Electrospinning Superassembled Mesoporous AIEgen-Organosilica Frameworks Featuring Diversified Forms and Superstability for Wearable and Washable Solid-State Fluorescence Smart Sensors

Article abstract of DOI:10.1021/acs.analchem.0c04226

Flexible optical sensors are widely studied and applied in many fields. However, developing highly stable and washable wearable sensors in optics is still facing significant challenges. Here, we demonstrate an AIEgen-organosilica framework (TPEPMO) hybrid

Full text of DOI:10.1021/acs.analchem.0c04226

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Article
Electrospinning Superassembled Mesoporous AIEgen−Organosilica
Frameworks Featuring Diversified Forms and Superstability for
Wearable and Washable Solid-State Fluorescence Smart Sensors
Meng Gao, Guanchen Xu, Runhao Zhang, Zhanjie Liu, Hongming Xia, Bing Shao, Changhu Xue,
Jichao Li, Shihai Miao, Wenlong Fu, Xingshuang Zhang, Junjie Zhou, Xiaoping Jiang, Kang Liang,
and Biao Kong*
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ABSTRACT: Flexible optical sensors are widely studied and
applied in many fields. However, developing highly stable and
washable wearable sensors in optics is still facing significant
challenges. Here, we demonstrate an AIEgen−organosilica frame-
work (TPEPMO) hybrid nanostructure-based flexible optical
sensor, which is prepared by a two-step co-condensation and
electrospinning superassembly process. Organosilica precursors
with aggregation-induced emission (AIE) features are covalently
linked into periodic mesoporous organosilica (PMO) frameworks
with high fluorescent efficiency due to the restriction of
intramolecular motion. The three-dimensional space of ordered
porous materials provides abundant reaction sites, allowing rapid and sensitive monitoring of analytes. TPEPMOs exhibit good
properties as acidic pH fluorescent sensors with a pKa of 4.3. A flexible film is obtained by dispersing TPEPMO nanospheres in a
poly(lactic-co-glycolic acid) (PLGA) and polyacrylonitrile (PAN) hybrid fibrous matrix (TPEPMO-CFs) using the electrospinning
superassembly technique and is successfully served as an efficient fluorescent probe for the naked eye detection of ammonia gas and
HCl vapor by emission changes. The fluorescence of TPEPMO-CFs can be reversed in the presence of volatile acidic/alkaline gas for
more than five cycles, exhibiting excellent recyclability. In addition, TPEPMO-CF sensors show excellent washability and long-term
photostability (fluorescence was maintained above 94% after washing 10 times). These stimuli-responsive AIEgen−organosilica
frameworks featuring diversified forms and superstability for wearable and washable solid-state fluorescence exhibit great potential
for smart gas sensors, wearable devices, and solid-state lighting applications.
the restriction of the intramolecular motion of AIE fluorogens
by the inorganic matrix.24−27 Periodic mesoporous organosilica
(PMO) is an interesting class of inorganic−organic hybrid
materials,28−31 which possesses not only the excellent
structural features of inorganic porous materials but also
excellent optical properties that originate from the functional
organic units embedded within the framework.32,33 Moreover,
the density and fixation of the optical bridging groups within
pore walls could be controlled by the molecular design of the
precursors and the preparation conditions of materials, which
creates a fascinating platform for hosting fluorogens.34,35
Nevertheless, most of the reported fluorescent PMO materials
INTRODUCTION
■
Flexible solid-state sensing materials have attracted enormous
attention recently due to their use in wide application fields,1−4
including wearable sensors,5,6 biochemical probes,7−11 light-
emitting diodes,12,13 and display devices.14 Fluorescent probes
have been extensively explored for their facile and noninvasive
operations using fluorescent molecules with desired optical
properties.15,16 However, traditional organic fluorophores often
suffer from aggregation-caused quenching (ACQ) in the solid
state, which leads to severe negative effects on the stability and
efficiency of the sensors.17 Promisingly, fluorogens demon-
strating aggregation-induced emission (AIE) characteristics
have drawn great attention, as they can overcome the
drawbacks of ACQ of traditional dyes in the solid state.18−20
Restriction of intramolecular rotation (RIR) upon aggregation
is proposed as the main cause of the AIE effect.21−23 Based on
this, many AIE fluorogens have been incorporated into
inorganic materials to construct inorganic−organic hybrid
materials, which exhibit excellent photoactive properties due to
Received: October 7, 2020
Accepted: December 17, 2020
Published: January 6, 2021
https://dx.doi.org/10.1021/acs.analchem.0c04226
© 2021 American Chemical Society
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Scheme 1. Illustration of the Construction of TPEPMOs and TPEPMO-CF Films for Sensing Applications
are mainly based on the ACQ bridging molecules, which
results in relatively low fluorescence efficiency.36 Thus, the
design of PMO materials with highly efficient emission
appropriate for solid-state optical applications is required.37−39
Fluorescent pH indicators are indispensable tools for
measuring changes in the immediate environment and
intracellular proton concentrations.40,41 In addition, volatile
acidic and alkaline gases, such as ammonia gas and hydro-
chloric acid gas, are common gases generated in the process of
industrial production, causing great harm to the human body
due to their toxic, hazardous, and corrosive nature.42
Therefore, the detection of acidic and alkali gases is equally
important along with the measurements of the pH value in the
solution. Although a variety of responsive sensors for pH levels
in the solution or acid/alkali gases have been developed
recently,43 solid sensors with effective fluorescence emission
that could detect solution pH values and acid/alkali gases
simultaneously are rarely reported. For practical applications,
solid-state sensors of acid−base monitoring are more
worthwhile that can provide wearability, washability, opera-
tional simplicity, and good stability.44
assembly. The TPEPMOs were made from TPE-Si4 bridged
organosilane precursors using a covalent linking strategy, which
could avoid the typical fluorescence quenching suffered by
conventional dyes (Scheme 1). The reactivity of TPE-Si4 with
OH−/H+ enables TPEPMOs to sense pH variations in the
acidic range, meeting the desire of “acidic” probes. A linear
relationship of its fluorescence intensity with pH is established
in the pH range of 3.9−4.7 with a pKa of 4.3, making it
promising as a fluorescent indicator for pH sensing.45 Flexible
solid-state fluorescent sensors, by immobilization of a
fluorescent functional group on solid substrates like fibrous
membranes, can provide more advantages over fluorescent
probes used only in solution, such as reusability and
tailorability. Here, TPEPMOs are embedded into poly(lactic-
co-glycolic acid) nanofibers (TPEPMO-CF1) and polyacrylo-
nitrile nanofibers (TPEPMO-CF2), separately, prepared using
two different flexible electrospun fibers. Using the as-fabricated
TPEPMO-CF1 as a free-standing solid sensor, the fluorescence
of TPEPMO-CF1 increased immediately upon exposure to
ammonia vapor. The TPEPMO-CF1 sensor shows a response
with the concentration of ammonia as low as 30 mg/m3, which
is the highest permissible concentration in air. Meanwhile, the
TPEPMO-CF1 fluorescent sensor is capable of in situ
responding to the amine vapors emitted from a shrimp sample.
These results demonstrate that TPEPMO-CF1 can be used as
In this work, we report a flexible, wearable, and washable
optical acid/alkali gas sensor fabricated by incorporating pH-
sensitive AIE performance PMOs (TPEPMOs) in polymer-
based optical fibers (TPEPMO-CFs) via spinning super-
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an excellent solid-state sensor for the detection of volatile
alkaline/acid gases for environmental and food spoilage
monitoring. Furthermore, the TPEPMO-CF2 sensor shows
excellent recyclability, optical stability, and washability.
TPEPMO-CF2 can be reversed in the presence of volatile
acidic/alkaline gas more than five cycles and it maintains up to
94% fluorescence performance even after washing 10 times.
We construct sensing materials step by step using AIE
molecules from nanoparticles (zero-dimensional (0D)), fibers
(one-dimensional (1D)) to films (two-dimensional (2D)).
This design strategy provides new perspectives for developing
high-performance fluorescence materials by combinations of
AIE characteristics and stimuli-responsive properties in
chemosensors, bioprobes, and solid-state lighting.
mixture was stirred at 80 °C for 1 h and then left to cool to
room temperature with stirring. The precipitate was collected
by centrifugation before being washed with water and ethanol
several times. To remove CTAB, the as-synthesized samples
were extracted with a methanol solution for 48 h.
Fabrication of Composite Electrospun Nanofibers.
PLGA scaffold nanocomposites were fabricated by the
electrospinning technique. PLGA (0.85 g) was dissolved in
HFIP (5 mL) under magnetic stirring for 12 h. Then,
TPEPMOs (0.25 g) were added into the mixture and
ultrasonicated for 30 min to obtain a homogeneous precursor
sol for further electrospinning. The distance between the
spinneret (metallic needle) and the collector (grounded
conductor) was fixed at 13 cm, and a high-voltage supply
was maintained at 14 kV. The spinning rate was controlled at
0.5 mL/h.46 The resulting samples are denoted as TPEPMO-
CF1.
PAN fiber membranes were fabricated through a facile
electrospinning method. PAN (15 wt %, Mw = 150 000) was
dissolved in 5 mL of N,N-dimethylformamide (DMF) under
stirring at 70 °C for 6 h. Then, TPEPMOs (0.25 g) were added
into the mixture, followed by ultrasonic treatment for 1 h. The
PAN solution was electrospun onto the rotating collector with
a spinning rate of 1.0 mL/h. The obtained PAN fibers were
dried at 60 °C for 4 h.47 The resulting samples are denoted as
TPEPMO-CF2. The synthetic method of TPE-CF2 was the
same as that of TPEPMO-CF2, except that TPEPMOs were
replaced by TPE-Si4 (25 mg).
Immobilization of TPEPMO-CF2 Membranes onto
Various Substrates. TPEPMO-CF2 membranes were
immobilized onto various substrates using a polydopamine
(PDA)-mediated surface coating method.48 Dopamine hydro-
chloride (100 mg) was dissolved in 1 mL of PVA aqueous
solutions (10 wt %) under stirring. After polydopamine
formation, coating these solutions onto the surfaces of spinning
membranes and film sensors can be immobilized onto a variety
of substrates, including gloves, papers, leaves, masks, and
clothes.
Response Measurements of Nanofibers to Vapors.
For the response measurements of electrospinning films to
vapors, the films were fixed in the sample cell and put into a
closed container filled with ammonia and hydrochloric acid
vapors. The fluorescence changes in the films were recorded by
fluorescence spectroscopy, keeping the test angle of the sample
cell consistent before and after exposure to the gas. The time-
dependent fluorescence of films was tested by sticking the films
to the cover of the quartz cell. Hydrochloric acid and ammonia
were added into the cell. After the vapors were saturated, the
film samples were put back into the cell. It was made sure that
the film did not touch any liquid and that the same point was
tested before and after exposure to vapors. The fluorescence
intensity was recorded at different times.42 For selective
experiments, films were exposed to different vapors generated
from their corresponding solvents for 10 min. For the
fluorescence change tests toward different concentrations of
vapors, ammonia and hydrochloric acid vapors are produced
using a water bath at 333 K. The films were fixed in the sample
cell. Then, a certain volume of the vapors was extracted using a
syringe and immediately injected into the sealed bottle (500
mL) containing the sensing films (in the sample cell). After 10
min, the films with the sample cell were taken out for
fluorescence measurement.
EXPERIMENTAL SECTION
■
Chemicals and Reagents. 4,4′-Diaminobenzophenone,
aniline, 3-(triethoxysilyl) propyl isocyanate, N,N′-dimethylfor-
mamide (DMF), granulated tin, and triethylamine were
purchased from Shanghai Jingchun Reagent Company.
Tetrahydrofuran (THF), dichloromethane, chloroform, meth-
anol, and ethanol were obtained from Tianjin Fuyu Reagent
Company. Tetraethoxy orthosilane (TEOS), polyacrylonitrile
(PAN) (Mw = 150 000), methanal, poly(vinyl alcohol) (PVA),
hexafluoro-2-propanol (HFIP), and aqueous ammonia were
bought from Shanghai Macklin Biochemical Company.
Hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid,
and acetone were obtained from Yantai Far East Fine Chemical
Company. Poly(lactic-co-glycolic acid) (PLGA) (Mw
=
88 000−117 000, 50:50) was purchased from Jinan Daigang
Biomaterial Company.
Synthesis. Preparation of TPE-A4. 4,4′,4″,4‴-(Ethene-
1,1,2,2-tetrayl)tetraaniline (TPE-A4) was synthesized accord-
ing to the literature procedures. 4,4′-Diaminobenzophenone
(0.85 g, 4.0 mmol) was dissolved in concentrated hydrochloric
acid (35 mL) and heated under reflux for 24 h with granulated
tin (3.3 g, 28.0 mmol). After cooling, the mixture was vacuum-
filtrated and washed with water. After drying overnight at room
temperature, a grayish product was obtained (yield 62%).
1
Spectral data for H NMR (400 MHz, DMSO-d6, δ) were as
follows: 7.10 (2H, ArH), 7.07 (2H, ArH), and 3.85 (2H, NH2)
(Figure S1). 13C NMR (400 MHz, DMSO-d6, δ): 121.49,
132.39, 137.10, 132.79, 139.59, and 140.70 (Figure S2).
Preparation of TPE-Si4. TPE-A4 (0.39 g, 1.0 mmol) and 3-
isocyanatopropyltriethoxysilane (1.28 g, 5.2 mmol) were
dissolved in 50 mL of anhydrous THF. Then, triethylamine
(0.6 g, 6.0 mmol) was added followed by 24 h reflux under a
nitrogen atmosphere. After cooling to room temperature, the
solvent was concentrated under reduced pressure and the solid
was washed by n-hexane to afford the TPE-Si4 product (yield,
1
68%). Spectral data for H NMR (400 MHz, DMSO-d6, δ)
were as follows: 8.54 (4H, PhNHCO), 7.11 (8H, ArH), 6.75
(8H, ArH), 6.29 (4H, NHCH2), 3.73 (24H, ethoxy CH2), 2.99
(8H, CH2NH), 1.45 (8H, CH2CH2Si), 1.14 (36H, ethoxy
CH3), and 0.56 (8H, Si−CH2) (Figure S3). 13C NMR (400
MHz, DMSO-d6, δ): 7.73, 18.67, 23.80, 46.27, 58.18, 117.22,
131.70, 137.11, 138.54, 138.98, and 155.55 (Figure S4).
Synthesis of TPE-Bridged Periodic Mesoporous Organo-
silicas (TPEPMOs). A mixture of cetyltrimethylammonium
bromide (CTAB) (200 mg), NaOH (700 μL, 2 M), and
distilled water (100 mL) was stirred at 80 °C for 30 min in a
250 mL round bottom flask. Then, TEOS (1.0 mL) was added
along with TPE-Si4 (0.1 g in 1 mL ethanol). The resulting
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Figure 1. (a) Fluorescence emission spectra of TPE-Si4 in THF−water mixtures with different water fractions. (b) Plot of relative fluorescence
intensities and the corresponding photographs under a UV lamp of TPE-Si4 in THF−water mixtures. Excitation wavelength: 365 nm and
concentration: 0.2 mM. (c) UV−vis absorption spectra of TPE-Si4 and TPEPMO in THF. (d) Fluorescence emission spectra of TPE-Si4 and
TPEPMO in THF. Excitation wavelength: 360 nm.
1
Characterization. H NMR spectra were acquired on a
Bruker AVANCE III 400 MHz nuclear magnetic resonance
(NMR) spectrometer. Fourier transform infrared (FT-IR)
spectra were recorded on a Nicolet 710 FT-IR spectrometer
(Thermo Nicolet Corporation) with samples prepared as KBr
compressing tablets. Small-angle X-ray scattering (SAXS) was
carried out using SAXSess mc2 by Anton Paar. Brunauer−
Emmet−Teller (BET) surface areas were tested using an SSA
6000 instrument. The powder PMO sample was pretreatment
at 150 °C for 5 h in vacuum. Scanning electron microscopy
(SEM) images were recorded on a SUPRATM55 scanning
electron microscope with an accelerating voltage of 5 kV.
Transmission electron microscopy (TEM) observations were
obtained from an FEI Talos F200S electron microscope. Prior
to the TEM measurement, the sample was first dispersed in
ethanol and then deposited on a copper grid. Ultraviolet−
visible (UV−vis) spectra were measured using a PerkinElmer
Lambda 750 UV/vis/near-infrared (NIR) spectrometer.
Fluorescence spectra were obtained using an FS5 fluorescence
spectrofluorometer. Using quinine sulfate in 0.1 M H2SO4 as a
standard, fluorescence quantum yields were calculated
according to the formula Φu = Φs(Fu/Fs)(As/Au), where
Φ, F, and A are the fluorescence quantum yield, integral
fluorescence intensity, and absorbance at a fixed wavelength,
respectively.
pounds were confirmed using nuclear magnetic resonance
(NMR) and the spectra are given in the Supporting
Information. The observed AIE properties of TPE-Si4 were
supported by the fluorescence studies of TPE-Si4 in H2O/THF
mixtures with different contents of water (Figure 1a,b). The
pure THF solution of TPE-Si4 is weakly emissive, while the
fluorescence intensity increases dramatically when the volume
fraction of water is higher than 70%. Significantly fluorescence
enhancements were observed with a further increase in the
water fraction, and TPE-Si4 displays its maximal emission
intensities with a water fraction of 90%.
The TPE molecules were introduced into PMOs through
covalent interaction by a one-pot condensation process using
CTAB as the template. After removing the surfactant,
TPEPMOs functionalized with AIE luminogen was obtained.
The UV−vis spectrum of TPE-Si4 in solution exhibits two
absorption bands at 280 and 350 nm (Figure 1c),
corresponding to CC and aromatic and the π−π* transition
of the TPE-Si4 molecule structure, respectively.49 A slight red
shift occurs for the π−π* transition after TPE-Si4 was
incorporated into the PMO materials, which could be due to
a higher conjugation or coplanarity degree of the molecules. In
addition, the broadened band could be attributed to the light
scattering against the TPEPMO particles.
The fluorescence spectrum is applied to understand the
origin of the fluorescence behaviors of TPE groups in the
PMOs. TPEPMOs exhibited strong fluorescence centered at
approximately 495 nm (Figure 1d), which has a 10 nm red
shift compared with that of TPE-Si4 in solution. The possible
interpretation for this phenomenon is that the immobilization
effect of the framework induces the formation of a more planar
RESULTS AND DISCUSSION
■
Optical Properties. The TPE-bridged precursor (TPE-Si4)
with four polymerizable silyl groups was synthesized by the
reaction of 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetraaniline with
3-isocyanatopropyltriethoxysilane (Figure S5). The com-
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Figure 2. (a) SAXS pattern of TPEPMOs and the inset showing the enlarged region. (b) N2 adsorption−desorption isotherms of TPEPMOs and
its corresponding pore-size distributions (inset). (c) TEM and (d) SEM images of TPEPMOs.
and rigid structure.50 When TPE-Si4 bonded with the rigid
skeleton of the mesostructure, a highly emissive signal could be
achieved by the restriction of the intramolecular rotation
(RIR) mechanism.51 To further contrast the fluorescence
emission efficiency of TPE-Si4 and TPEPMOs, their relative
fluorescence quantum yields were measured using quinine
sulfate in 0.1 M H2SO4 as a standard (Table S1). Specifically,
the fluorescence quantum yield of TPE-Si4 in solution is 0.30,
whereas the fluorescence quantum yields increased to 0.54 for
TPEPMOs. As the TPE units are incorporated into the silica
framework of the mesoporous materials, the internal rotation is
restricted by the silica framework walls of the hybrid materials,
significantly enhancing the fluorescence emission.
Structural Properties. Fabrication of luminescent AIE
PMOs was achieved by coassembly and covalent linking of
TEOS and TPE-Si4. Evidence for the incorporation of TPE
groups into the frameworks of PMOs is further confirmed by
FT-IR studies. The FT-IR spectrum of TPE-Si4 shows the
characteristic CO stretching vibration of the imide group at
1695 cm−1 and the CC stretching vibration of aromatic rings
at 1530 cm−1 (Figure S6). The characteristic bands of TPE-Si4
moieties remain in the spectrum of TPEPMOs, and the bands
at 1350−870 cm−1 corresponding to the stretching vibrations
of the Si−OH and Si−O−Si frameworks are observed, which
suggested that the TPE groups are incorporated into the pore
walls of PMO materials.
The curve belonged to the type IV isotherm with no hysteresis
loops, indicating the existence of the mesostructure and
uniform cylindrical pores in the hybrid materials. The pore-size
distribution of TPEPMOs calculated with the BJH method is
centered at 1.90 nm, and the BET surface area and the total
pore volume are calculated to be 617 m2/g and 0.97 cm3/g,
respectively (Table 1).
a
Table 1. Structural and Textural Data for TPEPMOs
d100
[nm]
a0
[nm]
DBJH
[nm]
SBET
Vt
b
sample
[m2/g]
[cm3/g] [nm]
TPEPMOs
4.42
5.10
1.90
617
0.97 3.20
a
d100 = interplanar distance between two (100) planes; a0 = lattice
parameter; DBJH = pore diameter, calculated from adsorption branches
by the Barrett−Joyner−Halenda (BJH) method; SBET = BET surface
area; Vt = total pore volume; wall thickness b = a0 − DBJH
.
The formation of mesoscale porous structures is further
confirmed by TEM images of the extracted TPEPMOs (Figure
2c), which show a highly ordered 2D hexagonal phase along
the (110) lattice direction. The observed pore diameter is ca.
1.9 nm, and the thickness of the pore walls is about 2.8 nm,
which are in good agreement with the nitrogen sorption
results. A representative nanosphere morphology can be seen
from the SEM observation, and the average particle size is
about 87 nm (Figure 2d).
Sensing Properties. Herein, the pH-responsive TPEP-
MOs are developed, which exhibit AIE features, overcoming
the limitations exposed by the concentration quenching
problem encountered by most conventional dyes. The
reactivity of TPE-Si4 with OH−/H+ enables TPEPMOs to
sense pH in a meta-acid range. With reduction in pH values
from 7 to 1, the TPEPMO emission peak intensity decreases
(Figure 3a). However, the fluorescent intensity changed
The small-angle X-ray scattering (SAXS) pattern of
TPEPMOs (Figure 2a) shows an intense diffraction peak in
the low-angle region, indicating the formation of the ordered
mesostructure. In addition, three peaks are shown at q = 1.42,
2.60, and 2.90 nm−1, which could be indexed as (100), (110),
and (200), belonging to a 2D hexagonal symmetry. The lattice
parameter a0 of the 2D hexagonal system is around 5.10 nm,
which is calculated using a0 = 2d100/√3. Nitrogen adsorption/
desorption isotherms of TPEPMOs are shown in Figure 2b.
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Figure 3. (a) Fluorescence spectra of TPEPMOs at pH values between 1 and 7. The inset is the photograph of the TPEPMO aqueous dispersion at
different pH values in UV light. (b) Fluorescence spectra of TPEPMOs at pH values between 4 and 5. (c) Sensing mechanism of TPEPMOs.
Figure 4. (a) SEM image of the flexible film of TPEPMO-CF1 hybrid nanofibers. (b) Fluorescence spectra and photograph of the TPEPMO-CF1
film after exposure to ammonia vapor (ca. 1.5 v %) for different times (excitation wavelength: 365 nm). (c) SEM image of the flexible film of
TPEPMO-CF2 hybrid nanofibers. (d) Fluorescent recovery cycles of the TPEPMO-CF2 film. Points HCl and NH3 are corresponding to MSNF
films after exposure to HCl and NH3 vapor alternatively.
remarkably in the pH range of 4−5. As shown in Figure 3b, an
aqueous dispersion of TPEPMOs exhibits greatly intensified
fluorescence emission once pH values increase from 4 to 5.
The plots of peak fluorescence emission intensities against pH
values are shown in Figure S7, and the sigmoidal fit to the data
indicates a pKa of about 4.3 for the TPEPMO probe. The
drastic change in the fluorescent intensity of the TPEPMO
material is attributed to the protonation of the carbonyl of
ureido units in TPE-Si4 molecules (Figure 3c). Fluorescence
quenching of TPE induced by protonation has been already
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discovered in previous reports.42 In addition, the pKa values
obtained here are consistent with the result in our previous
studies for the similar urea groups.52 A linear relationship of its
fluorescence intensity with pH is established in the pH range of
3.9−4.7 (Figure S8), which makes it promising as a fluorescent
indicator for pH sensing and also suggests that TPEPMO
probes could find potential applications as acidic cellular
compartments and pH meters.
However, it showed no remarkable fluorescence changes
when TPEPMO-CF1 was kept at room temperature without
the shrimps (Figure S13). This result highlights that the
flexible fiber membrane is an appropriate sensing material for
practical applications in monitoring food spoilage. The
reproducibility and recycling are the outstanding advantages
of solid sensors. The enhanced fluorescence of TPEPMO-CF
by NH3 could be quenched with the treatment of hydrochloric
acid vapor (Figure S14). These results not only imply the
excellent reproducibility of TPEPMO-CF1 but also indicate
that the film can be used as an environmentally friendly
detector for both ammonia and hydrochloric acid vapor.
For the practical application of wearable sensors, many
polymer membrane-based sensor substrates suffer from low
corrosion resistance, weak washability, and poor photostability.
Polyacrylonitrile (PAN) is widely used as a support for the
assembly of composite membranes due to its good lightfast-
ness, good solvent resistance, and acid−alkali resistance.
Therefore, the wearable sensor film TPEPMO-CF2 is
fabricated by the incorporation of TPEPMOs in PAN-based
fibers. Figure 4c shows a typical SEM image of the as-prepared
TPEPMO-CF2. The diameter of the individual TPEPMO-CF2
fiber is about 250 nm. Some TPEPMOs are observed on
nanofiber surfaces (Figure 4c, enlarged section). Besides, the
BET surface area and total pore volume TPEPMO-CF2 are
estimated to be 18.2 m2/g and 0.085 cc/g, respectively (Figure
S9b). The flexible TPEPMO-CF2 membrane exhibited a
strong fluorescence due to the neutral precursor sol of
electrospun, which is different from TPEPMO-CF1. As
illustrated in Figure 4d, the TPEPMO-CF2 film could quench
its fluorescence after exposure to hydrochloric acid vapor (ca.
0.025 v %), and the respective fluorescent spectra could
coincide nicely during the treatment of hydrochloric acid. The
time-dependent fluorescence response studies of TPEPMO-
CF2 show that the fluorescence changes can be detected
within 15 s, and 50% of fluorescence intensity is quenched in
90 s (Figure S15). The detection limit for hydrochloric acid
vapor is estimated to be 32 mg/m3 (Figure S16). We have
further examined the recovery of the TPEPMO-CF2 film for
the successive detection of NH3 gas. Upon exposing the
TPEPMO-CF2 film to NH3 gas, the fluorescence intensity of
the film increases immediately. After NH3 exposure, the
TPEPMO-CF2 film exhibits stronger fluorescence than the
original film because of the higher alkalinity of the film after
ammonia treatment. After five cycles of quenching and
recovery, the fluorescence property of the TPEPMO-CF2
was well maintained (Figure 4d), indicating that the film
exhibits good acid−alkali resistance and reversibility, which
could be used as a detector for harmful gases.
Flexible Sensor. Electrospinning is a facile and convenient
technique to produce a flexible fibrous membrane in which
functional nanoparticles can be embedded. First, we used the
electrospinning technique to embed TPEPMOs into PLGA
nanofibers, interweaving a flexible, free-standing, and tailorable
composite fibrous membrane TPEPMO-CF1, which could be
used as a solid sensor to detect alkaline/acidic gases. The
morphology of the TPEPMO-CF1 nanofibrous membrane was
evaluated using SEM (Figure 4a). The fibers of the membrane
are randomly interconnected with diameters of about 320 nm.
It is clear that TPEPMOs have been distributed almost
uniformly in the electrospun nanofibers and without obvious
agglomeration. Meanwhile, some of the spherical particles are
observable on the nanofiber surfaces.53 In addition, the BET
surface area and the total pore volume of TPEPMO-CF1 are
calculated to be 20.3 m2/g and 0.092 cc/g, respectively (Figure
S9a). After assembly of the TPEPMOs into the PLAG
nanofibers, the flexible membrane exhibited a weak fluo-
rescence under UV light because the precursor sol of
electrospun nanofibers is acidic. A reversible transition from
the weakly emissive to highly emissive was achieved by
exposure of the TPEPMO-CF1 membrane to ammonia vapor.
Upon exposing the TPEPMO-CF1 membrane to ammonia
vapor, the fluorescence intensity of the membrane increases
immediately; meanwhile, the corresponding fluorescence
emission is brighter. The fluorescence enhancement ratio of
the TPEPMO-CF1 membrane is measured with the increased
exposure time to saturated ammonia vapor (Figure S10).
Increased fluorescence changes can be quickly detected and a
more than 2-fold higher light-up process is observed within 2
min. In addition, Figure 4b shows the fluorescence spectra of
the TPEPMO-CF1 membrane upon exposure to ammonia
vapor (ca. 1.5 v %) for different times. The as-synthesized
TPEPMO-CF1 membrane was almost nonemissive under UV
irradiation, but its fluorescence intensity gradually increased
with time after exposure to ammonia vapor. The fluorescence
in 10 min is 4-fold larger than that at the beginning. When
TPEPMO-CF1 was exposed to a low concentration of
ammonia vapor (30 mg/m3, the maximum allowed concen-
tration at a working place), the fluorescence intensity increases
nearly 3-fold after 6 h, which could be directly observed by the
naked eye under a UV lamp (Figure S11). Furthermore, the
detection limit (3σ/S) for ammonia vapor is estimated to be
15 mg/m3 based on the signal-to-noise ratio method (Figure
S12).
The detection ability of the TPEPMO-CF2 sensor toward
various acids is then investigated (Figure S17a). The obvious
fluorescence quenching for hydrochloric acid and nitric acid
indicates that these volatile strong-acid vapors interact more
efficiently with the sensor, whereas the TPEPMO-CF2 film
reveals a low response to some weak volatile acids (H3PO4 and
H2SO4). In addition, since ammonia is a volatile gas, different
organic amines and volatile organic compounds are selected for
the selective test of fluorescence-enhanced sensing (Figure
S17b). After the TPEPMO-CF2 sensor is quenched by acid,
ammonia vapor shows the most effective fluorescence
enhancement sensing effect. Triethylamine also reveals a
certain light-up effect. However, for aniline and various organic
solvent vapors, almost no obvious fluorescence response is
The safety and quality control of food attracted increasing
attention due to health and economic concerns. In particular,
the sensitive detection of the released ammonia and biogenic
amines is crucial to the assessment of the safety and quality of
seafood and meat products during storage and transportation.
Accordingly, we investigated whether the TPEPMO-CF1 film
can be used to detect ammonia gas generated by food spoilage.
When TPEPMO-CF1 was sealed with shrimps and stored at
room temperature (25 °C) for 24 h, a significantly enhanced
fluorescent emission from TPEPMO-CF1 was observed.
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Figure 5. (a) Illustration of TPEPMOs in the TPEPMO-CF film and their corresponding optical properties. (b) Dewashing experiment of
TPEPMO-CF2, TPE-CF2, and soaked-CF for 10 times using 1/1 ethanol/PBS (7.4). (c) Photobleaching behaviors of TPEPMO-CF2 and TPE-
CF2.
observed. Moreover, the stability of the sensor is estimated in
different environments. The sensing properties of the
TPEPMO-CF2 sensor at different temperatures are shown in
Figure S18. The TPEPMO-CF2 film exhibits similar
fluorescence quenching phenomena after exposure to hydro-
chloric acid vapor at 20, 30, and 40 °C. Moreover, the
fluorescence could be effectively recovered by the effect of
ammonia at different temperatures.
This kind of integrated gas-sensing sensor may not only ensure
practical applications of wearable materials but also inspire the
future development of wearable sensors with simpler
preparation and wide applicability.
CONCLUSIONS
■
In summary, we have demonstrated a wearable and washable
optical sensor for monitoring of NH3 and HCl vapor based on
AIEgen−organosilica framework (TPEPMO)-incorporated
nanofibre composites. For effective light performance, a
mesoporous nanoparticle TPEPMO with AIE characteristics
was employed for the fabrication of TPEPMO-CF nanofibres.
TPEPMOs exhibited pH sensing capability in solution due to
the reactivity of TPE-Si4 with OH−/H+. The covalent linking
strategy enables the fluorescence efficiency of TPEPMOs due
to the restriction of intramolecular rotation of TPE-bridged
groups, and the three-dimensional interface of the mesopores
ensured the higher sensitivity for sensing. Stable and washable
sensors were fabricated by embedding the TPEPMOs in
superassembled electrospun nanofibres, which served as a
flexible light-guiding platform for detecting environmental
acidity/alkalinity. The fluorescence of the as-prepared
TPEPMO-CF1 increased immediately and became brighter
upon exposure to ammonia vapor, which renders them capable
of monitoring the ammonia vapor with low concentrations in
air and assessing the spoilage of food. Furthermore, the
TPEPMO-CF2 sensor built from polyacrylonitrile as a
substrate showed excellent reversibility and acid−alkali
resistance in the presence of volatile acidic/alkaline gases for
more than five cycles. The fluorescence performance of
TPEPMO-CF2 was maintained up to 94% even after being
washed 10 times, which is much higher than the TPE direct-
assembled TPE-CF2. The covalent linking and electrospinning
superassembly strategy provides better optical stability and
washability for fabricating wearable sensors. In addition, the
TPEPMO-embedded PAN polymer was coated onto a wide
variety of material surfaces, including chemigum (gloves),
To further prove the practical values of TPEPMO-CF2 film
sensors, their washability and photostability were thoroughly
investigated. As shown in Figure 5b, the fluorescence emission
intensity of the TPEPMO-CF2 film was well retained after
continuous washing. Even after being washed 10 times, 94% of
the fluorescence emission can still be maintained. However, for
the reference sample TPE-CF2 in which TPE-derivative was
assembled in PAN directly, only 85% of the fluorescence
intensity was retained. In addition, for another control sample,
which was prepared by the PAN film soaked in a TPE
derivative solution, the fluorescent groups were rapidly
removed during the washing process. After three and ten
washing cycles, the film has only 79 and 64% of fluorescence
left, respectively. These results suggest that the TPEPMO-CF2
sensor exhibits good washability. A salient advantage of PMOs
as chemosensors is their high photostability. Figure 5c exhibits
the emission intensity changes in TPEPMO-CF2 and TPE-
CF2 with time against light irradiation (λ = 365 nm) over 100
min. The results showed that TPE-CF2 lost 23% of its initial
fluorescence. However, TPEPMO-CF2 exhibited hardly any
photobleaching under strong excitation. The high photo-
stability of TPEPMO-CF2 could be attributed to the
protective silica network, which can protect TPE groups
from oxygen or other exoteric quenching factors.
Considering the requirement of wearable materials, it is
better to assemble TPEPMO-CF sensors onto various material
surfaces. To this end, the TPEPMO-embedded PAN polymer
was coated onto a wide variety of material surfaces, including
chemigum (gloves), masks, clothes, and papers (Figure S19).
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masks, clothes, and papers, further proving the practical values
of TPEPMO-CFs. These superassembled AIEgen−organosilica
frameworks featuring diversified forms and superstability with
wearable and washable solid-state fluorescence can be used as
excellent solid-state sensors with solid-state lighting in various
fields, which is environmentally friendly for future smart
sensing applications.
Shihai Miao − National Supercomputer Research Center of
Advanced Materials, Advanced Materials Institute, Qilu
University of Technology (Shandong Academy of Sciences),
Jinan 250014, P. R. China
Wenlong Fu − National Supercomputer Research Center of
Advanced Materials, Advanced Materials Institute, Qilu
University of Technology (Shandong Academy of Sciences),
Jinan 250014, P. R. China
Xingshuang Zhang − National Supercomputer Research
Center of Advanced Materials, Advanced Materials Institute,
Qilu University of Technology (Shandong Academy of
Sciences), Jinan 250014, P. R. China
Junjie Zhou − Collaborative Innovation Center of Chemistry
for Energy Materials (iChEM), Department of Chemistry,
Laboratory of Advanced Materials, Shanghai Key Laboratory
of Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200433, P. R. China
Xiaoping Jiang − National Supercomputer Research Center of
Advanced Materials, Advanced Materials Institute, Qilu
University of Technology (Shandong Academy of Sciences),
Jinan 250014, P. R. China
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.analchem.0c04226.
1H NMR and 13C NMR spectra; details of the synthesis
of TPE-Si4; fluorescence quantum yield of TPE-Si4 and
TPEPMOs; figures of FT-IR spectra and linear
calibration plot of TPEPMOs; time-dependent fluo-
rescence, selective test, regeneration test, and sensing
stability of TPEPMO-CFs; and film sensors immobilized
onto a variety of materials (PDF)
Kang Liang − School of Chemical Engineering and Graduate
School of Biomedical Engineering, The University of New
South Wales, Sydney, NSW 2052, Australia; orcid.org/
0000-0003-3985-7688
AUTHOR INFORMATION
■
Corresponding Author
Biao Kong − Collaborative Innovation Center of Chemistry for
Energy Materials (iChEM), Department of Chemistry,
Laboratory of Advanced Materials, Shanghai Key Laboratory
of Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200433, P. R. China; orcid.org/
0000-0002-3251-5071; Email: bkong@fudan.edu.cn
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.analchem.0c04226
Notes
The authors declare no competing financial interest.
Authors
ACKNOWLEDGMENTS
■
Meng Gao − Collaborative Innovation Center of Chemistry for
Energy Materials (iChEM), Department of Chemistry,
Laboratory of Advanced Materials, Shanghai Key Laboratory
of Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200433, P. R. China; National
Supercomputer Research Center of Advanced Materials,
Advanced Materials Institute, Qilu University of Technology
(Shandong Academy of Sciences), Jinan 250014, P. R. China
Guanchen Xu − National Supercomputer Research Center of
Advanced Materials, Advanced Materials Institute, Qilu
University of Technology (Shandong Academy of Sciences),
Jinan 250014, P. R. China
Runhao Zhang − Collaborative Innovation Center of
Chemistry for Energy Materials (iChEM), Department of
Chemistry, Laboratory of Advanced Materials, Shanghai Key
Laboratory of Molecular Catalysis and Innovative Materials,
Fudan University, Shanghai 200433, P. R. China
Zhanjie Liu − Fudan-Haier Joint Research Center, Haier
Biomedical Company, Qingdao 266101, P. R. China
Hongming Xia − Fudan-Haier Joint Research Center, Haier
Biomedical Company, Qingdao 266101, P. R. China
Bing Shao − Beijing Key Laboratory of Diagnostic and
Traceability Technologies for Food Poisoning, Beijing Center
for Disease Prevention and Control, Beijing 100013, P. R.
China; orcid.org/0000-0001-5631-6889
This work was supported by the National Key Research and
Development Program of China (2019YFC1604601,
2019YFC1604600, 2018YFC1602301, 2017YFA0206901, and
2017YFA0206900), the National Natural Science Foundation
of China (21705027, 22005160, 61903235, and 51808328),
the Natural Science Foundation of Shanghai (18ZR1404700),
the Natural Science Foundation of Shandong Province
(ZR2017PEM004 and ZR2017LEM011), the Major Scientific
and Technological Innovation Projects of Shandong Province
(2018CXGC1406), the Australia National Health and Medical
Research Council (NHMRC, APP1163786), the Australian
Research Council (ARC, DP190101008), and the Recruitment
Program of Global Experts of China and the Thousand Talent
Plan of Shanghai.
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