A highly sensitive fluorescent sensor based on small molecules doped in electrospun nanofibers: Detection of explosives as well as color modulation

Article abstract of DOI:10.1039/c5tc00819k

Fluorescent polymer nanofibers have wide applications in the fields of nano-photonics, nano-optoelectronics, chemical sensors and light-emitting diodes. The doping of small fluorophores into low-cost polymers is a proven alternative route to produce cost-

Full text of DOI:10.1039/c5tc00819k

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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC00819K
Journal of Materials Chemistry C
RSC Publishing
Paper
A Highly Sensitive Fluorescent Sensor Based on Small
Molecule Doped in Electrospun Nanofibers: Detection
of Explosives as well as Color Modulation
Cite this: DOI: 10.1039/x0xx00000x
Wei Xue,a Yang Zhang,a Juanjuan Duan,a Dong Liu,a Yawei Ma,a Naien Shi,*a
Shufen Chen,a Linghai Xie,*a Yan Qiana and Wei Huang*a,b
Received 00th January 2015,
Accepted 00th January 2015
DOI: 10.1039/x0xx00000x
Fluorescent polymer nanofibers have wide applications in the fields of nanoꢀphotonics, nanoꢀ
optoelectronics, chemical sensors and lightꢀemitting diodes. It is a distinguished substitute
route to dope small fluorophore into a type of lowꢀcost polymer in case of acquiring costꢀ
effective and highꢀperformance optical materials. In order to get a deep insight into the
photophysical process in the small molecule accommodating polymer nanofiber system, and
obtain a highly sensitive and costꢀeffective explosive fluorescent sensor, a new type of highly
sensitive lowꢀcost sensor towards nitroꢀcompounds based on PEO/MePyCz (polyethylene
oxide/ 4ꢀ(2ꢀ(2ꢀ(2ꢀmethoxyethoxy)ethoxy)ethoxy)ꢀ9ꢀ(pyrenꢀ1ꢀyl)ꢀ9Hꢀcarbazole) composite
nanofibers was synthesized. It exhibited fast response and high quenching efficiency towards
DNT vapor, which may be attributed to an improved exciton migration for MePyCz in PEO,
apart from the large driving force of the electron transfer and the nanofibrous structures.
Besides, it promoted a fluorescence resonance energy transfer process in the polymer fibrous
matrix with a greenꢀemitting material of 2ꢀ(thiophenꢀ2ꢀyl)ꢀfluorenꢀ9ꢀone. The sensing
composite nanofibrous film had good sensitivity and selectivity and might be constructed into
a portable detector for explosives.
www.rsc.org/
spectroscopy,21 and Xꢀray diffraction.22 It is a great challenge to
explore highly effective explosive vapor sensing materials since
Introduction
sensing in vapor of explosives is very important whereas the vapor
pressure is extremely low, and great efforts have been paid on
prominent fluorescent sensors. Conjugated polymers (pentriptycene,
diarylpoly(acetylene), poly(phenylꢀcoꢀfluorene) or spirofluorene),23,
24 organic nanorods (alkoxycarbonylꢀsubstituted, carbazoleꢀcornered,
aryleneꢀethynylenetetracycles),25 quantum dots26 (for example,
amineꢀcapped ZnSꢀMn2+ nanocrystals etc.) and microporous metal–
organic frameworks27ꢀ29 (benzene carboxlates, triphenylene
carboxylates, and etc.) are proven to be highꢀperformance
fluorescent sensing materials. However, the wide applications of the
reported materials are limited by costly, complicated and lowꢀyield
synthesis procedures, or harsh synthetic environment. There is still a
great need not only to fabricate highly sensitive and costꢀeffective
fluorescent sensors, but also to realize the largeꢀscale fabrication.
Considering the intrinsic delocalized piꢀelectrons and blue
emission of pyrene and good chargeꢀtransporting capability of
carbazole as well as the convenience and low cost of the two units, a
new kind of fluorescence sensing material of alkoxy substituted
carbazole pyrene was designed, synthesized and doped into a widely
applied lowꢀcost polymer of polyethylene oxide (PEO), so as to
obtain a type of highꢀperformance and costꢀeffective explosive
fluorescent sensor, which is also employed as a model system to
study the photophysical process in hybrid fluorescent polymer
nanofibers. The structure of the sensing material, 4ꢀ(2ꢀ(2ꢀ(2ꢀ
methoxyethoxy)ethoxy)ethoxy)ꢀ9ꢀ(pyrenꢀ1ꢀyl)ꢀ9Hꢀcarbazole
Fluorescent polymer nanofibers are of great interest because they
can serve as important elements in many fields, such as nanoꢀ
photonics, nanoꢀoptoelectronics, chemical and biological sensors,
organic semiconductors and lightꢀemitting diodes, due to their
fascinating properties of flexibility, charge transport ability and
hospitality.1ꢀ8 Electrospinning is an efficient high throughput method
to prepare lightꢀemitting conjugated polymer nanofibers, which is
based on stretching a polymer solution under electrostatic forces,
although there are also other approaches of supramolecular
assembly, covalent synthesis, physical drawing, nanolithography.9, 10
Generally, artificial synthetic conjugated fluorescent polymers are
generally of high cost with cumber preparation procedures. It is a
distinguished substitute route to dope small optoelectronic molecules
into a type of lowꢀcost polymer to act as the active components in
optoelectronic devices and sensors.11ꢀ15 In this case, it is of great
importance to investigate the photophysical process (exciton
migration, electron transfer, energy transfer, etc.) in the small
molecule accommodating polymer nanofibers.
As we know, the explosive detection is very important for the
applications in homeland safety, industrial process safety control,
and daily life. Fluorescent sensing is an effective and extensively
applied method to detect explosives, because of its short response
time, excellent sensitivity, simplicity, and costꢀeffectiveness, in
comparison with the currently widely used analytical laboratory
methods of chromatography,16ꢀ19 ion mobility spectrometry,20 Raman
(MePyCz), was shown in Scheme 1. At a doping content of 0.50
This journal is © The Royal Society of Chemistry 2015
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DOI: 10.1039/C5TC00819K
wt% for MePyCz into PEO, the hybrid fluorescent polymer 3.56 (m, 2H), 3.41 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm, Fig.
nanofibers exhibit a high quenching efficiency of 73% at 2h S2): δ 155.43, 143.95, 141.84, 131.44, 131.15, 130.99, 128.74,
exposure towards DNT, which is 2.6 fold as compared with that of 128.61, 128.25, 127.20, 126.78, 126.67, 126.49, 125.79, 125.73,
reported porphyrin TMOPP/ polystyrene solid nanofibers of ca. 125.51, 125.16, 124.61, 123.31, 122.74, 122.67, 120.13, 112.80,
28%.30 The exciton migration in the hybrid materials played a key 109.65, 103.40, 101.93, 72.00, 71.04, 70.86, 70.69, 70.07, 67.74,
role in the fast response to the analyte agents. Besides, the doped 59.07. MALDIꢀTOF MS, m/z (Fig. S3): Calcd. for C35H31NO4
MePyCz are found to perform efficient fluorescence energy transfer 529.23, Found 529.62. Anal. calcd for C35H31NO4: C, 79.37; H, 5.90;
with an energy acceptor of ThFO in the PEO polymer nanofibers.
N, 2.64. Found: C, 79.23; H, 5.87; N, 2.65.
Synthesis of 2-(thiophen-2-yl)-fluoren-9-one (ThFO, Scheme 2)
Experimental
O
O
Materials
thiophenꢀ2ꢀylboronic acid, Pd(PPh3)4
THF/toluene, K2CO3/KF, 48h, 95 o
S
Br
2ꢀ(2ꢀ(2ꢀmethoxyethoxy)ethoxy)ethanol, 4ꢀmethylbenzeneꢀ1ꢀsulfonyl
chloride (TsCl), 9Hꢀcarbazolꢀ4ꢀol, 9Hꢀcarbazole, 1ꢀbromopyrene,
cuprous iodide (CuI), 18ꢀcrownꢀ6, potassium carbonate (KCO3), 1,2ꢀ
dichlorobenzene, sodium hydroxide (NaOH), potassium fluoride
(KF), sodium chloride (NaCl), naphathalene, urea and magnesium
sulfate (MgSO4) were purchased from Shanghai Sinopharm
Chemical Reagent Co., Ltd. 2ꢀbromoꢀ9Hꢀfluorenꢀ9ꢀone was offered
by Puyang HuiCheng Chemical Co., Ltd. Thiophenꢀ2ꢀylboronic acid
(ThB(OH)2) and Pd(PPh3)4 were supplied by Sahn Chemical
Technology Co., Ltd. Polyethylene oxide (PEO) with Mw=200000
was purchased in the United States. 2,4ꢀdinitrotoluene (DNT) was
obtained from SigmaꢀAldrich Chemical Reagent. pꢀnitrotoluene
(pꢀNT) was purchased from Shanghai Sinopharm Chemical Reagent
Co., Ltd. Nitrobenzene (NB), nitromethane (NM) and benzophenone
(BP) were purchased from Shanghai Lingfeng Chemical Reagent
Co., Ltd. Picric acid (PA) was purchased from Guangdong Xilong
Chemical Co., Ltd. Tetrahydrofuran (THF), ethanol (EtOH),
petroleum ether (PE), 1,2ꢀdichloromethane (CH2Cl2), toluene, 1, 2ꢀ
dichlorobenzene and chloroform (CHCl3) were purchased from
Shanghai Lingfeng Chemical Reagent Co., Ltd. All reagents and
solvents were used as received without further purification.
C
ThFO
Scheme 2. Synthesis of 2ꢀ(thiophenꢀ2ꢀyl)ꢀfluorenꢀ9ꢀone (ThFO) by
Suzuki reaction.
Typically, 2ꢀbromoꢀ9Hꢀfluorenꢀ9ꢀone (4.0 g, 15.5 mmol), thiophenꢀ
2ꢀylboronic acid (5.0 g, 38.8 mmol), Pd(PPh3)4 (0.5 g, 0.05 mmol)
were combined with the solvent of THF/toluene (25 mL/25 mL) in
dark in N2 atmosphere and stirred. Then it was heated to 95°C and
refluxed. After stirring for 30 min, 30 mL K2CO3/KF (10.764
g/4.524 g) in THF/toluene (19.5 mL/19.5 mL) was added. It should
be noted that the aqueous solution of K2CO3/KF, THF/toluene mixed
solvent should be blowed by N2 for 2 h to eliminate oxygen. Next,
the reaction mixture was stirred for 48 h, cooled to room
temperature, quenched with saturated brine (50 mL) and extracted
with CH2Cl2. The organic phase was dried over MgSO4, filtered, and
concentrated under reduced pressure. The crude was purified by
column chromatography (PE/CH2Cl2, 4/1) to furnish ThFO as a
1
yellow solid(3.6 g, 89%). H NMR (400 MHZ, CDCl3, ppm, Fig.
S1): δ 7.91 (s, 1H), 7.74ꢀ7.67 (m, 2H), 7.52 (t, J=7.2 Hz, 3H), 7.39
(d, J =2.8 Hz, 1H), 7.31 (t, J=6.0 Hz, 2H), 7.11 (t, J=5.2 Hz, 1H).
13CNMR (100 MHz, CDCl3, ppm, Fig. S2): δ 193.59, 144.21,
143.12, 142.96, 135.40, 134.87, 134.31, 131.53, 128.99, 128.25,
125.44, 124.40, 123.71, 121.45, 121.43, 120.76, 120.35. GCꢀMS
(EIꢀm/z): 262 (M+). 13CNMR (100 MHz, CDCl3, ppm, Fig. S2): δ
193.59, 144.21, 143.12, 142.96, 135.40, 134.87, 134.31, 131.53,
128.99, 128.25, 125.44, 124.40, 123.71, 121.45, 121.43, 120.76,
120.35. GCꢀMS (EIꢀm/z, Fig. S3): Calcd. for C17H10OS (M+) 262.05,
Found 262.03. Anal. calcd for C17H10OS: C, 77.64; H, 3.84. Found:
C, 77.85; H, 3.86.
H
N
N
1ꢀbromopyrene, CuI, 18ꢀcrownꢀ6
O
O
K2CO3, 1,2ꢀdichlorobenzene
O
O
O
O
O
O
MePyCz
MeCz
Scheme
1.
The
synthesis
procedure
of
4ꢀ(2ꢀ(2ꢀ(2ꢀ
methoxyethoxy)ethoxy)ethoxy)ꢀ9ꢀ(pyrenꢀ1ꢀyl)ꢀ9Hꢀcarbazole
(MePyCz).
Preparation of the hybrid polymer film
Synthesis of 4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-9-(pyren-
1-yl)-9H-carbazole (MePyCz, Scheme 1)
For the PEO/MePyCz composite, 200 mg PEO was added into 2 mL
chloroform solution of MePyCz (1 mM) to get the PEO/MePyCz
mixed solution (MePyCz, 0.5 wt% PEO). For the MePyCz and
ThFO mixtures, different volume ratios of MePyCz/CHCl3 (1 mM)
to ThFO/CHCl3 (10 mM) solution (vMePyCz:vThFO=16:0, 16:1, 16:2,
16:4, 16:8, 0:16) were employed under a fixed amount of MePyCz
4ꢀ(2ꢀ(2ꢀ(2ꢀmethoxyethoxy)ethoxy)ethoxy)ꢀ9Hꢀcarbazole
(MeCz,
1.65 g, 5 mmol, Experimental section in ESI, Scheme S1), 1ꢀ
bromopyrene (1.69 g, 6 mmol), CuI (0.095 g, 0.5 mmol), 18ꢀcrownꢀ
6 (0.132 g, 0.5 mmol), and K2CO3 (0.76 g, 5.5 mmol) were
combined with 1,2ꢀdichlorobenzene (1.0 mL). Then the mixture was
heated to 190 °C, refluxed 24 h, cooled to room temperature, washed
with water, extracted with CH2Cl2. The organic layer was dried over
MgSO4, filtered, and concentrated in vacuum. The crude was
purified by silica gel chromatography (PE/CH2Cl2, 4/1)to furnish the
of
2 mL, and then 250 mg PEO was added to get the
PEO/MePyCz/ThFO composite.
For the electrospinning process, the asꢀprepared solution was
loaded into a standard 5 mL glass syringe. The open end of the
syringe was attached to a blunt stainless steel hypodermic needle
(OD=0.9 mm), which was used as the nozzle. A HighꢀVoltage DWꢀ
P403ꢀ1ACCC DC power supply was used to charge the solution by
attaching the positive electrode to the nozzle and the negative
grounding electrode to the aluminum collector. An electrical
potential of 19 kV was applied across a distance of 15 cm between
the needle tip and the collector. The electrospun solution was fed at a
constant rate of 1.0 mL/ h by a syringe pump. The electrospun fiber
1
pure product of MePyCz (2.5 g, 95%). H NMR (400 MHZ, CDCl3,
ppm, Fig. S1): δ 8.55 – 8.53 (m, 1H), 8.35 (d, J = 8.0 Hz, 1H), 8.28
(d, J = 7.6 Hz, 1H), 8.20 (s, 2H), 8.19 (d, J = 8.0 Hz, 1H), 8.10 (d, J
= 8.0 Hz, 1H), 8.06 (t, J = 7.6 Hz, 1H), 7.94 (d, J = 9.2 Hz, 1H), 7.54
(d, J = 9.2 Hz, 1H), 7.36 – 7.30 (m, 2H), 7.25 (t, J = 8.1 Hz, 1H),
7.00 (dd, J = 6.6, 2.1 Hz, 1H), 6.78 (d, J = 7.9 Hz, 1H), 6.64 (d, J =
8.0 Hz, 1H), 4.53 – 4.50 (m, 2H), 4.17– 4.15 (m, 2H), 3.92 (dd, J =
5.7, 3.8 Hz, 2H), 3.82 – 3.76 (m, 2H), 3.75 – 3.70 (m, 2H), 3.61 –
2 | J. Name., 2015, 00, 1-3
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ARTICLE
DOI: 10.1039/C5TC00819K
sensing films were collected on ITO glass slides mounted on
aluminum foil with a collection time of 5 min.
For PEO/MePyCz spinꢀcoating film, one drop of the above
PEO/MePyCz chloroform solution with MePyCz content of 0.5 wt%
was dipped onto an ITO glass slide and spinꢀcoated at 2,000 rpm for
30s at room temperature. Then it was dried in air for 12 h.
The fluorescence quenching experiments of the sensing films
towards several and other common interferents vapors are conducted
as follows: a small amount of explosives (15 mg for solid or 15 ꢁL
for liquid) was loaded in an aluminum container (5 mm×5 mm×3
mm), placed in a 1 cm×1 cm×3.5 cm quartz cell with a screw cover,
and closed overnight to reach a saturation vapor pressure. The
fluorescence data before and after exposure towards the analyte
vapor were collected at an excitation wavelength of 330 nm at room
temperature. Smoke was collected from the burning cigarette.
Fig. 2 Absorbance and fluorescence spectra of (a) MePyCz in
chloroform and solid film in dash and solid line, respectively
(λex=330 nm), and (b) PEO/MePyCz nanofibers (λex=330 nm).
carbazole groups32, 33. As for the PEO/MePyCz hybrid nanofibers,
the absorption peaks located at 278, 338 nm with a new shoulder at
380 nm indicated that the existence of aggregated molecules in the
composite fibers. The emission band of the fibers is at 440 nm which
is similar to that in the solution state. This may be attributed to much
weaker intermolecular interaction among the MePyCz molecules in
the fibers than that in solid film.
Characterization
1H NMR and 13C NMR spectra were recorded on a Varian Mercury
Plus 400M nuclear resonance spectrometer. GCꢀMS spectra were
collected on a Shimadzu GCMSꢀQP 2010 spectrometer. MALDI
mass spectrum was collected using a Microflex MALDIꢀTOF mass
spectrometer (Bruker Daltonics, Germany). Elemental analyses were
performed on a CHNꢀOꢀRapid analyzer (Heraeus, Germany).
FESEM images were taken on a Hitachi Sꢀ4800 field emission
scanning electron microscope. Fluorescence microscope (FLM)
images were taken on an Olympus IX71 (λex=365 nm) optical
microscope. UV–Vis absorption spectra were conducted at a
Shimadzu UV3600ꢀNIR recording spectrophotometer operated at
resolution of 2 nm. The fluorescence spectra were conducted on a
Shimadzu RFꢀ5301 fluorescence spectrometer. The fluorescence
lifetime measurements were carried out at room temperature on
Edinburgh FLSP920 fluorescence spectrophotometer.
Results and discussion
Fig. 3 (a) Timeꢀdependent fluorescence quenching process of
PEO/MePyCz fibrous film towards DNT in vapor (from i to x: 0, 1,
2, 3, 4, 5, 30, 60, 90, 120 min; λex=330 nm) (b) Time course curve of
PEO/MePyCz hybrid fibrous film in ambient air without DNT. (c)
Timeꢀdependent fluorescence quenching process of PEO/PyCz
fibrous film towards DNT in vapor (from i to x: 0, 1, 2, 3, 4, 5, 30,
60, 90, 120 min; λex=330 nm). (d) The quenching efficiency curves
versus exposure time with standard deviation error bars of three
batches prepared at different times.
Fig. 1 FESEM and FLM images of PEO/MePyCz hybrid nanofibers.
According to the FESEM image shown in Fig. 1a, the electrospun
composite nanofibers of PEO/MePyCz are relatively uniform and
smooth morphology with diameters ranging from 0.8 µm to 1.6 µm
(Fig. S5). Corresponding FLM image clearly shows that the blue
emitting sensing molecules of MePyCz are distributed uniformly in
the PEO matrix (Fig. 1b). As shown in Fig. 2, the absorption
spectrum of MePyCz in chloroform showed two individual
absorption bands at 260 nm and 334 nm, which result from the
higher excited state of pyrene or nonꢀconjugated carbazole groups31,
32. The emission spectra exhibited a distinct fluorescence band at 446
nm. In contrast, as for the MePyCz solid film, the absorption bands
slightly redꢀshifted to 267 nm and 339 nm, with a new band at ca.
380 nm in the longer wavelength, whereas the emission band shifted
to 463 nm, which suggests the aggregated form of pyrene and
Fig. 3a shows the fluorescence spectra of PEO/MePyCz
composite nanofibers towards DNT vapor. The timeꢀdependent
fluorescence quenching was measured over 2 h of exposure towards
the saturated DNT vapor in air. The fluorescence of the electrospun
fibrous film quenched 24% at the exposure time of 5 min (24%,
Quenching efficiency, and it equals (I0−I)/I0 × 100%, I0 and I are
luminescence intensity of the sensor before and after exposure to the
analyte), and the value reached as high as 73.2 % at 2h, which is 2.6
fold as compared with that of reported tetrakis(4ꢀmethoxylphenyl)
porphyrin (TMOPP) of 28%, and besides, it should be noted that the
doping content of MePyCz here is 0.5 wt%, much lower than that of
TMOPP of 10 wt% to the matrix (Fig. 3a).30 When the doping
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VJieowuArrtnicalelONnlainme e
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content of MePyCz increased to 5.0 wt%, the quenching efficiency
further raised to 89% (Fig. S6). As seen in Fig. 3b, the fluorescence
of PEO/MePyCz composite nanofibers doesn’t decay a little under
the excitation light in ambient air without DNT. This proves that the
fluorescence quenching arises from the interaction with DNT vapor.
This indicates that photoꢀinduced electron transfer (PIET) occurred
between the singlet excited state of sensor molecules and the
quencher of DNT molecules in the composite nanofibers, which is in
accord with their front molecular orbital energy values (Fig. S7)25.
Besides, the intrinsic porous structure of the electrospun nanofibers
plays an important role in the high quenching efficiency of the
PEO/MePyCz composite. As a comparison, the sensing performance
of PEO/MePyCz spinꢀcoating film with the MePyCz content 0.5
wt% was investigated and the quenching efficiency was much lower
(58%, Fig. S8), as compared to that of electrospun nanofibers (73%,
Fig. 3a). The nanofibrous film can accelerate the diffusion of DNT
25
to encounter with MePyCz and thus benefit the detection.6, In
order to get an insight into the molecular structure effect, 9ꢀ(Pyrenꢀ
1ꢀyl)ꢀ9Hꢀcarbazole (PyCz), another compound without the alkoxy
group, was synthesized from 1ꢀbromo pyrene and 9Hꢀcarbazole by
Ulmann Reaction (Experimental section in ESI, Fig. S4),34 and was
employed as the doped sensing matter with the same mass content
with MePyCz (PyCz, 0.5 wt% PEO). The quenching efficiency of
PEO/PyCz composite nanofibers towards DNT vapor in air at 5 min
and 2 h were 9% and 68%, which is obviously lower than that of
MePyCz of 24% and 73.2%, respectively (Fig. 3c). The LUMO and
HOMO energy level of PyCz is calculated to be ꢀ1.71 and ꢀ5.15 eV,
respectively, which lies in a higher position in comparison with that
of MePyCz with LUMO of ꢀ2.16 eV and HOMO of ꢀ5.56 eV (Fig.
S7). The electron transfer driving force from the sensing molecules
of PyCz to DNT (the difference between their LUMO levels, 1.70
eV) is larger than MePyCz (1.25 eV). Therefore, PEO/MePyCz gets
a definitely higher quenching efficiency with a smaller electron
driving force. Swager et al. previously reported that exciton energy
could migrate among the piꢀconjugated polymer interchains, and
Zang et al. reported there are bulky exciton diffusion along the piꢀ
conjugated molecule stacked nanowires, which enables efficient
fluorescence quenching by gaseous quenchers.25, 35 This discrepancy
between the electron driving force and quenching efficiency for
MePyCz and PyCz might be due to their difference in the exciton
migration. There are two kinds of exciton migration ways: one is the
exciton migration directly through adjacent chromophores (e.g.
MePyCz), the other is the exciton migration through the donorꢀ
bridgeꢀacceptor (e.g. MePyCzꢀPEOꢀMePyCz') route. The former one
have been widely exploited in the sensing system of piꢀconjugated
small molecule nanowires.25,36 The latter exciton energy migration in
PEO matrix may arise from an effective tunneling through energetic
bridge states barriers.37 The higher quenching efficiency for MePyCz
might arise from an improved exciton migration (faster speed or
longer length) for MePyCz along the above two ways than PyCz.
Next, the sensing selectivity of the PEO/MePyCz is investigated
towards nine substances, including pꢀNT, NB, DNT, NM, PA,
naphathalene, urea, EtOH, and smoke. As seen in fluorescence
quenching result in Fig. 4, the fluorescent composite nanofibers
exhibit obvious quenching towards the group of nitro compounds,
whereas almost no quenching was observed for the nonꢀnitro group
compounds of naphathalene, urea and EtOH. The quenching effect
towards the nitro compounds is mainly due to the photoinduced
electron transfer between the MePyCz molecules and the quenchers,
which is in accord with the lower LUMO energy level for DNT, p-
NT, PA, NB, NM (from DNT to NM: ꢀ3.409 eV, ꢀ3.10 eV, ꢀ4.321
eV, ꢀ2.915 eV, ꢀ2.486 eV) than that of MePyCz (ꢀ2.16 eV) (Fig. S7).
In addition, we can see that the quenching efficiencies result in
various degrees towards different explosives, which is not in accord
Fig.
4
Fluorescence quenching efficiencies of PEO/MePyCz
electrospun nanofibrous films towards the analytes.
with the electron driving forces between MePyCz and the analyte.
The quenching efficiencies of pꢀNT and NB are relatively much
higher than those of three ones, which are 91.98% for pꢀNT and
85.45% for NB. This can be attributed to the difference in vapor
pressure. The vapor pressure of p-NT was about 103 ꢀ fold and 108 ꢀ
fold that of DNT and PA (200 ppm, 180 ppb, 0.0077 ppb at 25°C for
pꢀNT, DNT and PA, respectively).38 In comparison with pꢀNT, the
relatively weak quenching observed for NB (300 ppm, 25°C) is
likely due to its lower partition into the film than that of pꢀNT.25, 39
Fig. 5 Absorbance and fluorescence spectra of (a) ThFO chloroform
solution (λex=440 nm) and (b) PEO/ThFO nanofibers.
Unlike the above photoꢀinduced electron transfer (PIET) that
processes in the detection of electronꢀdeficient nitroꢀcompounds,
fluorescence resonance energy transfer (FRET) is a common energy
transfer phenomenon that accompanies the exciton formation and
migration and takes place between the excited state of the donor and
ground state of the acceptor via intermolecular dipolar interactions.40
It is necessary to fundamental science and industrial applications to
tune the emitting color of a nanocomposite, especially based on a
nonꢀfluorescent lowꢀcost matrix material, and study the interactions
among the donor, acceptor and matrix. Herein, 2ꢀ(thiophenꢀ2ꢀyl)ꢀ
fluorenꢀ9ꢀone (ThFO) serves as the energy acceptor. Its absorption
spectrum showed a good overlap with the emission spectrum of
MePyCz in the 400ꢀ500 nm range for its chloroform solution as well
as composite polymer nanofibers with PEO (Fig. 2, 5 and Fig. S9).
Fig. 6 showed the fluorescence emission spectra of the composite
nanofibers doped with MePyCz and ThFO at different ratios. With
increasing the content of ThFO, the fluorescence intensity of the
band at 442 nm, the characteristic emission from MePyCz, gradually
decreased, and simultaneously, the 540 nm band for ThFO gradually
increased. To determine the mechanism of the FRET process,
fluorescence lifetime measurement of PEO/MePyCz/ThFO
electrospun composite nanofibers was done. The decay of the
MePyCz emission becomes faster with increasing the content of
4 | J. Name., 2015, 00, 1-3
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ARTICLE
DOI: 10.1039/C5TC00819K
ThFO (Fig. S10). This indicates that nonꢀradiative FRET
process between MePyCz and ThFO is the dominant
mechanism for energy transfer.41 Corresponding fluorescence
microscopy images clearly showed that the color of the
nanofibers changed from blue, cyan to green for the
PEO/MePyCz/ThFO composite nanofibers with different
component ratios (Fig. 1b and Fig. 7a,b,c). Here, by doping 0.5
wt% small fluorophore molecule of MePyCz at most, highly
fluorescent polymer nanofibers with high quantum yield of 0.29
were successfully obtained in a lowꢀcost, largeꢀscale and
convenient way, and the color can be tuned to cyan and green
by adding another simple small molecule of ThFO. To the best
of our knowledge, hitherto this is the first work to control the
color of polymer nanofibers by doping two small molecules
between which energy transfer took place. FRET based colorꢀ
tuning composite materials may found potential applications in
fluorescent sensors or multifunctional photoelectric devices,
such as molecular sensors, biological fluorescent labels and
multicolor lightꢀemitting diodes.42,43
Conclusions
Here, a new type of highly sensitive lowꢀcost sensor towards
nitroꢀcompounds based on PEO/MePyCz (polyethylene oxide/
4ꢀ(2ꢀ(2ꢀ(2ꢀmethoxyethoxy)ethoxy)ethoxy)ꢀ9ꢀ(pyrenꢀ1ꢀyl)ꢀ9Hꢀ
carbazole) composite nanofibers was reported. The quenching
effect arises from the photoinduced electron transfer from the
fluorophores to the analytes. The much faster response and high
quenching efficiency can be attributed to an improved exciton
migration for MePyCz in PEO, apart from the large driving
force of the electron transfer and the nanofibrous structures.
apart from the large driving force of the electron transfer.
Besides, it promoted an fluorescence resonance energy transfer
process in the polymer fibrous matrix including a greenꢀ
emitting material of 2ꢀ(thiophenꢀ2ꢀyl)ꢀfluorenꢀ9ꢀone. The
sensing composite nanofibrous film had good sensitivity and
selectivity and might be constructed into a portable detector for
explosives. The hybrid nanomaterials may also be applied in
nanoꢀoptoelectronics devices, lightꢀemitting diodes or
biological sensors.
Acknowledgements
This work was financially supported by National Natural
Science Foundation of China (21471082, 21101095, 21144004,
51173081), the National Basic Research Program of China
(2012CB933301, 2012CB723402), Program for Postgraduates
Research Innovation in University of Jiangsu Province
(CXZZ13_0466), Scientific Research Fund Program sponsored
by Nanjing University of Posts and Telecommunications
(NY214128), Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD, YX03001), the
Natural Science Foundation of Jiangsu Province (Grant No.
BM2012010), the Ministry of Education of China (No.
IRT1148), and Synergetic Innovation Center for Organic
Electronics and Information Displays. The authors thank Mr.
Changjin Ou, Mr. Zhengdong Liu, and Dr Lei Yang in Nanjing
University of Posts and Telecommunications, and Prof.
Shengbiao Li in Huazhong Normal University for their help in
the synthesis of the materials.
Fig.
6 Fluorescence spectra of PEO/MePyCz/ThFO hybrid
nanofibers at different volume ratio of MePyCz and ThFO
chloroform solution (vMePyCz:vThFO= 16:0, 16:1, 16:2, 16:4, 16:8, 0:16)
(λex=380 nm).
Notes and references
aKey Laboratory for Organic Electronics and Information Displays &
Institute of Advanced Materials (IAM), Jiangsu National Synergistic
Innovation Center for Advanced Materials (SICAM), Nanjing University
of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023,
China. Eꢀmail: iamneshi@njupt.edu.cn; iamlhxie@njupt.edu.cn
bKey Laboratory of Flexible Electronics (KLOFE)
& Institute of
Advanced Materials (IAM), National Jiangsu Synergistic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University
(NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. Eꢀmail:
weiꢀhuang@njtech.edu.cn
†Electronic Supplementary Information (ESI) available: Synthesis part of
MeCz and PyCz; 1H NMR (400 MHz, CDCl3, ppm), 13C NMR (100 MHz,
CDCl3, ppm), and MS of the compounds; The diameter distribution of the
PEO/MePyCz fibers; Timeꢀdependent fluorescence quenching process of
PEO/MePyCz fibrous film with the MePyCz content of 5.0 wt%; Energy
levels of HOMO and LUMO orbitals of MePyCz, PyCz and the target
explosives; Timeꢀdependent fluorescence quenching process of
PEO/MePyCz spinꢀcoating film with the MePyCz content of 0.5 wt%;
FESEM images of PEO/ThFO fibers and PEO/MePyCz/ThFO hybrid
fibers; Fluorescence decay profiles of PEO/MePyCz/ThFO hybrid
nanofibers. See DOI: 10.1039/b000000x/
Fig. 7 FLM images of (a) PEO/MePyCz/ThFO (vMePyCz:vThFO=16:1),
(b) PEO/MePyCz/ThFO (vMePyCz:vThFO= 4:1) and (c) PEO/ThFO
hybrid nanofibers.
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X. Lu, C. Wang and Y. Wei, Small, 2009, 5, 2349.
This journal is © The Royal Society of Chemistry 2015
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Page 6 of 6
VJieowuArrtnicalelONnlainme e
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DOI: 10.1039/C5TC00819K
2
3
P. Wang, Y. Wang and L. Tong, Light-Sci. Appl., 2013, 2, 1.
42
V. Vohra, U. Giovanella, R. Tubino, H. Murata and C. Botta, ACS
Nano, 2011, 5, 5572.
L. M. Tong, R. R. Gattass, J. B. Ashcom, S. L. He, J. Y. Lou, M. Y.
Shen, I. Maxwell and E. Mazur, Nature, 2003, 426, 816.
M. Law, D. J. Sirbuly, J. C. Johnson, J. Goldberger, R. J. Saykally and
P. D. Yang, Science, 004, 305, 1269.
A. Camposeo, L. Persano and D. Pisignano, Macromol. Mater. Eng.,
2013, 298, 487.
43 A. Camposeo, L. Persano and D. Pisignano, Macromol. Mater. Eng.,
2013, 298, 487.
4
5
6
Y. Long, H. Chen, Y. Yang, H. Wang, Y. Yang, N. Li, K. Li, J. Pei
and F. Liu, Macromolecules., 2009, 42, 6501.
7
8
9
10
R. Yan, D. Gargas and P. Yang, Nat. Photonics, 2009, 3, 569.
K. Peters, Smart. Mater. Struct., 2011, 20, 013002.
D. Li and Y. N. Xia, Adv. Mater., 2004, 16, 1151.
L. Persano, A. Camposeo, C. Tekmen and D. Pisignano, Macromol.
Mater. Eng., 2013, 298, 504.
11
12
13
A. Camposeo, F. Di Benedetto, R. Stabile, A. A. R. Neves, R.
Cingolani and D. Pisignano, Small, 2009, 5, 562.
A. Camposeo, F. Di Benedetto, R. Stabile, R. Cingolani and D.
Pisignano, Appl. Phys. Lett., 2007, 90.
F. Gu, H. Yu, P. Wang, Z. Yang and L. Tong, Acs Nano, 2010, 4,
5332.
14
15
16
Q. Song, L. Liu and L. Xu, J. Lightwave. Technol., 2009, 27, 4374.
F. Gu, L. Zhang, X. Yin and L. Tong, Nano Lett., 2008, 8, 2757.
E. Holmgren, H. Carlsson, P. Goede and C. Crescenzi, J. Chromatogr.
A, 2005, 1099, 127.
17
B. Paull, C. Roux, M. Dawson and P. Doble, J. Forensic. Sci., 2004,
49, 1181.
18
19
20
J. M. F. Douse, J. Chromatogr., 1982, 234, 415.
M. E. Sigman and C. Y. Ma, J. Forensic. Sci., 2001, 46, 6.
R. G. Ewing, D. A. Atkinson, G. A. Eiceman and G. J. Ewing,
Talanta, 2001, 54, 515.
21
22
J. M. Sylvia, J. A. Janni, J. D. Klein and K. M. Spencer, Anal. Chem.,
2000, 72, 5834.
R. D. Luggar, M. J. Farquharson, J. A. Horrocks and R. J. Lacey, X-
ray. Spectrom., 1998, 27, 87.
23
24
S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871.
S. W. Thomas, III, G. D. Joly and T. M. Swager, Chem. Rev., 2007,
107, 1339.
25
26
27
28
29
30
31
T. Naddo, Y. Che, W. Zhang, K. Balakrishnan, X. Yang, M. Yen, J.
Zhao, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2007, 129, 6978.
R. Tu, B. Liu, Z. Wang, D. Gao, F. Wang, Q. Fang and Z. Zhang,
Anal. Chem., 2008, 80, 3458.
R. Li, Y. P. Yuan, L. G. Qiu, W. Zhang and J. F. Zhu, Small, 2012, 8,
225.
C. Zhang, Y. Che, Z. Zhang, X. Yang and L. Zang, Chem. Commun.,
2011, 47, 2336.
A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong and J.
Li, Angew. Chem. Int. Edit., 2009, 48, 2334.
Y. Yang, H. Wang, K. Su, Y. Long, Z. Peng, N. Li and F. Liu, J.
Mater. Chem., 2011, 21, 11895, and fig. 5d therein.
Y. Wan, L. Yan, Z. Zhao, X. Ma, Q. Guo, M. Jia, P. Lu, G. Ramosꢀ
Ortiz, J. Luis Maldonado, M. Rodriguez and A. Xia, J. Phys. Chem. B,
2010, 114, 11737.
32
33
34
35
36
C. Zhang, Y. Che, X. Yang, B. R. Bunes and L. Zang, Chem.
Commun., 2010, 46, 5560.
X. J. Zhang, X. H. Zhang, W. S. Shi, X. M. Meng, C. Lee and S. Lee,
J. Phys. Chem. B, 2005, 109, 18777.
S. Jeong, S. H. Park, K. S. Kim, Y. Kwon, K. R. Ha, B. D. Choi and
Y. S. Han, J. Nanosci. Nanotechno., 2011, 11, 4351.
I. A. Levitsky, J. S. Kim and T. M. Swager, J. Am. Chem. Soc., 1999,
121, 1466.
Y. Che, D. E. Gross, H. Huang, D. Yang, X. Yang, E. Discekici, Z.
Xue, H. Zhao, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2012, 134,
4978.
37
38
S. Jang, T. C. Berkelbach and D. R. Reichman, New J. Phys., 2013, 15,
105020.
G. Tang, S. S. Y. Chen, P. E. Shaw, K. Hegedus, X. Wang, P. L. Burn
and P. Meredith, Polym. Chem., 2011, 2, 2360.
J. S. Yang and T. M. Swager, J. Am. Chem. Soc., 1998, 120, 11864.
N. Banerji, S. Cowan, M. Leclerc, E. Vauthey, A. J. Heeger, J. Am.
Chem. Soc., 2010, 132, 17459.
39
40
41
A. D. Peng, D. B. Xiao, Y. Ma, W. S. Yang and J. N. Yao, Adv.
Mater., 2005, 17, 2070.
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