N-hydroxypropyl substituted 4-hydroxynaphthalimide: Differentiation of solvents and discriminative determination of water in organic solvents

Article abstract of DOI:10.1016/j.saa.2021.119559

A naphthalimide-based fluorophore (HONIOH) was designed by introducing a hydroxy unit into the 4th position of the aromatic core and a hydroxypropyl unit into the N-imide site. Photophysical properties of HONIOH were highly dependent on solvents, which was ascribed to the excited state proton transfer (ESPT) coupled with intramolecular charge transfer (ICT) mechanism. Further studies demonstrated that HONIOH can be used to recognize N, N-dimethylformamide (DMF) qualitatively and differentiate methanol from ethanol. Three control compounds were synthesized, their photophysical properties were investigated in various solvents, and experimental results revealed that hydroxyl and hydroxypropyl units contribute to the solvents differentiation ability of HONIOH. In addition, HONIOH was successfully applied as a colorimetric, fluorescent probe for the discriminative detection of trace water in organic solvents, such as fluorescence turn-on response accompanied by fluorescent color changes from light yellow to purple in DMF, and fluorescence turn-off response and blue to yellow fluorescent color changes in acetonitrile, tetrahydrofuran, and acetone. We believe that N-substituted 4-hydroxynaphthalimide derivatives may find widespread applications in chemical and biochemical sensing and imaging.

Full text of DOI:10.1016/j.saa.2021.119559

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
N-hydroxypropyl substituted 4-hydroxynaphthalimide: Differentiation
of solvents and discriminative determination of water in organic
solvents
⇑
Jing Huang, Yuehui Liang, Hai-Bo Liu, Xiangmin Zhang, Jing Wang
School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
N-hydroxypropyl and 4-hydroxy incorporated 1,8-naphthalimide fluorescent probe, HONIOH, can be uti-
lized to differentiate organic solvents and discriminatively detect trace water in organic solvents. Three
model compounds, namely, PNI, HONI, and PNIOH were also investigated in order to demonstrate the
functions of N-hydroxypropyl and 4-hydroxy groups.
ꢀ
ꢀ
ꢀ
ꢀ
4-hydroxynaphthalimide derivative
was developed as colorimetric and
fluorescent probe.
The probe can be used to recognize
DMF and differentiate methanol from
ethanol.
Distinct colors changes were
observed in DMF and other organic
solvents.
The probe was successfully applied
for the discriminative detection of
trace water.
a r t i c l e i n f o
a b s t r a c t
Article history:
A naphthalimide-based fluorophore (HONIOH) was designed by introducing a hydroxy unit into the 4th
position of the aromatic core and a hydroxypropyl unit into the N-imide site. Photophysical properties of
HONIOH were highly dependent on solvents, which was ascribed to the excited state proton transfer
(ESPT) coupled with intramolecular charge transfer (ICT) mechanism. Further studies demonstrated that
HONIOH can be used to recognize N, N-dimethylformamide (DMF) qualitatively and differentiate metha-
nol from ethanol. Three control compounds were synthesized, their photophysical properties were inves-
tigated in various solvents, and experimental results revealed that hydroxyl and hydroxypropyl units
contribute to the solvents differentiation ability of HONIOH. In addition, HONIOH was successfully
applied as a colorimetric, fluorescent probe for the discriminative detection of trace water in organic sol-
vents, such as fluorescence turn-on response accompanied by fluorescent color changes from light yellow
to purple in DMF, and fluorescence turn-off response and blue to yellow fluorescent color changes in ace-
tonitrile, tetrahydrofuran, and acetone. We believe that N-substituted 4-hydroxynaphthalimide deriva-
tives may find widespread applications in chemical and biochemical sensing and imaging.
Ó 2021 Elsevier B.V. All rights reserved.
Received 2 December 2020
Received in revised form 17 January 2021
Accepted 27 January 2021
Available online 6 February 2021
Keywords:
Fluorescent probe
Naphthalimide
Trace water
Discriminative detection
Colorimetric
⇑
Corresponding author.
E-mail address: wjwyj82@gxu.edu.cn (J. Wang).
https://doi.org/10.1016/j.saa.2021.119559
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

J. Huang, Y. Liang, Hai-Bo Liu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
1. Introduction
Water (H2O) is the most important component of organisms and
a vital resource for all life. However, even trace amounts of H2O are
detrimental and dangerous in the chemical industry, food process-
ing, and many other fields where H2O is not needed [1–2]. There-
fore, the development of simple, fast, and sensitive detection
methods for trace water is of great demand in these fields. In recent
years, considerable effort has been devoted to developing fluores-
cent probes for water detection due to the apparent advantages of
the fluorescence method, including simple fabrication, easy opera-
tion, fast response, high selectivity, and sensitivity [2–6].
Until now, various fluorophores have been employed as the sig-
naling unit in designing fluorescent probes [2–6], and naphthalim-
ide (NI) fluorophore is one of the most promising candidates due to
its easy functionalization and tunable photophysical properties [7–
10]. The introduction of electron-donating or -withdrawing groups
at the N-imide site and the 4th position of the aromatic core of the
NI unit is the most common design approach [7–10]. NIs with
donor groups attached to the 4th position of the aromatic core,
which result in the characteristic intramolecular charge transfer
(ICT)-based structures, contribute substantially to diverse chemical
and biological sensing applications [7–16].
In the past 10 years, fluorophore 4-hydroxy(OH)-1,8-NI was fre-
quently employed as the fluorescent reporter by taking advantage
of its excellent ICT fluorescence properties of 4-OH-1,8-NI in alka-
line environment, such as phosphate or HEPES buffer at pH 7.4,
which emits green fluorescence due to the deprotonation of the
OH group [17–20]. For example, by the 4-position substitution,
4-OH-1,8-NI-derived ethers [21–28] and esters [29–37] have been
extensively developed as fluorescent probes (chemodosimeters)
for tracing various analytes via the analyte-triggered ether or ester
bond cleavage mechanism. Based on this mechanism, the inhibited
ICT process of ethers and esters is effectively recovered by releas-
ing N-substituted 4-OH-1,8-NI in the anionic forms, thus realizing
the successful fluorescence determination of the analytes [21–37].
Photophysical properties of the ICT-type structure are highly sol-
vent dependent and can be used for the determination of water
content [2,9,38–40]. Moreover, 4-OH-1,8-NI can exhibit excited-
state proton transfer (ESPT) because its structure contains a hydro-
gen bond donor (–OH) and a hydrogen bond acceptor (C=O) [18].
Photophysical properties of ESPT-type structure are also sensitive
to the surrounding medium such as the solvent type [18,41–42].
However, to the best of our knowledge, only limited reports on flu-
orescent probes that derived N-substituted 4-OH-1,8-NI have been
used for the discrimination of solvents as well as determination of
water content [43].
Inspired by the investigations described above, we designed an
N-hydroxypropyl and 4-hydroxy incorporated 1,8-naphthalimide
fluorescent probe, HONIOH, which can be utilized to differentiate
organic solvents and monitor H2O content in organic solvents.
Three model compounds, namely, PNI, HONI, and PNIOH were also
synthesized for comparison (Fig. 1) to examine the effects of N-
hydroxypropyl and 4-hydroxy on solvent differentiation ability.
HONIOH possesses several attractive features, such as qualita-
tive recognition of N, N-dimethylformamide (DMF), discrimination
between methanol (MeOH) and ethanol (EtOH), and water-induced
distinguishable fluorescent responses in organic solvents
(Table S1).
Fig. 1. Molecular structures and solid-state photographs of HONIOH, PNI, HONI,
and PNIOH under day light and 365 nm UV light.
methane (CH2Cl2), chloroform (CHCl3), N, N-dimethylformamide
(DMF), dimethyl sulfoxide (DMSO), methanol (MeOH), ethyl acet-
ate (EA) and tetrahydrofuran (THF) were distilled prior to the mea-
surements. All other reagents were used without further
purification. 1H NMR and 13C NMR spectra were recorded on the
Bruker AVANCE III HD600 spectrometer in DMSO d6 (chemical
shifts of 2.50 ppm (1H) and 39.52 ppm (13C)). High Resolution mass
spectra were recorded using a Thermo Fisher LCQ Fleet spectrom-
eter. UV–visible absorption spectra were collected using a UV 1901
spectrophotometer. Fluorescence spectra were collected with a
960 MC spectrophotometer.
2.2. Synthesis of 4-hydroxy-N-(3-hydroxypropyl)-1,8-naphthalimide
(HONIOH)
A
mixture of 4-bromo-1,8-naphthalic anhydride (l.050 g,
3.79 mmol) and 3-aminopropan-1-ol (0.3102 g, 4.13 mmol) were
dissolved in EtOH (25 mL) and then heated at 85 °C for 15 h. After
cooling the mixture, the precipitate was collected and washed with
H2O (3 ꢁ 15 mL) to give compound 1. Compound 1 (0.2000 g,
0.60 mmol) and K2CO3 (0.1650 g, 1.2 mmol) were dissolved
DMF/H2O (1:1, 12 mL) and then heated at 120 °C for 6 h. Upon
cooling down, the reaction mixture was diluted with H2O
(60 mL). Dichloroform (3 ꢁ 50 mL) was used to extract the aqueous
phase, sodium chloride solution (3 ꢁ 20 mL) was used to wash the
combined organic layer. After drying with anhydrous MgSO4 and
removing the solvent, HONIOH was obtained as a yellow powder
(0.142 g, 87.3%). 1H NMR: 1.77 (m, 2H), 3.49 (t, 2H), 4.08 (m,
2H), 4.50 (s, 1H), 7.16 (d, 1H), 7.77 (dd, 1H), 8.37 (d, 1H), 8.51
(ddd, 2H), 11.87 (s, 1H) (Fig. S1 in the ESIy); 13C NMR: 164.19,
163.53, 160.68, 134.01, 131.58, 129.64, 129.34, 126.08, 122.84,
122.32, 113.14, 110.42, 59.49, 39.49, 30.29 (Fig. S2 in the ESIy);
mass spectra (m/z): calculated for C15H13NO4 [M ꢂ H]ꢂ 270.27,
Found 270.14 (Fig. S3 in the ESIy).
2. Experimental
2.1. Materials and instruments
All chemical materials and solvents were purchased from com-
mercial suppliers, acetonitrile (AN), ethanol (EtOH), dichloro-
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J. Huang, Y. Liang, Hai-Bo Liu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
2.3. Synthesis of N-propyl-1,8-naphthalimide (PNI)
3. Results and discussion
A mixture of 1,8-naphthalic anhydride (0.3944 g, 2.0 mmol) and
aminopropane (0.3 mL, 3.6 mmol) were dissolved in EtOH (25 mL)
and then heated at 85 °C for 0.5 h. After cooling down, the precip-
itate was collected and washed with cold ethanol to give PNI as a
white powder (0.3648 g, 76%). 1H NMR: 0.94 (t, 3H), 1.67 (m, 2H),
4.03 (m, 2H), 7.89 (dd, 2H), 8.48 (dd, 2H), 8.52 (dd, 2H) (Fig. S4 in
the ESIy); mass spectra (m/z): calculated for C15H13NO2 [M+H]+
240.09, Found 240.18 (Fig. S5 in the ESIy).
The synthesis and characterization of naphthalimide-based
compounds HONIOH, PNI, HONI, and PNIOH are provided in the
experimental section and the Supporting Information (Figs. S1-
S10). In the solid state, HONIOH, PNI, HONI, and PNIOH showed
orange-yellow, white, light yellow, and yellow color under daylight
and green, blue, blue, and blue-green color when exposed to
365 nm UV light, respectively (Fig. 1).
3.1. Differentiation of solvents
2.4. Synthesis of 4-hydroxy-N-(3-hydroxypropyl)-naphthalimide
(HONI)
HONIOH is an NI derivative with N-hydroxypropyl and 4-
hydroxy substitutions. Photophysical properties of HONIOH dis-
persed in different solvents were investigated (Fig. 2, Fig. 3 and
Table S2). In AN, CH2Cl2, CHCl3, acetone, EA, toluene, and THF,
A mixture of 1,8-naphthalic anhydride (0.8043 g, 3.79 mmol)
and 3-aminopropan-1-ol (0.3102 g, 4.13 mmol) were dissolved in
EtOH (25 mL) and then heated at 85 °C for 15 h. After cooling
the mixture to room temperature, glacial water (50 mL) was added.
Then, the precipitate was collected and washed with H2O
(3 ꢁ 15 mL) to give HONI as a brown powder (0.76 g, 78.6%). 1H
NMR: 1.81 (m, 2H), 3.51 (td, 2H), 4.12 (m, 2H), 7.89 (dd, 2H),
8.50 (m, 4H) (Fig. S6 in the ESIy); 13C NMR: 163.86, 134.69,
131.72, 131.11, 127.76, 127.64, 122.53, 59.49, 38.04, 31.47
(Fig. S7 in the ESIy); mass spectra (m/z): calculated for C15H13NO3
[M + H]+ 256.27, Found 256.06 (Fig. S8 in the ESIy).
HONIOH (U = 0.67) showed blue emission color and one fluores-
cence emission band at around 436–453 nm (Fig. 2), and one
absorption band at around 365–370 nm (Fig. 3), indicating that
HONIOH exists as neutral forms in the excited state and the ground
state in these solvents [18–19]. In EtOH, DMF, DMSO, and MeOH,
HONIOH showed yellow or faint yellow emission color and two flu-
orescence emission bands at approximately 450 and 550 nm,
which are assigned to the excited neutral species and anionic spe-
cies, respectively, indicating that HONIOH undergoes ESPT actions
efficiently in EtOH, DMF, DMSO, and MeOH [41] (Fig. 2 and
Fig. S11). When the hydroxy group was replaced by a negative oxy-
gen moiety, HONIOH existed in the anionic species, which facili-
tated the ICT process owing to the stronger electron-donor
ability of the oxygen anions, with the appearance of fluorescence
emission peak centered at approximately 550 nm [17–37]. In
MeOH, neutral species and anionic species existed in the ground
state, and two absorption bands centered at 377 and 452 nm
(Fig. 3). In DMF, HONIOH predominately existed as anionic forms
with an absorption band of 480 nm in the ground state (Fig. 3)
due to the strong basicity of DMF [41]. In EtOH and DMSO, only
one absorption band appeared at 377 nm (Fig. 3), which is attrib-
uted to the neutral species [18]. In aqueous solution, HONIOH
2.5. Synthesis of 4-hydroxy-N-propyl-1,8-naphthalimide (PNIOH)
A mixture of 4-bromo-1,8-naphthalic anhydride (0.5541 g,
2.0 mmol) and aminopropane (0.2 mL, 3.0 mmol) were dissolved
in EtOH (25 mL) and then heated at 85 °C for 0.5 h. After cooling
the mixture, the precipitate was collected and washed with H2O
(3 ꢁ 15 mL) to obtain compound 2. Compound 2 (0.1900 g,
0.60 mmol) and K2CO3 (0.1650 g, 1.2 mmol) were dissolved in
DMF/H2O (1:1, 12 mL) and then heated at 120 °C for 6 h. Upon
cooling the reaction mixture, H2O (60 mL) was added. Ddichloro-
form (3 ꢁ 50 mL) was used to extract the aqueous phase, then
the organic layer was combined and washed with sodium chloride
solution (3 ꢁ 20 mL). After drying with anhydrous MgSO4, the sol-
vent was then removed under vacuum and PNIOH was obtained as
a yellow powder (0.142 g, 92.8%). 1H NMR: 0.92 (t, 3H), 1.69 (tq,
2H), 3.88 (t, 2H), 7.10 (dd, 1H), 7.38 (t, 1H), 8.02 (d, 1H), 8.14
(dd, 1H), 8.20 (ddd, 1H) (Fig. S9 in the ESIy); mass spectra (m/z):
calculated for C15H13NO3 [MꢂH]ꢂ 254.1, Found 254.22 (Fig. S10
in the ESIy).
2.6. Determination of the fluorescence quantum yields
The fluorescence quantum yields (
and PNIOH were determined in acetonitrile using rhodamine B as
standard ( = 0.89 in ethanol) [21]. All compounds were excited
U) of HONIOH, PNI, HONI,
U
at 360 nm and the area under each emission spectrum integrated.
The quantum yields for all compounds were obtained using the fol-
lowing equation (1) [35,44].
I1ABkexBgacetonitrile
IBA1kex1gethanol
U1
¼
UB
ð1Þ
U
is the quantum yield; I is the area under the emission spectra; A
is the absorbance at the excitation wavelength; kex is the excitation
wavelength; is the refractive index of the solution; the subscripts
g
1 and B refer to HONIOH (or PNI, or HONI, or PNIOH) and the stan-
Fig. 2. Fluorescence spectra of HONIOH (20
lM) in various solvents. The photos
dard, respectively.
were taken under 365 nm UV light irradiation. Inset: Magnification.
3

J. Huang, Y. Liang, Hai-Bo Liu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
have a slight solvent dependence (Figs. S12 and S13), confirming
the important role of the 4-hydroxy group. The solvents were cat-
egorized into two groups through the FL channel by observing the
distinct emission wavelengths and colors of PNIOH (Fig. S14 and
Table S3), which are ascribed to efficient ESPT in AN, EtOH, MeOH,
DMF, DMSO, and H2O in the excited state [2,41,45]. However, in
the case of PNIOH, qualitative recognition of DMF and differentia-
tion of solvents having similar physicochemical properties, such as
MeOH and EtOH, were not achieved by utilizing the UV–vis chan-
nel (Figs. S15 and Scheme S1), confirming the important role of the
N-hydroxypropyl group. Compared with PNI, HONI, and PNIOH,
HONIOH possesses desirable abilities for solvent differentiation,
which should be attributed to the synergy of N-hydroxypropyl
and 4-hydroxy groups.
3.2. Determination of trace water in organic solvents
The above results reveal that the fluorescence emission proper-
ties of HONIOH in H2O are distinct from organic solvents, such as
DMF, AN, THF, and acetone (Fig. 2 and Fig. S11). Therefore, it is pre-
dicted that the presence of trace amounts of H2O in these solvents
would result in fluorescence alteration.
Trace amounts of H2O in organic solvents were determined to
evaluate the accuracy of HONIOH as a water probe. As shown in
Fig. 3. UV–vis absorption spectra of HONIOH (20
lM) in various solvents. The
photos were taken under day light.
existed as anionic species in the ground state and excited state,
with an absorption band at 447 nm (Fig. 3) and a fluorescence
emission band at 552 nm (Fig. 2) [18–19].
According to the above experimental results, solvent-
dependent spectral changes in HONIOH were further analyzed.
First, HONIOH classified the investigated solvents into yellow
emission group and blue emission group through the fluorescence
channel (Fig. 2 and Fig. S11). Next, DMF and H2O could be selec-
tively recognized by monitoring the absorbance at 480 and
~450 nm, respectively, by taking advantage of the UV–vis channel
(Fig. 3). MeOH and EtOH could be discriminated by monitoring the
absorbance at 377 and ~450 nm (Scheme 1).
Photophysical properties of three model NI derivatives (Fig. 1),
namely, PNI
(U = 0.025), HONI (U = 0.014), and PNIOH
(U
= 0.063), in various solvents, were investigated to gain a better
insight into the solvents’ differentiation ability for HONIOH. Of
these three derivatives, PNI and HONI possess N-propyl and N-
hydroxypropyl substitution, respectively; PNIOH possesses a 4-
hydroxy group as in the case of HONIOH, whereas N-
hydroxypropyl is replaced with the N-propyl functionality. Fluo-
rescent emission wavelengths and emission colors of PNI and HONI
Fig. 4. Color changes of HONIOH in DMF solution (a) and AN solution (b) with
different water fractions (%) under 365 nm UV light; and fluorescence spectra of
HONIOH in DMF solution (c) and AN solution (d) with increasing amounts of water.
Concentration of HONIOH: 30 lM; excitation wavelength: 350 nm.
Scheme 1. Schematic representation of the two-step approach used to discriminate 12 kinds of solvents by HONIOH.
4

J. Huang, Y. Liang, Hai-Bo Liu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
Fig. 5. Plot of fluorescence intensity of HONIOH at 436 nm in DMF solution (a), 450 nm in AN solution (b), 444 nm in THF solution (c), and 447 nm in acetone solution (d) as a
function of H2O contents (v%). Concentration of HONIOH: 30 lM; excitation wavelength: 350 nm.
Table 1
Summary of the water sensing performance of HONIOH.
Solvent
R2
Calibration equation
Linear range/v%
DL/v%
DMF
AN
THF
0.9992
0.9941
0.9955
0.9942
F = 146.8390[H2O] + 41.4453
0.2–8.0
0.1–2.0
0.06–1.0
0.3–2.0
0.0817
0.0315
0.0186
0.0925
F = ꢂ1585.9767[H2O] + 4116.2646
F = ꢂ2665.2865[H2O] + 4123.7600
F = ꢂ1553.1011[H2O] + 4032.0730
Acetone
F: fluorescence peak intensity; DL: Detection limit.
Table 2
Determination of H2O in practical DMF samples (n = 3).
Samples
Water contenta (%)
Spiked (%)
Found (%)
Recovery (%)
1
2
3
2.5
2.5
2.5
2.0
4.0
6.0
4.45
6.61
8.54
98.9
101.7
100.5
a
Water content measured in practical DMF using the proposed probe HONIOH.
Fig. 4 and Figs. S16-S18, water induced the fluorescence turn-on
response property of HONIOH in DMF and the fluorescence turn-
off response property of HONIOH in AN, THF, and acetone. The
changes of water content (0–8%) in DMF of HONIOH induced fluo-
rescence color variations from light yellow to purple (Fig. 4a),
accompanied by an increased fluorescence emission intensity at
414, 430, and 551 nm (Fig. 4c and Fig. S16). HONIOH showed
one fluorescence emission band at 450 nm and emitted blue colors
in AN, the fluorescence intensity at 450 nm gradually decreased
upon increasing the water concentration (0–10%), and one new
emission centered at approximately 550 nm emerged (Fig. 4d),
thus leading to continuous emission color shifts from blue to yel-
5

J. Huang, Y. Liang, Hai-Bo Liu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119559
low (Fig. 4b). Similar to the fluorescence response of HONIOH in
AN, the fluorescence emission intensity of HONIOH at 444 and
447 nm enhanced gradually in THF and acetone, respectively, with
increasing amounts of H2O (0–10%), and one new emission
appeared at approximately 550 nm as in the case of AN (Fig. S17
and Fig. S18).
alization, Investigation. Hai-Bo Liu: Supervision, Writing - review
& editing, Funding acquisition. Xiangmin Zhang: Formal analysis,
Investigation. Jing Wang: Conceptualization, Methodology, Writ-
ing - original draft, Resources, Visualization, Project administra-
tion, Funding acquisition.
Good linear relationships were observed between water content
and fluorescence emission intensity at 436, 450, 444, and 447 nm in
DMF, AN, THF, and acetone, respectively, and the corresponding lin-
ear ranges were the H2O content of 0.2%–8%, 0.1%–2%, 0.06%–1%, and
0.3%–2%, respectively (Fig. 5 and Table 1). The detection limit of
HONIOH to trace H2O in DMF, AN, THF and acetone was calculated
to be 0.0817% (v/v), 0.0315% (v/v), 0.0186% (v/v) and 0.0925% (v/v),
respectively (Table 1). Moreover, the plot of I/I0 versus [H2O] in
DMF, and the Stern ꢂ Volmer plots of I0/I versus [H2O] in AN, THF,
and acetone displayed good linear relationships (Fig. S19), where
I0 and I represent the fluorescence intensity of HONIOH in the
absence and presence of water concentration, respectively.
Compared with the previously reported fluorescent probes for
determination of the water content in organic solvents
(Table S1), the most valuable performance of the present work is
that HONIOH can discriminatively detect the water content with
distinguishable fluorescence emission profile and emission color
changes, such as fluorescence ‘‘turn-on” mode in DMF and ‘‘turn-
off” mode in AN, THF, and acetone (Figs. S16-S18); and light yellow
to purple color changes in DMF and blue to yellow color changes in
AN, THF, and acetone (Fig. 4).
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge the financial supports from the National Nat-
ural Science Foundation of China (Grant Numbers 21966006,
31560014, 21567002), Natural Science Foundation of Guangxi Pro-
vince (Grant Number 2018GXNSFAA050015).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.saa.2021.119559.
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