A tryptophan-containing fluoroionophore sensor with high sensitivity to and selectivity for lead ion in water

Article abstract of DOI:10.1039/b604623a

We report herein a fluoroionophore sensor derivated from tryptophan that shows high sensitivity (detection limit up to 0.15 μM) and specific selectivity for lead ion (Pb²⁺) over Ca²⁺, Cd ²⁺, Co²⁺, Cr³⁺, Cu²⁺, K⁺, Mg²⁺, Na⁺, Fe²⁺, Mn²⁺, Ni ²⁺ and Zn²⁺ in aqueous solution. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604623a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
A tryptophan-containing fluoroionophore sensor with high sensitivity to
and selectivity for lead ion in water{
Li-Jun Ma, Yi-Fu Liu and Yuqing Wu*
Received (in Cambridge, UK) 30th March 2006, Accepted 8th May 2006
First published as an Advance Article on the web 23rd May 2006
DOI: 10.1039/b604623a
We report herein a fluoroionophore sensor derivated from
tryptophan that shows high sensitivity (detection limit up to
0.15 mM) and specific selectivity for lead ion (Pb2+) over Ca2+,
Cd2+, Co2+, Cr3+, Cu2+, K+, Mg2+, Na+, Fe2+, Mn2+, Ni2+ and
Zn2+ in aqueous solution.
Fluoroionophore chemosensors are becoming increasingly popular
due to their easy use in solution as well as their high sensitivity to
and selectivity for trace analytes.1 Many efficient fluoroionophores
Fig. 1 The chemical structure and atom numbering of N-[4(1-pyrene)-
have been developed for the specific recognition of metal ions
including alkali metals, alkaline earth metals and zinc ions.2
butyroyl]-L-tryptophan (PLT).
However, the development of fluoroionophores for Pb2+ detection
and lead ion, but also distinguishes Pb2+ from other metal ions
has been limited due to the high fluorescence quenching abilities of
heavy metal ions. The detection of Pb2+ in the environment and in
and exhibits a very high sensitivity (0.15 mM) in aqueous
solution. Another advantage afforded by PLT is that its
biological systems is important because of its toxicity.3 Hayashita
interactions with metal ions closely mimic those of a natural
et al. reported a podand fluoroionophore4a and a PD-18C6/Triton
tryptophan-containing peptide or protein. Thus, these studies
X-100 complex sensor4b that both exhibited high selectivities for
using PLT as a model will provide a foundation for future
lead ion in water, but the sensitivities were insufficient for practical
biophysical examination of recognition mechanisms between
Pb2+ analyses. Two other fluorescent chemosensors developed by
proteins and metal ions.
Me´tivier et al.4c and Kwon et al.4d exhibited much higher
Fluorescence and NMR spectra were used to assess the
sensitivities and selectivities for Pb2+, but required a high
cooperative interactions between metal ions and the tryptophan
concentration or a pure organic medium for analysis. Therefore,
moiety in PLT. Fig. 2(a) shows changes in the fluorescence
development of a specific fluoroionophore for the real-time and
spectrum of PLT with the addition of lead ion in 98% water–2%
in situ detection of Pb2+ in water needs to be explored for practical
DMSO (v/v; pH 6.4). Obvious differences in fluorescence spectra
analytical applications.
were observed in the presence of different concentrations of Pb2+.
Pyrene, with its defined emission spectrum and aggregation
The increase in the broad emission band at 470 nm corresponding
capability, is often employed as a fluorescent probe.5 In
to pyrene excimer,4 and the concomitant decrease in monomer
addition, noncovalent binding forces within the metal ion-
emission in the region of 370–410 nm, are clearly observed. In the
indole ring of tryptophan (Trp, W) have recently been the
presence of Pb2+, the absorption spectrum of PLT shows
subject of much investigation due to the functional and
characteristic features (see ESI{) attributed to ground-state
structural importance of such interactions in chemistry and
interactions between two pyrene rings.4,8,9 These results strongly
biology.6 Various types of unique metal–indole bonds, depen-
demonstrate that a pyrene dimer is formed upon binding of Pb2+
dent upon both the binding-site environment of the indole and
to PLT.
the nature of the metal ion, have been identified.7 Here we
Fig. 2(b) shows plots of the fluorescence intensity ratio (I470/I377
)
report a fluoroionophore sensor for lead ion, N-[4(1-pyrene)-
butyroyl]-L-tryptophan (PLT; Fig. 1), which was synthesized
by combining a pyrene-containing fluorophore, 4-(1-pyrenyl)-
butyric acid, with a versatile functional tryptophan (see ESI{).
Although PLT is structurally simple and easily synthesized, it
not only displays a strong interaction between the indole moiety
as a function of metal ion concentration. If it is assumed that the
fluorescence change is induced solely by 2 : 1 complex formation
between PLT and the metal ion (M2+), the fluorescence intensity
ratio (I470/I377) can be expressed by eqn (1),4,9 where [PLT]0 is the
initial concentration of the fluoroionophore, and wf1 and wf2 are
the fluorescence quantum yields for PLT at 470 and 377 nm,
respectively. Similarly, wc1 and wc2 are the quantum yields for the
2 : 1 complex at 470 and 377 nm, respectively. The value of wf1/wf2
can be evaluated from the intensity ratio (I470/I377) when [M2+] =
0 M.9 K21 [eqn (2)] is the apparent 2 : 1 binding constant of PLT
with M2+. As shown in Fig. 2(b), the observed I470/I377 values for
Pb2+ are well-fitted by eqn (1) (curved line), with a binding
constant of 1.09 6 106 M22 (R2 = 0.9935).
Key Laboratory for Supramolecular Structure and Material of Ministry
of Education, Jilin University, 2699 Qianjin Street, Changchun,
P. R. China. E-mail: yqwu@jlu.edu.cn; Fax: +86-431-5193421;
Tel: +86-431-5168730
{ Electronic supplementary information (ESI) available: Synthesis and
characterization of PLT and PLP, spectroscopic and NMR data of PLT
with Pb2+, effect of pH and competing metal ions on the interaction of
PLT and Pb2+. See DOI: 10.1039/b604623a
2702 | Chem. Commun., 2006, 2702–2704
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Fig. 2 (a) Fluorescence spectra of PLT (5.0 6 1026 M in 98% water–2% DMSO (v/v)) with concentration changes of Pb2+. Excitation wavelength is
340 nm. (b) Dependence of I470/I377 on the concentration of various metal ions.
ꢀ
ꢁ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
interference of the potentially competing metal ions on lead ion
detection has been also investigated in the presence of other kinds
of metal ions and the results demonstrate that the presence of Cr3+
or Cu2+ reduces its sensitivity, while no obvious interference of
other metal ions as Mg2+, Ca2+, Fe2+, Zn2+, Mn2+, Cd2+ etc. (in a
concentration of 10 mM) can be observed (see ESI{). Meanwhile,
pH-dependent study on the interaction between PLT and lead ion
indicates that pH around 6.4 is the ideal condition because low pH
prevent the formation of COO2, while high pH will cause
precipitation of metal ions (see ESI{).
w
wc1
4
f1 z
wf2 wf2
{1z 1z8K21½M2zꢀ½PLTꢀ0
I470
I377
ꢀ
ꢁ
~
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
(1)
(2)
wc2
wf2
4z
{1z 1z8K21½M2zꢀ½PLTꢀ0
½M2z(PLT)2ꢀ
K21~
2
½M2zꢀ½PLTꢀ
As also shown in Fig. 2(b), the fluorescence spectra of PLT
upon addition of Ca2+, Co2+, Cu2+, K+, Mg2+, Na+, Fe2+, Mn2+
and Ni2+ were characterized by certain extent quenching and a lack
of dimer peak at 470 nm. Only a very weak I470/I377 response was
observed for PLT upon addition of Cd2+, Cr3+ and Zn2+ (see
Fig. 2(b) and ESI{). Therefore, it can be concluded that PLT
exhibits high selectivity for Pb2+ over the other examined metal
ions in water. In addition, it should be noted that high sensitivity
for Pb2+ (detection limit up to 0.15 mM) is achieved by PLT in the
present study. This represents the lowest reported detection limit
for Pb2+ by a fluoroionophore in aqueous solution and is well
within the practical range (0.02–1.0 mM) for Pb2+ analysis.3 The
When the indole moiety in PLT was changed to benzene, only
detectable low-region fluorescence quenching other than the
470 nm emission response was detected (see ESI{). This result
demonstrates that indole moiety of PLT plays a crucial role in the
specific recognition of Pb2+.
NMR experiments were performed to explore the coordination
1
mechanism of PLT and Pb2+. Fig. 3 shows H and 13C NMR
spectra of indole before and after the addition of Pb2+
(10 equivalents). Indole protons shifted downfield upon Pb2+
binding, and obvious chemical shifts were also observed for
carbons.10 These findings not only are consistent with a direct
coordination interaction between indole and Pb2+, but also
1
Fig. 3 Region of the H NMR (left) and 13C NMR (right) spectra of (a) PLT (5.5 mM), (b) PLT/Pb2+ in D2O–DMSO-d6 (1 : 2, v/v) solution.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2702–2704 | 2703

View Article Online
specific interactions between metal ions and a variety of amino
acids, modification of the recognition site in other sensors is being
actively undertaken in our laboratory to develop more advanced
fluoroionophore sensors for metal ion recognition in water. These
studies will provide a foundation for future investigations of metal
ion–peptide or ion–protein interactions.
The authors are grateful to the projects of NSFC (No. 20373017
and 20473028), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT0422) and the
Program of Introducing Talents of Discipline to Universities
(B06009).
Fig. 4 Proposed Pb2+–indole interaction model for Pb2+–PLT.
indicate that the entire indole ring binds to the Pb2+ in a
noncovalent interaction (possibly due to spatial positioning).
Because the chemical shift of Trp indolyl C(3) is exquisitely
sensitive to changes in the torsion angles of the side-chain, the
observed chemical shift (DdC3 = 0.318 ppm) reflects substantial
reorientation mobility of the indole unit.11 Chelation of Pb2+ with
carboxylate leads to reorganization of the indole unit to achieve
Pb2+ coordination, leading to changes in the torsion angles of the
Trp indole group (Fig. 4). This coordination mechanism was
further supported by the downfield shift of a-carbon C(9) proton
(DdH9 = 0.066 ppm) and its splitting into four peaks (Fig. 3) due to
the influence of nonequivalent hydrogen C(8)H2, with the rotatory
movement of the indole ring being limited upon binding of
Pb2+ with carboxylate. Furthermore, the chemical shifts of C(12)
(DdC12 = 20.2 ppm) and C(11) (DdC11 = 0.416 ppm) are consistent
with the formation of a hydrogen bond between the carbonyl
oxygen and amide hydrogen (Fig. 4) based on the similar proposed
model.12 Thus, the electrostatic interaction between Pb2+ and
carboxylate induced coordination of Pb2+ within the Trp moiety
and subsequent hydrogen bonding between amide groups, thereby
generating a pyrene dimer. The formation of dimer induced a
C(14) upfield shift of 2.142 ppm due to the high shielding of the
pyrene (see ESI{).
In conclusion, we have characterized a fluoroionophore sensor,
PLT, that shows high selectivity for Pb2+ over Ca2+, Cd2+, Co2+,
Cr3+, Cu2+, K+, Mg2+, Na+, Fe2+, Mn2+, Ni2+ and Zn2+ in aqueous
solution. The sensor exhibited changes in its dimer/monomer
fluorescence emission ratio (I470/I377) in response to Pb2+ in water,
allowing highly sensitive Pb2+ detection (detection limit 0.15 mM).
The high sensitivity and selectivity observed for PLT are very
important for the real-time monitoring and in situ detection of
toxic Pb2+ in the environment.3 In addition, we found that the
indole ring of PLT as well as the formation of pyrene-dimer is
crucially important for the selective recognition of Pb2+. Due to the
Notes and references
1 A. P. de Silva, D. B. Fox, A. J. M. Huxley and T. S. Moody, Coord.
Chem. Rev., 2000, 205, 41; V. Amendola, L. Fabbrizzi, M. Licchelli,
C. Mangano, P. Pallavicini, L. Parodi and A. Poggi, Coord. Chem. Rev.,
1999, 192, 649.
2 H. Ueyama, M. Takagi and S. Takenaka, J. Am. Chem. Soc., 2002, 124,
14286; A. Miyawaki, J. Llopis, R. Heim, J. M. McCaffery, J. A. Adams,
M. Ikura and R. Y. Tsien, Nature, 1997, 388, 882; K. R. Gee,
Z.-L. Zhou, W.-J. Qian and R. Kennedy, J. Am. Chem. Soc., 2002, 124,
776.
3 A. Fetch, Crit. Rev. Anal. Chem., 1998, 28, 267.
4 (a) T. Hayashita, D. Qing, M. Minagawa, J. C. Lee, C. H. Ku and
N. Teramae, Chem. Commun., 2003, 2160; (b) T. Hayashita, D. Qing,
R. A. Bartsch, S. Elshani, R. E. Hanes and N. Teramae, Supramol.
Chem., 2005, 17, 141; (c) R. Me´tivier, I. Leray and B. Valeur,
Chem. Commun., 2003, 996; (d) J. Y. Kwon, Y. J. Jang, Y. J. Lee,
K. M. Kim, M. S. Seo, W. Nam and J. Yoon, J. Am. Chem. Soc., 2005,
127, 10107.
5 L. Pu, Chem. Rev., 2004, 104, 1687.
6 S. L. de Wall, E. S. Meadows, L. J. Barbour and G. W. Gokel, J. Am.
Chem. Soc., 1999, 121, 5613; H. Kumita, T. Kato, K. Jitsukawa,
H. Einaga and H. Masuda, Inorg. Chem., 2001, 40, 3936.
7 N. V. Kaminskaia, G. M. Ullmann, D. B. Fulton and N. M. Kostic,
Inorg. Chem., 2000, 39, 5004; T. Motoyama, Y. Shimazaki, T. Yajima,
Y. Nakabayashi, Y. Naruta and O. Yamauchi, J. Am. Chem. Soc.,
2004, 126, 7378.
8 A. Ueno, I. Suzuki and T. Osa, J. Am. Chem. Soc., 1989, 111, 6391.
9 A. Yamauchi, T. Hayashita, S. Nishizawa, M. Watanabe and
N. Teramae, J. Am. Chem. Soc., 1999, 121, 2319.
10 DdH2, DdH4, DdH5, DdH6 and DdH7 are 0.033, 0.039, 0.064, 0.084 and
0.063 ppm, respectively; DdC2, DdC5, DdC6, DdC7, DdC7a and DdC3, DdC3a
and DdC4 are 0.171, 0.203, 0.194, 0.154, 0.098 and 20.318, 20.3 (peak is
part of overlap) and 20.162 ppm, respectively.
11 H. Sun and E. Oldfield, J. Am. Chem. Soc., 2004, 126, 4726; Y. Li and
C. M. Yang, J. Am. Chem. Soc., 2005, 127, 3527; O. Jardetzky,
K. Akasaka, D. Vogel, S. Morris and K. C. Holmes, Nature, 1978, 273,
564.
12 I. I. Schuster, J. Org. Chem., 1985, 50, 1656.
2704 | Chem. Commun., 2006, 2702–2704
This journal is ß The Royal Society of Chemistry 2006