Microwave-assisted synthesis of a novel steroid-derived Schiff base chemosensor for detection of Al3+ in aqueous media

Article abstract of DOI:10.1177/1747519820914827

A novel steroid-derived Schiff base chemosensor, N′-(2-hydroxybenzylidene)-3α-hydroxy-cholanhydrazide (LA), has been designed and prepared via microwave irradiation. The sensor LA showed highly selective fluorescent sensing for Al³⁺ with a low detection limit of 34 nM in the pH range from 6.05 to 9.32 in ethanol/water (1:2, v/v) solution. The binding stoichiometry between LA and Al³⁺ was determined as 1:1 by Job’s plot and further verified with ¹H NMR studies. Under a UV lamp, the fluorescence color changes could be easily detected by the naked eye. In addition, the sensor LA has been applied in detection of Al³⁺ in real water samples.

Full text of DOI:10.1177/1747519820914827

9
research-arti
1cle2020 4827CHL0010.1177/1747519820914827Journal of Chemical ResearchChen
Research Paper
Journal of Chemical Research
–6
1
Microwave-assisted synthesis of a novel
steroid-derived Schiff base chemosensor
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3+
for detection of Al in aqueous media
Yu Chen
Abstract
A novel steroid-derived Schiff base chemosensor, N′-(2-hydroxybenzylidene)-3α-hydroxy-cholanhydrazide (LA), has
been designed and prepared via microwave irradiation. The sensor LA showed highly selective fluorescent sensing for
3
+
Al with a low detection limit of 34nM in the pH range from 6.05 to 9.32 in ethanol/water (1:2, v/v) solution. The
3
+
1
binding stoichiometry between LA and Al was determined as 1:1 by Job’s plot and further verified with H NMR
studies. Under a UV lamp, the fluorescence color changes could be easily detected by the naked eye. In addition, the
3
+
sensor LA has been applied in detection of Al in real water samples.
Keywords
aluminum(III) ions, chemosensor, microwave irradiation, Schiff base, steroid
Date received: 25 November 2019; accepted: 3 March 2020
of this isomerization, are responsible for fluorescence emis-
Introduction
1
0–12
sion behavior of the Schiff bases.
Binding with metal
Aluminum is the most abundant metal in the earth’s crust,
forms approximately 8% of earth mass. Aluminum has
been widely used in modern life, such as water treatment
plants, food additives, textiles, and pharmaceuticals.
However, with the rapid development of aluminum indus-
try, the content of aluminum in natural water and soil is
ions, the C=N isomerization would be inhibited and pro-
vides a significant change in the fluorescence emission,
which is a crucial advantage in practical applications.
13
“
Green chemistry” is the frontier of international chem-
ical science research today and has received widespread
concern and attention from governments, business, and
chemical circles around the world. As a new technology in
line with the concept of “green chemistry,” microwave
1
gradually increasing. As we all know, aluminum is not a
biologically essential element, and excessive accumulation
3
+
of Al would be toxic and cause many serious diseases,
irradiation has attracted considerable interest in recent
2
such as neuronal disorder, loss of memory, listlessness,
14,15
years.
Microwave-assisted organic synthesis is a
3
,4
and Alzheimer’s disease. Therefore, the design and syn-
thesis of chemosensors with high selectivity and excellent
sensitivity in detecting aluminum is of great significance.
valuable technology that helps reduce reaction times,
Schiff bases are good ligands for metal ions, with which Pingdingshan Polytechnic College, Pingdingshan, P.R. China
they can form strong coordinate bonds. This characteristic
Corresponding author:
Yu Chen, Pingdingshan Polytechnic College, Pingdingshan 467001,
P.R. China.
has resulted in their extensive use in the design and synthe-
5
–9
sis of chemosensors. Two possible mechanisms that have
been reported, the C=N isomerization and the suppression Email: pdschenyu@163.com

2
Journal of Chemical Research 00(0)
Scheme 1. Synthetic protocol for sensor LA.
Table 1. Optimization of the reaction conditions synthesis
sensor LA.
procedures. The reaction was monitored with or without
additives, such as acetic acid, Sulfuric acid (H SO ), or
2
4
hydrochloride (HCl) under refluxing in Tetrahydrofuran
THF), Dimethyl Sulfoxide (DMSO), N,N-Dimethyl-
Entry
Solvent
Additive
Time/h
Yield/%
(
a
1
2
3
4
5
6
7
8
9
EtOH
EtOH
EtOH
EtOH
AcOH
THF
MeOH
DMF
DMSO
MeOH
–
5
5
5
5
5
5
5
5
12
formamide (DMF), ethanol, methanol, or acetic acid. The
result demonstrated that the yield of sensor LA was in the
range of 12%–73%, while longer reaction time was
required. With better catalytic property, HCl was used in
the synthesis of sensor LA under microwave irradiation.
Fortunately, it showed a dramatic improvement of the
yield and remarkable reduction in reaction time (0.17h)
when we employed a microwave-assisted method.
Compared with conventional thermal heating, microwave
irradiation technique decreased the reaction time from 5 to
0.17h and enhanced the yield from 73% to 86%. The study
data were given in Table 1.
a
HCl
AcOH
H SO
69
a
51
a
43
2
4
a
–
65
a
HCl
HCl
HCl
HCl
HCl
36
a
51
a
73
a
5
0.17
35
b
1
0
86
a
Conventional method.
Microwave irradiation: 300W, ordinary pressure.
b
The UV-Vis titration spectra of LA upon addition of
various concentrations of Al were carried out in ethanol
increase yields, achieve cleaner reactions, simplify
3+
1
6–18
workups, and design energy-saving protocols.
Although various Schiff base chemosensors have been
and water (1:2, v/v) at room temperature. As shown in
Figure 1, the absorption spectra of receptor LA have three
absorption maxima, centered at 254, 264, and 294nm.
1
9–23
designed and synthesized,
to the best of our knowledge
and according to the literature survey that few efforts have
been made in the efficient synthesis of chemosensors with
3+
Upon addition of increasing concentrations of Al (0–1.0
equiv.) to the solution of sensor LA (50μM), those three
“green chemistry” strategy. Besides, steroids are rarely
absorption bands were enhanced gradually, which clearly
used as a skeleton for fluorescent sensor. In previous
3+
suggests that LA participates in a coordination with Al .
2
4–27
studies,
we reported a microwave mediated, efficient,
The fluorescence emissions of sensor LA (50μM) on add-
high yield, and environmentally friendly synthesis of Schiff
base derivatives. Herein, a novel steroidal Schiff base fluo-
rescent sensor, N′-(2-hydroxybenzylidene)-3α-hydroxy-
cholanhydrazide (LA), was designed and prepared via
+
ing different metal ions (1.0 equiv. of LA) including Ag ,
3+
2+
2+
2+
3+
2+
3+
2+
2+
2+
Al , Ba , Ca , Co , Cr , Cu , Fe , Hg , Mg , Mn ,
2+
2+
2+
Ni , Pb , and Zn were determined in ethanol/water (1:2,
v/v). After excitation at 400nm, the sensor LA exhibited very
3
+
microwave irradiation (Scheme 1), and Al detection capa-
bility of LA has been investigated. The free LA exhibited
almost no detectable fluorescence emission due to the C=N
isomerization and the photo-induced electron transfer
3+
week fluorescence emission. Upon addition of Al (50μM),
a remarkable fluorescence emission peak at 460nm was
observed (Figure 2). In contrast, no significant fluorescence
change was detected after addition of other metal ions under
the same conditions. These results indicate that LA exhibits
3
+
(
PET) phenomena; when coordinating with Al , the C=N
isomerization and PET effects were inhibited and the
molecular structure changed from flexible to rigid, which
3+
high selectivity for Al over other metal ions. Under the 365-
nm UV lamp, the remarkable light-blue fluorescence emis-
1
2,28–30
dramatically increased the fluorescence intensity.
3+
sion associated with the reactions between LA and Al was
observed by the naked eye, but no significant fluorescence
change was detected for LA solution with other metal ions.
The quantitative sensing abilities of LA (50μM) toward
Results and discussion
To develop an optimal reaction condition, 3α-hydrox- Al3+ were studied, and a working curve was obtained
ycholan-24-hydrazide (3) and salicylaldehyde (4) were (Figure 3). With addition of increasing concentrations ofAl3
employed in conventional and microwave-assisted ions (0, 10, 20, 30, 40, 50, 60, and 70μM), the fluorescence
+

Chen
3
Figure 1. Absorption spectra of sensor LA (50μM) in ethanol
Figure 3. Fluorescence titrations of 50-μM LA (λex =400nm)
and water (1:2, v/v) solution obtained by adding aliquots of
in ethanol and water (1:2, v/v) in the presence of different
3
+
3+
Al ] (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 equiv.).
equivalents of Al
ions (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4
[
equiv.). Inset: Graph of the fluorescence intensity at 460nm as a
function of the concentration ratio of Al3 and LA.
+
Figure 2. Fluorescent spectra of sensor LA (50μM) with 1.0
equiv. of various metal ions in ethanol and water (1:2, v/v). Inset:
The colors of sensor LA and LA-Al as viewed by the naked
eye under a 365-nm UV lamp.
3
+
Figure 4. Benesi–Hildebrand analysis of the emission changes
for the complexation between LA and Al3 in ethanol and
water (1:2, v/v). (λ =400nm, λ =460nm).
+
ex
em
intensity of LA-Al3+ enhanced remarkably and showed a
maximum fluorescence emission at 460nm. Besides, a good
linear correlation (R2=0.99078) between the emission inten-
3+
for Al reached 34nM, which is low for the detection of
3+
3+
Al comparing with other published Al binding sensors
and much lower than World Health Organization (WHO)
recommended 3.7–7.4μMAl standard drinking water.
These results demonstrate that sensor LA could be poten-
tially used for selective detection of Al in analytical
chemistry, especially in water quality monitoring.
To determine the utility of sensor LA as an Al -selective
receptor in the complex background of competing species,
3+
sity of LA and the concentration ratio of [Al ]/[LA] was
3+
observed on addition of 0–1.4 equiv. of Al ions. According
3+
32,33
to the titration profile (Figure 4), the binding constant of LA
3
+
3
-1
for Al could be calculated as 4.13×10 M based on the
3+
31
modified Benesi–Hildebrand equation (equation (1)), in
where F, F , and Fmax represents the fluorescence intensities
3+
min
3+
of LA-Al complex, free LA, and the maximum fluores-
3+
cence intensity of LA-Al complex, respectively.
3+
the fluorescence emissions of LA-Al were examined in the
presence of different metal ions (Figure 5). Upon addition of
different metal ions (1 equiv. of Al ), the fluorescence emis-
1
1
1
3+
=
+
(1)
3
+
3+
2+
F − Fmin K F − Fmin Al  Fmax − Fmin
sions of LA-Al were almost not interfered, except for Cu
(
)
max


3+
and Fe . The obvious reduction of fluorescence intensities
2
+
3+
According to the 3σ method (limit of detection caused by Cu and Fe might due to an energy or electron
34
35,36
(
LOD)=3σ/K, σ represents the standard deviation of a transfer. As reported in literatures,
blank solution and K represents the slope of the calibration because the newly formed complex LA-Fe or LA-Cu
curve in Supplemental Figure S1), the LOD of sensor LA was too stable to be replaced by Al .
it was possibly
3+
2+
3+

4
Journal of Chemical Research 00(0)
LA and Al3+ was 50μM, with a continuous variable molar
3
+
3+
fraction of guest [Al ]/([LA+Al ]). The maximum fluo-
rescence emission intensity at 460nm was observed at the
molar ratio of 0.51, establishing a 1:1 binding stoichiome-
3
+
try between sensor LA and Al .
Additional evidence was given by NMR (nuclear mag-
3
+
netic resonance) studies of sensor LA and LA-Al com-
1
plex. The H NMR spectra of LA in the absence and
3
+
presence of Al were recorded in DMSO-d . Significant
6
spectral changes were observed (Figure 7). Signals for the
3
1
protons of –NH– (H ) at 11.57ppm and –OH (H ) at
1.19ppm disappeared, which indicates the enolization and
1
3
+ 37,38
deprotonation of LA in the presence of Al .
the signals of H2 and H4 were downfield shifted 0.08 and
.3 respectively. These phenomena indicate that the
Besides,
0
Figure 5. Relative fluorescence intensity ratio of LA-Al3+
hydroxyl group, carbonyl group, and C=N group of sensor
LA participate in a coordination with Al , which in return
(
50μM) in the absence and presence of different metal ions (1.0
3+
equiv.). (λ =400nm, λ =460nm).
ex
em
changes the electron distribution in the chemosensor.
Based on the above results, a binding mechanism for the
3
+
fluorescence response of LA toward Al is proposed. As
shown in Scheme 2, the fluorescence emission of free LA
is nearly quenched, which may due to the PET phenome-
non causing by lone pair electron from Schiff-base nitrogen
atom to the phenol moiety and C=N isomerization process.
3
+
However, upon addition of Al ions (2.0 equiv.), two nitro-
gen atoms of the azomethine groups and the hydroxyl group
3
+
on the phenol participate in the coordination with Al ,
forming a 1:1 complex and resulting in a remarkable fluo-
rescence enhancement of LA.
3
9,40
By utilizing a previously reported method,
sensor
3
+
LA was used to measure the Al content in actual water
samples, including tap water (from our laboratory), domes-
tic sewage (from student residences at our college), and
industrial sewage (from industrial areas in Pingdingshan
City). All the water samples were filtered through a
0.2-mm filter membrane to remove large particular impu-
rities, followed by removal of remaining organics by
extraction processes. The resulting samples were diluted
with ethanol and water (1:2, v/v) in a 10.0-mL volumetric
flask. Table 2 shows the results acquired using sensor LA
Figure 6. Job’s plot for the complexation of LA with Al3+
(
λex =400nm, λ =460nm).
em
To investigate the applicability of sensor LA for detect-
3
+
ing Al in different environments, the pH effects were
studied (see Supplemental Figure S2). The fluorescence
3
+
3+
with the appropriate concentration gradient of Al added.
emissions of LA (50μM) and LA-Al complex (50μM,
:1, M/M) were detected in various buffer solutions (see
The results indicate that sensor LA had good recovery
1
3
+
and demonstrated high accuracy in the analysis of Al .
Supplemental Table S1). The free sensor LA exhibited very
Therefore, sensor LA can measure the concentration of
week fluorescence emissions in the pH range of 3.92–9.84.
3
+
3+
Al in real water samples and has practical value in envi-
ronmental analysis.
Upon addition of Al , a remarkable fluorescence enhance-
ment of LA was observed and the pH value increased from
6.05 to 9.32. On the contrary, in acidic solutions (pH<6.05),
the fluorescence intensities significantly decreased, which
might due to the protonation of Schiff base group. Moreover,
Conclusion
3+
In summary, a highly selective and sensitive Schiff
base chemosensor, N′-(2-hydroxybenzylidene)-3α-hydroxy-
cholanhydrazide (LA), has been designed and synthesized
under microwave irradiation. LA shows significant “turn-
on” fluorescence behavior with Al in ethanol/water (1:2,
v/v) solution, giving strong light-blue emission. The detec-
the fluorescence intensities of LA-Al were sharply reduced
in strong alkaline solutions (pH>9.32), which mainly result
from the decomposition of Schiff base group. Besides, the
formation of aluminum hydroxide in strong alkaline solu-
tions could possibly be another reason for this significant
inactivation of probing process. These results demonstrate
3+
-8
3+
tion limit was calculated to be 3.4×10 molL-1 and
remained stable, detectingAl in the pH range of 6.05–9.32.
that sensor LA is highly selective toward Al in a relatively
wide pH range (6.05–9.32).
3+
The binding stoichiometry between sensor LA and Al3+
In addition, the sensor LA exhibits satisfactory results for
3+
Al detection in the analysis of real water samples and can
be further used in potential applications for the detection of
(
Figure 6) was studied by the method of continuous varia-
tion (Job’s plot). In the Job’s plot, the total concentration of

Chen
5
1
3+
Figure 7. H NMR spectra of sensor LA and LA-Al in DMSO-d6.
3+
Scheme 2. Proposed sensing mechanism between sensor LA and Al .
Table 2. Determination of Al3+ in real water samples with sensor LA.
b
3+
Sample
pHa
Added Al3+ (nM)
40
Found Al (nM)
Recovery (%)
RSD (%)
Tap water
6.71
39.18
60.51
79.76
100.25
39.66
60.33
80.20
99.89
39.41
60.39
79.67
100.52
97.95
100.85
99.70
100.25
99.15
100.55
100.25
99.89
1.25
0.60
0.98
6
8
0
0
1
1
1
00
40
Domestic sewage
Industrial sewage
7.96
8.43
6
8
0
0
00
40
98.53
100.65
99.59
6
8
0
0
00
100.52
RSD: relative standard deviation.
a
3+
Determined with a pH meter before treating with Al .
b
Results are based on three measurements.
nanomolar concentrations of Al3+ in chemical and environ- infrared spectrum was recorded on a Perkin-Elmer 1700
mental systems.
FTIR spectrophotometer. Mass spectra were obtained using
a Finnigan LCQ DECA mass spectrometer in electrospray
ionization (ESI) positive mode. A Mapada UV-6100 UV-Vis
spectrophotometer was used for recording the UV-Vis
Experimental
All chemicals and solvents were obtained from commercial spectra. Fluorescence emission spectra were recorded on a
sources and used without further purification, except when Dual-FL fluorescence spectrophotometer. Elemental analy-
specified. Solvents used in fluorescent experiments were of sis (C, H, and N) was performed with an Elementar Vario
1
13
spectroscopic grade. The H NMR and C NMR spectra of MICRO cube, and the results are within ±0.4% of the
the ligand were recorded on an Agilent 400-NMR DDR2 calculated values. All the reactions were performed in a
spectrometer. The chemical shifts (δ) are recorded in ppm, commercial microwave apparatus (XH-100A, 100–1000W;
relative to tetramethylsilane (SiMe ). The Fourier transform Beijing Xianghu Science and Technology Development Co.
4

6
Journal of Chemical Research 00(0)
Ltd, Beijing, China). All the solvents were purified before
4. Good PF, Olanow CW and Perl DP. Brain Res 1992; 593:
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from lithocholic acid according to a literature procedure.
2
6
Preparation of N′-(2-hydroxybenzylidene)-
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α-hydroxy-cholanhydrazide (sensor LA)
2
09: 359.
To a solution of compound 3 (1mmol) in methanol (5mL),
salicylaldehyde (0.95mmol) and acetic acid (2 drops) were
added successively. The mixture was heated to reflux in the
9
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NMR (400MHz, DMSO-d ): δ 11.57 (s, 1H, NH), 11.19 (s,
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Declaration of conflicting interests
4
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The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
2
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Funding
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0. Fujikawa Y, Urano Y, Komatsu T, et al. J Am Chem Soc
The author(s) received no financial support for the research,
authorship, and/or publication of this article.
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ORCID iD
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Yu Chen
https://orcid.org/0000-0002-2711-6535
4
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Supplemental material
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003; 125: 1778.
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Supplemental material for this article is available online.
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