Tetraphenylpyrene-bridged silsesquioxane-based fluorescent hybrid porous polymer with selective metal ions sensing and efficient phenolic pollutants adsorption activities

Article abstract of DOI:10.1016/j.polymer.2021.124083

A silsesquioxane based fluorescent porous polymer (PCS-TPPy) was easily prepared by Friedel-Crafts reaction of tetraphenylpyrene (TPPy) with octavinylsilsesquioxane (OVS) using AlCl₃ as catalyst. PCS-TPPy exhibited a high porosity with a Brunau

Full text of DOI:10.1016/j.polymer.2021.124083

Polymer 230 (2021) 124083
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Tetraphenylpyrene-bridged silsesquioxane-based fluorescent hybrid porous
polymer with selective metal ions sensing and efficient phenolic pollutants
adsorption activities
a
a,b,*
Nan Yang , Hongzhi Liu
a
International Center for Interdisciplinary Research and Innovation of Silsesquioxane Science, Key Laboratory of Special Functional Aggregated Materials, Ministry of
Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, PR China
b
Key Laboratory of Organosilicon and Materials Technology Of the Ministry of Education, Hangzhou Normal University Hangzhou, 31112, Shandong University, Jinan,
PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A silsesquioxane based fluorescent porous polymer (PCS-TPPy) was easily prepared by Friedel-Crafts reaction of
Silsesquioxane
Porous polymer
Phenolic pollutants
Ion sensing
3
tetraphenylpyrene (TPPy) with octavinylsilsesquioxane (OVS) using AlCl as catalyst. PCS-TPPy exhibited a high
porosity with a Brunauer-Emmet-Teller surface area of 1236 m2
-1
and total pore volume of 0.98 cm3 g . The
-1
g
3+
excellent fluorescence of PCS-TPPy has enabled it to have sensitive and selective sensing capability for Ru and
3
+
Fe with good linear response from 10 to 1000
M, respectively. Moreover, high surface area, hierarchical microporous/mesoporous structure and conjugated
structure of tetraphenylpyrene unit in this material favor phenolic pollutants adsorption with an excellent
μ
M and the corresponding detection limits are 3.12 M and 6.78
μ
μ
ꢀ 1
ꢀ 1
adsorption capacity of 1.53 mmol g for 4-bromo-phenol (BP), 0.68 mmol g for hydroquinone (HQ) and 0.50
ꢀ 1
mmol g for phenol (PH), respectively. Importantly, PCS-TPPy exhibits an excellent regeneration performance.
1
. Introduction
Air, water and soil are now polluted by human activities to varying
fluorescent porous polymers are extremely interesting because the
fluorescence could endow the porous polymer with new functions that
are of great importance in different applications, for example, sensor,
bio-imaging, photodynamic therapy and light-emitting diode [21–24].
Different kinds of fluorescent units, such as tetraphenylethene (TPE),
hexaphenylsilolev (HPS), triphenylamine (TPA), spirobifluorene and
carbazole have been successfully introduced to prepare covalently-
linked fluorescent porous materials [25–29]. Besides, the large surface
area and pore volume can significantly facilitate adsorption through the
rapid diffusion of guest molecules into the porous internal surface.
Therefore, it is desirable to further design and prepare novel fluorescent
porous polymers for efficient adsorption and detection of pollutants for
environment protection.
degrees [1–3], therefore, the world is facing serious challenges from
environmental pollution [4]. As two kinds of important pollutants,
metal ions (Hg2 , Pb , Ru , Gd ) and phenolic substances (organic
+
2+
3+
3+
pollutants) have serious impacts on environmental safety, human life
2
+
2+
2+
,
and health [2,5–10]. Although some metal ions (Zn , Ca , Mg
3
+
Fe ) are beneficial and essential to human body, they will also have an
adverse effect on human body due to the oversaturation caused by
bioaccumulation [11–13]. Therefore, it is of great significance to accu-
rately detect and remove these pollutants for environmental protection
and human health [14].
Compared with traditional porous materials such as activated carbon
and zeolite [15,16], covalently-linked porous polymers as new
fast-growing solid materials have attracted a lot of attention for their
potential applications in catalysis, detection, gas storage and adsorption
due to the flexibility of synthesis methods, a wide range of selected
monomers and excellent performances [17–20]. Among them,
Cage silsesquioxanes (SQs) have proven to be ideal building blocks to
construct hybrid porous polymers due to their rigid structure and mul-
tifunctionality [30–33]. A series of novel organic-inorganic hybrid
silsesquioxanes-based porous polymers containing multiple organic
functional groups could be prepared by Friedel-crafts reaction, Heck
reaction, thiol-ene reaction and cationic polymerization using
*
Corresponding author. International Center for Interdisciplinary Research and Innovation of Silsesquioxane Science，Key Laboratory of Special Functional
Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering Shandong University, Jinan, PR China.
E-mail address: liuhongzhi@sdu.edu.cn (H. Liu).
https://doi.org/10.1016/j.polymer.2021.124083
Received 31 May 2021; Received in revised form 25 July 2021; Accepted 6 August 2021
Available online 7 August 2021
0
032-3861/© 2021 Elsevier Ltd. All rights reserved.

N. Yang and H. Liu
Polymer 230 (2021) 124083
octavinylsilsesquioxane (OVS) as a starting material [25,27,34,35].
Friedel-Crafts reaction of OVS with different fluorescent monomers is a
feasible method to prepare hybrid fluorescent porous polymers [36]. For
example, Meng et al. successfully prepared a series of silsesquiox-
aneꢀ carbazole-corbelled hybrid porous polymers with controllable
porosity and excellent luminescence property, which could serve as a
aluminum chloride (AlCl
3
) (0.14 g, 1 mmol) were added to a three-
necked flask with 25.0 mL 1, 2-dichloroethane under nitrogen protec-
tion. The mixture was stirred at room temperature for 1 h, then heated to
◦
85 C for one day. After the reaction, the mixture was cooled to room
temperature and filtrated. The solid was washed with methanol, THF
and acetone successively to remove unreacted monomer and catalyst.
The resulting product was further purified in a Soxhlet extractor with
methanol for 24 h and dichloromethane for another 24 h, finally dried
metal-free heterogeneous catalyst for the cycloaddition reaction of CO
2
with epoxide to form cyclic carbonate [37]. Due to their rigidity and
excellent luminescence, a series of pyrene derivatives have been used to
prepare COFs or MOFs as the fluorescent probes in the past decade
◦
under vacuum at 75 C to obtain a grey solid (0.56 g, 98%).
[
38–41]. However, the combination of pyrene unit and silsesquioxane
2
.4. Detection of metal ions
unit, which can offer hybrid porous polymers with improved fluores-
cence and stability, has not received adequate attention by far. Our
group once prepared two kinds of fluorescent hybrid porous polymers by
Friedel-Crafts and Heck reaction of OVS with pyrene and brominated
pyrene [28,42], respectively, which could be applied in the field of
detection and adsorption. Compared with pristine pyrene, 1,3,6,8-tetra-
phenylpyrene (TPPy) is a kind of fluorescent chromophore with stronger
fluorescence performance, which has high hole transport performance
with high quantum yield in both solution and solid state [43]. In order to
further improve the fluorescence of silsesquioxanes-based porous poly-
mer, we select octavinylsilsesquioxane (OVS) and 1,3,6,8-tetraphenyl-
The detection of metal ions is of great significance in ecological and
environmental safety. PCS-TPPy is a cross-linked polymer with an
excellent fluorescence property; therefore, it could act as a sensor to
3+
3+
2+
2+
detect metal ions heterogeneously. Metal ions (Ru , Fe , Fe , Ca ,
2+
2+
2+
2+
2+
2+
2+
Mg , Mn , Ni , Co , Cu , Sr , Cd ) were dissolved in deionized
water to form a solution, then PCS-TPPy (3.0 mg) was added to 10.0 mL
of metal ion solution, which consisted of equal volumes of water and
3+
3+
absolute ethanol. Ru and Fe were selected for sensitivity analysis.
3+
Taking Ru
fluorescence detection sensitivity experiment as an
example, PCS-TPPy (3.0 mg) was added to 10 mL equal volume of water
pyrene (TPPy) as building blocks to construct
a
new hybrid
3+
and absolute ethanol in Ru solutions to prepare suspensions with
fluorescent porous polymer (PCS-TPPy) by Friedel-Crafts reaction in this
paper, which shows more sensitive and selective sensing properties
compared with other similar polymers (Table S1). After introducing the
TPPy unit into the hybrid cross-linked network, as shown in Table S3, it
can be found that the specific surface area, micropore area and micro-
pore volume of PCS-TPPy increase except improved fluorescence
compared with the pyrene-based porous materials [28b,42]. The
detection of metal ions and adsorption of phenolic pollutants are further
investigated using PCS-TPPy. More importantly, it offers an excellent
regeneration performance.
3+
different concentrations. Finally, the limit of detection (LOD) for Ru
was calculated using eq (1) as follows:
LOD = 3 ×
σ
/S
(1)
where represents the standard deviation of blank measurement, which
σ
was obtained by recording the fluorescence intensity of the PCS-TPPy
suspension in aqueous solution ten times. and S is the slope of the
calibration of PCS-TPPy to Ru3 or Fe ions. Moreover, the BR buffer
solution was chosen to explore the influence of pH changes on lumi-
nescence properties test. And then the effect of pH on the detection ef-
ficiency was investigated by adjusting the pH of the mixture containing
+
3+
2
. Experimental
3
+
3+
ꢀ 3
Ru or Fe (10 mol/L) and PCS-TPPy suspension (3 mg/10 mL) in
the pH range from 1 to 7. Hydrochloric acid and sodium hydroxide were
used to adjust the pH.
2
.1. Materials
Unless otherwise mentioned, all reagents were purchased from
2
.5. Adsorption experiment
commercial suppliers and used without further purification. 1,2-dichlo-
roethane was dried by distillation over CaH about two days.
2
Phenols are typical refractory aromatic compounds in wastewater
[
9]. 4-bromo-phenol (BP), hydroquinone (HQ), and phenol (PH) were
2
.2. Synthesis of 1,3,6,8-tetraphenylpyrene (TPPy)
selected as representative pollutants to investigate the adsorption pro-
cess of PCS-TPPy in this work. The pollutant samples to be tested were
prepared by diluting the deionized jelly stock solution of BP, HQ and PH
with the known amount. 3 mg PCS-TPPy was added to the pollutant
solutions with different initial concentrations, stirred at room temper-
ature for one day, and filtered to remove the adsorbent. Finally, the UV
absorbance was measured and the equilibrium concentrations of
phenolic pollutants in the filtrate were calculated.
TPPy was synthesized as described in a previous publication [44]. 1,
,6,8-Tetrabromopyrene (200 mg, 0.386 mmol), phenylboronic acid
3
(
3
254 mg, 2.08 mmol), Pd(PPh )4 (50 mg, 0.04 mmol) and 2.0 M NaOH
aqueous solution (2 mL) were mixed in a flask containing nitrogen
◦
saturated toluene (10 mL). The reaction mixture was stirred at 90 C for
2
4 h. After cooling to room temperature, the reaction mixture was
extracted with dichloromethane (2 × 40 mL). The combined organic
extracts were dried with anhydrous magnesium sulfate and evaporated.
The crude product was purified by column chromatography using a 1:5
petroleum ether/ethyl acetate mixture as the eluent to provide a
light-colored powder and recrystallized from hexane to obtain 1,3,6,
2.6. Detection and adsorption in real water samples
OVS and TPPy are rigid and multifunctional molecules, which make
them easy to construct stable hybrid porous polymer by Friedel-Crafts
reaction. TPPy endows the porous polymer with excellent fluorescence
and the introduction of silica-like cubic cages enhances the thermal
stability of the hybrid polymers. In addition, cubic cage could prevent
8
-tetraphenylpyrene (TPPy) as a light yellow powder (125 mg, 63%):
IR max (KBr) 2960, 1600, 1513, 1494, 1459, 1286, 1245, 1176, 1106,
ν
1
1
035, 835, 549, 476 cmꢀ 1; H NMR (400 MHz, CD
2
Cl
2
) δ 7.36–7.40 (m,
4
H), 7.47 (t, J = 7.6 Hz, 8H), 7.61 (d, J = 8.0 Hz, 8H), 7.98 (s, 2H), 8.12
(
s, 4H); 13C NMR (125 MHz, CDCl
3
) δ 141.10, 137.28, 130.68, 129.57,
π
-
π
stacking and enhance the fluorescence intensity. Therefore, the
1
28.37, 128.15, 127.32, 125.97, 125.35; m/z 506.1948(M+).
prepared silsesquioxane-based TPPy-functionalized porous polymers
show high stability and fluorescence besides good porosity. Considering
the porosity and fluorescence (Table S2, Fig. S7†), PCS-TPPy is chosen to
be a representative material to explore its practical application. Detec-
tion and adsorption experiments were carried out by applying PCS-TPPy
in three different types of real water (lake water of Daming Lake, spring
2
.3. Synthesis of TPPy-bridged silsesquioxane-based hybrid porous
polymer (PCS-TPPy)
OVS (0.32 g, 0.5 mmol), TPPy (0.13 g, 0.25 mmol), and anhydrous
2

N. Yang and H. Liu
Polymer 230 (2021) 124083
water of Black Tiger Spring and pool water of Five Dragons Pool in
1). Theorily, the starting molar ratio of reactants and the final molar
ratio in the product should be same because the Friedel-Crafts reaction is
a polyaddition reaction. According to the elemental analysis of the final
product, the ratio of C:H is 10.04:1 (mass ratio), as shown in Table S4,
which is a little lower than starting ratio of C:H (mass ratio is 11.68:1).
This is probably ascribed to the enwrapped catalyst in PCS-TPPy. In
order to explore the influence of molar ratio of OVS to TPPy on the
porosity and fluorescent properties, we have prepared another two
materials with the molar ratio of 1:1 and 3:1. The other two synthesized
materials are named as PCS-TPPy-a (molar ratio of OVS to TPPy is 1:1)
and PCS-TPPy-b (molar ratio of OVS to TPPy is 3:1), respectively.
The identification of PCS-TPPy is characterized by FTIR, solid-state
Jinan, China) with different concentrations of metal ions (Ru3 , Fe ) or
phenolic pollutants (BP, HQ, PH). After applying PCS-TPPy in different
real samples, the fluorescence emission intensity of the prepared sam-
ples is recorded. Then, a certain concentration of metal ions is added to
the sample to test its fluorescence change (Fig. S10†). The results show
that even in different actual water samples, PCS-TPPy can still effec-
+
3+
tively detect Ru3 and Fe . Moreover, three certain amounts of
phenolic pollutants are added to the three actual water samples, then
PCS-TPPy is added to the aqueous solution to perform a simulation
experiment on the adsorption of phenolic pollutants in the actual water
sample (Table S11, Fig. S11†).
+
3+
1
3
29
C and Si NMR spectroscopy. The FTIR absorption bands at 2960
cm 1 and 1458 cm might be attributed to the stretching and bending
vibrations of saturated C–H, indicating aliphatic methylene fragment
was formed after the successful Friedel–Crafts reaction. Moreover, one
ꢀ
ꢀ 1
2
.7. Material characterization
1H NMR (500 MHz) and 13C NMR (125 MHz) spectra of TPPy were
ꢀ 1
recognizes the characteristic peaks of PCS-TPPy at 3034 cm (part of
recorded by Bruker Avance spectrometer, and dissolved in CD
2
Cl
2
. The
ꢀ 1
the remaining vinyl) and 1120 cm
Fig. S1†) [25,27].
The solid-state 13C NMR spectrum of PCS-TPPy along with OVS is
(Si–O–Si absorption peak)
molecular weight of TPPy was determined by electrospray high reso-
lution mass spectrometry(Bruker Impact II). Fourier-transform infrared
(
(
FTIR) spectra were recorded with a Bruker Tensor 27 spectrophotom-
shown in (Fig. 1). The signals from PCS-TPPy are located at 128.2 ppm
ꢀ 1
ꢀ 1
eter with a disc of KBr from 4000 to 400 cm at a resolution of 4 cm .
2
and 138.2 ppm, which are considered to be the result of overlapping sp
1
3
Solid-state C cross-polarization/magic-angle-spinning (CP/MAS) NMR
carbon atoms originating from the aromatic rings of TPPy unit and
unreacted vinyl groups. And the signals appearing at the aliphatic region
2
9
and Si MAS NMR spectra were performed with a Bruker Avance-500
NMR spectrometer operating at a magnetic field strength of 9.4 T. Ni-
trogen sorption isotherms were measured on a Micro Meritics surface
(
10–60 ppm) are due to the formation of Si–CH
linkers between cage and TPPy after reaction. Two peaks appeared at
1.2 and 39.0 ppm, corresponding to the methylene carbons of
Si–CH –CH -TPPy or Si–CH –CH -TPPy, respectively; the peaks at 49.0
and 12.8 ppm should be assigned to the methine carbon of Si–CH(CH )-
)-TPPy, respectively. This result
2
–CH
2
- or Si–CH(CH
3
)
◦
area and pore size analyzer. The samples were degassed at 150 C for 12
1
h prior to measurement. A sample of approximately 400 mg and a UHP-
grade nitrogen (99.999%) gas source were used in the nitrogen sorption
measurements at 77 K and collected on a Quantachrome Quadrasorb
apparatus. The Brunauer-Emmett-Teller surface areas (BET) were
2
2
2
2
3
TPPy and methyl carbon of Si–CH(CH
3
indicated that the generation of regioisomers occurred in the Friedel–-
investigated in the range 0.01≤ P/P
0
≤ 0.2. Pore size distributions were
29
Crafts reaction [28a]. In the solid-state Si NMR spectrum of PCS-TPPy
calculated using the Nonlinear Density Functional Theory (NL-DFT)
kernel as implemented in the carbon/slit-cylindrical pore mode of the
Quadrawin software. Powder X-ray diffraction (PXRD) were performed
(
Fig. 2), there were two peaks at ꢀ 66.5 and ꢀ 69.4 ppm, which were
attributed to T
units newly formed after the Friedel–Crafts reaction and
–CH -TPPy and Si–CH(CH )-TPPy, respectively
3
corresponded to Si–CH
[
2
2
3
on a Riguku D/MAX 2550 diffractometer using Cu-K
α
radiation at 40 kV
and 200 mA with a scan rate of 10 min . Thermal gravimetric analysis
TGA) was measured on a Mettler-Toledo SDTA-854 TGA system with
26,27]. Compared with OVS, the smaller peak at ꢀ 80.2 ppm was
◦
ꢀ 1
assigned toT
3
silicon linked with the unreacted vinyl, which also proved
or
(OH)3-n], indicating that the
intact cage structure is retained in the process of Friedel–Crafts reaction
(
the incomplete reaction of vinyl in OVS due to steric hindrance. No T
peak was observed [T
= CSi (OSi)
1
◦
ꢀ 1
◦
heating rate of 10 C min from room temperature to 800 C in nitrogen
atmosphere. High-resolution transmission electron microscopy (HR-
TEM) experiments were recorded with a JEM 2100 electron microscope
T
2
n
n
13
29
[
28b,42]. The results of FTIR, solid-state C and Si NMR spectroscopy
(
JEOL, Japan) with an acceleration voltage of 200 kV. Field-emission
demonstrated that OVS reacted with 1,3,6,8-Tetraphenylpyrene suc-
cessfully to produce PCS-TPPy through facile Friedel–Crafts reaction.
Similar to silsesquioxanes-based porous materials previously
scanning electron microscopy (FE-SEM) experiments were recorded
with a HITACHI S4800 spectrometer. The luminescence excitation
spectra were measured on Hitachi F-4500 fluorescence spectropho-
tometer equipment. The UV–Vis absorption spectrum was obtained by
Cary 5000 UV–Vis Spectrophotometer.
3
. Results and discussion
3
.1. Characterization of PCS-TPPy
The synthesis of PCS-TPPy is successfully achieved by Friedel–Crafts
reaction of OVS with TPPy (molar ratio of OVS to TPPy is 2:1 ) (Scheme
1
3
Scheme 1. Synthetic route of PCS-TPPy.
Fig. 1. Solid-state C CP/MAS NMR spectra of PCS-TPPy and OVS.
3

N. Yang and H. Liu
Polymer 230 (2021) 124083
2
9
Fig. 2. Solid-state Si MAS NMR spectra of PCS-TPPy and OVS.
reported, the powder X ray diffraction profile of PCS-TPPy (Fig. S2†)
◦
showed a wide-range diffraction peak at approximate 20 , which was
attributed to the Si–O–Si linkage [45], indicating that the hybrid ma-
terial owned an amorphous characteristic. At the same time, the
morphology of PCS-TPPy was further characterized by field emission
scanning electron microscopy (FE-SEM) and high-resolution trans-
mission microscopy (HR-TEM). From the FE-SEM and HR-TEM images,
non-uniform nanopores can be observed and no long-range ordered
nanostructure aggregates are formed (Fig. S3†, S4†), which is consistent
with the characterization result of PXRD. These results comprehensively
show that PCS-TPPy is an amorphous material [27,42].
Thermogravimetric analysis (TGA) is an important method to eval-
uate the thermal stability of porous polymers. One can see in Fig. S5†,
compared with TPPy and OVS, PCS-TPPy shows a higher thermal sta-
bility. This can be attributed to the highly cross-linked network and the
introduction of inorganic silica-like cage, which improve the pyrolysis
temperature of organic moieties in the network.
3
.2. Porosity of PCS-TPPys (PCS-TPPy, PCS-TPPy-a, PCS-TPPy-b)
Fig. 3. (a) N
2
adsorption-desorption isotherm of PCS-TPPy mearsured at 77 K
(
inset was the pore size distribution curve of the PCS-TPPy calculated by NL-
DFT.) (b) N adsorption-desorption isotherm of PCS-TPPy@BP mearsured at
7 K (inset was the pore size distribution curve of the PCS-TPPy@BP calculated
In order to optimize the molar ratio of OVS to TPPy, 1:1, 2:1, and 3:1
2
7
were chosen to explore the influence of reactant ratio on the final
property of the material. The porosities of PCS-TPPys are measured by
by NL-DFT.).
the N
and Fig. S6†, at a lower P/P
rapidly, and with the increase of P/P
2
adsorption-desorption experiment at 77 K. As presented in Fig. 3
, the amount of N adsorption increased
, the adsorption capacity gradually
2
m /g was achieved due to the lower crosslink-density, as shown in
Table S2. PCS-TPPy has the highest micropore specific surface area and
micropore volume, although its total specific surface area is lower than
PCS-TPPy-a. An increase in the molar ratio of OVS to TPPy means an
increase in the reactive sites of TPPy, which will increase the local
crosslink density and result in a higher surface area. However, a further
increase in the molar ratio of OVS to TPPy greater than 2:1 will increase
the number of unreacted vinyl groups due to steric effect [25], resulting
in a high free volume of porous polymers and a reduced surface area. In
addition, due to the average crosslink density between OVS and TPPy,
inter-framework may occur and cause pore filling in the network, which
also reduces the surface area of PCS-TPPy-b.
0
2
0
slowed down. The apparent hysteresis in the desorption curve indicates
the presence of microporous and mesoporous structures in PCS-TPPy
[
46]. The specific surface area of PCS-TPPy-a, PCS-TPPy, PCS-TPPy-b
can be determined by the Brunauer-Emmet-Teller (BET) method to be
2
2
2
1
564 m /g, 1236 m /g and 54 m /g, respectively, and the corre-
2
2
sponding micropore area is calculated as 348 m /g, 455 m /g, and 25
2
m /g by the t-plot method. The pore size distribution (PSDs) of
PCS-TPPys is obtained by non-local density functional theory (NL-DFT)
analysis. All porous materials showed similar pore-size distributions
curves, for example, PCS-TPPy has bimodal pore structure of micropores
centered at around 1.69 nm and mesopores centered at about 2.77 nm.
The respective total pore volume (Vtotal) of PCS-TPPy-a, PCS-TPPy,
3.3. Luminescence properties
PCS-TPPy-b was 1.16, 0.98 and 0.052 cm3
micropore volume (Vmicro) to the Vtotal of PCS-TPPys was 0.13, 0.20,
.21, respectively. They could be regarded as mesoporous materials.
g
ꢀ 1
and the ratio of the
We also investigated the fluorescence changes of PCS-TPPys with
different molar ratios of OVS to TPPy. Three hybrid polymers were
investigated in 10 mL N,N-dimethylformamide (DMF), and the test
conditions were 340 nm for excitation wavelength, 5 nm and 10 nm for
excitation and emission slit. As shown in Fig. S7†, it is observed that the
fluorescence intensity increases with increasing OVS loading and the
0
Usually, higher local crosslinking density will result in higher surface
area and different molar ratio of OVS to TPPy could result in different
local crosslinking-density. It was observed that the surface area de-
creases with increasing OVS loading and the lowest surface area of 54
4

N. Yang and H. Liu
Polymer 230 (2021) 124083
+
3+
fluorescence intensity reaches maximum when the molar ratio of OVS to
TPPy is 3:1 (PCS-TPPy-b). This is because the introduction of more cubic
The UV absorption spectra of Ru3 and Fe show that both ions
have a peak at ~220 nm with a broad shoulder at ~300 nm, which
partially overlaps the excitation peak of PCS-TPPy at ~375 nm
(Fig. S9†). Therefore, one can explain the quenching mechanism by the
internal filtering effect (IFE) [47]. The excitation energy of PCS-TPPy
cages greatly prevent
π
- stacking of TPPy units and enhances fluores-
π
cence intensity. This further confirms the influence of OVS and TPPy
content on the porosity and fluorescence properties of synthesized
materials.
can be transferred to Ru3 or Fe indicating that there is a competi-
+
3+
The strong fluorescence of PCS-TPPy prompted us to explore its
application in metal ions detection. 3.0 mg PCS-TPPy was added to
ethanol/water solution (volume ratio 1:1) in the presence of various
tive absorption between Ru3 or Fe and PCS-TPPy [48]. Moreover,
+ 3+
3
+
compared with other metal ions, Ru shows stronger UV absorption
intensity in the range of 500–700 nm, which overlaps with the fluores-
cence emission peak of PCS-TPPy (560 nm), which shows energy
transfer also takes place between the two units [7]. This is consistent
2
+
2+
ꢀ
2+
2+
2+
2+
2ꢀ
cations and anions (cations: blank, Fe , Ca , Mg , Mn , Ni , Co
,
,
2
+
2+
2+
ꢀ
ꢀ
ꢀ
3ꢀ
ꢀ
Cu , Sr and Cd ; anions: F , Cl , Br , I , PO
3
, BF
ꢀ 3
4
, SO
4
ꢀ
3ꢀ
ꢀ 1
3+
HSO
3
and PO
4
). The concentration of all ions is 10 mol L . As
with the result that Ru has a better quenching effect on PCS-TPPy than
+
shown in Fig. 4, it is observed that PCS-TPPy shows strong detection
other metal ions (including Fe3 ).
sensitivity to Ru3 and Fe . Moreover, the introduction of other metal
+
3+
Same as selectivity, sensitivity to metal ions also plays an important
role in the development of environmental science [49,50]. In order to
further determine the effect of metal ion concentration on the fluores-
3
+
ions or anions has basically no effect on the quenching behavior of Ru
3
+
and Fe , which further demonstrates the high selectivity of PCS-TPPy
3
+
3+
3+
to Ru and Fe
.
cence performance of PCS-TPPy, the fluorescence intensity in Ru and
3
+
Moreover, in order to investigate the effect of pH on the detection
efficiency of PCS-TPPy. The BR buffer solution was chosen to explore the
influence of pH changes on luminescence properties test. As shown in
Fig. S8†, the pH barely influenced the detection efficiency over the pH
range of 1–7 as evidenced by the negligible effect on the fluorescence
Fe solutions with different concentrations are measured. As shown in
3
+
3+
Fig. 5, with the concentration of Ru and Fe increasing, the fluo-
rescence intensity of PCS-TPPy gradually decreases. When the concen-
tration of Ru3 and Fe ions is 3 × 10 and 5 × 10 mol L
+
3+
ꢀ 3
ꢀ 3
ꢀ 1
,
respectively, the fluorescence of PCS-TPPy is completely quenched.
Obviously, PCS-TPPy shows better fluorescence quenching sensitivity to
intensity, which reveals that the detection of Ru3 or Fe is very stable.
Therefore, we choose deionized water (neutral) to conduct subsequent
luminescence properties test.
+
3+
3
+
Ru . This may be attributed to the fact that the Ru(III)/Ru(0) couple
(0.4V) has a higher reduction potential than Fe(III)/Fe(0) couple
(
ꢀ 0.036V) [51,52]. As shown in Fig. 6, PCS-TPPy had a good linear
Fig. 4. The fluorescence intensity ratio of PCS-TPPy in the presence of various
Fig. 5. The fluorescence emission spectra of PCS-TPPy with different concen-
3
+
3+
3+
3+
metal ions and anions before and after the addition of Ru and Fe
.
trations of Ru and Fe .
5

N. Yang and H. Liu
Polymer 230 (2021) 124083
phenolic substances pose a huge threat to human health due to their
good water solubility. Therefore, the removal of toxic phenolic pollut-
ants from water is a very important issue [59,60]. Here, 4-bromo-phenol
(
BP), hydroquinone (HQ) and phenol (PH) were chosen to investigate
the adsorption behaviors of PCS-TPPy for phenolic pollutants in this
work.
The standard curves of three phenolic pollutants are shown in
Fig. S13-S15†. In the adsorption experiment, the adsorption capacity of
PCS-TPPy was detected by measuring the UV absorbance intensity
changes of the filtrate of phenolic pollutants. Fig. 7 depicts the effect of
different concentrations of BP, HQ, and PH on the equilibrium adsorp-
tion capacity (Qe). The results show that at low concentrations, the Qe
growth rate is obvious, and as the concentration increases, the equilib-
rium adsorption capacity tends to be flat and reach a saturated state. It
can be seen from the Fig. 7 that PCS-TPPy shows the highest adsorption
ꢀ
1
ꢀ 1
capacity of 1.53 mmol g (265 mg g ) for BP, which is higher than
other adsorbents reported (Table S7). And the adsorption capacities of
ꢀ 1
ꢀ 1
PCS-TPPy for HQ and PH are 0.68 mmol g (75 mg g ) and 0.50 mmol
ꢀ 1
ꢀ 1
g
(47 mg g ), respectively.
The understanding of interaction mechanism can be achieved
3
+
3+
Fig. 6. The detection linearity of PCS-TPPy to Ru and Fe .
through comprehensive experimental observations and theoretical cal-
culations. Adsorption performance is not only highly affected by a va-
riety of weak molecular interactions (such as electrostatic interaction,
hydrophobicity, hydrophilicity, hydrogen bonding and van der Waals
interaction), but also by the inherent characteristics of various adsor-
bents (such as pore size, shape, molecular scale, crystallinity, modified
functional groups) [61]. As we know, silsesquioxane-based hybrid
porous materials can be used as excellent adsorbents, which has been
confirmed by previous reports [62]. By analyzing the nitrogen
adsorption-desorption isotherm of PCS-TPPy@BP, one can see that its
specific surface area and total pore volume are significantly reduced
compared with PCS-TPPy (Fig. 3b and Table S2). And the reduction of
the mesopore area is more obvious than micropore area, which proves
that the adsorption of phenols in the mesopores is more obvious [63].
Besides, we have compared the porosity of PCS-TPPy with other similar
adsorbents and found that whether the specific surface area or the pore
volume of PCS is larger than other adsorbents listed in Table S7†. This
also explains the high adsorption capacity of PCS-TPPy for BP, HQ and
BP, showing the importance of porosity in the adsorption activity. One
can see that the molecular size of these three phenolic pollutants follows
the order: HQ > BP > PH (Table S8, Fig. S12†). The surface area of
response to Ru3 from 10 to 1000
+
μ
M with detection limit of 3.12 M
μ
ꢀ
4
(
S/N = 3), and the linear range equation was (I
0
/I)-1 = 0.480C(10 ) -
0
.298 (R2 = 0.9972). Also, PCS-TPPy displays selective fluorescent
3
+
sensing for Fe with a low detection limit of 6.78 M over other metal
μ
ions with the linear response ranging from 10 to 1000 M. And the
μ
ꢀ 4
2
equation was denoted as (I
0
/I)-1 = 0.221C(10 ) - 0.002 (R = 0.9973)
[
53,54]. Therefore, the results can further confirm the high sensitivity of
3
+
3+
3+
PCS-TPPy for Ru and Fe , especially for Ru , which is comparable
to some other similar sensors (Tables S5–S6).
Therefore, the mechanisms of fluorescent quenching behavior can be
explained in detail as the followings: firstly, PCS-TPPy is considered to
be electron-rich due to the presence of TPPy unit with
π
conjugation
system and cage core with Si-O-Si linkage in the crosslinked network,
while Ru3 and Fe are both electron-deficient, which is prone to
electron transfer and causes fluorescence annihilation [52]; secondly,
from the UV absorption spectra of PCS-TPPy and metal ions, one can see
+
3+
that except for Ru3 and Fe , the UV absorption spectra of other metal
ions also overlap with the excitation or emission spectra of PCS-TPPy to
a certain extent (Fig. S9†). However, these metal ions cannot cause the
annihilation of PCS-TPPy, which can be attributed to their lower stan-
dard reduction potential (Mg(II)/Mg(0) couple (-2.37V), Sr(II)/Sr(0)
couple (-2.89V), Fe(II)/Fe(0) couple (-0.45V), Ca(II)/Ca(0) couple
+
3+
PCS-TPPy calculated as 1236 m2
g
ꢀ 1
(Table S1), provides sufficient
storage space for the adsorption of phenolic pollutants. From the
3
ꢀ 1
(
-2.87V), Co(II)/Co(0) couple(-0.28V), Mn(II)/Mn(0) couple (-1.03V),
perspective of porosity, the pore volume (0.98 cm g ) of PCS-TPPy is
much bigger than the molecular structure size of these three phenolic
Cu(II)/Cu(0) couple (0.34V), Cd(II)/Cd(0) couple (-0.40V), Ni(II)/Ni(0)
couple (-0.25V)) [55,56]. Meanwhile, the fluorescence quenching effect
3
+
3+
of Ru for PCS-TPPy is better than Fe , which may also be related to
the higher standard reduction potential of the Ru(III)/Ru(0) couple [51,
+
2]. For Cu2 with high standard electrode potential, it cannot quench
5
the fluorescence of PCS-TPPy, which could be attributed to its larger
diameter (Cu2 , 1.5 Å). The diameters of other metal ions are collected
+
3
+
2+
3+
2+
2+
2+
as follows: Ru ,1.4 Å; Fe , 1.1 Å; Mg , 1.4 Å; Sr , 2.4 Å; Fe , 1.6 Å;
2
+
2+
2+
2+
Ca , 2.0 Å; Co , 1.3 Å; Mn , 1.3 Å; Cd , 1.4 Å; Ni , 1.4 Å [55,56].
This is due to the extensive cross-linked structure and the twisted
arrangement of the peripheral aromatic ring preventing the larger
diameter metal ions from approaching PCS-TPPy [57]. In summary,
internal filtering effect (IFE), electron transfer, energy transfer, reduc-
tion potential and diameter could be the main factors that induced the
high selectivity to Ru3 and Fe by PCS-TPPy.
+
3+
3
.4. Adsorption of phenolic pollutants
Environmental pollution is an important problem that human is
facing nowadays [58]. According to the United Nations’ survey, billions
of cubic meters of human-made sewage are discharged into seawater
and rivers every year. Among these pollutants, very dangerous and toxic
Fig. 7. Adsorption isotherms of BP, HQ and PH onto PCS-TPPy.
6

N. Yang and H. Liu
Polymer 230 (2021) 124083
pollutants; hence the adsorbate can easily penetrate into the adsorbent
were simulated to evaluate the reusability and stability of PCS-TPPy as
an adsorbent. PCS-TPPy is added to an aqueous solution of phenolic
[
60]. It can be distinctly seen from the molecular structure that the
ꢀ 1
two-dimensional dimensions of BP (0.651 nm * 0.432 nm), HQ (0.654
nm * 0.432 nm) and PH (0.568 nm * 0.432 nm) are significantly smaller
than that of the micropores (1.41 nm). Therefore, PCS-TPPy could offer
an excellent adsorption capacity for phenolic pollutants.
pollutants (BP) with an initial concentration of 30 mg L . The adsorbed
material is repeatedly washed with ethanol and dichloromethane suc-
cessively, and can be used after drying. Fascinatingly, it can be seen from
Fig. 9 that even after 5 cycles of experiments, PCS-TPPy still maintains
an ultra-high removal efficiency.
In order to understand the difference in adsorption of BP, HQ and PH
by PCS-TPPy in more detail, we have also compared the affinity of the
three phenolic substances with water. The solubility of PH, HQ and BP in
3.5. Adsorption of phenolic pollutants in real water samples
◦
water are 83 g/L, 60 g/L, 14 g/L (20 C), respectively [64], which is
consistent with the difference in adsorption of BP, HQ and PH by
The detection and adsorption experiments were carried out by
applying PCS-TPPy in three different types of water (lake water of
Daming Lake, spring water of Black Tiger Spring and pool water of Five
Dragons Pool in Jinan, China) with different concentrations of metal
PCS-TPPy. Meanwhile, the
PCS-TPPy is another important factor for its good adsorption. The
phenolic molecules could be adsorbed onto PCS-TPPy by interac-
π
-π interaction of phenolic substances with
πꢀ π
ions (Ru3 , Fe ) or phenolic pollutants (BP, HQ, PH). As shown in
+
3+
tion [65], resulting in excellent adsorption capacities for three phenolic
substances.
ꢀ 3
Fig. S10†, after adding PCS-TPPy and a certain concentration (10 mol
ꢀ 1
To further verify the adsorption mechanism, three common adsorp-
tion isotherms were selected to fit the data, namely Langmuir, Freund-
lich and Temkin equations [65,66]. The results are shown in
Fig. S16-S18† and Table S9. The adsorption behaviour of PCS-TPPy on
L
) of metal ions to different real water samples, one can see that even
in different actual water samples, PCS-TPPy can still effectively detect
3
+
3+
Ru and Fe . Moreover, after three certain molar amounts of phenolic
pollutants were added to the three actual water samples, and then PCS-
TPPy was added to the aqueous solution to perform a simulation
experiment on the adsorption of phenolic pollutants in the actual water
samples (Table S11, Fig. S11†). And the results show that PCS-TPPy has
a slightly lower equilibrium adsorption capacity (Qe) for phenolic pol-
lutants than corresponding value under the same conditions in the
deionized water. This may be related to the natural organic matter
(NOM) that has similar structure to phenolic pollutants in actual water
samples, which may have effects on the adsorption performance for the
phenolic pollutants [10]. Besides, the adsorption behaviour of PCS-TPPy
to these three phenolic pollutants in real water samples is also more in
line with the Langmuir model, indicating that the adsorption behaviour
of PCS-TPPy on the three phenolic pollutants is indeed close to mono-
layer and relatively uniform adsorption.
these three phenolic pollutants is more in line with the Langmuir model,
2
and its linear correlation coefficients (R
L
) are 0.9799 (BH), 0.9249
(
HQ) and 0.9557 (PH), respectively, which are significantly higher than
2
2
Freundlich’s R
F
T
and Temkin R , indicating that the adsorption
behaviour of PCS-TPPy on the three phenolic pollutants is close to
monolayer and relatively uniform adsorption. According to theoretical
calculations, the maximum adsorption capacity of PCS-TPPy for BP, HQ
ꢀ 1
ꢀ 1
ꢀ 1
ꢀ 1
and PH is 1.88 mmol g (324.7 mg g ), 0.86 mmol g (94.87 mg g )
ꢀ 1
ꢀ 1
and 0.71 mmol g (67.25 mg g ), respectively, which is slightly higher
than the corresponding experimental Qe value.
In view of the highest adsorption capacity of PCS-TPPy on BP, BP was
chosen to explore the adsorption kinetics with an initial concentration of
ꢀ 1
3
0 mg L . Fig. 8 depicts the effect of contact time on the absorption of
BP by PCS-TPPy. One can see that the adsorption rate of PCS-TPPy is
extremely fast and the system can reach adsorption equilibrium in 1 h.
In order to further explore the kinetic adsorption behavior of PCS-
4. Conclusion
TPPy on BP [67–68], two common models were used for simulation.
In summary, we use octavinylsilsesquioxane (OVS) and tetraphe-
nylpyrene (TPPy) as reaction units to successfully prepare
silsesquioxane-based hybrid fluorescent porous polymer (PCS-TPPy)
through efficient and concise Friedel-Crafts reaction. The resulting
2
As shown in Fig. S19† and Table S10,
R
2
(0.9992) of the
2
pseudo-second-order kinetic model is higher than R
1
(0.9658) of the
pseudo-first-order kinetic model; and the fitted equilibrium adsorption
ꢀ 1
2
ꢀ 1
value (63.86 mg g ) is basically consistent with the experimental value
polymer shows a high surface area of 1236 m g , total pore volume of
ꢀ
1
0.98 cm3
ꢀ 1
g , and hierarchical structure with micropores and meso-
(
64 mg g ), which proves that the adsorption of BP by PCS-TPPy is
more in line with the pseudo-second-order kinetic model.
pores. The excellent fluorescence of PCS-TPPy makes it to selectively
detect Ru3 and Fe with good linear response from 10 to 1000
+
3+
μ
M and
M,
Simultaneously, the regeneration and circulation of the adsorbent
the corresponding detection limits are 3.12 and 6.78
μM
μ
Fig. 8. Effect of contact time on the absorption of BP by PCS-TPPy.
Fig. 9. The removal efficiency of PCS-TPPy for BP after five cycles.
7

N. Yang and H. Liu
Polymer 230 (2021) 124083
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