A new rhodamine-based fluorescent chemosensor for mercury in aqueous media

Article abstract of DOI:10.1016/j.cclet.2014.04.033

A new fluorescent on-off chemosensor for Hg²⁺ initiated by a derivative of rhodamine B was designed and synthesized. Compound 1 exhibited high sensitivity and selectivity for Hg²⁺ over other commonly coexistent metal ions in aqueous media. Upon the addition of Hg ²⁺, the spirocyclic ring of probe is opened and a significant enhancement of visible color and fluorescence in the range of 500-600 nm is observed. The colorimetric and fluorescent response to Hg²⁺ can be conveniently detected by the naked eye, which provides a facile method for visual detection of Hg²⁺. From the molecular structure and spectral results of 1, an irreversible, hydrolysis, desulfurization reaction mechanism is proposed.

Full text of DOI:10.1016/j.cclet.2014.04.033

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CCLET 2977 1–4
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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A new rhodamine-based fluorescent chemosensor
for mercury in aqueous media
a,b,c
Q1 Yao Liu a, Er-Bing Yang a, Rui Han a, Di Zhang a, Yong Ye a,b, , Yu-Fen Zhao
*
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a Phosphorus Chemical Engineering Research Center of Henan Province, the College of Chemistry and Molecular Engineering,
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Zhengzhou University, Zhengzhou 450001, China
b Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100086, China
c Key Lab Chem Biol, Fujian Prov. Coll. Chem. & Chem. Engn., Xiamen University, Xiamen 361005, China
A R T I C L E I N F O
A B S T R A C T
A new fluorescent ‘‘on–off’’ chemosensor for Hg2+ initiated by a derivative of rhodamine B was designed
and synthesized. Compound 1 exhibited high sensitivity and selectivity for Hg2+ over other commonly
coexistent metal ions in aqueous media. Upon the addition of Hg2+, the spirocyclic ring of probe is
opened and a significant enhancement of visible color and fluorescence in the range of 500–600 nm is
observed. The colorimetric and fluorescent response to Hg2+ can be conveniently detected by the naked
eye, which provides a facile method for visual detection of Hg2+. From the molecular structure and
spectral results of 1, an irreversible, hydrolysis, desulfurization reaction mechanism is proposed.
ß 2014 Yao Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Article history:
Received 11 November 2013
Received in revised form 21 March 2014
Accepted 22 April 2014
Available online xxx
Keywords:
Fluorescent probe
Rhodamine
Mercury
On–off
Chemosensor
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1. Introduction
(Scheme 1) for Hg2+, which shows very high sensitivity and 32
selectivity toward Hg2+ over other metal ions.
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During the recent two decades, the design and synthesis of
compounds for sensing relevant environmentally and biologically
important ionic species, particularly for heavy metal and transition
metal cations, are currently of great interest [1]. Numerous
excellent works have focused on the selective and sensitive
detection of transition metal ions (such as, Cu2+, Pb2+, Zn2+ and
Fe3+) [2]. Governmental restrictions on the levels of residual
heavy metals in end products are also very strict. Therefore,
detection of Hg2+ levels has attracted tremendous attention. Many
probes for Hg2+ have been proposed thus far [3]. However, most of
the probes act only in organic media. Consequently, there is an
urgent need to develop chemical sensors capable of detecting
the presence of Hg2+ in aqueous media. As everyone knows,
rhodamine derivatives are non-fluorescent and colorless,
whereas ring-opening of the corresponding spirolactam gives rise
to strong fluorescence emission and a pink color [4]. Recently,
some rhodamine-based fluorescent probes were reported for
the detection of Hg2+ metal ions [5]. In this paper, we report a
new rhodamine-based derivative, turn-on fluorescent sensor 1
2. Experimental
34
All the materials for synthesis were purchased from commercial 35
suppliers and used without further purification. The solutions of 36
metal ions were prepared from their nitrate salts, except for FeCl2, 37
CrCl3 and MnCl2. The use of Fe(NO3)3 and FeCl3 yielded nearly the 38
same results. Rhodamine
B derivative (2) was synthesized 39
according to the literature [6]. A Hitachi F-4500 spectrofluorimeter 40
was used for fluorescence measurements. The absorption spectra 41
were recorded with a Techcomp UV-8500 spectrophotometer 42
(Shanghai, China). The NMR spectra were measured on a Bruker 43
AMX-400 spectrometer at 400 MHz in CDCl3. Elemental analyses 44
*
Corresponding author at: Phosphorus Chemical Engineering Research Center of
Q2
Henan Province, the College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou 450001, China.
E-mail address: yeong@zzu.edu.cn (Y. Ye).
Scheme 1. The synthesis of title compound.
http://dx.doi.org/10.1016/j.cclet.2014.04.033
1001-8417/ß 2014 Yao Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: Y. Liu, et al., A new rhodamine-based fluorescent chemosensor for mercury in aqueous media, Chin.
Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.04.033

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Y. Liu et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
0.16
L
Mg2+
K +
0.14
0.12
0.10
Co2+
Cu2+
Mn2+
Ba2+
Pb2+
Na+
Hg2+
Fig. 1. Color change of the neutral buffer solution of 1 in the presence of metal ions.
Ca2+
Hg2+
Cr3+
Ag+
Ni2+
Cd2+
Zn2+
Fe2+
0.08
0.06
0.04
0.02
0.00
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were carried out with a Flash EA 1112 instrument. Mass spectra
were acquired in positive ion mode using a Bruker ESQUIRE 3000
ion trap spectrometer equipped with a gas nebulizer probe,
capable of analyzing ions up to m/z 6000.
+
Synthesis of 1: Benzoyl chloride (0.16 g, 1.1 mmol) was slowly
added to the mixture of 2 (0.5 g, 1.0 mmol) and triethylamine
(1 mL) in 10 mL CH2Cl2 at 0 8C. After the addition, the mixture
was stirred at room temperature for 4 h. The solvent was removed
and the residue was purified by silica gel column chromatography
with CH2Cl2/MeOH (15/1, v/v) as eluent to afford 1, yield 85%. Mp
Li
Fe3+
480
500
520
540
560
580
600
620
640
Wavelength (nm)
Fig. 3. Changes in the absorption spectra of 1 in the presence of different metal ions
116–118 8C; 1H NMR (400 MHz, CDCl3, ppm):
d 8.17–8.19 (m, 1H),
in ethanol–water (4:1, v/v).
7.92 (s, 1H), 7.84 (d, 2H, J = 7.2 Hz), 7.40–7.52 (m, 5H), 7.10–7.13
(m, 1H), 6.34–6.41 (m, 2H), 6.24 (d, 2H, J = 8.8 Hz), 3.95(s, 2H),
3.32–3.37 (m, 8H), 1.17–1.20 (m, 12H); ESI-MS: m/z 605.6 [M+H]+;
Anal. calcd. for C37H40N4O2S: C 73.48, H 6.67, N 9.26, found: C
73.42, H 6.69, N 9.30.
To further investigate the interaction of Hg2+ and 1, an
ultraviolet photometric titration experiment was carried out. An
increasing, linear absorption intensity of 1 could be observed with
increasing Hg2+ concentration accompanied by color changes from
colorless to pink. To determine the stoichiometry of the Hg-ligand
complex, Job’s method for absorbance measurement was applied.
Keeping the sum of the initial concentration of Hg2+ and 1 at
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3. Results and discussion
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m
mol/L, the molar ratio of Hg2+ was varied from 0 to 2. A plot of
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Compound 1 was facilely synthesized from rhodamine B
according to procedures in the literature and obtained as light
orange crystals. As shown in Scheme 1, compound 1 was prepared
in high yield by reacting 1 with benzoyl chloride in the presence of
triethylamine. Its structure has been confirmed using 1H NMR, ESI
mass spectrometry, and elemental analysis. Although 1 is a
derivative of rhodamine B, it forms a nearly colorless solution in
either water or absolute ethanol, indicating that the spirocyclic
form exists predominantly. Besides, neither the color nor the
fluorescence (excited at 530 nm) characteristics of rhodamine
could be observed for 1 in ethanol–water (4:1, v/v), suggesting that
the spirocyclic form was still preferred. Addition of mercuric ion to
the solution of 1 causes instantaneous development of a pink color
and a strong fluorescence (Fig. 1). This observation shows that the
mercury-induced, ring-opening reaction takes place rapidly at
room temperature.
[Hg2+]/{[Hg2+] + [1]} versus the molar fraction of Hg2+ was
provided in Fig. 2. It showed that the [Hg2+]/{[Hg2+] + [1]} value
went through a maximum at a molar fraction of 0.5, indicating a
1:1 stoichiometry of the Hg2+ to 1 in the complex.
Fig. 3 shows the absorption spectra of 1 in the presence of
various metal ions in ethanol-water. When no metal ion was added
to the solution of 1 (10
mmol/L), almost no absorption above
562 nm could be observed, whereas a significant enhancement of
the characteristic absorption of rhodamine B emerged soon after
Hg2+ was injected into the solution. There was a large enhance-
ment factor (34-fold) of absorbance at
lmax = 562 nm upon the
addition of 20 equiv. of Hg2+. Other cations of interest gave no
response (Fig. 4).
Changes in the fluorescence properties of 1 caused by other
metal ions, including Mg2+, K+, Co2+, Cu2+, Mn2+, Ba2+, Pb2+, Na+,
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
[
0.6
]
0.8
1.0
Hg2+]/[Hg2++L
Fig. 2. Plots according to the method for continuous variations, indicating the 1:1
stoichiometry for 1–Hg2+ (the total concentration of 1 and Hg2+ is 100
mol/L).
Fig. 4. Change in the absorbance at 562 nm of 1 (10
mmol/L) in presence of 20 equiv.
m
of various different metal ions in ethanol–water (4:1,v/v).
Please cite this article in press as: Y. Liu, et al., A new rhodamine-based fluorescent chemosensor for mercury in aqueous media, Chin.
Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.04.033

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Y. Liu et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Fig. 5. Fluorescence spectra of 1 (10
mmol/L) in ethanol–water (4:1,v/v) in the
presence of 1 equiv. of Mg2+, K+, Co2+, Cu2+, Mn2+, Ba2+, Pb2+, Na+, Ca2+, Hg2+, Zn2+
Ni2+, Cd2+, Ag+, Fe2+, Li+, Cr3+ and Fe3+ Ex: 560 nm, Em: 580 nm, slit: 5.
,
Scheme 2. Proposed mechanism.
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Ca2+, Cr3+, Ag+, Ni2+, Cd2+, Zn2+, Fe2+, Li+ and Fe3+ were also
measured. Fluorescence spectra of solutions of 1 (10 mol/L),
m
recorded within 5 min after the addition of 1 equiv. of each of these
metal ions, are displayed in Fig. 5. Only Hg2+ ion promote
significant fluorescence intensity changes, while other metal ions
did not cause any changes under identical conditions. In addition,
the enhancement in fluorescence intensity resulting from addition
of Hg2+ is not influenced by subsequent addition of other metal
ions. Finally, while the colorless to pink color change associated
with the reaction of 1 with Hg2+ is readily detected visually, no
significant color changes are promoted by other metal ions. This
interesting feature reveals that 1 can serve as a selective ‘‘naked-
eye’’ chemosensor for Hg2+
.
Generally, one of the most important and useful applications for
a fluorescent chemosensor is the detection of metal ions, especially
for heavy metals. The limit of the chemosensor for Hg2+ ion has
Fig. 7. ESI-MS of 1 + Hg2+.
been tested. When 1 was employed at 0.1
mmol/L in EtOH/H2O
(4:1, v/v), the fluorescent intensity of 1 was proportional to the
added concentration of Hg2+ (Fig. 6). The detection limit was
measured to be 1.0 ppb, establishing 1 was capable of distinguish-
ing safe and toxic levels of Hg2+ in drinking water according to the
U.S. (EPA) standard (2 ppb).
results of 1, it is concluded that the reaction mechanism should 125
involve two steps. First, the addition of the Hg2+ ion induced 126
the N atom of amide to attack the C atom of C55S bond, and 127
thus a ring opening of the spirolactam of rhodamine took place. 128
Both UV –vis and fluorescence data lead to a significant
OFF –ON signal. From the molecular structure and spectral
Second,
a water-promoted hydrolysis followed. After the 129
removal of HgS and phthalic acid, an intramolecular guanylation 130
took place. Finally, a cyclic product 5 was formed through an 131
irreversible hydrolysis, desulfurization reaction, as depicted 132
in Scheme 2. In order to prove this reaction mechanism of 133
the present system, the reaction products of 1 with Hg2+ were 134
subjected to electrospray ionization mass spectral analyses. 135
In ESI-MS, a major ion peaks were detected at m/z 471.4 (Fig. 7). 136
It was characterized as the ring opened product of 137
rhodamine derivative, indicating the generation of 5 as a final 138
2.5x106
2.0x106
1.5x106
1.0x106
product.
139
5.0x105
0.0
4. Conclusion
140
In conclusion, we synthesized
a
new rhodamine-based 141
fluorescent probe for Hg2+. The colorimetric and fluorescent 142
response to Hg2+ can be conveniently detected by the naked 143
0
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20
Hg2+ (ppb)
eye, which provides a facile method for visual detection of Hg2+
.
144
The selectivity of this system for Hg2+ over other metal ions is 145
excellent. 146
Fig. 6. The fluorescence intensity at 580 nm of compound 1 (0.1
function of the Hg2+ concentration (0–20 ppb) in ethanol–water (4:1,v/v).
mmol/L) as a
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Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.04.033