A novel highly selective colorimetric and “turn-on” ?uorimetric chemosensor for detecting Hg2+ based on Rhodamine B hydrazide derivatives in aqueous media

Article abstract of DOI:10.1016/j.jphotochem.2019.04.031

A novel chemical sensor SD based on Rhodamine B hydrazide derivative was designed and synthesized, which displayed both colorimetric and “turn-on” ?uorescence responses for Hg²⁺ in DMSO/H₂O (1: 9, v/v) solution. In the presence of me

Full text of DOI:10.1016/j.jphotochem.2019.04.031

JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A novel highly selective colorimetric and “turn-on” fluorimetric
chemosensor for detecting Hg2+ based on Rhodamine B hydrazide
derivatives in aqueous media
Jing-Han Hu⁎, Chen Long, Qing-Qing Fu, Peng-Wei Ni, Zhi-Yuan Yin
College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu, 730070, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Chemosensor
Colorimetric
Fluorescence
Hg2+
A novel chemical sensor SD based on Rhodamine B hydrazide derivative was designed and synthesized, which
displayed both colorimetric and “turn-on” fluorescence responses for Hg2+ in DMSO/H2O (1 : 9, v/v) solution.
In the presence of mercury ions, the sensor SD showed an obvious change in color from colorless to distinct rose
red by naked eyes and rapidly provided bright orange fluorescence under UV lamp in the aqueous solution. The
detection limit on fluorescence response of SD to Hg2+ was as low as 2.71 × 10−7M. Compared to other metal
ions such as Fe3+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Cu2+, Ag+, Ca2+and Mg2+, SD could only selectively
recognizes Hg2+.The results of the Job’s plot and infrared spectra analysis suggested that the combined stoi-
chiometry between SD and Hg2+ was 1:1. Notably, SD could be used as molecular switch controlled by EDTA
and Hg2+ cyclically. In addition, SD-based test strips were produced which could be used as a convenient and
efficient Hg2+ test kits. SD also could qualitatively detect Hg2+ in tap water.
Test strip
DFT
1. Introduction
Therefore, designing dual-channel probes with naked eye recognition
and turn-on fluorescent signal in aqueous solution has attracted more
With the development of science and technology, mercury com-
pounds are widely used in electrical, medical, military, casting and
other fields [1–3]. Mercury is liquid at room temperature and has
toxicity, volatility and bioaccumulation along the trophic chain [4,5].
Mercury vapor formed by volatilization is highly toxic and easily enters
the human body through the respiratory tract, causing sever diseases
like deafness, vision lost, insanity, paralysis coma and Alzheimer's
[6–8]. In addition, the same highly toxic mercury-containing com-
pounds are even more difficult to prevent. After the waste liquid con-
taining mercury enters the ecosystem, mercury with a strong migration
and low degradability lead to mercury pollution [9–11]. Mercury was
listed as one of the priority pollutants according to the U.S. EPA
[12,13]. The maximum U.S. EPA limit for allowable levels of Hg2+ ions
in drinking water is 2 ppb [14].
attention.
Our team is also very interested in such probes [31–39]. We have
found that designing probe based on Rhodamine B is ideal. Rhodamine
B has good spectral characteristics, such as high fluorescence quantum
yield, large molar extinction coefficient, high stability, etc [40–43]. In
addition, favourable water solubility extends the application range of
these probes [44]. We designed a Rhodamine B hydrazide-based probe
SD for higher selectivity, sensitivity and reversibility of Hg2+. After
adding Hg2+, based on the open-ring process of the spirolactam of the
probe SD, the color of the probe changed from colorless to purplish red,
and the fluorescence changed from off to on at λex =540 nm. The de-
tection limit of mercury ions in probe SD is down to 2.71 × 10−7M.
Recyclable projects can also be implemented by adding EDTA. Com-
pared with other reported Hg2+ sensors (in S1), sensor SD has excellent
sensitivity and water solubility and has been fabricated into test strips
for practical testing. In addition, SD could be used to qualitatively de-
tecting Hg2+ in tap water samples. The mechanism of the process was
verified by 1H NMR, UV–vis and other spectroscopy methods, and DFT
calculation was performed.
Fluorescent sensors have become powerful tools for the detection
and imaging of trace ions in the environment, food and biological
samples [15–17]. There have been many reports of mercury ion probes
in recent years [18,19]. However, since mercury ions are generally used
as quenchers due to the spin-orbit coupling effects and have sulpho-
phile, most of the probes have problems such as high detection limit
[20,21], poor water solubility [22–26] and toxicity of Sulfur [27–30].
⁎ Corresponding author.
E-mail address: hujinghan62@163.com (J.-H. Hu).
https://doi.org/10.1016/j.jphotochem.2019.04.031
Received 15 February 2019; Received in revised form 18 March 2019; Accepted 19 April 2019
Availableonline04May2019
1010-6030/©2019PublishedbyElsevierB.V.

J.-H. Hu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
2. Experimental
2.1. Materials and physical methods
All reagents and solvents were reagent grade at analytical grade
without further purification. Fresh double distilled water was purified
by standard methods and was used throughout the analytical experi-
ment. Perchlorate salt of cations (Cu2+, Fe3+, Co2+, Ni2+, Cd2+, Pb2+
,
Zn2+, Cr3+, Hg2+, Ag+, Ca2+and Mg2+) were purchased from Alfa-
Aesar Chemical Reagent Co. and stored in a vacuum desiccator. UV–vis
spectra were recorded on
a Shi-madzu UV-2550 spectrometer.
Fluorescence spectra were obtained by using a Shimadzu RF-5301
Fluorescence Spectrometer equipped with a xenon lamp. Melting points
were measured on an X-4 digital melting-point apparatus. Electrospray
ionization mass spectra (ESI-MS) was conducted on an Agilent 1100 LC-
MSD-Trap-VL system.1H NMR and 13C NMR were recorded on a
Mercury-400BB at 400 MHz spectra, and samples were dissolved in
DMSO-d6. 1H chemical shifts are reported in ppm downfield from tet-
ramethyl silane (TMS, δscale) with the solvent resonances as internal
standards. Melting points were measured on an X-4 digital melting-
point apparatus. Electrospray ionization mass spectra (ESI-MS) was
conducted on an Agilent 1100 LC-MSD-Trap-VL system. Infrared
Fig. 1. Single-crystal X-ray structure of sensor SD.
shown in Scheme 1. Melting point > 300℃. 1H NMR (CDCl3, 400 MHz,
ppm) d: 10.36 (s, 1 H), 9.24 (s, 1 H), 7.97 (m, 1 H), 7.53 (m, 2 H), 7.18
(d, J =7.2 Hz, 1 H), 6.78 (d, J =2.6 Hz, 2 H), 6.63 (d, J =2.2 Hz, 1 H),
6.48(m, 4 H), 6.26 (dd, J = 8.9, 2.5 Hz, 2 H), 3.71(s,3 H), 3.32 (q, J
=7.0 Hz, 8 H), 1.15 (t, J =7.0 Hz, 12 H). (Fig. S2). 13C NMR (DMSO-d6,
100 MHz, ppm) d: 163.90, 153.34, 152.51, 151.72, 149.46, 148.99,
134.41, 129.47, 128.16, 123.52, 119.79, 118.97, 117.77,112.27,
108.57, 105.56, 97.76, 66.14, 55.82, 44.13, 12.87 (Fig. S3). m/z (ES+)
calcd. for: [C36H38N4O4] + = 591.29, found: 591.3724 (Fig. S4). The
structure of sensor SD was further confirmed by single-crystal X-ray.
Diffraction the single crystal of probe suitable was obtained by using
DMF solvent diffusion method for X-ray crystal-lography. The distance
of H (3A)⋯ N (4) was 1.914 nm (Fig. 1).
spectra were performed on
photometer.
a Digilab FTS-3000 FTIR spectro-
2.2. Synthesis of intermediate rhodamine B hydrazide [45]
Rhodamine B hydrazide was prepared by a one-step condensation
reaction of Rhodamine B (0.96 g, 2 mmol) and hydrazine hydrate (1 ml,
0.02 mol) in methanol (30 ml). The reaction was refluxed for 3 h at 65
℃ in air. After the reaction was completed, it was cooled to room
temperature. The pH was adjusted to 8 with NaOH solution and a lot of
solid precipitate appeared. Then washed three times with distilled
water to give a pale pink compound in 83% yield, melting point:
214–216°C.
3. Results and discussion
A series of recognition experiments were carried out to SD in
DMSO/H2O (v/v = 1:9) solution. In the UV–vis experiment, when 50
equiv. of Hg2+ were added to SD (2.0 × 10−5 M), a broad absorption
band appeared at 565 nm, while the absorption band at 363 nm was
weakened (Fig. 2). No significant spectral changes occurred after the
addition of other cations to SD. It was also observed by the naked eye
that the SD solution changed from colorless to distinct rose red after the
addition of Hg2+. In the fluorescence experiment, only when 50 equiv.
of Hg2+ was added, SD (2.0 × 10−5 M) in DMSO/H2O (v/v = 1:9)
solution at the excitation wavelength of 540 nm was significant
2.3. Synthesis of SD
Rhodamine B hydrazide (0.46 g, 1 mmol) and 2-Hydroxy-5-meth-
oxybenzaldehyde (0.15 g, 1 mmol) was dissolved in EtOH (30 mL) and
refluxed for 6 h. After completion of the reaction, the mixture was
cooled to room temperature, and the precipitate was washed with
distilled water. Then filtered the mixture and recrystallized the pre-
cipitate with DMF-H2O to give a crimson compound in 79% yield. As
Scheme 1. Synthetic procedures for receptor SD.
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J.-H. Hu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
Fig. 2. Absorbance spectra of SD (2.0 × 10−5 M) in DMSO/
H2O (1:9, v/v) in the presence of Hg2+ and other ions: Fe3+
,
Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Cu2+, Ag+, Ca2+and
Mg2+ (50 equiv.). Inset: The photograph showing the color
change of SD solution in DMSO/H2O (1:9, v/v) after addition
of Hg2+ and other ions at room temperature.
Fig. 3. Fluorescence intensity of SD (2.0 × 10−5 M) in
DMSO/H2O (1:9, v/v) in the presence of Hg2+ and other ions:
Fe3+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Cu2+, Ag+
,
Ca2+and Mg2+ (50 equiv.). Inset: The photograph showing
the color change of SD solution in DMSO/H2O (1:9, v/v) after
addition of Hg2+ and other ions at room temperature under
UV light.
enhanced, and the change of the fluorescence of SD solution from
temperature to investigate the change in the response of SD
(2.0 × 10−5M) to the ring opening of rhodamine derivatives in DMSO/
H2O (v/v = 1:9) solution. As shown in Fig. 4, mercury ion (0.1 M) were
added dropwise to adjust the concentration to 0–21.7 equiv., which was
observed that the absorption peak at 368 nm gradually weakened while
the absorption peak at 565 nm increased. When 31.2 equivalents of
colorless to bright orange was observed under the ultraviolet lamp at
the same time. None of the other cations (Fe3+, Co2+, Ni2+, Cd2+
,
Pb2+, Zn2+, Cr3+, Cu2+, Ag+, Ca2+and Mg2+) caused any significant
change in fluorescence color and emission intensity (Fig. 3).
Titration experiments with mercury ions were carried out at room
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J.-H. Hu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
Fig. 7. The reversibility of fluorescence of SD controlled by alternating addition
Fig. 4. Absorbance spectra titration of SD (2.0 × 10−5M) in the presence of
of EDTA and Hg2+
.
different concentration of Hg2+ (0.0–21.7 equiv.) in DMSO/H2O (v/v = 1:9).
the UV–vis spectrum and the fluorescence spectrum indicated that the
sensor SD could be an effective probe for distinguishing Hg2+
.
In order to further study the practicality of SD for Hg2+ mixed
aqueous solution, the effect of pH on SD was studied by fluorescence
spectroscopy. As shown in Fig. S5, Hg2+ was added to the SD solution
at different pH values, and the results showed that SD could work in the
pH range of 4.0–9.0.
With further investigation, after adding EDTA to SD-Hg2+ in
DMSO/H2O (v/v = 1:9) solution, the fluorescence intensity of the so-
lution returned to the original state of SD. This indicated that the re-
versibility of the binding process between Hg2+ and SD can be de-
monstrated (Fig. 7). Since EDTA has a strong affinity to Hg2+, when
EDTA complexed to Hg2+, it leads to de-metallization of the sensor SD
and regeneration of the spironolactam ring, which caused the fluores-
cence quenched. The fluorescence of SD can be repeated as an “OFF-
ON-OFF” switching process multiple times.
As shown in Fig. 8, the changes in fluorescence intensity of SD with
Hg2+ were almost linear. As calculated on the basis of 3SB/S (where SB
is the standard deviation of the blank solution and S is the slope of the
calibration curve) [46], the detection limit was down to 2.71 × 10−7M,
indicating that SD can detect low concentration of Hg2+ in practical
applications. To determine the combined stoichiometry between SD
and Hg2+, job's plot was drawn (Fig. 9). When the molar fraction of
Hg2+ was 0.5, the fluorescence intensity reached an extreme value,
indicating that the stoichiometric ratio of SD and Hg2+ was 1:1.
To confirm the complexation mechanism between SD and Hg2+, the
1H NMR titration spectrum in DMSO-d6 was investigated (Fig.10). SD
showed two single peaks at 10.36 ppm and 9.01 ppm, corresponding to
Fig. 5. Fluorescence intensity titration of SD (2.0 × 10−5M) in the presence of
different concentration of Hg2+ (0.0–31.2 equiv.) in DMSO/H2O (v/v = 1:9).
Hg2+ (0.1 M) were added, when excited at 540 nm, the fluorescence of
SD was significantly enhanced at 588 nm (Fig. 5).
In order to further verify the selectivity of SD, anti-interference tests
of various metal ions (Fe3+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+
,
Cu2+, Ag+, Ca2+and Mg2+) were carried out. As shown in Fig. 6, in the
absence of mercury ions, the SD solution containing various cations did
not cause significant spectral changes, but all of the solutions showed
significant spectral changes after the addition of Hg2+. The results in
Fig. 6. (a) Absorbance spectra changes for SD (2.0 × 10−5 M) in presence of selected metal ions in DMSO/H2O (v/v = 1:9) solution. (b) Fluorescence intensity
changes for SD (2.0 × 10−5 M) in presence of selected metal ions in DMSO/H2O (v/v = 1:9) solution. The red bars represent the intensity of SD in the presence of
other cations (Fe3+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Cu2+, Ag+, Ca2+and Mg2+). The green bars represent the change that occurs after addition of Hg2+ to the
previously solutions, respectively.
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JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
Fig. 8. Fluorescence detection limit spectra of SD in DMSO/H2O (v/v = 1:9)
Fig. 11. Comparison between the FT-IR spectral data for SD and SD-Hg2+
solution upon addition of an increasing concentration of Hg2+
.
complex.
−OH and −CH = N, respectively. After Hg2+ was added, the
−CH = N signal downfield shifted to 9.86 ppm and the −OH signal
upfield shifted to 9.21 ppm. In addition, the proton peak in the benzene
ring also had a splitting phenomenon due to the change of electron
density caused by the ring-opening of the spirolactam of the SD. The
ring-opening was also confirmed in the Fourier transform infrared
spectrum (Fig.11). After the addition of Hg2+, the absorption peak of
carbonyl band in the spironolactone ring disappeared at 1691 cm−1
,
while
a new absorption peak of amide carbonyl appeared at
1589 cm−1, indicating that the spironolactam ring was opened. Based
on these results, Scheme 2 can be used to describe the possible reaction
mechanism between SD and Hg2+
.
In order to better confirm the proposed mechanism of SD and Hg2+
,
DFT calculations were performed at B3LYP / 6–311 g (2d, p) level [47].
When SD was complexed with Hg2+, the -CN bond of the rhodamine
spironolactone ring was destroyed, which promoted the coordination
between the carbonyl oxygen of the ionophore and the mercury ion.
Compared to the π electrons in LUMO, the π electrons in the HOMO of
Fig. 9. Job’s plots of SD and Hg2+
.
Fig. 10. 1H NMR spectra of SD and in the presence of different amounts of Hg2+ in DMSO-d6 at room temperature.
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JournalofPhotochemistry&PhotobiologyA:Chemistry379(2019)105–111
Scheme 2. The proposed reaction mechanism between SD
and Hg2+
.
Fig. 12. The DFT computed HOMO and LUMO diagram of SD and SD-Hg2+
system.
Fig. 14. linear fluorescence response of SD (0.1 M) in water samples. The re-
sponse (F) is normalized to the emission of the free sensor (F0).
SD-Hg2+ system was transferred to mercury ion, and the energy gap
(ΔE) between them was 0.049 au and 0.036 au, respectively, indicating
that the coordination of SD with Hg2+ reduced the HOMO- LUMO
energy gap and stabilizes the system (Fig.12). These calculations were
consistent with the experimental results.
water environments, we added 0–5 ppb of Hg2+ to the SD (0.1 M) so-
lution. At a concentration of 2 ppb (the maximum U.S. EPA limit for
allowable levels of Hg2+ ions in drinking water), the fluorescence
emission intensity of SD increased by 3.2 times (Fig. 14). Under the
experimental conditions, the fluorescence intensity of the SD solution
was almost proportional to the amount of Hg2+ (R2 = 0.99), indicating
that SD can distinguish the safety and toxicity levels of inorganic
mercury in drinking water.
4. Application
In order to study the application value of sensor SD in real life, the
test strips were prepared by immersing the filter papers in SD (0.1 M)
DMSO/H2O (1:9, v / v) solution and then exposing them into air to dry
them. As shown in Fig. 13, when Hg2+ was added to the test papers, a
remarkable color change could be observed under visible light and ul-
traviolet light. Therefore, SD test papers had excellent application value
The practicability of the probe SD was demonstrated by analysing
the water sample. The content of Hg2+ in tap water was measured by
fluorimetry under the same conditions, and the tests was repeated three
times, as shown in Table 1. The detected Hg2+ concentration was close
to the added concentration. The recovery rate was about 100%, and the
relative standard deviation (RSD) of the three measurements were less
than 4.20%, which indicates that it is feasible and accurate to de-
termine the Hg2+ in the environmental sample using SD.
in the detection of Hg2+
.
To further demonstrate that SD can detect Hg2+ concentrations in
5. Conclusions
In summary, we designed and synthesized a simple and effective
sensor SD, which could selectively recognize Hg2+ through dual-
channel within the pH range of 4–10. The detection limit of fluores-
cence response of SD to Hg2+ was as low as 2.71 × 10−7M based on
the basis of 3SB/S. The fluorescence process could be reversed by
adding Hg2+and EDTA, and the switching process could be repeated
several times without a large fluorescence loss. By studying the 1H NMR
and IR spectra, the proposed reaction mechanism between SD and
Hg2+ was due to the ring opening of the spironolactone ring of rho-
damine. In addition, the SD-based test strips were prepared which could
be used as a colorimetric and fluorescence measurement tool for rapid
detection of Hg2+. The qualitative detection of mercury ions in tap
Fig. 13. Photographs of SD (0.1 M) on test strips (a) only SD, (b) after im-
mersion into water solutions with Hg2+, (c) only SD under UV lamp, (d) after
immersion into water solutions with Hg2+ under UV lamp, at room temperature
and irradiation under UV light at 365 nm.
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Table 1
determination of Hg2+ in the tap water.
Sample
Hg2+added concentration / (μM)
Hg2+found concentration /(μM)
recovery rate /%
RSD / (%, n = 3)
Tap water
2
1.97
5.12
10.21
98.5
4.2
3.5
3.3
5
102.4
102.1
10
water successfully verified the feasibility of SD in practical applications.
We believe that SD as a Hg2+ sensor makes it more conspicuous for its
potential application.
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