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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 1961–1965 1961
Cite this: New J. Chem., 2012, 36, 1961–1965
A bis(rhodamine)-based highly sensitive and selective fluorescent
chemosensor for Hg(II) in aqueous mediaw
Rui Han,aXian Yang,
aDi Zhang,
aMin Fan,
aYong Ye*
acand Yufen Zhao
abc
Received (in Gainesville, FL, USA) 17th March 2012, Accepted 23rd August 2012
DOI: 10.1039/c2nj40638a
A new bis(rhodamine)-based fluorescent probe 3 was syn-
thesized, and it exhibited high selectivity for Hg2+
over other
commonly coexistent metal ions in aqueous media with a broad
pH span (3–9).
Hg2+ is one of the most hazardous components in the
environment. Its frequent and fruitful use can result in a high
level of residues, which may result in the contamination of
water systems and soil, and therefore cause health hazards.1
Governmental restrictions on the levels of residual heavy
metals in end products are very strict. Therefore, Hg detection
has attracted tremendous attention. The design and develop-
ment of fluorescent Hg2+ probes has therefore attracted a
great deal of attention. A lot of Hg2+ probes have been
proposed so far;2 however, most of the probes act only in
organic media. Hg2+ probes capable of acting in aqueous
media have also been proposed; however, many of these
probes show insufficient selectivity for Hg2+.3 The design of
Hg2+ probes with high selectivity in aqueous media is there-
fore of current focus. In addition, Hg2+ exists in various
places such as in living organisms, soil, rivers, and seas.4
Versatile Hg2+ probes must therefore be applicable to samples
with a broad pH range. Several Hg2+ probes showing high
selectively in aqueous media have been proposed; however,
most of these probes act only at neutral pH,5 and only a few
probes act at an acidic–neutral pH6 or at a relatively broader
pH range.7 On the basis of the well-known spirolactam (non-
fluorescent) to ring-open amide (fluorescent) equilibrium,
rhodamine frameworks have been considered as an ideal mode
for the construction of the OFF-ON systems that have fre-
quently been utilized to design fluorescence-enhanced probes
for metal ions.8 Here, we report a new bis(rhodamine)-based
fluorescent probe that exhibited high selectivity for Hg2+ over
other commonly coexistent metal ions in aqueous media and
in a broad pH range.
As shown in Scheme 1, compound 2 was facilely synthesized
from rhodamine B derivative 1 and Lawesson reagent in
moderate yield in toluene. Compound 3 was prepared by the
reaction of compound 2 and benzenedicarbonyl dichloride in
the presence of triethylamine. Their structures have been
confirmed using 1H NMR, 13C NMR, ESI mass spectrometry,
and elemental analysis (see ESIw). Although 3 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. The characteristic peak near
65.6 ppm (9-carbon) in the 13C NMR spectrum of 3 also
supports this consideration. Besides, neither the color nor
the fluorescence (excited at 580 nm) characteristics of
rhodamine could be observed for 3 in water or ethanol,
suggesting that the spirocyclic form was still preferred in this
range. Addition of mercuric ion to the solution of 3 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.
To further investigate the interaction of Hg2+ and 3, an
ultraviolet photometric titration experiment was carried out
(Fig. 2). A linear increase of absorption intensity of 3 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.10 Keeping the sum of
Scheme 1 The synthesis of the title compound 3.
a Key Lab of Chemical Biology and Organic Chemistry of HenanProvince, Department of Chemistry, Zhengzhou University,Zhengzhou 450052, China. E-mail: yeyong03@tsinghua.org.cn;Fax: +86 371 67767051; Tel: +86 371 67767050
bDepartment of Chemistry and Key Laboratory for Chemical biologyof Fujian Province, Xiamen University, Xiamen 361005, China
c Key Laboratory of Bioorganic Phosphorus Chemistry & ChemicalBiology (Ministry of Education), Department of Chemistry,Tsinghua University, Beijing 100084, Chinaw Electronic supplementary information (ESI) available. See DOI:10.1039/c2nj40638a
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1962 New J. Chem., 2012, 36, 1961–1965 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
the initial concentration of Hg2+ and 3 at 100 mM, the molar
ratio of Hg2+ was varied from 0 to 2. A plot of [Hg2+]/
{[Hg2+] + [3]} versus the molar fraction of Hg2+ is provided
in Fig. S5 (ESIw). It showed that the [Hg2+]/{[Hg2+] + [3]}
value went through a maximum at a molar fraction of
0.36, indicating a 1 : 2 stoichiometry of Hg2+ to 3 in the
complex.
Further evidence for this stoichiometry comes from the
fluorescence experiment. A fluorescence titration of Hg2+
was conducted using 80 mM solution of 3 in water–ethanol
(20/80 v/v) at pH 7. The observed fluorescence intensity was
nearly proportional to the Hg2+ concentration. The satura-
tion behavior of the fluorescence intensity after 2 equiv. of
Hg2+ reveals that the Hg2+ chemodosimeter has a 1 : 2
stoichiometry (Fig. 3).
Fig. 4 shows the absorption spectra of 3 in the presence of
various metal ions in ethanol–water. When no metal ion was
added to the solution of 3 (10 mM), 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 enhancement factor (500-fold) of absorbance at
lmax = 562 nm upon the addition of 20 equiv. of Hg2+. Other
cations of interest gave no response (Fig. 5).
Changes in the fluorescence properties of 3 caused by other
metal ions, including Ag+, Na+, Cr3+, Pb2+, Mg2+, K+,
Co2+, Mn2+, Ba2+, Ca2+, Al3+, Li+, Cu2+, Fe2+, Fe3+,
Ni2+, Zn2+, Cd2+, were also measured. The fluorescence
spectra of solutions of 3 (10 mM), recorded within 5 min after
the addition 2 equiv. of each of these metal ions, are displayed
in Fig. 6. Only Ag+ ions promote small fluorescence intensity
changes, while other metal ions did not cause any significant
changes under identical conditions. The selectivity observed
for Hg2+ over other ions is remarkably high. In addition, the
enhancement in fluorescence intensity resulting from the addi-
tion 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 3 with Hg2+ is visually readily
detectable, no significant color changes are promoted by other
metal ions (Fig. S6, ESIw). This interesting feature reveals that
3 can serve as a selective ‘‘naked-eye’’ chemosensor for Hg2+.
Fig. 1 The color (top) and the fluorescent (bottom) changes of 3 to
Hg2+ in EtOH–water solution (80 : 20, v/v) [3] = 40 mM, [Hg2+] =
40 mM.
Fig. 2 Absorption spectra of 3 in C2H5OH–H2O (80/20, v/v) upon
addition of different amounts of Hg2+ ions.
Fig. 3 Fluorescence intensities of 3 with gradual addition of different
amounts of Hg2+ (from bottom 0–8 eq. Hg2+).
Fig. 4 Changes in the absorption spectra of 3 in the presence of
different metal ions in ethanol–water (80 : 20, v/v).
Fig. 5 Change in the absorbance at 562 nm of 3 (10 mM) in presence
of 20 eq. of various different metal ions in ethanol–water (80 : 20, v/v).
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 1961–1965 1963
To check the practical ability of compound 3 as a Hg2+
selective fluorescent sensor, we carried out competitive experi-
ments in the presence of Hg2+ at 10 equiv. mixed with Mg2+,
K+, Co2+, Cu2+, Mn2+, Ba2+, Na+, Ca2+, Pb2+, Cr3+,
Ag+, Ni2+, Cd2+, Zn2+, Fe2+, Fe3+ and Li+ (10 equiv.
each). No significant variation in the fluorescence emission
was observed by comparison, with or without the other metal
ions (Fig. S7, ESIw). The fluorescence enhancement of 3 with
HgCl2 has almost no change with the addition of different
anions, such as CH3COO�, NO3�, SO4
2�, F�, ClO4� and Br�
(Fig. S8, ESIw).To investigate the practical applicability of 3, the detection
limit of this new chemodosimeter system was evaluated. The
fluorescence titration profile of 3 (10 mM) with Hg2+, shown in
Fig. 7, demonstrates that the detection of Hg2+ is at the parts
per billion level. Under these conditions, the fluorescence
intensity of the solution of 3 was nearly proportional to the
amount of Hg2+ added (Fig. 7b). The detection limit was
measured to be 0.128 ppm. Therefore, the proposed probe 3
was sensitive enough to detect Hg2+ in industrial wastewater,
which has a discharge limit of 0.25 mM (50 ppb) defined by the
Standardization Administration (SA) of the People’s Republic
of China.11
For practical applicability, the optimized pH conditions of
this new probe was also evaluated (Fig. S9, ESIw). For free 3,at acid conditions (pH o 3), the ring opening of rhodamine
took place because of the strong protonation. When pH > 3,
no significant ring opening was observed. However, in the
presence of Hg2+ ions, there was an obvious fluorescence
OFF-ON change between pH 3 and 9. Thus, probe 3 can
detect Hg2+ ions with a wide pH span (3–9), because in this
region, 3 with the Hg2+ ions induces a remarkable fluores-
cence OFF-ON, whereas 3 without the Hg2+ ions does not
lead to such a change.
Both UV-vis and fluorescence data lead to a significant
OFF-ON signal. From the molecular structure and spectral
results of 3, it is concluded that the reaction mechanism should
involve two steps. First, the addition of the Hg2+ ions induces
the N atom of the amide to attack the C atom of the CQS
bond, and thus a ring opening of the spirolactam of rhodamine
took place. Secondly, water-promoted hydrolysis took place.
After the removal of HgS and m-phthalic acid, an intra-
molecular guanylation took place. Finally, a cyclic product 5
was formed through an irreversible hydrolysis desulfurization
reaction, as depicted in Scheme 2.
In order to prove the reaction mechanism of the present
system, the reaction products of 3 with Hg2+ were subjected to
electrospray ionization mass spectral analyses. In the ESI-MS
spectra, a major ion peak was detected at m/z 471.4 (Fig. S10,
ESIw). It was characterized to be the ring opening product of
Fig. 6 Fluorescence spectra of 3 (10 mM) in ethanol–water (80 : 20,
v/v) in the presence of 2 equiv. of Hg2+, Ag+, Na+, Cr3+, Pb2+,
Mg2+, K+, Co2+, Mn2+, Ba2+, Ca2+, Al3+, Li+, Cu2+, Fe2+, Fe3+,
Ni2+, Zn2+, Cd2+. Ex: 560 nm, Em: 580 nm, slit: 5.
Fig. 7 (a) Fluorescence emission changes of 3 (10 mM) upon addition
of Hg2+ in water–ethanol (20/80 v/v) at 25 1C. (b) The fluorescence
intensities at 560 nm.
Scheme 2 The proposed mechanism for the fluorescence OFF-ON.
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1964 New J. Chem., 2012, 36, 1961–1965 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
rhodamine, indicating the generation of 5 as a final product.
According to ESI-MSn, the fragment ion at m/z 443, derived
from the precursor ion at m/z 471, can be further fragmented
into the ions at m/z 399. The fragmentation pathway of
compound 5 is shown in Fig. S11. High-resolution FTICR-
MS indicated that the exact mass of the ion at m/z 471 is
471.3136, corresponding to the formula C30H39N4O (calcu-
lated 471.3118).
In conclusion, we synthesized a new bis(rhodamine)-based
fluorescent probe for Hg2+ detection. The colorimetric and
fluorescent response to Hg2+ can be conveniently detected
even by the naked eye, which provides a facile method for
visual detection of Hg2+. The selectivity of this system for
Hg2+ over other metal ions is excellent, and the detection of
Hg2+ at the 0.128 ppm level is still available. There are two
good features of this system: (i) a remarkably high selectivity
toward Hg2+ ions over miscellaneous competitive cations, and
(ii) a wide pH span (3–9). These features make it a promising
candidate for the determination of Hg2+ ions in aqueous
solution for practical analysis. The cell-permeable property
and synthesis of a series of similar compounds were con-
ducted. The results will be published elsewhere.
Experimental
General
All the materials for the syntheses were purchased from
commercial suppliers and used without further purification.
The solutions of metal ions were prepared from their nitrate
salts, except for FeCl2, CrCl3 and MnCl2. The use of Fe(NO3)3and FeCl3 yielded nearly the same results. Rhodamine B
derivative (1) was synthesized according to the literature.9 A
Hitachi F-4500 spectrofluorimeter was used for fluorescence
measurements. The absorption spectra were recorded with a
Techcomp UV-8500 spectrophotometer (Shanghai, China).
NMR spectra were measured on a Bruker AMX-400 spectro-
meter at 400 MHz in CDCl3. Elemental analyses 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 2
Lawesson reagent (0.5055 g, 1.25 mmol) was added in three
portions to a solution of 1 (1.21 g, 2.5 mmol) in 30 mL dry
toluene under argon. The mixture was refluxed for 12 h. After
removal of the solvent, the residue was purified by silica gel
column chromatography with CH2Cl2/MeOH (35/1, v/v),
yielding the desired product 2 as a red solid. Yield 45.7%,
ESI-MS: m/z 500.3 [M + H]+.
Synthesis of 3
A solution of benzenedicarbonyl dichloride (0.202 g, 1 mmol)
in 20 mL CH2Cl2 was slowly added to a mixture of 2 (0.500 g,
1 mmol) and triethylamine (0.5 mL) in 15 mL CH2Cl2 at
0 1C. 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 (20/1, v/v) as eluent to afford 3, yield 83.5%.
M.p. 174 1C–176 1C. 1H NMR (400 MHz, CDCl3, d ppm):
8.16–8.20 (m, 3H), 7.93 (d, 2H, J = 8.4 Hz), 7.76 (s, 2H),
7.45–7.50 (m, 5H), 7.10 (d, 2H, J = 7.6 Hz), 6.35–6.41
(m, 8H), 6.25 (d, 4H, J = 8.4 Hz), 4.32 (bs, 4H), 3.30–3.34
(m, 20H), 1.16–1.19 (m, 24H); 13C NMR (100 MHz, CDCl3,
d ppm): 192.8, 166.3, 153.2, 151.4, 149.3, 137.5, 134.3, 132.8,
130.9, 130.1, 128.8, 128.6, 128.4, 125.9, 125.4, 123.3, 108.4,
103.2, 97.9, 65.6, 44.4, 43.3, 41.0, 12.6; ESI-MS: m/z 1130.5
[M + H]+; Anal. calcd for C68H74N8O4S2: C 72.18, H 6.59,
N 9.90, found: C 72.22, H 6.71, N 9.87.
Acknowledgements
This work was financially supported by the National
Science Foundation of China (nos. 20972143, 20972130) and
Program for New Century Excellent Talents in University
(NCET-11-0950).
Notes and references
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