Hyperbranched copolymer based photoluminescent vesicular probe conjugated with tetraphenylethene: Synthesis, aggregation-induced emission and explosive detection

Article abstract of DOI:10.1016/j.molliq.2020.113034

A carboxyl functionalized hyperbranched polymer, poly(3-ethyl-3-oxetanemethanol)-star-poly(ethylene oxide) (HSP-COOH) was subjected to addition reaction with tetraphenylethene (TPE) containing thioethanol (i.e. 2-((4-(1,2,2 triphenylvinyl)phenethyl)thio)ethanol (TPE-OH) to afford HSP-TPE copolymer with high yields. The TPE grafted hyperbranched copolymer forms homogeneous solution in common organic solvents such as THF. In aqueous solution, the copolymer self-assemble to form aggregates. The aggregates display maximum photoluminescence (PL) in THF/water (3:7 v/v) mixture. The introduction of TPE units into polymer makes it highly sensitive probe for picric acid (PA) with detection limit as low as 20 ppb via aggregation induced emission. The PL was effectively quenched by possible π-π interactions between TPE units of HSP-TPE and hydroxyl containing phenolic acid. The quenching efficiency significantly increased nonlinearly at higher quencher concentration. The present study offers details for design and synthesis of new hyperbranched polymeric fluorescent vesicle probe with high sensitivity.

Full text of DOI:10.1016/j.molliq.2020.113034

Journal of Molecular Liquids 308 (2020) 113034
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Hyperbranched copolymer based photoluminescent vesicular probe
conjugated with tetraphenylethene: Synthesis, aggregation-induced
emission and explosive detection
a,
b
c
Faran Nabeel a, , Tahir Rasheed , Muhammad Farhan Mahmood , Salah Ud-Din Khan
⁎
⁎
a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Department for Nanostructured Materials, k-7,Jozef Stefen Institute Jamova Cesta 39, SI 1000 Ljubljana, Slovenia
Sustainable Energy Technologies Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A carboxyl functionalized hyperbranched polymer, poly(3-ethyl-3-oxetanemethanol)-star-poly(ethylene oxide)
(HSP-COOH) was subjected to addition reaction with tetraphenylethene (TPE) containing thioethanol (i.e. 2-((4-
(1,2,2 triphenylvinyl)phenethyl)thio)ethanol (TPE-OH) to afford HSP-TPE copolymer with high yields. The TPE
grafted hyperbranched copolymer forms homogeneous solution in common organic solvents such as THF. In
aqueous solution, the copolymer self-assemble to form aggregates. The aggregates display maximum
photoluminescence (PL) in THF/water (3:7 v/v) mixture. The introduction of TPE units into polymer makes it
highly sensitive probe for picric acid (PA) with detection limit as low as 20 ppb via aggregation induced emission.
The PL was effectively quenched by possible π-π interactions between TPE units of HSP-TPE and hydroxyl con-
taining phenolic acid. The quenching efficiency significantly increased nonlinearly at higher quencher concentra-
tion. The present study offers details for design and synthesis of new hyperbranched polymeric fluorescent
vesicle probe with high sensitivity.
Received 31 January 2020
Received in revised form 24 March 2020
Accepted 31 March 2020
Available online 1 April 2020
Keywords:
Hyperbranched polymer
Photoluminescent vesicles
Explosive detection
Aggregation induced emission
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Raman spectroscopy [10,14] in solution phase. However, most of
these analytical instruments based detections becomes inappropriate
Explosive materials and their detection are one of the pressing con-
cerns in homeland security and global peace during these days.
Nitroaromatic compounds (NACs) such as picric acid (PA) and trinitro-
toluene (TNT) have been used on a large scale during World War-II [1].
According to the United Nations, there are over 60 regions and countries
throughout the world where millions of landmines do exist [2]. Among
several explosives, PA has wide-ranging use in military facilities; such as
in the dye, leather, pharmaceuticals, rocket fuel production and blasting
industries. The PA is highly acidic, readily soluble in water and trace
amounts of it can severely effect eyes, skin, liver, kidneys, metabolism,
cardiovascular and respiratory systems in both human and wildlife
[3–7]. In marine environment, the biotransformation products of PA
i.e., picramic acid and 2,4-dinitrophenol are even more toxic than PA
[8]. Sensitive and selective detection of PA in aqueous media therefore
becomes essential due to its toxicity and investigation of unexploded
landmines areas for the global environment and social security. Various
methods have been developed for the detection of NACs, for example
high pressure liquid chromatography [9,10] electrochemical methods,
[11,12] gas chromatography coupled with mass spectrometry [13] and
due to expensive, bulky, labor intensive and time-taking operations
for practical applications. Therefore, new recognitions approaches for
sensitive detection of explosives are desired. Sensors for nitroaromatic
explosive materials based on photoluminescence (PL) have attained
great attention due to their high sensitivity, stability, selectivity, cost-
effectiveness and portability [15,16]. Usually, the PL intensity is de-
creased by an increase in aggregation from fluorophores. This aggrega-
tion of fluorophores is caused by the increase in concentration. Contrary
to this aggregation-caused quenching (ACQ) aggregation-induced
emission (AIE) is another phenomenon which can also be used for the
detection of various analytes. Tang et al., have developed variety of
TPE fluorophores which show intense/induced fluorescence emission
upon aggregation and phenomenon is termed as “aggregation-induced
emission (AIE)” effect [17]. TPE along with its derivatives has gained
more importance due to its characteristic better photo stability, simple
preparation, ease of functionalization and good photoluminescence
properties [18–22]. Most of the investigations on TPE as AIE fluorophore
were carried out for detection of explosives [23–28] including metal
ions [23,29] and biomedical probes [17,20,30–33]. Tang et al. prepared
a number of TPE-containing polymers for the detection of explosive ma-
terials i.e. PA in aqueous medium [23,24,27,28]. Wang and coworkers
used TPE with block copolymers for aqueous detection of Pb2+ [29].
⁎
Corresponding authors.
E-mail addresses: frnnbl@sjtu.edu.cn (F. Nabeel), masil@sjtu.edu.cn (T. Rasheed).
https://doi.org/10.1016/j.molliq.2020.113034
0167-7322/© 2020 Elsevier B.V. All rights reserved.

2
F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
Compared to single-molecule probe, sensors based on self-assembly
tetrakis-(triphenylphosphine) palladium [Pd(PPh3)4] were procured
from Alfa Aesar Co. China. All other chemicals and solvents were pro-
cured from, Shanghai Chemical & Reagents Co. China, Adamas beta
and Aldrich USA. Triethylamine (TEA) and Tetrahydrofuran (THF)
were refluxed in CaH2 before use. The PA stock solution was prepared
in deionized water. The GPC of the polymers was determined by HLC-
8320GPC (TOSOH, EcoSEC) instrument at 40 °C using DMF as an eluent
at 0.6 mL min−1. PerkinElmer LS 55 fluorescence spectrophotometer
was used to perform fluorescence experiments. The DSC of the polymer
sample was determined via DSC Q2000 (TA Instruments, made of USA)
with Thermal Advantage fitted by a cooling system of RCS refrigerator.
1H and 13C NMR spectra were carried on mercury plus 400-MHz
(Varian, USA) spectrometer using deuterated solvents (chloroform
and DMSO) with TMS (tetramethylsilane, an internal standard).
Malvern Zetasizer (Malvern Instruments, Ltd.) was used to perform dy-
namic light scattering (DLS) experiments in the aqueous medium at
25 °C. A Tecnai G2 Spirit Biotwin instrument was used for transmission
electron microscopy (TEM) measurements with 120 kV/voltage. The
atomic-force microscopy (AFM) observations were recorded from Mul-
timode Nanoscope-IIIa instrument, USA (Digital Instruments) at
300 kHz using tapping mode. Magnetic stirrer was used during the re-
actions and silica gel column chromatography was performed to purify
the product.
of molecularly imprinted nanoparticles or self-assembled systems are
more advantageous due to spatial displayed multivalent chromophores
onto the surface of nanoaggregates are easily accessible for the target
analyte and leads to higher sensitivity [34–47]. Multivalent probe-
functionalized vesicles have also been reported for aqueous detection
of different analytes, for example, metal ions, toxins, proteins and bio-
molecules [35,36,38–43]. For instance, Wang et al., developed fluores-
cent amphiphilic cellulose nanoaggregates for the detection of 2,4-
dinitrotoluene (DNT) and PA in aqueous solution [37]. Stevens and co-
workers, have reported multivalent nanoparticles prepared form lipo-
somes as enzyme substrate for the detection of phospholipase-A2
[43]. Recently, vesicle-based polymeric chemical sensors for the detec-
tion of Hg2+ and SO2 in aqueous medium have been reported [38–40].
Further, the polymeric vesicle-based probe has its higher concentration
within a local volume which can definitely increase the sensitivity of de-
tection system [36,41,42].
Benefiting from above advantages, a Polymersome-based sensing
system enriched in TPE molecules was developed for sensitive detection
PA in aqueous medium as shown in Scheme 1. The solution of HSP-TPE
(hyperbranched polymer functionalized with TPE) in THF produced
AIE-active HSP-TPE aggregates by introducing various water fractions.
These AIE-active HSP-TPE aggregates could detect PA in aqueous solu-
tion, with a lower limit of detection (LOD) of 20 ppb and quenching con-
stant of 2.4 × 104 M−1
.
2.2. Synthesis of (2-(4-vinylphenyl) ethene-1,1,2-triyl) tribenzene (TPE)
2. Experimental section
The TPE was prepared by the method already reported (Scheme 2)
in excellent yield [31]. The 4-vinylphenylboronic acid (2.120 g,
14.32 mmol), Ph3Br (4 g, 11.94 mmol) were dissolved in toluene
(100 mL) in a round bottom flask then TBAB (0.320 g, 0.1 mmol) and
K2CO3 (aqueous solution, 2 M, 24 mL) were added with stirring
followed by addition of Pd(PPh3)4 (0.010 g, 8.65 × 10−3 mmol) under
nitrogen gas. The reaction lasted for 24 h at 90 °C. After cooled down
to room temperature, the toluene was removed via vacuum distillation
and then dissolved the product in dichloromethane (DCM, 150 mL). The
product was extracted by washing with deionized water (three times)
and drying over magnesium sulfate (anhydrous). The obtained
crude product was further purified by from silica column chromatog-
raphy using the mixture of DCM and petroleum ether (4: 1 v/v) and
TPE was obtained with excellent yield (88%) after the solvent was
evaporated. The 1H NMR of TPE is given in supporting information
(Fig. S4).
2.1. Reagents and instrumentation
The HSP sample (degree of polymerization of PEO arms, DParm ≈ 10)
was prepared according to the method previously reported [48,49] by
cationic ring opening polymerization. The characterization of HSPs are
provided in the supporting information (Fig. S1–S3). The mole fraction
of PEO arm (fEO) was 90.9% and the degree of branching (DB) of the
core was 38%. The average number molar mass (Mn) and dispersity
(Mw/Mn) of HSP were 9600 Da and 2.5 respectively. The 2,2-Azobis
(isobutyronitrile) (AIBN) was procured from Shanghai Reagent Co.
China and was recrystallized using ethanol two times before use. Tetra-
n-butylammonium bromide (TBAB), 4-dimethylaminopyridine (DMAP),
4-vinylphenylboronic acid, N,N′-Dicyclohexylcarbodiimide (DCC), (2-
bromoethene-1,1,2-triyl)tribenzene (Ph3Br) and mercaptoethanol were
procured from TCI Co., Shanghai, China. Potassium carbonate and
Scheme 1. Illustration of self-assembly of the copolymer (HSP-TPE) containing fluorescent TPE probe and sensing behavior of HSP-TPE aggregates towards picric acid (PA). In HSP-TPE the
blue, red and grey color present hydrophobic core, hydrophilic arms, and probe molecules resp. The PL intensity of HSP-TPE aggregates is quenched upon addition of picric acid.

F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
3
of HSP-COOH was about 85%. The HSP-COOH was used in the second
step for the preparation of HSP-TPE. To a flask of 100 mL, dissolved
HSP-COOH (400 mg, 2.66 mmol of COOH) in dimethylformamide,
DMF (40 mL, dried). Then added TPE-OH (1.3 g, 2.98 mmol), 4-
Dimethylaminopyridine (DMAP) (25 mg, 0.2 mmol) and DCC
(0.823 g, 4 mmol) to the solution and reacted for 24 h at room tem-
perature. The solvent was evaporated from the solution by vacuum
distillation and the crude product was obtained. The crude HSP-TPE
was then purified by dissolving in THF and precipitating with petro-
leum ether (three times) and dried for 24 h at 30 °C using vacuum
oven to obtain HSP-TPE. The percentage yield of HSP-TPE was
about 76%. For further confirmation of HSP, HSP-COOH and HSP-
TPE FTIR spectroscopy was carried out (fig. S6 supporting
information).
Scheme 2. Synthesis of (2-(4-vinylphenyl)ethene-1,1,2-triyl)tribenzene (TPE) and 2-((4-
(1,2,2-triphenylvinyl)phenethyl)thio)ethanol (TPE-OH).
2.5. Self-assembly of HSPs in water
Self-assembly of HSPs was achieved according to previously re-
ported method [48,49] by slowly introducing 10 mL of deionized
water into a 50 mL flask containing 10 mg of the polymer while stirring.
The polymer solution was kept on stirring at room temperature for
30 min resulting HSPs vesicles (concentration = 1 mg/mL).
2.3. Synthesis of 2-((4-(1,2,2-triphenylvinyl)phenethyl)thio)ethanol (TPE-
OH)
The TPE-OH was prepared by reacting TPE and mercapto ethanol in
the presence of AIBN as initiator as shown in Scheme 2. To a round bot-
tom flask added TPE (2.255 g, 6.3 mmol), mercapto ethanol (3.938 g,
50.4 mmol), and AIBN (517.3 mg, 3.15 mmol) and toluene (160 mL)
and dissolved under nitrogen gas. The reaction flask sealed using rubber
stopper after purging (30 min) with nitrogen gas. Then reacted for 24 h
at 70 °C. After cooling to room temperature, excess mercapto ethanol
and toluene were removed by vacuum distillation. The product TPE-
OH was obtained after placing overnight in a vacuum oven at 40 °C
(yield 70% approximately). The 1H NMR of TPE-OH is given in
supporting information (Fig. S5).
2.6. Self-assembly of HSP-TPEs probe in THF/water mixture
For self-assembly of HSP-TPEs, the polymer was first dissolved in
THF and then deionized water was introduced dropwise into its THF so-
lution while vigorous stirring. The THF solution of the polymer was pre-
pared with concentration = 5 μg × mL−1. Aggregates of HSP-TPE were
prepared with the increasing amount of water content from 0 to 90% in
THF solution.
2.7. Limit of detection for picric acid (PA)
2.4. Synthesis of HSP-TPE
Limit of detection (LOD) for PA was determined by reported
method [50,51] via fluorescence titration of PA aliquots and HSP-
TPE (concentration = 5 μg × mL−1) aggregates mixture with 70%
water content in THF solution. The fluorescence titration was then
plotted in order to obtain corresponding PA concentration (CT)
where fluorescence intensity changed sharply. The product of CT
and CL (the HSP-TPE concentration) gave LOD as shown by the equa-
tion below:
The HSP-TPE was synthesized in two steps as illustrated in Scheme 3.
The prepared HSP sample (degree of polymerization of PEO arms,
DParm ≈ 10) was used to prepare HSP-COOH in the first step. To a
flask (250 mL), added HSP (1.901 g, 3.419 mmol of OH) and dissolved
the polymer in THF (70 mL). Then added butanedioic anhydride
(683.8 mg, 6.838 mmol) and TEA (0.95 mL, 6.838 mmol and refluxed
for 20 h at 70 °C. After cooling the reaction mixture to room temperature
the product was precipitated by petroleum ether (three times) at
30–60 °C. The obtained product (HSP-COOH) was dried for 24 h at
room temperature by placing in a vacuum oven. The percentage yield
LOD ¼ CL ꢀ CT
Scheme 3. The synthesis of HSP-TPE by esterification reaction from TPE-OH and HSP-COOH produced from carboxylation of HSP.

4
F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
2.8. Determination of Stern-Volmer constant
2.87 ppm (l, l′) in HSP-TPE. Further signal peak at δ 12.3 ppm (proton
a) due to –COOH, has become almost negligible. These results con-
firmed the synthesis of HSP-TPE. The percentage of TPE grafted to HSP
was calculated by comparing the integral peaks of protons (A) of –CH3
groups of HSP core and integral peaks of the protons of TPE using the re-
lation ‘(STPE/19)/(SA/3) or (3STPE/19SA)’ and it was about 31%. The dif-
ferential scanning calorimetry (DSC) was conducted of HSP-TPE
copolymer and glass transition (Tg) was observed at temperature =
29.37 °C from DSC curve given in Fig. S7 (supporting information).
Another useful information regarding the sensitivity of the probe for
PA was obtained by plotting Stern-Volmer plot and determining the
Stern-Volmer constant (KSV) [28,50,51]. The plot is obtained as a func-
tion of [PA] using the following relationship:
Io=I ¼ 1 þ KSV ꢀ ½PAꢁ
where ‘Io’ is the initial fluorescence intensity when [PA] is zero and ‘I' is
the fluorescence intensity at any point.
3.2. Self-assembly and morphology of HSP-TPE aggregates
The self-assembled aggregates of HSPs (concentration = 1 mg/mL)
obtained by direct hydration [48,49] were subjected to TEM, DLS and
AFM spectroscopy which confirmed the vesicle configuration (Fig. S8
supporting information). The HSP-TPE self-assemble into vesicles
(Fig. 2) when water is introduced to their THF solution (5 μg × mL−1).
The TEM image of HSP-TPE (3:7 THF/Water v/v) (Fig. 2a) displays a ve-
sicular morphology with a clear dark boundary outside compared to the
internal pool. The average size of the spherical vesicles measured by DLS
is Ca. 712 nm with PDI of 0.38 (Fig. 2b). The AFM image (Fig. 2c) shows
spherical morphology of the HSP-TPE vesicles. The height profile
(Fig. 2d) shows an average diameter and height of HSP-TPE vesicles is
704 nm and 3.6 nm, respectively (diameter to height ratio is 1:196).
One reason for the large size vesicles is due to aggregation from direct
hydration method. Another possible reason is the increase in the hydro-
phobic character by the introduction of TPE molecules in the amphi-
philic hyperbranched HSPs which further increased hydrophobic
interaction between large-sized TPE molecules inside the vesicle.
3. Results and discussion
3.1. Characterization of HSPs and HSP-TPEs
The synthetic route for the HSP-TPE copolymer is given in Scheme 3.
The monomers TPE (i.e., 2-(4-vinylphenyl)ethene-1,1,2-triyl)
tribenzene) was prepared by using Suzuki coupling reaction employing
catalyst (i.e., tetrakis-(triphenylphosphine) palladium) [31] whereas
TPE-OH was prepared by simple addition reaction using AIBN as an ini-
tiator. The TPE-OH was successfully grafted to carboxylic acid termi-
nated poly(ethylene oxide) (PEO) arms of HSP-COOH by esterification.
The 1H NMR spectrum of HSP is given in Fig. S2 in supporting informa-
tion. When compared, 1H NMR peaks signals of the samples HSP-COOH
(DParm ≈ 10), TPE-OH and HSP-TPE in Fig. 1, spectral signals A-E and f-k
were from HBPO core and PEO arms respectively. The 1H NMR spectra
for HSP-COOH (Fig. 1) displayed new signals at δ 12.3 ppm from –
COOH, protons a, δ 4.39 ppm from protons f and δ 2.43 ppm from
CH2– (butanedioic anhydride) protons b, b' when compared to 1H
NMR for HSP (Fig. S2) confirmed successful conversion of –OH (end
group) of HSP to –COOH groups [52,53]. The conversion percentage of
–OH groups to –COOH was calculated to be approximately 77% by inte-
gral peaks comparison from A and b, b' using the relation ‘(Sb, b′/4)/(SA/
3) or (3Sb, b′/4SA)’. The 1H NMR spectra of HSP-TPE when compared
with HSP-COOH, new signals peaks are observed at δ 7.08 to 6.93 ppm
are assigned to protons from TPE. Moreover, signals at δ 4.03 (proton
c), 2.75 (proton d, d') and 2.67 (proton e) ppm are resultant of ester
linkage formation. Also the protons b, b' in HSP-COOH are shifted to
3.3. Photoluminescence of HSP-TPE aggregates
TPE molecules with multiple aryl groups are soluble in good solvents
where the aryl groups can rotate freely around the central stator linked
through single bonds are virtually non-luminescent. These intramolec-
ular free rotations are responsible for the conversion of photonic energy
into heat thus deactivates the excited states non-radioactively. How-
ever, in a poor solvent (i.e. water), it forms aggregates and becomes
AIE active due to restriction in intramolecular free rotation. In the cur-
rent case, aggregates of HSP-TPE at concentration = 5 μg × mL−1
were obtained by introducing water into their THF solution while vigor-
ous stirring. The electronic absorption from UV spectra and fluorescence
emission intensity from emission spectra of HSP-TPE aggregates in THF/
water (3:7 v/v) mixture is observed at peaks ~333 nm and 390 nm re-
spectively (Fig. S9 in supporting information).
Representatively, as shown in Fig. 3a, the HSP-TPE in solution form
show a weak PL at excitation wavelength λ = 333 nm, whereas, HSP-
TPE solutions in THF become photoluminescent when water in different
concentrations is used as precipitant. As the amount of water is in-
creased up to 70% in the THF solution, the value of PL is also increased
and an intense PL peak is recorded at ~390 nm under similar measure-
ment conditions. TPE unit in HSP-TPE polymer is hydrophobic in nature
therefore, the polymer must show aggregation in THF/water mixtures.
Consequently, HSP-TPE polymer is induced to turn on light by aggrega-
tion and is AIE active under high water content. The restricted intramo-
lecular rotation of TPE in HSP-TPE aggregates resulted in excimers
formation and become emissive by blockage of non-radiative channels.
By adding water more than 70% in THF solution of HSP-TPE, decreased
the PL intensity, this was caused by the low solubility of the polymer.
Another reason for this behavior is the decrease in the number of AIE-
active molecules per unit volume [1]. The PL intensity of the THF/
water aggregates with different amount of water is shown in
Fig. 3b and shows maximum PL intensity at 3:7 (v/v) of THF/water
mixture.
The fluorescence quantum yields (ΦF) of HSP-TPE aggregates (con-
Fig. 1. 1H NMR spectra of (a) HSP-COOH, (b) TPE-OH and (c) HSP-TPE in chloroform‑d.
centration = 5 μg × mL−1) were estimated in order to determine the

F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
5
Fig. 2. Morphology of HSP-TPE self-assemblies with THF/water (3:7 v/v) mixture. (a) TEM image (b) DLS curve (c) AFM image and (d) vesicle height profile.
quantitative picture of the HSP-TPE using standard conditions. The ΦF
values for 0 to 20% water content were almost same (~0.1) and increased
gradually up to a maximum of 7% ΦF (approximately) for 70% of water
content (Fig. 3c) which is about 70 fold greater than ΦF in THF solution
alone. The optical microscopic image of the aggregates of HSP-TPE
under bright and fluorescent light have already been discussed in Fig. 3d.
Fig. 3. Photoluminescence (PL) spectra of (a) HSP-TPE in THF/water mixtures using different water percentage with an excitation wavelength of λ = 333 nm, (b) PL intensity and
(c) quantum yields (ΦF) of HSP-TPE with the increasing amount of water content (polymer concentration: 5 μg × mL−1) and (d) optical microscopic image of vesicular aggregates
under bright light and fluorescent micrographs under blue light (inset figure).

6
F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
[PA]
(
µ
g
x
mL-1
)
3.4. Sensing ability of HSP-TPE aggregates
700
600
0
The sensing ability of the HSP-TPE aggregates was investigated in
terms of quenching efficiency (by reduction in fluorescence emission)
of the probe (with 5 μg × mL−1 in water/THF; 7:3 v/v dispersion)
upon addition of 10 equiv. of picric acid (PA), nitrobenzene (NB), cate-
chol, benzoic acid (BA), toluene, p-nitrophenol (p-NP) and metal ions
(Fe2+, Fe3+, K+, Hg2+, Cu2+, Cr3+, Ba2+ and Ni2+). Among interfering
organic compounds, PA has shown most effective quenching response
towards HSP-TPE polymers with quenching efficiency of 98% as
shown in Fig. 4. The considerable high quenching efficiency from PA
was due to donor-acceptor intermolecular photoinduced mechanism
of electron rich fluorophore and electron deficient nitroaromatic [54].
Other NACs (NB and p-NP) also have shown some quenching compared
to toluene since it possess no electron withdrawing group and conse-
quently show no quenching response. For practical applications aspect,
there could be presence of metal ions as impurities in environmental
samples. Therefore, fluorescence quenching response of HSP-TPE
probe was further analyzed against different metal ions. Form the
Fig. 4 it can be concluded that metal ions have no fluorescence
quenching impact for HSP-TPE probe.
500
400
300
200
100
0
40
350 400 450 500 550 600
Wavelength (nm)
Fig. 5. Fluorescent spectra of (a) HSP-TPE in THF/water (3:7 v/v) mixture with varied
amount of picric acid (PA) at an excitation wavelength of λ = 333 nm.
CL = Concentration of HSP-TPE, CT = Concentration of PA at which
change is observed.
LOD = 5 μg × mL−1 × 0.004 equiv.
= 20 × 10−3 μg × mL−1
.
= 20 ppb.
3.5. Sensitive detection of picric acid (PA) by HSP-TPE
The result shows that HSP-TPE probe is more sensitive towards PA
compared to most of the PA detection probes [50,51,55].
The NACs, for example, 2,4,6-trinitrotoluene (TNT), 2,4-
dinitrotoluene (DNT) and PA are used as explosive materials in warfare.
Their sensitive detection is of anti-terrorism and homeland-safety im-
plications. Due to commercially unavailability of TNT and DNT, the PA
was used as a model explosive compound in the present study. The PL
properties of HSP-TPE aggregates in THF/water mixtures can be
employed as a probe for sensitive detection of PA. Upon PA addition to
HSP-TPE (concentration = 5 μg × mL−1) aggregates with 3:7 (v/v) of
THF/water mixture, HSP-TPE shows PL at same wavelength but with de-
creasing intensity sensitively as shown in Fig. 5. Clearly discerned from
the PL spectrum data that PL intensity is quenched significantly by in-
troducing even a very small concentration of PA and is fully quenched
with PA concentration of 40 μg × mL−1 or 40 ppm.
3.7. Stern-Volmer plot and Stern-Volmer constant (KSV
)
The relative PL intensities quenched by PA concentrations was plot-
ted in the Stern-Volmer plot as shown in Fig. 7. The quenching constant
or Stern-Volmer constant (KSV) was determined according to the
method reported in the literature [28,50,51] and corresponded to
2.4 × 104 M−1 which is among the best values [28,55]. The Stern-
Volmer plot shows a linear quenching trend by PA below 20 μg ×
mL−1 of PA concentration as shown in Fig. 7 with a correlation factor
equal to 0.995. The Stern-Volmer plot deviated from linearity towards
upside at PA concentration above 20 μg × mL−1 (Fig. 7). The deviation
from linearity was due to the higher amount of PA which caused
“super quenching” by self-absorption along with energy transfer
[55,56]. Other possible reason could be an interaction between PA and
TPE domains (Lewis acid-base type) [27].
3.6. Calculation of limit of detection (LOD) for picric acid (PA)
Limit of detection (LOD) for PA by HSP-TPE was measured as re-
ported in the literature [50,51] by plotting PL intensity from HSP-TPE
aggregates with 3:7 (v/v) of THF/water mixture against PA concentra-
tion in equivalents as shown in Fig. 6. Taking the datum value as CT =
0.004 equiv., LOD for PA can be determined by using the equation as
given below:
3.8. Visual detection of PA under UV Light
From practical applicability point of view, visual detection of PA in
aqueous solution with high sensitivity is important. Test paper strips
(dia 2 cm2) prepared by immersing into HSP-TPE solution (5 μg ×
mL−1) could suffice the desired task. Different concentrations of PA
(10, 0.1, 0.01 and 0 μM) were introduced to HSP-TPE test paper strips
LOD ¼ CL ꢀ CT:
100
80
60
40
20
0
2+
3+
PA
PA
Fe
Fe
670
660
650
640
NB
Catechol
BA
Toluene
p-NP
+
K
2+
Hg
2+
Cu
3+
2+
Cr
Ba
2+
Ni
630
0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008
[PA] (in equivalents)
Fig. 4. Extent of fluorescence quenching efficiency of HSP-TPE (5 μg × mL−1 in water/THF;
7:3 v/v dispersion) by adding 10 equiv. of picric acid (PA), nitrobenzene (NB), catechol,
benzoic acid (BA), toluene, p-nitrophenol, Fe2+, Fe3+, K+, Hg2+, Cu2+, Cr3+, Ba2+ and
Fig. 6. Plot of fluorescence intensity of HSP-TPE at an excitation wavelength of λ = 333 nm
Ni2+
.
versus PA concentration.

F. Nabeel et al. / Journal of Molecular Liquids 308 (2020) 113034
7
12
10
8
Acknowledgmwnt
OH
O2N
NO2
Authors are grateful to the School of Chemistry and Chemical Engi-
neering, Shanghai Jiao Tong University, Shanghai 200,240, China for
providing the experimental facilities. The authors would like to sin-
cerely appreciate funding from Researchers Supporting Project number
(RSP- 2019/58), King Saud University, Riyadh, Saudi Arabia.
NO2
6
Picric Acid (PA)
4
Appendix A. Supplementary data
2
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.113034.
2
R
= 0.995
0
0
10
20
30
40
[PA] (
µ
g
x
mL-1)
References
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where Io is PL intensity at [PA] = 0 and I is the PL intensity at a specific concentration with
HSP-TPE in THF/water mixture.
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