Highly sensitive and selective turn-on fluorescent chemosensor for Pb 2+ and Hg2+ based on a rhodamine-phenylurea conjugate

Article abstract of DOI:10.1039/c001587c

A novel easily available turn-on fluorescent chemosensor RPU based on a rhodamine-phenylurea conjugate was synthesized, which showed highly selective and sensitive recognition toward Pb²⁺ in CH₃CN and Hg ²⁺ in aqueous media over a wide range of tested metal ions with remarkably enhanced fluorescent intensities and also clear color changes from colorless to pink.

Full text of DOI:10.1039/c001587c

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Highly sensitive and selective turn-on fluorescent chemosensor for Pb2+
and Hg2+ based on a rhodamine–phenylurea conjugatew
Zhi-Qiang Hu,*a Cun-sheng Lin,a Xiao-Ming Wang,a Lei Ding,a Cun-Li Cui,a
Shu-Feng Liu*a and Hai Yan Lu*b
Received 25th January 2010, Accepted 23rd March 2010
First published as an Advance Article on the web 16th April 2010
DOI: 10.1039/c001587c
A novel easily available turn-on fluorescent chemosensor RPU
based on a rhodamine–phenylurea conjugate was synthesized,
which showed highly selective and sensitive recognition toward
Pb2+ in CH3CN and Hg2+ in aqueous media over a wide range
of tested metal ions with remarkably enhanced fluorescent
intensities and also clear color changes from colorless to pink.
applications, the development of a small molecule probe that
can sense both Pb2+ and Hg2+ simultaneously is more
desirable.
The rhodamine framework is an ideal mode to construct
fluorescent chemosensors due to its excellent photophysical
properties such as long absorption and emission wavelength,
large absorption coefficient and high fluorescence quantum
yield.8 Several rhodamine-based probes for mercury ion have
been recently developed.9 In contrast, only few rhodamine
based probes for lead(II) has been reported.10 Most of the
reported rhodamine fluorescent probes are based on rhodamine
hydrazide derivatives or rhodamine amine conjugates.11 Herein,
we report a new turn-on fluorescent chemosensor based on
rhodamine–phenylurea conjugate (RPU), which can sensitively
and selectively detect Pb2+ in CH3CN, and Hg2+ in aqueous
media and display remarkably enhanced fluorescence intensities
and clear color changes upon recognition. To the best of our
knowledge, this is the first example of a fluorescent sensor
based on a small molecule that allows the detection of both
Pb2+ and Hg2+ ions.
The detection of heavy metal ions is an important issue
because contamination with heavy metal ions may have severe
effects on human health and the environment.1 Lead(II) and
mercury ions are two of the most toxic heavy metal ions. The
accumulation of lead in the body can result in neurological,
reproductive, cardiovascular and developmental disorders2
while mercury can cause a wide variety of diseases such as
prenatal brain damage, serious cognitive and motion disorders.3
Currently, the common techniques for detection of lead(II) ion
and mercury ion, such as atomic absorption spectrometry and
inductively coupled plasma mass spectrometry, are often
expensive, complex, and unsuitable for on-site analyses.
Therefore, the development of new analytical methods for
the sensitive and selective determination of Pb2+ and Hg2+, is
highly desirable. Because of its operational simplicity, low
cost, real time monitoring and high selectivity, fluorescent
detection has become the promising strategy used for Pb2+
and Hg2+ detection.4
Synthesis of RPU is shown in Scheme 1. Reaction of
rhodamine B base 1 with POCl3 and then NH3 afforded 2,
which further reacted with phenyl isocyanate to give RPU in
47% yield. Similarly, rhodamine–phenylthiourea conjugate
(RPTU) was also synthesized by the reaction of 2 and phenyl
isothiocyanate. The structures of RPU and RPTU were
confirmed by their 1H NMR, 13C NMR, MS spectra and
EA (Fig. S1–S4, ESIw).
All solutions of RPU in CH3CN and CH3CN–H2O are
colorless. The absorption spectra of RPU (10 mM) in CH3CN
exhibited only very weak bands over 500 nm. Addition of
50 equiv. Pb2+ into CH3CN solution immediately resulted in a
significant enhancement of absorbance at about 560 nm and a
shoulder peak appeared at 525 nm (Fig. S5, ESIw). Under the
identical condition, only a very mild increase of absorbance at
564 nm was observed with the same amount of Hg2+ while no
Recently, considerable efforts have been undertaken to
develop fluorescent probes for lead(II) ion,5 and mercury ions.6
However, many reported fluorescent chemosensors generally
undergo fluorescence quenching upon binding with these two
metal ions, which is not as sensitive as a fluorescence enhancement
response. In addition, the fluorescence enhancement in most
cases is small and usually suffers from a high background. In
particular, most of the reported fluorescent chemosensors can
selectively sense only one of them. The only example reported
by Liu et al. that can detect these two metal ions are involved
in the use of DNA,7 which usually lack the simplicity that a
small-molecule fluorescence probe can offer. For practical
a College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 266042, China.
E-mail: huzhiqiang@iccas.ac.cn; Fax: +86-0532-84023405
b College of Chemistry and Chemical Engineering, Graduate
University of Chinese Academy of Sciences, Beijing 100049, China
w Electronic supplementary information (ESI) available: Apparatus
and reagents; synthesis and structural characterization of RPU and
RPTU, absorption spectral responses of RPU for Pb2+ and Hg2+, Job
plot of complex, reversibility analysis of RPU and Hg2+, competitive
experiments, fluorescence response of RPTU for Hg2+. See DOI:
10.1039/c001587c
Scheme 1 Synthesis of RPU and RPTU.
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established a 2 : 1 stoichiometry for the RPU–Pb2+ complex and
a 1:1 stoichiometry for the RPU–Hg2+ complex (Fig. S10 and
S11, ESIw). The different binding stoichiometry between
RPU–Pb2+ and RPU–Hg2+ was also confirmed by the Benesi–
Hidebrand method (Fig. S12 and S13, ESIw). The association
constants were further determined to be 7.4 Â 107 MÀ1 for the
RPU–Pb2+ complex and 3.08 Â 107 MÀ2 for the RPU–Hg2+
complex. The inset pictures (Fig. 2) show relative intensities (I/I0)
vs. the concentrations of Pb2+ and Hg2+ in the low concentra-
tion region up to 7 Â 10À9 and 3.5 Â 10À8 M, respectively. These
revealed the detection limits of RPU for Pb2+ and Hg2+ around
7 Â 10À9 and 3.5 Â 10À8 M, respectively.
Fig. 1 Fluorescence intensity of RPU (1 mM) in (a) CH3CN upon the
addition of 100 equiv. metal ions and (b) CH3CN–H2O (3/7, v/v) upon
The possible interferences by other metal ions were assessed
through competitive experiments. The fluorescence changes of
RPU in CH3CN were measured by the treatment of 10 equiv.
Pb2+ ions in the presence of 200 equiv. other interfering metal
ions including Zn2+, Mg2+, Ca2+, Cd2+, Cu2+, Ba2+, K+,
Co2+, Ce3+, Na+, Ag+, Ni2+, Hg2+. The tested background
metal ions showed small or no interference with the detection
of Pb2+ ion. Similarly, the competitive experiments also
confirmed that background metal ions showed small or no
interference with the detection of Hg2+ ion in CH3CN–H2O
(3/7, v/v) (Fig. S14, ESIw).
the addition of 130 equiv. metal ions (Pb2+, Hg2+, Mg2+, Ce3+, Cd2+
,
Na+, Cu2+, K+, Ag+, Co2+, Zn2+, Ni2+, Ba2+, Ca2+). lex = 530 nm.
obvious response could be observed upon the addition of
Mg2+, Ce3+, Cd2+, Na+, Zn2+, K+, Cu2+, Ag+, Co2+
,
Ni2+, Ca2+, Ba2+ into CH3CN solution. Fig. 1a shows the
fluorescence spectra (lex = 530 nm) of RPU (1 mM) measured
in CH3CN with given metal cations (100 equiv.). RPU shows a
very weak fluorescence in the absence of metal ions. Its
fluorescence quantum yield is about 0.003 with rhodamine B
in methanol as standard. However, the addition of Pb2+
resulted in a remarkably enhanced fluorescence at 579 nm
(about 1000-fold), and the fluorescence quantum yield is 0.32.
The color of the solution also changed from colorless to
pink-red. This strongly suggested that RPU can serve as a
‘‘naked eye’’ probe for Pb2+. Under the same conditions,
additions of Hg2+ only induced about 40-fold fluorescence
enhancement at 575 nm but no color change of the solution.
Addition of other metals led to no obvious fluorescent enhance-
ments and color changes. Similar results could be obtained
upon changing the solvent from CH3CN to CH3COCH3.
The response of RPU to Pb2+ is significantly affected by the
content of water in solution. No obvious absorption and
emission response could be observed in CH3CN–H2O (3/7, v/v).
This suggests that water molecule could compete with RPU for
coordination to Pb2+ and result in degradation of the emission
response.12 However, addition of Hg2+ into CH3CN–H2O
(3/7, v/v) solution of RPU resulted in a significant enhancement
of absorbance (Fig. S6, ESIw) and fluorescence at 584 nm
(Fig. 1b) with the change of color from colorless to pink.
Additions of other metal ions to CH3CN–H2O (3/7, v/v)
solution of RPU caused no obvious response. Herein, it should
be pointed out that the same phenomenon could be also
observed in solutions with other volume ratios of water and
CH3CN (1 : 4 to 4 : 1) (Fig. S7, ESIw). Moreover, addition of
Hg(ClO4)2 made the fluorescence of RPU increase as strongly as
that of Hg(NO3)2 and Hg(AcO)2 (Fig. S8, ESIw). Furthermore,
addition of KI to a solution containing RPU and Hg2+ led to
immediate disappearance of both pink color and fluorescence
(Fig. S9, ESIw). This indicates that RPU reversibly coordinates
with Hg2+. These results show that RPU can selectively and
sensitively sense Pb2+ and Hg2+ in the different media.
The binding mechanism was preliminarily investigated
through 1H NMR. In the spectrum of RPU, the resonance
Fig. 2 (a) Fluorescence titration spectra of RPU (1 mM) with Pb2+
(0–100 mM) in CH3CN; Inset: the fluorescence at 579 nm of RPU
(0.1 mM) as a function of the Pb2+ concentration (0–7 Â 10À9 M); (b)
Fluorescence titration spectra of RPU (1 mM) with Hg2+ (0–130 mM)
in CH3CN–H2O (3/7, v/v); Inset: the fluorescence at 575 nm of RPU
(0.1 mM) as a function of the Hg2+ concentration (0–3.5 Â 10À8 M).
lex = 530 nm.
Fluorescence titrations of RPU with Pb2+ in CH3CN and
Hg2+ in CH3CN–H2O (3 : 7, v/v) were then performed. As
shown in Fig. 2, an increase of fluorescence intensity could be
observed with increasing Pb2+ and Hg2+ concentration. Binding
analysis using the method of continuous variations (Job plot)
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for NH appears at 10.92 ppm. Addition of Hg2+ into the
solution led to an apparent upfield shift to 10.63 ppm. From
this observation it can be deduced that NH does not participate
in the complexation of RPU and Hg2+. The downshift peaks
near 6.3 ppm, which were assigned to the proton signals of
xanthene, proved the delocalization of xanthene. With the
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1
addition of Pb2+ instead of Hg2+, similar results in H NMR
of RPU could be also obtained (Fig. S15, ESIw). To further
explore the binding mechanism, RPTU was synthesized as a
substitute of RPU and tested for fluorescence changes upon
the addition of Hg2+ and Pb2+ ions. Unlike RPU, RPTU
exhibited high selectivity and sensitivity for Hg2+ in CH3CN
(Fig. S16–S19, ESIw). However, in the CH3CN–H2O system,
additions of the tested metal ions including Hg2+ or Pb2+ ions,
created no obvious fluorescence enhancements and color
changes. This confirmed that the CQO group in RPU played
a crucial role in the complexation with Hg2+ or Pb2+. From
these data, it could be deduced that the possible coordination
atoms for Pb2+ and Hg2+ were two O atoms. The exact binding
modes between RPU and Hg2+ or Pb2+ are being investigated.
In conclusion, we have reported a novel easily available
turn-on fluorescent chemosensor based on a rhodamine–
phenylurea conjugate. It displayed an excellent selectivity
and a high sensitivity toward the detection of Pb2+ in CH3CN
and Hg2+ in aqueous media over a wide range of tested metal
ions with remarkably enhanced fluorescent intensities and also
clear color changes from colorless to pink. The background
metal ions showed small or no interference with the detection
of Pb2+ or Hg2+ ion. Moreover, compared with the reported
probes for Pb2+ or Hg2+, this is the first chemosensor based
on a small molecule that can detect both Pb2+ and Hg2+ at
1 nM and 10 nM level, respectively. We believe that the RPU
can be used for many practical applications in chemical,
environmental and biological systems.
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The authors are grateful for the financial support from the
National Natural Science Foundation of China No. 20602022
and Natural Science Foundation of Shandong province No.
Y2007B39.
Notes and references
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