A new reversible fluorescent chemosensor based on rhodamine for rapid detection of Al(III) in natural environmental water samples and living organisms

Article abstract of DOI:10.1016/j.tetlet.2020.152407

A fluorescence sensor RBKB with high selectivity and sensitivity to Al³⁺ was designed and synthesized. UV–Vis and fluorescence spectra showed that the probe had a good response to Al³⁺ in EtOH/H₂O (1:1, v/v) solution and t

Full text of DOI:10.1016/j.tetlet.2020.152407

Tetrahedron Letters 61 (2020) 152407
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new reversible fluorescent chemosensor based on rhodamine for rapid
detection of Al(III) in natural environmental water samples and living
organisms
Chun Kan a,1, , Linyun Wu a,1, Xiaotao Shao a, Xing Wang a, Yao Zhang a, Jing Zhu b, Siyan Qiu b
⇑
a Department of Chemistry and Material Science, College of Science, Nanjing Forestry University, 159 LongpanRoad, Nanjing 210037, PR China
b Department of Pharmacy, Jiangsu Key Laboratory for Pharmacology and Safety Evaluation of Chinese Materia Medica, Nanjing University of Chinese Medicine,
138 XianlinDadao, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
A fluorescence sensor RBKB with high selectivity and sensitivity to Al3+ was designed and synthesized.
UV–Vis and fluorescence spectra showed that the probe had a good response to Al3+ in EtOH/H2O (1:1,
v/v) solution and the response time is very short. The process of detecting Al3+ is accompanied by the
probe solution changing from colorless to pink, which has a good visual effect. The reversible type of
the probe and Al3+ was confirmed by adding EDTA. Meanwhile, the probe has a low detection limit of
Article history:
Received 15 July 2020
Revised 10 August 2020
Accepted 25 August 2020
Available online 1 September 2020
0.177 l
M. In addition, the probe can be used for the detection of Al3+ in actual environmental water sam-
Keywords:
ples with a high recovery rate. It is worth mentioning that RBKB can be used to detect Al3+ in living cells,
zebrafish, and plant tissues, which has high biological significance.
Ó 2020 Elsevier Ltd. All rights reserved.
Sensitive fluorescent probe
Al3+
Real water sample
Biological imagine
Introduction
fluorescent probes are the most effective tool to monitor detecting
Al3+ in natural environment and living systems [16,22–25]. Fluo-
Aluminum is one of the most abundant (8.3% by weight) metal
elements in the earth’s crust. Meanwhile, the advantages of alu-
minum are small density, good ductility, and good thermal conduc-
tivity. Therefore, it is used in various fields, such as aerospace
materials, engineering materials, food additives, medical equip-
ment, drinking water purification and so on [1–4]. On the other
hand, excessive aluminum can also negatively affect people’s lives,
long-term exposure to or ingestion of Al3+ will cause to accumulate
in the skin or organs and induce various human diseases. Particu-
larly, an excess amount of Al3+ in brain tissues can seriously dam-
age the central nervous system, increasing the risk of neurological
diseases [5]. Al3+ can enter the body through the food chain and
eventually accumulate in the human body, which may be the root
reason of various health problems (such as Alzheimer’s disease,
memory loss, bone softening and Parkinson’s disease) [6–12].
Therefore, selective and sensitive methods for detection of Al3+ in
biological systems are in great demand [13].
rescence sensing is a detection tool with high selectivity and sen-
sitivity, which can be used to detect specific substances in
various analytes, with very powerful functions [26,27]. Since Al3+
plays an important role in the industrial, environmental and bio-
logical fields, it has been subjected to numerous studies in a simple
and efficient manner for real-time detection [28–31]. In recent
years, the technology of fluorescence sensing has been favored
by many scholars at home and abroad. They are committed to
building fluorescent probes with specific recognition performance
because they have better detection performance and application
value than traditional detection technologies [32,33].
As we all know, rhodamine is an ideal dye for the development
of fluorescent probe because of its excellent photophysical
properties [34,35], such as high extinction coefficient, great photo-
stability, and excellent quantum yields. Rhodamine spirolac-
tamderivatives have no fluorescence. However, when certain
substances such as metal ions or some active substances are
encountered, the rhodamine system will trigger the ring-opening
reaction and emit its characteristic emission peak [36–39]. In addi-
tion, rhodamine and its derivatives show interchangeability
between colorless and non-fluorescent closed-spiro rings and col-
ored and highly fluorescent open-loop forms. The reaction system
has high selectivity and sensitivity, showing its great potential in
the field of environment and biology. In the context of long-term
Many techniques have been applied to the detection of Al3+
such as spectrophotometry [14–16], high-performance liquid chro-
matography (HPLC) [17–19] and electrochemistry [20,21]. Ion-type
⇑
Corresponding author.
E-mail address: kanchun@njfu.edu.cn (C. Kan).
1
The two authors contributed equally to this paper.
https://doi.org/10.1016/j.tetlet.2020.152407
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

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C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
interests in searching for fluorescent chemosensors for Al3+, rho-
damine spirolactam-based chemosensors for ‘‘turn-on” detection
of Al3+ had been designed [40].
(m, 8H), 1.17 (t, J = 14.04 Hz, 12H) ppm. 13C NMR (CDCl3),
d167.93, 154.06, 152.98, 148.60, 148.53, 148.27, 139.28, 135.13,
133.62, 131.03, 128.32, 128.16, 127.88, 127.77, 125.74, 124.50,
123.12, 110.76, 108.74, 108.38, 107.13, 97.07, 66.02, 44.30, 38.90,
Herein, we have designed and synthesized a new novel fluores-
cent probe RBKB on the platform of rhodamine B. It was proved to
have high selectivity and short response time for Al3+ [41]. Addi-
tionally, the proposed probe can successfully detect Al3+ in real
environmental water samples. It was also demonstrated these
applications of Al3+ fluorescence imaging in living cells, animals
and plant tissues. These results indicated that the probe had good
biocompatibility and could broaden the application of bioscience
and materials fields.
12.73 ppm. ESI-MS calculated for
C39H39N5O3S: 657.2867,
(M + H)+ found: 658.2873.
Response time and influence of the pH
The specific ring structure of rhodamine B determines whether
fluorescence occurs or not. By exploring the response time of the
probes RBKB and Al3+, the change of the rhodamine spirolactam
ring state can be judged. Fig. 1A reveals the change in RBKB fluo-
rescence intensity after adding Al3+ (100
lM) to the system solu-
Results and discussion
tion within 30 min. The fluorescence degree of the RBKB rapidly
augmented to a higher level within one minute, and continued to
increase to the highest value with time and then remained
unchanged. Therefore, the probe RBKB can be used to sensitively
detect Al3+. In order to continue to explore the effect of pH value
on the structure of the rhodamine ring, the study was carried out
under the system solution. Fig. 1B shows that when pH is between
6 and 8, the fluorescence degree of RBKB at 582 nm is very weak
and does not change with the change of pH value. At this time,
the probe is in a closed-loop state. On the other hand, after Al3+
was added to the probe RBKB, the mixed solution had a strong flu-
orescence intensity and remained stable when the pH was less
than 10. Under these circumstances, the probe was induced by
Al3+ to open the loop. In summary, the probe RBKB can detect
Al3+ in physiological pH range.
Synthesis of RBKB
Scheme 1 shows the synthesis process of RBKB. Refer to the
methods reported in the previous literature to synthesize interme-
diate compound RS1 [42]. RS1 (502 mg, 0.94 mmol) was dissolved
in 50 ml of CH2Cl2, then 2-Thiopheneacetylchloride (151 mg,
0.98 mmol) and triethylamine (116
lL, 0.99 mmol) were added
dropwise to stir for 6 h under anhydrous and nitrogen atmosphere
(room temperature). The reaction process was monitored by TLC.
After the reaction was completed, the solvent in the bottle was
spin-dried and extracted three times with dichloromethane and
saturated brine (100 ml each). The combined organic phase was
dried over anhydrous Na2SO4 and evaporated to dryness and con-
centrated. Finally purified by column chromatography using CH3-
OH/CH2Cl2 (1:99, v/v) as an eluent to afford bright red solid
(391 mg, 60%). 1H NMR (CDCl3), d8.31 (d, J = 8.28 Hz, 1H), 7.99
(m, 1H), 7.85 (s, 1H), 7.71 (d, J = 7.92 Hz, 1H), 7.51 (m, 3H), 7.45
(d, J = 5.04 Hz, 1H), 7.24 (s, 1H), 7.20 (d, J = 2.94 Hz, 1H), 7.09 (d,
J = 6.84 Hz, 1H), 6.32 (d, J = 8.76 Hz, 2H), 6.22 (d, J = 2.46 Hz,
2H), 6.11 (dd, J1 = 2.46 Hz, J2 = 2.52 Hz, 2H), 3.94 (s, 2H), 3.31
Ions selectivity
In order to verify the specific detection of specific metal ions by
the probe RBKB, characterization was performed by ultraviolet–
visible spectroscopy and fluorescence spectroscopy by adding
Scheme 1. Synthesis of RBKB.

C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
3
Fig. 1. (A) Fluorescence spectra of RBKB (10
l
M) – Al3+ (100
l
M) over a period of 30 min in the system solution. (B) Influence of pH on Al3+ detection by RBKB (10
l
M).
Fig. 2. System solutions is the same as above, RBKB concentration is 10 lM. (A) UV absorption chart of various cations. (B) Fluorescence intensity graph of various cations. (C)
Experiment diagram of cation coexistence competition. The black bar means only containing various metal cations, and the red bar means that each metal ion coexists with
Al3+ (D) Experimental diagram of stability of anion coexistence solution.
some common metal ions in the system solution. As shown in
Fig. 2A, only the sample with Al3+ added shows an obvious UV
absorption peak at 560 nm, and the color of the probe solution
changed from colorless to pink. After adding other ions, there is
only a weak absorption at 560 nm in the solution containing
Fe2+, which can be ignored. The fluorescence spectrum (Fig. 2B)

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C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
Fig. 3. (A) Fluorescence titration of RBKB (10
l
M) in solutions of different concentrations of Al3+ (0–50 equivalent). (B) Benesi-Hildebrand plot (kem = 582 nm) of 1/(F–F0) vs
1/[Al3+]. (C) Fluorescent intensity at 582 nm of RBKB (10 M) in system solution with different amounts of Al3+. The excitation wavelength is 520 nm.
l
shows that when the system solution contains only the probe solu-
tion, the probe’s fluorescence intensity is very weak, almost no flu-
orescence. This is because the rhodamine five-membered ring
structure is in a closed-loop state now. However, the fluorescence
intensity at 582 nm was significantly increased in samples with
AI3+ added. This is because AI3+ induced the ring-opening of the
rhodamine five-membered ring structure and produced a strong
fluorescence. Although Fe2+ also has fluorescence absorption, its
intensity is not at the same level as Al3+, it can be ignored. In sum-
of the probe and Al3+ complexed in the original test, these common
anions would not interfere with the complexation of the probe
with Al3+
.
Fluorescence titration experiment
The influence of Al3+ concentration on the probe RBKB fluores-
cence enhancement was investigated by changing its content in
the system solution (Fig. 3A). When Al3+ is not added, the fluores-
cence intensity of the sample is very weak, and there is almost no
fluorescence. As the concentration of Al3+ increased, the fluores-
cence intensity of the sample also increased, and the color of the
solution gradually changed from colorless to pink. When the con-
centration of Al3+ reached 50 equivalent, the fluorescence intensity
of the sample reached the maximum value, which indicated that
the addition of Al3+ induced the opening of the RBKB five-mem-
bered ring structure. The fluorescence intensity values at different
concentrations of Al3+ were selected, and the binding constant
between the probe and Al3+ was calculated using the Benesi-Hilde-
brand plot formula. Through calculation (the specific method is in
mary, the probe RBKB can specifically detect Al3+
.
Further explore whether Al3+ and other cations coexist, whether
these cations will affect the results of Al3+ detection by the probe
(Fig. 2C). When the sample contains only the probe and other
cationic solutions except for Al3+, the fluorescence intensity of
every sample is weak, and after the Al3+ solution is continuously
added to each sample, the fluorescence intensity is significantly
strengthened. Therefore, the detection of Al3+ by the probe RBKB
is not interfered with by other cations.
Finally, we explored the stability of probe RBKB for the detec-
tion of Al3+ in complex solutions formed by Al3+ and other common
anions (Fig. 2D). In the end, it was found that the fluorescence
intensity was not much different from the fluorescence intensity
the supplementary material), the detection limit is 0.177
lM and
the binding constant is 3.69 Â 103 MÀ1. These results demon-

C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
5
Fig. 4. (A) Job’s plot of RBKB with Al3+ ([RBKB] + [Al3+] = 50 M) in system solution. (B) Reversibility of Al3+ to RBKB by adding EDTA by fluorescence method. (C) Reversible
l
stability of the probe.
strated that the probe RBKB can detect Al3+ at very low concentra-
tions and has good stability.
can peel Al3+ from the binding region and probe RBKB is rever-
sible. Fig. 4C reveals the reversible stability of the probe. The
response mechanism between the RBKB and Al3+ is shown in
Scheme 2.
Sensing mechanism
The binding mode of compound RBKB and aluminum ions were
confirmed by Job’s plot evaluation of the fluorescence titration
experiment, As shown in Fig. 4A, with the increase of the ratio of
Al3+ concentration to RBKB-Al3+ concentration, the fluorescence
intensity of the system continued to increase. When the concentra-
tion ratio reached 0.5, it peaked and then showed a downward
trend, which proved that the complexation ratio of RBKB and
Al3+ was 1:1.
The unique fluorescence response mechanism of Rhodamine B
was used to establish an ‘‘off- on-off” reversible fluorescent
probe. We selected EDTA with a strong affinity for Al3+ as a
chelating agent to explore the fluorescence reversibility of RBKB.
The fluorescence intensity decreased with the increase of EDTA
Application in real environmental water samples
In order to broaden the practical application of the RBKB, we
investigated its detection performance for Al3+ in real environmen-
tal water samples. We collected different real water samples from
Xuanwu and Zixi Lake respectively, then treated with ultrasonic
waves, centrifugation, and filtration to remove excess Al3+. Then
a certain amount of Al3+ was added to water samples. We can con-
clude from Table 1, the recovery rate of Al3+ in two different envi-
ronments is 90.5–96.8. Even under the interference of complex
natural water samples, the recovery rate of Al3+ remains high level,
indicating that probe RBKB can be used to trace the detection of
Al3+ in real water samples.
in a 100
lM probe solution (Fig. 4B). It indicated that EDTA

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C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
Scheme 2. Response mechanism between the RBKB and Al3+.
Fluorescence imaging
Table 1
The recovery of Al3+ in real water samples.
We selected the 20 lM probe RBKB solution for live-cell imag-
Sample
A13 + added (pNI)
Detected (11114)
Recovery (%)
94.5
ing experiments. As shown in Fig. 5C1, after incubating the cells
with the probe for 6 h (37 ℃), observation through a confocal flu-
orescence microscope showed no fluorescence. The cells were pre-
Xuanwu Lake
50
20
18.89 + 0.05
94.7
47.35 0.07
100
150
200
20
50
100
150
200
91.38 0.11
133.51 + 0.06
188.42 + 0.12
18.11 + 0.13
48.38 0.07
94.71 0.17
135.73 + 0.08
190.47 + 0.12
91.4
91.3
94.2
90.6
96.8
94.7
90.5
95.2
treated with 100 l
M Al3+ for 1 h and then incubated with the probe
in the medium for 6 h. It was found that the probe RBKB success-
fully stained the cells (Fig. 5C2). The blue channel used to label the
cells and the red field showing fluorescence successfully over-
lapped and complemented each other (Fig. 5D2). When the exper-
iment was complete, the cells were still in good condition
(Fig. 5A2). In summary, the probe RBKB has excellent cell cytocom-
patibility and can be used in cell staining imaging experiments.
Whether the probe RBKB can be used for imaging of animal and
plant tissues determines its application value. Zebrafish and
human genes have a high degree of similarity, which means that
their experimental results are mostly applicable to humans and
have great practical significance. Zebrafish did not show fluores-
cence after 1 h of incubation in pure water containing RBKB only,
but after continued incubation in Al3+ for 1 h, we found significant
fluorescence in its stomach and head (Fig. 6). We selected the soy-
bean root tissue cultivated with Al3+ aqueous solution for plant tis-
sue imaging, and the results showed no fluorescence. After
immersing the sample in the probe RBKB for 1 h, the sample
Zixi Lake
MTT assay
In order to use the probe RBKB in the human body to achieve
the detection of Al3+ in the human body, we first need to explore
the toxicity of the probe itself in the cell, so we selected human
breast cancer cells and used the MTT assay to detect the probe
RBKB cytotoxicity. We can see in the Fig. S4, as the probe concen-
tration increases, the cell activity increases, indicating that the
compound RBKB is not toxic and has no effect on cell survival.
When the concentration had increased to 10
cell activity remains at about 100%. Therefore, RBKB can be used
for research on living cells.
lM and 20 lM, the
Fig. 5. Confocal fluorescence images of RBKB in living MCF-7 cells (kex = 546 nm). (1) cells with RBKB (20
lM), (2) cells with RBKB (20
l lM), (A) bright field, (B)
M)-Al3+(20
blue channel (kem = 420–480 nm), (C) red channel (kem = 560–680 nm), (D) the overlay images. Scale bar: 50
l
M.

C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
7
Fig. 6. RBKB fluorescence imaging of Al3+ in zebrafish (kex = 546 nm). (A) fluorescence images, (B) bright field, (C) the overlay field. (1) Zebrafish with probe RBKB (20
lM)
only, (2) A sample of zebrafish containing RBKB (20 l lM).
M)-Al3+ (20
Fig. 7. RBKB fluorescence imaging of Al3+ in soybean root tissue (kex = 546 nm). (A) fluorescence images, (B) bright field, (C) the overlay field. (1) Zebrafish with probe RBKB
(20 M) only, (2) A sample of soybean root tissue containing RBKB (20 M).
M)-Al3+ (20
l
l
l

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C. Kan et al. / Tetrahedron Letters 61 (2020) 152407
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Conclusion
In summary, 1H NMR, 13C NMR, and ESI-MS tests confirmed that
the probe RBKB proposed in this paper had been successfully
designed and synthesized. RBKB can detect Al3+ with high selectiv-
ity and sensitivity in EtOH/H2O (1:1, v/v, HEPES, 1.0 mM, pH = 7.40)
system solution without interference from other ions. Job’s plot
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was 1:1. The reversible fluorescence response to the probe and
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detection limit is 0.177 lM. Meanwhile, RBKB has a good detection
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
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This work was supported by the Jiangsu Six Talent Peak Award
(2015SWYY013) and The Priority Academic Program Development
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152407.
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