A simple turn-on fluorescent chemosensor based on Schiff base-terminated water-soluble polymer for selective detection of Al3+ in 100% aqueous solution

Article abstract of DOI:10.1016/j.saa.2019.02.062

A simple water-soluble polymer PEGBHB based on polyethylene glycol bearing a Schiff base derivative moiety was successfully designed and synthesized. PEGBHB showed high selectivity and sensitivity towards Al³⁺ as a turn-on fluorescent chemosensor without influence by other competitive metal ions in 100% aqueous solution. The detection limit of PEGBHB for Al³⁺ was found to be 9.67 × 10^?9 M. A 1:1 stoichiometry between PEGBHB and Al³⁺ has been confirmed by Job plot analysis. PEGBHB could detect Al³⁺ over a wide pH range from 4 to 10. The chemosensor was reversible by adding EDTA to the solution of PEGBHB-Al³⁺ complex. An INHIBIT molecular logic gate was constructed with the help of OFF-ON-OFF signal on alternate addition of Al³⁺ and EDTA to the chemosensor. Furthermore, test papers were fabricated facilely using PEGBHB for convenient and visual detection of practical Al³⁺.

Full text of DOI:10.1016/j.saa.2019.02.062

Accepted Manuscript
A simple turn-on fluorescent chemosensor based on Schiff base-
terminated water-soluble polymer for selective detection of Al3+
in 100% aqueous solution
Liping Bai, Farong Tao, Leixuan Li, Aixia Deng, Chunna Yan,
Guang Li, Liping Wang
PII:
S1386-1425(19)30182-9
https://doi.org/10.1016/j.saa.2019.02.062
SAA 16838
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 30 September 2018
Revised date:
Accepted
date:
5 January 2019
1
7 February 2019
Please cite this article as: L. Bai, F. Tao, L. Li, et al., A simple turn-on fluorescent
chemosensor based on Schiff base-terminated water-soluble polymer for selective
detection of Al3+ in 100% aqueous solution, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.02.062
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ACCEPTED MANUSCRIPT
A simple turn-on fluorescent chemosensor based on Schiff
base-terminated water-soluble polymer for selective detection of Al3+
in 100% aqueous solution
Liping Bai, Farong Tao, Leixuan Li, Aixia Deng, Chunna Yan, Guang Li*, Liping Wang*
School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
*
Corresponding authors.
E-mail addresses: lglzsd@126.com (G. Li), wangliping5@163.com (L. Wang).
ABSTRACT
A simple water-soluble polymer PEGBHB based on polyethylene glycol bearing a Schiff base
derivative moiety was successfully designed and synthesized. PEGBHB showed high selectivity
and sensitivity towards Al3+ as a turn-on fluorescent chemosensor without influence by other
competitive metal ions in 100% aqueous solution. The detection limit of PEGBHB for Al3+ was
-9
3+
found to be 9.67 × 10 M. A 1 : 1 stoichiometry between PEGBHB and Al has been confirmed
by Job plot analysis. PEGBHB could detect Al3+ over a wide pH range from 4 to 10. The
chemosensor was reversible by adding EDTA to the solution of PEGBHB-Al3+ complex. An
INHIBIT molecular logic gate was constructed with the help of OFF-ON-OFF signal on alternate
addition of Al3+ and EDTA to the chemosensor. Furthermore, test papers were fabricated facilely
3
+
using PEGBHB for convenient and visual detection of practical Al .
3+
Keywords: Al ions; Fluorescent chemosensor; Schiff base; Water-soluble polymer; Test paper
1. Introduction
As the most abundant metallic element in the earth’s crust, aluminum (Al) is widely used in

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many fields, such as aluminum-based pharmaceuticals, food additives, cooking utensils, water
purification instruments and computers [1-4]. However, environmental acidification has greatly
increased the amount of free Al3+ ions, which are harmful to environment and humans. About 40%
of the acidic soils, which are detrimental to growing plants, are caused by Al3+ toxicity [5-7]. Al3+
has been assigned as prime food pollutants by World Health Organization (WHO), and the
average daily human intake range of Al3+ has been recommended to be around 3-10 mg [8,9].
Unregulated overloading of Al3+ can cause aluminum-related bone disease, reversible microcytic
hypochromic anaemia, encephalopathy and neurodegenerative diseases [10-14]. Therefore, the
highly sensitive and selective detection of Al3+ ions in aqueous media has significant importance
for environment protection and human health. Some conventional methods such as inductively
coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry,
graphite furnace atomic absorption spectrometry and electrochemical methods, are expensive,
relatively complex and time consuming in practice [15-18]. Recently, fluorescent chemosensors
for metal ions have attracted great interest due to their high sensitivity, low detection limit, low
cost, operational simplicity, real-time detection and time saving [19,20]. Unfortunately, the
development of fluorescent chemosensors for Al3+ ions in aqueous media is still challenging
because of the lack of spectroscopic characteristics, poor coordination ability and strong hydration
ability of Al3+ ions [21,22]. Therefore, it is highly essential to develop effective fluorescent
3
+
chemosensors for the sensitive and selective detection of Al .
To date, a large number of Schiff base derivatives have been developed for the detection of
various metal ions including Al3+ due to their ease of synthesis, low cost and excellent
photophysical properties [23-36]. Most of the Schiff base based fluorescent chemosensors are

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constructed by small-molecule Schiff base derivatives, which exhibit poor solubility in aqueous
solution, and organic cosolvent must be applied in the detection process. To some extent, the use
of organic solvent may limit the potential applications of fluorescent chemosensors in
environmental and biological fields. Furthermore, it is still a challenge to fabricate fluorescent
chemosensor devices using small molecules. Over the past few years, polymer has been proved as
promising matrices for the construction of fluorescent chemosensors [37-40]. Compared with
small-molecule fluorescent chemosensors, fluorescent chemosensors based on polymer exhibit
several distinct advantages including better water solubility, signal amplification and ease of
fabrication into devices. Zhang et al. prepared a well-defined alternating copolymer
P(VBCDEG-alt-SaAEMI) based on a salicylaldimine Schiff base via alternating RAFT
copolymerization for highly sensitive Zn2+ detection [41]. Patra et al. synthesized π-conjugated
polymers poly(metaphenylene-alt-fluorene) with pendant terpyridyl binding sites through Suzuki
2+
coupling polymerization for Cu detection [42]. In our previous works, two kind of water-soluble
polymer-based chemosensors, P(NIPAM-co-RhBBA) with pendant rhodamine B moieties and
PEGRh terminated with rhodamine B derivative, have been reported, which both showed high
selectivity to Cu2+ in pure aqueous solution [43,44]. Thus, the poor water solubility of
small-molecule Schiff base derivatives can also be overcome by covalently incorporating small
amounts of Schiff base derivatives into hydrophilic polymers, and the secondary pollution caused
by organic cosolvent can be prevented completely in metal ion detection. To the best of our
knowledge, the development of fluorescent chemosensors based on water-soluble polymers
containing Schiff base derivatives for selective detection of Al3+ ions in pure aqueous solution is
relatively backward.

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In continuation to our interest on water-soluble polymer chemosensors [43-46], we have
designed and synthesized a simple Schiff base-terminated water-soluble polymer PEGBHB, which
3
+
could serve as an efficient and highly selective fluorescent chemosensor for Al in 100% aqueous
solution. The detection properties of PEGBHB towards Al3+ were investigated using UV-Vis
absorption spectroscopy and fluorescence emission spectroscopy. PEGBHB exhibited excellent
recognition for Al3+ with no influence by other metal ions, and could detect Al3+ in a wide pH
range. An INHIBIT molecular logic gate was constructed based on the reversible sensing property
of PEGBHB. Meanwhile, test papers coated with PEGBHB were prepared for the practical
3+
detection of Al in real water samples.
2. Experimental
2.1. Materials
Polyethylene glycol monomethyl ether (PEG, Mn = 5000) was purchased from
Sigma-Aldrich. Benzoyl hydrazine, 2,4-dihydroxybenzaldehyde, 4-(dimethylamino)
3
+
pyridine (DMAP), N,N’-dicyclohexylcarbodiimide (DCC), nitrate salts of metal ions (Al ,
2
+
3+
2+
2+
3+
2+
3+
2+
3+
+
2+
+
2+
2+
2+
Ba , Ce , Cd , Co , Cr , Cu , Fe , Hg , In , K , Mn , Na , Ni , Pb and Zn ) and
all other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
China), and used as received. Carboxylated PEG was synthesized according to the
published procedure [47], and dried by azeotropic distillation in toluene for 6 h before use.
The normal filter papers for the preparation of test papers were purchased from Hangzhou
Special Paper Industry Co., Ltd. (Hangzhou, China).
2.2. Instruments

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1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured on a Varian
Mercury 400 spectrometer using DMSO-d
6
or CDCl as the solvents and TMS as the
3
internal standard. The ESI-MS spectrum was recorded on a LTQ Orbitrap mass
spectrometer. UV-Vis absorption spectra were recorded on a Shimadzu UV-3600
spectrophotometer. Fluorescent spectra were obtained on a Hitachi F-7000 fluorescence
spectrophotometer. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet
IR-100 infrared spectrometer using KBr pellets. The pH value was measured with a
Mettler-Toledo FE20 pH-meter.
2
2
.3. Synthesis
.3.1. Synthesis of the compound BHB
Benzoyl hydrazine (1.0 g, 7.3 mmol) and 2,4-dihydroxybenzaldehyde (1.064 g, 7.7 mmol) were
dissolved in ethanol (25 mL). After refluxing for 6 h under nitrogen, the solution was allowed to
cool and stand overnight at room temperature. The precipitate was filtered and washed three times
with 15 mL cold ethanol. After drying under vacuum overnight, the final product BHB was
1
obtained as a light pink solid (1.56 g, yield 83%). H NMR (400MHz, DMSO-d
6
), δ (ppm): 6.30
(d, 1H, J = 2 Hz), 6.32 (dd, 1H, J = 8.4 Hz, J = 2 Hz), 7.19 (d, 1H, J = 8.4 Hz), 7.47-7.54 (m, 3H),
7
.89 (d, 2H, J = 6.8 Hz), 8.46 (s, 1H), 9.86 (s, 1H), 11.52 (s, 1H), 11.87 (s, 1H); 13C NMR
(100MHz, DMSO-d
6
), δ (ppm): 78.91, 79.24, 79.57, 103.15, 107.99, 110.75, 127.92, 128.72,
+
1
2
31.91, 133.39, 149.94, 160.11, 161.14, 162.91; ESI-MS m/z: [M + Na] calcd. for C14
H
12
N
2
O :
3
79.0746; found: 279.0735.
2.3.2. Synthesis of PEGBHB

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To the solution of carboxylated PEG (1.0 g, 0.196 mmol) in dry THF (20 mL), BHB (52.8 mg,
.206 mmol), a catalytic amount of DMAP (10 mg) and DCC (61.9 mg, 0.3 mmol) were added
0
rapidly. The reaction mixture was stirred for 48 h at room temperature. After filtration, the filtrate
was concentrated under reduced pressure and precipitated in cold diethyl ether three times. The
desired white solid product PEGBHB was obtained by filtration and dried under vacuum for 24 h
at room temperature (0.91 g, yield 87%).
2.4. General spectroscopic measurements
The stock solutions of various metal salts (0.015 M) were prepared using deionized water as
-4
solvent. PEGBHB was dissolved in deionized water to form the stock solution (1 × 10 M). 3 mL
solutions containing required concentration of PEGBHB and metal salts were made by mixing
appropriate amount of stock solutions and deionized water, and the spectra of the mixed solutions
were recorded 30 s after each preparation at room temperature. The pH value of the solutions was
adjusted using HCl for pH 2-6 and NaOH for 8-13.
2.5. Application as test paper
Test papers were prepared by immersing normal filter papers into the methanol solution of
-
1
PEGBHB (10 mg·mL ) for 5 min, and then dried in air at room temperature. The solutions of
-4
various metal salts (10 M) were prepared using deionized water as solvent. Three solutions of
Al3+ (10 M) were prepared using deionized water, tap water and river water (Tuhai River) as
solvent, respectively. After being dipped in different aqueous solution for 5 s, the test papers were
taken out and observed under a 365 nm UV lamp.
-4
3. Results and discussion
3.1. Synthesis of the chemosensor PEGBHB

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The synthesis of PEGBHB is shown in Scheme 1. A simple Schiff base derivative BHB was
firstly synthesized by a one-step condensation reaction of benzoyl hydrazine with
1
13
2
,4-dihydroxybenzaldehyde, and the structure was supported by H NMR, C NMR and ESI-MS
spectral data (Fig. S1-S3). As most of Schiff base derivatives, BHB also exhibited poor solubility
in water. The post-modification approach has been proven to be a facile strategy for the
preparation of functional polymers [48]. In order to prepare water-soluble polymer chemosensor,
the hydrophilic polymer PEG was chosen as water-soluble polymer matrix. And then, Schiff base
derivative BHB-terminated polymer PEGBHB was obtained by post-modification of carboxylated
1
PEG with BHB. The formation of PEGBHB was verified by H NMR analysis (Fig. S4).
Compared to small-molecule Schiff base derivatives, PEGBHB showed excellent solubility in 100%
water.
Scheme 1. Synthetic route for the chemosensor PEGBHB.
3.2. Absorption studies of PEGBHB
UV-Vis absorption spectrum recorded for PEGBHB in pure aqueous solution showed an
absorption maximum at 328 nm, while the intensity of the 328 nm band of PEGBHB decreased
3
+
with the concomitant appearance of a new band at 367 nm upon the addition of Al ions (Fig. S5).
3+
2+
3+
2+
2+
3+
2+
3+
2+
3+
+
2+
+
2+
Various metal ions (Al , Ba , Ce , Cd , Co , Cr , Cu , Fe , Hg , In , K , Mn , Na , Ni ,

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Pb2+ and Zn ) have been added to aqueous solutions of PEGBHB and the absorption spectra
2+
showed new bands for Al , Cu , Zn , Ni , Co and Pb2+ (Fig. S6). The mixture of PEGBHB
3
+
2+
2+
2+
2+
3
+
3+
and Al gave the highest absorbance centered at 367 nm. And then, the Al -binding behaviors of
PEGBHB in aqueous solution were first studied by UV-Vis absorption titration. Upon the addition
of increasing concentrations of Al3+ ions, the absorption intensity at 328 nm decreased gradually,
and a new strong absorption band appeared at 367 nm with increase of the intensity (Fig. 1a). As
shown in Fig. 1b, the changes in absorbance of its two bands both became steady when about 1
3+
equiv. of Al ions were added. The result suggested that the stoichiometry between PEGBHB and
Al3+ was 1 : 1. According to the linear Benesi–Hildebrand expression, the measured absorbance
3
+
[
1/(A − A )] at 367 nm varied as a function of 1/[Al ] in a linear relationship, and the association
0
3+
5
-1
constant of PEGBHB with Al was calculated to be 2.14 × 10 M (Fig. S7).
Fig. 1. (a) UV-Vis absorption spectra of PEGBHB (10 μM) in aqueous solution with variable
3+
concentration of Al in range of 0-5 equiv.; (b) plot of absorbance intensity of PEGBHB (10 μM)
3
+
in aqueous solution at 328 nm and 367 nm depending on the equivalents of Al .
3.3. Fluorescence studies and binding behavior
The fluorescence selectivity of PEGBHB was examined in the presence of various metal ions in
pure aqueous solution (Fig. 2a). An excitation maximum of 345 nm did not show any significant

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change in fluorescence emission spectra of free PEGBHB in aqueous solution. Upon the addition
of 2 equiv. of Al3+ ions, a significant turn-on fluorescence response at 435 nm occurred with the
change of the solution from non-fluorescence to strong bright blue fluorescence within 20 s under
2
+
3+
2+
a 365 nm UV lamp. In contrast, the addition of 2.0 equiv. of other metal ions (Ba , Ce , Cd ,
2
+
3+
2+
3+
2+
3+
+
2+
+
2+
2+
2+
Co , Cr , Cu , Fe , Hg , In , K , Mn , Na , Ni , Pb and Zn ) caused negligible spectral
changes, and led to no obvious fluorescence changes of the solutions under a 365 nm UV lamp
(
Fig. 2b). The changes in fluorescence of PEGBHB before and after addition of Al3+ could be
explained by a combination of excited state intramolecular proton transfer (ESIPT), C=N
isomerization, and chelation enhanced fluorescence (CHEF) mechanism [49-52] (Scheme 2). In
3+
the absence of Al , free PEGBHB exhibited no fluorescence emission due to ESIPT and C=N
3
+
isomerization of imine double bond. After addition of Al , a stable chelation of PEGBHB with
Al3+ prevented ESIPT and C=N isomerization, resulting in the enhancement of fluorescence
emission intensity, which could also be ascribed to the CHEF mechanism. The fluorescence
change indicates that PEGBHB can act as a “turn-on” fluorescent chemosensor for Al3+ in pure
aqueous solution.
Fig. 2. (a) Fluorescence emission spectra of PEGBHB (10 μM) in aqueous solution upon the
addition of various metal ions (2 equiv. of each, excited at 345 nm); (b) the emission photographs

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of PEGBHB (10 μM) in the presence of various metal ions (2 equiv.) under a 365 nm UV lamp.
3
+
Scheme 2. Proposed mechanism for the detection of Al by PEGBHB.
3+
The Al -binding behaviors of PEGBHB in aqueous solution were further evaluated by
fluorescence titration experiments. In the absence of Al3+ ions, the aqueous solution of free
PEGBHB exhibited no obvious fluorescence emission, which could be attributed to the ESIPT
mechanism. Upon the addition of an increasing amount of Al3+ ions, the fluorescence of the
aqueous solution changed from non-fluorescence to bright blue fluorescence gradually under a
365 nm UV lamp (Fig. S8), and the fluorescence band centered at 435 nm increased obviously
(
Fig. 3). The complexation of Al3+ with PEGBHB increased the rigidity of the complex and
restricted the ESIPT effect and C=N isomerization of the complex, resulting in the enhancement
of the fluorescence intensity. The gradual enhancement of the fluorescence intensity with the
increase amount of Al3+ ions further confirmed the CHEF mechanism. When the amount of Al3+
ions was increased to 1 equiv., the fluorescence intensity was saturated. The fluorescence emission
3
+
intensity of the aqueous solution did not change obviously on excess addition of Al . The Job plot
analysis also confirmed the 1 : 1 stoichiometry for complexation of Al3+ with PEGBHB (Fig. 4).
Based on the result of fluorescence titration, the stoichiometry was further supported by the
Benesi-Hildebrand equation, and the association constant was determined to be 1.30 × 10 M-1
5
(
Fig. S9). The detection limit of PEGBHB as a fluorescent chemosensor for Al3+ ions was found

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-9
to be 9.67 × 10 M according to the equation 3σ/K, where σ is the standard deviation of 10
replicate measurements of blank PEGBHB solution and K is the slope of the calibration curve (Fig.
S10). The detection limits of some previously reported Schiff base-based chemosensors for Al3+
ions were compared with this work (Table 1).
The binding mode of PEGBHB with Al3+ was further confirmed by H NMR titration
1
experiments and FT-IR titration experiments. As shown in Fig. S11, the proton peak of phenolic
3
+
hydroxyl group (-OH) at 11.44 ppm disappeared after addition of Al , meaning the deprotonation
3
+
of the hydroxyl group and the binding of Al . Additionally, the proton peak of -CH=N- at 8.45
3
+
ppm shifted to 8.36 ppm, which could also be attributed to the binding of Al . The FT-IR
-1
-1
spectrum of PEGBHB showed characteristic peaks of –OH at 3326 cm and -CH=N- at 1627 cm
3+
(
Fig. S12). Upon the addition of Al , the peak of –OH disappeared and the peak of -CH=N-
-1
3+
shifted to 1602 cm . These results suggest that PEGBHB complexes with Al as proposed
mechanism in Scheme 2.
Fig. 3. Fluorescence emission spectra of PEGBHB (10 μM) in aqueous solution with variable
concentration of Al3+ in range of 0-5 equiv. Inset: plot of fluorescence intensity of PEGBHB (10
3
+
μM) in aqueous solution at 435 nm against the equivalents of Al .

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Table 1
3
+
Comparison of PEGBHB with some reported Schiff base-based chemosensors for Al ions.
Detection limit
Sl. No. Chemosensor
1
Testing media
Refs.
(
M)
Methanol/H
2
O
1.93 × 10-7
[23]
(1/999, V/V)
5
2
.25 × 10-7
.38 × 10-6
2
Methanol
Near
[24]
100%
3
4
5
6
7
4.00 × 10-6
1.12 × 10-6
5.00 × 10-8
6.70 × 10-6
9.00 × 10-9
[52]
[29]
[30]
[32]
[33]
aqueous solution
Nearly
perfect
aqueous solution
Methanol/H
3/7, V/V)
2
O
(
DMF/H2O
9/1, V/V)
(
DMSO/H O
2
(7/3, V/V)
DMSO/H O
2
8
9
1.05 × 10-8
[34]
(1/5, V/V)
Ethanol
2.78×10-6
[36]
1
00%
aqueous
10
9.67 × 10-9
This work
solution

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Fig. 4. Job plot of PEGBHB and Al3+ in aqueous solution. The total concentration of PEGBHB
3
+
and Al is 10 μM.
3
.4. Selectivity of PEGBHB to Al3+
3
+
To check the applicability of PEGBHB as a selective fluorescent chemosensor for Al ,
competitive experiments were performed in the presence of 2 equiv. of Al3+ ions mixed with 2
2
+
3+
2+
2+
3+
2+
3+
2+
3+
equiv. of other competitive metal ions, such as Ba , Ce , Cd , Co , Cr , Cu , Fe , Hg , In ,
+
2+
+
2+
2+
2+
K , Mn , Na , Ni , Pb and Zn . As shown in Fig. 5, no significant variation of the
fluorescence intensity at 435 nm was observed by comparison with or without other metal ions,
indicating that the presence of other metal ions has no influence on the selectivity of the
fluorescent chemosensor to Al3+ ions. All these selective results demonstrate that PEGBHB has
3+
excellent recognition for Al ions, and can satisfy the practical application requirement.

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Fig. 5. Competition analysis of PEGBHB (10 μM) to various metal ions in aqueous solutions. The
black bars represent the intensity of PEGBHB mixed with 2 equiv. of other metal ions; the red
bars represent the intensity of PEGBHB mixed with 2 equiv. of Al3+ and 2 equiv. of other metal
ions.
3.5. Effect of pH
The wide applicable range of pH was also an important factor for the property evaluation of the
fluorescent chemosensors. The effect of pH on the fluorescence response of PEGBHB to Al3+ was
evaluated in the pH range from 2 to 13 (Fig. 6). The aqueous solutions of the free PEGBHB
showed no obvious changes in the fluorescence intensity at 435 nm over the pH range of 2-13,
which meaning that PEGBHB was stable over the wide pH range. Upon the addition of Al3+ ions,
the fluorescence intensity at 435 nm enhanced, indicating the formation of the PEGBHB-Al3+
complex. The fluorescence intensity of the complex maintained stable with pH value from 4 to 10.
Under strong acidic conditions, the protonation of PEGBHB occurred and hampered its binding
3+
ability with Al . The fluorescence intensity decreased significantly with the continuous increase
of pH value because of the formation of Al(OH) under higher pH conditions. Both strong acidic
and strong alkaline conditions could block the complexation of PEGBHB with Al . However, the
3
3
+

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high activity of PEGBHB over such a wide pH range makes it favorable for the analysis of
environmental samples over this extended range of pH, and suitable for its application in
unbuffered solutions.
Fig. 6. Effect of pH on the fluorescence intensity (at 435 nm) of PEGBHB (10 μM) in aqueous
3
+
solutions in the absence and presence of Al (2 equiv.).
3.6. Reversibility studies
The reversibility of the fluorescent chemosensor to Al3+ was explored by titration of the
PEGBHB-Al3+ complex with EDTA, which was known as a strong chelating agent for Al . As
shown in Fig.7a, the fluorescence intensity of PEGBHB-Al3+ complex in aqueous solution was
decreased to that of free PEGBHB after addition of 1 equiv. of EDTA. The quenching of the
bright blue fluorescence was also observed by naked eyes under a 365 nm UV lamp (Fig. 7b). The
fluorescence quenching could be attributed to the decomplexation of Al3+ and the formation of
more stable EDTA-Al3+ complex, leading to regaining of free PEGBHB. The addition of Al3+ to
the non-fluorescent aqueous solution of free PEGBHB displayed a bright blue fluorescence, which
could act as ON switch. On the other hand, the titration of PEGBHB-Al3+ complex with EDTA
resulted in the complete quenching of fluorescence, and so serving as OFF switch. The ON/OFF
3+

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behavior of the fluorescent chemosensor, supported by the distinguished fluorescence changes
from non-fluorescence to bright blue fluorescence and then again to non-fluorescent state, clearly
3
+
demonstrates the reversible recognition property of PEGBHB to Al .
Fig. 7. (a) Reversible sensing nature of PEGBHB (10 μM) with Al3+ (1 equiv.) and EDTA (1
equiv.); (b) visual fluorescence changes of PEGBHB (10 μM) after alternate addition of Al3+ and
EDTA under a 365 nm UV lamp.
3.7. Construction of logic gate
Recently, great attention has been paid to the development of molecular logic gates based on the
transformation of chemically encoded information into fluorescent signals. The above study of
fluorescence emission clearly demonstrated that PEGBHB could serve as molecular system which
exhibited logic gate properties [53,54]. The OFF-ON-OFF behavior of the chemosensor offered
the scope to construct INHIBIT type molecular logic gate [24,34,55-59]. Here, Al3+ and EDTA

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were used as two chemical inputs whereas the change of fluorescence emission at 435 nm was
considered as output signal. Truth table was depicted for the input responses where “1” and “0”
represented the presence and absence of the input signals which were Al3+ or EDTA respectively.
Similarly for the output responses, “1” was defined as fluorescence appearance (ON) and “0” as
fluorescence quenching (OFF). The output value was 1 only when individual Al3+ was present,
and the output values of all other actions were 0. Thus the change of emission at 435 nm with Al3+
and EDTA as chemical inputs can be interpreted as a monomolecular circuit exhibiting an
INHIBIT molecular logic gate (Fig. 8).
3
+
Fig. 8. Truth table and the monomolecular circuit based on Al and EDTA as chemical inputs and
fluorescence emission as output.
3.8. Application as test paper
As a facile and efficient strategy to detect metal ions in real water samples, the test paper
method has attracted considerable attention [60-62]. To further evaluate the practical application
of the fluorescent chemosensor, test papers coated with PEGBHB were prepared by a simple
immersion and drying process. As shown in Fig. 9a, only when the test paper was dipped in the
aqueous solution of Al3+ ions, the strong bright blue fluorescence was visualized immediately by
naked eyes under a 365 nm UV lamp. No obvious changes of the test papers were observed after
being immersed into the aqueous solutions of other potentially competitive metal ions.
3
+
Furthermore, the practicability of the test paper was demonstrated using Al samples in deionized
water, tap water and river water (Tuhai River). The color of the test papers all changed to bright

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blue immediately under a 365 nm UV lamp after being dipped into different solutions of Al3+ ions
(
Fig. 9b). Therefore, test papers coated with PEGBHB are extremely suitable for on-site detection
3
+
of Al ions in real water samples.
Fig. 9. Fluorescent detection of Al3+ by test paper under a 365 nm UV lamp: (a) after being
-4
3+
2+
3+
immersed into different aqueous solutions (10 M) (from left to right, up: blank, Al , Co , Cr ,
Cu2+ and Fe ; down: Hg , Mn , Na , Ni , Pb and Zn ); (b) after being dipped into different
3+
2+
2+
+
2+
2+
2+
3+
-4
solutions of Al (10 M) (from left to right: deionized water, tap water and river water).
4. Conclusions
In conclusion, we have successfully designed and developed a novel and simple water-soluble
polymer based fluorescent chemosensor PEGBHB by incorporating Schiff base derivative moiety
into the hydrophilic polymer PEG. PEGBHB showed a remarkable turn-on fluorescence response
for the detection of Al3+ with high selectivity and sensitivity in pure aqueous solution. PEGBHB
maintained excellent fluorescence sensing ability to Al3+ over a wide pH range from 4 to 10, and
the detection limit for Al3+ was calculated to be 9.67 × 10-9 M. In addition, the reversible
fluorescence response of PEGBHB to Al3+ has been proved by the EDTA-titration experiments,
and an INHIBIT molecular logic gate was constructed based on two inputs (Al3+ and EDTA) and
one output. Furthermore, the test papers coated with PEGBHB have been fabricated facilely,
which could act as a practical fluorescent chemosensor for on-site detection of Al3+ ions in real

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water samples. This work provides a new strategy to develop Schiff base-based water-soluble
chemosensors for metal ion detection in 100% aqueous solution.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51503094);
the Natural Science Foundation of Shandong Province (No. ZR2015BQ002); and the Shandong
Province Higher Education Science and Technology Program (No. J16LA01).
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Graphical abstract

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Highlights



A novel and simple Schiff base-terminated water-soluble polymer PEGBHB has been
successfully developed.
PEGBHB exhibited high selectivity and sensitivity toward Al3+ as turn-on fluorescent
chemosensor in 100% aqueous solution over a wide pH range.
Test papers coated with PEGBHB have been fabricated facilely for on-site detection of Al3+
ions in real water samples.