Two 8-hydroxyquinoline-based fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents with large Stokes shifts

Article abstract of DOI:10.1016/j.saa.2019.117956

It is of great significance to detect the moisture in organic solvents before used in water-sensitive reactions. Herein, two Schiff base quinoline derivatives, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone (HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine (HQPH), were designed and synthesized by a simple one-step reaction, and used as fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents. Based on excited-state intramolecular proton transfer (ESIPT) process, HQCT and HQPH exhibited strong fluorescence emissions with large Stokes shifts in dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) solvents compared to other various organic solvents, and their fluorescence quenching and fluorescent color changes were obviously observed with increasing water content. The experimental results revealed that the hydroxyl groups substituted at the 8-position of HQCT and HQPH played a key role in the fluorescence emission processes. Dynamic light scattering (DLS) and ¹H NMR titration indicated that the sensing mechanism for the detection of water was based on inhibition of the ESIPT by H₂O via forming hydrogen bonds. In the range of 0.0–1.8 wt%, the fluorescence intensity of chemosensors changed as a linear function of water content. The detection limits of water in DMSO by HQCT and HQPH were as low as 0.0220 wt% and 0.0274 wt%, respectively. Moreover, HQCT and HQPH are successfully applied for the detection of moisture content in real commercial organic solvents.

Full text of DOI:10.1016/j.saa.2019.117956

Journal Pre-proof
Two 8-hydroxyquinoline-based fluorescent chemosensors for
ultra-fast and sensitive detection of water content in strong polar
organic solvents with large Stokes shifts
Jin-Tao Wang, Ying-Ying Pei, Shang-Feng Ren, Ming-Yue Yan,
Wen Luo, Bo Zhang, Qing-Feng Li
PII:
S1386-1425(19)31347-2
https://doi.org/10.1016/j.saa.2019.117956
SAA 117956
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 21 October 2019
Revised date:
11 December 2019
3 December 2019
Accepted
date:
1
Please cite this article as: J.-T. Wang, Y.-Y. Pei, S.-F. Ren, et al., Two 8-hydroxyquinoline-
based fluorescent chemosensors for ultra-fast and sensitive detection of water content
in strong polar organic solvents with large Stokes shifts, Spectrochimica Acta Part A:
Molecular
and
Biomolecular
Spectroscopy(2019),
https://doi.org/10.1016/
j.saa.2019.117956
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Two 8-hydroxyquinoline-based fluorescent chemosensors for
ultra-fast and sensitive detection of water content in strong polar
organic solvents with large Stokes shifts
1
,2
*1,2
1,2
1,2
1,2
Jin-Tao Wang , Ying-Ying Pei , Shang-Feng Ren , Ming-Yue Yan , Wen Luo ,
1
,2
*1,2
Bo Zhang , Qing-Feng Li
1
Henan Key Laboratory of Rare Earth Functional Materials; The Key Laboratory of
Rare Earth Functional Materials and Applications, Zhoukou Normal University,
Zhoukou, 466001, Henan, P. R. China.
2
International Joint Research Laboratory for Biomedical Nanomaterials of Henan,
Zhoukou Normal University, Zhoukou, 466001, Henan, P. R. China.
*
Corresponding author
E-mail: peiyy0329@163.com (Y. Y. Pei); liqingf335@163.com (Q. F. Li).
1

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Abstract
It is of great significance to detect the moisture in organic solvents before used in
water-sensitive reactions. Herein, two Schiff base quinoline derivatives,
8
8
-hydroxyquinoline-2-carboxaldehyde
thiosemicarbazone
(HQCT)
and
-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine (HQPH),
were designed and synthesized by a simple one-step reaction, and used as fluorescent
chemosensors for ultra-fast and sensitive detection of water content in strong polar
organic solvents. Based on excited-state intramolecular proton transfer (ESIPT)
process, HQCT and HQPH exhibited strong fluorescence emissions with large Stokes
shifts in dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) solvents
compared to other various organic solvents, and their fluorescence quenching and
fluorescent color changes were obviously observed with increasing water content. The
experimental results revealed that the hydroxyl groups substituted at the 8-position of
HQCT and HQPH played a key role in the fluorescence emission processes. Dynamic
1
light scattering (DLS) and H-NMR titration indicated that the sensing mechanism for
the detection of water was based on inhibition of the ESIPT by H O via forming
2
hydrogen bonds. In the range of 0.0-1.8 wt %, the fluorescence intensity of
chemosensors changed as a linear function of water content. The detection limits of
water in DMSO by HQCT and HQPH were as low as 0.0220 wt % and 0.0274 wt %,
respectively. Moreover, HQCT and HQPH are successfully applied for the detection
of moisture content in real commercial organic solvents.
2

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Keywords: Fluorescent sensor; 8-hydroxyquinoline; water detection; large Stokes
shift
1
. Introduction
Water is of crucial importance for daily human life, but is considered as one of the
most common impurities in organic solvents. For instance, the presence of trace water
in organometallic reactions can decrease the reactivity of organometallic compounds
and lower the yields or even bring about catastrophic events such as fire and
explosion [1]. Therefore, sensitive and selective detection of water in organic solvents
is of considerable significance in chemical reactions and industrial production
processes. At present, Karl-Fischer method is a traditional analytical technique, and is
frequently used for quantitative measurement of water content in organic solvents
because of its absolute measurement, high sensitivity, wide applicability, and large
dynamic range [2]. However, there are some shortcomings of requiring specialized
instruments and skilled personnel, being time-consuming, using toxic reagents (e.g., I2,
and SO ) and being unsuitable for redox-active samples [3].
2
Up to now, a great number of fluorescent sensors with outstanding advantages,
such as operational simplicity, in-situ monitoring, rapid response and low cost, have
been reported. The sensing mechanisms are mainly based on intramolecular charge
transfer (ICT) [4-9], photo-induced electron transfer (PET) [10,11], solvatochromism
[
12-15], aggregation induced emission (AIE) [16,17] and excited state intramolecular
proton transfer (ESIPT) [18,19]. In particular, ESIPT has attracted much attention for
3

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its applications in chemosensors, biological imaging, luminescent materials and
molecular logic gates, etc. [20-23] Generally, molecules can exhibit ESIPT
fluorescence if their structures contain a hydrogen bond donor (-OH or NH ) and a
2
hydrogen bond acceptor (=N- or C=O) which can form an intramolecular hydrogen
bond [24]. Large Stokes shift is one of the most significant photophysical properties
of ESIPT-based chromophores compared to traditional fluorophores such as
fluorescein, rhodamine, coumarin or BODIPY. It is an ideal feature for fluorophores
due to the avoidance of self-absorption or inner filter effect, and the fluorescence
analysis can be effectively improved by using this kind of fluorophores [25].
Although a large number of organic dye-based fluorescent chemosensors have been
extensively developed for the determination of water content in organic solvents, most
of the solvents tested are either non-polar or weakly polar organic solvents. Compared
to other reported literatures (Table S1), few chemosensors for detection of trace water
in strong polar organic solvents such as DMSO have been reported so far, and those
with large Stokes shifts are even fewer.
As an ideal fluorophore and binding moiety, 8-hydroxyquinoline is one of the most
useful chelating agents. Thus, various 8-hydroxyquinoline-based chemosensors have
been prepared for the detection of metal cations, anions and pH [26-31]. To the best
of our knowledge, only one report has been published regarding a
8
-hydroxyquinoline-based fluorescent chemosensor for determination of water
content in organic solvents such as acetonitrile, THF and dioxane, but stronger polar
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solvents are not involved [32]. In this work, we synthesized two
8
-hydroxyquinoline-based fluorescent chemosensors (HQCT and HQPH) for the
detection of water content in strong polar organic solvents, (especially in DMSO and
DMF), via an ESIPT mechanism with large Stokes shifts (≥ 128 nm), where the
8
-hydroxyquinoline moiety acted as an ESIPT-based fluorophore. It is found that the
water-sensing behaviors of HQCT and HQPH were ultra-fast and sensitive, with
lower detection limits of 0.0220 wt % and 0.0274 wt % in DMSO, respectively. Based
1
on H-NMR titration and dynamic light scattering experiments, a plausible sensing
mechanism was proposed. Finally, we successfully performed detection of water
content in real samples with the chemosensors.
2
2
. Experimental Section
.1. Materials and methods
Unless otherwise stated, all chemicals and reagents were purchased from
commercial suppliers (Heowns, Aldrich, Energy Chemical) and used without further
purification. All solvents (i.e., DMSO, DMF, acetonitrile, tetrahydrofuran,
dichloromethane, ethanol, ethyl acetate and 1,4-dioxane) for spectral analysis were
dried by standard methods before using. Ultrapure water from a Millipore Milli-Q
1
13
purification system was used for the titration. H and C-NMR spectra were collected
on a Bruker Avance III 600 MHz instrument. Mass spectrometry was recorded on an
Agilent 6520 Q-TOF mass spectrometer. UV-vis spectra were collected on a Lambda
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3
5 (PerkinElmer) spectrometer using a quartz cell of 1 cm path length. The
fluorescence spectra were recorded on an Edinburgh FLS920 spectrometer.
2
.2. Synthesis and characterization of HQCT and HQPH
-Hydroxyquinoline-2-carboxaldehyde (302.2 mg,
8
1.75
mmol)
and
thiosemicarbazide (192.5 mg, 2.11 mmol) were dissolved in 20 mL of absolute
ethanol, and the solution was heated under reflux for 5 h. After cooling to room
temperature, the resulting precipitate was collected by filtration and washed three
°
times with cold absolute ethanol; then, the product was dried under vacuum at 45 C
°
for 48 h to produce 0.34 g of HQCT (79% yield) as pale yellow powder. m.p. 249 C.
1
H NMR (600 MHz, DMSO-d , Fig. S1), δ (ppm): 11.81 (s, 1H, -NH-), 9.81 (s, 1H,
6
-
OH), 8.39 (d, 1H, ArH), 8.38 (s, 1H, -NH ), 8.28 (s, 1H, -NH ), 8.23 (d, 1H, ArH),
2 2
1
3
8
.22 (s, 1H, -CH=N), 7.39 (t, 1H, ArH), 7.33 (d, 1H, ArH), 7.05 (d, 1H, ArH). C
NMR (150 MHz, DMSO-d , Fig. S2), δ (ppm): 179.30, 153.89, 152.36, 142.97,
6
1
38.71, 136.66, 129.29, 128.59, 118.99, 118.28, 112.54. MS (ESI, Fig. S3): m/z
+
[
M+H] calcd. for C H N OS: 247.06, found: 247.04.
1
1
11
4
8
-Hydroxyquinoline-2-carboxaldehyde (302.2 mg, 1.75 mmol) and 2-picolinyl
hydrazide (263.2 mg, 1.92 mmol) were dissolved in 20 mL of absolute ethanol, and
the solution was heated to reflux and stirred for 4 h. After cooling to room
temperature, the resulting precipitate was collected by filtration and washed three
°
times with cold absolute ethanol; then, the product was dried under vacuum at 45 C
°
1
for 48 h to produce 0.42 g of HQPH (82% yield) as white solid. m.p. 226 C. H NMR
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(
600 MHz, DMSO-d , Fig. S4), δ (ppm): 12.69 (s, 1H, -NH-), 9.88 (s, 1H, -OH), 8.83
6
(s, 1H, -CH=N), 8.77 (d, 1H, ArH), 8.35 (d, 1H, ArH), 8.17 (d, 1H, ArH), 8.13 (d, 1H,
ArH), 8.09 (t, 1H, ArH), 7.71 (t, 1H, ArH), 7.47 (t, 1H, ArH), 7.42 (d, 1H, ArH), 7.14
1
3
(
d, 1H, ArH). C NMR (150 MHz, DMSO-d , Fig. S5), δ (ppm): 161.57, 153.97,
6
1
1
2
52.49, 150.12, 149.67, 149.10, 138.79, 138.53, 137.06, 129.44, 128.80, 127.64,
+
23.46, 118.41, 112.69. MS (ESI, Fig. S6): m/z [M+H] calcd. for C H N O :
1
6
13
4
2
93.10, found: 293.10.
2
.3. The detection limit
The detection limit was calculated based on the following equation: DL = 3σ/|k|,
where σ is the standard deviation of the blank measurements and k is the slope
between fluorescence intensity versus water content.
3
3
. Results and discussion
.1. Synthesis and characterizations
Scheme 1. Synthetic routes of HQCT and HQPH.
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The synthetic routes of HQCT and HQPH are shown in Scheme 1. According to the
literatures [33-36], HQCT and HQPH were successfully prepared by a simple
condensation
reaction
of
8-hydroxyquinoline-2-carboxaldehyde
with
thiosemicarbazide or 2-picolinyl hydrazide in absolute ethanol under reflux conditions.
1
13
The structures of HQCT and HQPH were confirmed by H NMR, C NMR, and mass
spectroscopy (MS) (see supporting information, Fig. S1-S6). The spectral data are in
good agreement with the expected structures.
3
.2. Spectral characteristics in different solvents
Fig. 1. (a) Fluorescence spectra of HQCT (20 μM) in different solvents, λ = 350 nm.
ex
(
b) Fluorescence spectra of HQPH (20 μM) in different solvents, λ = 340 nm.
ex
Photographs of HQCT (c) and HQPH (d) in different solvents under a 365 nm UV
lamp.
With the chemosensors in hand, the fluorescence and absorption spectra of HQCT
and HQPH at a concentration of 20 μM were investigated in a variety of organic
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solvents including dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate
(
EA), ethanol (EtOH), dioxane (Dio), acetonitrile (ACN), N, N-dimethylformamide
DMF) and dimethyl sulfoxide (DMSO). The absorption spectra of HQCT and HQPH
(
in different pure organic solvents were first investigated. As can be seen from Fig.
S13, HQCT and HQPH displayed two major bands in the range of 290-310 nm and
3
30-360 nm, which can be attributed to π-π* and n-π* transitions [26].
Moreover, studies on the fluorescence of HQCT and HQPH in different organic
solvents were also performed, the corresponding photophysical data are summarized
in Table S2. The fluorescence spectra of HQCT showed a major emission peak
around 480 nm with a shoulder peak at 445 nm (Fig. 1a). On the other hand, the
HQPH exhibited a maximum emission peak around 486 nm with a shoulder peak at
4
50 nm (Fig. 1b). The emission peaks at 445 nm and 450 nm are assigned to the enol
form, and the emission bands at 480 nm and 486 nm are attributed to the keto
tautomer, produced by the ESIPT process [24]. The Stokes shifts of HQCT and
HQPH in DMSO are 130 and 146 nm, respectively. The above large Stokes shifts can
be attributed to the ESIPT [25]. Noticeably, there are slight changes of the absorption
spectra in all the evaluated solvents. In contrast, the type of solvent has a significant
effect on the fluorescence emission intensity of HQCT and HQPH. As shown in Fig.
1
a and 1b, as the polarity of solvents increased from DCM to DMSO, the emission
intensity changed significantly. For instance, the HQCT and HQPH in strong polar
organic solvents such as DMSO and DMF exhibited strong fluorescence emissions. In
contrast, in weakly polar organic solvents such as DCM, THF, EA and Dio, the
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fluorescence intensity was very weak, which may be attributed to the formation of
hydrogen bonds between high electronegativity oxygen atoms in solvent molecules
and -OH in HQCT or HQPH, resulting in inhibition of the ESIPT process. Fig. 1c and
1
d show the pictures of HQCT and HQPH in different organic solvents under
illumination of 365 nm UV light. Obviously, the HQCT and HQPH in DMSO or
DMF displayed strong blue-green fluorescence, but caused weak or no fluorescence in
DCM, THF, EA and Dio under identical conditions. These experimental results are
good consistent with the spectral data.
3
.3. The fluorescence response to water content
It is found that the HQCT and HQPH emitted bright blue-green fluorescence in
strong polar organic solvents, especially in DMSO and DMF, while their fluorescence
was strongly quenched by the addition of water. Therefore, this phenomenon inspires
us to detect water content in strong polar water-miscible organic solvents using
HQCT or HQPH as a fluorescence sensor.
Fig. 2. (a) Fluorescence intensities at 480 nm of HQCT (20 μM) after addition of 0,
0
.2, 5, 15 wt % water to DMSO as a function of time. (b) Fluorescence intensities at
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4
86 nm of HQPH (20 μM) after addition of 0, 0.2, 5, 20 wt % water to DMSO as a
function of time.
Firstly, DMSO is one of the most commonly used aprotic organic solvents in
organic synthesis with strong polarity, which is often used as a solvent to dissolve
water-insoluble compounds. DMSO has a very strong affinity for water. Once
exposed to air, pure DMSO will soon be diluted with water [37]. To evaluate the
practicality of HQCT and HQPH for water sensing, a variety of experiments were
carried out to measure the response time. As shown in Fig. 2, upon adding a certain
amount of H O to DMSO, the fluorescence intensity of HQCT and HQPH decreased
2
immediately in less than 5 s and remained stable over a period of 15 min, indicating
that both HQCT and HQPH show a fast response toward water and can be used for
practical applications.
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Fig. 3. Fluorescence spectra (a) and changes in fluorescence intensity at 480 nm (b) of
HQCT (20 μM) in DMSO with increasing water content, λex = 350 nm. Inset:
photographs of HQCT in DMSO with increasing water content under a 365 nm lamp.
Fluorescence spectra (c) and changes in fluorescence intensity at 486 nm (d) of
HQPH (20 μM) in DMSO with increasing water content, λex = 340 nm. Inset:
photographs of HQPH in DMSO with increasing water content under a 365 nm lamp.
Fig. 3a and 3c display the fluorescence responses of HQCT and HQPH with the
addition of water to anhydrous DMSO. With the increase of water content (0-15 wt%
for HQCT and 0-10 wt% for HQPH, respectively), the fluorescence intensity of
chemosensors (20 μM) was drastically decreased, and then decreased slightly with
further increase of water content (15-52 wt% for HQCT and 10-42 wt% for HQPH,
respectively). In addition, with gradual increase of water content in DMSO, obvious
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color changes from intense blue-green to weak yellow-green, to colorless were easily
observed by the naked eye under UV 365 nm excitation (insets in Fig. 3b and 3d),
suggesting that HQCT and HQPH can be used for qualitative analysis of water
content in organic solvents. Moreover, as seen from Fig. S14, with increasing water
content in DMSO, the normalized fluorescence emission peak showed a slight
bathochromic shift from 480 to 502 nm for HQCT, from 486 to 498 nm for HQPH,
respectively. The red-shift of emission peak can be attributed to the solvatochromic
emission property of chemosensors in DMSO/H O mixtures, which is induced by a
2
progressively increasing portion of the more polar water [38]. These results are in
accordance with the photos taken under a 365 nm UV lamp (insets in Fig. 3b and 3d).
Fig. 4. Fluorescence spectra (a) and changes in fluorescence intensity at 478 nm (b) of
HQCT (20 μM) in DMF with increasing water content, λ = 350 nm. Inset:
ex
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photographs of HQCT in DMF with increasing water content under a 365 nm lamp.
Fluorescence spectra (c) and changes in fluorescence intensity at 484 nm (d) of
HQPH (20 μM) in DMF with increasing water content, λex = 340 nm. Inset:
photographs of HQPH in DMF with increasing water content under a 365 nm lamp.
DMF is not only an excellent polar solvent for various compounds, but also can
react as a chemical reagent [39]. Hence, we further tested the fluorescence emission
behaviors of HQCT and HQPH in DMF. Fig. 4 shows the fluorescence changes of
HQCT and HQPH after adding different amounts of water to anhydrous DMF. It is
clear that the maximum emission of HQCT and HQPH in DMF is more sensitive to a
low-level water content. As the water content increased from 0 to 5 wt %, the
fluorescence intensity of HQCT and HQPH in DMF was reduced by 55% and 70%,
respectively, compared to 47% and 58% decreased in DMSO, respectively. In
addition to this, other spectral behaviors including a decrease in fluorescence intensity
(Fig. 4a and 4c) and a slight bathochromic shift with increasing water content in DMF
were similar to those observed in DMSO. In detail, as the water content increased
from 0 to 46 wt %, the emission peak of HQCT at 478 nm was gradually red shifted
to 498 nm (Fig. S15a), while the emission peak of HQPH at 484 nm was
progressively red shifted to 494 nm with increasing water content from 0 to 39 wt %
(Fig. S15b). Furthermore, as shown in the insets of Fig. 4b and 4d, HQCT and HQPH
exhibited apparent fluorescent color variations from intense blue-green to weak
yellow-green, to colorless under UV irradiation (365 nm). Finally, we evaluated the
fluorescence responses of chemosensors in dry ACN and EtOH. As shown in Fig. S16
and S17, the changes in fluorescence intensity of HQCT and HQPH in response to
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increasing water content in ACN and EtOH solvents followed a similar trend with that
in DMSO and DMF under the same conditions, but with relatively weaker
fluorescence intensity and lower fluorescence quenching percentage.
Fig. 5. Plot of the fluorescence intensity of HQCT (20 μM) vs. water content in
DMSO (a) and DMF (b). Plot of the fluorescence intensity of HQPH (20 μM) vs.
water content in DMSO (c) and DMF (d).
In order to assess the possibility of quantitative detection of trace water, we
investigated the sensing performance of chemosensors in different dry organic
solvents (i.e., DMSO, DMF, ACN and EtOH). Fig. 5 and S18 show that the plots of
the fluorescence intensity of HQCT and HQPH versus the water content (wt %)
exhibit good linearity over a range of different water contents, with a correlation
2
coefficient (R ) of 0.997, 0.996, 0.995, 0.996, 0.998, 0.996, 0.993 and 0.994 for
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HQCT/DMSO, HQCT/DMF, HQCT/ACN,
HQCT/EtOH, HQPH/DMSO,
HQPH/DMF, HQPH/ACN and HQPH/EtOH systems, respectively. Furthermore,
detection limits were calculated based on the 3σ/|k| method. The detection limits
(DLs) of HQCT for water content were calculated to be 0.0220 wt %, 0.0246 wt %,
0
.1404 wt %, and 0.1513 wt % in DMSO, DMF, ACN, and EtOH, respectively. While
the DL (wt %) values of HQPH for water content in DMSO, DMF, ACN, and EtOH
were determined to be 0.0274, 0.0461, 0.0221, and 0.0595, respectively. It is
noteworthy that the water-sensing performance of HQCT and HQPH slightly depends
on the examined solvent. HQCT displayed the highest sensitivity toward water in
DMSO, followed by DMF, ACN, and EtOH. However, the lowest detection limit for
HQPH was achieved in ACN, followed by DMSO, DMF, and EtOH. Compared to
previously reported chemosensors based on organic dyes (Table S1), the lower
detection limits of HQCT and HQPH for water content in DMSO are observed. These
results indicate that HQCT and HQPH can be used as fluorescent sensors to quantify
and accurately analyze the water content of some organic solvents, especially in
strong polar organic solvents such as DMSO and DMF.
3
.4. The detection mechanism toward water
In order to find out the cause of water-induced fluorescence quenching in organic
solvents, the aggregation states of HQCT and HQPH were first investigated by
dynamic light scattering (DLS). As shown in Fig. S19, when adding 6 wt % water to
HQCT or 35 wt % water to HQPH in DMSO, the hydrodynamic diameter was below
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2
nm, implying no aggregation formation in the solution. In contrast, significant
fluorescence quenching of chemosensors was observed under the same conditions.
The above results indicate that there is no direct causal relationship between
fluorescence quenching and aggregation, so the aggregation-caused quenching (ACQ)
can be ruled out.
1
Fig. 6. Partial H-NMR spectra of HQCT (a) and HQPH (b) in DMSO-d containing
6
different amounts of D O.
2
1
To further elucidate the mechanism for the detection of water, H NMR spectral
measurements of HQCT and HQPH in DMSO-d /D O mixtures at different contents
6
2
1
of D O were performed. As shown in Fig. S1, the H NMR spectrum of HQCT in
2
DMSO-d without adding D O were in good consistent with its structure. Upon
6
2
addition of D O, the signals at 11.81 ppm (the peak h), 9.81 ppm (the peak g), 8.38
2
ppm (the peak i) and 8.28 ppm (the peak i) representing the protons of -OH, -NH-,
and -NH disappeared completely owing to fast hydrogen/deuterium exchange (Fig.
2
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S20a) [40]. At the same time, in a magnified view (Fig. 6a), most of the proton signals
were shifted to the high field with increasing water content, especially the peaks a, b
and c from the quinoline ring and -CH=N-, indicating that the chemical environment
1
of protons has changed. Similar results were also observed in the H NMR titration
experiments of HQPH in DMSO-d with D O (Fig. 6b and S20b). The only difference
6
2
was that the proton signals (the peaks i, j, k and l) on the pyridine ring also moved to
the high field, which means that the nitrogen atom on the pyridine ring are also
involved in the interaction with water.
In addition to the above phenomena, we also note that the hydroxyl groups
substituted at the 8-position of HQCT and HQPH are likely to play an important role
in the fluorescence emission processes. Therefore, two control compounds (QCT and
QPH) without hydroxyl groups substituted at the 8-position were designed to confirm
this hypothesis. The synthetic routes of QCT and QPH are shown in Scheme S1.
Detailed synthesis procedures can be found in the supporting information. The
1
13
chemical structures of QCT and QPH were well characterized using H NMR, C
NMR and mass spectroscopy (MS) (Fig. S7-S12). As expected, QCT and QPH had no
fluorescence emission in DMSO, while HQCT and HQPH exhibited strong
fluorescence (Fig. S21).
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Scheme 2. Proposed detection mechanisms of HQCT (a) and HQPH (b) toward H O.
2
Based on the above experimental results, the sensing mechanisms of HQCT and
HQPH for the detection of water in DMSO are proposed as depicted in Scheme 2.
When trace amounts of water are added to HQCT/DMSO or HQPH/DMSO solution,
the intramolecular hydrogen bonding (-OH---N-) needed for the ESIPT process is
inhibited by H O via forming hydrogen bonds with -OH and N atoms, which can
2
result in fluorescence “turn off”.
3
.5. Interference studies from pH and metal ions
The influence of pH value of water in DMSO and DMF on fluorescence emission
intensity of HQCT and HQPH was investigated with a wide range of pH (pH
.0-14.0). As shown in Fig. 7, after adding water (5 wt %) with different pH values,
1
the fluorescence intensity of HQCT and HQPH in DMSO and DMF was quite stable
below pH 10.0, which indicated a wide range of practical usability of HQCT and
HQPH in sensing water. When the pH value was raised above 10.0, the fluorescence
intensity of HQCT and HQPH decreased sharply. This is likely because the hydroxyl
1
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group of HQCT and HQPH was deprotonated under high alkaline conditions,
resulting in the disruption of the ESIPT process.
Fig. 7. Fluorescence intensity changes of HQCT (20 μM) (a) and HQPH (20 μM) (b)
in DMSO and DMF after adding water (5 wt %) with different pH values.
The practicability of HQCT and HQPH for detecting water in organic solvent was
2
+
verified by fluorescence interference experiment, possible interference ions (Zn ,
2
+
2+
2+
2+
2+
2+
3+
2+
2+
+
+
+
3+
3+
2+
Cd , Mg , Co , Cu , Ni , Fe , Fe , Mn , Pb , K , Li , Na , Al , Cr , Hg )
were tested. First, the metal ions were separately added to the system containing only
HQCT or HQPH, and the fluorescence emission spectral data were collected after
being mixed. Then the water was added to the above system, and the fluorescence
spectrum was taken. As shown in Fig. 8, the results showed that the water-sensing
behaviors of HQCT and HQPH in DMSO were not obviously interfered by the
2
+
2+
coexistent metal ions (1.0 equiv.), except for Cu . The interference of Cu may be
related to its strong fluorescence quenching characteristics [41].
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Fig. 8. The fluorescent intensity of HQCT (20 μM) (a) and HQPH (20 μM) (b) in
DMSO with 1.0 equiv. of various metal ions, and then added 5 wt % water.
3
.6. Fluorescence detection of water content in real samples
Table 1. Determination of water content in real organic solvent samples.
Chemosensors
Samples
Water content (wt %)
Added
wt %)
Found (wt %)
Recovery (%)
(
HQCT
DMSO
DMF
ACN
≤ 0.005
≤ 0.005
≤ 0.005
≤ 0.005
≤ 0.005
≤ 0.005
≤ 0.005
≤ 0.005
0.230
0.320
0.750
0.750
0.900
0.500
0.370
0.750
0.237 ± 0.012
0.316 ± 0.023
0.730 ± 0.041
0.712 ± 0.033
0.885 ± 0.016
0.524 ± 0.029
0.365 ± 0.021
0.727 ± 0.037
103
99
97
EtOH
DMSO
DMF
ACN
95
HQPH
98
105
99
EtOH
97
2
1

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To explore the practical application of chemosensors to measure trace water in real
samples, four commercial solvents (DMSO, DMF, ACN and EtOH. Extra dry, with
molecular sieves, water ≤ 50 ppm) purchased from Energy Chemical were employed
to measure the recovery of water using standard addition method. The values of water
content derived from calibration equation and the recoveries were summarized in
Table 1. Seen from Table 1, the reasonable spiked recoveries were obtained both for
HQCT and HQPH, indicating that the present method can be used in real organic
solvent samples.
4
. Conclusions
In summary, two 8-hydroxyquinoline-based fluorescent chemosensors (HQCT and
HQPH) were designed and successfully synthesized. The fluorescence emission
intensity of chemosensors strongly depends on the solvent polarity. Specifically, the
chemosensors in strong polar organic solvents such as DMSO and DMF exhibited
strong fluorescence emission with large Stokes shifts, while displaying weak or no
fluorescence emission in weakly polar organic solvents. Furthermore, as the water
content increased in four water-miscible organic solvents, the pronounced
fluorescence quenching and color changes could be observed due to the inhibition of
ESIPT process. In addition, the low limit of detection allows HQCT and HQPH to be
utilized as ultra-sensitive water chemosensors for the quantitative detection of
low-level water in organic solvents (DMSO, DMF, ACN, and EtOH). We expect that
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the work in this paper will expand the scope of research and application of
water-sensitive sensors in organic solvents.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of
China (Grant No. 21805319 and 21503285), Key Research Projects of Henan Higher
Education Institutions (Grant No. 19A150050) and High-level Talent Research
Funding Projects of Zhoukou Normal University (Grant No. ZKNUC2017045).
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