Electrospun aggregation-induced emission active POSS-based porous copolymer films for detection of explosives

Article abstract of DOI:10.1039/c4cc06559j

Electrospun aggregation-induced emission (AIE)-active polyhedral oligomeric silsesquioxane (POSS)-based copolymer films exhibit an approximately 9-fold increase in response to explosive vapors compared to dense films although porous copolymer films have a thickness as high as 560 ± 60 nm. This journal is

Full text of DOI:10.1039/c4cc06559j

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Electrospun aggregation-induced emission active
POSS-based porous copolymer films for detection
of explosives†
Cite this:DOI: 10.1039/c4cc06559j
Received 20th August 2014,
Accepted 12th September 2014
a
a
a
a
a
a
Hui Zhou, Qun Ye, Wei Teng Neo, Jing Song, Hong Yan, Yun Zong,
b
ac
a
Ben Zhong Tang,* T. S. Andy Hor and Jianwei Xu*
DOI: 10.1039/c4cc06559j
www.rsc.org/chemcomm
11
Electrospun aggregation-induced emission (AIE)-active polyhedral with film thicker than 25 nm. To reduce the dependence of
oligomeric silsesquioxane (POSS)-based copolymer films exhibit an sensing performance on film thickness, a sensor based on a highly
approximately 9-fold increase in response to explosive vapors nanostructured film with a large surface-to-volume ratio, inherent
compared to dense films although porous copolymer films have a high porosity, and easy accessibility of sensing materials is needed.
thickness as high as 560 Æ 60 nm.
On the other hand, aggregation-caused quenching (ACQ) of fluores-
cence is commonly observed in fluorescent conjugated polymers in
In the past few decades, detection of explosives has become the solid state, which has undermined their potential as solid state
12
an internationally concerned issue for the anti-terrorism and sensors or probes for explosive detection.
homeland security. Most high explosives are nitro-substituted Recently, aggregation induced emission (AIE) materials in the
organic compounds. Typically, nitro-aromatics, such as 2,4,6- areas of optoelectronics and sensory systems
13
14–16
have attracted
trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT), are the increasing interest due to the absence of the ACQ effect. Fluores-
primary military explosives and also the principal components cence of the AIE active tetraphenylethene (TPE)-based polymers can
17
in the unexploded landmines worldwide.
be effectively quenched by nitroaromatics in solution. However,
Various analytical techniques have been used for the detection the solid state sensing materials with AIE properties have been
of explosives, such as chromatography using an ultraviolet absorp- rarely used for vapor detection mainly due to the lack of suitable
1
2
3
tion detector, mass spectrometry, Raman spectroscopy, X-ray processable AIE active polymers. Herein, we report a feasible
4
5
6
imaging, thermal neutron analysis, electrochemical assay, and method to prepare AIE active copolymers using free radical
ion mobility spectroscopy, which are always highly costly and copolymerization of polyhedral oligomeric silsesquioxane
7
difficult to use. Fluorescence based sensing has provided an (POSS) acrylate and TPE-containing acrylate monomers. POSSs
alternative detection approach, which is highly sensitive, con- have been demonstrated as building blocks for the generation
1
8a
18b
venient and cost-effective. Fluorescent conjugated polymers have of porous materials through thermolysis, hydrosilylation,
18c
been applied as film sensing materials for the vapor detection of other coupling reactions, as well as free radical copolymerization
1
8d
nitro compounds, because of their efficient exciton migration with other monomers.
Several porous materials derived from
8–10
along the polymer chains. However, the performance of most vinyl POSS exhibit Brunauer–Emmett–Teller (BET) surface areas as
fluorescent sensory materials is limited by film thickness due high as 700 m g and a maximum tunable mesopore volume of
2
À1
3
À1 18e–g
to the fact that diffusion of analyte vapor in dense films is slow. 2.0 cm g
.
In this communication, POSS based AIE active
A spin-coated conjugated polymer film achieves its optimum copolymers show pronounced AIE effects in solution as well as in
quenching efficiency towards TNT vapor with an ultra-thin film the solid state. Porous films fabricated from the copolymers by
(ca. 2.5 nm) and experiences a sharp drop in quenching efficiency electrospinning showed high sensitivity and selectivity for the
vapor detection of nitro-compounds.
a
The synthetic routes leading to acrylate monomers and
polymers P1-4 are shown in Scheme 1. The key intermediate
4-(1,2,2-triphenylvinyl)phenol (1) was prepared from 1-bromo-
triphenylethylene and 4-hydroxyphenylboronic acid via a Suzuki
Institute of Materials Research and Engineering, A*STAR (Agency for Science,
Technology and Research), 3 Research Link, Singapore 117602,
Republic of Singapore. E-mail: jw-xu@imre.a-star.edu.sg
b
c
Department of Chemistry, The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, Republic of Singapore
17f
coupling reaction according to our previous work. Compound 1
reacted with 12-bromododecan-1-ol to produce precursor 2 with
a reasonable yield of 86%, followed by treatment with acryloyl
chloride in the presence of triethylamine in dry dichloromethane
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc06559j
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gauge, the flow rate, voltage, the distance between the needle and
the collector and the type of collector; and (2) physicochemical
properties of polymer solutions, such as molecular weight, solvent
21
volatility, solution concentration and polymer–solvent interactions.
Generally, electrospinning the same polymer in different solvents
21
could lead to substantially different morphologies of membranes.
In the present study, acetone–chloroform was selected as co-solvents
because of the weak interaction between polymers and co-solvents
as well as the low volatility of solvents which accelerates the drying
Scheme 1 Synthetic routes leading to monomer 3 and its corresponding process to prevent the formation of interconnected coarse fibers
polymers (P1–P4).
and webs. The electrohydrodynamic preparation of the POSS–MMA
copolymer using acetone–chloroform co-solvent can success-
fully produce membranes with uniform fibers.
2
1a
to afford monomer 3 with 95% yield. Finally, polymers P1-4 were
prepared from a mixture of monomer 3 and acryloisobutyl POSS (4)
with different ratios by radical polymerization using AIBN as a
radical initiator with 48–53% yields. Polymers P1-4 were purified by
re-precipitation twice in methanol to give white solids. They are
soluble in most organic solvents such as tetrahydrofuran (THF),
dichloromethane, chloroform and toluene. Polymers P1-4 were
structurally characterized by various spectroscopic methods and
elemental analysis. The ratio of TPE to POSS was determined by
The films F1–F8 were fabricated by electrospinning, drop-
coating and spin-coating from different polymers, respectively.
Concentration of the polymer in co-solvents was calculated in
weight percent. At the beginning, we fabricated films using a
polymer with a concentration of 2.0% in an acetone/chloroform
mixture (1/1, v/v). SEM images of polymer films F1-8 are shown in
Fig. 1 and Fig. S2 (ESI†). The SEM results showed the morphological
transition of polymers films from small crumpled particles with
fibers (F1 in Fig. 1), to particles (F2 in Fig. S2, ESI†), to large
crumpled particles with fibers (F3 in Fig. S2, ESI†), to fibers (F4 in
Fig. 1). Films were fabricated using polymer P4 at higher concentra-
tions of 5.0% and 10.0%, and SEM images indicate the formation of
particles with fibers under both conditions (F5 and F6 in Fig. S2,
ESI†). It is observed that film F4 exhibits a uniform porous fiber
structure with the diameters of approximately 300 nm. Furthermore,
films F7 and F8 were also fabricated by drop-coating and spin-
coating using the solution of 2.0% P4 in acetone/chloroform
1
1
H NMR. Using P2 as an example, the H NMR spectrum of P2 is
illustrated in Fig. S1 in ESI.† The presence of signals at d 7.03, 6.90
and 6.61 with an integration ratio of 15 : 2 : 2, which are assigned to
the aromatic protons of the tetraphenylethene moiety, shows that
the TPE moiety remained intact during the radical polymerization
and purification process. The signal at d 0.96 belongs to the protons
of methyl groups in POSS (4). The presence of signals at d 6.61
and 0.96 with an integration ratio of 1 : 14 suggests the ratio of
monomer 3 to 4 be 3 : 2. The molecular weights of those polymers
against polystyrene standards were determined by gel permeation
chromatography (GPC) and the data are summarized in Table 1.
The polymers show reasonable molecular weights of 18 800–32 200
Dalton with a relatively narrow polydispersity index of 1.36–1.68.
Those polymers exhibited high thermal stability with decomposi-
tion temperatures ranging from 346 to 361 1C under nitrogen.
Electrohydrodynamic preparation is a convenient method used
to produce polymer particles and fibers with diameters ranging
(1/1, v/v). In both cases, SEM images showed different morphologies
from those obtained by electrohydrodynamic preparation. Film F7
formed a porous structure with different sizes of holes ranging from
2
mm to 10 mm due to solvent evaporation. In contrast, film F8 shows
19,20
from a few nanometers to several hundred micrometers.
The
morphology of prepared substrates strongly depends on two main
conditions including: (1) processing parameters, such as needle
a
Table 1 Properties of polymers P1-4 and their corresponding films
b
Ratio
M
w
Â
M
M
w
/
Thickness
Film (nm)
À3
Polymer of 3 : 4 (10
)
n
T
d
T
g
T
m
P1
P2
P3
P4
P4
P4
P4
P4
3 : 0
3 : 2
1 : 3
1 : 6
32.2
1.68 356 49
1.37 346 23
—
—
128 F3
141 F4
F5
F1
F2
1400 Æ 120
660 Æ 60
650 Æ 70
560 Æ 60
740 Æ 80
1580 Æ 130
1300 Æ 200
590 Æ 30
21.6
19.8
18.8
1.36 347
1.36 349
—
—
F6
F7
F8
Fig. 1 SEM images of films: (a) F1, (b) F4, (c) F7 and (d) F8, scale bar is
10 mm. See processing parameters in Table S1 (ESI†). The inserted photo in
a
b
Not detected. Experiments were performed in duplicate and mean values
were taken. Films F1–F6 are fabricated by electrospinning, and films F7 and b is the fluorescence image of film F2 taken under a fluorescence
F8 were fabricated by drop coating and spin coating, respectively.
microscope with a 337 nm excitation; scale bar is 50 mm.
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only a cracked surface. Thus, the morphology of polymer films with
a uniform porous nanostructure can be easily achieved by judicious
selection of suitable comonomers and their concentrations.
All polymers P1-4 were non-emissive when dissolved in good
solvents, such as THF, dichloromethane, chloroform and toluene.
Taking P4 solution in THF as an example (Fig. S3 in ESI†), the
2
fluorescence became obvious when a large amount of H O (440%)
was added into THF solution, indicating that TPE maintained its AIE
activity after being incorporated into the polymer structure with
Fig. 3 (a) Fluorescence quenching of F4 upon exposure to DNT saturated
POSS residues. However, these particles were much less stable than vapor, the insets display the photos of F4 before and after exposure to DNT
17d
our reported AIE polymer nanoparticles. They tend to precipitate saturated vapor for 4 min under UV light (365 nm) illumination, at 25 1C.
(
b) Fluorescence quenching of films F1–F8 in the presence of DNT
out even when stored at 4 1C for one day, which is likely caused by
the hydrophobic nature of big POSS blocks in copolymers. Similar
observations were seen for other formulations. For porous film F1,
an obvious bathochromic/red shift of 11.5 nm was observed in the
UV-Vis spectrum (Fig. 2a) in the aggregated state compared to its
solution. This could be attributed to the enhanced planar conforma-
tion or aggregation of TPE units in the aggregated state. In contrast,
for porous film F4, almost no bathochromic/red shift except a
shoulder peak was found in the UV-vis spectra when compared to
its solution, indicating the weak stacking of TPE residues in porous
morphology structure, which was likely to be caused by the intro-
duction of POSS into the polymer. The porous film F4 displayed
strong emission in the range from 375 to 600 nm with an emission
maximum of 469 nm, and no shift was found compared to other
polymers P1–P3.
saturated vapor for 4.0 min, at 25 1C.
without POSS residues showed a low quenching efficiency of 23%.
Therefore, porous film F4 displayed higher sensitivity to DNT vapor
than other low porous films under the same conditions. Therefore,
the sensitivity of the electrospun porous film is much higher than
that of dense films. In conjunction with porous fibers created by
electrospinning, the presence of POSS moieties in the copolymers
enables the formation of porous structure, leading to an increase in
total surface area and subsequently facilitating the diffusion of the
gaseous analytes inside the polymer film and as a result improving
its sensitivity.
The fluorescence response of the porous film F4 to other nitro
compound vapors was also investigated. As shown in Fig. 4a, the
response of F4 to TNT vapor was slower than that of DNT, and its
fluorescence quenching reaches 15% in 60 s and 26% in 4.0 min,
respectively. Finally, F4 lost nearly 50% of fluorescent intensity
after 20 min of exposure to saturated TNT vapor. Porous film
F4 showed higher sensing ability to DNT vapor probably due
to its higher vapor concentration (ca. 100 ppb) relative to TNT
The BET test displayed in Fig. 2b revealed that P4 porous
material produced by electrospinning had a BET surface area of
2
À1
1
70 m g , which was almost 57 fold greater than that of P4 dense
2
À1
material produced by drop coating (SBET = 4 m g ). The significant
increase in the BET surface area indicates the highly porous
structure of film F4 compared to film F7, which is in fact consistent
with SEM observation. Typically, most porous materials with regular
porous morphologies will be open and accessible to analyte mole-
cules. In order to test the sensing ability, the copolymer films were
investigated by exposing films to saturated DNT vapors at room
temperature. As shown in Fig. 3a, fluorescence quenching was
immediately observed by exposing porous film F4 to saturated
DNT vapor, and the fluorescence quenching of F4 reached 52% in
(
4
ca. 5 ppb). F4’s fluorescence was only quenched by 12% in
.0 min for NT vapor, while almost no fluorescence response to
highly volatile toluene vapor, demonstrating its good selectivity
to explosive vapors. It is worth noting that the quenched
fluorescence could be recovered by exposure to hydrazine vapor,
and the recovered film still displayed sensing ability to explo-
sives. After 5 off–on cycles, the florescence quenching towards to
DNT vapor still remained around 78%, suggesting high stability
and fluorescence reversibility of film F4 (Fig. 4b).
30 s, 79% in 90 s and 90% in 4 min, respectively. In contrast, we also
studied the fluorescence responsiveness ability of other copolymer
films (Fig. 3b), and their quenching efficiencies upon exposure to
DNT vapor for 4.0 min were less than 22%, which is much lower
than 90% of film F4. Under the same conditions, the porous film F1
Fig. 4 (a) Time-dependent fluorescence quenching of porous film
F4 upon exposure to saturated vapor of TNT, DNT, NT and toluene,
respectively. The insets display the photos of F4 upon exposure to analyte
saturated vapor under UV light (365 nm) illumination, 25 1C. (b) The
fluorescence recovery of F4 upon exposure to saturated vapor of DNT
and hydrazine.
Fig. 2 (a) UV-Vis spectra of polymer P4 in THF and porous films F1 and F4,
together with fluorescence spectra of porous film F4. (b) N
2
absorption
isotherms at À196 1C for P4 before and after electrospinning.
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In summary, a new type of AIE active sensory material for 14 (a) J. Liu, Y. Zhong, P. Lu, Y. Hong, J. W. Y. Lam, M. Faisal, Y. Yu,
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Unlike fluorescent conjugated polymers, the porous AIE polymer
films showed less dependence of fluorescence response to nitro-
compounds on film thickness even though they have thickness as
high as 560 Æ 60 nm. This is due to the presence of POSS moieties,
facilitating the formation of porous structures and subsequently
leading to a large response to explosive vapors. Moreover, less
dependence of response to nitro-compounds on film thickness
avoids a tedious film fabrication method to control film thickness.
Finally, electrospun AIE active films displayed remarkable fluores-
cence quenching sensitivity to TNT and DNT vapors compared to
the corresponding dense films, making them promising for
potential applications in the detection of explosives.
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1
The authors would like to acknowledge the financial support
(
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