributed and packed SNs. The BET surface areas of AIE-
SNs-15 and AIE-SNs-7 were calculated to be only 21.9 and
62.7 m2/g, respectively. In contrast, the BET surface areas were
calculated to be 652.6 and 606.6 m2/g for AIE-MSNs-15 and
AIE-MSNs-7, respectively, which were much larger than those
of the unextracted AIE-SNs-15 and AIE-SNs-7. The greatly
increased surface areas of AIE-MSNs-15 and AIE-MSNs-7
were consistent with the porous features observed from their
TEM images (Figures 5 and S13), and their surface areas were
comparable in value with many other hollow MSNs reported in
the literature.14,15,26
of mesoporous silica in AIE-MSNs caused the anchored CTPE
−
C6−CTPE to be fully exposed to H2O, promoting the RIR effect
significantly on the TPE luminogens. The unchanged emission
(475 nm, Figure S14b) and absorption (319 nm, Figure S15a)
wavelengths of AIE-MSNs compared with AIE-SNs and CTPE
−
C6−CTPE further revealed the consistent and unrestricted state
of the AIEgens in SNs. A typical Tyndall phenomenon can also
be observed in AIE-SNs-15, AIE-SNs-7, AIE-MSNs-15, and
AIE-MSNs-7 (Figure S15b), confirming their excellent
dispersibility in an aqueous dispersion. Furthermore, the
AIE-MSNs possess good photostability, and the fluorescence
intensities remained above 70% after 60 min of UV irradiation
(Figure S16a). The stability for different types of AIE-MSNs
could also be determined by recording the time-dependent
evolution of the fluorescence intensity. Their emission
decreased after 1 week because of the spontaneous aggregation
of AIE-MSNs in aqueous dispersion (Figure S16b). However,
further aggregation of AIE-MSNs can induce the strengthening
of the AIE effect, leading to the recovery of their fluorescence
intensities. The superior fluorescence properties of AIE-MSNs
indicate their promising prospects in biomedical applications.
IR spectra of CTAB-SNs, AIE-SNs, CTAB-MSNs, and AIE-
MSNs are given in Figure 6e. Compared with CTAB-SNs and
AIE-SNs, the stretching vibration bands of CTAB located at
2925, 2856, and 1487 cm−1 disappeared in CTAB-MSNs and
AIE-MSNs, suggesting that CTAB has been removed
In the synthetic process of MSNs, the length and volume of
the hydrophobic alkyl chain of the surfactants can indicate
their pore diameter and volume.28 The pore volumes of AIE-
SNs-15 and AIE-SNs-7 were calculated to be 0.047 and 0.127
cm3/g, while the pore volumes of AIE-MSNs-15 and AIE-
MSNs-7 were calculated to be 0.614 and 0.634 cm3/g,
respectively. Because of the presence of more CTPE−C6−
CTPE, the pore volume of AIE-SNs-7 has always been larger
K
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
than that of AIE-SNs-15 before and after the removal of
CTAB. In particular, as shown in Figure 7b, AIE-MSNs-7
exhibited larger holes with the pore size center around 3.2 nm.
Comparatively, the pore sizes of AIE-MSNs-15 and most
reported MSNs are limited to less than 3.0 nm because of the
low molecular weight of CTAB.10,29 Because CTPE−C6−CTPE
contains the bulky TPE luminogens connecting at the tails, the
large hydrophobic TPE groups help to organize the micelles,
having an empty volume at the center, and are responsible for
the larger pore size of AIE-MSNs-7. On the basis of the
aforementioned observations, the potential pore size and
volume can also be effectively improved by tuning the molar
ratio of CTAB/CTPE−C6−CTPE in the coassembled process. By
AIE-MSNs-CPT are higher than those of MSNs-CPT because
of the larger loading capacities of CPT. Meanwhile, AIE-
MSNs-15-CPT and AIE-MSNs-7-CPT exhibited similar
sustained release curves and drug release reached a plateau
after 72 h. However, the amount of CPT released from AIE-
MSNs-7-CPT was higher than that of AIE-MSNs-15-CPT at
the beginning, although their loading capacity was not
outstanding, which may be due to the presence of the larger
pore size of AIE-MSNs-7. The same result can also be clearly
observed in the release curves of MSNs-15-CPT and MSNs-7-
CPT. To better understand their behavior of drug release, AIE-
MSNs-CPT and MSNs-CPT were further investigated by
constant incubation with fresh PBS instead of the tested
supernatant and monitored every 2 h (Figure S18b). The
sustained release can be well realized, and CPT can be
continuously released for more than 90 h. Meanwhile, the
adsorbed CPT can be easily released from AIE-MSNs-7-CPT
and MSNs-7-CPT because of their advantageous pore size,
resulting in the larger release amount at the beginning than
that of AIE-MSNs-15-CPT and MSNs-15-CPT. In addition,
the release curves were both up and down accompanied by
several releasing upsurges instead of a constant decrease as the
time goes on, which strongly suggested their complex internal
structures.
taking advantage of the bulky TPE groups of CTPE−C6−CTPE
,
the modified AIE-MSNs with simple operation, high surface
areas, and larger pore volumes and sizes would be favorable
and promising for further applications in host−guest
interaction.
3.6. Drug Loading and Release Behaviors of AIE-
MSNs. It is well-known that many anticancer drugs are
aromatic and hydrophobic, resulting in poor water solubility
and restricted clinical application.8 The anchored CTPE−C6−
CTPE in AIE-MSNs can provide not only fluorophore but also
available hydrophobic TPE units for aromatic drug loading. To
confirm their effectiveness in loading the hydrophobic drug,
CPT was used as the model guest molecule, and the loading
capacities of AIE-MSNs-15 and AIE-MSNs-7 were evaluated as
344.71 and 319.22 mg/g, respectively, which are much higher
than those of calcined MSNs-15 (231.12 mg/g) and MSNs-7
(218.54 mg/g). Although the morphologies and frames of
MSNs-15 and MSNs-7 were preserved well (Figure S17), the
original CTPE−C6−CTPE was absent in calcined MSNs,
resulting in the disappearance of TPE units and corresponding
fluorescent properties. The data suggest that AIE-MSNs are
superior to calcined MSNs in loading hydrophobic anticancer
drugs because of the presence of additional hydrophobic
interaction and π−π stacking. Additionally, an obvious peak of
CPT (438 nm) was observed in the fluorescence spectra of
AIE-MSNs-CPT because of the covering of a large amount of
CPT (Figure S18a), suggesting their good performance in
CPT loading.
4. CONCLUSIONS
In summary, a TPE-functionalized gemini surfactant CTPE
−
C6−CTPE was designed and synthesized to develop a
straightforward method for the controllable preparation of
hollow AIE-MSNs. The versatile CTPE−C6−CTPE possesses
superior water solubility, surface activity, stability, and
sensitivity to CTAB and organic solvents and can realize
photoluminescence simultaneously in dispersed, aggregated,
and solid states. In the preparation of tunable AIE-SNs, CTPE
−
C6−CTPE will not only provide an anchored fluorophore but
also serve as an intimate partner of CTAB to regulate their
construction. By proper tuning of the molar ratio of CTAB/
CTPE−C6−CTPE, the morphologies of AIE-SNs could be tuned
effectively and two kinds of silica nanorods (AIE-SNs-15 and
AIE-SNs-7) were produced directly. After the extraction of
CTAB, AIE-SNs can evolve into AIE-MSNs successfully and
bring about abundant mesoporous silica and high accessible
surface area. It is noteworthy that the incorporated bulky TPE
luminogens can not only provide enhanced fluorescence for
AIE-MSNs to overcome the inevitable weakening of the
fluorescence intensity caused by leakage but also improve their
internal structure, endowing AIE-MSNs-7 with larger pore size
(3.2 nm) and pore volume (0.634 cm3/g). Moreover, the
resultant AIE-MSNs-15 and AIE-MSNs-7 can both serve as the
desired nanocarriers of the hydrophobic anticancer drug for
their high loading capacities and long release times. The
proposed CTPE−C6−CTPE enriches the family of gemini
surfactants and opens new perspectives to controllably prepare
advanced fluorescent mesoporous materials with simple
operation for the future large-scale industrial production and
various applications in bioimaging, drug delivery, bioenrich-
ment and separation, sensing, and so on.
The in vitro release profiles of CPT from AIE-MSNs-CPT
and MSNs-CPT are shown in Figure 8. The release values of
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02252.
■
S
Figure 8. In vitro cumulative release profiles of CPT from AIE-MSNs-
15-CPT, AIE-MSNs-7-CPT, MSNs-15-CPT, and MSNs-7-CPT.
L
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Supplementary figures and tables regarding the syn-
Article
Anionic-Neutral Block Copolymer Ternary Surfactants. Langmuir
2006, 22, 6−9.
thesis, characterization of CTPE−C6−CTPE, AIE-SNs, and
AIE-MSNs, and other experiments (PDF)
(14) Zhang, Y.; Zhou, G.; Sun, B.; Zhao, M.; Zhang, J.; Chen, F. A
Cationic-Cationic Co-Surfactant Templating Route for Synthesizing
Well-Defined Multilamellar Vesicular Silica with an Adjustable
Number of Layers. Chem. Commun. 2014, 50, 2907−2909.
(15) Yang, Y.; Yu, M.; Song, H.; Wang, Y.; Yu, C. Preparation of
Fluorescent Mesoporous Hollow Silica-Fullerene Nanoparticles Via
Selective Etching for Combined Chemotherapy and Photodynamic
Therapy. Nanoscale 2015, 7, 11894−11898.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gzn@whu.edu.cn.
■
ORCID
Zhinong Gao: 0000-0003-2855-5334
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
(16) Li, L.; Tang, F.; Liu, H.; Liu, T.; Hao, N.; Chen, D.; Teng, X.;
He, J. In Vivo Delivery of Silica Nanorattle Encapsulated Docetaxel
for Liver Cancer Therapy with Low Toxicity and High Efficacy. ACS
Nano 2010, 4, 6874−6882.
(17) Wang, S.; Chen, M.; Wu, L. One-Step Synthesis of Cagelike
Hollow Silica Spheres with Large through-Holes for Macromolecule
Delivery. ACS Appl. Mater. Interfaces 2016, 8, 33316−33325.
The authors declare no competing financial interest.
̈
(18) Chiu, H.-Y.; Goßl, D.; Haddick, L.; Engelke, H.; Bein, T.
ACKNOWLEDGMENTS
This work was supported by the Fundamental Research Funds
for the Chinese Central Universities (Grant 2012203020211).
■
Clickable Multifunctional Large-Pore Mesoporous Silica Nano-
particles as Nanocarriers. Chem. Mater. 2018, 30, 644−654.
(19) Li, Y.; Li, N.; Pan, W.; Yu, Z.; Yang, L.; Tang, B. Hollow
Mesoporous Silica Nanoparticles with Tunable Structures for
Controlled Drug Delivery. ACS Appl. Mater. Interfaces 2017, 9,
2123−2129.
(20) Lin, L. S.; Song, J.; Yang, H. H.; Chen, X. Yolk-Shell
Nanostructures: Design, Synthesis, and Biomedical Applications. Adv.
Mater. 2018, 30, 1704639−1704668.
(21) Liang, J.; Tang, B. Z.; Liu, B. Specific Light-up Bioprobes Based
on AIEgen Conjugates. Chem. Soc. Rev. 2015, 44, 2798−2811.
(22) Yan, L.; Zhang, Y.; Xu, B.; Tian, W. Fluorescent Nanoparticles
Based on AIE Fluorogens for Bioimaging. Nanoscale 2016, 8, 2471−
2487.
(23) Zhou, Z.; Zhang, C. C.; Zheng, Y.; Wang, Q. Aggregation
Induced Emission Mediated Controlled Release by Using a Built-in
Functionalized Nanocluster with Theranostic Features. J. Med. Chem.
2016, 59, 410−418.
(24) Fang, X.; Zhang, Y.-M.; Chang, K.; Liu, Z.; Su, X.; Chen, H.;
Zhang, S. X.-A.; Liu, Y.; Wu, C. Facile Synthesis, Macroscopic
Separation, E/Z Isomerization, and Distinct AIE Properties of Pure
Stereoisomers of an Oxetane-Substituted Tetraphenylethene Lumi-
nogen. Chem. Mater. 2016, 28, 6628−6636.
(25) Li, D.; Yu, J. AIEgens-Functionalized Inorganic-Organic Hybrid
Materials: Fabrications and Applications. Small 2016, 12, 6478−6494.
(26) Fan, Z.; Li, D.; Yu, X.; Zhang, Y.; Cai, Y.; Jin, J.; Yu, J. AIE
Luminogen-Functionalized Hollow Mesoporous Silica Nanospheres
for Drug Delivery and Cell Imaging. Chem. - Eur. J. 2016, 22, 3681−
3685.
(27) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.;
Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a
Liquid-Crystal Template Mechanism. Nature 1992, 359, 710−712.
(28) Kim, J.; Kim, B.; Anand, C.; Mano, A.; Zaidi, J. S.; Ariga, K.;
You, J.; Vinu, A.; Kim, E. A Single-Step Synthesis of Electroactive
Mesoporous Prodot-Silica Structures. Angew. Chem., Int. Ed. 2015, 54,
8407−8410.
(29) Zhang, Y.; Yue, Q.; Yu, L.; Yang, X.; Hou, X.; Zhao, D.; Cheng,
X.; Deng, Y. Amphiphilic Block Copolymers Directed Interface
Coassembly to Construct Multifunctional Microspheres with
Magnetic Core and Monolayer Mesoporous Aluminosilicate Shell.
Adv. Mater. 2018, 30, 1800345−1800354.
REFERENCES
■
(1) Wang, F.; Xiao, J.; Chen, S.; Sun, H.; Yang, B.; Jiang, J.; Zhou,
X.; Du, J. Polymer Vesicles: Modular Platforms for Cancer
Theranostics. Adv. Mater. 2018, 30, 1705674−1705681.
(2) Wang, H.; Chen, Q.; Zhou, S. Carbon-Based Hybrid Nanogels:
A Synergistic Nanoplatform for Combined Biosensing, Bioimaging,
and Responsive Drug Delivery. Chem. Soc. Rev. 2018, 47, 4198−4232.
(3) Xing, P.; Zhao, Y. Supramolecular Vesicles for Stimulus-
Responsive Drug delivery. Small Methods 2018, 2, 1700364−1700379.
(4) Zhang, X.; Zhang, X.; Wang, S.; Liu, M.; Zhang, Y.; Tao, L.; Wei,
Y. Facile Incorporation of Aggregation-Induced Emission Materials
into Mesoporous Silica Nanoparticles for Intracellular Imaging and
Cancer Therapy. ACS Appl. Mater. Interfaces 2013, 5, 1943−1947.
(5) Wang, Y.-F.; Che, J.; Zheng, Y.-C.; Zhao, Y.-Y.; Chen, F.; Jin, S.-
B.; Gong, N.-Q.; Xu, J.; Hu, Z.-B.; Liang, X.-J. Multi-Stable
Fluorescent Silica Nanoparticles Obtained from in Situ Doping with
Aggregation-Induced Emission Molecules. J. Mater. Chem. B 2015, 3,
8775−8781.
(6) Croissant, J. G.; Fatieiev, Y.; Almalik, A.; Khashab, N. M.
Mesoporous Silica and Organosilica Nanoparticles: Physical Chem-
istry, Biosafety, Delivery Strategies, and Biomedical Applications. Adv.
Healthcare Mater. 2018, 7, 1700831−1700905.
(7) Yan, S.; Hu, F.; Hong, X.; Shuai, Q. Water-Soluble Ferrocene
Complexes (WFCs) Functionalized Silica Nanospheres for WFC
Delivery in HepG2 Tumor Therapy. Mater. Sci. Eng., C 2018, 90,
397−406.
(8) Li, N.; Niu, D.; Jiang, Y.; Xu, C.; Pan, S.; He, J.; Chen, J.; Zhang,
L.; Li, Y. Morphology Evolution and Spatially Selective Functionaliza-
tion of Hierarchically Porous Silica Nanospheres for Improved
Multidrug Delivery. Chem. Mater. 2017, 29, 10377−10385.
(9) He, Q.; Shi, J.; Chen, F.; Zhu, M.; Zhang, L. An Anticancer Drug
Delivery System Based on Surfactant-Templated Mesoporous Silica
Nanoparticles. Biomaterials 2010, 31, 3335−3346.
(10) Coti, K. K.; Belowich, M. E.; Liong, M.; Ambrogio, M. W.; Lau,
Y. A.; Khatib, H. A.; Zink, J. I.; Khashab, N. M.; Stoddart, J. F.
Mechanised Nanoparticles for Drug Delivery. Nanoscale 2009, 1, 16−
39.
(30) Liao, X.; Gao, Z.; Xia, Y.; Niu, F.; Zhai, W. Rational Design and
Synthesis of Carboxylate Gemini Surfactants with an Excellent
Aggregate Behavior for Nano-La2o3Morphology-Controllable Prep-
aration. Langmuir 2017, 33, 3304−3310.
(31) Xia, Y.; Gao, Z.; Liao, X.; Pan, C.; Zhang, Y.; Feng, X. Rapid
Synthesis of Hierarchical, Flower-Like Ag Microstructures with a
Gemini Surfactant as a Directing Agent for SERS Applications.
CrystEngComm 2017, 19, 6547−6555.
(32) Yan, S.; Wei, W.; Gao, Z.; Xia, Y.; Han, J. Gemini Surfactant
with Pyrrolidinium Head Groups and a Hydroxyl-Substituted Spacer:
(11) Xuan, M.; Shao, J.; Zhao, J.; Li, Q.; Dai, L.; Li, J. Magnetic
Mesoporous Silica Nanoparticles Cloaked by Red Blood Cell
Membranes: Applications in Cancer Therapy. Angew. Chem., Int. Ed.
2018, 57, 6049−6053.
(12) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y.
Stimuli-Responsive Controlled Drug Release from a Hollow
Mesoporous Silica Sphere/Polyelectrolyte Multilayer Core-Shell
Structure. Angew. Chem. 2005, 117, 5213−5217.
(13) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Synthesis of
Hollow Silica Spheres with Mesostructured Shell Using Cationic-
M
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Surface Properties and Assisted One-Pot Synthesis of Dendritic Au
Nanocrystals. New J. Chem. 2018, 42, 11573−11582.
(33) Yan, S.; Gao, Z.; Xia, Y.; Liao, X.; Han, J.; Pan, C.; Zhang, Y.;
Zhai, W. Aggregation-Induced Emission Gemini Surfactant-Assisted
Fabrication of Shape-Controlled Fluorescent Hollow Mesoporous
Silica Nanoparticles. Eur. J. Inorg. Chem. 2018, 2018, 1891−1901.
(34) Yatcilla, M. T.; Herrington, K. L.; Laura, L.; Brasher, a.; Kaler,
E. W.; Chiruvolu, S.; Zasadzinski, J. A. Phase Behavior of Aqueous
Mixtures of Cetyltrimethylammonium Bromide (CTAB) and Sodium
Octyl Sulfate (SOS). J. Phys. Chem. 1996, 100, 5874−5879.
(35) Shi, L.; Lundberg, D.; Musaev, D. G.; Menger, F. M.
[12]Annulene Gemini Surfactants: Structure and Self-Assembly.
Angew. Chem., Int. Ed. 2007, 46, 5889−5891.
(36) Guan, W.; Zhou, W.; Lu, C.; Tang, B. Z. Synthesis and Design
of Aggregation-Induced Emission Surfactants: Direct Observation of
Micelle Transitions and Microemulsion Droplets. Angew. Chem., Int.
Ed. 2015, 54, 15160−15164.
(37) Guan, W.; Wang, S.; Lu, C.; Tang, B. Z. Fluorescence
Microscopy as an Alternative to Electron Microscopy for Microscale
Dispersion Evaluation of Organic-Inorganic Composites. Nat.
Commun. 2016, 7, 11811−11817.
(38) Sing, K. S. W. Reporting Physisorption Data for Gas/Solid
Systems-with Special Reference to the Determination of Surface Area
and Porosity. Pure Appl. Chem. 1985, 57, 603−619.
N
DOI: 10.1021/acs.inorgchem.8b02252
Inorg. Chem. XXXX, XXX, XXX−XXX